- ID:
- ivo://CDS.VizieR/J/A+AS/131/197
- Title:
- F, G and K stars BVRI photometry
- Short Name:
- J/A+AS/131/197
- Date:
- 15 Dec 2021 08:12:40
- Publisher:
- CDS
- Description:
- We present accurate BV(RI)c photometry for a sample of F, G and K stars detected in selected areas of the ROSAT all-sky survey (RASS). We have used the photometry, in addition to low-resolution spectroscopy, to estimate spectral classifications, distances and X-ray luminosities. The log(L_X_/L_V_) in the sample lies below -2. Although the sample contains also nearby, inactive stars, it is dominated by active objects. The median X-ray luminosity in our sample is <L_X_>=29.88 and the mean value of the hardness ratios <HR1>=0.13+/-0.35. We compare the derived X-ray luminosity function with similar functions obtained from the serendipitous samples of the Einstein Observatory medium sensitivity survey (EMSS, Cat. <IX/15>) and EXOSAT (Cat. <J/A+AS/115/41>). Our sample is completely consistent with the EMSS sample of solar type stars, indicating that both our sources and the EMSS sources are representative of the high galactic latitude X-ray stellar population. We do not find extremely active stars (log(L_X_)>=32), as are found in the EMSS sample, and we argue that these objects are rare.
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- ID:
- ivo://CDS.VizieR/J/ApJS/117/319
- Title:
- Northern RASS X-ray sources catalog
- Short Name:
- J/ApJS/117/319
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a catalog of optical identifications of a representative sample of northern ({delta})>-9{deg}) ROSAT All-Sky Survey (RASS) sources. A full identification has been carried out for a count-rate- and area-limited complete RASS subsample comprising 674 sources. All sources are within six study areas outside the galactic plane (|b|>19.6{deg}), one area being near the north Galactic pole and one near the north ecliptic pole.
- ID:
- ivo://CDS.VizieR/J/A+A/310/384
- Title:
- Optical brightness of 3 ROSAT Seyfert galaxies
- Short Name:
- J/A+A/310/384
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The printed version of this paper describes the X-ray and optical properties of four new active galaxies discovered in the ROSAT All-Sky Survey. While one object (RX J1257.5+2412) is identified as a BL Lac object, the other three (RX J1239.3+2431, RX J1225.7+2055, and RX J1250.2+1923) are narrow line Seyfert 1 galaxies. Three of these four objects are bright enough to be studied on archival photographic plates of Sonneberg Observatory. The present list contains the individual brightness measurements of these objects all of which turned out to be variable on timescales of weeks to years. Some lightcurves (long-term as well as short-term) are given in the printed version.
- ID:
- ivo://CDS.VizieR/J/A+AS/146/323
- Title:
- RASS young sources around R CrA
- Short Name:
- J/A+AS/146/323
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the ROSAT All-Sky Survey data in a 126 deg^2^ area in and around the CrA star forming region. With low-resolution spectroscopy of unidentified ROSAT sources we could find 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new off-cloud weak-line T Tauri stars may have formed in similar cloudlets, which have dispersed recently. High-resolution spectra of our new T Tauri stars show that they have significantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion - south relative to the galactic plane - of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.
- ID:
- ivo://CDS.VizieR/J/A+A/341/751
- Title:
- ROSAT HRI observations of the Pleiades
- Short Name:
- J/A+A/341/751
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In a deep X-ray survey of the Pleiades open cluster, we use the ROSAT High Resolution Imager to explore a region of the cluster formerly surveyed with the PSPC. These new observations substantially improve upon both the sensitivity and the spatial resolution for this region of the Pleiades, allowing us to detect 18 cluster members not detected before and 16 members not included in the catalogs used in previous surveys. The high sensitivity of the present observations permits us to obtain more stringent upper limits for 72 additional members and also provides sufficient numbers of stars to enable us to explore the dependence of L_x_ on stellar rotation for the slow rotators of the Pleiades. Using the new high sensitivity X-ray observations and the recent rotational measurements we discuss the activity-rotation relationship in the Pleiades solar type stars. We also present new photometric observations of optical counterparts of a number of X-ray sources detected in previous surveys but not yet identified.
- ID:
- ivo://CDS.VizieR/J/AJ/110/1229
- Title:
- ROSAT X-ray survey in NGC 6475
- Short Name:
- J/AJ/110/1229
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A ROSAT X-ray survey, with complementary optical photometry, of the open cluster NGC 6475 has enabled the detection of ~50 late-F to K0 and ~70 K/M dwarf new candidate members, providing the first reliable detection of low-mass stars in this low galactic latitude, 220Myr old cluster. The X-ray observations reported here have a typical limiting sensitivity of L_X_ ~ 10^29^erg/s. The detection frequency of early type cluster members is consistent with the hypothesis that the X-ray emitting early type stars are binary systems with an unseen, low-mass secondary producing the X-rays. The ratio between X-ray and bolometric luminosity among NGC 6475 members saturates at a spectral-type/color which is intermediate between that in much younger and in much older clusters, consistent with rotational spindown of solar-type stars upon their arrival on the ZAMS. The upper envelope of X-ray luminosity as a function of spectral type is comparable to that of the Pleiades, with the observed spread in X-ray luminosity among low-mass members being likely due to the presence of binaries and relatively rapid rotators. However, the list of X-ray selected candidate members is likely biased against low-mass, slowly rotating single stars. While some preliminary spectroscopic information is given in an appendix, further spectroscopic observations of the new candidate members will aid in interpreting the coronal activity among solar-type NGC 6475 members and their relation to similar stars in older and younger open clusters.
