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Context. Historically, due to instrumental limitations and a lack of disk detections, the structure of the transition from the envelope to the rotationally supported disk has been poorly studied. This is now possible with ALMA through observations of CO isotopologues and tracers of freezeout. Class 0 sources are ideal for such studies given their almost intact envelope and young disk. Aims. The structure of the disk-envelope interface of the prototypical Class 0 source, VLA1623A, which has a confirmed Keplerian disk, is constrained through modeling and analysis of ALMA observations of DCO+ (3-2) and C18O (2-1) rotational lines. Methods. The physical structure of VLA1623 is obtained from the large-scale spectral energy distribution (SED) and continuum radiative transfer. An analytic model using a simple network coupled with radial density and temperature profiles is used as input for a 2D line radiative transfer calculation for comparison with the ALMA Cycle 0 12-m array and Cycle 2 ACA observations of VLA1623. Results. The DCO+ emission shows a clumpy structure bordering VLA1623A's Keplerian disk. This suggests a cold ring-like structure at the disk-envelope interface. The radial position of the observed DCO+ peak is reproduced in our model only if the region's temperature is between 11 K and 16 K, lower than expected from models constrained by continuum data and source SED. Altering the density profile has little e ect on the DCO+ peak position, but increased density is needed to reproduce the observed C18O tracing the disk. Conclusions. The observed DCO+ (3-2) emission around VLA1623A is the product of shadowing of the envelope by the disk observed in C18O. Disk-shadowing causes a drop in the gas temperature outside of the disk on >200 AU scales, encouraging the production of deuterated molecules. This indicates that the physical structure of the disk-envelope interface differs from the rest of the envelope, highlighting the drastic impact that the disk has on the envelope and temperature structure. The results presented here show that DCO+ is an excellent cold temperature tracer. [ABSTRACT FROM AUTHOR]

Context. Disks are observed around pre-main sequence stars, but how and when they form is still heavily debated. While disks around young stellar objects have been identified through thermal dust emission, spatially and spectrally resolved molecular line observations are needed to determine their nature. Only a handful of embedded rotationally supported disks have been identified to date. Aims. We identify and characterize rotationally supported disks near the end of the main accretion phase of low-mass protostars by comparing their gas and dust structures. Methods. Subarcsecond observations of dust and gas toward four Class I low-mass young stellar objects in Taurus are presented at significantly higher sensitivity than previous studies. The 13CO and C18O J = 2-1 transitions at 220 GHz were observed with the Plateau de Bure Interferometer at a spatial resolution of ⩽0.8" (56 AU radius at 140 pc) and analyzed using uv-space position velocity diagrams to determine the nature of their observed velocity gradient. Results. Rotationally supported disks (RSDs) are detected around 3 of the 4 Class I sources studied. The derived masses identify them as Stage I objects; i.e., their stellar mass is higher than their envelope and disk masses. The outer radii of the Keplerian disks toward our sample of Class I sources are ⩽100 AU. The lack of on-source C18O emission for TMR1 puts an upper limit of 50 AU on its size. Flattened structures at radii >100 AU around these sources are dominated by infalling motion (υ ∝ r-1). A large-scale envelope model is required to estimate the basic parameters of the flattened structure from spatially resolved continuum data. Similarities and differences between the gas and dust disk are discussed. Combined with literature data, the sizes of the RSDs around Class I objects are best described with evolutionary models with an initial rotation of Ω = 10-14 Hz and slow sound speeds. Based on the comparison of gas and dust disk masses, little CO is frozen out within 100 AU in these disks. Conclusions. Rotationally supported disks with radii up to 100 AU are present around Class I embedded objects. Larger surveys of both Class 0 and I objects are needed to determine whether most disks form late or early in the embedded phase. [ABSTRACT FROM AUTHOR]

Aims. Our aim is to study the response of the gas-to-energetic processes associated with high-mass star formation and compare it with previously published studies on low- and intermediate-mass young stellar objects (YSOs) using the same methods. The quantified far-IR line emission and absorption of CO, H2O, OH, and [Oi] reveals the excitation and the relative contribution of different atomic and molecular species to the gas cooling budget. Methods. Herschel/PACS spectra covering 55-190 μm are analyzed for ten high-mass star forming regions of luminosities Lbol ~ 104-106 L☉ and various evolutionary stages on spatial scales of ~104 AU. Radiative transfer models are used to determine the contribution of the quiescent envelope to the far-IR CO emission. Results. The close environments of high-mass protostars show strong far-IR emission from molecules, atoms, and ions. Water is detected in all 10 objects even up to high excitation lines, often in absorption at the shorter wavelengths and in emission at the longer wavelengths. CO transitions from J = 14-13 up to typically 29-28 (Eu/kB ~ 580-2400 K) show a single temperature component with a rotational temperature of Trot ~ 300 K. Typical H2O excitation temperatures are Trot ~250 K, while OH has Trot ~ 80 K. Far-IR line cooling is dominated by CO (~75%) and, to a smaller extent, by [Oi] (~20%), which becomes more important for the most evolved sources. H2O is less important as a coolant for high-mass sources because many lines are in absorption. Conclusions. Emission from the quiescent envelope is responsible for ~45-85% of the total CO luminosity in high-mass sources compared with only ~10% for low-mass YSOs. The highest-J lines (Jup ⩾ 20) originate most likely in shocks, based on the strong correlation of CO and H2O with physical parameters (Lbol, Menv) of the sources from low- to high-mass YSOs. The excitation of warm CO described by Trot ~ 300 K is very similar for all mass regimes, whereas H2O temperatures are ~100 K high for high-mass sources compared with low-mass YSOs. The total far-IR cooling in lines correlates strongly with bolometric luminosity, consistent with previous studies restricted to low-mass YSOs.Molecular cooling (CO, H2O, and OH) is ~4 times greater than cooling by oxygen atoms for all mass regimes. The total far-IR line luminosity is about 10-3 and 10-5 times lower than the dust luminosity for the low-and high-mass star forming regions, respectively. [ABSTRACT FROM AUTHOR]

Context. For stars to form, material must fall inwards from core scales through the envelope towards the central protostar. While theories of how this takes place have been around for some time, the velocity profile around protostars is poorly constrained. The combination of observations in multiple transitions of a tracer which is sensitive to kinematics and radiative transfer modelling of those lines has the potential to break this deadlock. Aims. Seven protostars observed with the Heterodyne Instrument for the Far-Infrared (HIFI) on board the Herschel Space Observatory as part of the "Water in star-forming regions with Herschel" (WISH) survey show infall signatures in water line observations. We aim to constrain the infall velocity and the radii over which infall is taking place within the protostellar envelopes of these sources. We will also use these data to constrain the chemistry of cold water. Methods. We use 1-D non-LTE ratran radiative transfer models of the observed water lines to constrain the infall velocity and chemistry in the protostellar envelopes of six Class 0 protostars and one Class I source. We assume a free-fall velocity profile and, having found the best fit, vary the radii over which infall takes place. Results. In the well-studied Class 0 protostar NGC1333-IRAS4A we find that our observations probe infall over the whole envelope to which our observations are sensitive (r ≳ 1000 AU). For L1527, L1157, BHR71 and IRAS 15398 infall takes place on core to envelope scales (i.e. ~10 000-3000 AU). In Serpens-SMM4 and GSS30 the inverse P-Cygni profiles seen in the ground-state lines are more likely due to larger-scale motions or foreground clouds. Models including a simple consideration of the chemistry are consistent with the observations, while using step abundance profiles are not. The non-detection of excited water in the inner envelope in six out of seven protostars is further evidence that water must be heavily depleted from the gas-phase at these radii. Conclusions. Infall in four of the sources is supersonic and in all sources must take place at the outer edge of the envelope, which may be evidence that collapse is global or outside-in rather than inside-out. The mass infall rate in NGC1333-IRAS4A is large (≳10-4 M☉ yr-1), higher than the mass outflow rate and expected mass accretion rates onto the star. This suggests that any flattened disk-like structure on small scales will be gravitationally unstable, potentially leading to rotational fragmentation and/or episodic accretion. [ABSTRACT FROM AUTHOR]

Aims. Herschel-HIFI spectra of H2O towards low-mass protostars show a distinct velocity component not seen in observations from the ground of CO or other species. The aim is to characterise this component in terms of excitation conditions and physical origin. Methods. A velocity component with an offset of ~10 km s-1 detected in spectra of the H2O 110-101 557 GHz transition towards six low-mass protostars in the "Water in star-forming regions with Herschel" (WISH) programme is also seen in higher-excited H2O lines. The emission from this component is quantified and local excitation conditions are inferred using 1D slab models. Data are compared to observations of hydrides (high-J CO, OH+, CH+, C+, OH) where the same component is uniquely detected. Results. The velocity component is detected in all six targeted H2O transitions (Eup ~ 50-250 K), as well as in CO 16-15 towards one source, Ser SMM1. Inferred excitation conditions imply that the emission arises in dense (n ~ 5 × 106-108 cm-3) and hot (T ~ 750 K) gas. The H2O and CO column densities are ≳1016 and 1018 cm-2, respectively, implying a low H2O abundance of ~10-2 with respect to CO. The high column densities of ions such as OH+ and CH+ (both ≳1013 cm-2) indicate an origin close to the protostar where the UV field is strong enough that these species are abundant. The estimated radius of the emitting region is 100 AU. This component likely arises in dissociative shocks close to the protostar, an interpretation corroborated by a comparison with models of such shocks. Furthermore, one of the sources, IRAS 4A, shows temporal variability in the offset component over a period of two years which is expected from shocks in dense media. High-J CO gas detected with Herschel-PACS with Trot ~ 700 K is identified as arising in the same component and traces the part of the shock where H2 reforms. Thus, H2O reveals new dynamical components, even on small spatial scales in low-mass protostars. [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] and OH 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. The OH radical is a key species in the water chemistry network of star-forming regions, because its presence is tightly related to the formation and destruction of water. Previous studies of the OH far-infrared emission from low- and intermediate-mass protostars suggest that the OH emission mainly originates from shocked gas and not from the quiescent protostellar envelopes. Aims. We aim to study the excitation of OH in embedded low- and intermediate-mass protostars, determine the influence of source parameters on the strength of the emission, investigate the spatial extent of the OH emission, and further constrain its origin. Methods. This paper presents OH observations from 23 low- and intermediate-mass young stellar objects obtained with the PACS integral field spectrometer on-board Herschel in the context of the "Water In Star-forming regions with Herschel" (WISH) key program. Radiative transfer codes are used to model the OH excitation. Results. Most low-mass sources have compact OH emission (≲5000 AU scale), whereas the OH lines in most intermediate-mass sources are extended over the whole 47."0 × 47."0 PACS detector field-of-view (≳20 000 AU). The strength of the OH emission is correlated with various source properties such as the bolometric luminosity and the envelope mass, but also with the [OI] and H2O emission. Rotational diagrams for sources with many OH lines show that the level populations of OH can be approximated by a Boltzmann distribution with an excitation temperature at around 70 K. Radiative transfer models of spherically symmetric envelopes cannot reproduce the OH emission fluxes nor their broad line widths, strongly suggesting an outflow origin. Slab excitation models indicate that the observed excitation temperature can either be reached if the OH molecules are exposed to a strong far-infrared continuum radiation field or if the gas temperature and density are sufficiently high. Using realistic source parameters and radiation fields, it is shown for the case of Ser SMM1 that radiative pumping plays an important role in transitions arising from upper level energies higher than 300 K. The compact emission in the low-mass sources and the required presence of a strong radiation field and/or a high density to excite the OH molecules points toward an origin in shocks in the inner envelope close to the protostar. [ABSTRACT FROM AUTHOR]

Context. Water is a key tracer of dynamics and chemistry in low-mass star-forming regions, but spectrally resolved observations have so far been limited in sensitivity and angular resolution, and only data from the brightest low-mass protostars have been published. Aims. The first systematic survey of spectrally resolved water emission in 29 low-mass (L <40 Lʘ) protostellar objects is presented. The sources cover a range of luminosities and evolutionary states. The aim is to characterise the line profiles to distinguish physical components in the beam and examine how water emission changes with protostellar evolution. Methods. H2O was observed in the ground-slate 110-101 transition at 557 GHz (Eup/kB -∼ 60K) as single-point observations with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel in 29 deeply embedded Class 0 and I low-mass protostars. Complementary far-R and tub-mm continuum data (including PACS data from our programme) are used to constrain the spectral energy distribution (SED) of each source. H20 intensities are compared to inferred envelope properties, e.g., mass and density, outflow properties and CO 3-2 emission. Results. H2O emission is detected in all objects except one (TMCIA). The line profiles are complex and consist of several kinematic components tracing different physical regions in each system. In particular, the profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not in the quiescent envelope. Several sources show multiple shock components appearing in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in seven sources (nix Class 0, one Class 1) indicative of infalling envelopes, and regular P-Cygni profiles in four sources (three Class 1, one Class 0) indicative of expanding envelopes. Molecular "bullets" moving at ≳50 km s-1 with respect to the source are detected in four Class 0 sources; three of these sources were not known to harbour bullets previously. In the outflow, the H2O/CO abundance ratio an a function of velocity in nearly the name for all line wings, increasing from 10-3 at low velocities (<5 km s-1) to ≳ 10-1 at high velocities (>10 km s-1). The water abundance in the outer cold envelope in low, ≳10-10. The different H2O profile components show a clear evolutionary trend: in the younger Class 0 sources the emission in dominated by outflow components originating inside an infalling envelope. When large-scale infall diminishes during the Class I phase, the outflow weakens and H2O emission all but disappears. [ABSTRACT FROM AUTHOR]

Aims. Young stars interact vigorously with their surroundings, as evident from the highly rotationally excited CO (up to Eu/k = 4000 K) and H2O emission (up to 600 K) detected by the Herschel Space Observatory in embedded low-mass protostars. Our aim is to construct a model that reproduces the observations quantitatively, to investigate the origin of the emission, and to use the lines as probes of the various heating mechanisms. Methods. The model consists of a spherical envelope with a power-law density structure and a bipolar outflow cavity. Three heating mechanisms are considered: passive heating by the protostellar luminosity, ultraviolet irradiation of the outflow cavity walls, and small-scale C-type shocks along the cavity walls. Most of the model parameters are constrained from independent observations; the two remaining free parameters considered here are the protostellar UV luminosity and the shock velocity. Line fluxes are calculated for CO and H2O and compared to Herschel data and complementary ground-based data for the protostars NGC 1333 IRAS2A, HH 46 and DK Cha. The three sources are selected to span a range of evolutionary phases (early Stage 0 to late Stage I) and physical characteristics such as luminosity and envelope mass. Results. The bulk of the gas in the envelope, heated by the protostellar luminosity, accounts for 3-10% of the CO luminosity summed over all rotational lines up to J = 40-39; it is best probed by low-J CO isotopologue lines such as C18O 2-1 and 3-2. The UV-heated gas and the C-type shocks, probed by 12CO 10-9 and higher-J lines, contribute 20-80% each. The model fits show a tentative evolutionary trend: the CO emission is dominated by shocks in the youngest source and by UV-heated gas in the oldest one. This trend is mainly driven by the lower envelope density in more evolved sources. The total H2O line luminosity in all cases is dominated by shocks (>99%). The exact percentages for both species are uncertain by at least a factor of 2 due to uncertainties in the gas temperature as function of the incident UV flux. However, on a qualitative level and within the context of our model, both UV-heated gas and C-type shocks are needed to reproduce the emission in far-infrared rotational lines of CO and H2O. [ABSTRACT FROM AUTHOR]

Molecules containing one or a few hydrogen atoms and a heavier atom (hydrides) have been predicted to trace FUV radiation. In some chemical models, FUV emission by the central object or protostar of a star forming region greatly enhances some of the hydride abundances. Two massive regions, W3 IRS5 and AFGL 2591, have been observed in hydride lines by HIFI onboard the Herschel Spce Observatory. We use published results as well as new observations of CH+ towards W3 IRS5. Molecular column densities are derived from ground state absorption lines, radiative transfer modeling or rotational diagrams. Models assuming no internal FUV are compared with two-dimensional models including FUV irradiation of outflow walls. We confirm that the effect of FUV is clearly noticeable and greatly improves the fit. The most sensitive molecules to FUV irradiation are CH+ and OH+, enhanced in abundance by many orders of magnitude. Modeling in addition also full line radiative transfer, Bruderer et al. (2010b) achieve good agreement of a two-dimensional FUV model with observations of CH+ in AFGL 2591. It is concluded that CH+ and OH+ are good FUV tracers in star-forming regions. [ABSTRACT FROM AUTHOR]

Water is a key molecule for tracing physical and chemical processes in star-forming regions. The key program "Water in star-forming regions with Herschel" is observing several water transitions towards low-mass protostars with HIFI. Results regarding the 557 GHz transition of water are reported here showing that the line is surpris- ingly broad, and consists of several different velocity components. The bulk of the emission comes from shocks, where the abundance is increased by several orders of magnitude to ~10-4. The abundance of water in the outer envelope is determined to ~10-8, whereas only an upper limit of 10-5 is derived for the inner, warm envelope. [ABSTRACT FROM AUTHOR]