Search

We present new criteria for selecting H  ii regions from the Infrared Astronomical Satellite (IRAS) Point Source Catalogue (PSC), based on an H  ii region catalogue derived manually from the all-sky Wide-field Infrared Survey Explorer (WISE). The criteria are used to augment the number of H  ii region candidates in the Milky Way. The criteria are defined by the linear decision boundary of two samples: IRAS point sources associated with known H  ii regions, which serve as the H  ii region sample, and IRAS point sources at high Galactic latitudes, which serve as the non-H  ii region sample. A machine learning classifier, specifically a support vector machine, is used to determine the decision boundary. We investigate all combinations of four IRAS bands and suggest that the optimal criterion is |$\mathrm{log}\left(F_{60}/F_{12}\right)\geqq \left(-0.19 \times \mathrm{log}\left(F_{100}/F_{25}\right)+ 1.52\right)$|⁠, with detections at 60 and 100  |${\mu}$| m. This selects 3041 H  ii region candidates from the IRAS PSC. We find that IRAS H  ii region candidates show evidence of evolution on the two-colour diagram. Merging the WISE H  ii catalogue with IRAS H  ii region candidates, we estimate a lower limit of approximately 10 200 for the number of H  ii regions in the Milky Way. [ABSTRACT FROM AUTHOR]

Methyl isocyanate has been recently detected in comet 67P/Churyumov-Gerasimenko (67P/CG) and in the interstellar medium. New physicochemical studies on this species are now necessary as tools for subsequent studies in astrophysics. In this work, infrared spectra of solid CH3NCO have been obtained at temperatures of relevance for astronomical environments. The spectra are dominated by a strong, characteristic multiplet feature at 2350-2250 cm-1, which can be attributed to the asymmetric stretching of the NCO group. A phase transition from amorphous to crystalline methyl isocyanate is observed at -90 K. The band strengths for the absorptions of CH3NCO in ice at 20 K have been measured. Deuterated methyl isocyanate is used to help with the spectral assignment. No X-ray structure has been reported for crystalline CH3NCO. Here we advance a tentative theoretical structure, based on density functional theory (DFT) calculations, derived taking the crystal of isocyanic acid as a starting point. A harmonic theoretical spectrum is then calculated for the proposed structure and compared with the experimental data. A mixed ice of H2O and CH3NCO was formed by simultaneous deposition of water and methyl isocyanate at 20 K. The absence of new spectral features indicates that methyl isocyanate and water do not react appreciably at 20 K, but form a stable mixture. The high CH3NCO/H2O ratio reported for comet 67P/CG, and the characteristic structure of the 2350-2250 cm-1 band, makes it a very good candidate for future astronomical searches. [ABSTRACT FROM AUTHOR]

The degradation of spatial resolution in star-forming regions, observed at large distances (d ≳ 1 kpc) with Herschel, can lead to estimates of the physical parameters of the detected compact sources (clumps), which do not necessarily mirror the properties of the original population of cores. This paper aims at quantifying the bias introduced in the estimation of these parameters by the distance effect. To do so, we consider Herschel maps of nearby star-forming regions taken from the Herschel Gould Belt survey, and simulate the effect of increased distance to understand what amount of information is lost when a distant star-forming region is observed with Herschel resolution. In the maps displaced to different distances we extract compact sources, and we derive their physical parameters as if they were original Herschel infrared Galactic Plane Survey maps of the extracted source samples. In this way, we are able to discuss how the main physical properties change with distance. In particular, we discuss the ability of clumps to form massive stars: we estimate the fraction of distant sources that are classified as high-mass stars-forming objects due to their position in the mass versus radius diagram, that are only 'false positives'. We also give a threshold for high-mass star formation M >1282 (r/[pc])1.42 M☉. In conclusion, this paper provides the astronomer dealing with Herschel maps of distant star-forming regions with a set of prescriptions to partially recover the character of the core population in unresolved clumps. [ABSTRACT FROM AUTHOR]

In this paper we derive and discuss several implications of the analytic form of a modified blackbody, also called greybody, which is widely used in Astrophysics, and in particular in the study of star formation in the far-infrared/submillimetre domain. The research in this area has been greatly improved thanks to recent observations taken with the Herschel satellite, so that it became important to clarify the sense of the greybody approximation, to suggest possible further uses, and to delimit its intervals of validity. First, we discuss the position of the greybody peak, making difference between the optically thin and thick regimes. Second, we analyse the behaviour of bolometric quantities as a function of the different greybody parameters. The ratio between the bolometric luminosity and the mass of a source, the ratio between the so-called 'submillimetre luminosity' and the bolometric one, and the bolometric temperature are observables used to characterize the evolutionary stage of a source, and it is of primary importance to have analytic equations describing the dependence of such quantities on the greybody parameters. Here we discuss all these aspects, providing analytic relations, illustrating particular cases, and providing graphical examples. Some equations reported here are well known in Astrophysics, but are often spread over different publications. Some of them, instead, are brand new and represent a novelty in Astrophysics literature. Finally, we indicate an alternative way to obtain, under some conditions, the greybody temperature and dust emissivity index directly from an observing spectral energy distribution, avoiding a bestfitting procedure. [ABSTRACT FROM AUTHOR]

The 11.2-μmemission band belongs to the family of the 'unidentified' infrared emission bands seen in many astronomical environments. In thiswork, we present a theoretical interpretation of the band characteristics and profile variation for a number of astrophysical sources in which the carriers are subject to a range of physical conditions. The results of Density Functional Theory calculations for the solo out-of-plane vibrational bending modes of large polycyclic aromatic hydrocarbon (PAH) molecules are used as input for a detailed emission model which includes the temperature and mass dependence of PAH band wavelength, and a PAH mass distribution that varies with object. Comparison of the model with astronomical spectra indicates that the 11.2-μm band asymmetry and profile variation can be explained principally in terms of the mass distribution of neutral PAHs with a small contribution from anharmonic effects. [ABSTRACT FROM AUTHOR]

We apply the Finkbeiner et al. two-component thermal dust emission model to the Planck High Frequency Instrument maps. This parameterization of the far-infrared dust spectrum as the sum of two modified blackbodies (MBBs) serves as an important alternative to the commonly adopted single-MBB dust emission model. Analyzing the joint Planck/DIRBE dust spectrum, we show that two-component models provide a better fit to the 100-3000 GHz emission than do single-MBB models, though by a lesser margin than found by Finkbeiner et al. based on FIRAS and DIRBE. We also derive full-sky 6'1 resolution maps of dust optical depth and temperature by fitting the two-component model to Planck 217-857 GHz along with DIRBE/IRAS 100 µm data. Because our two-component model matches the dust spectrum near its peak, accounts for the spectrum's flattening at millimeter wavelengths, and specifies dust temperature at 6'1 FWHM, our model provides reliable, high-resolution thermal dust emission foreground predictions from 100 to 3000 GHz. We find that, in diffuse sky regions, our two-component 100-217 GHz predictions are on average accurate to within 2.2%, while extrapolating the Planck Collaboration et al. single-MBB model systematically underpredicts emission by 18.8% at 100 GHz, 12.6% at 143 GHz, and 7.9% at 217 GHz. We calibrate our two-component optical depth to reddening, and compare with reddening estimates based on stellar spectra. We find the dominant systematic problems in our temperature/reddening maps to be zodiacal light on large angular scales and the cosmic infrared background anisotropy on small angular scales. [ABSTRACT FROM AUTHOR]

Context. Planetary nebulae (PNe) are powerful tracers of evolved stellar populations. Among the 3000 known PNe in the Galaxy, about 600 are located within the 520 square-degree area covered by the VVV survey. The VVV photometric catalogue provides an important new dataset for the study of PNe, with high-resolution imaging in five near-infrared bands. Aims. There are various colour-colour diagrams that can be obtained from the VVV filters. We investigate the location of PNe in these diagrams and the separation from other types of objects. This includes the new Y-J vs. Z-Y diagram. Methods. Aperture photometry of known PNe in the VVV area was retrieved from source catalogues. Care was taken to minimise any confusion with field stars. The colours of the PNe we are determined for (H-Ks), (J-H), (Z-Y), and (Y-J), and compared to stars and to other types of emission line objects. Cloudy photo-ionisation models were used to predict colours for typical PNe. Results. We present near-infrared photometry for 353 known PNe. The best separation from other objects is obtained in the (H-Ks) vs. (J-H) diagram. We calculated the emission-line contribution to the in-band flux based on a model for NGC 6720: we find that this is highest in the Z and Y bands at over 50%, lower in the J band at 40%, and lowest in the H and Ks bands at 20%. A new view of PNe in the wavelength domain of the Z and Y bands is shown. Photo-ionisation models are used to explore the observed colours in these bands. The Y band is shown to be dominated by He i 1.083 μm and Heii 1.012 μm, and colours involving this band are very sensitive to the temperature of the ionizing star. Conclusions. The VVV survey represents a unique dataset for studying crowded and obscured regions in the Galactic plane. The diagnostic diagrams presented here allow one to study the properties of known PNe and to uncover objects not previously classified. [ABSTRACT FROM AUTHOR]

Context. Studying the H I line profiles in the Leiden-Argentina-Bonn (LAB) all-sky database, we have found a population of very cold H I clouds. So far, the role of these clouds in the interstellar medium (ISM) has remained unclear. Aims. In this paper, we attempt to confirm the existence of the narrow-line H I emission (NHIE) clouds by using the data from the Parkes Galactic all-sky survey (GASS) and try to find their place among other coldest constituents of the ISM. Methods. We repeat the search of NHIE with the GASS data and derive or compile some preliminary estimates for the distribution, temperatures, distances, linear sizes, column and number densities, masses and the composition of NHIE clouds, and compare these data with corresponding estimates for H I self-absorption (HISA) features, the Planck cold clumps (CC) and infrared dark clouds (IRDC). Results. We demonstrate that from LAB and GASS we can separate comparable NHIE complexes and the properties of the obtained NHIE clouds are very similar to those of HISA features, but both of these types of clouds are somewhat warmer, more extended and of lower densities than the cores in the Planck CC and IRDC. Conclusions. We conclude that NHIE may be of the same type of clouds as HISA, but in different observing conditions, as the Planck CC and IRDC are most likely similar ISM structures in different observing conditions and probably in slightly different evolutionary stages. Both NHIE and HISA may be an intermediate phase between the diffuse cold neutral medium and star forming molecular clumps represented by the Planck CC and IRDC. [ABSTRACT FROM AUTHOR]

We derive the distribution of the synchrotron spectral index across NGC6946 and investigate the correlation between the radio continuum (synchrotron) and far-infrared (FIR) emission using the KINGFISH Herschel-PACS and SPIRE data. The radio-FIR correlation is studied as a function of star formation rate, magnetic field strength, radiation field strength, and the total gas surface density. The synchrotron emission follows both star-forming regions and the so-called magnetic arms present in the inter-arm regions. The synchrotron spectral index is steepest along the magnetic arms (αn ~ 1), while it is flat in places of giant Hii regions and in the center of the galaxy (αn ~ 0.6-0.7). The map of αn provides observational evidence for aging and energy loss of cosmic ray electrons (CREs) propagating in the disk of the galaxy. Variations in the synchrotron-FIR correlation across the galaxy are shown to be a function of both star formation and magnetic field strength. We find that the synchrotron emission correlates better with cold rather than with warm dust emission, when the diffuse interstellar radiation field is the main heating source of dust. The synchrotron-FIR correlation suggests a coupling between the magnetic field and the gas density. NGC6946 shows a power-law behavior between the total (turbulent) magnetic field strength B and the star formation rate surface density ΣSFR with an index of 0.14 (0.16) ± 0.01. This indicates an efficient production of the turbulent magnetic field with the increasing gas turbulence expected in actively star forming regions. Moreover, it is suggested that the B-ΣSFR power law index is similar for the turbulent and the total fields in normal galaxies. On the other hand, for galaxies interacting with the cluster environment this index is steeper for turbulent magnetic fields than it is for the total magnetic fields. The scale-by-scale analysis of the synchrotron-FIR correlation indicates that the ISM affects the propagation of old/diffused CREs, resulting in a diffusion coefficient of D0 = 4.6 × 1028 cm2 s-1 for 2.2 GeV CREs. [ABSTRACT FROM AUTHOR]

Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope. Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages. Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H2O, OH, and atomic lines. For four targets the spectra span the entire 55-200 μm region. The PACS field-of-view covers ~47" with the resolution of 9.4". Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H2O 818-707 63.3 μm line (Eu/kB = 1071 K) is detected in 7 sources. CO transitions from J = 14-13 up to J = 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 that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on ⩾10 000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H2O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [Oi] fluxes. In contrast, [Oi] andOH often peak together at the central position. Conclusions. The PACS data probe at least two physical components. The H2O and CO emission very 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, most 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 the fraction contributed by [OI] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state. [ABSTRACT FROM AUTHOR]

Context. It has been suggested that the ratio of total-to-selective extinction RV in dust in the interstellar medium differs in the Galactic bulge from its value in the local neighborhood. Aims. We attempt to test this suggestion. Methods. The mid-infrared hydrogen lines in 16 Galactic bulge PNe measured by the Spitzer Space Telescope are used to determine the extinction corrected Hβ flux. This is compared to the observed Hβ flux to obtain the total extinction at Hβ. The selective extinction is obtained from the observed Balmer decrement in these nebulae. The value of RV can then be found. Results. The ratio of total-to-selective extinction in the Galactic bulge is consistent with the value RV = 3.1, which is the same as has been found in the local neighborhood. Conclusions. The suggestion that RV is different in the Galactic bulge is incorrect. The reasons for this are discussed. [ABSTRACT FROM AUTHOR]

Context. Measuring the mass distribution of infrared dark clouds (IRDCs) over the wide dynamic range of their column densities is a fundamental obstacle in determining the initial conditions of high-mass star formation and star cluster formation. Aims. We present a new technique to derive high-dynamic-range, arcsecond-scale resolution column density data for IRDCs and demonstrate the potential of such data in measuring the density variance - sonic Mach number relation in molecular clouds. Methods. We combine near-infrared data from the UKIDSS/Galactic Plane Survey with mid-infrared data from the Spitzer/GLIMPSE survey to derive dust extinction maps for a sample of ten IRDCs. We then examine the linewidths of the IRDCs using 13CO line emission data from the FCRAO/Galactic Ring Survey and derive a column density - sonic Mach number relation for them. For comparison, we also examine the relation in a sample of nearby molecular clouds. Results. The presented column density mapping technique provides a very capable, temperature independent tool for mapping IRDCs over the column density range equivalent to AV ≃ 1-100 mag at a resolution of 2″ . Using the data provided by the technique, we present the first direct measurement of the relationship between the column density dispersion, σN/ , and sonic Mach number, Ms, in molecular clouds. We detect correlation between the variables with about 3-σ confidence. We derive the relation ≃N/ ≃ (0.047 ± 0.016)Ms, which is suggestive of the correlation coefficient between the volume density and sonic Mach number, σ /< > ≃ (0.20 -0.22+0.37)Ms, in which the quoted uncertainties indicate the 3-s range. When coupled with the results of recent numerical works, the existence of the correlation supports the picture of weak correlation between the magnetic field strength and density in molecular clouds (i.e., B ∝ ?0.5). While our results remain suggestive because of the small number of clouds in our demonstration sample, the analysis can be improved by extending the study to a larger number of clouds. [ABSTRACT FROM AUTHOR]

We present the first complete ∼55-671 μm spectral scan of a low-mass Class 0 protostar (Serpens SMM1) taken with the PACS and SPIRE spectrometers onboard Herschel. More than 145 lines have been detected, most of them rotationally excited lines of 12CO (full ladder from Ju = 4-3 to 42-41 and Eu/k = 4971 K), H2O (up to 818-707 and Eu/k = 1036 K), OH (up to 2⨅1/2 J = 7/2-5/2 and Eu/k = 618 K), 13CO (up to Ju = 16-15), HCN and HCO+ (up to Ju = 12-11). Bright [OI]63, 145 μm and weaker [C II]158 and [C I]370, 609 μm lines are also detected, but excited lines from chemically related species (NH3, CH+, CO+, OH+ or H2O+) are not. Mid-infrared spectra retrieved from the Spitzer archive are also first discussed here. The ∼10-37 μm spectrum has many fewer lines, but shows clear detections of [Ne II], [Fe II], [Si II] and [S I] fine structure lines, as well as weaker H2 S(1) and S(2) pure rotational lines. The observed line luminosity is dominated by CO (∼54%), H2O (∼22%), [OI] (∼12%) and OH (∼9%) emission. A multi-component radiative transfer model allowed us to approximately quantify the contribution of the three different temperature components suggested by the 12CO rotational ladder (Thot k ≈ 800 K, Twarm k ≈ 375 K and Tcool k ≈ 150 K). Gas densities n(H2) ∼ 5 × 106 cm-3 are needed to reproduce the observed far-IR lines arising from shocks in the inner protostellar envelope (warm and hot components) for which we derive upper limit abundances of x(CO) ≲ 10-4, x(H2O) ≲ 0.2 × 10-5 and x(OH) ≲ 10-6 with respect to H2. The lower energy submm 12CO and H2O lines show more extended emission that we associate with the cool entrained outflow gas. Fast dissociative Jshocks (vs > 60 km s-1) within an embedded atomic jet, as well as lower velocity small-scale non-dissociative shocks (vs ≲ 20 km s-1) are needed to explain both the atomic fine structure lines and the hot CO and H2O lines respectively. Observations also show the signature of UV radiation (weak [CII] and [CI] lines and high HCO+/HCN abundance ratios) and thus, most observed species likely arise in UV-irradiated shocks. Dissociative J-shocks produced by a jet impacting the ambient material are the most probable origin of [OI] and OH emission and of a significant fraction of the warm CO emission. In addition, H2O photodissociation in UV-irradiated non-dissociative shocks along the outflow cavity walls can also contribute to the [OI] and OH emission. [ABSTRACT FROM AUTHOR]

Context. Dust emission is increasingly used as a tracer of the mass in the interstellar medium. With the combination of Planck and Herschel observatories, we now have both the spectral coverage and the angular resolution required to observe dense and cold molecular clouds. However, as these clouds are optically thick at short wavelengths but optically thin at long wavelengths, it is tricky to conclude anything about dust properties without a proper treatment of the radiative transfer. Aims. Our aim is to disentangle the effects of radiative transfer and dust properties on the variations in the dust emission at long wavelengths. This enables us to provide observers with tools to analyse the dust emission arising from dense clouds. Methods. We model cylindrical clouds with visual extinctions between 1 and 20 mag, illuminated by the standard interstellar radiation field, and carry out full radiative transfer calculations using a Monte Carlo code. Dust temperatures are solved using the DustEM code for amorphous carbons and silicates representative of dust at high Galactic latitude (DHGL), carbon and silicate grains coated with carbon mantles, and mixed aggregates of carbon and silicate. We also allow for variations in the optical properties of the grains with wavelength and temperature. We determine observed colour temperatures, Tcolour, and emissivity spectral indices, βcolour, by fitting the dust emission with modified blackbodies using a standard χ2 fitting method, in order to compare our models with observational results. Results. Radiative transfer effects can explain neither the low Tcolour, the increased submillimetre emissivity measured at the centre of dense clouds, nor the observed βcolour - Tcolour anti-correlation for the models considered. Adding realistic noise to the modelled data, we show that it is unlikely to be the only explanation of the βcolour - Tcolour anti-correlation observed in starless clouds, which may instead be explained by intrinsic variations in the grain optical properties with temperature. Similarly the higher submillimetre emissivity and the low Tcolour have to originate in variations in the grain optical properties, probably caused by their growth to form porous aggregates. We find it difficult to infer the nature of the grains from the spectral variations in their emission, owing to radiative transfer effects for λ ≲300 pm, and to the mixture of different grain populations for longer wavelengths. Finally, the column density is underestimated when determined with modified blackbody fitting because of the discrepancy between Tcolour and the "true" dust temperature in the innermost layers of the clouds. [ABSTRACT FROM AUTHOR]

We present infrared observations of the supernova remnant G21.5-0.9 with the Very Large Telescope, the Canada-France-Hawaii Telescope and the Spitzer Space Telescope. Using the VLT/ISAAC camera equipped with a narrow-band [Fe II] 1.64 pm filter the entire pulsar wind nebula in SNR G21.5-0.9 was imaged. This led to detection of iron line-emitting material in the shape of a broken ring-like structure following the nebula's edge. The detected emission is limb-brightened. We also detect the compact nebula surrounding PSR J1833-1034, both through imaging with the CFHT/AOB-KIR instrument (K' band) and the IRAC camera (all bands) and also through polarimetric observations performed with VLT/ISAAC (Ks band). The emission from the compact nebula is highly polarised with an average value of the linear polarisation fraction PavgL ≈ 0.47, and the swing of the electric vector across the nebula can be observed. The infrared spectrum of the compact nebula can be described as a power law of index αIR = 0.7 ± 0.3, and suggests that the spectrum flattens between the infrared and X-ray bands. [ABSTRACT FROM AUTHOR]

Context. Observing supernova remnants (SNRs) and modelling the shocks they are associated with is the best way to quantify the energy SNRs re-distribute back into the interstellar medium (ISM). Aims. We present comparisons of shock models with CO observations in the F knot of the W28 supernova remnant. These comparisons constitute a valuable tool to constrain both the shock characteristics and pre-shock conditions. Methods. New CO observations from the shocked regions with the APEX and SOFIA telescopes are presented and combined. The integrated intensities are compared to the outputs of a grid of models, which were combined from an MHO shock code that calculates the dynamical and chemical structure of these regions and a radiative transfer module based on the large velocity gradient (LVG) approximation. Results. We base our modelling method on the higher J CO transitions, which unambiguously trace the passage of a shock wave. We provide fits for the blue- and red-lobe components of the observed shocks. We find that only stationary, C-type shock models can reproduce the observed levels of CO emission. Our best models are found for a pre-shock density of 104 cm-3, with the magnetic field strength varying between 45 and 100 μG, and a slightly higher shock velocity for the so-called blue-shock (∼ 25 km s-1) than for the red one (∼20 km-1). Our models also satisfactorily account for the pure rotational H2 emission that is observed with Spitzer. [ABSTRACT FROM AUTHOR]

To investigate the fundamental principles of H2 formation in a giant molecular cloud, we derive the HI and H2 surface density (∑HI and ∑H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the Hi data from the Galactic Arecibo L-band Feed Array Hi Survey and an estimate of the local dust-to-gas ratio, we then derive the ∑H2 distribution across Perseus. We find a relatively uniform ∑HI ~ 6-8 Mʘ pc-2 for both dark and star-forming regions, suggesting a minimum Hi surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between ∑H2/∑HI and ∑HI + ∑H2. The transition between the HI- and H2-dominated regions occurs at N(Hi) + 2N(H2) ~ (8-14) x 1020 cm-2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of "CO-dark" gas, fDG ~ 0.3. While significant spatial variations of FDG are found, we do not find a clear correlation with the mean V-band extinction. [ABSTRACT FROM AUTHOR]

We present an analysis of super-giant molecular complexes (SGMCs) in the overlap region of the Antennae galaxy merger, based on ALMA CO(3-2) interferometry and VLT/SINFONI imaging spectroscopy of H2 1-0 S(1) at angular resolutions of 0.9"and 0.7" , respectively. All but one SGMC have multiple velocity components offset from each other by up to 150 km s-1. H2 line emission is found in all SGMCs and the kinematics of H2 and CO are well matched. H2/CO line ratios vary by up to a factor of 10 among SGMCs and different velocity components of the same SGMCs. We also identify the CO counterpart of a bright, compact source of near-IR H2 line emission, which shows no Brγ, and was first identified with SINFONI. This source has the highest H2/CO line ratio, and coincides with the steepest CO velocity gradient of the entire overlap region. With a size of 50 pc and a virial mass of a few 107 M☉ it is perhaps a pre-cluster cloud that has not yet formed significant numbers of massive stars. We present observational evidence that the H2 emission is powered by shocks, and demonstrate how the H2 1-0 S(1) and the CO(3-2) lines can be used as tracers of energy dissipation and gas mass, respectively. The variations in the H2/CO line ratio may indicate that the SGMCs are dissipating their turbulent kinetic energy at different rates. The compact source could represent a short (∼1 Myr) evolutionary stage in the early formation of super-star clusters [ABSTRACT FROM AUTHOR]

Context. Circumstellar disks and envelopes of low-mass young stellar objects (YSOs) contain significant amounts of ice. Such icy material will evolve to become volatile components of planetary systems, such as comets in our solar system. Aims. To investigate the composition and evolution of circumstellar ice around low-mass young stellar objects (YSOs), we observed ice absorption bands in the near infrared (NIR) towards eight YSOs ranging from class 0 to class II, among which seven are associated with edge-on disks. Methods. We performed slit-less spectroscopic observations using the grism mode of the InfraRed Camera (IRC) on board AKARI, which enables us to obtain full NIR spectra from 2.5 μm to 5 μm, including the CO2 band and the blue wing of the H2O band, which are inaccessible from the ground. We developed procedures to carefully process the spectra of targets with nebulosity. The spectra were fitted with polynomial baselines to derive the absorption spectra. The molecular absorption bands were then fitted with the laboratory database of ice absorption bands, considering the instrumental line profile and the spectral resolution of the grism dispersion element. Results. Towards the class 0-I sources (L1527, IRC-L1041-2, and IRAS 04302), absorption bands of H2O, CO2, CO, and XCN are clearly detected. Column density ratios of CO2 ice and CO ice relative to H2O ice are 21-28% and 1346%, respectively. If XCN is OCN, its column density is as high as 2-6% relative to H2O ice. The HDO ice feature at 4.1 μm is tentatively detected towards the class 0-I sources and HV Tau. Non-detections of the CH-stretching mode features around 3.5 μm provide upper limits to the CH3OH abundance of 26% (L1527) and 42% (IRAS 04302) relative to H2O. We tentatively detect OCS ice absorption towards IRC-L1041-2. Towards class 0-I sources, the detected features should mostly originate in the cold envelope, while CO gas and OCN- could originate in the region close to the protostar, where there are warm temperatures and UV radiation. We detect H2O ice band towards ASR41 and 2MASSJ 1628137-243139, which are edge-on class II disks. We also detect H2O ice and CO2 ice towards HV Tau, HK Tau, and UY Aur, and tentatively detect CO gas features towards HK Tau and UY Aur [ABSTRACT FROM AUTHOR]

We present chemical and radiative transfer models for the many far-infrared ortho- and para-H2O lines that were observed from the Orion-KL region in high resolution Fabry-Pérot (FP) mode by the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). The chemistry of the region was first studied by simulating the conditions in the different known components of Orion-KL: chemical models for a hot core, a plateau and a ridge were coupled with an accelerated Λ-iteration radiative transfer model to predict H2O line fluxes and profiles. Our models include the first 45 energy levels of ortho- and para-H2O. We find that lines arising from energy levels below 560 K were best reproduced by a gas of density at a temperature of 70–90 K, expanding at a velocity of 30 km s−1 and with a H2O/H2 abundance ratio of the order of , similar to the abundance derived by Cernicharo et al. However, the model that best reproduces the fluxes and profiles of H2O lines arising from energy levels above 560 K has a significantly higher H2O/H2 abundance, , originating from gas of similar density, in the Plateau region, that has been heated to 300 K, relaxing to 90–100 K. We conclude that the observed water lines do not originate from high-temperature shocks. [ABSTRACT FROM AUTHOR]

The motion of planetary nebulae through the interstellar medium (ISM) is thought to lead to a variety of observational consequences, including the formation of bright rims, deformation and fragmentation of the shells, and a shift of the central stars away from the geometric centres of the envelopes. These and other characteristics have been noted through imaging in the visual wavelength regime. We report further observations of such shells taken in the mid-infrared (MIR), acquired through programmes of Infrared Array Camera imaging undertaken using the Spitzer Space Telescope. NGC 2440 and NGC 6629 are shown to possess likely interacting haloes, together with ram-pressure-stripped material to one side of their shells. Similarly, the outer haloes of NGC 3242 and NGC 6772 appear to have been fragmented through Rayleigh–Taylor (RT) instabilities, leading to a possible flow of ISM material towards the inner portions of their envelopes. If this interpretation is correct, then it would suggest that NGC 3242 is moving towards the NE, a suggestion which is also supported through the presence of a 60 μm tail extending in the opposite direction, and curved bands of H emission in the direction of motion – components which may arise through RT instabilities in the magnetized ISM. NGC 2438 possesses strong scalloping at the outer limits of its asymptotic giant branch (AGB) halo, probably reflecting RT instabilities at the nebular/ISM interface We also note that the interior structure of the source has been interpreted in terms of a recombining shell, a hypothesis which may not be consistent with the central star luminosities. Finally, we point out that two of the rims (and likely shock interfaces) appear to have a distinct signature in the MIR, whereby relative levels of 8.0 μm emission are reduced. This may imply that the grain emission agents are depleted in the post-shock AGB regimes. [ABSTRACT FROM AUTHOR]

By systematically searching regions around planetary nebulae (PNe) for signs of interactions of their precursors’ wind with ambient matter we found a number of huge IRAS dust structures. Some of them may be chance projections, but a few appear to be real, like those around NGC 6826 and NGC 2899. In the case of NGC 6826 we noticed a giant (∼2°) bipolar dust emission, whose axis is along the proper motion of the central star. The PN itself is offset in the direction of motion both as to the center of this ∼30 pc large dust structure and to the center of a similarly large new H α nebula. NGC 2899 was found in the center of a 14×11 pc quadrupolar cavity, whose directions of axes coincide with the directions of the main axes of the optical PN. In both cases, the formation of these structures appears to have commenced in the asymptotic giant branch (AGB) phase. [ABSTRACT FROM AUTHOR]

We present mid-infrared (MIR) photometry for 367 Galactic disc, bulge and Large Magellanic Cloud (LMC) planetary nebulae (PNe), determined using data acquired with the Spitzer Space Telescope, and through the Legacy Programs GLIMPSE II (Galactic Legacy Infrared Mid-plane Survey Extraordinaire II) and SAGE (Surveying the Agents of the Galaxy's Evolution). This has permitted us to make a comparison between the luminosity functions of bulge and LMC PNe, and between the MIR colours of all three categories of source. It is determined that whilst the luminosity functions of the LMC and bulge sources are likely to be closely similar, the [3.6]–[5.8] and [5.8]–[8-0] indices of LMC nebulae are different from those of their disc and bulge counterparts. This may arise because of enhanced polycyclic aromatic hydrocarbon emission within the LMC sources, and/or as a result of further, and more radical differences between the spectra of LMC and Galactic PNe. We also determine that the more evolved disc sources listed in the Macquarie/AAO/Strasbourg (MASH) catalogues of Parker et al. and Miszalski et al. have similar colours to those of the less evolved (and higher surface brightness) sources in the catalogue of Acker et al., a result which appears at variance with previous studies of these sources. [ABSTRACT FROM AUTHOR]

The far-infrared (FIR) emissivity of dust is an important parameter characterizing the physical properties of the grains. With the availability of stellar data bases and FIR data from Infrared Space Observatory ( ISO), it is possible to compare the optical and infrared properties of dust, and derive the FIR emissivity with respect to the optical extinction. In this paper, we present the results of a systematic analysis of the FIR emissivity of interstellar clouds observed with ISOPHOT (the photometer onboard ISO) at least at two infrared wavelengths, one close to and the other at 200 μm. We constructed FIR emission maps, determined dust temperatures, created extinction maps using Two-Micron All-Sky Survey data and calculated FIR emissivity for each of these clouds. We present the largest homogeneously reduced data base constructed so far for this purpose. During the data analysis, special care was taken on possible systematic errors. We find that FIR emissivity has a clear dependence on temperature. The emissivity is enhanced by a factor of usually less than 2 in the low dust temperature regime of 12 K . This result suggests larger grain sizes in those regions. However, the emissivity increase of typically below 2 restricts the possible grain growth processes to ice mantle formation and coagulation of silicate grains, and excludes the coagulation of carbonaceous particles on the scales of the regions we investigated. In the temperature range 14 K ≤ , a systematic decrease of emissivity is observed with respect to the values of the diffuse interstellar matter. Possible scenarios for this behaviour are discussed in this paper. [ABSTRACT FROM AUTHOR]

We present high spatial resolution radio and near-infrared hydrogen recombination line observations of the southern massive star forming region G333.6−0.2. The 3.4-cm continuum peak is found slightly offset from the infrared (IR) source. The H90α spectra show for the first time a double-peak profile at some positions. The complex velocity structure may be accounted for by champagne outflows, which may also explain the offset between the radio and IR sources. The 2.17-μm Brγ image and H90α map are combined to construct an extinction map which shows a trend probably set by the blister nature of the H ii region. The total number of Lyman continuum photons in the central 50-arcsec is estimated to be equivalent to that emitted by up to 19 O7V stars. [ABSTRACT FROM AUTHOR]

We note that H2 emitting planetary nebulae tend to have Zanstra temperatures . This is shown to be consistent with a large evolutionary lifetime, and the kinematic ages of the envelopes. Non-local thermodynamic equilibrium stellar atmospheric modelling also shows that levels of soft X-ray emission increase more rapidly than has previously been assumed, and are preferentially large in H2 emitting sources. It is suggested that this may hold the key to explaining the strengths of the H2 transitions. [ABSTRACT FROM AUTHOR]

Near-infrared imaging of the emission from molecular hydrogen is a powerful method for discovering outflows in star-forming regions. We present new near-infrared images, and long-slit and integral-field spectroscopy of the ultracompact H ii region G25.65+1.05. These new observations reveal shocked emission associated with a bipolar outflow from a young high-mass star at the centre of the source. The physical parameters of the outflow are discussed and compared with those of outflows from lower mass stars. [ABSTRACT FROM AUTHOR]

The vertical profiles of disc galaxies are built by the material trapped around stable periodic orbits, which form their ‘skeletons’. Therefore, knowledge of the stability of the main families of periodic orbits in appropriate 3D models enables one to predict possible morphologies for edge-on disc galaxies. In a pilot survey we compare the orbital structures that lead to the appearance of ‘peanut’- and ‘X’-like features with the edge-on profiles of three disc galaxies (IC 2531, NGC 4013 and UGC 2048). The subtraction from the images of a model representing the axisymmetric component of the galaxies reveals the contribution of the non-axisymmetric terms. We find a direct correspondence between the orbital profiles of 3D bars in models and the observed main morphological features of the residuals. We also apply a simple unsharp masking technique in order to study the sharpest features of the images. Our basic conclusion is that the morphology of the boxy ‘bulges’ of these galaxies can be explained by considering disc material trapped around stable 3D periodic orbits. In most models, these building-block periodic orbits are bifurcated from the planar central family of a non-axisymmetric component, usually a bar, at low-order vertical resonances. In such a case, the boxy ‘bulges’ are parts of bars seen edge-on. For the three galaxies we study, the families associated with the ‘peanut’ or ‘X’-shape morphology are probably bifurcations at the vertical 2/1 or 4/1 resonance. [ABSTRACT FROM AUTHOR]

The advent of modern infrared astronomy has brought into evidence the role played by the interstellar dust in galaxy formation and evolution. Therefore, to fully exploit modern data, realistic spectrophotometric models of galaxies must include this important component of the interstellar medium (ISM). In this paper, the first of a series of two devoted to modelling the spectra of galaxies of different morphological type in the presence of dust, we present our description of the dust both in the diffuse ISM and in the molecular clouds (MCs). Our galaxy model contains three interacting components: the diffuse ISM, made of gas and dust, the large complexes of MCs in which active star formation occurs and, finally, the populations of stars that are no longer embedded in the dusty environment of their parental MCs. Our model for the dust takes into account three components, i.e. graphite, silicates and polycyclic aromatic hydrocarbons (PAHs). We consider and adapt to our aims two prescriptions for the size distribution of the dust grains and two models for the emission of the dusty ISM. We cross-check the emission and extinction models of the ISM by calculating the extinction curves and the emission for the typical environments of the Milky Way (MW) and the Large and Small Magellanic Clouds (LMC and SMC) and by comparing the results with the observational data. The final model we have adopted is a hybrid one which stems from combining the analysis of Guhathakurta & Draine for the emission of graphite and silicates and Puget, Leger & Boulanger for the PAH emission, and using the distribution law of Weingartner & Draine and the ionization model for PAHs of Weingartner & Draine. We apply the model to calculate the spectral energy distribution (SED) of single stellar populations (SSPs) of different age and chemical composition, which may be severely affected by dust at least in two types of stars: the young, massive stars while they are still embedded in their parental MCs and the intermediate- and low-mass asymptotic giant branch (AGB) stars when they form their own dust shell around. We use the ‘ray-tracing’ method to solve the problem of radiative transfer and to calculate extended libraries of SSP SEDs. Particular care is taken to model the contribution from PAHs, introducing different abundances of C in the population of very small carbonaceous grains (VSGs) and different ionization states in PAHs. The SEDs of young SSPs are then compared with observational data of star-forming regions of four local galaxies successfully reproducing their SEDs from the ultraviolet (UV)-optical regions to the mid- and far-infrared region (MIR and FIR, respectively). [ABSTRACT FROM AUTHOR]

We point out that many planetary nebulae (PNe) have large infrared indices ( H– KS), and that this is likely to result from the presence of hot grains, and/or H2 S(1) line emission at . We are able to identify two groups of sources associated with each of these mechanisms, both of which appear to possess distinct physical characteristics. One difference between these groups concerns the near-infrared dimensions of the sources. It appears that hot dust outflows frequently have sizes . Four of the sources are particularly extreme in this regard, and show emission extending well outside of the primary shells. We propose that this is likely to arise from high-temperature grains located in low-density haloes. The location of such grains at large distances from the central stars represents a challenge for any mechanism purporting to explain this phenomenon. The most likely explanation appears to be in terms of photon heating of very small grains. [ABSTRACT FROM AUTHOR]

We have observed Hubble V, the brightest H ii region complex in the dwarf irregular galaxy NGC 6822, at near-infrared (near-IR; 1.8–2.4 μm) wavelengths. The line emission maps of Hubble V show the typical structure of a photodissociation region (PDR) where an ionized core, traced by compact He i emission (2.0587 μm) and Brγ emission (2.1661 μm), is surrounded by an outer layer traced by molecular hydrogen (H2) emission. The measured line ratios of from 0.2 to 0.6 and the unshifted and unresolved line profiles suggest that the H2 emission originates purely from a PDR. We find no evidence for shock activity. By comparing the H2 results with a PDR model, we conclude that Hubble V includes dense and warm PDRs. In this environment, most of the H2 molecules are excited by far-ultraviolet photons (with a field strength of 102–4 times that of the average interstellar field), although collisional processes de-excite H2 and contribute significantly to the excitation of the first vibrational level. We expect that Hubble V is in the early stage of molecular cloud dissolution. [ABSTRACT FROM AUTHOR]

Fluctuations in the brightness of the background radiation can lead to confusion with real point sources. This type of confusion with background emission is relevant when making infrared (IR) observations with relatively large beam sizes, since the amount of fluctuation tends to increase with the angular scale. To quantitively assess the effect of the background emission on the detection of point sources for current and future far-IR observations by space-borne missions such asSpitzer,ASTRO-F,HerschelandSpace Infrared Telescope for Cosmology and Astrophysics(SPICA), we have extended the Galactic emission map to a higher level of angular resolution than that of the currently available data. Using this high-resolution map, we estimate the sky confusion noise owing to the emission from interstellar dust clouds or cirrus, based on fluctuation analysis and detailed photometry over realistically simulated images. We find that the confusion noise derived by simple fluctuation analysis agrees well with the results from realistic simulations. Although sky confusion noise becomes dominant in long wavelength bandswith 60–90 cm aperture missions, it is expected to be two orders of magnitude lower for the next generation of space missions (with larger aperture sizes) such asHerschelandSPICA. [ABSTRACT FROM AUTHOR]