arXiv subjects

V. Ossenkopf

Publications and source records attributed to V. Ossenkopf.

At least 19 recordsLinked to original sources

arXiv2020

Description of turbulent dynamics in the interstellar medium: multifractal/microcanonical analysis I. Application to Herschel observations of the Musca filament

Observations of the interstellar medium (ISM) show a complex density and velocity structure which is in part attributed to turbulence. We here present a self-contained introduction to the multifractal formalism in a microcanonical version which allows us for the first time to compute precise turbulence characteristic parameters from a single observational map without the need for averages in a grand ensemble of statistical observables. We focus on studying the 250 mu-m Herschel map of the Musca filament and make use of MHD simulations. We find a clear signature of a multiplicative cascade in Musca with an inertial range from 0.05 to 0.65 pc. We show that the proposed microcanonical approach provides singularity spectra which are truly scale invariant as required to validate any method to analyze multifractality. The obtained, for the first time precise enough, singularity spectrum of Musca is clearly not as symmetric as usually observed in log-normal behavior. We claim that the ISM towards Musca features more a log-Poisson shape of its singularity spectrum. Since log-Poisson behavior is claimed to exist when dissipation is stronger for rare events in turbulent flows in contrast to more homogeneous (in volume and time) dissipation events, we suggest that this deviation from log-normality could trace enhanced dissipation in rare events at small scales, which may explain or is at least consistent with the dominant filamentary structure in Musca. Moreover we find that sub-regions in Musca tends to show different multifractal properties. It strongly suggests that different types of dynamics exist inside the Musca cloud. These differences between sub-regions appear only after eliminating noise features which have the tendency to "log-normalize" an observational map. Our study sets up fundamental tools which will be applied to other galactic clouds and simulations in forthcoming studies.

astro-ph.GA↗

arXiv2017

Data processing pipeline for Herschel HIFI

{Context}. The HIFI instrument on the Herschel Space Observatory performed over 9100 astronomical observations, almost 900 of which were calibration observations in the course of the nearly four-year Herschel mission. The data from each observation had to be converted from raw telemetry into calibrated products and were included in the Herschel Science Archive. {Aims}. The HIFI pipeline was designed to provide robust conversion from raw telemetry into calibrated data throughout all phases of the HIFI missions. Pre-launch laboratory testing was supported as were routine mission operations. {Methods}. A modular software design allowed components to be easily added, removed, amended and/or extended as the understanding of the HIFI data developed during and after mission operations. {Results}. The HIFI pipeline processed data from all HIFI observing modes within the Herschel automated processing environment as well as within an interactive environment. The same software can be used by the general astronomical community to reprocess any standard HIFI observation. The pipeline also recorded the consistency of processing results and provided automated quality reports. Many pipeline modules were in use since the HIFI pre-launch instrument level testing. {Conclusions}. Processing in steps facilitated data analysis to discover and address instrument artefacts and uncertainties. The availability of the same pipeline components from pre-launch throughout the mission made for well-understood, tested, and stable processing. A smooth transition from one phase to the next significantly enhanced processing reliability and robustness.

astro-ph.IM↗

arXiv2016

Understanding star formation in molecular clouds III. Probability distribution functions of molecular lines in Cygnus X

Column density (N) PDFs serve as a powerful tool to characterize the physical processes that influence the structure of molecular clouds. Star-forming clouds can best be characterized by lognormal PDFs for the lower N range and a power-law tail for higher N, commonly attributed to turbulence and self-gravity and/or pressure, respectively. We report here on PDFs obtained from observations of 12CO, 13CO, C18O, CS, and N2H+ in the Cygnus X North region and compare to a PDF derived from dust observations with the Herschel satellite. The PDF of 12CO is lognormal for Av~1-30, but is cut for higher Av due to optical depth effects. The PDFs of C18O and 13CO are mostly lognormal up for Av~1-15, followed by excess up to Av~40. Above that value, all CO PDFs drop, most likely due to depletion. The high density tracers CS and N2H+ exhibit only a power law distribution between Av~15 and 400, respectively. The PDF from dust is lognormal for Av~3-15 and has a power-law tail up to Av~500. Absolute values for the molecular line column densities are, however, rather uncertain due to abundance and excitation temperature variations. Taken the dust PDF face value, we 'calibrate' the molecular line PDF of CS to the one of the dust and determined an abundance [CS]/[H2] of 10^-9. The slopes of the power-law tails of the CS, N2H+, and dust PDFs are consistent with free-fall collapse of filaments and clumps. A quasi static configuration of filaments and clumps can possibly also account for the observed N-PDFs, as long as they have a sufficiently condensed density structure and external ram pressure by gas accretion is provided. The somehow flatter slopes of N2H+ and CS can reflect an abundance change and/or subthermal excitation at low column densities.

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arXiv2015

Velocity resolved [CII] spectroscopy of the center and the BCLMP302 region of M33 (HerM33es)

We aim to understand the contribution of the ionized, atomic and molecular phases of the ISM to the [CII] emission from clouds near the dynamical center and the BCLMP302 HII region in the north of the nearby galaxy M33 at a spatial resolution of 50pc. We combine high resolution [CII] spectra taken with the HIFI spectrometer onboard the Herschel satellite with [CII] Herschel-PACS maps and ground-based observations of CO(2-1) and HI. All data are at a common spatial resolution of 50pc. Typically, the [CII] lines have widths intermediate between the narrower CO(2-1) and broader HI line profiles. We decomposed the [CII] spectra in terms of contribution from molecular and atomic gas detected in CO(2-1) and HI, respectively. We find that the relative contribution of molecular and atomic gas traced by CO(2-1) and HI varies depends mostly on the local physical conditions and geometry. We estimate that 11-60% and 5-34% of the [CII] intensities in the center and in BCLMP302, respectively, arise at velocities showing no CO(2-1) or HI emission and could arise in CO-dark molecular gas. The deduced strong variation in the [CII] emission not associated with CO and HI cannot be explained in terms of differences in A_v, far-ultraviolet radiation field, and metallicity between the two studied regions. Hence the relative amounts of diffuse (CO-dark) and dense molecular gas possibly vary on spatial scales smaller than 50pc. Based on the emission measure observed at radio wavelengths we estimate the contribution of ionized gas at a few positions to lie between 10-25%. The correlations between the intensities of tracers corresponding to the same velocity range as [CII], differ from the correlation derived from PACS data. The results in this paper emphasize the need for velocity-resolved observations to discern the contribution of different components of the ISM to [CII] emission. (abridged)

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arXiv2015

Velocity-resolved [CII] emission and [CII]/FIR Mapping along Orion with Herschel

We present the first 7.5'x11.5' velocity-resolved map of the [CII]158um line toward the Orion molecular cloud-1 (OMC-1) taken with the Herschel/HIFI instrument. In combination with far-infrared (FIR) photometric images and velocity-resolved maps of the H41alpha hydrogen recombination and CO J=2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [CII] luminosity (~85%) is from the extended, FUV-illuminated face of the cloud G_0>500, n_H>5x10^3 cm^-3) and from dense PDRs (G_0~10^4, n_H~10^5 cm^-3) at the interface between OMC-1 and the HII region surrounding the Trapezium cluster. Around 15% of the [CII] emission arises from a different gas component without CO counterpart. The [CII] excitation, PDR gas turbulence, line opacity (from [13CII]) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the [CII]/FIR and FIR/M_Gas ratios and show that [CII]/FIR decreases from the extended cloud component (10^-2-10^-3) to the more opaque star-forming cores (10^-3-10^-4). The lowest values are reminiscent of the "[CII] deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing [CII]/FIR ratio correlates better with the column density of dust through the molecular cloud than with FIR/M_Gas. We conclude that the [CII] emitting column relative to the total dust column along each line of sight is responsible for the observed [CII]/FIR variations through the cloud.

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arXiv2015

Wavelet-based cross-correlation analysis of structure scaling in turbulent clouds

We propose a statistical tool to compare the scaling behaviour of turbulence in pairs of molecular cloud maps. Using artificial maps with well defined spatial properties, we calibrate the method and test its limitations to ultimately apply it to a set of observed maps. We develop the wavelet-based weighted cross-correlation (WWCC) method to study the relative contribution of structures of different sizes and their degree of correlation in two maps as a function of spatial scale, and the mutual displacement of structures in the molecular cloud maps. We test the WWCC for circular structures having a single prominent scale and fractal structures showing a self-similar behavior without prominent scales. Observational noise and a finite map size limit the scales where the cross-correlation coefficients and displacement vectors can be reliably measured. For fractal maps containing many structures on all scales, the limitation from the observational noise is negligible for signal-to-noise ratios >5. (abbrev). Application of the WWCC to the observed line maps of the giant molecular cloud G333 allows to add specific scale information to the results obtained earlier using the principle component analysis. It confirms the chemical and excitation similarity of $^{13}$CO and C$^{18}$O on all scales, but shows a deviation of HCN at scales of up to 7' (~7 pc). This can be interpreted as a chemical transition scale. The largest structures also show a systematic offset along the filament, probably due to a large-scale density gradient. The WWCC can compare correlated structures in different maps of molecular clouds identifying scales that represent structural changes such as chemical and phase transitions and prominent or enhanced dimensions.

astro-ph.SR↗

arXiv2015

Disentangling the excitation conditions of the dense gas in M17 SW

We probe the chemical and energetic conditions in dense gas created by radiative feedback through observations of multiple CO, HCN and HCO$^+$ transitions toward the dense core of M17 SW. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain maps of the $J=16-15$, $J=12-11$, and $J=11-10$ transitions of $^{12}$CO. We compare these maps with corresponding APEX and IRAM 30m telescope data for low- and mid-$J$ CO, HCN and HCO$^+$ emission lines, including maps of the HCN $J=8-7$ and HCO$^+$ $J=9-8$ transitions. The excitation conditions of $^{12}$CO, HCO$^+$ and HCN are estimated with a two-phase non-LTE radiative transfer model of the line spectral energy distributions (LSEDs) at four selected positions. The energy balance at these positions is also studied. We obtained extensive LSEDs for the CO, HCN and HCO$^+$ molecules toward M17 SW. The LSED shape, particularly the high-$J$ tail of the CO lines observed with SOFIA/GREAT, is distinctive for the underlying excitation conditions. The critical magnetic field criterion implies that the cold cloudlets at two positions are partially controlled by processes that create and dissipate internal motions. Supersonic but sub-Alfvénic velocities in the cold component at most selected positions indicates that internal motions are likely MHD waves. Magnetic pressure dominates thermal pressure in both gas components at all selected positions, assuming random orientation of the magnetic field. The magnetic pressure of a constant magnetic field throughout all the gas phases can support the total internal pressure of the cold components, but it cannot support the internal pressure of the warm components. If the magnetic field scales as $B \propto n^{2/3}$, then the evolution of the cold cloudlets at two selected positions, and the warm cloudlets at all selected positions, will be determined by ambipolar diffusion.

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arXiv2015

Detection of two power-law tails in the probability distribution functions of massive GMCs

We report the novel detection of complex high-column density tails in the probability distribution functions (PDFs) for three high-mass star-forming regions (CepOB3, MonR2, NGC6334), obtained from dust emission observed with Herschel. The low column density range can be fit with a lognormal distribution. A first power-law tail starts above an extinction (Av) of ~6-14. It has a slope of alpha=1.3-2 for the rho~r^-alpha profile for an equivalent density distribution (spherical or cylindrical geometry), and is thus consistent with free-fall gravitational collapse. Above Av~40, 60, and 140, we detect an excess that can be fitted by a flatter power law tail with alpha>2. It correlates with the central regions of the cloud (ridges/hubs) of size ~1 pc and densities above 10^4 cm^-3. This excess may be caused by physical processes that slow down collapse and reduce the flow of mass towards higher densities. Possible are: 1. rotation, which introduces an angular momentum barrier, 2. increasing optical depth and weaker cooling, 3. magnetic fields, 4. geometrical effects, and 5. protostellar feedback. The excess/second power-law tail is closely linked to high-mass star-formation though it does not imply a universal column density threshold for the formation of (high-mass) stars.

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arXiv2015

C$_2$H observations toward the Orion Bar

C$_2$H is one of the first radicals to be detected in the interstellar medium. Its higher rotational transitions have recently become available with the Herschel Space Observatory. We aim to constrain the physical parameters of the C$_2$H emitting gas toward the Orion Bar. We analyse the C$_2$H line intensities measured toward the Orion Bar CO$^+$ Peak and Herschel/HIFI maps of C$_2$H, CH, and HCO$^+$, and a NANTEN map of [CI]. We interpret the observed C$_2$H emission using radiative transfer and PDR models. Five rotational transitions of C$_2$H have been detected in the HIFI frequency range toward the CO$^+$ peak. A single component rotational diagram gives a rotation temperature of ~64 K and a beam-averaged C$_2$H column density of 4$\times$10$^{13}$ cm$^{-2}$. The measured transitions cannot be explained by any single parameter model. According to a non-LTE model, most of the C$_2$H column density produces the lower-$N$ C$_2$H transitions and traces a warm ($T_{\rm{kin}}$ ~ 100-150 K) and dense ($n$(H$_2$)~10$^5$-10$^6$ cm$^{-3}$) gas. A small fraction of the C$_2$H column density is required to reproduce the intensity of the highest-$N$ transitions ($N$=9-8 and N=10-9) originating from a high density ($n$(H$_2$)~5$\times$10$^6$ cm$^{-3}$) hot ($T_{\rm{kin}}$ ~ 400 K) gas. The total beam-averaged C$_2$H column density in the model is 10$^{14}$ cm$^{-2}$. Both the non-LTE radiative transfer model and a simple PDR model representing the Orion Bar with a plane-parallel slab of gas and dust suggest, that C$_2$H cannot be described by a single pressure component, unlike the reactive ion CH$^+$, which was previously analysed toward the Orion Bar CO$^+$ peak. The physical parameters traced by the higher rotational transitions ($N$=6-5,...,10-9) of C$_2$H may be consistent with the edges of dense clumps exposed to UV radiation near the ionization front of the Orion Bar.

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arXiv2015

Temperatures of dust and gas in S~140

In dense parts of interstellar clouds (> 10^5 cm^-3), dust & gas are expected to be in thermal equilibrium, being coupled via collisions. However, previous studies have shown that the temperatures of the dust & gas may remain decoupled even at higher densities. We study in detail the temperatures of dust & gas in the photon-dominated region S 140, especially around the deeply embedded infrared sources IRS 1-3 and at the ionization front. We derive the dust temperature and column density by combining Herschel PACS continuum observations with SOFIA observations at 37 $μ$m and SCUBA at 450 $μ$m. We model these observations using greybody fits and the DUSTY radiative transfer code. For the gas part we use RADEX to model the CO 1-0, CO 2-1, 13CO 1-0 and C18O 1-0 emission lines mapped with the IRAM-30m over a 4' field. Around IRS 1-3, we use HIFI observations of single-points and cuts in CO 9-8, 13CO 10-9 and C18O 9-8 to constrain the amount of warm gas, using the best fitting dust model derived with DUSTY as input to the non-local radiative transfer model RATRAN. We find that the gas temperature around the infrared sources varies between 35 and 55K and that the gas is systematically warmer than the dust by ~5-15K despite the high gas density. In addition we observe an increase of the gas temperature from 30-35K in the surrounding up to 40-45K towards the ionization front, most likely due to the UV radiation from the external star. Furthermore, detailed models of the temperature structure close to IRS 1 show that the gas is warmer and/or denser than what we model. Finally, modelling of the dust emission from the sub-mm peak SMM 1 constrains its luminosity to a few ~10^2 Lo. We conclude that the gas heating in the S 140 region is very efficient even at high densities, most likely due to the deep UV penetration from the embedded sources in a clumpy medium and/or oblique shocks.

astro-ph.SR↗

arXiv2015

Understanding star formation in molecular clouds II. Signatures of gravitational collapse of IRDCs

We analyse column density and temperature maps derived from Herschel dust continuum observations of a sample of massive infrared dark clouds (G11.11-0.12, G18.82-0.28, G28.37+0.07, G28.53-0.25). We disentangle the velocity structure of the clouds using 13CO 1-0 and 12CO 3-2 data, showing that these IRDCs are the densest regions in massive giant molecular clouds and not isolated features. The probability distribution function (PDF) of column densities for all clouds have a power-law distribution over all (high) column densities, regardless of the evolutionary stage of the cloud: G11.11-0.12, G18.82-0.28, and G28.37+0.07 contain (proto)-stars, while G28.53-0.25 shows no signs of star formation. This is in contrast to the purely log-normal PDFs reported for near/mid-IR extinction maps. We only find a log-normal distribution for lower column densities, if we perform PDFs of the column density maps of the whole GMC in which the IRDCs are embedded. By comparing the PDF slope and the radial column density profile, we attribute the power law to the effect of large-scale gravitational collapse and to local free-fall collapse of pre- and protostellar cores. Independent from the PDF analysis, we find infall signatures in the spectral profiles of 12CO for G28.37+0.07 and G11.11-0.12, supporting the scenario of gravitational collapse. IRDCs are the densest regions within GMCs, which may be the progenitors of massive stars or clusters. At least some of the IRDCs are probably the same features as ridges (high column density regions with N>1e23 cm-2 over small areas), which were defined for nearby IR-bright GMCs. Because IRDCs are only confined to the densest (gravity dominated) cloud regions, the PDF constructed from this kind of a clipped image does not represent the (turbulence dominated) low column density regime of the cloud.

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arXiv2015

Understanding star formation in molecular clouds I. Effects of line-of-sight contamination on the column density structure

Column-density maps of molecular clouds are one of the most important observables in the context of molecular cloud- and star-formation (SF) studies. With the Herschel satellite it is now possible to determine the column density from dust emission. We use observations and simulations to demonstrate how LOS contamination affects the column density probability distribution function (PDF). We apply a first-order approximation (removing a constant level) to the molecular clouds of Auriga, Maddalena, Carina and NGC3603. In perfect agreement with the simulations, we find that the PDFs become broader, the peak shifts to lower column densities, and the power-law tail of the PDF flattens after correction. All PDFs have a lognormal part for low column densities with a peak at Av~2, a deviation point (DP) from the lognormal at Av(DP)~4-5, and a power-law tail for higher column densities. Assuming a density distribution rho~r^-alpha, the slopes of the power-law tails correspond to alpha(PDF)=1.8, 1.75, and 2.5 for Auriga, Carina, and NGC3603 (alpha~1.5-2 is consistent gravitational collapse). We find that low-mass and high-mass SF clouds display differences in the overall column density structure. Massive clouds assemble more gas in smaller cloud volumes than low-mass SF ones. However, for both cloud types, the transition of the PDF from lognormal shape into power-law tail is found at the same column density (at Av~4-5 mag). Low-mass and high-mass SF clouds then have the same low column density distribution, most likely dominated by supersonic turbulence. At higher column densities, collapse and external pressure can form the power-law tail. The relative importance of the two processes can vary between clouds and thus lead to the observed differences in PDF and column density structure.

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arXiv2015

Detection of a large fraction of atomic gas not associated with star-forming material in M17 SW

We probe the column densities and masses traced by the ionized and neutral atomic carbon with spectrally resolved maps, and compare them to the diffuse and dense molecular gas traced by [C I] and low-$J$ CO lines toward the star-forming region M17SW. We mapped a 4.1pc x 4.7pc region in the [C I] 609 m$μ$ line using the APEX telescope, as well as the CO isotopologues with the IRAM 30m telescope. We analyze the data based on velocity channel maps that are 1 km/s wide. We correlate their spatial distribution with that of the [C II] map obtained with SOFIA/GREAT. Optically thin approximations were used to estimate the column densities of [C I] and [C II] in each velocity channel. The spatial distribution of the [C I] and all CO isotopologues emission was found to be associated with that of [C II] in about 20%-80% of the mapped region, with the high correlation found in the central (15-23 km/s ) velocity channels. The excitation temperature of [C I] ranges between 40 K and 100 K in the inner molecular region of M17 SW. Column densities in 1 km/s channels between ~10$^{15}$ and ~10$^{17}$ cm$^{-2}$ were found for [C I]. Just ~20% of the velocity range (~40 km/s) that the [C II] line spans is associated with the star-forming material traced by [C I] and CO. The total gas mass estimated from the [C II] emission gives a lower limit of ~4.4x10$^3$ $M_{\odot}$. At least 64% of this mass is not associated with the star-forming material in M17SW. We also found that about 36%, 17%, and 47% of the [C II] emission is associated with the HII, HI, and H_2 regimes, respectively. Comparisons with the H41$α$ line shows an ionization region mixed with the neutral and part of the molecular gas, in agreement with the clumped structure and dynamical processes at play in M17SW. These results are also relevant to extra-galactic studies in which [C II] is often used as a tracer of star-forming material.

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arXiv2014

Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen fraction, f(H2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f(H2) in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042+-0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer zeta_H throughout our sample, and find a log-normal distribution with mean log(zeta_H)=-15.75, (zeta_H=1.78x10^-16 s^-1), and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H3+ observations. Ionization rates in the Galactic center tend to be 10--100 times larger than found in the Galactic disk, also in accord with prior studies.

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arXiv2014

Carbon in different phases ([CII], [CI], and CO) in infrared dark clouds: Cloud formation signatures and carbon gas fractions

Context: How do molecular clouds form out of the atomic phase? And what are the relative fractions of carbon in the ionized, atomic and molecular phase? These are questions at the heart of cloud and star formation. Methods: Using multiple observatories from Herschel and SOFIA to APEX and the IRAM 30m telescope, we mapped the ionized, atomic and molecular carbon ([CII]@1900GHz, [CI]@492GHz and C18O(2-1)@220GHz) at high spatial resolution (12"-25") in four young massive infrared dark clouds (IRDCs). Results: The three carbon phases were successfully mapped in all four regions, only in one source the [CII] line remained a non-detection. Both the molecular and atomic phases trace the dense structures well, with [CI] also tracing material at lower column densities. [CII] exhibits diverse morphologies in our sample, from compact to diffuse structures probing the cloud environment. In at least two out of the four regions, we find kinematic signatures strongly indicating that the dense gas filaments have formed out of a dynamically active and turbulent atomic/molecular cloud, potentially from converging gas flows. The atomic-to-molecular carbon gas mass ratios are low between 7% and 12% with the lowest values found toward the most quiescent region. In the three regions where [CII] is detected, its mass is always higher by a factor of a few than that of the atomic carbon. The ionized carbon emission depends as well on the radiation field, however, we also find strong [CII] emission in a region without significant external sources, indicating that other processes, e.g., energetic gas flows can contribute to the [CII] excitation as well.

astro-ph.SR↗

arXiv2014

Deuteration around the ultracompact HII region Mon R2

The massive star-forming region Mon R2 hosts the closest ultra-compact HII region that can be spatially resolved with current single-dish telescopes. We used the IRAM-30m telescope to carry out an unbiased spectral survey toward two important positions (namely IF and MP2), in order to studying the chemistry of deuterated molecules toward Mon R2. We found a rich chemistry of deuterated species at both positions, with detections of C2D, DCN, DNC, DCO+, D2CO, HDCO, NH2D, and N2D+ and their corresponding hydrogenated species and isotopologs. Our high spectral resolution observations allowed us to resolve three velocity components: the component at 10 km/s is detected at both positions and seems associated with the layer most exposed to the UV radiation from IRS 1; the component at 12 km/s is found toward the IF position and seems related to the molecular gas; finally, a component at 8.5 km/s is only detected toward the MP2 position, most likely related to a low-UV irradiated PDR. We derived the column density of all the species, and determined the deuterium fractions (Dfrac). The values of Dfrac are around 0.01 for all the observed species, except for HCO+ and N2H+ which have values 10 times lower. The values found in Mon R2 are well explained with pseudo-time-dependent gas-phase model in which deuteration occurs mainly via ion-molecule reactions with H2D+, CH2D+ and C2HD+. Finally, the [H13CN]/[HN13C] ratio is very high (~11) for the 10 km/s component, which also agree with our model predictions for an age of ~0.01-0.1 Myr. The deuterium chemistry is a good tool for studying star-forming regions. The low-mass star-forming regions seem well characterized with Dfrac(N2H+) or Dfrac(HCO+), but it is required a complete chemical modeling to date massive star-forming regions, because the higher gas temperature together with the rapid evolution of massive protostars.

astro-ph.GA↗

arXiv2013

Kinematics of the ionized-to-neutral interfaces in Monoceros R2

Context. Monoceros R2 (Mon R2), at a distance of 830 pc, is the only ultra-compact H ii region (UC H ii) where its associated photon-dominated region (PDR) can be resolved with the Herschel Space Observatory. Aims. Our aim is to investigate observationally the kinematical patterns in the interface regions (i.e., the transition from atomic to molecular gas) associated with Mon R2. Methods. We used the HIFI instrument onboard Herschel to observe the line profiles of the reactive ions CH+, OH+ and H2O+ toward different positions in Mon R2. We derive the column density of these molecules and compare them with gas-phase chemistry models. Results. The reactive ion CH+ is detected both in emission (at central and red-shifted velocities) and in absorption (at blue-shifted velocities). OH+ is detected in absorption at both blue- and red-shifted velocities, with similar column densities. H2O+ is not detected at any of the positions, down to a rms of 40 mK toward the molecular peak. At this position, we find that the OH+ absorption originates in a mainly atomic medium, and therefore is associated with the most exposed layers of the PDR. These results are consistent with the predictions from photo-chemical models. The line profiles are consistent with the atomic gas being entrained in the ionized gas flow along the walls of the cavity of the H ii region. Based on this evidence, we are able to propose a new geometrical model for this region. Conclusions. The kinematical patterns of the OH+ and CH+ absorption indicate the existence of a layer of mainly atomic gas for which we have derived, for the first time, some physical parameters and its dynamics.

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arXiv2013

Large scale IRAM 30m CO-observations in the giant molecular cloud complex W43

We aim to give a full description of the distribution and location of dense molecular clouds in the giant molecular cloud complex W43. It has previously been identified as one of the most massive star-forming regions in our Galaxy. To trace the moderately dense molecular clouds in the W43 region, we initiated an IRAM 30m large program, named W43-HERO, covering a large dynamic range of scales (from 0.3 to 140 pc). We obtained on-the-fly-maps in 13CO (2-1) and C18O (2-1) with a high spectral resolution of 0.1 km/s and a spatial resolution of 12". These maps cover an area of ~1.5 square degrees and include the two main clouds of W43, as well as the lower density gas surrounding them. A comparison with Galactic models and previous distance calculations confirms the location of W43 near the tangential point of the Scutum arm at a distance from the Sun of approximately 6 kpc. The resulting intensity cubes of the observed region are separated into sub-cubes, centered on single clouds which are then analyzed in detail. The optical depth, excitation temperature, and H2 column density maps are derived out of the 13CO and C18O data. These results are then compared with those derived from Herschel dust maps. The mass of a typical cloud is several 10^4 solar masses while the total mass in the dense molecular gas (>100 cm^-3) in W43 is found to be about 1.9e6 solar masses. Probability distribution functions obtained from column density maps derived from molecular line data and Herschel imaging show a log-normal distribution for low column densities and a power-law tail for high densities. A flatter slope for the molecular line data PDF may imply that those selectively show the gravitationally collapsing gas.

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