Search

One of the long-term goals of exoplanet science is the atmospheric characterization of dozens of small exoplanets in order to understand their diversity and search for habitable worlds and potential biosignatures. Achieving this goal requires a space mission of sufficient scale. We seek to quantify the exoplanet detection performance of a space-based mid-infrared nulling interferometer that measures the thermal emission of exoplanets. For this, we have developed an instrument simulator that considers all major astrophysical noise sources and coupled it with Monte Carlo simulations of a synthetic exoplanet population around main-sequence stars within 20 pc. This allows us to quantify the number (and types) of exoplanets that our mission concept could detect over a certain time period. Two different scenarios to distribute the observing time among the stellar targets are discussed and different apertures sizes and wavelength ranges are considered. Within a 2.5-year initial search phase, an interferometer consisting of four 2 m apertures with a total instrument throughput of 5% covering a wavelength range between 4 and 18.5 $\mu$m could detect up to ~550 exoplanets with radii between 0.5 and 6 R$_\oplus$ with an integrated SNR$\ge$7. At least ~160 of the detected exoplanets have radii $\le$1.5 R$_\oplus$. Depending on the observing scenario, ~25-45 rocky exoplanets (objects with radii between 0.5 and 1.5 $_{\oplus}$) orbiting within the empirical habitable zone (eHZ) of their host stars are among the detections. With an aperture size of 3.5 m, the total number of detections can increase to up to ~770, including ~60-80 rocky, eHZ planets. With 1 m aperture size, the maximum detection yield is ~315 exoplanets, including $\le$20 rocky, eHZ planets. In terms of predicted detection yield, such a mission can compete with large single-aperture reflected light missions. (abridged), Comment: Accepted for publication by A&A - some typos corrected and affiliations updated; 14 pages main text (incl. 14 figures); first paper in the LIFE paper series; papers II (arXiv:2203.00471) and III (arXiv:2112.02054) are also available

While exoplanets are now routinely detected, the detection of small bodies in extrasolar systems remains challenging. Since the discovery of sporadic events interpreted as exocomets (Falling Evaporating Bodies) around $\beta$ Pic in the early 80s, only $\sim$20 stars have been reported to host exocomet-like events. We aim to expand the sample of known exocomet-host stars, as well as to monitor the hot-gas environment around stars with previously known exocometary activity. We have obtained high-resolution optical spectra of a heterogeneous sample of 117 main-sequence stars in the spectral type range from B8 to G8. The data have been collected in 14 observing campaigns expanding over 2 years from both hemispheres. We have analysed the Ca ii K&H and Na i D lines in order to search for non-photospheric absorptions originated in the circumstellar environment, and for variable events that could be caused by outgassing of exocomet-like bodies. We have detected non-photospheric absorptions towards 50% of the sample, attributing a circumstellar origin to half of the detections (i.e. 26% of the sample). Hot circumstellar gas is detected in the metallic lines inspected via narrow stable absorptions, and/or variable blue-/red-shifted absorption events. Such variable events were found in 18 stars in the Ca ii and/or Na i lines; 6 of them are reported in the context of this work for the first time. In some cases the variations we report in the Ca ii K line are similar to those observed in $\beta$ Pic. While we do not find a significant trend with the age or location of the stars, we do find that the probability of finding CS gas in stars with larger vsin i is higher. We also find a weak trend with the presence of near-infrared excess, and with anomalous ($\lambda$ Boo-like) abundances, but this would require confirmation by expanding the sample., Comment: Accepted in A&A on March 23rd 2020

The debris disk surrounding $\beta$~Pictoris has been observed with ALMA to contain a belt of CO gas with a distinct peak at $\sim$85 au. This CO clump is thought to be the result of a region of enhanced density of solids that collide and release CO through vaporisation. The parent bodies are thought to be comparable to solar system comets, in which CO is trapped inside a water ice matrix. Since H$_2$O should be released along with CO, we aim to put an upper limit on the H$_2$O gas mass in the disk of $\beta$ Pictoris. We use archival data from the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory to study the ortho-H$_2$O 1$_{10}$-1$_{01}$ emission line. The line is undetected. Using a python implementation of the radiative transfer code RADEX, we convert upper limits on the line flux to H$_2$O gas masses. The resulting lower limits on the CO/H$_2$O mass ratio are compared to the composition of solar system comets. Depending on the assumed gas spatial distribution, we find a 95% upper limit on the ortho-H$_2$O line flux of $7.5 \times 10^{-20}$ W m$^{-2}$ or $1.2 \times 10^{-19}$ W m$^{-2}$. These translate into an upper limit on the H$_2$O mass of $7.4 \times 10^{16}$-$1.1 \times 10^{18}$ kg depending on both the electron density and gas kinetic temperature. The range of derived gas-phase CO/H$_2$O ratios is marginally consistent with low-ratio solar system comets., Comment: 7 pages, 3 figures; accepted for publication in Astronomy & Astrophysics

The quest for other habitable worlds and the search for life among them are major goals of modern astronomy. One way to make progress towards these goals is to obtain high-quality spectra of a large number of exoplanets over a broad range of wavelengths. While concepts currently investigated in the United States are focused on visible/NIR wavelengths, where the planets are probed in reflected light, a compelling alternative to characterize planetary atmospheres is the mid-infrared waveband (5-20~$\mu$m). Indeed, mid-infrared observations provide key information on the presence of an atmosphere, the surface conditions (e.g., temperature, pressure, habitability), and the atmospheric composition in important species such as H$_2$O, CO$_2$, O$_3$, CH$_4$, and N$_2$O. This information is essential to investigate the potential habitability of exoplanets and to make progress towards the search for life in the universe. Obtaining high-quality mid-infrared spectra of exoplanets from the ground is however extremely challenging due to the overwhelming brightness and turbulence of Earth's atmosphere. In this paper, we present a concept of space-based mid-infrared interferometer that can tackle this observing challenge and discuss the main technological developments required to launch such a sophisticated instrument., Comment: 20 page, 3 figures, 1 table

The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and other stars are at the focus of intense contemporary research. Observations at submm/mm wavelengths are particularly useful to obtain information about the run of the temperature in the upper atmosphere of Sun-like stars. We used ALMA to study the chromospheric emission of the aCen binary system in all six available frequency bands during Cycle2 in 2014/15. Since ALMA is an interferometer, the multi-telescope array is particularly suited for the observation of point sources. With its large collecting area, the sensitivity is high enough to allow the observation of nearby main-sequence stars at submm/mm wavelengths for the first time. The comparison of the observed spectral energy distributions with theoretical model computations provides the chromospheric structure in terms of temperature and density above the stellar photosphere and the quantitative understanding of the primary emission processes. Both stars were detected and resolved at all ALMA frequencies. For both aCenA and B, the existence and location of the temperature minima, firstly detected from space with Herschel, are well reproduced by the theoretical models of this paper. For aCenB, the temperature minimum is deeper than for A and occurs at a lower height in the atmosphere, but for both stars, Tmin/Teff is consistently lower than what is derived from optical and UV data. The submm/mm emission of the aCen stars is indeed very well reproduced by modified chromospheric models of the Quiet Sun. This most likely means that the non-radiative heating mechanisms of the upper atmosphere that are at work in the Sun are operating also in other solar-type stars., Comment: 10 pages, 7 figures, accepted for Astron. & Astrophys., Section 8

Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. The full sample of 177 FGK stars with d<20 pc proposed for the DUNES survey is presented. Herschel/PACS observations at 100 and 160 micron complemented with data at 70 micron, and at 250, 350 and 500 micron SPIRE photometry, were obtained. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the DEBRIS consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age. The subsample of 105 stars with d<15 pc containing 23 F, 33 G and 49 K stars, is complete for F stars, almost complete for G stars and contains a substantial number of K stars to draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type 0.26 (6 objects with excesses out of 23 F stars), 0.21 (7 out of 33 G stars) and 0.20 (10 out of 49 K stars), the fraction for all three spectral types together being 0.22 (23 out of 105 stars). Uncertainties corresponding to a 95% confidence level are given in the text for all these numbers. The medians of the upper limits of L_dust/L_* for each spectral type are 7.8E-7 (F), 1.4E-6 (G) and 2.2E-6 (K); the lowest values being around 4.0E-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts., Comment: 31 pages, 15 figures, 10 tables, 2 appendices

The young star beta Pictoris is well known for its dusty debris disk, produced through the grinding down by collisions of planetesimals, kilometre-sized bodies in orbit around the star. In addition to dust, small amounts of gas are also known to orbit the star, likely the result from vaporisation of violently colliding dust grains. The disk is seen edge on and from previous absorption spectroscopy we know that the gas is very rich in carbon relative to other elements. The oxygen content has been more difficult to assess, however, with early estimates finding very little oxygen in the gas at a C/O ratio 20x higher than the cosmic value. A C/O ratio that high is difficult to explain and would have far-reaching consequences for planet formation. Here we report on observations by the far-infrared space telescope Herschel, using PACS, of emission lines from ionised carbon and neutral oxygen. The detected emission from C+ is consistent with that previously reported being observed by the HIFI instrument on Herschel, while the emission from O is hard to explain without assuming a higher-density region in the disk, perhaps in the shape of a clump or a dense torus, required to sufficiently excite the O atoms. A possible scenario is that the C/O gas is produced by the same process responsible for the CO clump recently observed by ALMA in the disk, and that the re-distribution of the gas takes longer than previously assumed. A more detailed estimate of the C/O ratio and the mass of O will have to await better constraints on the C/O gas spatial distribution., Comment: Accepted by A&A; 6 pages including 3 figures

Hydrides are simple compounds containing one or a few hydrogen atoms bonded to a heavier atom. They are fundamental precursor molecules in cosmic chemistry and many hydride ions have become observable in high quality for the first time thanks to the Herschel Space Observatory. Ionized hydrides, such as CH+ and OH+, and also HCO+ that affect the chemistry of molecules such as water, provide complementary information on irradiation by far UV (FUV) or X-rays and gas temperature. The targeted lines of CH+, OH+, H2O+, C+ and CH are detected mostly in blue-shifted absorption. H3O+ and SH+ are detected in emission and only toward some high-mass objects. The observed line parameters and correlations suggest two different origins, related to gas entrained by the outflows and to the circumstellar envelope. The column density ratios of CH+/OH+ are estimated from chemical slab models, assuming that the H2 density is given by the specific density model of each object at the beam radius. For the low-mass YSOs the observed ratio can be reproduced for an FUV flux of 2-400 times the ISRF at the location of the molecules. In two high-mass objects, the UV flux is 20-200 times the ISRF derived from absorption lines, and 300-600 ISRF using emission lines. If the FUV flux required for low-mass objects originates at the central protostar, a substantial FUV luminosity, up to 1.5 L_sun, is required. There is no molecular evidence for X-ray induced chemistry in the low-mass objects on the observed scales of a few 1000 AU. For high-mass regions, the FUV flux required to produce the observed molecular ratios is smaller than the unattenuated flux expected from the central object(s) at the Herschel beam radius. This is consistent with an FUV flux reduced by circumstellar extinction or by bloating of the protostar., Comment: 40 pages, 97 figures, accepted by Astronomy and Astrophysics

Protoplanetary disks dissipate rapidly after the central star forms, on time-scales comparable to those inferred for planet formation. In order to allow the formation of planets, disks must survive the dispersive effects of UV and X-ray photoevaporation for at least a few Myr. Viscous accretion depletes significant amounts of the mass in gas and solids, while photoevaporative flows driven by internal and external irradiation remove most of the gas. A reasonably large fraction of the mass in solids and some gas get incorporated into planets. Here, we review our current understanding of disk evolution and dispersal, and discuss how these might affect planet formation. We also discuss existing observational constraints on dispersal mechanisms and future directions., Comment: Chapter in International Space Science Institute (ISSI) Book on "The Disk in Relation to the Formation of Planets and their Proto-atmospheres", to be published in Space Science Reviews by Springer

We recently announced the detection of an unknown submillimeter source in our ALMA observations of alpha Cen AB. The source was detected in two epochs, a strong detection at 445~GHz and one at lower significance at 343.5~GHz. After valuable feedback of the community, it turns out that the detection at 343.5~GHz could not be reproduced with a different reduction software nor with fitting within the $(u,v)$-plane. The detection at 445~GHz has been further confirmed with modeling of the $(u,v)$-data and was shown to be robust at $>12\sigma$, confirming our detection of this unknown source. However, based on only one epoch, further analysis and preferably new data are needed, before publication of an article in which the nature of the new source can be discussed. The analysis has indicated the importance of both $(u,v)$-plane fitting and alternative data reduction when dealing with low signal to noise source detections., Comment: withdrawn until further data is available

To date O2 has definitely been detected in only two sources, namely rho Oph A and Orion, reflecting the extremely low abundance of O2 in the interstellar medium. One of the sources in the HOP program is the +50 km/s Cloud in the Sgr A Complex in the centre of the Milky Way. The Herschel HIFI is used to search for the 487 and 774 GHz emission lines of O2. No O2 emission is detected towards the Sgr A +50 km/s Cloud, but a number of strong emission lines of methanol (CH3OH) and absorption lines of chloronium (H2Cl+) are observed. A 3 sigma upper limit for the fractional abundance ratio of (O2)/(H2) in the Sgr A +50 km/s Cloud is found to be X(O2) less than 5 x 10(-8). However, since we can find no other realistic molecular candidate than O2 itself, we very tentatively suggest that two weak absorption lines at 487.261 and 487.302 GHz may be caused by the 487 GHz line of O2 in two foreground spiral arm clouds. By considering that the absorption may only be apparent, the estimated upper limit to the O2 abundance of less than (10-20) x 10(-6) in these foreground clouds is very high. This abundance limit was determined also using Odin non-detection limits. If the absorption is due to a differential Herschel OFF-ON emission, the O2 fractional abundance may be of the order of (5-10) x 10(-6). With the assumption of pure absorption by foreground clouds, the unreasonably high abundance of (1.4-2.8) x 10(-4) was obtained. The rotation temperatures for CH3OH-A and CH3OH-E lines in the +50 km/s Cloud are found to be 64 and 79 K, respectively, and the fractional abundance of CH3OH is approximately 5 x 10(-7)., Comment: Accepted by Astronomy & Astrophysics, 11 pages, 12 figures

We aim at determining the spatial distribution of the gas and dust in star-forming regions and address their relative abundances in quantitative terms. We also examine the dust opacity exponent beta for spatial and/or temporal variations. Using mapping observations of the very dense rho Oph A core, we examined standard 1D and non-standard 3D methods to analyse data of far-infrared and submillimeter (submm) continuum radiation. The resulting dust surface density distribution can be compared to that of the gas. The latter was derived from the analysis of accompanying molecular line emission, observed with Herschel from space and with APEX from the ground. As a gas tracer we used N2H+, which is believed to be much less sensitive to freeze-out than CO and its isotopologues. Radiative transfer modelling of the N2H+(J=3-2) and (J=6-5) lines with their hyperfine structure explicitly taken into account provides solutions for the spatial distribution of the column density N(H2), hence the surface density distribution of the gas. The gas-to-dust mass ratio is varying across the map, with very low values in the central regions around the core SM 1. The global average, =88, is not far from the canonical value of 100, however. In rho Oph A, the exponent beta of the power-law description for the dust opacity exhibits a clear dependence on time, with high values of 2 for the envelope-dominated emission in starless Class -1 sources to low values close to 0 for the disk-dominated emission in Class III objects. beta assumes intermediate values for evolutionary classes in between. Since beta is primarily controlled by grain size, grain growth mostly occurs in circumstellar disks. The spatial segregation of gas and dust, seen in projection toward the core centre, probably implies that, like C18O, also N2H+ is frozen onto the grains., Comment: 21 pages, 18 figures

The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and those of other stars constitute an active field of research. By studying stellar chromospheres one aims at identifying the relevant physical processes. Defining the permittable extent of the parameter space can also serve as a template for the Sun-as-a-star. Earlier observations with Herschel and APEX have revealed the temperature minimum of alpha Cen, but these were unable to spatially resolve the binary into individual components. With the data reported here, we aim at remedying this shortcoming. Furthermore, these earlier data were limited to the wavelength region between 100 and 870mu. In the present context, we intend to extend the spectral mapping to longer wavelengths, where the contrast between stellar photospheric and chromospheric emission becomes increasingly evident. ALMA is particularly suited to point sources, such as unresolved stars. ALMA provides the means to achieve our objectives with both its high sensitivity of the collecting area for the detection of weak signals and the high spatial resolving power of its adaptable interferometer for imaging close multiple stars. This is the first detection of main-sequence stars at a wavelength of 3mm. Furthermore, the individual components of the binary alpha CenAB are clearly detected and spatially well resolved at all ALMA wavelengths. The high S/N of these data permit accurate determination of their relative flux ratios. The previously obtained flux ratio of 0.44, which was based on measurements in the optical and at 70mu, is consistent with the present ALMA results, albeit with a large error bar. Given the distinct difference in their cyclic activity, the similarity of their submm SEDs appears surprising., Comment: 5 pages, 3 figures, Letter to Astronomy and Astrophysics

The HH54 shock is a Herbig-Haro object, located in the nearby Chamaeleon II cloud. Observed CO line profiles are due to a complex distribution in density, temperature, velocity, and geometry. Resolving the HH54 shock wave in the far-infrared cooling lines of CO constrain the kinematics, morphology, and physical conditions of the shocked region. We used the PACS and SPIRE instruments on board the Herschel space observatory to map the full FIR spectrum in a region covering the HH54 shock wave. Complementary Herschel-HIFI, APEX, and Spitzer data are used in the analysis as well. The observed features in the line profiles are reproduced using a 3D radiative transfer model of a bow-shock, constructed with the Line Modeling Engine code (LIME). The FIR emission is confined to the HH54 region and a coherent displacement of the location of the emission maximum of CO with increasing J is observed. The peak positions of the high-J CO lines are shifted upstream from the lower J CO lines and coincide with the position of the spectral feature identified previously in CO(10-9) profiles with HIFI. This indicates a hotter molecular component in the upstream gas with distinct dynamics. The coherent displacement with increasing J for CO is consistent with a scenario where IRAS12500-7658 is the exciting source of the flow, and the 180 K bow-shock is accompanied by a hot (800 K) molecular component located upstream from the apex of the shock and blueshifted by -7 km s$^{-1}$. The spatial proximity of this knot to the peaks of the atomic fine-structure emission lines observed with Spitzer and PACS ([OI]63, 145 $\mu$m) suggests that it may be associated with the dissociative shock as the jet impacts slower moving gas in the HH54 bow-shock., Comment: 6 pages, 5 figures

Context: Outflows are an important part of the star formation process as both the result of ongoing active accretion and one of the main sources of mechanical feedback on small scales. Water is the ideal tracer of these effects because it is present in high abundance in various parts of the protostar. Method: We present \textit{Herschel} HIFI spectra of multiple water-transitions towards 29 nearby Class 0/I protostars as part of the WISH Survey. These are decomposed into different Gaussian components, with each related to one of three parts of the protostellar system; quiescent envelope, cavity shock and spot shocks in the jet and at the base of the outflow. We then constrain the excitation conditions present in the two outflow-related components. Results: Water emission is optically thick but effectively thin, with line ratios that do not vary with velocity, in contrast to CO. The physical conditions of the cavity and spot shocks are similar, with post-shock H$_{2}$ densities of order 10$^{5}-$10$^{8}$\,cm$^{-3}$ and H$_{2}$O column densities of order 10$^{16}-$10$^{18}$\,cm$^{-2}$. H$_{2}$O emission originates in compact emitting regions: for the spot shocks these correspond to point sources with radii of order 10-200\,AU, while for the cavity shocks these come from a thin layer along the outflow cavity wall with thickness of order 1-30\,AU. Conclusions: Water emission at the source position traces two distinct kinematic components in the outflow; J shocks at the base of the outflow or in the jet, and C shocks in a thin layer in the cavity wall. Class I sources have similar excitation conditions to Class 0 sources, but generally smaller line-widths and emitting region sizes. We suggest that it is the velocity of the wind driving the outflow, rather than the decrease in envelope density or mass, that is the cause of the decrease in H$_{2}$O intensity between Class 0 and I., Comment: Abstract is abridged. Accepted to Astronomy & Astrophysics. 50 pages (21 excluding appendices), 15 figures and 7 tables in the main paper, 18 figures and 10 tables in the appendices. Table 4 updated to Corrigendum version

Context: Herschel observations suggest that the H$_2$O distribution in outflows from low-mass stars resembles the H$_2$ emission. It is still unclear which of the different excitation components that characterise the mid- and near-IR H$_2$ distribution is associated with H$_2$O. Aim: The aim is to spectrally resolve the different excitation components observed in the H$_2$ emission. This will allow us to identify the H$_2$ counterpart associated with H$_2$O and finally derive directly an H$_2$O abundance estimate with respect to H$_2$. Methods: We present new high spectral resolution observations of H$_2$ 0-0 S(4), 0-0 S(9), and 1-0 S(1) towards HH 54, a bright nearby shock region in the southern sky. In addition, new Herschel-HIFI H$_2$O (2$_{12}$$-$1$_{01}$) observations at 1670~GHz are presented. Results: Our observations show for the first time a clear separation in velocity of the different H$_2$ lines: the 0-0 S(4) line at the lowest excitation peaks at $-$7~km~s$^{-1}$, while the more excited 0-0 S(9) and 1-0 S(1) lines peak at $-$15~km~s$^{-1}$. H$_2$O and high-$J$ CO appear to be associated with the H$_2$ 0-0 S(4) emission, which traces a gas component with a temperature of 700$-$1000 K. The H$_2$O abundance with respect to H$_2$ 0-0 S(4) is estimated to be $X$(H$_2$O)$<$1.4$\times$10$^{-5}$ in the shocked gas over an area of 13$^{\prime\prime}$. Conclusions: We resolve two distinct gas components associated with the HH 54 shock region at different velocities and excitations. This allows us to constrain the temperature of the H$_2$O emitting gas ($\leq$1000 K) and to derive correct estimates of H$_2$O abundance in the shocked gas, which is lower than what is expected from shock model predictions., Comment: Astronomy & Astrophysics Letter, in press

We present the study of the H2O spatial distribution at two bright shocked regions along IRAS4A, one of the strongest H2O emitters among the Class 0 outflows. We obtained Herschel-PACS maps of the IRAS4A outflow and HIFI observations of two shocked positions. The largest HIFI beam of 38 arcsec at 557 GHz was mapped in several key water lines with different upper energy levels, to reveal possible spatial variations of the line profiles. We detect four H2O lines and CO (16-15) at the two selected positions. In addition, transitions from related outflow and envelope tracers are detected. Different gas components associated with the shock are identified in the H2O emission. In particular, at the head of the red lobe of the outflow, two distinct gas components with different excitation conditions are distinguished in the HIFI emission maps: a compact component, detected in the ground-state water lines, and a more extended one. Assuming that these two components correspond to two different temperature components observed in previous H2O and CO studies, the excitation analysis of the H2O emission suggests that the compact (about 3 arcsec) component is associated with a hot (T~1000 K) gas with densities ~(1-4)x10^5 cm^{-3}, whereas the extended one (10-17 arcsec) traces a warm (T~300-500 K) and dense gas (~(3-5)x10^7 cm^{-3}). Finally, using the CO (16-15) emission observed at R2, we estimate the H2O/H2 abundance of the warm and hot components to be (7-10)x10^{-7} and (3-7)x10^{-5}. Our data allowed us, for the first time, to resolve spatially the two temperature components previously observed with HIFI and PACS. We propose that the compact hot component may be associated with the jet that impacts the surrounding material, whereas the warm, dense, and extended component originates from the compression of the ambient gas by the propagating flow., Comment: 13 pages, 11 figures. Accepted for publication in Astronomy and Astrophysics

[Abridged] Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby binary aCentauri have higher than solar metallicities, which is thought to promote giant planet formation. We aim to determine the level of emission from debris in the aCen system. Having already detected the temperature minimum, Tmin, of aCenA, we here attempt to do so also for the companion aCenB. Using the aCen stars as templates, we study possible effects Tmin may have on the detectability of unresolved dust discs around other stars. We use Herschel and APEX photometry to determine the stellar spectral energy distributions. In addition, we use APEX for spectral line mapping to study the complex background around aCen seen in the photometric images. Models of stellar atmospheres and discs are used to estimate the amount of debris around these stars. For solar-type stars, a fractional dust luminosity fd 2e-7 could account for SEDs that do not exhibit the Tmin-effect. Slight excesses at the 2.5 sigma level are observed at 24 mu for both stars, which, if interpreted to be due to dust, would correspond to fd (1-3)e-5. Dynamical disc modelling leads to rough mass estimates of the putative Zodi belts around the aCen stars, viz. <~4e-6 MMoon of 4 to 1000 mu size grains, distributed according to n a^-3.5. Similarly, for filled-in Tmin emission, corresponding EKBs could account for ~1e-3 MMoon of dust. Light scattered and/or thermally emitted by exo-Zodi discs will have profound implications for future spectroscopic missions designed to search for biomarkers in the atmospheres of Earth-like planets. The F-IR SED of aCenB is marginally consistent with the presence of a minimum temperature region in the upper atmosphere. We also show that an aCenA-like temperature minimum may result in an erroneous apprehension about the presence of dust around other stars., Comment: 16 pages, 11 figures, accepted by A&A 21 Jan 2014; updated affiliations

[abridged] Aims. Our Herschel Open Time Key Programme DUNES aims at detecting and characterizing debris disks around nearby, sun-like stars. In addition to the statistical analysis of the data, the detailed study of single objects through spatially resolving the disk and detailed modeling of the data is a main goal of the project. Methods. We obtained the first observations spatially resolving the debris disk around the sun-like star HIP 17439 (HD23484) using the instruments PACS and SPIRE on board the Herschel Space Observatory. Simultaneous multi-wavelength modeling of these data together with ancillary data from the literature is presented. Results. A standard single component disk model fails to reproduce the major axis radial profiles at 70 um, 100 um, and 160 um simultaneously. Moreover, the best-fit parameters derived from such a model suggest a very broad disk extending from few au up to few hundreds of au from the star with a nearly constant surface density which seems physically unlikely. However, the constraints from both the data and our limited theoretical investigation are not strong enough to completely rule out this model. An alternative, more plausible, and better fitting model of the system consists of two rings of dust at approx. 30 au and 90 au, respectively, while the constraints on the parameters of this model are weak due to its complexity and intrinsic degeneracies. Conclusions. The disk is probably composed of at least two components with different spatial locations (but not necessarily detached), while a single, broad disk is possible, but less likely. The two spatially well-separated rings of dust in our best-fit model suggest the presence of at least one high mass planet or several low-mass planets clearing the region between the two rings from planetesimals and dust., Comment: 12 pages, 4 figures, accepted for publication in A&A

Context: The dusty debris disk around the $\sim$20 Myr old main-sequence A-star {\beta} Pictoris is known to contain gas. Evidence points towards a secondary origin of the gas as opposed to being a direct remnant form the initial protoplanetary disk, although the dominant gas production mechanism is so far not identified. The origin of the observed overabundance of C and O compared to solar abundances of metallic elements, e.g. Na and Fe, is also unclear. Aims: Our goal is to constrain the spatial distribution of C in the disk, and thereby the gas origin and its abundance pattern. Methods: We used the HIFI instrument onboard Herschel to observe and spectrally resolve CII emission at 158 $\mu$m from the {\beta} Pic debris disk. Assuming Keplerian rotation, we use the spectrally resolved line profile to constrain the spatial distribution of the gas. Results: We show that most of the gas is located around $\sim$100 AU or beyond. We estimate a total C gas mass of $1.3\times10^{-2}$ M$_\oplus$. The data suggest that more gas is located on the southwest side of the disk than on the northeast side. The data are consistent with the hypothesis of a well-mixed gas (constant C/Fe ratio throughout the disk). Assuming instead a spatial profile expected from a simplified accretion disk model, we found it to give a significantly worse fit to the observations. Conclusions: Since the bulk of the gas is found outside 30 AU, we argue that the cometary objects known as "falling evaporating bodies" are unlikely to be the dominant source of gas; production from grain-grain collisions or photodesorption seems more likely. The incompatibility of the observations with a simplified accretion disk model could favour a preferential depletion explanation for the overabundance of C and O. More stringent constraints on the spatial distribution will be available from ALMA observations of CI at 609 $\mu$m., Comment: 10 pages, 7 figures, published in A&A. v2: Revised section 5.4 and figure 6, results unchanged. Changed x- and y-axis of figure 5. Some other minor changes. Fixed typos v3, v4: Language correction, some typos

In the framework of the WISH key program, several H2O (E_u>190 K), high-J CO, [OI], and OH transitions are mapped with PACS in two shock positions along the two prototypical low-luminosity outflows L1448 and L1157. Previous HIFI H2O observations (E_u=53-249 K) and complementary Spitzer mid-IR H2 data are also used, with the aim of deriving a complete picture of the excitation conditions. At all selected spots a close spatial association between H2O, mid-IR H2, and high-J CO emission is found, whereas the low-J CO emission traces either entrained ambient gas or a remnant of an older shock. The excitation analysis at L1448-B2 suggests that a two-component model is needed to reproduce the H2O, CO, and mid-IR H2 lines: an extended warm component (T~450 K) is traced by the H2O emission with E_u =53-137 K and by the CO lines up to J=22-21, and a compact hot component (T=1100 K) is traced by the H2O emission with E_u>190 K and by the higher-J CO lines. At L1448-B2 we obtain an H2O abundance (3-4)x10^{-6} for the warm component and (0.3-1.3)x10^{-5} for the hot component; we also detect OH and blue-shifted [OI] emission, spatially coincident with the other molecular lines and with [FeII] emission. This suggests a dissociative shock for these species, related to the embedded atomic jet. On the other hand, a non-dissociative shock at the point of impact of the jet on the cloud is responsible for the H2O and CO emission. The other examined shock positions show an H2O excitation similar to L1448-B2, but a slightly higher H2O abundance (a factor of 4). The two gas components may represent a gas stratification in the post-shock region. The extended and low-abundance warm component traces the post-shocked gas that has already cooled down to a few hundred Kelvin, whereas the compact and possibly higher-abundance hot component is associated with the gas that is currently undergoing a shock episode., Comment: Accepted for publication in Astronomy and Astrophysics

(abridged) Infrared excesses associated with debris disk host stars detected so far peak at wavelengths around ~100{\mu}m or shorter. However, six out of 31 excess sources in the Herschel OTKP DUNES have been seen to show significant - and in some cases extended - excess emission at 160{\mu}m, which is larger than the 100{\mu}m excess. This excess emission has been suggested to stem from debris disks colder than those known previously. Using several methods, we re-consider whether some or even all of the candidates may be associated with unrelated galactic or extragalactic emission and conclude that it is highly unlikely that none of the candidates represents a true circumstellar disk. For true disks, both the dust temperatures inferred from the SEDs and the disk radii estimated from the images suggest that the dust is nearly as cold as a blackbody. This requires the grains to be larger than ~100{\mu}m, regardless of their material composition. To explain the dearth of small grains, we explore several conceivable scenarios: transport-dominated disks, disks of low dynamical excitation, and disks of unstirred primordial macroscopic grains. Our qualitative analysis and collisional simulations rule out the first two of these scenarios, but show the feasibility of the third one. We show that such disks can survive for gigayears, largely preserving the primordial size distribution. They should be composed of macroscopic solids larger than millimeters, but smaller than kilometers in size. Thus planetesimal formation, at least in the outer regions of the systems, has stopped before "cometary" or "asteroidal" sizes were reached., Comment: Astrophysical Journal (in press). 22 pages, 13 figures

GASPS is a far-infrared line and continuum survey of protoplanetary and young debris disks using PACS on the Herschel Space Observatory. The survey includes [OI] at 63 microns, as well as 70, 100 and 160um continuum, with the brightest objects also studied in [OI]145um, [CII]157um, H2O and CO. Targets included T Tauri stars and debris disks in 7 nearby young associations, and a sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide disk sample, systemically comparing the results with models. In this paper we review the main aims, target selection and observing strategy. We show initial results, including line identifications, sources detected, and a first statistical study. [OI]63um was the brightest line in most objects, by a factor of ~10. Detection rates were 49%, including 100% of HAeBe stars and 43% of T Tauri stars. Comparison with published dust masses show a dust threshold for [OI]63um detection of ~1e-5 M_solar. Normalising to 140pc distance, 32% with mass 1e-6 - 1e-5 M_solar, and a small number with lower mass were also detected. This is consistent with moderate UV excess and disk flaring. In most cases, continuum and line emission is spatially and spectrally unresolved, suggesting disk emission. ~10 objects were resolved, likely from outflows. Detection rates in [OI]145um, [CII]157um and CO J=18-17 were 20-40%, but [CII] was not correlated with disk mass, suggesting it arises instead from a compact envelope. [OI] detection rates in T Tauri associations of ages 0.3-4Myr were ~50%. ~2 stars were detectable in associations of 5-20Myr, with no detections in associations of age >20Myr. Comparing with the total number of young stars, and assuming a ISM-like gas/dust ratio, this indicates that ~18% of stars retain a gas-rich disk of total mass >1M_Jupiter for 1-4Myr, 1-7% keep such disks for 5-10Myr, and none remain beyond 10-20Myr., Comment: Accepted for PASP

Ground based radial velocity (RV) searches continue to discover exoplanets below Neptune mass down to Earth mass. Furthermore, ground based transit searches now reach milli-mag photometric precision and can discover Neptune size planets around bright stars. These searches will find exoplanets around bright stars anywhere on the sky, their discoveries representing prime science targets for further study due to the proximity and brightness of their host stars. A mission for transit follow-up measurements of these prime targets is currently lacking. The first ESA S-class mission CHEOPS (CHaracterizing ExoPlanet Satellite) will fill this gap. It will perform ultra-high precision photometric monitoring of selected bright target stars almost anywhere on the sky with sufficient precision to detect Earth sized transits. It will be able to detect transits of RV-planets by photometric monitoring if the geometric configuration results in a transit. For Hot Neptunes discovered from the ground, CHEOPS will be able to improve the transit light curve so that the radius can be determined precisely. Because of the host stars' brightness, high precision RV measurements will be possible for all targets. All planets observed in transit by CHEOPS will be validated and their masses will be known. This will provide valuable data for constraining the mass-radius relation of exoplanets, especially in the Neptune-mass regime. During the planned 3.5 year mission, about 500 targets will be observed. There will be 20% of open time available for the community to develop new science programmes.

Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. We used {\it Herschel}/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 $\mu$m were obtained, complemented in some cases with observations at 70 $\mu$m, and at 250, 350 and 500 $\mu$m using SPIRE. The observing strategy was to integrate as deep as possible at 100 $\mu$m to detect the stellar photosphere. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of $\sim$ 12.1% $\pm$ 5% before \emph{Herschel} to $\sim$ 20.2% $\pm$ 2%. A significant fraction ($\sim$ 52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70-160$\mu$m range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.

Context. The observed physical properties of outflows from low-mass sources put constraints on possible ejection mechanisms. Historically, these quantities have been derived from CO using ground-based observations. It is thus important to investigate whether parameters such as momentum rate (thrust) and mechanical luminosity (power) are the same when different molecular tracers are used. Aims. We aim at determining the outflow momentum, dynamical time-scale, thrust, energy and power using CO and H2O as tracers of outflow activity. Methods. Within the framework of the WISH key program, three molecular outflows from Class 0 sources have been mapped using the HIFI instrument aboard Herschel. We use these observations together with previously published H2 data to infer the physical properties of the outflows. We compare the physical properties derived here with previous estimates based on CO observations. Results. Inspection of the spatial distribution of H2O and H2 confirms that these molecules are co-spatial. The most prominent emission peaks in H2 coincide with strong H2O emission peaks and the estimated widths of the flows when using the two tracers are comparable. Conclusions. For the momentum rate and the mechanical luminosity, inferred values are independent of which tracer that is used, i.e., the values agree to within a factor of 4 and 3 respectively., Comment: Accepted for publication in A&A, 5 pages, 2 figures

(Abridged) We present a survey of the water emission in a sample of more than 20 outflows from low mass young stellar objects with the goal of characterizing the physical and chemical conditions of the emitting gas. We have used the HIFI and PACS instruments on board the Herschel Space Observatory to observe the two fundamental lines of ortho-water at 557 and 1670 GHz. These observations were part of the "Water In Star-forming regions with Herschel" (WISH) key program, and have been complemented with CO and H2 data. We find that the emission from water has a different spatial and velocity distribution from that of the J=1-0 and 2-1 transitions of CO, but it has a similar spatial distribution to H2, and its intensity follows the H2 intensity derived from IRAC images. This suggests that water traces the outflow gas at hundreds of kelvins responsible for the H2 emission, and not the component at tens of kelvins typical of low-J CO emission. A warm origin of the water emission is confirmed by a remarkable correlation between the intensities of the 557 and 1670 GHz lines, which also indicates the emitting gas has a narrow range of excitations. A non-LTE radiative transfer analysis shows that while there is some ambiguity on the exact combination of density and temperature values, the gas thermal pressure nT is constrained within less than a factor of 2. The typical nT over the sample is 4 10^{9} cm^{-3}K, which represents an increase of 10^4 with respect to the ambient value. The data also constrain within a factor of 2 the water column density. When this quantity is combined with H2 column densities, the typical water abundance is only 3 10^{-7}, with an uncertainty of a factor of 3. Our data challenge current C-shock models of water production due to a combination of wing-line profiles, high gas compressions, and low abundances., Comment: 21 pages, 13 figures. Accepted for publication in A&A

(Abridged) Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied. We quantify their far-infrared line emission and the contribution of different atomic and molecular species to the gas cooling budget during protostellar evolution. We also determine the spatial extent of the emission and investigate the underlying excitation conditions. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 objects, including 5 Class I sources. The high-excitation H2O line at 63.3 micron is detected in 7 sources. CO transitions from J=14-13 up to 49-48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~350 K and ~700 K. H2O has typical excitation temperatures of ~150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern depending on the species and the transition. The H2O line fluxes correlate strongly with those of the high-J CO lines, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH and not with [OI] fluxes. The PACS data probe at least two physical components. The H2O and CO emission likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, likely resulting from H2O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [OI] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H2O and CO, with [OI] increasing for Class I sources., Comment: Accepted for publication in A&A, 48 pages with appendix

Chromospheres and coronae are common phenomena on solar-type stars. Understanding the energy transfer to these heated atmospheric layers requires direct access to the relevant empirical data. Study of these structures has, by and large, been limited to the Sun thus far. The region of the temperature reversal can be directly observed only in the far infrared and submm. We aim at the determination of the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star alpha Cen A. For the nearby binary system alpha Centauri, stellar parameters are known with high accuracy from measurements. For the basic model parameters Teff, log g and [Fe/H], we interpolate in the grid of GAIA/PHOENIX stellar model atmospheres and compute the corresponding model for the G2 V star alpha Cen A. Comparison with photometric measurements shows excellent agreement between observed photospheric data in the optical and infrared. For longer wavelengths, the modelled spectral energy distribution is compared to MIPS, PACS, SPIRE and LABOCA photometry. A specifically tailored Uppsala model based on the MARCS code and extending further in wavelength is used to gauge the emission characteristics of alpha Cen A in the FIR. Similar to the Sun, the FIR emission of alpha Cen A originates in the minimum temperature region above the stellar photosphere in the visible. However, in comparison with the solar case, the FIR photosphere of alpha Cen A appears marginally cooler, Tmin=T160mu=3920+/-375 K. Beyond the minimum near 160mu, the brightness temperatures increase and this radiation likely originates in warmer regions of the chromosphere of alpha Cen A. To the best of our knowledge this is the first time a temperature minimum has been directly measured on a main-sequence star other than the Sun., Comment: 5 pages, 1 figure, A&A Letters, in press

Some planetary systems harbour debris disks containing planetesimals such as asteroids and comets. Collisions between such bodies produce small dust particles, the spectral features of which reveal their composition and, hence, that of their parent bodies. A measurement of the composition of olivine crystals has been done for the protoplanetary disk HD100546 and for olivine crystals in the warm inner parts of planetary systems. The latter compares well with the iron-rich olivine in asteroids (x<0.29). In the cold outskirts of the Beta Pictoris system, an analogue to the young Solar System, olivine crystals were detected but their composition remained undetermined, leaving unknown how the composition of the bulk of Solar System cometary olivine grains compares with that of extrasolar comets. Here we report the detection of the 69-micrometre-wavelength band of olivine crystals in the spectrum of Beta Pictoris. Because the disk is optically thin, we can associate the crystals with an extrasolar proto-Kuiper belt a distance of 15-45 astronomical units from the star (one astronomical unit is the Sun-Earth distance), determine their magnesium-rich composition (x=0.01+/-0.001) and show that they make up 3.6+/-1.0 per cent of the total dust mass. These values are strikingly similar to those for the dust emitted by the most primitive comets in the Solar System, even though Beta Pictoris is more massive and more luminous and has a different planetary system architecture.

We investigate on the spatial and velocity distribution of H2O along the L1448 outflow, its relationship with other tracers, and its abundance variations, using maps of the o-H2O 1_{10}-1_{01} and 2_{12}-1_{01} transitions taken with the Herschel-HIFI and PACS instruments, respectively. Water emission appears clumpy, with individual peaks corresponding to shock spots along the outflow. The bulk of the 557 GHz line is confined to radial velocities in the range \pm 10-50 km/s but extended emission associated with the L1448-C extreme high velocity (EHV) jet is also detected. The H2O 1_{10}-1_{01}/CO(3-2) ratio shows strong variations as a function of velocity that likely reflect different and changing physical conditions in the gas responsible for the emissions from the two species. In the EHV jet, a low H2O/SiO abundance ratio is inferred, that could indicate molecular formation from dust free gas directly ejected from the proto-stellar wind. We derive averaged Tkin and n(H2) values of about 300-500 K and 5 10^6 cm-3 respectively, while a water abundance with respect to H2 of the order of 0.5-1 10^{-6} along the outflow is estimated. The fairly constant conditions found all along the outflow implies that evolutionary effects on the timescales of outflow propagation do not play a major role in the H2O chemistry. The results of our analysis show that the bulk of the observed H2O lines comes from post-shocked regions where the gas, after being heated to high temperatures, has been already cooled down to a few hundred K. The relatively low derived abundances, however, call for some mechanism to diminish the H2O gas in the post-shock region. Among the possible scenarios, we favor H2O photodissociation, which requires the superposition of a low velocity non-dissociative shock with a fast dissociative shock able to produce a FUV field of sufficient strength., Comment: 16 pages, 13 figures, accepted for publication on Astronomy & Astrophysics

Apart from being an important coolant, H2O is known to be a tracer of high-velocity molecular gas. Recent models predict relatively high abundances behind interstellar shockwaves. The dynamical and physical conditions of the H2O emitting gas, however, are not fully understood yet. We aim to determine the abundance and distribution of H2O, its kinematics and the physical conditions of the gas responsible for the H2O emission. The observed line profile shapes help us understand the dynamics in molecular outflows. We mapped the VLA1623 outflow, in the ground-state transitions of o-H2O, with the HIFI and PACS instruments. We also present observations of higher energy transitions of o-H2O and p-H2O obtained with HIFI and PACS towards selected outflow positions. From comparison with non-LTE radiative transfer calculations, we estimate the physical parameters of the water emitting regions. The observed water emission line profiles vary over the mapped area. Spectral features and components, tracing gas in different excitation conditions, allow us to constrain the density and temperature of the gas. The H2O emission originates in a region where temperatures are comparable to that of the warm H2 gas (T\gtrsim200K). Thus, the H2O emission traces a gas component significantly warmer than the gas responsible for the low-J CO emission. The H2O column densities at the CO peak positions are low, i.e. N(H2O) \simeq (0.03-10)x10e14 cm-2. The H2O abundance with respect to H2 in the extended outflow is estimated at X(H2O)<1x10e-6, significantly lower than what would be expected from most recent shock models. The H2O emission traces a gas component moving at relatively high velocity compared to the low-J CO emitting gas. However, other dynamical quantities such as the momentum rate, energy and mechanical luminosity are estimated to be the same, independent of the molecular tracer used, CO or H2O., Comment: 14 pages, 13 figures, 4 tables

Circumstellar debris disks older than a few Myr should be largely devoid of primordial gas remaining from the protoplanetary disk phase. Tracing the origin of observed atomic gas in Keplerian rotation in the edge-on debris disk surrounding the ~12 Myr old star {\beta} Pictoris requires more detailed information about its spatial distribution than has previously been acquired by limited slit spectroscopy. Especially indications of asymmetries and presence of Ca II gas at high disk latitudes call for additional investigation. We set out to recover a complete image of the Fe I and Ca II gas emission around {\beta} Pic by spatially resolved, high-resolution spectroscopic observations to better understand the morphology and origin of the gaseous disk component. The multiple fiber facility FLAMES/GIRAFFE at the VLT, with the large IFU ARGUS, was used to obtain spatially resolved optical spectra in four regions covering the northeast and southwest side of the disk. Emission lines from Fe I and Ca II were mapped and could be used to fit a parametric function for the disk gas distribution, using a gas-ionisation code for gas-poor debris disks. Both Fe I and Ca II emission are clearly detected, with the former dominating along the disk midplane, and the latter revealing vertically more extended gas. The surface intensity of the Fe I emission is lower but more extended in the northeast (reaching the 210 AU limit of our observations) than in the southwest, while Ca II shows the opposite asymmetry. The modelled Fe gas disk profile shows a linear increase in scale height with radius, and a vertical profile that suggests dynamical interaction with the dust. We also qualitatively demonstrate that the Ca II emission profile can be explained by optical thickness in the disk midplane, and does not require Ca to be spatially separated from Fe. [ABRIDGED], Comment: 12 pages, 14 figures, accepted for publication in Astronomy & Astrophysics

Fomalhaut is a young, nearby star that is suspected to harbor an infant planetary system, interspersed with one or more belts of dusty debris. We present far-infrared images obtained with the Herschel Space Observatory with an angular resolution between 5.7 and 36.7 arcsec at wavelengths between 70 and 500 micrometer. The images show the main debris belt in great detail. Even at high spatial resolution, the belt appears smooth. The region in between the belt and the central star is not devoid of material; thermal emission is observed here as well. Also at the location of the star, excess emission is detected. We use a dynamical model together with radiative-transfer tools to derive the parameters of the debris disk. We include detailed models of the interaction of the dust grains with radiation, for both the radiation pressure and the temperature determination. Comparing these models to the spatially resolved temperature information contained in the images allows us to place strong constraints on the presence of grains that will be blown out of the system by radiation pressure. We use this to derive the dynamical parameters of the system. The appearance of the belt points towards a remarkably active system in which dust grains are produced at a very high rate by a collisional cascade in a narrow region filled with dynamically excited planetesimals. Dust particles with sizes below the blow-out size are abundantly present. The equivalent of 2000 one-km-sized comets are destroyed every day, out of a cometary reservoir amounting to 110 Earth masses. From comparison of their scattering and thermal properties, we find evidence that the dust grains are fluffy aggregates, which indicates a cometary origin. The excess emission at the location of the star may be produced by hot dust with a range of temperatures, but may also be due to gaseous free-free emission from a stellar wind., Comment: 9 pages, 6 figures

(Abridged) Water is a key tracer of dynamics and chemistry in low-mass protostars, but spectrally resolved observations have so far been limited in sensitivity and angular resolution. In this first systematic survey of spectrally resolved water emission in low-mass protostellar objects, H2O was observed in the ground-state transition at 557 GHz with HIFI on Herschel in 29 embedded Class 0 and I protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our program) are used to constrain the spectral energy distribution of each source. H2O intensities are compared to inferred envelope and outflow properties and CO 3-2 emission. H2O emission is detected in all objects except one. The line profiles are complex and consist of several kinematic components. The profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not the quiescent envelope. Several sources show multiple shock components in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in 7 sources (6 Class 0, 1 Class I) indicative of infalling envelopes, and regular P-Cygni profiles in 4 sources (3 Class I, 1 Class 0) indicative of expanding envelopes. "Bullets" moving at >50 km/s are seen in 4 Class 0 sources; 3 of these are new detections. In the outflow, the H2O/CO abundance ratio as a function of velocity is nearly the same for all sources, increasing from 10^-3 at <5 km/s to >10^-1 at >10 km/s. The H2O abundance in the outer envelope is low, ~10^-10. The different H2O profile components show a clear evolutionary trend: in the Class 0 sources, emission is dominated by outflow components originating inside an infalling envelope. When the infall diminishes during the Class I phase, the outflow weakens and H2O emission disappears., Comment: Accepted for publication in A&A

Aims. We present photometric data of debris disks around HIP 103389 (HD 199260), HIP 107350 (HN Peg, HD206860), and HIP 114948 (HD 219482), obtained in the context of our Herschel Open Time Key Program DUNES (DUst around NEarby Stars). Methods. We used Herschel/PACS to detect the thermal emission of the three debris disks with a 3 sigma sensitivity of a few mJy at 100 um and 160 um. In addition, we obtained Herschel/PACS photometric data at 70 um for HIP 103389. Two different approaches are applied to reduce the Herschel data to investigate the impact of data reduction on the photometry. We fit analytical models to the available spectral energy distribution (SED) data. Results. The SEDs of the three disks potentially exhibit an unusually steep decrease at wavelengths > 70 um. We investigate the significance of the peculiar shape of these SEDs and the impact on models of the disks provided it is real. Our modeling reveals that such a steep decrease of the SEDs in the long wavelength regime is inconsistent with a power-law exponent of the grain size distribution -3.5 expected from a standard equilibrium collisional cascade. In contrast, a very distinct range of grain sizes is implied to dominate the thermal emission of such disks. However, we demonstrate that the understanding of the data of faint sources obtained with Herschel is still incomplete and that the significance of our results depends on the version of the data reduction pipeline used. Conclusions. A new mechanism to produce the dust in the presented debris disks, deviations from the conditions required for a standard equilibrium collisional cascade (grain size exponent of -3.5), and/or significantly different dust properties would be necessary to explain the potentially steep SED shape of the three debris disks presented. (abridged), Comment: 14 pages, 4 figures, accepted by A&A

Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O2, and water, H2O. Contrary to expectation, the space missions SWAS and Odin found only very small amounts of water vapour and essentially no O2 in the dense star-forming interstellar medium. Only toward rho Oph A did Odin detect a weak line of O2 at 119 GHz in a beam size of 10 arcmin. A larger telescope aperture such as that of the Herschel Space Observatory is required to resolve the O2 emission and to pinpoint its origin. We use the Heterodyne Instrument for the Far Infrared aboard Herschel to obtain high resolution O2 spectra toward selected positions in rho Oph A. These data are analysed using standard techniques for O2 excitation and compared to recent PDR-like chemical cloud models. The 487.2GHz line was clearly detected toward all three observed positions in rho Oph A. In addition, an oversampled map of the 773.8GHz transition revealed the detection of the line in only half of the observed area. Based on their ratios, the temperature of the O2 emitting gas appears to vary quite substantially, with warm gas (> 50 K) adjacent to a much colder region, where temperatures are below 30 K. The exploited models predict O2 column densities to be sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these model, the observationally determined O2 column densities seem not to depend strongly on the derived gas temperatures, but fall into the range N(O2) = (3 to >6)e15/cm^2. Beam averaged O2 abundances are about 5e-8 relative to H2. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz of about 4 - 5 arcmin, encompassing the entire rho Oph A core., Comment: 10 pages, 8 figures intended for publication in A&A

As part of the WISH (Water In Star-forming regions with Herschel) key project, we report on the observations of several ortho- and para-H2O lines performed with the HIFI instrument towards two bright shock spots (R4 and B2) along the outflow driven by the L1448 low-mass proto-stellar system, located in the Perseus cloud. These data are used to identify the physical conditions giving rise to the H2O emission and infer any dependence with velocity. These observations provide evidence that the observed water lines probe a warm (T_kin~400-600 K) and very dense (n 10^6 - 10^7 cm^-3) gas, not traced by other molecules, such as low-J CO and SiO, but rather traced by mid-IR H2 emission. In particular, H2O shows strong differences with SiO in the excitation conditions and in the line profiles in the two observed shocked positions, pointing to chemical variations across the various velocity regimes and chemical evolution in the different shock spots. Physical and kinematical differences can be seen at the two shocked positions. At the R4 position, two velocity components with different excitation can be distinguished, with the component at higher velocity (R4-HV) being less extended and less dense than the low velocity component (R4-LV). H2O column densities of about 2 10^13 and 4 10^14 cm^-2 have been derived for the R4-LV and the R4-HV components, respectively. The conditions inferred for the B2 position are similar to those of the R4-HV component, with H2O column density in the range 10^14 - 5 10^14 cm^-2, corresponding to H2O/H2 abundances in the range 0.5 - 1 10^-5. The observed line ratios and the derived physical conditions seem to be more consistent with excitation in a low velocity J-type shock with large compression rather than in a stationary C-shock, although none of these stationary models seems able to reproduce all the characteristics of the observed emission., Comment: Accepted for publication in A&A

In the framework of the Herschel-WISH key program, several ortho-H2O and para-H2O emission lines, in the frequency range from 500 to 1700 GHz, were observed with the HIFI instrument in two bow-shock regions (B2 and R) of the L1157 cloud. The primary aim is to analyse water emission lines as a diagnostic of the physical conditions in the blue (B2) and red-shifted (R) lobes to compare the excitation conditions. A total of 5 ortho- and para-H216O plus one o-H218O transitions were observed in B2 and R with a wide range of excitation energies (27 K<=Eu<=215 K). The H2O spectra, observed in the two shocked regions, show that the H2O profiles are markedly different in the two regions. In particular, at the bow-shock R, we observed broad (~30 km s-1 with respect to the ambient velocity) red-shifted wings where lines at different excitation peak at different red-shifted velocities. The B2 spectra are associated with a narrower velocity range (~6 km s-1), peaking at the systemic velocity. The excitation analysis suggests, for B2, low values of column density NH2O <=5{\times}1013 cm-2, a density range of 105 <=nH2 <=107 cm-3, and warm temperatures (>=300 K). The presence of the broad red-shifted wings and multiple peaks in the spectra of the R region, prompted the modelling of two components. High velocities are associated with relatively low temperatures (~100K),NH2O{\simeq}5{\times}1012-5{\times}1013 cm-2 and densities nH2{\simeq}106-108 cm-3.Lower velocities are associated with higher excitation conditions with Tkin>=300 K, very dense gas (nH2 ~108 cm-3) and low column density (NH2O<5{\times}1013 cm-2)., Comment: 9 pages, 8 figures, A&A in press

We present Herschel PACS 100 and 160 micron observations of the solar-type stars alpha Men, HD 88230 and HD 210277, which form part of the FGK stars sample of the Herschel Open Time Key Programme (OTKP) DUNES (DUst around NEarby S tars). Our observations show small infrared excesses at 160 micron for all three stars. HD 210277 also shows a small excess at 100 micron, while the 100 micron fluxes of alpha Men and HD 88230 agree with the stellar photospheric predictions. We attribute these infrared excesses to a new class of cold, faint debris discs. alpha Men and HD 88230 are spatially resolved in the PACS 160 micron images, while HD 210277 is point-like at that wavelength. The projected linear sizes of the extended emission lie in the range from ~ 115 to ~ 250 AU. The estimated black body temperatures from the 100 and 160 micron fluxes are $\lesssim$ 22 K, while the fractional luminosity of the cold dust is Ldust/Lstar ~ 10E-6, close to the luminosity of the Solar-System's Kuiper belt. These debris discs are the coldest and faintest discs discovered so far around mature stars and cannot easily be explained by invoking "classical" debris disc models., Comment: Accepted by Astronomy and Astrophysics Letters

We are aiming at the observational estimation of the relative contribution to the cooling by CO and H2O, as this provides decisive information for the understanding of the oxygen chemistry behind interstellar shock waves. Methods. The high sensitivity of HIFI, in combination with its high spectral resolution capability, allows us to trace the H2O outflow wings at unprecedented signal-to-noise. From the observation of spectrally resolved H2O and CO lines in the HH52-54 system, both from space and from ground, we arrive at the spatial and velocity distribution of the molecular outflow gas. Solving the statistical equilibrium and non-LTE radiative transfer equations provides us with estimates of the physical parameters of this gas, including the cooling rate ratios of the species. The radiative transfer is based on an ALI code, where we use the fact that variable shock strengths, distributed along the front, are naturally implied by a curved surface. Based on observations of CO and H2O spectral lines, we conclude that the emission is confined to the HH54 region. The quantitative analysis of our observations favours a ratio of the CO-to-H2O-cooling-rate >> 1. From the best-fit model to the CO emission, we arrive at an H2O abundance close to 1e-5. The line profiles exhibit two components, one of which is triangular and another, which is a superposed, additional feature. This additional feature likely originates from a region smaller than the beam where the ortho-water abundance is smaller than in the quiescent gas. Comparison with recent shock models indicate that a planar shock can not easily explain the observed line strengths and triangular line profiles.We conclude that the geometry can play an important role. Although abundances support a scenario where J-type shocks are present, higher cooling rate ratios than predicted by these type of shocks are derived., Comment: Accepted for publication in A&A

The molecular species hydrogen peroxide, HOOH, is likely to be a key ingredient in the oxygen and water chemistry in the interstellar medium. Our aim with this investigation is to determine how abundant HOOH is in the cloud core {\rho} Oph A. By observing several transitions of HOOH in the (sub)millimeter regime we seek to identify the molecule and also to determine the excitation conditions through a multilevel excitation analysis. We have detected three spectral lines toward the SM1 position of {\rho} Oph A at velocity-corrected frequencies that coincide very closely with those measured from laboratory spectroscopy of HOOH. A fourth line was detected at the 4{\sigma} level. We also found through mapping observations that the HOOH emission extends (about 0.05 pc) over the densest part of the {\rho} Oph A cloud core. We derive an abundance of HOOH relative to that of H_2 in the SM1 core of about 1\times10^(-10). To our knowledge, this is the first reported detection of HOOH in the interstellar medium., Comment: 5 pages, 4 figures, accepted for publication in Astronomy & Astrophysics, new version corrects a typo in Table 1 (and consequently in Fig 4)

Herschel-HIFI observations of water in the low-mass star-forming object L1448-MM, known for its prominent outflow, are presented, as obtained within the `Water in star-forming regions with Herschel' (WISH) key programme. Six H2-16O lines are targeted and detected (E_up/k_B ~ 50-250 K), as is CO J= 10-9 (E_up/k_B ~ 305 K), and tentatively H2-18O 110-101 at 548 GHz. All lines show strong emission in the "bullets" at |v| > 50 km/s from the source velocity, in addition to a broad, central component and narrow absorption. The bullets are seen much more prominently in H$_2$O than in CO with respect to the central component, and show little variation with excitation in H2O profile shape. Excitation conditions in the bullets derived from CO lines imply a temperature >150 K and density >10^5 cm^-3, similar to that of the broad component. The H2O/CO abundance ratio is similar in the "bullets" and the broad component, ~ 0.05-1.0, in spite of their different origins in the molecular jet and the interaction between the outflow and the envelope. The high H2O abundance indicates that the bullets are H2 rich. The H2O cooling in the "bullets" and the broad component is similar and higher than the CO cooling in the same components. These data illustrate the power of Herschel-HIFI to disentangle different dynamical components in low-mass star-forming objects and determine their excitation and chemical conditions., Comment: Accepted for publication in A&A

Dusty debris discs around main sequence stars are thought to be the result of continuous collisional grinding of planetesimals in the system. The majority of these systems are unresolved and analysis of the dust properties is limited by the lack of information regarding the dust location.vThe Herschel DUNES key program is observing 133 nearby, Sun-like stars (<20 pc, FGK spectral type) in a volume limited survey to constrain the absolute incidence of cold dust around these stars by detection of far infrared excess emission at flux levels comparable to the Edgeworth-Kuiper belt (EKB). We have observed the Sun-like star HD 207129 with Herschel PACS and SPIRE. In all three PACS bands we resolve a ring-like structure consistent with scattered light observations. Using {\alpha} Bo\"otis as a reference point spread function (PSF), we deconvolved the images, clearly resolving the inner gap in the disc at both 70 and 100 {\mu}m. We have resolved the dust-producing planetesimal belt of a debris disc at 100 {\mu}m for the first time. We measure the radial profile and fractional luminosity of the disc, and compare the values to those of discs around stars of similar age and/or spectral type, placing this disc in context of other resolved discs observed by Herschel/DUNES., Comment: 6 pages, 3 figures

`Water In Star-forming regions with Herschel' (WISH) is a key program on the Herschel Space Observatory designed to probe the physical and chemical structure of young stellar objects using water and related molecules and to follow the water abundance from collapsing clouds to planet-forming disks. About 80 sources are targeted covering a wide range of luminosities and evolutionary stages, from cold pre-stellar cores to warm protostellar envelopes and outflows to disks around young stars. Both the HIFI and PACS instruments are used to observe a variety of lines of H2O, H218O and chemically related species. An overview of the scientific motivation and observational strategy of the program is given together with the modeling approach and analysis tools that have been developed. Initial science results are presented. These include a lack of water in cold gas at abundances that are lower than most predictions, strong water emission from shocks in protostellar environments, the importance of UV radiation in heating the gas along outflow walls across the full range of luminosities, and surprisingly widespread detection of the chemically related hydrides OH+ and H2O+ in outflows and foreground gas. Quantitative estimates of the energy budget indicate that H2O is generally not the dominant coolant in the warm dense gas associated with protostars. Very deep limits on the cold gaseous water reservoir in the outer regions of protoplanetary disks are obtained which have profound implications for our understanding of grain growth and mixing in disks., Comment: 71 pages, 10 figures, PASP, in press

From mapping observations of H2CO, HDCO, and D2CO, we have determined how the degree of deuterium fractionation changes over the central 3'x3' region of rho Oph A. The multi-transition data of the various H2CO isotopologues, as well as from other molecules (e.g., CH3OH and N2D+) present in the observed bands, were analysed using both the standard type rotation diagram analysis and, in selected cases, a more elaborate method of solving the radiative transfer for optically thick emission. In addition to molecular column densities, the analysis also estimates the kinetic temperature and H2 density. Toward the SM1 core in rho Oph A, the H2CO deuterium fractionation is very high. In fact, the observed D2CO/HDCO ratio is 1.34+/-0.19, while the HDCO/H2CO ratio is 0.107+/-0.015. This is the first time, to our knowledge, that the D2CO/HDCO abundance ratio is observed to be greater than 1. The kinetic temperature is in the range 20-30 K in the cores of rho Oph A, and the H2 density is (6-10)x10^5 cm-3. We estimate that the total H2 column density toward the deuterium peak is (1-4)x10^23 cm-2. As depleted gas-phase chemistry is not adequate, we suggest that grain chemistry, possibly due to abstraction and exchange reactions along the reaction chain H2CO -> HDCO -> D2CO, is at work to produce the very high deuterium levels observed., Comment: 17 pages, 11 figures, accepted for publication in Astronomy & Astrophysics

We report observations of the reactive molecular ions OH$^+$, H$_2$O$^+$, and H$_3$O$^+$ towards Orion KL with Herschel/HIFI. All three $N=1-0$ fine-structure transitions of OH$^+$ at 909, 971, and 1033GHz and both fine-structure components of the doublet {\it ortho}-H$_2$O$^+$ $1_{11}-0_{00}$ transition at 1115 and 1139GHz were detected; an upper limit was obtained for H$_3$O$^+$. OH$^+$ and H$_2$O$^+$ are observed purely in absorption, showing a narrow component at the source velocity of 9 kms$^{-1}$, and a broad blueshifted absorption similar to that reported recently for HF and {\it para}-H$_{2}^{18}$O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH$^+$ and H$_2$O$^+$ for the 9 km s$^{-1}$ component of $9 \pm 3 \times 10^{12}$cm$^{-2}$ and $7 \pm 2 \times 10^{12}$cm$^{-2}$, and those in the outflow of $1.9 \pm 0.7 \times 10^{13}$cm$^{-2}$ and $1.0 \pm 0.3 \times 10^{13}$cm$^{-2}$. Upper limits of $2.4\times 10^{12}$cm$^{-2}$ and $8.7\times 10^{12}$cm$^{-2}$ were derived for the column densities of {\it ortho} and {\it para}-H$_3$O$^+$ from transitions near 985 and 1657GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate., Comment: 5 pages, 1 figure. Accepted to and to be published in the Herschel HIFI special issue of A\&A Letters

HERSCHEL-HIFI observations of water from the intermediate mass protostar NGC7129 FIRS 2 provide a powerful diagnostic of the physical conditions in this star formation environment. Six spectral settings, covering four H216O and two H218O lines, were observed and all but one H218O line were detected. The four H2 16 O lines discussed here share a similar morphology: a narrower, \approx 6 km/s, component centered slightly redward of the systemic velocity of NGC7129 FIRS 2 and a much broader, \approx 25 km/s component centered blueward and likely associated with powerful outflows. The narrower components are consistent with emission from water arising in the envelope around the intermediate mass protostar, and the abundance of H2O is constrained to \approx 10-7 for the outer envelope. Additionally, the presence of a narrow self-absorption component for the lowest energy lines is likely due to self-absorption from colder water in the outer envelope. The broader component, where the H2O/CO relative abundance is found to be \approx 0.2, appears to be tracing the same energetic region that produces strong CO emission at high J., Comment: 6 pages, 4 figures, accepted by A&A