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Ýlgili Makaleler
A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars. II. A Survey
We have used archival CCD data, Fourier analysis, and self-correlationanalysis to study the photometric time variability of 162 T Tauri starsand related objects, including Herbig Ae/Be stars. Many show periodicvariability, presumably due to rotation of a spotted star in most cases.For the nonperiodic stars, we have estimated an upper limit to theperiodic variability. We have also analyzed 26 stars suspected to haveperiods longer than about 10 days. Of these 26 stars, at least 14 haveperiods significantly longer than 10 days; their variability may be dueto processes in the disk, or the effect of a companion, since most (butnot all) T Tauri stars have rotational periods less than this value. Fora few of the rotational variables, namely AA Tau, DK Tau, DL Tau, DNTau, GK Tau, GM Aur, Rox 29, V1121 Oph, V410 Tau, and V649 Ori, we havefound long-term variability of the amplitude, presumably due to activitycycles; the time scales are 1500-4000 days.
| A high-resolution spectroscopic survey of late-type stars: chromospheric activity, rotation, kinematics, and age
Aims: We present a compilation of spectroscopic data from asurvey of 144 chromospherically active young stars in the solarneighborhood, which may be used to investigate different aspects of itsformation and evolution in terms of kinematics and stellar formationhistory. The data have already been used by us in several studies. Withthis paper, we make all these data accessible to the scientificcommunity for future studies on different topics. Methods: Weperformed spectroscopic observations with echelle spectrographs to coverthe entirety of the optical spectral range simultaneously. Standard datareduction was performed with the IRAF echelle package. We applied thespectral subtraction technique to reveal chromospheric emission in thestars of the sample. The equivalent width of chromospheric emissionlines was measured in the subtracted spectra and then converted tofluxes using equivalent width-flux relationships. Radial and rotationalvelocities were determined by the cross-correlation technique.Kinematics, equivalent widths of the lithium line ?6707.8 Åand spectral types were also determined. Results: A catalog ofspectroscopic data is compiled: radial and rotational velocities, spacemotion, equivalent widths of optical chromospheric activity indicatorsfrom Ca II H & K to the calcium infrared triplet and the lithiumline in ?6708 Å. Fluxes in the chromospheric emission linesand R'_HK are also determined for each observation of a star in thesample. We used these data to investigate the emission levels of ourstars. The study of the H? emission line revealed two differentpopulations of chromospheric emitters in the sample, clearly separatedin the logFH?/Fbol - (V-J) diagram. Thedichotomy may be associated with the age of the stars.Based on observations made with the 2.2 m telescope of theGerman-Spanish Astronomical Centre, Calar Alto (Almería, Spain),operated jointly by the Max-Planck-Institute for Astronomy, Heidelberg,and the Spanish National Commission for Astronomy; the Nordic OpticalTelescope (NOT), operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway and Sweden, in the Spanish Observatorio delRoque de Los Muchachos of the Instituto de Astrofísica deCanarias; the Isaac Newton Telescope (INT) operated on the island of LaPalma by the Isaac Newton Group in the Spanish Observatorio del Roque deLos Muchachos of the Instituto de Astrofísica de Canarias; withthe Italian Telescopio Nazionale Galileo (TNG) operated on the island ofLa Palma by the Centro Galileo Galilei of the INAF (Istituto Nazionaledi Astrofisica) at the Spanish Observatorio del Roque de Los Muchachosof the Instituto de Astrofísica de Canarias; and with theHobby-Eberly Telescope (HET) operated by McDonald Observatory on behalfof The University of Texas at Austin, the Pennsylvania State University,Stanford University, Ludwig-Maximilians-Universität München,and Georg-August-Universität Göttingen. This research has madeuse of the SIMBAD database and VizieR catalog access tool, operated atCDS, Strasbourg, France.Tables A.1-A.4 and reduced spectra are alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A97
| AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs
Context. The AKARI, a Japanese infrared space mission, has performed anAll-Sky Survey in six infrared-bands from 9 to 180 ?m with higherspatial resolutions and better sensitivities than IRAS. Aims: Weinvestigate the mid-infrared (9 and 18 ?m) point source catalog (PSC)obtained with the infrared camera (IRC) onboard AKARI, in order tounderstand the infrared nature of the known objects and to identifypreviously unknown objects. Methods: Color-color diagramsand a color-magnitude diagram were plotted with the AKARI-IRC PSCand other available all-sky survey catalogs. We combined the Hipparcosastrometric catalog and the 2MASS all-sky survey catalog with theAKARI-IRC PSC. We furthermore searched literature and SIMBADastronomical database for object types, spectral types, and luminosityclasses. We identified the locations of representative stars and objectson the color-magnitude and color-color diagram schemes. Theproperties of unclassified sources can be inferred from their locationson these diagrams. Results: We found that the (B-V) vs.(V-S9W) color-color diagram is useful for identifying thestars with infrared excess emerged from circumstellar envelopes ordisks. Be stars with infrared excess are separated well from other typesof stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W)diagram is a powerful tool for classifying several object types.Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars formdistinct sequences in this color-color diagram. Young stellarobjects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, andplanetary nebulae (PNe) have the largest mid-infrared color excess andcan be identified in the infrared catalog. Finally, we plot the L18W vs.(S9W-L18W) color-magnitude diagram, using the AKARI data togetherwith Hipparcos parallaxes. This diagram can be used to identify low-massYSOs and AGB stars. We found that this diagram is comparable to the [24]vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources usingthe Spitzer Space Telescope data. Our understanding of Galactic objectswill be used to interpret color-magnitude diagram of stellar populationsin the nearby galaxies that Spitzer Space Telescope observed. Conclusions: Our study of the AKARI color-color andcolor-magnitude diagrams will be used to explore properties ofunknown objects in the future. In addition, our analysis highlights afuture key project to understand stellar evolution with a circumstellarenvelope, once the forthcoming astronometrical data with GAIA areavailable.Catalog (full Tables 3 and 4) are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A2
| 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.
| Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association
Context: The associations and moving groups of young stars are excellentlaboratories for investigating stellar formation in the solarneighborhood. Previous results have confirmed that a non-negligiblefraction of old main-sequence stars is present in the lists of possiblemembers of young stellar kinematic groups. A detailed study of theproperties of these samples is needed to separate the young stars fromold main-sequence stars with similar space motion, and identify theorigin of these structures. Aims: Our intention is tocharacterize members of the young moving groups, determine their agedistribution, and quantify the contamination by old main-sequence stars,in particular, for the Local Association. Methods: We used starspossible members of the young (~10-650 Myr) moving groups from theliterature. To determine the age of the stars, we used several suitableage indicators for young main sequence stars, i.e., X-ray fluxes fromthe Rosat All-sky Survey database, photometric data from the Tycho-2,Hipparcos, and 2MASS database. We also used spectroscopic data, inparticular the equivalent width of the lithium line Li i ?6707.8Å and H?, to constrain the range of ages of the stars. Results: By combining photometric and spectroscopic data, we were ableto separate the young stars (10-650 Myr) from the old (>1 Gyr) fieldones. We found, in particular, that the Local Association iscontaminated by old field stars at the level of ~30%. This value must beconsidered as the contamination for our particular sample, and not ofthe entire Local Association. For other young moving groups, it is moredifficult to estimate the fraction of old stars among possible members.However, the level of X-ray emission can, at least, help to separate twoage populations: stars with <200 Myr and stars older than this. Conclusions: Among the candidate members of the classical movinggroups, there is a non-negligible fraction of old field stars thatshould be taken into account when studying the stellar birthrate in thesolar neighborhood. Our results are consistent with a scenario in whichthe moving groups contain both groups of young stars formed in a recentstar-formation episode and old field stars with similar space motion.Only by combining X-ray and optical spectroscopic data is it possible todistinguish between these two age populations.On-line Tables with the data are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/499/129
| 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.
| Identification of New M Dwarfs in the Solar Neighborhood
We present the results from a spectroscopic study of 1080 nearby activeM dwarfs, selected by correlating the Two Micron All Sky Survey andROSAT catalogs. We have derived the spectral types and estimateddistances for all of our stars. The spectral types range between K5 andM6. Nearly half of our stars lie within 50 pc. We have measured theequivalent width of the Hα emission line. Our targets show anincrease in chromospheric activity from early to midspectral types, witha peak in activity around M5. Using the count rate and hardness ratiosobtained from the ROSAT catalog, we have derived the X-ray luminosities.Our stars display a ``saturation-type'' relation between thechromospheric and coronal activity. The relation is such thatlogLX/Lbol remains ``saturated'' at a value ofapproximately -3 for varying Hα equivalent width. We have found568 matches in the USNO-B catalog and have derived the tangentialvelocities for these stars. There is a slight trend of decreasingchromospheric activity with age, such that the stars with highervtan have lower Hα equivalent widths. The coronalemission, however, remains saturated at a value oflogLX/Lbol~-3 for varying tangential velocities,suggesting that the coronal activity remains saturated with age. We donot find any break in the saturation-type relation at the spectral typeat which stars become fully convective (~M3.5). Most of the stars in oursample show more coronal emission than the dMe stars in the Hyades andPraesepe and have vtan<40 km s-1, suggesting ayoung population.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| Tycho-2 stars with infrared excess in the MSX Point Source Catalogue
Stars of all evolutionary phases have been found to have excess infraredemission due to the presence of circumstellar material. To identify suchstars, we have positionally correlated the infrared Mid-Course SpaceExperiment (MSX) Point Source Catalogue and the Tycho-2 opticalcatalogue. Near-mid-infrared colour criteria have been developed toselect infrared excess stars. The search yielded 1938 excess stars; overhalf (979) have never previously been detected by IRAS. The excess starswere found to be young objects such as Herbig Ae/Be and Be stars, andevolved objects such as OH/IR (infrared) and carbon stars. A number ofB-type excess stars were also discovered whose infrared colours couldnot be readily explained by known catalogued objects.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| Peculiarities of the UV Continuum Energy Distribution for T Tauri Stars
In the UV spectra of BP Tau, GW Ori, T Tau, and RY Tau obtained with theHubble Space Telescope, we detected an inflection near 2000 Angstrems inthe F^clambda(lambda) curve that describes the continuumenergy distribution. The inflection probably stems from the fact thatthe UV continuum in these stars consists of two components: the emissionfrom an optically thick gas with T < 8000 K and the emission from agas with a much higher temperature. The total luminosity of the hotcomponent is much lower than that of the cool component, but the hot-gasradiation dominates at lambda < 1800 Angstrems. Previously, otherauthors have drawn a similar conclusion for several young stars fromlow-resolution IUE spectra. However, we show that the short-wavelengthcontinuum is determined from these spectra with large errors. We alsoshow that, for three of the stars studied (BP Tau, GW Ori, and T Tau),the accretion-shock radiation cannot account for the observed dependenceF^clambda(lambda) in the ultraviolet. We argue that more than90% of the emission continuum in BP Tau at lambda > 2000 Angstremsoriginates not in the accretion shock but in the inner accretion disk.Previously, a similar conclusion was reached for six more classical TTau stars. Therefore, we believe that the high-temperature continuum canbe associated with the radiation from the disk chromosphere. However, itmay well be that the stellar chromosphere is its source.
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| The Dispersal of Young Stars and the Greater Sco-Cen Association
We review topics related to the dispersal of young stars from theirbirth-sites, and focus in particular on the entourage of young starsrelated to the ongoing star-formation event in the Sco-Cen OBassociation. We conduct a follow-up kinematic study to that presented inMamajek, Lawson, & Feigelson (2000; ApJ 544, 356) amongst nearby,isolated, young stars. In addition to the eta Cha and TW Hya groups, wefind several more intriguing Sco-Cen outlier candidates: most notablyβ Pic, PZ Tel, HD 199143, and HD 100546. We discuss the connectionbetween Sco-Cen and the southern ``150 pc Conspiracy'' molecular clouds,and in particular, Corona Australis. The kinematic evidence suggeststhat many of the nearby, isolated ~10 Myr-old stars were born nearSco-Cen during the UCL and LCC starbursts 10-15 Myr ago. We hypothesizethat these stars inherited 5-10 km/s velocities moving away fromSco-Cen, either through molecular cloud turbulence, or through formationin molecular clouds associated with the expanding Sco-Cen superbubbles(e.g. Loop I).
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 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/Abstract.html
| The far-UV spectrum of T Tauri stars - I. The relevance of the IUE Newly Extracted Spectra
The far-UV spectrum of the T Tauri stars (TTSs) provides important cluesabout the structure of the stellar atmospheres, winds and accretionshocks. The IUE (International Ultraviolet Explorer) Final Archivecontains the most complete data base for such studies. A new extractionsystem, the IUE Newly Extracted Spectra (ines), has been developed toovercome the disadvantages of the extraction system used in the IUEFinal Archive, the Signal Weighted Extraction Technique (swet). We havecompared the ines spectra of the whole sample of TTSs in the far-UVrange (1200-2000Å) with the swet low-resolution spectra availablein the IUE Final Archive. Although in most of the cases there is a goodagreement between both samples, an important enhancement of the inesline fluxes with respect to the swet line fluxes is reported forparticular spectra. The line fluxes are enhanced by as much as a factorof ~2.5 in some objects, which is significant for variability studies ofTTSs because the variations of the UV lines are typically of this order.The emission-measure distributions built to study the atmospheres ofthese stars are based on the UV emission line fluxes, so the new systemis susceptible to introduce changes in these models. Moreover, thenon-linear enhancement of the ines line fluxes produces variations indiagnostic line ratios usually taken as temperature and density tracersin late-type stars. These line ratios can vary by as much as a factor of3 when the ines data are compared with the swet, with the subsequentvariation of the physical parameters derived from them.
| HD 98800: A Unique Stellar System of Post-T Tauri Stars
HD 98800 is a system of four stars, and it has a large infrared excessthat is thought to be due to a dust disk within the system. In thispaper we present new astrometric observations made with Hipparcos, aswell as photometry from Hubble Space Telescope WFPC2 images. Combiningthese observations and reanalyzing previous work allow us to estimatethe age and masses of the stars in the system. Uncertainty in these agesand masses results from uncertainty in the temperatures of the stars andany reddening they may have. We find that HD 98800 is most probablyabout 10 Myr old, although it may be as young as 5 Myr or as old as 20Myr. The stars in HD 98800 appear to have metallicities that are aboutsolar. An age of 10 Myr means that HD 98800 is a member of the post-TTauri class of objects, and we argue that the stars in HD 98800 can helpus understand why post-T Tauris have been so elusive. HD 98800 may haveformed in the Centaurus star-forming region, but it is extraordinary inbeing so young and yet so far from where it was born.
| Photometric variability of southern T Tauri stars
We present photometric light curves of T Tauri Stars (TTS) observed inthe Southern hemisphere. Our list includes 26 TTS, half of which werediscovered by the Pico dos Dias survey (PDS). The majority of theobserved PDS stars are weak TTS and present, on average, the low rangeof variability typical of such stars. We monitored some of the stars forseveral seasons in order to detect possible changes in rotationalmodulation. In particular, we find that the classical TTS, AS 216, showsa stable period of 3.12d for three consecutive years (between 1985 and1987) which then does not reveal itself during the observing season of1989 and 1990. Coupled with this change during the last two observingseasons is an overall increase in the amplitude of the photometricvariability in all colors. We derive photometric periods for 13previously unmonitored objects, and confirm the previously publishedrotational period of SY Cha. In addition, we confirm that TW Cha and SZ82 do not reveal any signs of periodic modulation. Based on observationsmade at Laboratorio Nacional de Astrofisica/CNPq/MCT, Brazil. Thecomplete data set of the photoelectric photometry is only available inelectronic form at the CDS, via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Parallaxes and proper motions of prototypes of astrophysically interesting classes of stars
Hipparcos data are presented for 13 stars belonging to six differentevolutionary types that present various puzzles. There are three FKComae stars, one Wolf-Rayet binary, the 'galloping giant' FG Sagittae,two high-velocity B stars, two runaway T Tauri stars, and fourcataclysmic variables. Most of the numbers are of limited statisticalsignificance, because even the best-known examples of rare classes ofstars are likely to be distant and faint. In most cases, the stars areconfirmed as being more or less what was expected. A few are not. Wepresent these data primarily to call attention to the fact that the118,226 stars in the Hipparcos Catalog include some that are ofindiviual, as well as of statistical, interest.
| Evidence for Cleared Regions in the Disks Around Pre-Main-Sequence Spectroscopic Binaries
Combining new infrared photometry with data in the literature, we havecompiled spectral energy distributions (SEDs) from optical to millimeterwavelengths for four low-mass pre-main-sequence spectroscopic binaries(a < 1 AU) showing infrared excesses. Three of these binaries havesmall or no excess emission at lambda = 1-5 \micron\ but large excessemission at longer wavelengths, suggesting cleared regions withincircumbinary disks. The inferred sizes of these gaps or holes areconsistent with theoretical predictions for dynamical clearing bystellar companions. In contrast, the binary AK Sco has a power-law SEDwith no evidence of reduced near-infrared emission. Nonetheless, astrong 9.8 \micron silicate emission feature suggests the presence ofoptically thin dust, and the near-infrared emission can be produced byvery small amounts ( ~ 10(-9) \msun) of material within a gap or hole.Thus a dynamically cleared region may exist in AK Sco but is notrequired by the data. That three of these binaries (and several othershort-period pre-main-sequence binaries) have near-infrared excessesindicative of hot material near the stars is significant given thatcircumstellar disk radii are limited to less than 0.06 AU (or severalstellar radii). The spatial distribution of the near-infrared-emittingmaterial may not be disk-like. Continued replenishment is necessary,perhaps from circumbinary disks. Circumbinary disk masses aroundshort-period pre-main-sequence binaries range from 0.06 \msun\ to lessthan 10(-4) \msun. The larger disk masses are sufficient tosubstantially influence the binaries' orbital eccentricities duringtheir pre-main-sequence lifetimes.
| Spottedness of the emission-line dwarf stars BF CVn, DT Vir, EQ Vir, and V1396 Cyg from photoelectric and photographic observations
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| Dispersed T Tauri Stars and Galactic Star Formation
Existing samples of low-mass T Tauri stars from nearby star formingregions are very deficient in stars older than 2 Myr. We argue that thisarises from the dispersal of stars outside well-surveyed regions and isnot due to a burst of star formation, erroneous theoretical isochrones,or survey flux limits. Evidence is accumulating, most dramatically fromthe ROSAT All-Sky Survey, that a large population of weak- lined T Tauri(WTT) stars is widely dispersed within and around star formingcomplexes.The spatial distribution, age distribution, and kinematics of T Tauristars, both close to and widely distributed around active clouds, arestudied using simple models of T Tauri dispersal. Models are compared toobservations of the Chamaeleon and Taurus-Auriga cloud complexes. Thedispersal of T Tauri stars appears to have two major causes: slowisotropic drifting of stars away from long-lived star forming clouds,and star formation in short-lived rapidly moving cloudlets. The firstmechanism is determined by the ?v ?1 km s-1thermal velocity dispersion of gas within molecular cloud cores. Thesecond mechanism is determined by the large-scale turbulent motions ofmolecular cloud complexes. A third mechanism for dispersal, dynamicalejection of high-velocity T Tauri stars, appears to be less important.The results have a number of implications for star formation in theGalaxy: star formation in at least one cloud (Chamaeleon I) has beencontinuous for ?20 Myr; star formation efficiencies of clouds mayoften be 20% or higher; a large fraction of low-mass stars may form insmall short-lived cloudlets each producing no more than a few stars; andT Tauri kinematics support molecular evidence for large-scale turbulencein molecular clouds.
| The Connection between Submillimeter Continuum Flux and Binary Separation in Young Binaries: Evidence of Interaction between Stars and Disks
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...458..312J&db_key=AST
| Spottedness of the emission-line dwarfs V775 Her, VY Ari, OU Gem, and V1005 Ori in 1974-1993
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| Coronal X-ray emission and rotation of cool main-sequence stars
We analyze the coronal X-ray emission of single main sequence stars ofspectral type F through M with photometrically (CaII H+K or broad-bandphotometry) determined rotation periods, using X-ray data from the ROSATall-sky-survey. Our sample contains both field stars in the solarneighborhood and members of the Pleiades and Hyades open clusters. Fieldstars and members of the two young open clusters follow the samerotation-activity relation, i.e., we find no intrinsic dependence ofcoronal activity on age. Assuming a power law relationship betweencoronal X-ray emission and stellar rotation, we estimate a power lawindex close to unity. With a high level of confidence (alpha = 0.99), wefind a qualitative change in behavior around Rossby number valuesR0 approximately equals 1. For R0 greater than 1,coronal activity drops more rapidly with increasing Rossby number as forR0 less than 1. Assuming an exponential relation between theLx/Lbol ratios and Rossby number, R0approximately equals 1/3 is the characteristic Rossby number for a dropof X-ray activity.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| The estimation of spottedness of three red dwarves (sic) stars : MS Ser, FK Ser, V775 HER by using multicolour photoelectric observations.
Not Available
| Optical Spectroscopy of the Dusty K5 V Star HD 98800
Not Available
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Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Yilan |
| Sað Açýklýk: | 18h20m22.74s |
| Yükselim: | -10°11'13.6" |
| Görünürdeki Parlaklýk: | 11.004 |
| özdevim Sað Açýklýk: | -3 |
| özdevim Yükselim: | -42.8 |
| B-T magnitude: | 13.096 |
| V-T magnitude: | 11.177 |
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