arXiv subjects

Philippe Andre

Publications and source records attributed to Philippe Andre.

23 records · Page 2Linked to original sources

arXiv2001

Wide-field (sub)millimeter continuum surveys of protoclusters: Clues to the origin of the IMF

Recent (sub)millimeter continuum surveys of nearby star-forming regions have revealed a wealth of new, cold cloud fragments. Those which are small-scale (diameter < 10000 AU), starless, and gravitationally bound are good candidates for being the direct progenitors of protostars, i.e., the structures within which individual protostellar collapse is initiated. The mass spectrum of these protocluster condensations is reminiscent of the stellar initial mass function (IMF), suggesting the IMF is partly determined by cloud fragmentation at the pre-stellar stage of star formation.

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arXiv2001

The Structure of Prestellar Cores as derived from ISO Observations

Observations of dark cloud cores have been carried out in the mid-infrared using ISOCAM and in the far-infrared using ISOPHOT, both aboard the Infrared Space Observatory. The cores are in most cases detected in emission at 200 and 170 micron, remain undetected at 90 micron and are seen in absorption against the diffuse mid-infrared background at 7 micron. The observations are consistent with the cores being pre-stellar and not having a central heating source, and yield core temperatures of ~ 11-13 K. We were able to determine the density structure of the cores up to radii that extend beyond the sensitivity limit of previous submillimetre studies. The column density profiles of the cores studied here flatten out in the centre, as shown by previous submillimetre continuum results, and interestingly, in a few cases, the profiles steepen with radius beyond ~ 5000-10000 AU and present sharp edges. This could indicate that these cores are decoupled from their parent molecular cloud and represent finite reservoirs of mass for subsequent accretion.

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arXiv2000

An ISOCAM Absorption Survey of the Structure of Pre-stellar Cloud Cores

We present the results of a mid-infrared (7 micron) imaging survey of a sample of 24 starless dense cores carried out at an angular resolution of 6 arcsec with the ISOCAM camera aboard the Infrared Space Observatory (ISO). The targeted cores are believed to be pre-stellar in nature and to represent the initial conditions of low-mass, isolated star formation. In previous submillimeter dust continuum studies of such pre-stellar cores, it was found that the derived column density profiles did not follow a single power-law such as N[H2] \propto r^(-1) throughout their full extent but flattened out near their center. These submillimeter observations however could not constrain the density profiles at radii greater than ~ 10000 AU. The present absorption study uses ISOCAM's sensitivity to map these pre-stellar cores in absorption against the diffuse mid-infrared background. The goal was to determine their structure at radii that extend beyond the limits of sensitivity of the submillimeter continuum maps and at twiceas good an angular resolution. Among the 24 cores observed in our survey, a majority of them show deep absorption features. The starless cores studied here all show a column density profile that flattens in the center, which confirms the submillimeter emission results. Moreover, beyond a radius of ~ 5000-10000 AU, the typical column density profile steepens with distance from core center and gets steeper than N[H2] \propto r^(-1), until it eventually merges with the low-density ambient molecular cloud. At least three of the cores present sharp edges at R ~ 15000-30000 AU and appear to be decoupled from their parent clouds, providing finite reservoirs of mass for subsequent star formation.

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arXiv2000

X-rays and regions of star formation: a combined ROSAT-HRI/near-to-mid IR study of the rho Oph dark cloud

We have obtained two deep exposures of the rho Oph cloud core region with the ROSAT HRI. The improved position accuracy (1"-6") with respect to previous recent X-ray observations (ROSAT PSPC, and ASCA) allows us to remove positional ambiguities for the detected sources. We also cross-correlate the X-ray positions with IR sources found in the ISO-ISOCAM survey of the same region at 6.7 and 14.3 \mum, in addition to sources known from ground-based observations, which are young stars. We find that there is no statistically significant difference between the X-ray luminosity functions of HRI-detected Class II and Class III sources, i.e., T Tauri stars with and without disks, confirming that the contribution of these disks to X-ray emission or absorption, must be small. Most of the sources are variable, and their variability is consistent with a solar-like (hence magnetic) flare origin. We use the information given both by the ISOCAM survey and by our HRI deep exposure to study the T Tauri star population of the rho Oph dense cores. We confirm that essentially all Class II and Class III sources are X-ray emitters, and that a strong correlation exists between their X-ray luminosity, L_X, and their stellar luminosity, L_*, with L_X/L_* ~ 1E-4. Most of the new ISOCAM Class II sources are not detected, however, which we explain by the fact that their X-ray luminosities ``predicted'' on the basis of this correlation are too faint to be detected by the HRI. We predict that ~40 unknown faint or embedded Class III sources remain to be discovered in X-rays in the HRI/ISOCAM overlapping area, down to a limit of L_X ~ 3 \times 1E28 erg/s. We show that the bulk of these unknown Class III sources should be made of low- to very low-mass stars. Prospects for future detections with XMM-Newton and Chandra are discussed.

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arXiv1999

From Pre-Stellar Cores to Protostars: The Initial Conditions of Star Formation

The last decade has witnessed significant advances in our observational understanding of the earliest stages of low-mass star formation. The advent of sensitive receivers on large radio telescopes such as the JCMT and IRAM 30m MRT has led to the identification of young protostars at the beginning of the main accretion phase (`Class 0' objects), and has made it possible to probe, for the first time, the inner density structure of pre-collapse cores. Class 0 objects are characterized by strong, centrally-condensed dust continuum emission at submillimeter wavelengths, very little emission shortward of 10 microns, and powerful jet-like outflows. Direct evidence for gravitational infall has been observed toward several of them. They are interpreted as accreting protostars which have not yet accumulated the majority of their final stellar mass. In contrast to protostars, pre-stellar cores have flat inner density profiles, suggesting the initial conditions for fast protostellar collapse depart sometimes significantly from a singular isothermal sphere. In the case of non-singular initial conditions, the beginning of protostellar evolution is expected to feature a brief phase of vigorous accretion/ejection which may coincide with Class 0 objects. In addition, submillimeter continuum imaging surveys of regions of multiple star formation such as Ophiuchus and Serpens suggest a picture according to which each star in an embedded cluster is built from a finite reservoir of mass and the associated IMF is primarily determined at the pre-stellar stage of evolution.

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