Search

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.

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. [ABSTRACT FROM AUTHOR]

Context. The abundance of key molecules determines the level of cooling that is necessary for the formation of stars and planetary systems. In this context, one needs to understand the details of the time dependent oxygen chemistry, leading to the formation of O2 and H2O. Aims. We aim to determine the degree of correlation between the occurrence of O2 and HOOH (hydrogen peroxide) in star-forming molecular clouds. We first detected O2 and HOOH in ρ Oph A, we now search for HOOH in Orion OMC A, where O2 has also been detected. Methods. We mapped a 3'×3' region around Orion H2-Peak 1 with the Atacama Pathfinder Experiment (APEX). In addition to several maps in two transitions of HOOH, viz. 219.17 GHz and 251.91 GHz, we obtained single-point spectra for another three transitions towards the position of maximum emission. Results. Line emission at the appropriate LSR-velocity (Local Standard of Rest) and at the level of ≥4σ was found for two transitions, with lower signal-to-noise ratio (2.8-3.5σ) for another two transitions, whereas for the remaining transition, only an upper limit was obtained. The emitting region, offset 18'' south of H2Peak 1, appeared point-like in our observations with APEX. Conclusions. The extremely high spectral line density in Orion makes the identification of HOOH much more difficult than in ρ Oph A. As a result of having to consider the possible contamination by other molecules, we left the current detection status undecided. [ABSTRACT FROM AUTHOR]

Aims. 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 for spatial and/or temporal variations. Methods. Using mapping observations of the very dense ρ OphAcore, we examined standard 1D and non-standard 3D methods to analyse data of far-infrared and submillimetre (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. Results. The gas-to-dust mass ratio is varying across the map, with very low values in the central regions around the core SM1. The global average, =88, is not far from the canonical value of 100, however. In ρ Oph A, the exponent β 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. β assumes intermediate values for evolutionary classes in between. Conclusions. Since β 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. [ABSTRACT FROM AUTHOR]

Context. 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. This feedback will probably also help identify stars that potentially host planetary systems that are reminiscent of our own. Aims. Earlier observations with Herschel and APEX have revealed the temperature minimum of ff Cen, but these were unable to spatially resolve the binary into individual components. With the data reported in this Letter, we aim at remedying this shortcoming. Furthermore, these earlier data were limited to the wavelength region between 100 and 870 μm. In the present context, we intend to extend the spectral mapping (SED) to longer wavelengths, where the contrast between stellar photospheric and chromospheric emission becomes increasingly evident. Methods. The Atacama Large Millimeter/submillimeter Array (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. Results. This is the first detection of main-sequence stars at a wavelength of 3 mm. Furthermore, the individual components of the binary α Cen AB are clearly detected and spatially well resolved at all ALMA wavelengths. The high signal-to-noise ratios of these data permit accurate determination of their relative flux ratios, i.e., SvB/SvA = 0.54 ± 0.04 at 440 μm, =0.46 ± 0.01 at 870 μm, and =0.47 ± 0.006 at 3.1 mm, respectively. Conclusions. The previously obtained flux ratio of 0.44 ± 0.18, which was based on measurements in the optical and at 70 μm, is consistent with the present ALMA results, albeit with a large error bar. The observed 3.1 mm emission greatly exceeds what is predicted from the stellar photospheres, and undoubtedly arises predominantly as free-free emission in the ionized chromospheric plasmas of both stars. Given the distinct difference in their cyclic activity, the similarity of their submm SEDs appears surprising. [ABSTRACT FROM AUTHOR]

Context. 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. Aims. Resolving the HH54 shock wave in the far-infrared (FIR) cooling lines of CO constrain the kinematics, morphology, and physical conditions of the shocked region. Methods. 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). Results. 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 kms-1. The spatial proximity of this knot to the peaks of the atomic fine-structure emission lines observed with Spitzer and PACS ([O i]63, 145 аm) suggests that it may be associated with the dissociative shock as the jet impacts slower moving gas in the HH54 bow-shock. [ABSTRACT FROM AUTHOR]

Context. Herschel observations suggest that the H2O distribution in outflows from low-mass stars resembles the H2 emission. It is still unclear which of the different excitation components that characterise the mid- and near-IR H2 distribution is associated with H2O. Aims. The aim is to spectrally resolve the different excitation components observed in the H2 emission. This will allow us to identify the H2 counterpart associated with H2O and finally derive directly an H2O abundance estimate with respect to H2. Methods. We present new high spectral resolution observations of H2 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 H2O (212-101) observations at 1670 GHz are presented. Results. Our observations show for the first time a clear separation in velocity of the different H2 lines: the 0-0 S(4) line at the lowest excitation peaks at -7 kms-1, while the more excited 0-0 S(9) and 1-0 S(1) lines peak at -15 km s-1. H2O and high-J CO appear to be associated with the H2 0-0 S(4) emission, which traces a gas component with a temperature of 700-1000 K. The H2O abundance with respect to H2 0-0 S(4) is estimated to be X(H2O) < 1.4 × 10-5 in the shocked gas over an area of 13" . 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 H2O emitting gas (≤1000 K) and to derive correct estimates of H2O abundance in the shocked gas, which is lower than what is expected from shock model predictions. [ABSTRACT FROM AUTHOR]

Context. Water is a key molecule in protostellar environments because its line emission is very sensitive to both the chemistry and the physical conditions of the gas. Observations of H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observatory have highlighted the complexity of H2O line profiles, in which different kinematic components can be distinguished. Aims. The goal is to study the spatial distribution of H2O, in particular of the different kinematic components detected in H2O emission, at two bright shocked regions along IRAS 4A, one of the strongest H2O emitters among the Class 0 outflows. Methods. We obtained Herschel-PACS maps of the IRAS 4A outflow and HIFI observations of two shocked positions. The largest HIFI beam of 38" at 557 GHz was mapped in several key water lines with different upper energy levels, to reveal possible spatial variations of the line profiles. A large velocity gradient (LVG) analysis was performed to determine the excitation conditions of the gas. Results. We detect four H2O lines and CO (16–15) at the two selected shocked 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 LVG analysis of the H2O emission suggests that the compact (about 3" , corresponding to about 700 AU) component is associated with a hot (T ~ 1000 K) gas with densities nH2 ~ (1–4) × 105 cm-3, whereas the extended (10"–17" , corresponding to 2400–4000 AU) one traces a warm (T ~ 300–500 K) and dense gas (nH2 ~ (3–5) × 107 cm-3). Finally, using the CO (16–15) emission observed at R2 and assuming a typical CO/H2 abundance of 10-4, we estimate the H2O/H2 abundance of the warm and hot components to be (7–10) × 10-7 and (3–7) × 10-5. Conclusions. 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. [ABSTRACT FROM AUTHOR]

Context. Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby, solar-type binary α Centauri have metallicities that are higher than solar, which is thought to promote giant planet formation. Aims. We aim to determine the level of emission from debris around the stars in the α Cen system. This requires knowledge of their photospheres. Having already detected the temperature minimum, Tmin, of α Cen A at far-infrared wavelengths, we here attempt to do the same for the more active companion α Cen B. Using the α Cen stars as templates, we study the possible effects that Tmin may have on the detectability of unresolved dust discs around other stars. Methods. We used Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry to determine the stellar spectral energy distributions in the far infrared and submillimetre. In addition, we used APEX-SHeFI observations for spectral line mapping to study the complex background around α Cen seen in the photometric images. Models of stellar atmospheres and of particulate discs, based on particle simulations and in conjunction with radiative transfer calculations, were used to estimate the amount of debris around these stars. Results. For solar-type stars more distant than α Cen, a fractional dust luminosity fd ≡ Ldust=Lstar ~ 2 × 10-7 could account for SEDs that do not exhibit the Tmin effect. This is comparable to estimates of fd for the Edgeworth-Kuiper belt of the solar system. In contrast to the far infrared, slight excesses at the 2.5σ level are observed at 24 μm for both α CenA and B, which, if interpreted as due to zodiacal-type dust emission, would correspond to fd ~ (1-3) × 10-5, i.e. some 102 times that of the local zodiacal cloud. Assuming simple power-law size distributions of the dust grains, dynamical disc modelling leads to rough mass estimates of the putative Zodi belts around the α Cen stars, viz.≲4×10-6 M◐ of 4 to 1000 μm size grains, distributed according to n(a) ∝ α-3.5. Similarly, for filled-in Tmin emission, corresponding Edgeworth-Kuiper belts could account for ~10-3 M◐ of dust. Conclusions. Our far-infrared observations lead to estimates of upper limits to the amount of circumstellar dust around the stars α Cen A and B. 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 far-infrared spectral energy distribution of α Cen B is marginally consistent with the presence of a minimum temperature region in the upper atmosphere of the star. We also show that an α Cen A-like temperature minimum may result in an erroneous apprehension about the presence of dust around other, more distant stars. [ABSTRACT FROM AUTHOR]

Context. The dusty debris disk around the ~20 Myr old main-sequence A-star β Pictoris is known to contain gas. Evidence points towards a secondary origin of the gas as opposed to being a direct remnant from 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 with solar abundances of metallic elements such as 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 on board the Herschel Space Observatory to observe and spectrally resolve C ii emission at 158 μm from the β Pic debris disk. Assuming a disk in Keplerian rotation and a model for the line emission from the disk, we used the spectrally resolved line profile to constrain the spatial distribution of the gas. Results. We detect the Cii 158 μm emission. Modelling the shape of the emission line shows that most of the gas is located at about ~100 AU or beyond. We estimate a total C gas mass of 1:3+1:3 -0:5 ~ 10-2 M⊗ (central 90% confidence interval). The data suggest that more gas is located on the south-west side of the disk than on the north-east side. The shape of the emission line is 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 poorer 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 probably not 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 might favour a preferential depletion explanation for the overabundance of C and O, although it is unclear how much this conclusion is affected by the simplifications made. More stringent constraints on the spatial distribution will be available from ALMA observations of Ci emission at 609 μm. [ABSTRACT FROM AUTHOR]

Context. The dust observed in debris disks is produced through collisions of larger bodies left over from the planet/planetesimal formation process. Spatially resolving these disks permits to constrain their architecture and thus that of the underlying planetary/ planetesimal system. Aims. Our Herschel open time key program 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 µm, 100 µm, and 160 µm 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. [ABSTRACT FROM AUTHOR]

Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, several H2O (Eu > 190 K), high-J CO, [Oi], and OH transitions are mapped with Herschel-PACS in two shock positions along two prototypical outflows around the low-luminosity sources L1448 and L1157. Previous Herschel-HIFI H2O observations (Eu = 53-249 K) are also used. The aim is to derive a complete picture of the excitation conditions at the selected shock positions. Methods. We adopted a large velocity gradient analysis (LVG) to derive the physical parameters of the H2O and CO emitting gas. Complementary Spitzer mid-IR H2 data were used to derive the H2O abundance. Results. Consistent with other studies, at all selected shock 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, conducted in detail at the L1448-B2 position, 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 Eu = 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 Eu > 190 K and by the higher-J CO transitions. At L1448-B2 we obtain an H2O abundance (3-4)×10-6 for the warm component and (0.3-1.3)×10-5 for the hot component and a CO abundance of a few 10-5 in both components. In L1448-B2 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). Conclusions. 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. This hot gas component is more affected by evolutionary effects on the timescales of the outflow propagation, which explains the observed H2O abundance variations. [ABSTRACT FROM AUTHOR]

Context. Typical debris discs are composed of particles ranging from several micron sized dust grains to km sized asteroidal bodies, and their infrared emission peaks at wavelengths 60-100 μm. Recent Herschel DUNES observations have identified several debris discs around nearby Sun-like stars (F, G and K spectral type) with significant excess emission only at 160 μm. Aims.We observed HIP 92043 (110 Her, HD 173667) at far-infrared and sub-millimetre wavelengths with Herschel PACS and SPIRE. Identification of the presence of excess emission from HIP 92043 and the origin and physical properties of any excess was undertaken through analysis of its spectral energy distribution (SED) and the PACS images. Methods. The PACS and SPIRE images were produced using the HIPE photProject map maker routine. Fluxes were measured using aperture photometry. A stellar photosphere model was scaled to optical and near infrared photometry and subtracted from the farinfared and sub-mm fluxes to determine the presence of excess emission. Source radial profiles were fitted using a 2D Gaussian and compared to a PSF model based on Herschel observations of μ Boo to check for extended emission. Results. Clear excess emission from HIP 92043 was observed at 70 and 100 μm. Marginal excess was observed at 160 and 250 μm. Analysis of the images reveals that the source is extended at 160 μm. A fit to the source SED is inconsistent with a photosphere and single temperature black body. Conclusions. The excess emission from HIP 92043 is consistent with the presence of an unresolved circumstellar debris disc at 70 and 100 μm, with low probability of background contamination. The extended 160 μm emission may be interpreted as an additional cold component to the debris disc or as the result of background contamination along the line of sight. The nature of the 160 μm excess cannot be determined absolutely from the available data, but we favour a debris disc interpretation, drawing parallels with previously identified cold disc sources in the DUNES sample. [ABSTRACT FROM AUTHOR]

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, therefore, important to investigate whether parameters such as momentum rate (thrust) and mechanical luminosity (power) are the same when different molecular tracers are used. Aims. Our objective is to determine 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 Water In Star-forming regions with Herschel (WISH) key program, three molecular outflows from Class 0 sources have been mapped using the Heterodyne Instrument for the Far Infrared (HIFI) instrument aboard Herschel. We used these observations together with previously published H2 data to infer the physical properties of the outflows. We compared 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 not dependent on which tracer is used, i.e. the values agree to within a factor of 4 and 3, respectively. [ABSTRACT FROM AUTHOR]

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, therefore, important to investigate whether parameters such as momentum rate (thrust) and mechanical luminosity (power) are the same when different molecular tracers are used. Aims. Our objective is to determine 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 Water In Star-forming regions with Herschel (WISH) key program, three molecular outflows from Class 0 sources have been mapped using the Heterodyne Instrument for the Far Infrared (HIFI) instrument aboard Herschel. We used these observations together with previously published H2 data to infer the physical properties of the outflows. We compared 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 not dependent on which tracer is used, i.e. the values agree to within a factor of 4 and 3, respectively. [ABSTRACT FROM AUTHOR]

Context. Water is a potential tracer of outflow activity because it is heavily depleted in cold ambient gas and is copiously produced in shocks. Aims. 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. Methods. We 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. Results. The emission of water has a different spatial and velocity distribution from that of the J = 1-0 and 2-1 transitions of CO. On the other hand, 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 that is 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 that the emitting gas has a narrow range of excitations. A radiative transfer analysis shows that while there is some ambiguity in 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 4×109 cm-3K, which represents an increase of 104 with respect to the ambient value. The data also constrain the water column density within a factor of 2 and indicate values in the sample between 2 × 1012 and 1014 cm-2. When these values are combined with estimates of the H2 column density, the typical water abundance is only 3 × 10-7, with an uncertainty of a factor of 3. Conclusions. Our data challenge current C-shock models of water production through the combination of wing-line profiles, high gas compressions, and low abundances. [ABSTRACT FROM AUTHOR]

Context. 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. Aims. The region of the temperature reversal can be directly observed only in the far infrared and submillimetre spectral regime. We aim at determining the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star α Cen A. As a bonus this will also provide a detailed mapping of the spectral energy distribution, i.e. knowledge that is crucial when searching for faint, Kuiper belt-like dust emission around other stars. Methods. For the nearby binary system α Cen, stellar parameters are known with high accuracy from measurements. For the basic model parameters Teff, log g and [Fe/H], we interpolate stellar model atmospheres in the grid of Gaia/PHOENIX and compute the corresponding model for the G2V star α 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 Spitzer-MIPS, Herschel-PACS, Herschel-SPIRE, and APEX-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 α CenA in the far infared. Results. Similar to the Sun, the far infrared (FIR) emission of α CenA originates in the minimum temperature region above the stellar photosphere in the visible. However, in comparison with the solar case, the FIR photosphere of α CenA appears marginally cooler, Tmin ∼ T160 μm = 3920 ± 375 K. Beyond the minimum near 160 μm, the brightness temperatures increase, and this radiation very likely originates in warmer regions of the chromosphere of α Cen A. Conclusions. To the best of our knowledge, this is the first time a temperature minimum has been directly measured on a mainsequence star other than the Sun. [ABSTRACT FROM AUTHOR]

Context. Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with the ambient medium and the chemical processes at play. Aims. Our goal is to investigate the spatial and velocity distribution of H2O along outflows, its relationship with other tracers, and its abundance variations. In particular, this study focuses on the outflow driven by the low-mass protostar L1448-C, which previous observations have shown to be one of the brightest H2O emitters among the class 0 outflows. Methods. To this end, maps of the o-H2O 110-101 and 212-101 transitions taken with the Herschel-HIFI and PACS instruments, respectively, are presented. For comparison, complementary maps of the CO(3-2) and SiO(8-7) transitions, obtained at the JCMT, and the H2 S(0) and S(1) transitions, taken from the literature, were used as well. Physical conditions and H2O column densities were inferred using large velocity gradient radiative transfer calculations. Results. The water distribution appears to be 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 ±10-50 km s-1, but extended emission at extreme velocities (up to vr ∼ 80 km s-1) is detected and is associated with the L1448-C extreme high-velocity (EHV) jet. The H2O 110-101/CO(3-2) ratio shows strong variations as a function of velocity that likely reflect different and changing physical conditions in the gas that is responsible for the emissions from the two species. In the EHV jet, a low H2O/SiO abundance ratio is inferred, which could indicate molecular formation from dust-free gas directly ejected from the proto-stellar wind. The ratio between the two observed H2O lines and the comparison with H2 indicate averaged Tkin and n(H2) values of ∼300-500 K and 5 × 106 cm-3, respectively, while a water abundance with respect to H2 of about 0.5-1×10-6 along the outflow is estimated, in agreement with results found by previous studies. The fairly constant conditions found all along the outflow imply that evolutionary effects on the timescales of outflow propagation do not play a major role in the H2O chemistry. Conclusions. 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 already been cooled down to a few hundred K. The relatively low derived abundances, however, call for some mechanism that diminishes the H2O gas in the post-shock region. Among the possible scenarios, we favor H2O photodissociation, which requires the superposition of a low-velocity nondissociative shock with a fast dissociative shock able to produce a far-ultraviolet field of sufficient strength. [ABSTRACT FROM AUTHOR]

Context. 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 β 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 to exclude or confirm its connection to observed dust structures or suggested cometary bodies on inclined eccentric orbits. Aims. We set out to recover a complete image of the Fe I and Ca II gas emission around βPic by spatially resolved, high-resolution spectroscopic observations to better understand the morphology and origin of the gaseous disk component. Methods. The multiple fiber facility FLAMES/GIRAFFE at the Very Large Telescope (VLT), with the large integral-field-unit ARGUS, was used to obtain spatially resolved optical spectra (from 385.9 to 404.8 nm) in four regions covering the northeast and southwest side of the disk. Emission lines from Fel (at 386.0 nm) and Call (at 393.4 and 396.8 nm) 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. Results. 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 FeT 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. [ABSTRACT FROM AUTHOR]

Context. Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O2, and water, H2O. These high abundances imply high cooling rates, leading to relatively short timescales for the evolution of gravitationally unstable dense cores, forming stars and planets. Contrary to expectations, the dedicated space missions SWAS and Odin typically found only very small amounts of water vapour and essentially no O2 in the dense star-forming interstellar medium. Aims. Only toward ρ OphA did Odin detect a very weak line of O2 at 119 GHz in a beam of size 10 arcmin. The line emission of related molecules changes on angular scales of the order of some tens of arcseconds, requiring a larger telescope aperture such as that of the Herschel Space Observatory to resolve the O2 emission and pinpoint its origin. Methods. We use the Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel to obtain high resolution O2 spectra toward selected positions in the ρOphAcore. These data are analysed using standard techniques for O2 excitation and compared to recent PDR-like chemical cloud models. Results. The NJ = 33-12 line at 487.2 GHz is clearly detected toward all three observed positions in the ρ Oph Acore. In addition, an oversampled map of the 54-34 transition at 773.8 GHz reveals the detection of the line in only half of the observed area. On the basis of their ratios, the temperature of the O2 emitting gas appears to vary quite substantially, with warm gas (≳50 K) being adjacent to a much colder region, of temperatures lower than 30 K. Conclusions. The exploited models predict that the O2 column densities are sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these models, the observationally determined O2 column densities do not seem to depend strongly on the derived gas temperatures, but fall into the range N(O2) = 3 to ≳6 ×1015 cm-2. Beam-averaged O2 abundances are about 5 ×10-8 relative to H2. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz in the range of 4 to 5 arcmin, encompassing the entire ρ OphAcore. We speculate that one of the reasons for the generally very low detection rate of O2 is the short period of time during which O2 molecules are reasonably abundant in molecular clouds. [ABSTRACT FROM AUTHOR]

Context. The existence of debris disks around old main sequence stars is usually explained by continuous replenishment of small dust grains through collisions from a reservoir of larger objects. Aims. We present photometric data of debris disks around HIP 103389 (HD199260), HIP 107350 (HNPeg, HD206860), and HIP 114948 (HD219482), 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 3σ sensitivity of a few mJy at 100 μm and 160 μm. In addition, we obtained Herschel/PACS photometric data at 70 μm for HIP 103389. These observations are complemented by a large variety of optical to far-infrared photometric data. 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 using the fitting method of simulated thermal annealing as well as a classical grid search method. Results. The SEDs of the three disks potentially exhibit an unusually steep decrease at wavelengths ≥70 μm. We investigate the significance of the peculiar shape of these SEDs and the impact on models of the disks provided it is real. Using grain compositions that have been applied successfully for modeling of many other debris disks, 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 steep grain size distribution or, alternatively an upper grain size in the range of few tens of micrometers are implied. This suggests that a very distinct range of grain sizes would 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. [ABSTRACT FROM AUTHOR]

Aims. As part of the WISH (Water In Star-forming regions with Herschel) key project, systematic observations of H2O transitions in young outflows are being carried out with the aim of understanding the role of water in shock chemistry and its physical and dynamical properties. We report on the observations of several ortho- and para-H2O lines performed with the HIFI instrument toward 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 to infer any dependence on velocity. Methods. We used a large velocity gradient (LVG) analysis to derive the main physical parameters of the emitting regions, namely n(H2), Tkin, N(H2O) and emitting-region size. We compared these with other main shock tracers, such as CO, SiO and H2 and with shock models available in the literature. Results. These observations provide evidence that the observed water lines probe a warm (Tkin ∼ 400-600 K) and very dense (n ∼ 106-107 cm-3) gas that is not traced by other molecules, such as low-J CO and SiO, but is 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, of which the component at higher velocity (R4-HV) is less extended and less dense than the low velocity component (R4-LV). H2O column densities of about 2 × 1013 and 4 × 1014 cm-2 were 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 1014-5 × 1014 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 strong compression rather than in a stationary C-shock, although none of these stationary models seems able to reproduce the whole characteristics of the observed emission [ABSTRACT FROM AUTHOR]

Context. 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, which hosts what is considered to be the prototypical chemically-rich outflow. Aims. Our 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. Methods. For this purpose, we ran the non-LTE RADEX model for a plane-parallel geometry to constrain the physical parameters (Tkin, NH2O and nH2 ) of the water emission lines detected. Results. A total of 5 ortho- and para-H2 16O transitions were observed in B2 and R with a wide range of excitation energies (27K ⩽ Eu ⩽ 215 K). The H2 18O profiles differ markedly in the two regions. In particular, at the bow-shock R, we observed broad (∼30 km s2O spectra, observed in the two shocked regions, show that the H2O profiles differ markedly in the two regions. In particular, at the bow-shock R, we observed broad (∼30 km s-1 with respect to the ambient velocity) redshifted 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 × 1013 cm-2, a density range of 105 ⩽ nH2, 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 (∼100 K), N7 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 (∼100 K), NH2O ≃ 5 × 1012-5 × 1013 cm-2 and densities nH2 ∼ 106-108 cm-3. Lower velocities are associated with higher excitation conditions with Tkin ⩾ 300 K, very dense gas (nH2 ∼ 108). Conclusions. The overall analysis suggests that the emission in B2 comes from an extended (⩾15") region, whilst we cannot rule out the possibility that the emission in R arises from a smaller (>3") region. In this context, H-3) and low column density (NH2O < 5 × 1013 cm-2). Conclusions. The overall analysis suggests that the emission in B2 comes from an extended (⩾15") region, whilst we cannot rule out the possibility that the emission in R arises from a smaller (>3") region. In this context, H2O seems to be important in tracing different gas components with respect to other molecules, e.g. such as SiO, a classical jet tracer. We compare a grid of C- and J-type shocks spanning different velocities (10 to 40 km s-1) and two pre-shock densities (2 × 104 and 2 × 105 cm-3), with the observed intensities. Although none of these models seem to be able to reproduce the absolute intensities of the water emissions observed, it appears that the occurrence of J-shocks, which can compress the gas to very high densities, cannot be ruled out in these environments. [ABSTRACT FROM AUTHOR]