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ASAS Photometry of ROSAT Sources. I. Periodic Variable Stars Coincident with Bright Sources from the ROSAT All Sky Survey
Photometric data from the ASAS - South (declination less than 29°)survey have been used for identification of bright stars located nearthe sources from the ROSAT All Sky Survey Bright Source Catalog (RBSC).In total 6028 stars brighter than 12.5 mag in I- or V-bands have beenselected and analyzed for periodicity. Altogether 2302 variable starshave been found with periods ranging from 0.137 d to 193 d. Most ofthese stars have X-ray emission of coronal origin with a few cataclysmicbinaries and early type stars with colliding winds. Whenever it waspossible we collected data available in the literature to verify periodsand to classify variable objects.The catalog includes 1936 stars (1233 new) considered to be variable dueto presence of spots (rotationally variable), 127 detached eclipsingbinary stars (33 new), 124 contact binaries (11 new), 96 eclipsing starswith deformed components (19 new), 13 ellipsoidal variables (4 new), 5miscellaneous variables and one pulsating RR Lyr type star (blended withan eclipsing binary). More than 70% of new variable stars haveamplitudes smaller than 0.1 mag, but for ASAS 063656-0521.0 we havefound the largest known amplitude of brightness variations due to thepresence of spots (up to ?V=0.8 mag). The table with the compileddata and figures with light curves can be downloaded from Acta AstronomicaArchive.
| Distance determination for RAVE stars using stellar models. III. The nature of the RAVE survey and Milky Way chemistry
We apply the method of Burnett & Binney (2010, MNRAS, 407, 339) forthe determination of stellar distances and parameters to the internalcatalogue of the RAdial Velocity Experiment (RAVE; Steinmetz et al.2006, AJ, 132, 1645). Subsamples of stars that either have Hipparcosparallaxes or belong to well-studied clusters inspire confidence in theformal errors. Distances to dwarfs cooler than ~6000 K appear to beunbiased, but those to hotter dwarfs tend to be too small by ~10% of theformal errors. Distances to giants tend to be too large by about thesame amount. The median distance error in the whole sample of 216 000stars is 28% and the error distribution is similar for both giants anddwarfs. Roughly half the stars in the RAVE survey are giants. The giantfraction is largest at low latitudes and in directions towards theGalactic Centre. Near the plane the metallicity distribution isremarkably narrow and centred on [M/H] = -0.04 dex; with increasing |z|it broadens out and its median moves to [M/H] ? -0.5. Mean age as afunction of distance from the Galactic centre and distance |z| from theGalactic plane shows the anticipated increase in mean age with |z|.
| Random forest automated supervised classification of Hipparcos periodic variable stars
We present an evaluation of the performance of an automatedclassification of the Hipparcos periodic variable stars into 26 types.The sub-sample with the most reliable variability types available in theliterature is used to train supervised algorithms to characterize thetype dependencies on a number of attributes. The most useful attributesevaluated with the random forest methodology include, in decreasingorder of importance, the period, the amplitude, the V-I colour index,the absolute magnitude, the residual around the folded light-curvemodel, the magnitude distribution skewness and the amplitude of thesecond harmonic of the Fourier series model relative to that of thefundamental frequency. Random forests and a multi-stage scheme involvingBayesian network and Gaussian mixture methods lead to statisticallyequivalent results. In standard 10-fold cross-validation (CV)experiments, the rate of correct classification is between 90 and 100per cent, depending on the variability type. The main mis-classificationcases, up to a rate of about 10 per cent, arise due to confusion betweenSPB and ACV blue variables and between eclipsing binaries, ellipsoidalvariables and other variability types. Our training set and thepredicted types for the other Hipparcos periodic stars are availableonline.
| V474 Car: A Rare Halo RS CVn Binary in Retrograde Galactic Orbit
We report the discovery that the star V474 Car is an extremely active,high velocity halo RS CVn system. The star was originally identified asa possible pre-main-sequence star in Carina, given its enhanced stellaractivity, rapid rotation (10.3 days), enhanced Li, and absolutemagnitude which places it above the main sequence (MS). However, itsextreme radial velocity (264 km s-1) suggested thatthis system was unlike any previously known pre-MS system. Our detailedspectroscopic analysis of echelle spectra taken with the CTIO 4 m findsthat V474 Car is both a spectroscopic binary with an orbital periodsimilar to the photometric rotation period and metal-poor ([Fe/H]sime-0.99). The star's Galactic orbit is extremely eccentric (e ~=0.93) with a perigalacticon of only ~0.3 kpc of the Galacticcenter—and the eccentricity and smallness of its perigalacticonare surpassed by only ~0.05% of local F/G-type field stars. Theobserved characteristics are consistent with V474 Car being ahigh-velocity, metal-poor, tidally locked, chromospherically activebinary, i.e., a halo RS CVn binary, and one of only a few such specimensknown.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
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