Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| The XMM-Newton bright serendipitous survey . Identification and optical spectral properties
Aims.We present the optical classification and redshift of 348 X-rayselected sources from the XMM-Newton Bright Serendipitous Survey (XBS),which contains a total of 400 objects (identification level = 87%).About 240 are new identifications. In particular, we discuss in detailthe classification criteria adopted for the active galactic nuclei(AGNs) population. Methods: By means of systematic spectroscopiccampaigns using various telescopes and through the literature search, wehave collected an optical spectrum for the large majority of the sourcesin the XBS survey and applied a well-defined classification “flowchart”. Results: We find that the AGNs represent the mostnumerous population at the flux limit of the XBS survey(~10-13 erg cm-2 s-1) constituting 80%of the XBS sources selected in the 0.5-4.5 keV energy band and 95% ofthe “hard” (4.5-7.5 keV) selected objects. Galactic sourcespopulate the 0.5-4.5 keV sample significantly (17%) and only marginally(3%) the 4.5-7.5 keV sample. The remaining sources in both samples areclusters/groups of galaxies and normal galaxies (i.e. probably notpowered by an AGN). Furthermore, the percentage of type 2 AGNs (i.e.optically absorbed AGNs with A_V>2 mag) dramatically increases goingfrom the 0.5-4.5 keV sample (f=NAGN 2/N_AGN=7%) to the4.5-7.5 keV sample (f=32%). We finally propose two simple diagnosticplots that can be easily used to obtain the spectral classification forrelatively low-redshift AGNs even if the quality of the spectrum is notgood.Based on observations collected at the Telescopio Nazionale Galileo(TNG) and at the European Southern Observatory (ESO) and on observationsobtained with XMM-Newton, an ESA science mission with instruments andcontributions directly funded by ESA Member States and the USA (NASA).Table 3 is only available in electronic form at http://www.aanda.org
| 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.
| Exploring the X-ray sky with the XMM-Newton bright serendipitous survey
We present here ``The XMM-Newton Bright Serendipitous Survey'', composedof two flux-limited samples: the XMM-Newton Bright Source Sample (BSS,hereafter) and the XMM-Newton ``Hard'' Bright Source Sample (HBSS,hereafter) having a flux limit of f_x≃ 7 × 10-14erg cm-2 s-1 in the 0.5-4.5 keV and 4.5-7.5 keVenergy band, respectively. After discussing the main goals of thisproject and the survey strategy, we present the basic data on a completesample of 400 X-ray sources (389 of them belong to the BSS, 67 to theHBSS with 56 X-ray sources in common) derived from the analysis of 237suitable XMM-Newton fields (211 for the HBSS). At the flux limit of thesurvey we cover a survey area of 28.10 (25.17 for the HBSS) sq. deg. Theextragalactic number-flux relationships (in the 0.5-4.5 keV and in the4.5-7.5 keV energy bands) are in good agreement with previous and newresults making us confident about the correctness of data selection andanalysis. Up to now ˜ 71% (˜ 90%) of the sources have beenspectroscopically identified making the BSS (HBSS) the sample with thehighest number of identified XMM-Newton sources published so far. At theX-ray flux limits of the sources studied here we found that: a) theoptical counterpart in the majority (˜ 90%) of cases has amagnitude brighter than the POSS II limit (R ˜ 21mag);b) the majority of the objects identified so far are broad line AGN bothin the BSS and in the HBSS. No obvious trend of the source spectra (asdeduced from the Hardness Ratios analysis) as a function of the countrate is measured and the average spectra of the ``extragalactic''population corresponds to a (0.5-4.5 keV) energy spectral index of˜ 0.8 (˜ 0.64) for the BSS (HBSS) sample. Based on thehardness ratios we infer that about 13% (40%) of the sources in the BSS(HBSS) sample are described by an energy spectral index flatter thanthat of the cosmic X-ray background. Based on previous X-ray spectralresults on a small subsample of objects we speculate that all thesesources are indeed absorbed AGN with the N_H ranging from a few times1021 up to few times 1023 cm-2. We donot find strong evidence that the 4.5-7.5 keV survey is sampling acompletely different source population if compared with the 0.5-4.5 keVsurvey; rather we find that, as expected from the CXB synthesis models,the hard survey is simply picking up a larger fraction of absorbed AGN.At the flux limit of the HBSS sample we measure surface densities ofoptically type 1 and type 2 AGN of 1.63± 0.25 deg-2and 0.83± 0.18 deg-2, respectively; optically type 2AGN represent 34± 9% of the total AGN population. Finally, wehave found a clear separation, in the hardness ratio diagram and in the(hardness ratio) vs. (X-ray to optical flux ratio) diagram, betweenGalactic ``coronal emitting'' stars and extragalactic sources. Theinformation and ``calibration'' reported in this paper will make theexisting and incoming XMM-Newton catalogs a unique resource forastrophysical studies.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA). The majority of the newoptical spectroscopy data used here have been obtained using thefacilities of the Italian ``Telescopio Nazionale Galileo'' (TNG) and ofthe European Southern Observatory (ESO).Tables \ref{t2}-\ref{t4} and Appendices are only available in electronicform at http://www.edpsciences.org
| 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
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Микроскоп |
| Прямое восхождение: | 21h26m35.84s |
| Склонение: | -44°50'47.7" |
| Видимая звёздная величина: | 7.888 |
| Расстояние: | 114.155 парсек |
| Собственное движение RA: | -79.1 |
| Собственное движение Dec: | -28.2 |
| B-T magnitude: | 8.429 |
| V-T magnitude: | 7.933 |
Каталоги и обозначения:
|
