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The Chandra Variable Guide Star Catalog
Variable stars have been identified among the optical-wavelength lightcurves of guide stars used for pointing control of the Chandra X-rayObservatory. We present a catalog of these variable stars along withtheir light curves and ancillary data. Variability was detected to alower limit of 0.02 mag amplitude in the 4000-10000 Å range usingthe photometrically stable Aspect Camera on board the Chandraspacecraft. The Chandra Variable Guide Star Catalog (VGUIDE) contains827 stars, of which 586 are classified as definitely variable and 241are identified as possibly variable. Of the 586 definite variable stars,we believe 319 are new variable star identifications. Types of variablesin the catalog include eclipsing binaries, pulsating stars, and rotatingstars. The variability was detected during the course of normalverification of each Chandra pointing and results from analysis of over75,000 guide star light curves from the Chandra mission. The VGUIDEcatalog represents data from only about 9 years of the Chandra mission.Future releases of VGUIDE will include newly identified variable guidestars as the mission proceeds. An important advantage of the use ofspace data to identify and analyze variable stars is the relatively longobservations that are available. The Chandra orbit allows forobservations up to 2 days in length. Also, guide stars were often usedmultiple times for Chandra observations, so many of the stars in theVGUIDE catalog have multiple light curves available from various timesin the mission. The catalog is presented as both online data associatedwith this paper and as a public Web interface. Light curves with data atthe instrumental time resolution of about 2 s, overplotted with the databinned at 1 ks, can be viewed on the public Web interface and downloadedfor further analysis. VGUIDE is a unique project using data collectedduring the mission that would otherwise be ignored. The stars availablefor use as Chandra guide stars are generally 6-11 mag and are commonlyspectral types A and later. Due to the selection of guide stars entirelyfor positional convenience, this catalog avoids the possible bias ofsearching for variability in objects where it is to be expected.Statistics of variability compared to spectral type indicate theexpected dominance of A-F stars as pulsators. Eclipsing binaries areconsistently 20%-30% of the detected variables across all spectraltypes.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
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Osservazione e dati astrometrici
| Costellazione: | Ercole |
| Ascensione retta: | 17h22m47.77s |
| Declinazione: | +26°31'25.6" |
| Magnitudine apparente: | 9.11 |
| Moto proprio RA: | -6.8 |
| Moto proprio Dec: | 7.6 |
| B-T magnitude: | 11.057 |
| V-T magnitude: | 9.271 |
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