תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The stellar content of the XMM-Newton bright serendipitous survey
Context: .The comparison of observed counts in a given sky directionwith predictions by Galactic models yields constraints on the spatialdistribution and the stellar birthrate of young stellar populations. TheXMM-Newton Bright Serendipitous Survey (XBSS) is an unbiased survey thatincludes a total of 58 stellar sources selected in the 0.5-4.5 keVenergy band, having a limiting sensitivity of 10-2 cnts-1 and covering an area of 28.10 sq deg. Aims: .Wepresent the results of analysing the stellar content of the XBSS so asto understand the recent star formation history of the Galaxy in thevicinity of the Sun. Methods: .We compared the observations withthe predictions obtained with XCOUNT, a model of the stellar X-raycontent of the Galaxy. The model predicts the number and properties ofthe stars to be observed in terms of magnitude, colour, population andf_x/fv ratio distributions of the coronal sources detectedwith a given instrument and sensitivity in a specific sky direction. Results: .As in other shallow surveys, we observe an excess of starsnot predicted by our Galaxy model. Comparing the colours of theidentified infrared counterparts with the model predictions, we observethat this excess is produced by yellow (G+K) stars. The study of theX-ray spectrum of each source reveals a main population of stars withthe coronal temperature stratification typical of intermediate-agestars. As no assumptions have been made for the selection of the sample,our results must be representative of the entire solar neighbourhood.Some stars show infrared excess due to circumstellar absorption, whichis indicative of youth.The XMM-Newton Bright Serendipitous Survey was conceived by theXMM-Newton Survey Science Center (SSC), a consortium of 10 institutionsappointed by ESA to help the SOC in distinct technical aspects,including the exploitation of the XMM-Newton serendipitous detections(see http://xmmssc-www.star.le.ac.uk/). Tables 2 and 3 are onlyavailable in electronic form at http://www.aanda.org
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| The ROSAT International X-ray/Optical Survey (RIXOS): source catalogue
We describe the ROSAT International X-ray/Optical Survey (RIXOS), amedium-sensitivity survey and optical identification of X-ray sourcesdiscovered in ROSAT high Galactic latitude fields (|b|>28 deg) andobserved with the Position Sensitive Proportional Counter (PSPC)detector. The survey made use of the central 17arcmin of each ROSATfield. A flux limit of3x10-14ergcm-2s-1 (0.5-2keV) wasadopted for the survey, and a minimum exposure time of 8000s wasrequired for qualifying ROSAT observations. X-ray sources in the surveyare therefore substantially above the detection threshold of each fieldused, and many contain enough counts to allow the X-ray spectral slopeto be estimated. Spectroscopic observations of potential counterpartswere obtained of all sources down to the survey limit in 64 fields,totalling a sky area of 15.77deg2. Positive opticalidentifications are made for 94per cent of the 296 sources thusexamined. A further 18 fields (4.44deg2), containing 105sources above the 3x10-14ergcm-2s-1survey limit, are completely optically identified to a higher flux of8x10-14ergcm-2s-1 (0.5-2keV). Opticalspectroscopic data are supplemented by deep CCD imaging of many sourcesto reveal the morphology of the optical counterparts, and objects toofaint to register on Sky Survey plates. The faintest opticalcounterparts have R~22. This paper describes the survey method, andpresents a catalogue of the RIXOS sources and their opticalidentifications. Finding charts based on Sky Survey data are given foreach source, supplemented by CCD imaging where necessary.
| X-ray spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectralproperties of sources detected in the RIXOS survey, which is a large,nearly complete sample of objects detected serendipitously in ROSAT PSPCfields down to a flux limit of 3x10^-14ergcm^-2s^-1 (0.5-2keV). We showthat for X-ray surveys containing sources with low count rate, such asRIXOS, spectral slopes estimated using simple hardness ratios in theROSAT band can be biased. Instead, we analyse three-colour X-ray datausing statistical techniques appropriate to the Poisson regime whichremove the effects of this bias. We also show that the use ofthree-colour data enables some discrimination between thermal andnon-thermal spectra. We have then applied this technique to the RIXOSsurvey to study the spectral properties of the sample. For the AGN wefind an average energy index of 1.05+/-0.05, with no evidence forspectral evolution with redshift. Individual AGN are shown to have arange of properties, including soft X-ray excesses and intrinsicabsorption. Narrow-emission-line galaxies (NELGs) also seem to fit to apower-law spectrum, which may indicate a non-thermal origin for theirX-ray emission. We infer that most of the clusters in the sample have abremsstrahlung temperature >3keV, although some show evidence for acooling flow. The stars deviate strongly from a power-law model but fitto a thermal model. Finally, we have analysed the whole RIXOS sample(extending the flux cut-off to the sensitivity threshold of eachindividual observation) containing 1762 sources to study therelationship between spectral slope and flux. We find that the meanspectral slope of the sources hardens at lower fluxes, in agreement withresults from other samples. However, a study of the individual sourcesdemonstrates that the majority have relatively soft spectra even atfaint flux levels, and the hardening of the mean is caused by theappearance of a population of very hard sources at the lowest fluxes.This has implications for the nature of the soft X-ray background.
| Identification of a Complete Sample of Northern ROSAT All-Sky Survey X-Ray Sources. III. The Catalog
We present a catalog of optical identifications of a representativesample 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. Allsources are within six study areas outside the Galactic plane (| b |> 19.dg6), one area being near the north Galactic pole and one nearthe north ecliptic pole.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | ג'ירף |
| התרוממות ימנית: | 08h03m09.50s |
| סירוב: | +65°08'08.8" |
| גודל גלוי: | 7.702 |
| מרחק: | 114.025 פארסק |
| תנועה נכונה: | -6.6 |
| תנועה נכונה: | -54.2 |
| B-T magnitude: | 8.176 |
| V-T magnitude: | 7.742 |
קטלוגים וכינוים:
|
