arXiv subjects

A. Baudry

Publications and source records attributed to A. Baudry.

67 records · Page 4Linked to original sources

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.

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arXiv2010

Herschel-PACS spectroscopy of the intermediate mass protostar NGC7129 FIRS 2

Aims: We present preliminary results of the first Herschel spectroscopic observations of NGC7129 FIRS2, an intermediate mass star-forming region. We attempt to interpret the observations in the framework of an in-falling spherical envelope. Methods: The PACS instrument was used in line spectroscopy mode (R=1000-5000) with 15 spectral bands between 63 and 185 microns. This provided good detections of 26 spectral lines seen in emission, including lines of H2O, CO, OH, O I, and C II. Results: Most of the detected lines, particularly those of H2O and CO, are substantially stronger than predicted by the spherical envelope models, typically by several orders of magnitude. In this paper we focus on what can be learned from the detected CO emission lines. Conclusions: It is unlikely that the much stronger than expected line emission arises in the (spherical) envelope of the YSO. The region hot enough to produce such high excitation lines within such an envelope is too small to produce the amount of emission observed. Virtually all of this high excitation emission must arise in structures such as as along the walls of the outflow cavity with the emission produced by a combination of UV photon heating and/or non-dissociative shocks.

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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.

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arXiv2010

Origin of the hot gas in low-mass protostars: Herschel-PACS spectroscopy of HH 46

'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key Programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H2O emission is concentrated in the outflow. In addition, [OI] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H2O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [OI] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 \times 10-2 L_sun, with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope.

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arXiv2007

Dense gas in luminous infrared galaxies

Molecules that trace the high-density regions of the interstellar medium have been observed in (ultra-)luminous (far-)infrared galaxies, in order to initiate multiple-molecule multiple-transition studies to evaluate the physical and chemical environment of the nuclear medium and its response to the ongoing nuclear activity. The HCN(1-0), HNC(1-0), HCO+(1-0), CN(1-0) and CN(2-1), CO(2-1), and CS(3-2) transitions were observed in sources covering three decades of infrared luminosity including sources with known OH megamaser activity. The data for the molecules that trace the high-density regions have been augmented with data available in the literature. The integrated emissions of high-density tracer molecules show a strong relation to the far-infrared luminosity. Ratios of integrated line luminosities have been used for a first order diagnosis of the integrated molecular environment of the evolving nuclear starbursts. Diagnostic diagrams display significant differentiation among the sources that relate to initial conditions and the radiative excitation environment. Initial differentiation has been introduced between the FUV radiation field in photon-dominated-regions and the X-ray field in X-ray-dominated-regions. The galaxies displaying OH megamaser activity have line ratios typical of photon-dominated regions.

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arXiv2004

Densification of the International Celestial Reference Frame: Results of EVN Observations

The current realization of the International Celestial Reference Frame (ICRF) comprises a total of 717 extragalactic radio sources distributed over the entire sky. An observing program has been developed to densify the ICRF in the northern sky using the European VLBI network (EVN) and other radio telescopes in Spitsbergen, Canada and USA. Altogether, 150 new sources selected from the Jodrell Bank-VLA Astrometric Survey were observed during three such EVN+ experiments conducted in 2000, 2002 and 2003. The sources were selected on the basis of their sky location in order to fill the "empty" regions of the frame. A secondary criterion was based on source compactness to limit structural effects in the astrometric measurements. All 150 new sources have been successfully detected and the precision of the estimated coordinates in right ascension and declination is better than 1 milliarcsecond (mas) for most of them. A comparison with the astrometric positions from the Very Long baseline Array Calibrator Survey for 129 common sources indicates agreement within 2 mas for 80% of the sources.

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arXiv2002

A high-sensitivity OH 5-cm line survey in late-type stars

We have undertaken a comprehensive search for 5-cm excited OH maser emission from evolved stars representative of various stages of late stellar evolution. Observed sources were selected from known 18-cm OH sources. This survey was conducted with the 100-m Effelsberg telescope to achieve high signal to noise ratio observations and a sensitivity limit of about 0.05 to 0.1 Jy. A total of 64 stellar sources were searched for both main line and satellite line emission. We confirm the previous detection of 5 cm OH in Vy 2-2, do not confirm emission from NML-Cyg and do not report any other new detection within the above sensitivity limit. Implications of these results on the pumping mechanism of the OH radical in circumstellar envelopes are briefly discussed.

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arXiv2002

The stellar mass to light ratio in the isolated spiral NGC 4414

We present high resolution CO(1-0) interferometric observations and deep HST B-V-I images of the flocculent isolated Sc type spiral NGC 4414. The goal is to determine the stellar mass-to-light (M/L) ratio in a galactic disk. The stars are seen without a dust screen, the central gas mass is very low (undetected), and we show that the dark matter is negligible in the central regions. We have developed an axisymmetric analytical gravitational potential model to account for the central light (mass) profile, the dynamics of the molecular gas in the highly obscured molecular ring, and the stellar light profile outside the highly obscured region. The contribution of dark matter is constrained by the extremely extended HI rotation curve and is small, possibly negligible, at distances less than 5 -- 7 kpc from the center. Furthermore, the M/L ratios we derive are low, about 1.5 in I band and 0.5 in K' band. The B and V band M/L ratios vary greatly due to absorption by dust, reaching 4 in the molecular ring and decreasing to about 1.6 -- 1.8 at larger radii. This unequivocally shows that models, like most maximum disk models, assuming constant M/L ratios in an optical waveband, simply are not appropriate. We illustrate this by making mock maximum disk models with a constant V band M/L ratio. The key is having the central light distribution unobscured such that it can be used to trace the mass. A primitive attempt to determine the intrinsic M/L ratio yields values close to unity in the B,V, and I bands and slightly below 0.5 in K'.

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arXiv2000

28SiO, 29SiO and 30SiO excitation: effects of infrared line overlaps

We have investigated the influence of several line overlaps between 28SiO, 29SiO and 30SiO on maser emission from the envelopes of late-type stars using the Large Velocity Gradient radiative transfer formalism modified to account for overlaps. Our model includes: (i) 40 rotational levels for each of the v=0 to 4 vibrational states and each isotopic species; (ii) collisions with molecular and atomic hydrogen; and (iii) radiative pumping by a shell of dust grains surrounding the SiO masing layers. We have shown that several line overlaps may play a role on the excitation of the 28SiO v=3, J=1-0 maser, and on the anomalously weak v=3, J=2-1 emission. We suggest that 28SiO maser emission in the v=4, J=5-4 line, the only one observed in this vibrational level, may also result from line overlaps. For 29SiO, infrared line overlaps successfully explain observed maser emission in the v=0, J=5-4 and v=1, 2, J=6-5 lines. 30SiO, v=0, J=2-1, and v=0, J=4-3 maser emissions could also result from line overlaps, and we find that several overlaps could explain the newly discovered maser line 30SiO v=1, J=1-0. Finally, we make some predictions for new 28SiO, 29SiO and 30SiO maser lines.

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arXiv2000

VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser

The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6 epochs, spread over 4 years. Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources. The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy. This confirms the model in which one of the maser spots corresponds to the stellar continuum, amplified by the maser. The stellar proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.

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arXiv1998

29SiO (v=0) and 28SiO (v=1) J=2-1 maser emission from Orion IRc2

We have observed with the IRAM interferometer at two different epochs and simultaneously the two transitions v=0, J=2-1 and v=1, J=2-1 of 29SiO and 28SiO in Orion IRc2. We have made the first maps of 29SiO v=0, J=2-1 emission from Orion. These maps and properties of the 29SiO spectra attest to maser emission. Our 28SiO maps show the stable ring of maser spots observed in previous works. Combining our own data with published works we derive that the relative motion between the two ridges of the 28SiO emission ring is less than about 0.7 AU/yr over a period of 7 years. On the other hand, the weak high velocity maser features observed around 30 km/s seem to move with respect to the stable ring of 28SiO main emission. Our relative 29SiO (v=0) and 28SiO (v=1) spot maps show that most 29SiO and 28SiO emission features are closely related but have not the same spatial extent. We conclude that these masers are not excited in the same gas layers in agreement with pumping models which predict that various v state masers peak in different spatial regions. In addition, our maps of v=0 and v=1 emission suggest that local line overlaps due to turbulence and high gas temperature do not play a dominant role in the excitation of 28SiO and 29SiO, although excitation effects resulting from the overlap of Doppler-shifted ro-vibrational lines may still be significant.

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arXiv1998

On the origin of the high velocity SiO maser emission from late-type stars

We have undertaken toward 30 Mira or semi-regular variables and one OH/IR object highly sensitive observations of the v=1, J=2-1 and 3-2 transitions of SiO simultaneously with observations of the J=1-0 transition of CO during three observing sessions in the period 1995 to 1996. As in our previous observations of 1994, we observe that for several stars the SiO profiles exhibit unusually broad wings which sometimes exceed the terminal velocity of the expanding circumstellar envelope traced by the thermal CO emission. We have discovered a clear dependence of the SiO wing emission on the optical phase. These wings are probably due to peculiar gas motions and varying physical conditions in relation with the stellar pulsation. However, we cannot exclude other mechanisms contributing to the observed wings. In particular, SiO turbulent motions for the semi-regular variables or the asymmetric mass loss mechanism may play a role. We conclude that the SiO wing emission is due to masing processes and that this emission very likely arises from the inner part of the circumstellar envelope.

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arXiv1997

Identification of molecular complexes in M81

We report about high spatial resolution observations made with the IRAM interferometer of the CO emission from a 1.1x1.1 kpc plane-of-sky field on a spiral arm of Messier 81. With a beam of 5''(90 pc), we identify 6 giant molecular cloud complexes with virial masses of about 10^6 Msun, including one associated with a giant HII region. The deduced N(H2)/I(CO) ratios are about 3 times larger on average than those measured near the solar neighborhood, suggesting that the complexes are not self-gravitationally bound except, possibly, for the complex associated with the giant HII region; they could be the average of several clouds of mass a few 10^5 Msun and diameter < 100 pc. The linewidths are very narrow with respect to the measured sizes, so that the size-linewidth relation for M 81 clouds is very different from that in the Milky Way. The narrow linewidths imply smaller virial masses than for Galactic complexes of the same size, and this is consistent with the weaker CO emission from the GMCs in M 81. We conclude from these observations that the molecular medium in M 81 differs from that in the Milky Way.

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