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- ID:
- ivo://CDS.VizieR/J/ApJ/448/683
- Title:
- Hyades RASS observations
- Short Name:
- J/ApJ/448/683
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the results of a complete X-ray survey of the Hyades cluster region using the ROSAT All-Sky Survey (RASS). Our survey covers over 900deg^2^ of the sky. Over 185 optically identified Hyades were detected down to a limiting X-ray luminosity of ~1-2x10^28^ergs/s (0.1-1.8keV); among solar-like stars, i.e., main-sequence stars of spectral type G, the RASS detection rate is ~90%. The presence of many binary systems in the cluster is a key factor influencing the X-ray luminosity function. Short-period (~ a few days or less) binaries are anomalously X-ray bright, as might be expected; however, the X-ray luminosity functions of K and possibly M binaries of all types are significantly different from their single counterparts, confirming the results of Pye et al. (1994MNRAS.266..798P) for a smaller K star sample drawn from deep ROSAT pointings. Comparison with Einstein Observatory studies of a subset of Hyades stars demonstrates a general lack of significant (> a factor of 2) long-term X-ray variability. This may be the result of the dominance of a small-scale, turbulent dynamo in the younger Hyades stars compared to the large-scale, cyclic dynamo observed in the Sun.
- ID:
- ivo://CDS.VizieR/J/A+A/344/521
- Title:
- LMC variable X-ray sources
- Short Name:
- J/A+A/344/521
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The ROSAT PSPC (Cat. <IX/11>) performed more than 200 pointed observations of the Large Magellanic Cloud (LMC) between 1990 and 1994. Most of the LMC was covered more than once and a large fraction at least ten times, making the PSPC observations an in valuable archive to study long-term time variability in the LMC. We performed a systematic search for variable X-ray sources in the LMC and found 27 sources which showed flux variations by factors between 3 and more than 1000. A comparison of the temporal and spectral characteristics of 15 X-ray sources which are not yet identified optically with those of the previously known sources in our sample allows to draw first conclusions about their nature. Up to six of the unidentified sources are promising candidates for high mass X-ray binaries in the LMC. Six sources are probably foreground stars, two soft objects might be supersoft sources and one source could be a black hole candidate. Most of the X-ray binary candidates are found in the area of the LMC 4 supergiant shell leading to a significant concentration of such systems in that region, consistent with the idea of being born in the stellar formation event which created the shell.
- ID:
- ivo://CDS.VizieR/J/A+AS/139/277
- Title:
- LMC X-ray sources ROSAT PSPC catalogue
- Short Name:
- J/A+AS/139/277
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyzed more than 200 ROSAT PSPC observations in a 10 by 10 degree field centered on the Large Magellanic Cloud (LMC) and performed between 1990 and 1994 to derive a catalogue of X-ray sources. The list contains 758 sources with their X-ray properties. From cross-correlations of the PSPC catalogue with the SIMBAD data base and literature searches we give likely identifications for 144 X-ray sources based on positional coincidence, but taking into account X-ray properties like hardness ratios and source extent. 46 known sources are associated with supernova remnants (SNRs) and candidates in the LMC, most of them already detected by previous X-ray missions.
- ID:
- ivo://CDS.VizieR/J/A+AS/132/155
- Title:
- Main-sequence and subgiants ROSAT data
- Short Name:
- J/A+AS/132/155
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present X-ray data for all main-sequence and subgiant stars of spectral types A, F, G, and K and luminosity classes IV and V listed in the Bright Star Catalogue (Cat. <V/50>) that have been detected as X-ray sources in the ROSAT all-sky survey; In addition to count rates, source detection parameters, hardness ratios, and X-ray fluxes we also list X-ray luminosities derived from Hipparcos parallaxes.
- ID:
- ivo://CDS.VizieR/J/A+A/312/439
- Title:
- New T Tauri stars in Taurus-Auriga
- Short Name:
- J/A+A/312/439
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- On the basis of the ROSAT All-Sky-Survey, a study of the Taurus-Auriga star forming region has been performed in order to search for hitherto undiscovered T Tauri stars. Our study covers an area of about 280 square degrees, located between 4^h^ and 5^h^ in right ascension and between 15deg and 34deg in declination. Identification of ROSAT All-Sky Survey sources in this area by means of optical spectroscopy revealed 2 new classical T Tauri stars (CTTS) and 66 new weak-line-T Tauri stars (WTTS) with W_{lambda}_(H{alpha})<=10A. Additional pointed ROSAT observations led to the identification of 6 more WTTS and 2 CTTS, giving a total of 76 new T Tauri stars. The large area of our study, as compared with previous works, allows us to study the spatial distribution of WTTS in this star forming region. We find the WTTS of our survey to be distributed over the whole region investigated. There is a noticeable decline of the surface density from south to north within our study area, but the spatial distribution extends most probably beyond our study region. No clustering towards the population of T Tauri stars known prior to ROSAT in Taurus-Auriga could be observed. We suggest that the WTTS found in our study might in part be somewhat older than the previously known T Tauri stars in Taurus-Auriga, and that their broad spatial distribution is due to the typical velocity dispersion of a few km/s measured for Taurus T Tauri stars, in which case for some of our WTTS an age on the order of 10^7^years would be required for reaching the observed distances from the Taurus dark clouds. We estimate a WTTS/CTTS ratio of about 6 within our study area, but conclude that because of the different spatial distribution of WTTS and CTTS this ratio will be most probably significantly larger for a more extended area.
- ID:
- ivo://CDS.VizieR/J/A+AS/123/329
- Title:
- New weak-line T Tauri stars in Lupus
- Short Name:
- J/A+AS/123/329
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present first results obtained by a survey of the Lupus star forming region in search of new T Tauri stars. This study has been performed on the basis of deep pointed ROSAT observations in the Lupus dark clouds as well as data from the ROSAT All-Sky-Survey in the surrounding, less obscured regions. Our survey covers an area of about 230 square degrees, located between 15^h^6^m^ and 16^h^24^m^ in right ascension and between -47{deg} and -32{deg} in declination. Identification of ROSAT All-Sky-Survey sources in this area by means of optical spectroscopy revealed 89 T Tauri stars, 86 of them "weak-line" T Tauri stars (WTTS not known from previous studies of this region. Our pointed ROSAT observations led to the identification of 47 more T Tauri stars, giving a total of 136 new T Tauri stars. The large area of our study, as compared with previous works, allows us to study the spatial distribution of WTTS in this star forming region on alarge scale. We find the new WTTS to be distributed over the whole area of our survey, indicating that their spatial distribution might extend well beyond our study area. Contrary to the Lupus T Tauri stars known prior to this study, the WTTS discovered by the ROSAT All-Sky-Survey are not clustered in the regions of highest extinction, i.e. the dark clouds.
- ID:
- ivo://CDS.VizieR/J/A+A/319/184
- Title:
- New WTTS in the Chamaeleon complex
- Short Name:
- J/A+A/319/184
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyse the nature of the optical counterparts of the ROSAT all-sky survey (RASS) X-ray sources identified with new weak-line T Tauri (WTTS) stars in the Chamaeleon star forming region (SFR). The new WTTS are distributed throughout the whole SFR, while the classical T Tauri stars (CTTS) are found only in the cloud cores. Adopting a distance of 150pc we derive the stellar parameters and place the new WTTS in the HR diagram. By comparison with theoretical pre-main sequence (PMS) evolutionary tracks, we find masses in the range of 0.2-2.5M_{sun}_ and ages from a few 10^5^yr to 5x10^7^yr. Many of the youngest WTTS are located far away from the main Chamaeleon dark clouds.
- ID:
- ivo://CDS.VizieR/J/A+A/312/818
- Title:
- NGC 2516 X-ray sources
- Short Name:
- J/A+A/312/818
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The ROSAT PSPC detector was used to obtain a deep pointing with its center on the young open cluster NGC 2516 and total integration time of 9284 sec. Altogether 64 X-ray sources were detected showing strong concentration towards the optical cluster centre. This suggests that most of the sources are actually associated with NGC 2516. 42 of these sources could be identified with optical stars in the field of the cluster; after correction for a mean offset between optical and nominal ROSAT positions of about 13", optical star and X-ray source positions are found to agree, on the average, within about 9", while maximum observed positional differences amount to 27". X-ray sources identified with optical cluster stars include HR 3147 (=HD 66194; B2.5IVe), the visually brightest B star in the cluster, three close visual late B-type binaries (h 4027 A,B, h 4031 A,B, and I 1104 A,B) with projected separations ranging between about 4x10^16^cm and 6x10^16^cm, as well as several (six) chemically peculiar Bp/Ap(Si) and Ap(SrCrEu) stars.
- ID:
- ivo://CDS.VizieR/J/ApJS/129/435
- Title:
- NORAS galaxy cluster survey. I.
- Short Name:
- J/ApJS/129/435
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the construction of an X-ray-selected sample of galaxy clusters for cosmological studies, we have assembled a sample of 495 X-ray sources found to show extended X-ray emission in the first processing of the ROSAT All-Sky Survey. The sample covers the celestial region with declination {delta}>=0{deg} and Galactic latitude |b|>=20{deg} and comprises sources with a count rate >=0.06counts/s and a source extent likelihood of L>=7. In an optical follow-up identification program we find 378 (76%) of these sources to be clusters of galaxies.