Astron. Astrophys. 359, 113-130 (2000)

3. Nature and IR properties of the HRI sources

Following the results of CMFA, essentially all X-ray sources we found in the  Ophiuchi dark cloud should be young stars, a number of them being still embedded in the cloud. Embedded YSO are mainly studied at IR and millimeter wavelengths, but they may all be potential X-ray emitter regardless of their IR classification. Indeed, using the results of Wilking et al. (1989), AM, and GWAYL, CMFA analyzed their results in the light of the IR observations of the stars they observed in X-rays. We can then (i) use the published IR surveys to provide a list of recognized YSO members of the cluster to be compared to the observed X-ray properties, and (ii) use X-ray emission to discriminate between true cluster members and the many potential background stars seen in IR images.

3.1. New Class II and Class III source census after ISOCAM

Near-IR surveys of the  Ophiuchi cluster, sensitive enough to detect low-luminosity embedded young stars, have recently been published (Comerón et al. 1993; Strom et al. 1995- hereafter SKS; BKLT). However these ground-based surveys encountered limitations in recognizing the nature of all the embedded sources, and have therefore not much increased the number of bona fide members of the  Oph cluster. The mid-IR camera aboard ISO , ISOCAM , produced a map of the  Oph main cloud, used by Bontemps et al. (2000) to study the young star population. This mid-IR study resulted in a significantly more complete census of the  Oph cluster population. We here use this new census as a basis for discussion about the nature of the detected X-ray sources and to estimate the occurrence and properties of the X-ray emission of the different classes of YSO.

The mid-IR photometry at 6.7 and 14.3 µm appears invaluable to characterize sources with IR excesses, i.e., Class I sources and Class II sources (e.g., Nordh et al. 1996; Bontemps et al. 1998). In  Oph, Bontemps et al. (2000) have doubled the number of Class II sources known. These authors conclude that the sample is complete down to stellar luminosities, , as low as 0.03 , thus extending downwards the luminosity function obtained from the ground by about one order of magnitude. However, just as in the near-IR, these measurements alone cannot characterize the nature of sources without IR excess: these can be either Class III sources (diskless TTS) or background sources.

The tentative Class III classification of a number of sources coming from previous X-ray observation (CMFA), or from this article, is now confirmed by ISOCAM , which detected no IR excess. We call these sources "new" Class III sources (see Table B1, and 5).

Since we already identified one of the HRI sources as a foreground star (ROXF37 = HD148352, see Table B1), the question arises that a number of these new sources with Class III spectra may also be field stars, contaminating the genuine young star sample. Guillout et al. (1996) has estimated the stellar content of flux-limited X-ray surveys, based on the age-dependent stellar population model developed by the Besançon group. For the average sensitivity of our HRI observation ( cts s^-1, see Fig. 6), the  Oph galactic latitude (), and our total field of view (0.5 square degrees), this model yields an estimate of 5 contaminating stars (P. Guillout, private communication). Removing HD148352, this leaves 4 possible field star candidates among the 21 "new" Class III sources and "Class II-III" sources listed in Table B1. This number is small enough to not affect significantly our discussion, and in what follows we will simply neglect the possible contamination of our various Class III samples by field stars.

The Class III source population will be studied in detail below (4 and 6).

3.2. Cloud extinctions and stellar luminosities from near-IR photometry

YSO suffer from large amounts of extinction by dust - cloud dust plus circumstellar dust - (up to or more in Oph), which strongly affects their fluxes at all wavelengths of interest, but does not necessarily prevent their detection in soft X-rays (see, e.g., CMFA). Despite this effect, it is possible to estimate the total extinctions along the line of sight and stellar luminosities. Near-IR photometry data appear to provide the most reliable estimate for the luminosity of the embedded young stars (see discussion in Bontemps et al. 2000 and references therein): the J-band fluxes are usually almost purely photospheric and thus trace the stellar luminosities very well (e.g., GWAYL; SKS). Similarly the color is a sensitive tracer of interstellar extinction. We therefore use the extinctions and stellar luminosities derived from near-IR photometry for the ISOCAM sample by Bontemps et al. (2000).

In CMFA the bolometric luminosities were approximately estimated from the J-band fluxes using the GWAYL conversion based on an observational correlation between J fluxes and bolometric luminosities for Taurus and Chameleon TTS. The bolometric luminosity comprises in principle the total luminosity (accretion + stellar) of a YSO. However for young PMS stars like Class II sources and Class III sources, this luminosity should coincide with their stellar luminosity since their accretion luminosity (if any) has become significantly smaller than the photospheric luminosity. Finally we note that the conversion between the J-band absolute magnitude and the stellar luminosity used by Bontemps et al. (2000) is numerically similar to the CMFA conversion between the dereddened J-band and the bolometric luminosity.

Cols. 8-9 of Table B1 give for each source its more up-to-date values of interstellar extinction and stellar luminosity, determined by Bontemps et al. for 140 pc. The reader will find the details of the calculations in Bontemps et al. (2000).

3.3. Nature of the HRI sources from IR data

The YSO evolutionary stage, inferred from IR spectral energy distributions (see 3.1), is available for most of the X-ray sources. The resulting census of the 55 X-ray sources with stellar counterparts is: one Class I protostar (YLW15=IRS43); 23 Class II sources, including 4 new ISOCAM Class II sources; 21 Class III sources, including 13 new Class III sources ⁴; 8 whose classification is either Class II or Class III sources (see discussion in Appendix D); one Class III early-type star (S1, with spectral type B3; see André et al. 1988), and one main sequence foreground star (HD148352 with spectral type F2V; see the Hipparcos catalogue).

In Table B1, Col. 6 lists cross-identifications with the ISOCAM survey: red (blue ) means ISOCAM sources with (without) IR excess. Col. 7 gives the IR classification, Cols. 8-9 the extinctions and stellar luminosities (from Bontemps et al. 2000).

Therefore, the present analysis has revealed no new X-ray emitting Class I source, apart from the Class I protostar YLW15 which has been the subject of a specific study (Grosso et al. 1997). The HRI /ISOCAM sources are overwhelmingly TTS, for which improved results are described in the following sections.

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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