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Ýlgili Makaleler
Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| RACE-OC project: Rotation and variability of young stellar associations within 100 pc
Context. Examining the angular momentum of stars and its interplay withtheir magnetic fields represent a promising way to probe the stellarinternal structure and evolution of low-mass stars. Aims: Weattempt to determine the rotational and magnetic-related activityproperties of stars at different stages of evolution.We focused ourattention primarily on members of clusters and young stellarassociations of known ages. In this study, our targets are 6 young loosestellar associations within 100 pc and with ages in the range 8-70 Myr:TW Hydrae (~8 Myr), ? Pictoris (~10 Myr), Tucana/Horologium,Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additionalrotational data for ? Persei and the Pleiades from the literatureare also considered. Methods: Rotational periods of starsexhibiting rotational modulation due to photospheric magnetic activity(i.e., starspots) were determined by applying the Lomb-Scargleperiodogram technique to photometric time-series data obtained by theAll Sky Automated Survey (ASAS). The magnetic activity level was derivedfrom the amplitude of the V lightcurves. The statistical significance ofthe rotational evolution at different ages was inferred by applying atwo-sided Kolmogorov-Smirnov test to subsequent age-bins. Results: We detected the rotational modulation and measured the rotationperiods of 93 stars for the first time, and confirmed the periods of 41stars already known from the literature. For an additional 10 stars, werevised the period determinations by other authors. The sample wasaugmented with periods of 21 additional stars retrieved from theliterature. In this way, for the first time we were able to determinethe largest set of rotation periods at ages of ~8, ~10 and ~30 Myr, aswell as increase by 150% the number of known periodic members of AB Dor. Conclusions: The analysis of the rotation periods in youngstellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC2264 data from the literature, has allowed us to find that in the0.6-1.2 M? range the most significant variations in therotation period distribution are the spin-up between 9 and 30 Myr andthe spin-down between 70 and 110 Myr. Variations of between 30 and 70Myr are rather doubtful, despite the median period indicating asignificant spin-up. The photospheric activity level is found to becorrelated with rotation at ages greater than ~70 Myr and to show someadditional age dependence besides that related to rotation and mass.Tables 1.1-1.7 and Figs. 1.1-1.22 are only available in electronic format http://www.aanda.orgBased on theAll Sky Automated Survey photometric data.
| Rotational velocities of nearby young stars
Context. Stellar rotation is a crucial parameter driving stellarmagnetism, activity and mixing of chemical elements. Measuringrotational velocities of young stars can give additional insight in theinitial conditions of the star formation process. Furthermore, theevolution of stellar rotation is coupled to the evolution ofcircumstellar disks. Disk-braking mechanisms are believed to beresponsible for rotational deceleration during the accretion phase, androtational spin-up during the contraction phase after decoupling fromthe disk for fast rotators arriving at the ZAMS. On the ZAMS, stars getrotationally braked by solar-type winds. Aims: We investigate theprojected rotational velocities v sin i of a sample of young stars withrespect to the stellar mass and disk evolutionary state to search forpossible indications of disk-braking mechanisms. Furthermore, we searchfor signs of rotational spin-up of stars that have already decoupledfrom their circumstellar disks. Methods: We analyse the stellarspectra of 220 nearby (mostly <100 pc) young (2-600 Myr) stars fortheir v sin i, stellar age, H? emission, and accretion rates. Thestars have been observed with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile. The spectra have beencross-correlated with appropriate theoretical templates. We build a newcalibration to be able to derive v sin i values from thecross-correlated spectra. Stellar ages are estimated from the Li Iequivalent width at 6708 Å. The equivalent width and width at 10%height of the H? emission are measured to identify accretors andused to estimate accretion rates dot{M}_acc. The v sin i is thenanalysed with respect to the evolutionary state of the circumstellardisks to search for indications of disk-braking mechanisms in accretors. Results: We find that the broad v sin i distribution of ourtargets extends to rotation velocities of up to more than 100 kms-1 and peaks at a value of 7.8 ± 1.2 km s-1, and that 70% of our stars show v sin i < 30 kms-1. Furthermore, we can find indications for disk-braking inaccretors and rotational spin-up of stars which are decoupled from theirdisks. In addition, we show that a number of young stars are suitablefor precise radial-velocity measurements for planet-search surveys.Based upon observations with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile.
| Search for associations containing young stars (SACY). III. Ages and Li abundances
Context: Our study is a follow-up of the SACY project, an extended highspectral resolution survey of more than two thousand opticalcounterparts to X-ray sources in the southern hemisphere targeted tosearch for young nearby association. Nine associations have either beennewly identified, or have had their member list revised. Groupsbelonging to the Sco-Cen-Oph complex are not considered in the presentstudy. Aims: These nine associations, with ages of between about 6Myr and 70 Myr, form an excellent sample to study the Li depletion inthe pre-main sequence (PMS) evolution. In the present paper, weinvestigate the use of Li abundances as an independent clock toconstrain the PMS evolution. Methods: Using our measurements ofthe equivalent widths of the Li resonance line and assuming fixedmetallicities and microturbulence, we calculated the LTE Li abundancesfor 376 members of various young associations. In addition, weconsidered the effects of their projected stellar rotation.Results: We present the Li depletion as a function of age in the firsthundred million years for the first time for the most extended sample ofLi abundances in young stellar associations. Conclusions: A clearLi depletion can be measured in the temperature range from 5000 K to3500 K for the age span covered by the nine associations studied in thispaper. The age sequence based on the Li-clock agrees well with theisochronal ages, the ?Cha association being the only possibleexception. The lithium depletion patterns for the associations presentedhere resemble those of the young open clusters with similar ages,strengthening the notion that the members proposed for these loose youngassociations have indeed a common physical origin. The observed scatterin the Li abundances hampers the use of Li in determining reliable agesfor individual stars. For velocities above 20 km s-1,rotation seems to play an important role in inhibiting the Li depletion.Based on observations collected at the ESO - La Silla and at theLNA-OPD.Tables [see full textsee full text]-[see full textsee full text] areonly available in electronic form at http://www.aanda.org
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Newly-discovered young stars in Carina and Vela
Recent observations have shown that a substantial population ofpre-main-sequence stars exists within 100 pc of the Sun. Such stars areuseful for constraining disk evolution timescales, given their relativeproximity and their ages in the 10--50 Myr range, filling the gapbetween nearby low-mass star-forming regions with ages of a few Myr andZAMS stars at ages of 100 Myr. We present here the latest results fromour continuing search for such stars.In this work, we have undertaken a search for young stars too faint tobe included in the Hipparcos catalog by looking for x-ray bright Tycho-2stars with kinematics similar to the young Hipparcos stars HIP 33111,33455, 46063, and 48558. These stars lie in the vicinity of (but outsidethe conventional boundaries of) the Lower Centaurus Crux (LCC) OBassociation and the ˜30 Myr-old open cluster IC 2391. There issignificant overlap between the stars selected here and those proposedby Makarov & Urban (2000) as the Carina-Vela moving group, thoughthe stars around HIP 33111 and 33455 lie outside the proposed movinggroup, and also farther from LCC.We present high-resolution optical spectra of more than 60 such stars,from which we derive Li abundances, surface gravities, and preciseradial velocities. Many of these stars are late-type pre-main-sequencestars, showing strong x-ray emission (logLx}/L{bol > -4) and strong Li absorption atlevels similar to the late-type stars in IC 2602 and IC 2391. Most ofthe stars that show strong Li absorption also share similar radialvelocities, clustering around vhelio ? +20 km/s.We present a detailed analysis of these stars' ages and kinematics, andwe discuss their relationship to the other young stars and knownstar-forming regions in the vicinity.We gratefully acknowledge the support of this work by the NationalScience Foundation, through grant AST-0307830.
| A moving group of young stars in Carina-Vela
Accurate two-colour photometry and proper motions of 7096 young X-raystars in the ROSAT All-Sky Survey Bright Star Catalogue, version 1RXS,are extracted from the Tycho-2 Catalogue. The sample is dominated by redmain-sequence and possibly pre-main-sequence stars. On a global propermotion convergence map, two features are very prominent: the nearbysection of the Gould Belt and the Hyades convergent point. Theappearance of the Gould Belt feature with its peak at (l=244.3°,b=-12.6°) is quite similar to that of Hipparcos OB stars. When onlystars with proper motions drawing close to that point are selected,strong concentrations of stars in the direction of the Sco-Cen complexare found. Another concentration, not corresponding to any known OBassociation, is detected between the position of the Lower CentaurusCrux and Vela OB2 associations. It is a new young moving group locatedin Carina and Vela, and a near extension of the Sco-Cen complex.Contrary to the classical Gould Belt OB associations, the Carina-Velamoving group has a considerable geometric depth, the closest membersbeing as near as 30pc from the Sun. IC 2391, one of the youngest andclosest open clusters on the sky, is a part of the Carina-Vela movinggroup. The Carina-Vela moving group does not link the Sco-Cen complexwith the Vela OB associations, because the latter is much more distantthan the outer limit of the sample. It is more likely that the younglate-type population of the Scorpio-Centaurus-Carina moving groupstretches towards the Sun and possibly beyond it.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
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| Takýmyýldýz: | Karina |
| Sað Açýklýk: | 09h23m34.98s |
| Yükselim: | -61°11'35.9" |
| Görünürdeki Parlaklýk: | 10.045 |
| özdevim Sað Açýklýk: | -25.7 |
| özdevim Yükselim: | 16.8 |
| B-T magnitude: | 10.94 |
| V-T magnitude: | 10.119 |
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