ROSAT X-ray sources embedded in the rho Ophiuchi cloud core

Abstract

We present a deep ROSAT Position Sensitive Proportional Counter image of the central region of the rho Oph star-forming region. The selected area, about 35' x 35' in size, is rich with dense molecular cores and young stellar objects (YSOs). Fifty-five reliable X-ray sources are detected (and up to 50 more candidates may be present) above similar to 1 keV, doubling the number of Einstein sources in this area. These sources are cross-identified with an updated list of 88 YSOs associated with the rho Oph cloud core. A third of the reliable X-ray sources do not have optical counterparts on photographic plates. Most can be cross-identified with Class II and Class III infrared (IR) sources, which are embedded T Tauri stars, but three reliable X-ray sources and up to seven candidate sources are tentatively identified with Class I protostars. Eighteen reliable, and up to 20 candidate, X-ray sources are probably new cloud members. The overall detection rate of the bona fide cloud population is very high (73% for the Class II and Class III objects). The spatial distribution of the X-ray sources closely follows that of the molecular gas. The visual extinctions A(V) (estimated from near-IR data) of the ROSAT sources can be as high as 50 or more, confirming that most are embedded in the cloud core and are presumably very young. Using bolometric luminosities L(bol) estimated from J-magnitudes, a tight correlation between L(x) and L(bol) is found, similar to that seen for older T Tauri stars in the Cha I cloud: L(x) similar to 10(-4)L(bol). A general relation L(x) proportional to L(bol) proportional to L(J) seems to apply to all T Tauri-like YSOs. The near equality of the extinction in the IR J band and in the keV X-ray range implies that this relation is valid for the detected fluxes as well as for the dereddened fluxes. The X-ray luminosity function of the embedded sources in rho Oph spans a range of L(x) similar to 10(28.5) to greater than or similar to 10(31.5) ergs s(-1) and is statistically indistinguishable from that of X-ray-detected visible T Tauri stars. We estimate a total X-ray luminosity L(x,Oph)greater than or similar to 6 x 10(32) ergs s(-1) from approximate to 200 X-ray sources in the cloud core, down to L(bol) similar to 0.1 L. or M* similar to 0.3 M.. We discuss several consequences of in situ irradiation of molecular clouds by X-rays from embedded YSOs. These X-rays must partially ionize the inner regions of circumstellar disk coronae, possibly playing an important role in coupling magnetic fields and wind or bipolar outflows. Photon-stimulated desorption of large molecules by YSO X-rays may be partly responsible for the bright 12 mu m halos seen in some molecular clouds. Because YSO X-ray emission exceeds cosmic-ray ionization as the principal source of ionization in molecular cloud cores, it may play an important role as a feedback agent in the self-regulation of star formation.