Simultaneous Chandra and Very Large Array Observations of Young Stars and Protostars in ? Ophiuchus Cloud Core A

Abstract

A 96 ks Chandra X-ray observation of ? Ophiuchus cloud core A detected 87 sources, of which 60 were identified with counterparts at other wavelengths. The X-ray detections include 12 of 14 known classical T Tauri stars (CTTSs) in the field, 15 of 17 known weak-lined TTSs (WTTSs), and 4 of 15 brown dwarf candidates. The X-ray detections are characterized by hard, heavily absorbed emission. Most X-ray detections have visual extinctions in the range AV ? 10-20 mag, but several sources with visual absorptions as high as AV ? 40-56 mag were detected. The mean photon energy of a typical source is E ? 3 keV, and more than half of the detections are variable. Prominent X-ray flares were detected in the unusual close binary system Oph S1, the X-ray-bright WTTS DoAr 21, and the brown dwarf candidate GY 31 (M5.5). Time-resolved spectroscopic analysis of the DoAr 21 flare clearly reveals a sequence of secondary flares during the decay phase that may have reheated the plasma. We find that the X-ray luminosity distributions and spectral hardnesses of CTTSs and WTTSs are similar. We also conclude that the X-ray emission of detected brown dwarf candidates is less luminous than TTSs, but spectroscopically similar. Simultaneous multifrequency Very Large Array (VLA) observations detected 31 radio sources at 6 cm, of which 10 were also detected by Chandra. We report new radio detections of the optically invisible IR source WLY 2-11 and the faint H? emission line star Elias 24 (class II). We confirm circular polarization in Oph S1 and report a new detection of circular polarization in DoAr 21. We find no evidence that X-ray and radio luminosities are correlated in the small sample of TTSs detected simultaneously with Chandra and the VLA. We describe a new nonparametric method for estimating X-ray spectral properties from unbinned photon event lists that is applicable to both faint and bright X-ray sources. The method is used to generate fX, log T, and LX light curves. In addition, we provide a publicly available electronic database containing multiwavelength data for 345 known X-ray, IR, and radio sources in the core A region.