A new method for a simultaneous search for clusters of galaxies in X-ray photon maps and optical galaxy maps is described. The method appears ideally suited for the analysis of the recently proposed wide-angle X-ray missions like DUO and ROSITA. As a first application, clusters are extracted from the 3rd version of the ROSAT All-Sky Survey (<IX/10>) and the Early Date Release of the Sloan Digital Sky Survey (SDSS, <J/AJ/123/567>). The time-consuming computations are performed within the German Astrophysical Virtual Observatory (GAVO). On a test area of 140 square degrees, 75 X-ray clusters are detected down to an X-ray flux limit of 3-5*10^-13^erg/s/cm^2^ in the ROSAT energy band 0.1-2.4keV. The clusters have redshifts z<0.5.
By combining the REFLEX and NORAS cluster data sets with the NVSS radio catalogue, we obtain a sample of 145, z<0.3, X-ray-selected clusters brighter than 3x10^-12^erg/s/cm^-2^ that show a central radio emission above 3mJy. For virial masses M_vir_<10^14.5^M_{sun}_, 11 clusters out of 12 (corresponding to 92 per cent of the systems) are inhabited by a central radio source.
The physical origin of the strong magnetic activity in T Tauri stars and its relation to stellar rotation is not yet well-understood. We investigate the relation between the X-ray activity, rotation, and Rossby number for a sample of young stars in the ~3Myr old cluster IC 348. We use the data of four Chandra observations of IC 348 to derive the X-ray luminosities of the young stars. Basic stellar parameters and rotation rates are collected from the literature. This results in a sample of 82 X-ray detected stars with known rotation periods. We determine the Rossby numbers (i.e. the ratio of rotation period to convective turnover time) of 76 of these stars from stellar structure- and evolution-models for pre-main sequence stars.
Consistent modeling of protoplanetary disks requires the simultaneous solution of both continuum and line radiative transfer, heating and cooling balance between dust and gas and, of course, chemistry. Such models depend on panchromatic observations that can provide a complete description of the physical and chemical properties and energy balance of protoplanetary systems. Along these lines we present a homogeneous, panchromatic collection of data on a sample of 85 T Tauri and Herbig Ae objects for which data cover a range from X-rays to centimeter wavelengths. Datasets consist of photometric measurements, spectra, along with results from the data analysis such as line fluxes from atomic and molecular transitions. Additional properties resulting from modeling of the sources such as disc mass and shape parameters, dust size and PAH properties are also provided for completeness. The purpose of this data collection is to provide a solid base that can enable consistent modeling of the properties of protoplan- etary disks. To this end, we performed an unbiased collection of publicly available data that were combined to homogeneous datasets adopting consistent criteria. Targets were selected based on both their properties but also on the availability of data. Data from more than 50 different telescopes and facilities were retrieved and combined in homogeneous datasets directly from public data archives or after being extracted from more than 100 published articles. X-ray data for a subset of 56 sources represent an exception as they were reduced from scratch and are presented here for the first time. Compiled datasets along with a subset of continuum and emission-line models are stored in a dedicated database and distributed through a publicly accessible online system. All datasets contain metadata descriptors that allow to backtrack them to their original resources. The graphical user interface of the online system allows the user to visually inspect individual objects but also compare between datasets and models. It also offers to the user the possibility to download any of the stored data and metadata for further processing.
We determine the quantitative morphology and star formation properties of galaxies in six nearby X-ray-detected, poor groups using multiobject spectroscopy and wide-field R imaging. The mean recessional velocities of the galaxy groups range from 2843 to 7558km/s. Each group has 1538 confirmed members ranging in luminosity from dwarfs to giants (-13.7>=M_R_-5logh>=-21.9). We measure structural parameters for each galaxy by fitting a PSF-convolved, two-component model to their surface brightness profiles. To compare the samples directly, we fade, smooth, and rebin each galaxy image so that we effectively observe each galaxy at the same redshift (9000km/s) and physical resolution (0.87h^-1^kpc). The structural parameters are combined with [O II] measurements to test for correlations between morphological characteristics and current star formation in these galaxies. We compare results for the groups to a sample of field galaxies.
X-ray surveys contain sizable numbers of star forming galaxies, beyond the AGN which usually make the majority of detections. Many methods to separate the two populations are used in the literature, based on X-ray and multiwavelength properties. We aim at a detailed test of the classification schemes and to study the X-ray properties of the resulting samples. We build on a sample of galaxies selected at 1.4GHz in the VLA-COSMOS survey, classified by Smolcic et al. (2008ApJS..177...14S) according to their optical colours and observed with Chandra. A similarly selected control sample of AGN is also used for comparison. We review some X-ray based classification criteria and check how they affect the sample composition. The efficiency of the classification scheme devised by Smolcic et al. (2008ApJS..177...14S) is such that ~30% of composite/misclassified objects are expected because of the higher X-ray brightness of AGN with respect to galaxies. The latter fraction is actually 50% in the X-ray detected sources, while it is expected to be much lower among X-ray undetected sources. Indeed, the analysis of the stacked spectrum of undetected sources shows, consistently, strongly different properties between the AGN and galaxy samples. X-ray based selection criteria are then used to refine both samples. The radio/X-ray luminosity correlation for star forming (SF) galaxies is found to hold with the same X-ray/radio ratio valid for nearby galaxies. Some evolution of the ratio may be possible for sources at high redshift or high luminosity, tough it is likely explained by a bias arising from the radio selection. Finally, we discuss the X-ray number counts of star forming galaxies from the VLA- and C-COSMOS surveys according to different selection criteria, and compare them to the similar determination from the Chandra Deep Fields. The classification scheme proposed here may find application in future works and surveys.
We investigated the X-ray emission from young stars and brown dwarfs in the {sigma} Orionis cluster ({tau}=~3Ma, d=~385pc) and its relation to mass, the presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory.
We have used all tha available data from the Einstein Observatory Imaging Proportional Counter (IPC), and a critical compilation of catalogued optical data, to measure the 0.16-3.5 keV X-ray emission from 88 K and 169 M stars of luminosity classes IV, V and VI within 25 pc from the Sun. The IPC detected 54 out of the 88 K stars, 70 out of the 138 M stars with M_v_ less than 13.4 (corresponding approximatively to M5), and 15 out of the 31 fainter M stars. We have identified a subsample of surveyed stars that is statistically representative of the population of K and M stars in the solar neighborhood.
The results of ROSAT All-Sky Survey (RASS, see cat. <IX/10>) have been used to investigate the X-ray properties of a complete sample of Abell clusters within a 561 square degree region at high galactic latitude; the mean redshift of the sample is 0.17.
Being fully radiative, stars of spectral type A are not expected to harbor magnetic dynamos and hence such stars are not expected to produce X-ray emission. Indeed, while the X-ray detection rate of such stars in X-ray surveys is low, it is not zero and some of the brighter A-type stars have been detected on different occasions and with different instruments. To study systematically the puzzle of the X-ray emitting A-type stars, we carried out an X-ray study of all A-type stars listed in the Bright Star Catalogue using the ROSAT public data archive. We found a total of 312 bright A-type stars positionally associated with ROSAT X-ray sources; we analyzed the X-ray light curves as well as searched for evidence of RV variations to identify possible late-type companions producing the X-ray emission. In this paper we present a list of X-ray active A-type stars, including the collected data about multiplicity, X-ray luminosity and spectral peculiarities.
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