Modelling clumpy PDRs in 3D - Understanding the Orion Bar stratification

Context. Models of photon-dominated regions (PDRs) still fail to fully reproduce some of the observed properties, in particular the combination of the intensities of different PDR cooling lines together with the chemical stratification, as observed e.g. for the Orion Bar PDR. Aims. We aim to construct a numerical PDR model, KOSMA-\tau 3D, to simulate full spectral cubes of line emission from arbitrary PDRs in three dimensions (3D). The model is to reproduce the intensity of the main cooling lines from the Orion Bar PDR and the observed layered structure of the different transitions. Methods. We build up a 3D compound, made of voxels ("3D pixels") that contain a discrete mass distribution of spherical "clumpy" structures, approximating the fractal ISM. To analyse each individual clump the new code is combined with the KOSMA-\tau PDR model. Probabilistic algorithms are used to calculate the local FUV flux for each voxel as well as the voxel-averaged line emissivities and optical depths, based on the properties of the individual clumps. Finally, the computation of the radiative transfer through the compound provides full spectral cubes. To test the new model we try to simulate the structure of the Orion Bar PDR and compare the results to observations from HIFI/Herschel and from the Caltech Submillimetre Observatory (CSO). In this context new Herschel data from the HEXOS guaranteed-time key program is presented. Results. Our model is able to reproduce the line integrated intensities within a factor 2.5 and the observed stratification pattern within 0.016 pc for the [Cii] 158 \mu m and different 12/13 CO and HCO+ transitions, based on the representation of the Orion Bar PDR by a clumpy edge-on cavity wall. In the cavity wall, a large fraction of the total mass needs to be contained in clumps. The mass of the interclump medium is constrained by the FUV penetration. Furthermore, ...Comment: Major changes compared to v1. Also several references have been adde

The link between gas and stars in the S254-S258 star-forming region

The paper aims to study relation between the distributions of the young stellar objects (YSOs) of different ages and the gas-dust constituents of the S254-S258 star formation complex. This is necessary to study the time evolution of the YSO distribution with respect to the gas and dust compounds that are responsible for the birth of the young stars. For this purpose, we use correlation analysis between different gas, dust, and YSO tracers. We compared the large-scale CO, HCO+, near-IR extinction, and far-IR Herschel maps with the density of YSOs of the different evolutionary classes. The direct correlation analysis between these maps was used together with the wavelet-based spatial correlation analysis. This analysis reveals a much tighter correlation of the gas-dust tracers with the distribution of class I YSOs than with that of class II YSOs. We argue that class I YSOs that were initially born in the central bright cluster S255-IR (both N and S parts) during their evolution to class II stage (similar to 2 Myr) had enough time to travel through the whole S254-S258 star formation region. Given that the region contains several isolated YSO clusters, the evolutionary link between these clusters and the bright central S255-IR (N and S) cluster can be considered. Despite the complexity of the YSO cluster formation in the non-uniform medium, the clusters of class II YSOs in the S254-258 star formation region can contain objects born in the different locations of the complex.Peer reviewe

The first CO+ image: Probing the HI/H2 layer around the ultracompact HII region Mon R2

The CO+ reactive ion is thought to be a tracer of the boundary between a HII region and the hot molecular gas. In this study, we present the spatial distribution of the CO+ rotational emission toward the Mon R2 star-forming region. The CO+ emission presents a clumpy ring-like morphology, arising from a narrow dense layer around the HII region. We compare the CO+ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 mm, H2 S(3) rotational line at 9.3 mm, polycyclic aromatic hydrocarbons (PAHs) and HCO+. We find that the CO+ emission is spatially coincident with the PAHs and [CII] emission. This confirms that the CO+ emission arises from a narrow dense layer of the HI/H2 interface. We have determined the CO+ fractional abundance, relative to C+ toward three positions. The abundances range from 0.1 to 1.9x10^(-10) and are in good agreement with previous chemical model, which predicts that the production of CO+ in PDRs only occurs in dense regions with high UV fields. The CO+ linewidth is larger than those found in molecular gas tracers, and their central velocity are blue-shifted with respect to the molecular gas velocity. We interpret this as a hint that the CO+ is probing photo-evaporating clump surfaces.Comment: The main text has 4 pages, 2 pages of Appendix, 4 figures, 1 table. Accepted for publication in Astronomy and Astrophysics letter

The ionized and hot gas in M17 SW: SOFIA/GREAT THz observations of [C II] and 12CO J=13-12

With new THz maps that cover an area of ~3.3x2.1 pc^2 we probe the spatial distribution and association of the ionized, neutral and molecular gas components in the M17 SW nebula. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain a 5'.7x3'.7 map of the 12CO J=13-12 transition and the [C II] 158 um fine-structure line in M17 SW and compare the spectroscopically resolved maps with corresponding ground-based data for low- and mid-J CO and [C I] emission. For the first time SOFIA/GREAT allow us to compare velocity-resolved [C II] emission maps with molecular tracers. We see a large part of the [C II] emission, both spatially and in velocity, that is completely non-associated with the other tracers of photon-dominated regions (PDR). Only particular narrow channel maps of the velocity-resolved [C II] spectra show a correlation between the different gas components, which is not seen at all in the integrated intensity maps. These show different morphology in all lines but give hardly any information on the origin of the emission. The [C II] 158 um emission extends for more than 2 pc into the M17 SW molecular cloud and its line profile covers a broader velocity range than the 12CO J=13-12 and [C I] emissions, which we interpret as several clumps and layers of ionized carbon gas within the telescope beam. The high-J CO emission emerges from a dense region between the ionized and neutral carbon emissions, indicating the presence of high-density clumps that allow the fast formation of hot CO in the irradiated complex structure of M17 SW. The [C II] observations in the southern PDR cannot be explained with stratified nor clumpy PDR models.Comment: 4 pages, 4 figures, letter accepted for the SOFIA/GREAT A&A 2012 special issu

The Link between Gas and Stars in the S254-S258 Star-forming Region

The paper aims to study relation between the distributions of the young stellar objects (YSOs) of different ages and the gas-dust constituents of the S254-S258 star formation complex. This is necessary to study the time evolution of the YSO distribution with respect to the gas and dust compounds that are responsible for the birth of the young stars. For this purpose, we use correlation analysis between different gas, dust, and YSO tracers. We compared the large-scale CO, HCO+, near-IR extinction, and far-IR Herschel maps with the density of YSOs of the different evolutionary classes. The direct correlation analysis between these maps was used together with the wavelet-based spatial correlation analysis. This analysis reveals a much tighter correlation of the gas-dust tracers with the distribution of class I YSOs than with that of class II YSOs. We argue that class I YSOs that were initially born in the central bright cluster S255-IR (both N and S parts) during their evolution to class II stage (∼2 Myr) had enough time to travel through the whole S254-S258 star formation region. Given that the region contains several isolated YSO clusters, the evolutionary link between these clusters and the bright central S255-IR (N and S) cluster can be considered. Despite the complexity of the YSO cluster formation in the non-uniform medium, the clusters of class II YSOs in the S254-258 star formation region can contain objects born in the different locations of the complex. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.The work of DAL and SAK on correlation analysis in Section 4 was supported by the Russian Science Foundation (RSF) grant 19-72-10012. The work of MSK in the Section 2.1 was supported by the Russian Foundation for Basic Research (RFBR) grant 20-02-00643. The work by DAL in Section 3 was supported by the Russian Ministry of Science and Higher Education, No. FEUZ-2020-0030. The work of AMS in Section 5 was supported by the Large Scientific Project of the Russian Ministry of Science and Higher Education 'Theoretical and experimental studies of the formation and evolution of extrasolar planetary systems and characteristics of exoplanets' (No. 075-15-2020-780, contract 780-10). VO was supported by the Collaborative Research Centre 956, sub-project C1, funded by the Deutsche Forschungsgemeinschaft (DFG), project ID 184018867. The authors acknowledges support from Onsala Space Observatory for the provisioning of its facilities/observational support. The Onsala Space Observatory national research infrastructure is funded through Swedish Research Council grant No. 2017-00648

The far-infrared spectroscopic surveyor (FIRSS)

We are standing at the crossroads of powerful new facilities emerging in the next decade on the ground and in space like ELT, SKA, JWST, and Athena. Turning the narrative of the star formation potential of galaxies into a quantitative theory will provide answers to many outstanding questions in astrophysics, from the formation of planets to the evolution of galaxies and the origin of heavy elements. To achieve this goal, there is an urgent need for a dedicated space-borne, far-infrared spectroscopic facility capable of delivering, for the first time, large scale, high spectral resolution (velocity resolved) multiwavelength studies of the chemistry and dynamics of the ISM of our own Milky Way and nearby galaxies. The Far Infrared Spectroscopic Surveyor (FIRSS) fulfills these requirements and by exploiting the legacy of recent photometric surveys it seizes the opportunity to shed light on the fundamental building processes of our Universe

HIFI observations of warm gas in DR21: Shock versus radiative heating

The molecular gas in the DR21 massive star formation region is known to be affected by the strong UV field from the central star cluster and by a fast outflow creating a bright shock. The relative contribution of both heating mechanisms is the matter of a long debate. By better sampling the excitation ladder of various tracers we provide a quantitative distinction between the different heating mechanisms. HIFI observations of mid-J transitions of CO and HCO+ isotopes allow us to bridge the gap in excitation energies between observations from the ground, characterizing the cooler gas, and existing ISO LWS spectra, constraining the properties of the hot gas. Comparing the detailed line profiles allows to identify the physical structure of the different components. In spite of the known shock-excitation of H2 and the clearly visible strong outflow, we find that the emission of all lines up to > 2 THz can be explained by purely radiative heating of the material. However, the new Herschel/HIFI observations reveal two types of excitation conditions. We find hot and dense clumps close to the central cluster, probably dynamically affected by the outflow, and a more widespread distribution of cooler, but nevertheless dense, molecular clumps.Comment: Accepted for publication by A&

Herschel observations in the ultracompact HII region Mon R2: Water in dense Photon-dominated regions (PDRs)

Mon R2, at a distance of 830 pc, is the only ultracompact HII region (UC HII) where the photon-dominated region (PDR) between the ionized gas and the molecular cloud can be resolved with Herschel. HIFI observations of the abundant compounds 13CO, C18O, o-H2-18O, HCO+, CS, CH, and NH have been used to derive the physical and chemical conditions in the PDR, in particular the water abundance. The 13CO, C18O, o-H2-18O, HCO+ and CS observations are well described assuming that the emission is coming from a dense (n=5E6 cm-3, N(H2)>1E22 cm-2) layer of molecular gas around the UC HII. Based on our o-H2-18O observations, we estimate an o-H2O abundance of ~2E-8. This is the average ortho-water abundance in the PDR. Additional H2-18O and/or water lines are required to derive the water abundance profile. A lower density envelope (n~1E5 cm-3, N(H2)=2-5E22 cm-2) is responsible for the absorption in the NH 1_1-0_2 line. The emission of the CH ground state triplet is coming from both regions with a complex and self-absorbed profile in the main component. The radiative transfer modeling shows that the 13CO and HCO+ line profiles are consistent with an expansion of the molecular gas with a velocity law, v_e =0.5 x (r/Rout)^{-1} km/s, although the expansion velocity is poorly constrained by the observations presented here.Comment: 4 pages, 5 figure

The CARMA-NRO Orion Survey:The filamentary structure as seen in C18O emission

We present an initial overview of the filamentary structure in the Orion A molecular cloud utilizing a high angular and velocity resolution C$^{18}$O(1-0) emission map that was recently produced as part of the CARMA-NRO Orion Survey. The main goal of this study is to build a credible method to study varying widths of filaments which has previously been linked to star formation in molecular clouds. Due to the diverse star forming activities taking place throughout its $\sim$20 pc length, together with its proximity of 388 pc, the Orion A molecular cloud provides an excellent laboratory for such an experiment to be carried out with high resolution and high sensitivity. Using the widely-known structure identification algorithm, DisPerSE, on a 3-dimensional (PPV) C$^{18}$O cube, we identified 625 relatively short (the longest being 1.74 pc) filaments over the entire cloud. We study the distribution of filament widths using FilChaP, a python package that we have developed and made publicly available. We find that the filaments identified in a 2 square degree PPV cube do not overlap spatially, except for the complex OMC-4 region that shows distinct velocity components along the line of sight. The filament widths vary between 0.02 and 0.3 pc depending on the amount of substructure that a filament possesses. The more substructure a filament has, the larger is its width. We also find that despite this variation, the filament width shows no anticorrelation with the central column density which is in agreement with previous Herschel observations.Comment: 19 pages, 20 figures. Accepted for publication in Astronomy and Astrophysic