arXiv subjects

S. Cabrit

Publications and source records attributed to S. Cabrit.

107 records · Page 6Linked to original sources

arXiv2010

Spitzer spectral line mapping of the HH211 outflow

Aims: We employ archival Spitzer slit-scan observations of the HH211 outflow in order to investigate its warm gas content, assess the jet mass flux in the form of H2 and probe for the existence of an embedded atomic jet. Methods: Detected molecular and atomic lines are interpreted by means of emission line diagnostics and an existing grid of molecular shock models. The physical properties of the warm gas are compared against other molecular jet tracers and to the results of a similar study towards the L1448-C outflow. Results: We have detected and mapped the v=0-0 S(0) - S(7) H2 lines and fine-structure lines of S, Fe+, and Si+. H2 is detected down to 5" from the source and is characterized by a "cool" T~300K and a "warm" T~1000 K component, with an extinction Av ~ 8 mag. The amount of cool H2 towards the jet agrees with that estimated from CO assuming fully molecular gas. The warm component is well fitted by C-type shocks with a low beam filling factor ~ 0.01-0.04 and a mass-flux similar to the cool H2. The fine-structure line emission arises from dense gas with ionization fraction ~0.5 - 5 x 10e-3, suggestive of dissociative shocks. Line ratios to sulfur indicate that iron and silicon are depleted compared to solar abundances by a factor ~10-50. Conclusions: Spitzer spectral mapping observations reveal for the first time a cool H$_2$ component towards the CO jet of HH211 consistent with the CO material being fully molecular and warm at ~ 300 K. The maps also reveal for the first time the existence of an embedded atomic jet in the HH211 outflow that can be traced down to the central source position. Its significant iron and silicon depletion excludes an origin from within the dust sublimation zone around the protostar. The momentum-flux seems insufficient to entrain the CO jet, although current uncertainties on jet speed and shock conditions are too large for a definite conclusion.

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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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arXiv2009

Dust Properties of Protoplanetary Disks in the Taurus-Auriga Star Forming Region from Millimeter Wavelengths

We present the most sensitive 3 mm-survey to date of protoplanetary disks carried in the Taurus-Auriga star forming region (average rms of about 0.3 mJy), using the IRAM PdBI. With our high detection rate of 17/19, we provide the first detections at wavelengths longer than about 1 mm for 12 sources. This enables us to study statistically the mm SED slopes and dust properties of faint disks and compare them to brighter disks using a uniform analysis method. With these new data and literature measurements at sub-millimeter and millimeter wavelengths, we analyze the dust properties of a sample of 21 isolated disks around T Tauri stars in the Taurus-Auriga star forming region. Together with the information about the disks spatial extension from sub/mm-mm interferometric studies, we derive from the observed sub-mm/mm spectral energy distribution constraints on the dust opacity law at these wavelengths, using two-layer flared disk models and a self-consistent dust model that takes properly into account the variation of the dust opacity with grain growth. We find evidence for the presence in the disk midplane of dust particles that have grown to sizes as large as at least 1 millimeter in all the disks of our sample, confirming what was previously observed on smaller brighter objects. This indicates that the dust coagulation from ISM dust to mm-sized grains is a very fast process in protoplanetary disks, that appears to occur before a young stellar object enters the Class II evolutionary stage. Also, the amount of these large grains in the disk outer regions is stationary throughout all the Class II evolutionary stage, indicating that mechanisms slowing down the dust inward migration are playing an important role in the Taurus-Auriga protoplanetary disks.

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arXiv2008

SiO line emission from C-type shock waves : interstellar jets and outflows

We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si--bearing material in refractory grain cores, which are taken to be olivine; we calculate also the rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady--state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models has been generated. We compare our results with those of an earlier study (Schilke et al. 1997). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si being released into the gas phase. More realistic assumptions concerning the initial fractional abundance of O2 lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG `slab' models. The fractional abundance of SiO is deduced. Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam.

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arXiv2007

Discovery of a bipolar X-ray jet from the T Tauri star DG Tau

We have obtained and analyzed Chandra ACIS-S observations of the strongly accreting classical T Tauri star DG Tau. Our principal goals are to map the immediate environment of the star to characterize possible extended X-rays formed in the jet, and to re-visit the anomalous, doubly absorbed X-ray spectrum of DG Tau itself. We combine our new ACIS-S data with a data set previously obtained. The data are superimposed to obtain flux and hardness images. Separate X-ray spectra are extracted for DG Tau and areas outside its point spread function. We detect a prominent X-ray jet at a position angle of PA ~225 deg (tentatively suggested by Guedel et al. 2005), coincident with the optical jet axis. We also identify a counter jet at PA = 45 deg. The X-ray jets are detected out to a distance of ~5" from the star, their sources being extended at the ACIS-S resolution. The jet spectra are soft, with a best-fit electron temperature of 3.4 MK. We find evidence for excess absorption of the counter jet. The spectrum of the DG Tau point source shows two components with largely different temperatures and absorption column densities. The similar temperatures and small absorbing gas columns of the jet sources and the soft component of the "stellar" source suggest that these sources are related, produced either by shocks or by magnetic heating in the jets. Cooling estimates suggest that the pressure in the hot gas contributes to jet expansion. The hard "stellar" component, on the other hand, is associated with a stellar corona or magnetosphere. The excessive photoelectric absorption of this component suggests the presence of dust-depleted accretion streams above coronal magnetic fields.

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arXiv2007

The evolution of stars in the Taurus-Auriga T association

In a recent study, individual parallaxes were determined for many stars of the Taurus-Auriga T association that are members of the same moving group. We use these new parallaxes to re-address the issue of the relationship between classical T Tauri stars (CTTSs) and weak-emission line T Tauri stars (WTTSs). With the available spectroscopic and photometric information for 72 individual stars or stellar systems among the Taurus-Auriga objects with known parallaxes, we derived reliable photospheric luminosities, mainly from the Ic magnitude of these objects. We then studied the mass and age distributions of the stellar sample, using pre-main sequence evolutionary models to determine the basic properties of the stellar sample. Statistical tests and Monte Carlo simulations were then applied to studying the properties of the two T Tauri subclasses. We find that the probability of CTTS and WTTS samples being drawn from the same parental age and mass distributions is low; CTTSs are, on average, younger than WTTSs. They are also less massive, but this is due to selection effects. The observed mass and age distributions of both T Tauri subclasses can be understood in the framework of a simple disk evolution model, assuming that the CTTSs evolve into WTTSs when their disks are fully accreted by the stars. According to this empirical model, the average disk lifetime in Taurus-Auriga is 4 10**6 (Mstar/Msun)**0.75 yr.

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arXiv2007

PdBI sub-arcsecond study of the SiO microjet in HH212 - Origin and collimation of Class 0 jets

The bipolar HH 212 outflow has been mapped in SiO using the extended configuration of the Plateau de Bure Interferometer (PdBI), revealing a highly collimated SiO jet closely associated with the H2 jet component. We study at unprecedented resolution (0.34" across the jet axis) the properties of the innermost SiO ``microjet'' within 1000 AU of this young Class 0 source, to compare it with atomic microjets from more evolved sources and to constrain its origin. The SiO channel maps are used to investigate the microjet collimation and velocity structure. A large velocity gradient analysis is applied to SiO (2-1), (5-4) and (8-7) data from the PdBI and the Submillimeter Array to constrain the SiO opacity and abundance. The HH212 Class 0 microjet shows striking similarities in collimation and energetic budget with atomic microjets from T Tauri sources. Furthermore, the SiO lines appear optically thick, unlike what is generally assumed. We infer T(kin) ~ 50-500 K and an SiO/H2 abundance greater than 4 10(-8)-6 10(-5) for n(H2) = 10(7)-10(5) cm(-3), i.e. 0.05-90% of the elemental silicon. This similar jet width, regardless of the presence of a dense envelope, definitely rules out jet collimation by external pressure, and favors a common MHD self-collimation (and possibly acceleration) process at all stages of star formation. We propose that the more abundant SiO in Class 0 jets could mainly result from rapid (less than 25 yrs) molecular synthesis at high jet densities.

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arXiv2006

A highly-collimated SiO jet in the HH212 protostellar outflow

We mapped the HH212 Class 0 outflow in SiO(2--1, 5--4) and continuum using the PdBI in its extended configurations. The unprecedented angular resolution (down to 0.34") allows accurate comparison with a new, deep H2 image obtained at the VLT. The SiO emission is confined to a highly-collimated bipolar jet (width 0.35") along the outflow axis. The jet can be traced down to within 500 AU of the protostar, in a region that is heavily obscured in H2 images. Where both species are detected, SiO shows the same overall kinematics and structure as H2, indicating that both molecules are tracing the same material. We find that the high-velocity SiO gas near the protostar is not tracing a wide-angle wind but is already confined to a flow inside a narrow cone of half-opening angle < 6 deg.

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arXiv2006

Molecular line emission in HH54: a coherent view from near to far infrared

Aims. We present a detailed study of the infrared line emission (1-200 micron) in the Herbig-Haro object HH54. Our database comprises: high- (R~9000) and low- (R~600) resolution spectroscopic data in the near-infrared band (1-2.5 micron); mid-infrared spectrophotometric images (5-12 micron); and, far-IR (45-200 micron, R~200) spectra acquired with the ISO satellite. As a result, we provide the detection of and the absolute fluxes for more than 60 molecular features (mainly from H2 in the near- and mid-infrared and from H2O, CO and OH in the far-infrared) and 23 ionic lines. Methods. The H2 lines, coming from levels from v=0 to v=4 have been interpreted in the context of a state-of-art shock code, whose output parameters are adopted as input to a Large Velocity Gradient computation in order to interpret the FIR emission of CO, H2O and OH. Results. The H2 emission can be interpreted as originating in either steady-state J-type shocks or in quasi-steady J-type shocks with magnetic precursor. However, our multi-species analysis shows that only a model of a J-type shock with magnetic precursor (v_shock=18 km/s, n_H=10^4 cm^-3, B=100 microG, age=400 yr) can account for both the observed H2 emission and the CO and H2O lines. Such a model predicts a H2O abundance of ~ 7 10^-5, in agreement with estimations from other shock models for outflows associated with low mass protostars. We can exclude the possibility that the observed atomic lines arise in the same shock as the molecular lines, and give arguments in favour of the presence of a further high-velocity, fully dissociative shock component in the region. Finally, in view of the forthcoming spectroscopic facilities on board of the Herschel satellite, we provide predictions for H2O lines considered to be the most suitable for diagnostic purposes.

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arXiv2004

Temporal evolution of magnetic molecular shocks I. Moving grid simulations

We present time-dependent 1D simulations of multifluid magnetic shocks with chemistry resolved down to the mean free path. They are obtained with an adaptive moving grid implemented with an implicit scheme. We examine a broad range of parameters relevant to conditions in dense molecular clouds, with preshock densities between 10^3 and 10^5 cm-3, velocities between 10 and 40 km/s, and three different scalings for the transverse magnetic field: B=0,0.1,1 μG \sqrt{n.cm3}. We first use this study to validate the results of Chièze, Pineau des Forêts & Flower (1998), in particular the long delays necessary to obtain steady C-type shocks, and we provide evolutionary time-scales for a much greater range of parameters. We also present the first time-dependent models of dissociative shocks with a magnetic precursor, including the first models of stationary CJ shocks in molecular conditions. We find that the maximum speed for steady C-type shocks is reached before the occurrence of a sonic point in the neutral fluid, unlike previously thought. As a result, the maximum speed for C-shocks is lower than previously believed. Finally, we find a large amplitude bouncing instability in J-type fronts near the H2 dissociation limit (u ~ 25-30 km/s), driven by H2 dissociation/reformation. At higher speeds, we find an oscillatory behaviour of short period and small amplitude linked to collisional ionisation of H. Both instabilities are suppressed after some time when a magnetic field is present. In a companion paper, we use the present simulations to validate a new semi-analytical construction method for young low-velocity magnetic shocks based on truncated steady-state models.

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arXiv2004

Temporal evolution of magnetic molecular shocks II. Analytics of the steady state and semi-analytical construction of intermediate ages

In the first paper of this series (Paper I) we computed time dependent simulations of multifluid shocks with chemistry and a transverse magnetic field frozen in the ions, using an adaptive moving grid. In this paper, we present new analytical results on steady-state molecular shocks. Relationships between density and pressure in the neutral fluid are derived for the cold magnetic precursor, hot magnetic precursor, adiabatic shock front, and the following cooling layer. The compression ratio and temperature behind a fully dissociative adiabatic shock is also derived. To prove that these results may even hold for intermediate ages, we design a test to locally characterise the validity of the steady state equations in a time-dependent shock simulation. Applying this tool to the results of Paper I, we show that most of these shocks (all the stable ones) are indeed in a quasi-steady state at all times, i.e. : a given snapshot is composed of one or more truncated steady shock. Finally, we use this property to produce a construction method of any intermediate time of low velocity shocks (u < 20 km/s) with only a steady-state code. In particular, this method allows one to predict the occurrence of steady CJ-type shocks more accurately than previously proposed criteria.

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arXiv2004

Predicted rotation signatures in MHD disc winds and comparison to DG Tau observations

Motivated by the first detections of rotation signatures in the DG Tau jet (Bacciotti et al. 2002), we examine possible biases affecting the relation between detected rotation signatures and true azimuthal velocity for self-similar MHD disc winds, taking into account projection, convolution as well as excitation gradients effects. We find that computed velocity shifts are systematically smaller than the true underlying rotation curve. When outer slower streamlines dominate the emission, we predict observed shifts increasing with transverse distance to the jet axis, opposite to the true rotation profile. Determination of the full transverse rotation profile thus requires high angular resolution observations (< 5 AU) on an object with dominant inner faster streamlines. Comparison of our predictions with HST/STIS observations of DG Tau clearly shows that self-similar, warm MHD disc wind models with lambda = 13 and an outer radius of the disc ~ 3 AU are able to reproduce detected velocity shifts, while cold disc wind models (lambda > 50) are ruled out for the medium-velocity component in the DG Tau jet.

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arXiv2003

Near-IR [Fe II] emission diagnostics applied to cold disk winds in young stars

We investigate the emissivity properties of the main near-IR transitions of the Fe+ ion in the conditions prevailing in the inner regions of jets from young stars, based on a simplified 16-level atom model. We present new diagnostic diagrams involving prominent near-IR line ratios that allow us to constrain the electronic density, temperature, and Fe gas phase abundance ratio, independently of the heating process. Comparison with recent near-IR observations of a sample of HH objects indicates gas phase Fe abundances ranging from 15-50 % up to 100 % of the solar value in agreement with the moderate depletions previously derived from optical line ratios or shock models. Hence, it appears that Fe-bearing dust is efficiently destroyed in stellar jets. We then use our Fe+ emissivity model to predict near-IR [Fe II] emission maps for self-similar, cold MHD disk wind models. We show that observations in [Fe II] with AMBER on the VLTI could severely constrain the MHD solution and the inner launch radius of the jet. We also compare theoretical predictions with recent observations in the near-IR [Fe II] lines of the L1551-IRS5 and DG Tau jets. The cold disk wind model reproduces quite well the two velocity components observed at -100 and -300 km/s, although the high velocity component appears overestimated by a factor of 1.5 in the DG Tau jet. However, the model predicts too little emission at intermediate-velocity and insufficient densities. Denser disk winds with stronger heating at the jet base, which have been invoked for optical jets, also appear needed in younger, embedded Class I jet sources.

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arXiv2003

Proper motions and velocity asymmetries in the RW Aur jet

We present adaptive optics spectro-imaging observations of the RW Aur jet in optical forbidden lines, at an angular resolution of 0.4 arcsec. Comparison with HST data taken 2 years later shows that proper motions in the blueshifted and redshifted lobes are in the same ratio as their radial velocities, a direct proof that the velocity asymmetry in this jet is real and not an emissivity effect. The inferred jet inclination to the line of sight is i = 46 +/- 3 degrees. The inner knot spacing appears best explained by time variability with at least two modes: one irregular and asymmetric (possibly random) on timescales of <3-10 yr, and another more regular with ~ 20 yr period. We also report indirect evidence for correlated velocity and excitation gradients in the redshifted lobe, possibly related to the blue/red velocity and brightness asymmetry in this system.

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arXiv2003

Warm Molecular Hydrogen and Ionized Neon in the HH2 Outflow

We report on spectro-imaging observations of the Herbig-Haro 2 outflow with the ISOCAM camera onboard the Infrared Space Observatory (ISO). The [Ne II}] 12.81 microns and [Ne III]15.55 microns lines are detected only towards the jet working surface (HH 2H), consistent with the high excitation of this knot in the optical range, while H2 pure rotational emission is found all over the shocked region HH 2. The low energy transition S(2) traces warm gas (T approx. 400K) peaked towards knots E-F and extended ejecta (T approx. 250-380) with masses of a few 0.001 solar mass in the high-velocity CO outflow extending between the powering source and HH 2. Such emission could arise from low-velocity C-type shocks (v= 10-15 km/s). The higher transitions S(3)-S(7) trace the emission of hot shocked gas (T= 1000-1400K) from individual optical knots in the HH 2 region. The ortho to para (OTP) ratio exhibits large spatial variations between 1.2 (E) and 2.5 (H), well below its value at LTE. The emission of the S(3)-S(7) lines is well accounted for by planar C-shock models with a typical velocity V= 20-30 km/s propagating into a medium of density 10^4-10^5 cm-3 with an initial OTP ratio close to 1 in the pre-shock gas. In the leading edge of the jet, where the geometry of the emission allows a simple modelling, a good agreement is found with velocities derived from the optical proper motions measured in the ionized gas.

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arXiv2001

MIFS: A Mega Integral Field Spectrograph for the VLT

We describe MIFS, a second generation integral-field spectrograph for the VLT, operating in the visible wavelength range. It combines a 1'x1' field of view with the improved spatial resolution provided by multi-conjugate adaptive optics and covers a large simultaneous spectral range (0.6-1.0 microns). A separate mode exploits the highest spatial resolution provided by adaptive optics. With this unique combination of capabilities, MIFS has a wide domain of application and a large discovery potential. The MIFS low-spatial resolution mode (sampled at 0.2") combined with the initial MCAO capabilities planned for the VLT will provide ultra-deep fields with a limiting magnitude for spectroscopy of R~28. MIFS will improve the present day detection limit of Lyman alpha emitters by a factor of 100, and will detect low-mass star-forming galaxies to z~7. The MIFS high-spatial resolution mode (3"x3" field sampled at 0.01") is optimized for the next step in (MC)AO. It will probe, e.g., the relationship between supermassive central black holes and their host galaxy and the physics of winds from accretion disks in young stellar objects at unprecedented spatial resolution. MIFS will extend Europe's lead in integral-field spectroscopy. It capitalizes on new developments in adaptive optics, and is a key step towards instrumentation for OWL.

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arXiv2000

Windows through the Dusty Disks Surrounding the Youngest Low Mass Protostellar Objects

The formation and evolution of young low mass stars are characterized by important processes of mass loss and accretion ocurring in the innermost regions of their placentary circumstellar disks. Because of the large obscuration of these disks at optical and infrared wavelengths in the early protostellar stages (Class 0 Sources), they were previously detected only at radio wavelengths using interferometric techniques. We have detected with the Infrared Space Observatory (ISO) the mid-infrared emission associated with the Class 0 protostar VLA1 in the HH1-2 region located in the Orion nebula. The emission arises in the three wavelength windows at 5.3, 6.6 and 7.5 micras where the absorption due to ices and silicates has a local minimum that exposes the central parts of the youngest protostellar systems to mid-infrared investigations. The mid-infrared emission arises from a central source with 4 AU diameter at an averaged temperature of 700 K, deeply embedded in a dense region with a visual extinction of Av=80-100mag.

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