X-RAY vs.IR sources in regions of star formation: completing the census of young stellar objects

N. Grosso ¹, T. Montmerle ¹, S. Bontemps ², P. André ¹, & E.D. Feigelson ³

¹ Service d'Astrophysique, CEA/DSM/DAPNIA/SAp, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France

² Stockholm Observatory, 13336 Saltsjöbaden, Sweden

³ Observatoire de Bordeaux, BP89, 33270 Floirac, France

⁴ Dept. of Astronomy and Astrophysics, Penn State University, University Park, PA16802, USA

Combining IR (ground-based near-IR and ISOCAM mid-IR) and X-ray deep surveys appears to be the best, and may be the only way to get a fairly complete census of the population of young stars in extremely young cluster embedded in the nearby star-forming clouds. The IR surveys are required to recognize and to study (to get ages and luminosities) the young stars/protostars still surrounded by large amount of circumstellar dust, while X-rays appear to be extremely efficient at probing all the young stars, and in particular the numerous ones which do not have circumstellar dust disks and thus cannot be distinguished from main-sequence stars by IR surveys.

To demonstrate this statement, we have obtained two deep exposures of the Rho Oph cloud core region (d = 150 pc) in X-rays with the ROSAT High Resolution Imager. The improved angular accuracy (1-6") with respect to previous recent observations (ROSAT PSPC, Casanova et al. 1995: ApJ, 439, 752; and ASCA, Kamata et al. 1997: PASJ, 49, 461) allows to remove positional ambiguities for the 63 detected sources. We cross-correlate the X-ray positions with IR sources found in the ISOCAM LW2/LW3 survey of the same region (Nordh et al.; see also Bontemps et al., this Conference) in addition to sources (optical and IR) known from ground-based observations. These IR/optical sources are ``young stellar objects'' (``YSOs'': protostars, T Tauri stars, with and without circumstellar disks; respectively Class I, II, and III sources).

We thus obtain the best-studied sample of X-ray/IR sources in a star-forming region, which confirms and significantly improves the results obtained previously.

(1) We confirm that essentially all IR sources are X-ray emitters, with a large majority of T Tauri stars, and that a strong correlation

( )

exists between the stellar luminosity and the X-ray luminosity. Many of the new IR sources (from ISOCAM) are not detected, but this is readily explained by the fact that their X-ray luminosities ``predicted'' on the basis of this correlation are too faint to be detected by the HRI. This is not inconsistent with the idea that all YSOs are X-ray emitters. Conversely, a number of previously unclassified IR sources without IR excess can, via their detection in X-rays (and only so), be classified a new Class III sources.

(2) Protostars are not detected with the HRI, with the exception of the protostar YLW15, with could be detected in the course of an extraordinarily powerful flare ( , Grosso et al. 1997, Nature, 387, 56). This is very likely due to their very high extinction (circumstellar envelope + interstellar): . A few protostars were tentatively detected by the ROSAT PSPC, because of its better sensitivity, and by ASCA, which reaches X-ray energies , where the extinction effect ( ) is much less severe.

More (maybe all) protostars, faint ISOCAM sources, and even perhaps brown dwarfs, will soon be detectable with the new generation of X-ray satellites (AXAF, XMM). These satellites feature both an increase in sensitivity and an extension towards higher X-ray energies. We anticipate that ALL young stellar objects, including brown dwarfs, will be proved to be X-ray emitters. Some consequences on circumstellar dust and gas chemistry will be briefly reviewed (see Feigelson & Montmerle 1999, Ann.Rev. Astr.Ap., in press; Glassgold, Feigelson, & Montmerle 1999, in Protostars and Planets IV, in press).