arXiv subjects

Maryvonne Gerin

Publications and source records attributed to Maryvonne Gerin.

At least 91 records · Page 5Linked to original sources

arXiv2012

Herschel Search for O2 Toward the Orion Bar

We report the results of a search for molecular oxygen (O2) toward the Orion Bar, a prominent photodissociation region at the southern edge of the HII region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O2 N_J = 3_3 - 1_2 transition at 487 GHz and the 5_4 - 3_4 transition at 774 GHz using the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory. Neither line was detected, but the 3sigma upper limits established here translate to a total line-of-sight O2 column density < 1.5 10^16 cm^-2 for an emitting region whose temperature is between 30K and 250 K, or < 1 10^16 cm^-2 if the O2 emitting region is primarily at a temperature of ~< 100 K. Because the Orion Bar is oriented nearly edge-on relative to our line of sight, the observed column density is enhanced by a factor estimated to be between 4 and 20 relative to the face-on value. Our upper limits imply that the face-on O2 column density is less than 4 10^15 cm^-2, a value that is below, and possibly well below, model predictions for gas with a density of 10^4 - 10^5 cm^-3 exposed to a far ultraviolet flux 10^4 times the local value, conditions inferred from previous observations of the Orion Bar. The discrepancy might be resolved if: (1) the adsorption energy of O atoms to ice is greater than 800 K; (2) the total face-on Av of the Bar is less than required for O2 to reach peak abundance; (3) the O2 emission arises within dense clumps with a small beam filling factor; or, (4) the face-on depth into the Bar where O2 reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.

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arXiv2012

Herschel observations of interstellar chloronium

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six sources in the Galaxy. We detected interstellar chloronium absorption in foreground molecular clouds along the sight-lines to the bright submillimeter continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and para-H2-37Cl+ isotopologues were detected, through observations of their 1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio of 3, the observed optical depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the gas phase. We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of ~ 2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2 Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591. The chloronium abundances inferred in this study are typically at least a factor ~10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine. Several explanations for this discrepancy were investigated, but none has proven satisfactory, and thus the large observed abundances of chloronium remain puzzling.

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arXiv2011

Herschel Measurements of Molecular Oxygen in Orion

We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5\times1016 cm-2, and assuming that the source has an equal beam filling factor for all transitions (beam widths 44, 28, and 19"), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is 0.3 - 7.3\times10-6. The unusual velocity suggests an association with a ~ 5" diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~ 10 M\odot and the dust temperature is \geq 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T \geq 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature \geq 180 K. The inferred O2 column density \simeq 5\times1018 cm-2 can be produced in Peak A, having N(H2) \simeq 4\times1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2.

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arXiv2010

A Comprehensive Survey of Hydrogen Chloride in the Galaxy

We report new observations of the fundamental $J=1-0$ transition of HCl (at 625.918GHz) toward a sample of 25 galactic star-forming regions, molecular clouds, and evolved stars, carried out using the Caltech Submillimeter Observatory. Fourteen sources in the sample are also observed in the corresponding H\tscl\ $J=1-0$ transition (at 624.978GHz). We have obtained clear detections in all but four of the targets, often in emission. Absorptions against bright background continuum sources are also seen in nine cases, usually involving a delicate balance between emission and absorption features. From RADEX modeling, we derive gas densities and HCl column densities for sources with HCl emission. HCl is found in a wide range of environments, with gas densities ranging from $10^5$ to $10^7$~cm$^{-3}$. The HCl abundance relative to H$_2$ is in the range of $(3-30)\times10^{-10}$. Comparing with the chlorine abundance in the solar neighborhood, this corresponds to a chlorine depletion factor of up to $\sim$400, assuming that HCl accounts for one third of the total chlorine in the gas phase. The [\tfcl]/[\tscl] isotopic ratio is rather varied, from unity to $\sim$5, mostly lower than the terrestrial value of 3.1. Such variation is highly localized, and could be generated by the nucleosynthesis in supernovae, which predicts a \tscl\ deficiency in most models. The lower ratios seen in W3IRS4 and W3IRS5 likely confine the progenitors of the supernovae to stars with relatively large mass ($\ga$25M$_\sun$) and high metallicity (Z$\sim$0.02).

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arXiv2010

Molecular absorption lines toward star-forming regions : a comparative study of HCO+, HNC, HCN, and CN

Aims. The comparative study of several molecular species at the origin of the gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in unraveling the coupled chemical and dynamical evolution of the ISM. Methods. The lowest rotational lines of HCO+, HCN, HNC, and CN were observed at the IRAM-30m telescope in absorption against the λ3 mm and λ1.3 mm continuum emission of massive star-forming regions in the Galactic plane. The absorption lines probe the gas over kiloparsecs along these lines of sight. The excitation temperatures of HCO+ are inferred from the comparison of the absorptions in the two lowest transitions. The spectra of all molecular species on the same line of sight are decomposed into Gaussian velocity components. Most appear in all the spectra of a given line of sight. For each component, we derived the central opacity, the velocity dispersion, and computed the molecular column density. We compared our results to the predictions of UV-dominated chemical models of photodissociation regions (PDR models) and to those of non-equilibrium models in which the chemistry is driven by the dissipation of turbulent energy (TDR models). Results. The molecular column densities of all the velocity components span up to two orders of magnitude. Those of CN, HCN, and HNC are linearly correlated with each other with mean ratios N(HCN)/N(HNC) = 4.8 $\pm$ 1.3 and N(CN)/N(HNC) = 34 $\pm$ 12, and more loosely correlated with those of HCO+, N(HNC)/N(HCO+) = 0.5 $\pm$ 0.3, N(HCN)/N(HCO+) = 1.9 $\pm$ 0.9, and N(CN)/N(HCO+) = 18 $\pm$ 9. These ratios are similar to those inferred from observations of high Galactic latitude lines of sight, suggesting that the gas sampled by absorption lines in the Galactic plane has the same chemical properties as that in the Solar neighbourhood. The FWHM of the Gaussian velocity components span the range 0.3 to 3 km s-1 and those of the HCO+ lines are found to be 30% broader than those of CN-bearing molecules. The PDR models fail to reproduce simultaneously the observed abundances of the CN-bearing species and HCO+, even for high-density material (100 cm-3 < nH < 104 cm-3). The TDR models, in turn, are able to reproduce the observed abundances and abundance ratios of all the analysed molecules for the moderate gas densities (30 cm-3 < nH < 200 cm-3) and the turbulent energy observed in the diffuse interstellar medium. Conclusions. Intermittent turbulent dissipation appears to be a promising driver of the gas phase chemistry of the diffuse and translucent gas throughout the Galaxy. The details of the dissipation mechanisms still need to be investigated.

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arXiv2010

Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4

We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6$-$0.4 as well as in the molecular gas directly associated with that source. The OH+ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O+ lines remains lower than 1 in the same velocity range, and the H3O+ line shows only weak absorption. For LSR velocities between 7 and 50 kms$^{-1}$ we estimate total column densities of $N$(OH+) $> 2.5 \times 10^{14}$ cm$^{-2}$, $N$(H2O+) $\sim 6 \times 10^{13}$ cm$^{-2}$ and $N$(H3O+) $\sim 4.0 \times 10^{13}$ cm$^{-2}$. These detections confirm the role of O$^+$ and OH$^+$ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH+ by the H2O+ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.

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arXiv2009

The Horsehead nebula, a template source for interstellar physics and chemistry

We present a summary of our previous investigations of the physical and chemical structure of the Horsehead nebula, and discuss how these studies led to advances on the understanding of the impact of FUV radiation on the structure of dense interstellar clouds. Specific molecular tracers can be used to isolate different environments, that are more sensitive to changes in the FUV radiation or density than the classical tracers of molecular gas : the CO isotopologues or the dust (sub)millimeter continuum emission. They include the HCO or CCH radicals for the FUV illuminated interfaces, or the molecular ions H$^{13}$CO$^+$, DCO$^+$ and other deuterated species (DNC, DCN) for the cold dense core. We discuss future prospects in the context of Herschel and ALMA.

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arXiv2009

Detection of 15NH2D in dense cores: A new tool for measuring the 14N/15N ratio in the cold ISM

Ammonia is one of the best tracers of cold dense cores. It is also a minor constituent of interstellar ices and, as such, one of the important nitrogen reservoirs in the protosolar nebula, together with the gas phase nitrogen, in the form of N2 and N. An important diagnostic of the various nitrogen sources and reservoirs of nitrogen in the Solar System is the 14N/15N isotopic ratio. While good data exist for the Solar System, corresponding measurements in the interstellar medium are scarce and of low quality. Following the successful detection of the singly, doubly, and triply deuterated isotopologues of ammonia, we have searched for 15NH2D in dense cores, as a new tool for investigating the 14N/15N ratio in dense molecular gas. With the IRAM-30m telescope, we have obtained deep integrations of the ortho 15NH2D (1(1,1)-1(0,1)) line at 86.4 GHz, simultaneously with the corresponding ortho NH2D line at 85.9 GHz. o-15NH2D is detected in Barnard-1b, NGC1333-DCO+, and L1689N, while we obtained upper limits towards LDN1544 and NGC1333-IRAS4A, and a tentative detection towards L134N(S). The 14N/15N abundance ratio in NH2D ranges between 350 and 850, similar to the protosolar value of ~ 424, and likely higher than the terrestrial ratio of 270.

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arXiv2008

HCO mapping of the Horsehead : Tracing the illuminated dense molecular cloud surfaces

Far-UV photons strongly affect the physical and chemical state of molecular gas in the vicinity of young massive stars. We have obtained maps of the HCO and H13CO+ ground state lines towards the Horsehead edge at 5'' angular resolution with a combination of IRAM PdBI and 30m observations. These maps have been complemented with IRAM-30m observations of several excited transitions at two different positions. Bright formyl radical emission delineates the illuminated edge of the nebula, with a faint emission remaining towards the shielded molecular core. Viewed from the illuminated star, the HCO emission almost coincides with the PAH and CCH emission. HCO reaches a similar abundance than HCO+ in the PDR (~1-2 x10^{-9} with respect to H2). Pure gas-phase chemistry models fail to reproduce the observed HCO abundance by ~2 orders of magnitude, except if reactions of OI with carbon radicals abundant in the PDR (i.e., CH2) play a significant role in the HCO formation. Alternatively, HCO could be produced in the PDR by non-thermal processes such as photo-processing of ice mantles and subsequent photo-desorption of either HCO or H2CO, and further gas phase photodissociation. The measured HCO/H13CO+ abundance ratio is large towards the PDR (~50), and much lower toward the gas shielded from FUV radiation (<1). We propose that high HCO abundances (>10^{-10}) together with large HCO/H13CO+ abundance ratios (>1) are sensitive diagnostics of the presence of active photochemistry induced by FUV radiation.

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arXiv2008

Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201

We present the detection and characterization of a peculiar low-mass protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope surrounding the object has been mapped through its 1.2 mm dust continuum emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in its spectral energy distribution, but large submillimeter-to-bolometric luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source and/or a multiple stellar system. Targeted observations of several molecular lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line suggests that the protostar/disk system is still accreting material from its natal envelope. Indeed, optically thick line profiles from high density tracers such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of inward motions. We construct a preliminary physical model of the circumstellar envelope (including radial density and temperature gradients, velocity field and turbulence) that reproduces the observed line profiles and estimates the ionization fraction. The presence of both mechanical and (non-ionizing) FUV-radiative input makes the region an interesting case to study triggered star formation.

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arXiv2007

The Horsehead mane: Towards an observational benchmark for chemical models

After a discussion about the need for observational benchmark for chemical models, we explain 1) why the Horsehead western edge is well suited to serve as reference for models and 2) the steps we are taking toward this goal. We summarize abundances obtained to date and we show recent results.

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arXiv2007

Deuterium fractionation in the Horsehead edge

Deuterium fractionation is known to enhance the [DCO+]/[HCO+] abundance ratio over the D/H elemental ratio of about 1e-5 in the cold and dense gas typically found in pre-stellar cores. We report the first detection and mapping of very bright DCO+ J=3-2 and J=2-1 lines (3 and 4 K respectively) towards the Horsehead photodissociation region (PDR) observed with the IRAM-30m telescope. The DCO+ emission peaks close to the illuminated warm edge of the nebula (< 50" or about 0.1 pc away). Detailed nonlocal, non-LTE excitation and radiative transfer analyses have been used to determine the prevailing physical conditions and to estimate the DCO+ and H13CO+ abundances from their line intensities. A large [DCO+]/[HCO+] abundance ratio (>= 0.02) is inferred at the DCO+ emission peak, a condensation shielded from the illuminating far-UV radiation field where the gas must be cold (10-20 K) and dense (>= 2x10^5 cm-3). DCO+ is not detected in the warmer photodissociation front, implying a lower [DCO+]/[HCO+] ratio (< 1e-3). According to our gas phase chemical predictions, such a high deuterium fractionation of HCO+ can only be explained if the gas temperature is below 20 K, in good agreement with DCO+ excitation calculations.

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arXiv2006

Benchmarking PDR models against the Horsehead edge

To prepare for the unprecedented spatial and spectral resolution provided by ALMA and Herschel/HIFI, chemical models are being benchmarked against each other. It is obvious that chemical models also need well-constrained observations that can serve as references. Photo-dissociation regions (PDRs) are particularly well suited to serve as references because they make the link between diffuse and molecular clouds, thus enabling astronomers to probe a large variety of physical and chemical processes. At a distance of 400 pc (1" corresponding to 0.002 pc), the Horsehead PDR is very close to the prototypical kind of source (i.e. 1D, edge-on) needed to serve as a reference to models.

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arXiv2006

The distribution of ND2H in LDN1689N

Finding tracers of the innermost regions of prestellar cores is important for understanding their chemical and dynamical evolution before the onset of gravitational collapse. While classical molecular tracers, such as CO and CS, have been shown to be strongly depleted in cold, dense gas by condensation on grain mantles, it has been a subject of discussion to what extent nitrogen-bearing species, such as ammonia, are affected by this process. As deuterium fractionation is efficient in cold, dense gas, deuterated species are excellent tracers of prestellar cores. A comparison of the spatial distribution of neutral and ionized deuterated species with the dust continuum emission can thus provide important insights into the physical and chemical structure of such regions. We study the spatial distribution of the ground-state 335.5 GHz line of ND2H in LDN1689N, using APEX, and compare it with the distribution of the DCO+(3--2) line, as well as the 350 micron dust continuum emission observed with the SHARC~II bolometer camera at CSO. While the distribution of the ND2H emission in LDN1689N is generally similar to that of the 350 microns dust continuum emission, the peak of the ND2H emission is offset by ~10'' to the East from the dust continuum and DCO+ emission peak. ND2H and ND3 share the same spatial distribution. The observed offset between the ND2H and DCO+ emission is consistent with the hypothesis that the deuterium peak in LDN1689N is an interaction region between the outflow shock from IRAS16293--2422 and the dense ambient gas. We detect the J = 4 - 3 line of H13CO+ at 346.998 GHz in the image side band serendipitously. This line shows the same spatial distribution as DCO+(3--2), and peaks close to the 350 mic emission maximum which provides further support for the shock interaction scenario.

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arXiv2002

Interstellar triply deuterated ammonia

Research on interstellar ND3 is reviewed and updated from the discovery papers. Results from observations of a dozen sources at centimeter and submillimeter wavelengths are presented. The two data sets are consistent, but do not constrain the excitation conditions in the ND3-emitting gas. The column density ratios of NH3:NH2D, NH2D:ND2H and ND2H:ND3, observed in similar sized beams, are ~10, and present a challenge for both the gas-phase and the grain-surface chemistry scenarios of deuterium fractionation. The role of shocks and of CO depletion in deuterium chemistry is discussed.

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arXiv2002

Carbon Chemistry in interstellar clouds

We discuss new developments of interstellar chemistry, with particular emphasis on the carbon chemistry. We confirm that carbon chains and cycles are ubiquitous in the ISM and closely chemically related to ea ch other, and to carbon. Investigation of the carbon budget in shielded and UV illuminated gas shows that the inventory of interstellar molecules is not complete and more complex molecules with 4 or more carbon atoms must be present. Finally we discuss the consequences for the evolution of clouds and conclude that the ubiquitous presence of carbon chains and cycles is not a necessary consequence of a very young age for interstellar clouds.

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arXiv2002

Connection between PAHs and small hydrocarbons in the Horsehead Nebula Photo-Dissociation Region

We present recent observations of small hydrocarbons (C3H2, C2H, C4H) with high abundances in the Photo-Dissociation Region of the Horsehead nebula. Our results show for the first time observational indications that the small hydrocarbon distribution follows the Aromatic Infrared Bands (AIBs) emission traced by ISO-LW2 (5-8.5 microns), whereas it does not coincide with the CO and isotopes large-scale distribution. The derived abundances are significantly higher than in local clouds. This enhancement might be explained by an in situ formation assisted by the release of carbonaceous molecules from UV-irradiated aromatic particles.

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arXiv2000

Molecular Carbon Chains and Rings in TMC-1

We present mapping results in several rotational transitions of HC3N, C6H, both cyclic and linear C3H2 and C3H, towards the cyanopolyyne peak of the filamentary dense cloud TMC-1 using the IRAM 30m and MPIfR 100m telescopes. The spatial distribution of the cumulene carbon chain propadienylidene H2C3 (hereafter l-C3H2) is found to deviate significantly from the distributions of the cyclic isomer c-C3H2, HC3N, and C6H which in turn look very similar. The cyclic over linear abundance ratio of C3H2 increases by a factor of 3 across the filament, with a value of 28 at the cyanopolyyne peak. This abundance ratio is an order of magnitude larger than the range (3 to 5) we observed in the diffuse interstellar medium. The cyclic over linear abundance ratio of C3H also varies by ~2.5 in TMC-1, reaching a maximum value (13) close to the cyanopolyyne peak. These behaviors might be related to competitive processes between ion-neutral and neutral-neutral reactions for cyclic and linear species.

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