arXiv subjects

P. Hily-Blant

Publications and source records attributed to P. Hily-Blant.

52 records · Page 3Linked to original sources

arXiv2010

Nitrogen hydrides in interstellar gas: Herschel/HIFI observations towards G10.6-0.4 (W31C)

The HIFI instrument on board the Herschel Space Observatory has been used to observe interstellar nitrogen hydrides along the sight-line towards G10.6-0.4 in order to improve our understanding of the interstellar chemistry of nitrogen. We report observations of absorption in NH N=1-0, J=2-1 and ortho-NH2 1_1,1-0_0,0. We also observed ortho-NH3 1_0-0_0, and 2_0-1_0, para-NH3 2_1-1_1, and searched unsuccessfully for NH+. All detections show emission and absorption associated directly with the hot-core source itself as well as absorption by foreground material over a wide range of velocities. All spectra show similar, non-saturated, absorption features, which we attribute to diffuse molecular gas. Total column densities over the velocity range 11-54 km/s are estimated. The similar profiles suggest fairly uniform abundances relative to hydrogen, approximately 6*10^-9, 3*10^-9, and 3*10^-9 for NH, NH2, and NH3, respectively. These abundances are discussed with reference to models of gas-phase and surface chemistry.

astro-ph.GA↗

arXiv2010

Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: a ubiquitous tracer of molecular gas

We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschel's HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1 - 0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure - as opposed to obtain a lower limit on - the column density of HF for the first time. As in previous observations of HF toward the source G10.6-0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH)-N(H2) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H2 is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H2 column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of ~ 24kms-1, that had not been identified in molecular absorption line studies prior to the launch of Herschel.

astro-ph.GA↗

arXiv2010

Herschel/HIFI discovery of interstellar chloronium (H$_2$Cl$^+$)

We report the first detection of chloronium, H$_2$Cl$^+$, in the interstellar medium, using the HIFI instrument aboard the \emph{Herschel} Space Observatory. The $2_{12}-1_{01}$ lines of ortho-H$_2^{35}$Cl$^+$ and ortho-H$_2^{37}$Cl$^+$ are detected in absorption towards NGC~6334I, and the $1_{11}-0_{00}$ transition of para-H$_2^{35}$Cl$^+$ is detected in absorption towards NGC~6334I and Sgr~B2(S). The H$_2$Cl$^+$ column densities are compared to those of the chemically-related species HCl. The derived HCl/H$_2$Cl$^+$ column density ratios, $\sim$1--10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H$_2$Cl$^+$ column densities, in excess of $10^{13}$~cm$^{-2}$, are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.

astro-ph.GA↗

arXiv2010

Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H$_2^{16}$O and H$_2^{18}$O in absorption towards Sagittarius~B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of $2.35 \pm 0.35$, corresponding to a spin temperature of $\sim$27~K, towards Sagittarius~B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust.

astro-ph.GA↗

arXiv2010

Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1 - 0 transition of OH+ and the 1115 GHz 1(11) - 0(00) transition of ortho-H2O+. The resultant spectra show absorption by ortho-H2O+, and strong absorption by OH+, in foreground material at velocities in the range 0 to 70 km/s with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from ~ 3 to ~ 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the 2 - 8% range. This conclusion is confirmed by the distribution of OH+ and H2O+ in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH+/H abundance ratio of a few E-8 suggests a cosmic ray ionization rate for atomic hydrogen of (0.6 - 2.4) E-16 s-1, in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H3+ and other species.

astro-ph.GA↗

arXiv2010

Excitation and Abundance of C3 in star forming cores:Herschel/HIFI observations of the sight-lines to W31C and W49N

We present spectrally resolved observations of triatomic carbon (C3) in several ro-vibrational transitions between the vibrational ground state and the low-energy nu2 bending mode at frequencies between 1654-1897 GHz along the sight-lines to the submillimeter continuum sources W31C and W49N, using Herschel's HIFI instrument. We detect C3 in absorption arising from the warm envelope surrounding the hot core, as indicated by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C3 absorption due to diffuse foreground clouds. From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (T_rot) of ~50--70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on the derived T_rot, we get column densities N(C3) ~7-9x10^{14} cm^{-2} and abundance x(C3)~10^{-8} with respect to H2. For W31C, using a radiative transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find that a model with x(C3)=10^{-8}, T_kin=30-50 K, N(C3)=1.5 10^{15} cm^{-2} fits the observations reasonably well and provides parameters in very good agreement with the simple excitation analysis.

astro-ph.GA↗

arXiv2010

CHESS, Chemical Herschel surveys of star forming regions:Peering into the protostellar shock L1157-B1

The outflow driven by the low-mass class 0 protostar L1157 is the prototype of the so-called chemically active outflows. The bright bowshock B1 in the southern outflow lobe is a privileged testbed of magneto-hydrodynamical (MHD) shock models, for which dynamical and chemical processes are strongly interdependent. We present the first results of the unbiased spectral survey of the L1157-B1 bowshock, obtained in the framework of the key program "Chemical Herschel Surveys of Star Forming Regions" (CHESS). The main aim is to trace the warm and chemically enriched gas and to infer the excitation conditions in the shock region. The CO 5-4 and H2O lines have been detected at high-spectral resolution in the unbiased spectral survey of the HIFI-Band 1b spectral window (555-636 GHz), presented by Codella et al. in this volume. Complementary ground-based observations in the submm window help establish the origin of the emission detected in the main-beam of HIFI, and the physical conditions in the shock.}{Both lines exhibit broad wings, which extend to velocities much higher than reported up to now. We find that the molecular emission arises from two regions with distinct physical conditions: an extended, warm (100K), dense (3e5 cm-3) component at low-velocity, which dominates the water line flux in Band~1; a secondary component in a small region of B1 (a few arcsec) associated with high-velocity, hot (> 400 K) gas of moderate density ((1.0-3.0)e4 cm-3), which appears to dominate the flux of the water line at 179mu observed with PACS. The water abundance is enhanced by two orders of magnitude between the low- and the high-velocity component, from 8e-7 up to 8e-5. The properties of the high-velocity component agree well with the predictions of steady-state C-shock models.

astro-ph.GA↗

arXiv2010

Strong absorption by interstellar hydrogen fluoride: Herschel/HIFI observations of the sight-line to G10.6-0.4 (W31C)

We report the detection of strong absorption by interstellar hydrogen fluoride along the sight-line to the submillimeter continuum source G10.6-0.4 (W31C). We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1232.4763 GHz J=1-0 HF transition in the upper sideband of the Band 5a receiver. The resultant spectrum shows weak HF emission from G10.6-0.4 at LSR velocities in the range -10 to -3 km/s, accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km/s. The spectrum is similar to that of the 1113.3430 GHz 1(11)-0(00) transition of para-water, although at some frequencies the HF (hydrogen fluoride) optical depth clearly exceeds that of para-H2O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6E+14 cm-2 on the HF column density along the sight-line to G10.6-0.4. Our lower limit on the HF abundance, 6E-9 relative to hydrogen nuclei, implies that hydrogen fluoride accounts for between ~ 30 and 100% of the fluorine nuclei in the gas phase along this sight-line. This observation corroborates theoretical predictions that - because the unique thermochemistry of fluorine permits the exothermic reaction of F atoms with molecular hydrogen - HF will be the dominant reservoir of interstellar fluorine under a wide range of conditions.

astro-ph.GA↗

arXiv2010

Detection of interstellar oxidaniumyl: abundant H2O+ towards the star-forming regions DR21, Sgr B2, and NGC6334

We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret its astrophysical origin. The characteristic hyperfine pattern of the H2O+ ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H2O+ absorption against the dust continuum background and allows us to derive the velocity profile of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the frequency of the transition and the conditions for its formation. In DR21, the velocity distribution of H2O+ matches that of the [CII] line at 158μ\m and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the HII-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H2O+ column density of 7.2e12 cm^-2 in NGC 6334, 2.3e13 cm^-2 in DR21, and 1.1e15 cm^-2 in Sgr B2.

astro-ph.GA↗

arXiv2009

Intermittency of interstellar turbulence: extreme velocity-shears and CO emission on milliparsec scale

The condensation of diffuse gas into molecular clouds occurs at a rate driven largely by turbulent dissipation. This process still has to be caught in action and characterized. A mosaic of 13 fields was observed in the CO(1-0) line with the IRAM-PdB interferometer in the translucent environment of two low-mass dense cores. The large size of the mosaic compared to the resolution (4 arcsec) is unprecedented in the study of the small-scale structure of diffuse molecular gas. Eight weak and elongated structures of thicknesses as small as 3 mpc (600 AU) and lengths up to 70mpc are found. These are not filaments because once merged with short-spacing data, they appear as the sharp edges of larger-scale structures. Six out of eight form quasi-parallel pairs at different velocities and different position angles. This cannot be the result of chance alignment. The velocity-shears estimated for the three pairs include the highest ever measured far from star forming regions (780 km/s/pc). Because the large scale structures have sharp edges, with little or no overlap, they have to be thin CO-layers. Their edges mark a sharp transition between a CO-rich component and a gas undetected in the CO line because of its low CO abundance, presumably the cold neutral medium. We propose that these sharp edges are the first directly-detected manifestations of the intermittency of interstellar turbulence. The large velocity-shears reveal an intense straining field, responsible for a local dissipation rate several orders of magnitude above average, possibly at the origin of the thin CO-layers.

astro-ph.GA↗

arXiv2009

The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds

The ionization fraction plays a key role in the chemistry and dynamics of molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the Photodissociation Region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+ J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and new HOC+ and CO+ observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, 13C fractionation reactions and HCO+/HOC+ isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated. The detection of HOC+ reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few times ~10(-9) in the core. PAH^- anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH] >10(-8)).

astro-ph.GA↗

arXiv2009

CO emission and variable CH and CH+ absorption towards HD34078: evidence for a nascent bow shock ?

The runaway star HD34078, initially selected to investigate small scale structure in a foreground diffuse cloud has been shown to be surrounded by highly excited H2. We first search for an association between the foreground cloud and HD34078. Second, we extend previous investigations of temporal absorption line variations (CH, CH+, H2) in order to better characterize them. We have mapped the CO(2-1) emission at 12 arcsec resolution around HD34078's position, using the 30 m IRAM antenna. The follow-up of CH and CH+ absorption lines has been extended over 5 more years. In parallel, CH absorption towards the reddened star Zeta Per have been monitored to check the homogeneity of our measurements. Three more FUSE spectra have been obtained to search for N(H2) variations. CO observations show a pronounced maximum near HD34078's position, clearly indicating that the star and diffuse cloud are associated. The optical spectra confirm the reality of strong, rapid and correlated CH and CH+ fluctuations. On the other hand, N(H2, J=0) has varied by less than 5 % over 4 years. We also discard N(CH) variations towards Zeta Per at scales less than 20 AU. Observational constraints from this work and from 24 micron dust emission appear to be consistent with H2 excitation but inconsistent with steady-state bow shock models and rather suggest that the shell of compressed gas surrounding HD34078, is seen at an early stage of the interaction. The CH and CH+ time variations as well as their large abundances are likely due to chemical structure in the shocked gas layer located at the stellar wind/ambient cloud interface. Finally, the lack of variations for both N(H2, J=0) towards HD34078 and N(CH) towards Zeta Per suggests that quiescent molecular gas is not subject to pronounced small-scale structure.

astro-ph.GA↗

arXiv2008

Ram-pressure stripped molecular gas in the Virgo spiral galaxy NGC 4522

IRAM 30m 12CO(1-0) and 12CO(2-1) HERA observations are presented for the ram-pressure stripped Virgo spiral galaxy NGC 4522. The CO emission is detected in the galactic disk and the extraplanar gas. The extraplanar CO emission follows the morphology of the atomic gas closely but is less extended. The CO maxima do not appear to correspond to regions where there is peak massive star formation as probed by Halpha emission. The presence of molecular gas is a necessary but not sufficient condition for star formation. Compared to the disk gas, the molecular fraction of the extraplanar gas is 30% lower and the star formation efficiency of the extraplanar gas is about 3 times lower. The comparison with an existing dynamical model extended by a recipe for distinguishing between atomic and molecular gas shows that a significant part of the gas is stripped in the form of overdense arm-like structures. It is argued that the molecular fraction depends on the square root of the total large-scale density. Based on the combination of the CO/Halpha and an analytical model, the total gas density is estimated to be about 4 times lower than that of the galactic disk. Molecules and stars form within this dense gas according to the same laws as in the galactic disk, i.e. they mainly depend on the total large-scale gas density. Star formation proceeds where the local large-scale gas density is highest. Given the complex 3D morphology this does not correspond to the peaks in the surface density. In the absence of a confining gravitational potential, the stripped gas arms will most probably disperse; i.e. the density of the gas will decrease and star formation will cease.

astro-ph↗

arXiv2007

Sulfur chemistry in the Horsehead. An interferometric view of the Horsehead PDR

Sulfur is an abundant element which remains undepleted in diffuse interstellar gas (Av<1) but it is traditionally assumed to deplete on dust grains at higher densities and larger Av. Photodissociation regions (PDRs) are an interesting intermediate medium between translucent and dark clouds where the chemistry and energetics are dominated by the illuminating FUV radiation field. Thus they can provide new insights about the sulfur depletion problem. However, physical and chemical gradients in PDRs take place at small angular scales (~1 to 10''). Aperture synthesis observations are therefore required to resolve such gradients. Besides, a complete understanding of molecular excitation is needed to correctly determine molecular abundances but also the preavailing physical conditions. Hence, multi-J observations at increasing frequencies are also required. Such high angular resolution and broad frequency coverage observations will be provided by ALMA in the near future. In this work we present IRAM-PdBI observations of the CS J=2-1 line toward the Horsehead PDR complemented with IRAM-30m observations of several rotational lines of different sulfur bearing molecules (CS, HCS+, SO, H2S, etc.). Photochemical and nonlocal, non-LTE radiative transfer models adapted to the Horsehead geometry have been developed. The gas phase sulfur abundance has been inferred in the PDR.

astro-ph↗

arXiv2006

Low sulfur depletion in the Horsehead PDR

We present 3.65''x3.34'' angular-resolution IRAM PdBI observations of the CS J=2-1 line toward the Horsehead Photodissociation Region (PDR), complemented with IRAM-30m single-dish observations of several rotational lines of CS, C34S and HCS+. We analyse the CS and HCS+ photochemistry, excitation and radiative transfer to obtain their abundances and the physical conditions prevailing in the cloud edge. Since the CS abundance scales to that of sulfur, we determine the gas phase sulfur abundance in the PDR, an interesting intermediate medium between translucent clouds (where sulfur remains in the gas phase) and dark clouds (where large depletions have been invoked). A nonlocal non-LTE radiative transfer code including dust and cosmic background illumination adapted to the Horsehead geometry has been developed to carefuly analyse the CS, C34S, HCS+ and C18O rotational line emission. We use this model to consistently link the line observations with photochemical models to determine the CS/HCS+/S/S+ structure of the PDR. The combination of CS, C34S and HCS+ observations together with the inclusion of the most recent CS collisional and chemical rates in our models implies that sulfur depletion invoked to account for CS and HCS+ abundances is much smaller than in previous studies.

astro-ph↗

arXiv2006

Dissipative structures of diffuse molecular gas: I - Broad HCO$^+$(1-0) emission

Results: We report the detection of broad HCO+(1-0) lines (10 mK < T < 0.5 K). The interpretation of 10 of the HCO+ velocity components is conducted in conjunction with that of the associated optically thin 13CO emission. The derived HCO+ column densities span a broad range, $10^{11}< N(HCO+)/Δv <4 \times 10^{12} \rm cm^2/(km/s^{-1}$, and the inferred HCO+ abundances, $2 \times 10^{-10}<X(HCO+) < 10^{-8}$, are more than one order of magnitude above those produced by steady-state chemistry in gas weakly shielded from UV photons, even at large densities. We compare our results with the predictions of non-equilibrium chemistry, swiftly triggered in bursts of turbulence dissipation and followed by a slow thermal and chemical relaxation phase, assumed isobaric. The set of values derived from the observations, i.e. large HCO+ abundances, temperatures in the range of 100--200 K and densities in the range 100--1000 cm3, unambiguously belongs to the relaxation phase. The kinematic properties of the gas suggest in turn that the observed HCO+ line emission results from a space-time average in the beam of the whole cycle followed by the gas and that the chemical enrichment is made at the expense of the non-thermal energy. Last, we show that the "warm chemistry" signature (i.e large abundances of HCO+, CH+, H20 and OH) acquired by the gas within a few hundred years, the duration of the impulsive chemical enrichment, is kept over more than thousand years. During the relaxation phase, the \wat/OH abundance ratio stays close to the value measured in diffuse gas by the SWAS satellite, while the OH/HCO+ ratio increases by more than one order of magnitude.

astro-ph↗