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Register a new userThe HIFI spectral survey of AFGL2591 (CHESS). I. Highly excited linear rotor molecules in the high-mass protostellar envelope
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Matthijs H.D. van der Wiel
Publication date
2013
fields
Physics
and research's language is
English
Authors
M. H. D. van der Wiel
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We aim to reveal the gas energetics in the circumstellar environment of the prototypical high-mass protostellar object AFGL2591 using space-based far-infrared observations of linear rotor molecules. Rotational spectral line signatures of CO, HCO+, CS, HCN and HNC from a 490-1240 GHz survey with Herschel/HIFI, complemented by ground-based JCMT and IRAM 30m spectra, cover transitions with E(up)/k between 5 and ~300 K (750K for 12C16O, using selected frequency settings up to 1850 GHz). The resolved spectral line profiles are used to separate and study various kinematic components. The line profiles show two emission components, the widest and bluest of which is attributed to an approaching outflow and the other to the envelope. We find evidence for progressively more redshifted and wider line profiles from the envelope gas with increasing energy level, qualitatively explained by residual outflow contribution picked up in the systematically decreasing beam size. Integrated line intensities for each species decrease as E(up)/k increases from <50 to 700K. We constrain the following: n(H2)~10^5-10^6 cm^-3 and T~60-200K for the outflow gas; T=9-17K and N(H2)~3x10^21 cm^-2 for a known foreground absorption cloud; N(H2)<10^19 cm^-2 for a second foreground component. Our spherical envelope radiative transfer model systematically underproduces observed line emission at E(up)/k > 150 K for all species. This indicates that warm gas should be added to the model and that the models geometry should provide low optical depth pathways for line emission from this warm gas to escape, for example in the form of UV heated outflow cavity walls viewed at a favorable inclination angle. Physical and chemical conditions derived for the outflow gas are similar to those in the protostellar envelope, possibly indicating that the modest velocity (<10 km/s) outflow component consists of recently swept-up gas.
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We aim to understand the rich chemical composition of AFGL 2591, a prototypical isolated high-mass star-forming region. Based on HIFI and JCMT data, the molecular abundances of species found in the protostellar envelope of AFGL 2591 were derived with the Monte Carlo radiative transfer code RATRAN, assuming either constant values or 1D stepwise radial profiles as abundance distributions. The reconstructed 1D abundances were compared with the results of time-dependent gas-grain chemical modeling, considering ages of 10,000 to 50,000 years, cosmic-ray ionization rates of 0.5 to 50 times 10^-16 s^-1, uniformly-sized 0.1-1 micron dust grains, a dust/gas ratio of 1%, and several sets of initial molecular abundances with C/O <1 and >1. Constant abundance models give good fits to the data for CO, CN, CS, HCO+, H2CO, N2H+, C2H, NO, OCS, OH, H2CS, O, C, C+, and CH. Models with an abundance jump at 100 K give good fits to the data for NH3, SO, SO2, H2S, H2O, HCl, and CH3OH. For HCN and HNC, the best models have an abundance jump at 230 K. The time-dependent chemical model can accurately explain abundance profiles of 15 out of these 24 species. The jump-like radial profiles for key species like HCO+, NH3, and H2O are consistent with the outcome of the time-dependent chemical modeling. The best-fit model has a chemical age of 10-50 kyr, a solar C/O ratio of 0.44, and a cosmic-ray ionization rate of 5 x 10^-17 s^-1; grain properties and external UV intensity do not affect the calculated chemical structure much. We thus demonstrate that simple constant or jump-like abundance profiles agree with time-dependent chemical modeling for most key C-, O-, N-, and S-bearing molecules. The main exceptions are species with very few observed transitions (C, O, C+, and CH), with a poorly established chemical network (HCl, H2S) or whose chemistry is strongly affected by surface processes (CH3OH).
The understanding of the formation process of massive stars (>8 Msun) is limited, due to theoretical complications and observational challenges. We investigate the physical structure of the large-scale (~10^4-10^5 AU) molecular envelope of the high-mass protostar AFGL2591 using spectral imaging in the 330-373 GHz regime from the JCMT Spectral Legacy Survey. Out of ~160 spectral features, this paper uses the 35 that are spatially resolved. The observed spatial distributions of a selection of six species are compared with radiative transfer models based on a static spherically symmetric structure, a dynamic spherical structure, and a static flattened structure. The maps of CO and its isotopic variations exhibit elongated geometries on scales of ~100, and smaller scale substructure is found in maps of N2H+, o-H2CO, CS, SO2, CCH, and methanol lines. A velocity gradient is apparent in maps of all molecular lines presented here, except SO, SO2, and H2CO. We find two emission peaks in warm (Eup~200K) methanol separated by 12, indicative of a secondary heating source in the envelope. The spherical models are able to explain the distribution of emission for the optically thin H13CO+ and C34S, but not for the optically thick HCN, HCO+, and CS, nor for the optically thin C17O. The introduction of velocity structure mitigates the optical depth effects, but does not fully explain the observations, especially in the spectral dimension. A static flattened envelope viewed at a small inclination angle does slightly better. We conclude that a geometry of the envelope other than an isotropic static sphere is needed to circumvent line optical depth effects. We propose that this could be achieved in envelope models with an outflow cavity and/or inhomogeneous structure at scales smaller than ~10^4 AU. The picture of inhomogeneity is supported by observed substructure in at least six species.
This paper presents the richness of submillimeter spectral features in the high-mass star forming region AFGL 2591. As part of the CHESS (Chemical Herschel Survey of Star Forming Regions) Key Programme, AFGL 2591 was observed by the Herschel/HIFI instrument. The spectral survey covered a frequency range from 480 up to 1240 GHz as well as single lines from 1267 to 1901 GHz (i.e. CO, HCl, NH3, OH and [CII]). Rotational and population diagram methods were used to calculate column densities, excitation temperatures and the emission extents of the observed molecules associated with AFGL 2591. The analysis was supplemented with several lines from ground-based JCMT spectra. From the HIFI spectral survey analysis a total of 32 species were identified (including isotopologues). In spite of the fact that lines are mostly quite week, 268 emission and 16 absorption lines were found (excluding blends). Molecular column densities range from 6e11 to 1e19 cm-2 and excitation temperatures range from 19 to 175 K. One can distinguish cold (e.g. HCN, H2S, NH3 with temperatures below 70 K) and warm species (e.g. CH3OH, SO2) in the protostellar envelope.
We derive the dense core structure and the water abundance in four massive star-forming regions which may help understand the earliest stages of massive star formation. We present Herschel-HIFI observations of the para-H2O 1_11-0_00 and 2_02-1_11 and the para-H2-18O 1_11-0_00 transitions. The envelope contribution to the line profiles is separated from contributions by outflows and foreground clouds. The envelope contribution is modelled using Monte-Carlo radiative transfer codes for dust and molecular lines (MC3D and RATRAN), with the water abundance and the turbulent velocity width as free parameters. While the outflows are mostly seen in emission in high-J lines, envelopes are seen in absorption in ground-state lines, which are almost saturated. The derived water abundances range from 5E-10 to 4E-8 in the outer envelopes. We detect cold clouds surrounding the protostar envelope, thanks to the very high quality of the Herschel-HIFI data and the unique ability of water to probe them. Several foreground clouds are also detected along the line of sight. The low H2O abundances in massive dense cores are in accordance with the expectation that high densities and low temperatures lead to freeze-out of water on dust grains. The spread in abundance values is not clearly linked to physical properties of the sources.
Broadband spectral surveys of protostars offer a rich view of the physical, chemical and dynamical structure and evolution of star-forming regions. The Herschel Space Observatory opened up the terahertz regime to such surveys, giving access to the fundamental transitions of many hydrides and to the high-energy transitions of many other species. A comparative analysis of the chemical inventories and physical processes and properties of protostars of various masses and evolutionary states is the goal of the Herschel CHEmical Surveys of Star forming regions (CHESS) key program. This paper focusses on the intermediate-mass protostar, OMC-2 FIR 4. We obtained a spectrum of OMC-2 FIR 4 in the 480 to 1902 GHz range with the HIFI spectrometer onboard Herschel and carried out the reduction, line identification, and a broad analysis of the line profile components, excitation, and cooling. We detect 719 spectral lines from 40 species and isotopologs. The line flux is dominated by CO, H2O, and CH3OH. The line profiles are complex and vary with species and upper level energy, but clearly contain signatures from quiescent gas, a broad component likely due to an outflow, and a foreground cloud. We find abundant evidence for warm, dense gas, as well as for an outflow in the field of view. Line flux represents 2% of the 7 L_Sol luminosity detected with HIFI in the 480 to 1250 GHz range. Of the total line flux, 60% is from CO, 13% from H2O and 9% from CH3OH. A comparison with similar HIFI spectra of other sources is set to provide much new insight into star formation regions, a case in point being a difference of two orders of magnitude in the relative contribution of sulphur oxides to the line cooling of Orion KL and OMC-2 FIR 4.
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