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

1. Introduction

The Ophiuchi dark cloud complex is one of the nearest active site of low-mass star formation (see Wilking 1992for a review). It is composed of two main dark clouds, L1688 and L1689, from which filamentary dark clouds, called streamers, extend to the north-east over tens of parsecs (e.g., Loren 1989; de Geus et al. 1990). The main star formation activity is observed in the westernmost dark cloud, L1688, which shows a rich cluster of low mass young stellar objects (YSO) around two dense molecular cores, "core A" and "core F" in the terminology of Loren (1989) and Loren et al. (1990).

The distance to the molecular complex remains somewhat controversial (see Wilking 1992), with a usually adopted distance pc from the Sun. From Hipparcos parallaxes and Tycho B-V colors of classes V and III stars, Knude & Hg (1998) have detected at pc an abrupt rise of the reddening as expected from a molecular cloud. Based on the Hipparcos positions, proper motions, and parallaxes, de Zeeuw et al. (1999) gives pc for the mean distance of the Upper Scorpius OB association. We adopt pc in this article, instead of 160 pc used in our previous work.

From infrared (IR) observations of star-forming regions, Lada and collaborators (e.g. Lada 1987; Wilking, Lada, & Young 1989, hereafter WLY) introduced an IR classification and distinguished different stages of evolution of young stellar objects (YSO). This classification was subsequently revisited by André & Montmerle (1994, hereafter AM) to incorporate results of millimeter continuum studies on circumstellar dust. The IR sources are classified in three classes, according to their spectral energy distributions (SEDs). This classification, initially defined empirically, is now well understood in terms of evolution of low-mass stars at their earliest stages. Submillimeter observations led to the discovery of cold objects, younger than the IR sources, and thus to the introduction of a fourth class named "Class 0" (André et al. 1993, 2000). Class 0 sources are very young protostars, peaking in the submillimeter range, at the beginning of the main accretion phase. Class I sources are evolved IR protostars, optically invisible, in the late accretion phase. Class II sources are YSO surrounded by optically thick circumstellar disks. Class III sources are YSO with an optically thin circumstellar disk or no circumstellar disk. Studies of optically visible YSO, T Tauri stars, led to another classification based on the line, which separates "classical" T Tauri stars (CTTS) from "weak-line" T Tauri stars (WTTS) according to their equivalent width in emission, with a boundary at EW[] Å, depending on the spectral type (Martín 1997). CTTS and WTTS are usually taken to be identical to Class II and Class III sources respectively, on the basis of their IR SED (see AM for a discussion about these two classifications). We will associate in this article Class II (Class III) sources with CTTS (WTTS).

Several ground-based near-IR surveys (e.g.Wilking et al. 1989; Greene et al. 1994, hereafter GWAYL; Barsony et al. 1997, hereafter BKLT; and references therein) discovered in a 1 square degree area around the densest regions (with survey completeness limit down to ), 100 low-luminosity embedded sources. More recently, the ISOCAM camera on-board the Infrared Space Observatory satellite imaged a half square degree centered on L1688 in the mid-IR (LW2 and LW3 filters, respectively centered at 6.7 µm and 14.3 µm - ISOCAM central programme surveys by Nordh et al.; see Abergel et al. 1996), and recognized 68 new faint young stars with infrared excess (Bontemps et al. 2000).

Near-IR spectroscopy has been used to determine spectral types of an increasingly large number of Oph YSO (see the pioneering works of Greene & Meyer 1995, and Greene & Lada 1996). Recently, Luhman & Rieke (1999) obtained K-band spectroscopy for 100 sources, combining a magnitude-limited sample in the cloud core () with a representative population from the outer region of the cluster ().

The Oph dark cloud YSO have also been extensively studied in X-rays. Early observations with the Einstein Observatory satellite showed that at the T Tauri star stage YSO are bright and variable X-ray emitters in the 0.2-4 keV energy band (Montmerle et al. 1983). When the S/N ratio is sufficient large, their X-ray spectra can be fitted by a thin thermal model, with temperatures keV and absorption column densities - cm^-2. Variability studies and modeling led to explain the X-ray emission in terms of bremsstrahlung from a hot ( K) plasma trapped in very large magnetic loops, in other words in terms of an enhanced solar-like flare activity (see reviews by Montmerle et al. 1993; and Feigelson & Montmerle 1999, hereafter FM). Casanova et al. (1995) - hereafter CMFA - reported deep ROSAT   Position Sensitive Proportional Counter (PSPC ) imaging of the Oph cloud dense cores A and F. They detected in the central portion of the field (the inner ring of the ROSAT detector entrance window support structure) 55 X-ray sources in the 1.0-2.4 keV energy band. For three X-ray sources, one or several Class I sources lie within the error boxes of X-ray peaks, but other counterparts are possible (unclassified IR sources, T Tauri stars). X-ray emission from one of these Class I sources, YLW15 (=IRS43 in WLY), was unambiguously confirmed with a follow-up ROSAT   High Resolution Imager (HRI ) observation by Grosso et al. (1997). The outer portion of the CMFA PSPC field, analyzed by Casanova (1994), contains 36 X-ray sources. The optical spectroscopic classification of these X-ray sources and other X-ray selected stars in the Oph dark cloud vicinity, based on and LiI (670.8 nm) spectroscopy, was made by Martín et al. (1998), doubling the number of PMS stars spectroscopically classified in the Ophiuchi area.

Observations of harder X-ray (4 keV) from the Oph dark cloud were initially only possible with non-imaging instruments. Tenma and Ginga revealed unresolved emission from the cloud core region, with a hard X-ray spectrum with keV and cm^-2 (Koyama 1987; Koyama et al. 1992). Wide-energy band imaging observations became possible with ASCA in the range 0.5-10 keV. In the Oph dark cloud, Koyama et al. (1994) detected hard X-rays from T Tauri stars, with up to 8 keV in the case of the WTTS DoAr21. There is also some evidence for unresolved hard X-ray emission from embedded young stars below the point source detection limit. From this ASCA observation, Kamata et al. (1997), found additional T Tauri stars and detected three X-ray sources associated with Class I sources, but with large X-ray error boxes (15"-30").

There is a deep connection between IR and X-ray observations of star-forming regions. Sensitive ground-based near-IR surveys penetrate dark clouds (except for dense cores) so that their source populations are frequently dominated by ordinary stars in the Galactic disk. Space-based mid-IR isolates YSO with significant circumstellar material and effectively eliminates the background star population, but they will miss the recognition of YSO with less massive or absent disks. X-ray emission, in contrast, is elevated by 1-4 orders of magnitude in YSO of all ages, irrespective of a disk presence. It thus provides a unique tool for improving the census of young star clusters.

In this article, we present the results from the HRI follow-up of the CMFA PSPC observation. The high angular resolution of these observations allows us to find counterparts to all X-ray sources without ambiguity. The comparison with the sensitive ISOCAM survey of the Oph dark cloud significantly improves the existing classification of these counterparts and allows us to do statistical studies on a well defined sample.

We first present the ROSAT   HRI observations: image analysis, source detection and identification (2). We incorporate the ISOCAM survey results from Bontemps et al. (2000) and we present the resulting IR classification for the HRI sources (3). The next sections discuss the X-ray luminosity of the HRI detected TTS (4), and the X-ray detectability of the embedded TTS population (5). Next (x6), we show that the HRI census of Class III sources cannot be complete, and that numerous unknown low-luminosity Class III sources, perhaps including brown dwarfs, must exist. Summary of the main results and conclusions are presented in 7, where prospects for improvements with XMM-Newton and Chandra , are also discussed.

Appendix A gives details about the HRI X-ray source detection, and lists the X-ray detections. Optical finding charts, and identification list of the HRI X-ray sources can be found in Appendix B. Appendix C compares these HRI observations with previous PSPC ones. Appendix D discusses the status of optical/IR counterparts without IR classification.

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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