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מאמרים קשורים
Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| 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.
| XMM-Newton probes the stellar population in Chamaeleon I South
We report on a 30 ks XMM-Newton observation of the central region of theCha I star forming cloud. The field includes a substantial fraction ofthe known pre-main-sequence population of Cha I South, including allthirteen known very-low mass Hα emitters. We detect two bona-fidebrown dwarfs (spectral types M 7.5 and M 8) and seven Hα emittingobjects near the hydrogen burning mass limit, including six of sevenearlier detections by ROSAT. Three objects classified as Cha I candidatemembers according to their NIR photometry are revealed by XMM-Newton,providing further evidence for them being truly young stars. A total of11 new X-ray sources without known optical/IR counterpart may comprisefurther as yet unrecognized faint cloud members. Spectral analysis ofthe X-ray bright stars shows that previous X-ray studies in Cha I haveunderestimated the X-ray luminosities, as a result of simplifiedassumptions on the spectral shape. In particular, the extinction isvariable over the field, such that the choice of a uniform value for thecolumn density is inappropriate. We establish that the X-ray saturationlevel for the late-type stars in Cha I is located nearLx/Lbol ˜ 10-2.5, with a possibledecline to Lx/Lbol ˜ 10-3 for thelowest mass stars. A group of strongly absorbed stars with unusuallyhard X-ray emission is clustered around HD 97048, a HAeBe star and theonly confirmed intermediate-mass star in the field. While the X-rayproperties of HD 97048 are indistinguishable from those of itslower-mass neighbors, another presumably A-type star (identified as suchbased on NIR photometry) stands out as the softest X-ray emitter in thewhole sample. This suggests that various X-ray emission mechanisms maybe at work in intermediate-mass pre-main-sequence stars. We find thatX-ray luminosity follows a tight correlation with age, effectivetemperature, and mass. No dramatic changes in these correlations areseen at the substellar boundary, suggesting that the same dynamomechanism operates in both low-mass stars and brown dwarfs, at least atyoung ages. The variability of the lowest-mass objects is also similarto that of higher-mass T Tauri stars. X-ray flares are seen in about1/10th of the Cha I members in the field.Tables \ref{tab:x-sources}-\ref{tab:lx} 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
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| Discovery of a New Companion and Evidence of a Circumprimary Disk: Adaptive Optics Imaging of the Young Multiple System VW Chamaeleon
Since a majority of young low-mass stars are members of multiplesystems, the study of their stellar and disk configurations is crucialto our understanding of both star and planet formation processes. Herewe present near-infrared adaptive optics observations of the youngmultiple star system VW Chamaeleon. The previously known 0.7" binary isclearly resolved already in our raw J- and K-band images. We report thediscovery of a new faint companion to the secondary, at an apparentseparation of only 0.1", or 16 AU. Our high-resolution photometricobservations also make it possible to measure the J-K colors of each ofthe three components individually. We detect an infrared excess in theprimary, consistent with theoretical models of a circumprimary disk.Analytical and numerical calculations of orbital stability show that VWCha may be a stable triple system. Using models for the age and totalmass of the secondary pair, we estimate the orbital period to be 74 yr.Thus, follow-up astrometric observations might yield direct dynamicalmasses within a few years and constrain evolutionary models of low-massstars. Our results demonstrate that adaptive optics imaging inconjunction with deconvolution techniques is a powerful tool for probingclose multiple systems. Based on observations collected at the EuropeanSouthern Observatory, Chile.
| A 3 μM Survey of the Chamaeleon I Dark Cloud
We describe an L-band photometric survey of ~ 0.5 deg2 of theChamaeleon I dark cloud. The survey has a completeness limit ofL<11.0. Our survey detects 124 sources, including all knownpre-main-sequence stars with L<=11. The fraction of sources withnear-IR excess emission is 58%+/-4% for K=9-11. Cha I sources have bluerH-K and K-L colors than pre-main-sequence stars in Taurus-Auriga. Thesesources also have a strong correlation between EW(Hα) and K-L.Stars with K-L<=0.6 have weak Hα emission; stars withK-L>=0.6 have strong Hα emission. Because many Cha I sourcesare heavily reddened, this division between weak emission T Tauri starsand classical T Tauri stars occurs at a redder K-L than inTaurus-Auriga. Based on observations obtained with the SPIREX/Abu systemin Antarctica. SPIREX/Abu was operated for the 1999 observing seasonunder agreement between the National Optical Astronomy Observatories(NOAO) and the Center for Astrophysical Research in Antarctica (CARA).
| On the distance to the Chamaeleon I and II associations
Constraints on the distances to the dark clouds Chamaeleon I and II areinvestigated in detail. A compilation of photometric data, spectraltypes and absolute magnitudes for field stars towards each cloud ispresented, and results are used to examine the distribution of reddeningwith distance along each line of sight. The distances to starsassociated with reflection nebulae in each cloud are examined in detail.On the basis of these results, we deduce the most probable distance ofCha I to be 160+/-15pc, and that of Cha II to be 178+/-18pc. Anexamination of the mean fluxes of T Tauri stars in each cloud providesindependent evidence to suggest that Cha II is significantly moredistant than Cha I. Both clouds appear to be embedded in a macroscopicsheet-like structure extending over much of the Chamaeleon-Musca-Cruxregion. The Chamaeleon III and DC\ts 300.2--16.9 clouds are probablypart of the same structure, with probable distances ~ 140--160pc.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| A Study of the Chamaeleon-I Dark Cloud and T-Association - Part Six - Interstellar Polarization Grain Alignment and Magnetic Field
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.268....1W&db_key=AST
| ROSAT X-Ray Study of the Chamaeleon I Dark Cloud. I. The Stellar Population
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...416..623F&db_key=AST
| A study of the Chamaeleon dark cloud and T-association. I - Extinction, distance and membership
The results of an observing program to investigate the nature and degreeof reddening of a number of stars in the field towards the dark cloudand T-association in Chamaeleon are presented. The observations compriseoptical (UBVRI) and infrared (JHK) photometry, and optical spectroscopy.New and previously published data are combined to assess membershipcriteria for 110 stars, and separate catalogues of association membersand field stars are presented. Optical identifications are listed for 27IRAS point sources in the region. The reddening/distance relation forfield stars indicates a distance to the cloud of 140 + or - 12 pc. Thisresult confirms that the extinction law is anomalous towards some starsin the cloud, notably the embedded A0 ZAMS star HD 97300, with values ofup to 5.5 for the ratio of total-to-selective extinction. A comparisonof 21-cm data with the reddening of background field stars indicatesthat the ratio N(H I)/E(B-V) is significantly less than the interstellarmean, indicating that most of the hydrogen in the cloud is molecular.
| Intermediate band, H-beta, and RI photometry of a large sample of stars unbiased with respect to their motion. I - The F-type stars
Intermediate band and H-beta data from a sample of 1000 stars earlierthan type G2 are presented and discussed relative to the stars'luminosities, metal abundances, and motion. Two groups of stars areconsidered, one within 10 arcsec of the South Galactic Pole and 8.5-8.6mag stars contained in the Moore-Paddock (1950) and Wayman (1960)samples (MP-W). The MP-W stars are mainly old disk population stars withmetal abundances of 4-0.25 solar values. Possible age-abundancedistribution-luminosity connections are explored, as are techniques forseparating old and young disk population objects.
| A study of the Chamaeleon dark cloud complex - Survey, structure and embedded sources
IR (JHK) photometry of CVF spectrophotometry of the sources found in a2-micron survey of a significant portion of the Chamaeleon dark cloudhas detected, through discrimination between background field stars andcloud members, an increase in the number of pre-main sequence (PMS)members, over that found optically, by more than 50%. This correspondsto a star formation efficiency of about 12% over the whole cloud, andsimilarly large numbers of hidden PMS objects are probably also presentin other cloud regions. The density structure of the cloud, derived fromthe reddening of background field stars, shows that small scaleclustering of small groups of PMS members occur preferentially nearsteep density gradients in the cloud. Dust shell models for reasonableoptical depths suggest that the observation of IR radiation from T Tauriobjects is due to some noncontinuous form of the shell densitydistribution.
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משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | זיקית |
| התרוממות ימנית: | 11h09m18.16s |
| סירוב: | -77°47'39.9" |
| גודל גלוי: | 8.502 |
| מרחק: | 93.897 פארסק |
| תנועה נכונה: | 38.4 |
| תנועה נכונה: | -25.1 |
| B-T magnitude: | 9.047 |
| V-T magnitude: | 8.547 |
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