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 A V % 10 20 mag, but several sources with visual absorptions as high as A V % 40 56 mag were detected. The mean photon energy of a typical source is hEi % 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 f X , log T , and L X 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.

We show that there has been a dramatic decline in the abundance of massive galaxies with strong H stellar absorption lines from z $ 1:2 to the present. These ''H -strong'' (HDS) galaxies have undergone a recent and rapid break in their star formation activity. Combining data from the Gemini Deep Deep and Sloan Digital Sky surveys to make mass-matched samples (M ? > 10 10:2 M ; with 25 and 50,255 galaxies, respectively), we find that the fraction of galaxies in an HDS phase has decreased from about 50% at z ¼ 1:2 to a few percent today. This decrease in fraction is due to an actual decrease in the number density of massive HDS systems by a factor of 2-4, coupled with an increase in the number density of massive galaxies by $30%. We show that this result depends only weakly on the threshold chosen for the H equivalent width (EW ) to define HDS systems (if greater than 4 8) and corresponds to a (1 þ z) 2:5AE0:7 evolution. Spectral synthesis studies of the high-redshift population using the PÉ GASE code, treating H A , EW[O ii], D n 4000, and rest-frame colors, favor models in which the Balmer absorption features in massive HDS systems are the echoes of intense episodes of star formation that faded '1 Gyr prior to the epoch of observation. The z ¼ 1:4 2 epoch appears to correspond to a time at which massive galaxies are in transition from a mode of sustained star formation to a relatively quiescent mode with weak and rare star formation episodes. We argue that the most likely local descendants of the distant massive HDS galaxies are passively evolving massive galaxies in the field and small groups.

Near the corotation resonance of a transient spiral arm, stellar orbital angular momenta may be changed without inducing significant kinematic heating, resulting in what has come to be known as radial migration. When radial migration is very efficient, a large fraction of disk stars experiences significant, permanent changes to their individual orbital angular momenta over the lifetime of the disk, having strong implications for the evolution of disk galaxies. The first step for a star in a spiral disk to migrate radially is to be captured in a "trapped" orbit, associated with the corotation resonance of the spiral pattern. An analytic criterion for determining whether or not a star is in a trapped orbit has previously been derived only for stars with zero random orbital energy in the presence of a spiral with fixed properties. In this first paper in a series, we derive an analytic criterion appropriate for a star that is on an orbit of finite random orbital energy. Our new criterion demonstrates that whether or not a star is in a "trapped" orbit primarily depends on the star's orbital angular momentum. This criterion could be a powerful tool in the interpretation of the results of N-body simulations. In future papers of this series, we apply our criterion to explore the physical parameters important to determining the efficiency of radial migration and its potential importance to disk evolution.

We present the analysis of a 38.4 and 23.6 ks observation of the core of the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory detected 99 X-ray sources in a 17 0 Â 17 0 region, including 18 of 23 Pleiades members. Five candidate Pleiades members have also been detected, confirming their cluster membership. Fifty-seven sources have no optical or near-infrared counterparts to limiting magnitudes V ¼ 22:5 and J ¼ 14:5. The unidentified X-ray sources are probably background active galactic nuclei and not stars. The Chandra field of view contains seven intermediate-mass cluster members. Five of these, HII 980 (B6+G), HII 956 (A7+F6), HII 1284 (A9+K), HII 1338 (F3+F6), and HII 1122 (F4+K), are detected in this study. All but HII 1284 have high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties comparable to nonflaring K-type stars. Since all five stars are visual or spectroscopic binaries with X-ray properties similar to F-G stars, the late-type binary companions are probably producing the observed coronal X-ray emission. Strengthening this conclusion is the nondetection by Chandra of two A stars, HII 1362 (A7, no known companion) and HII 1375 (A0+A SB) with X-ray luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7 stars with cooler companions. Despite the low number statistics, the Chandra data appear to confirm the expectation that late B and A stars are not strong intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual increase in coronal temperature with decreasing mass from F4 to K. M stars appear to have somewhat cooler coronae than active K stars.