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2MASS J06164006–6407194: The First Outer Halo L Subdwarf
We present the serendipitous discovery of an L subdwarf in the TwoMicron All Sky Survey (2MASS) J06164006–6407194, in a search ofthe 2MASS for T dwarfs. Its spectrum exhibits features indicative ofboth a cool and metal poor atmosphere including a heavilypressure-broadened K I resonant doublet, Cs I and Rb Ilines, molecular bands of CaH, TiO, CrH, FeH, and H2O, andenhanced collision induced absorption of H2. We assign 2MASSJ0616–6407 a spectral type of sdL5 based on a comparison of itsred optical spectrum to that of near solar-metallicity L dwarfs. Itshigh proper motion (μ = 1farcs405 ± 0.008yr–1), large radial velocity (V rad = 454± 15 km s–1), estimated u, v, wvelocities (94, –573, 125) km s–1 and Galacticorbit with an apogalacticon at ~29 kpc are indicative of membership inthe outer halo making 2MASS J0616–6407 the first ultracool memberof this population.
| Rotation periods of Post-T Tauri stars in Lindroos systems
We present a rotational study of Post-T Tauri stars (PTTSs) in Lindroossystems, defined as binaries with early type primaries on themain-sequence (MS) and late-type secondaries on the pre-main-sequence(PMS) phase. The importance of this study in comparison with previousones is that the Lindroos sample is not X-ray selected so we avoid apossible bias towards fast rotators. In this preliminary study we havemonitored eleven stars in the UBVRI bands during two campaigns of tenconsecutive nights each. Eight of the observed PTTSs show periodicmodulations in their lightcurves and the derived periods range from 1.9d to 8.0 d. The comparison of these results with theoretical rotationaltracks based on disk-star locking theory shows that star-disk decouplingtimes of 1-20 Myr could reproduce the rotational properties of thetargets, assuming an initial rotation period of 8 d and a mass of 1Mȯ. We have studied the rotation-activity relations ofLindroos PTTSs and compared them with those found in other groups of PMSand zero-age main-sequence (ZAMS) 1 Mȯ stars. TheLindroos sample displays activity-rotation relations very similar tothose found in TTSs. It contains a mixture of very active stars, withLX/Lbol ratios close to the saturation level of-3, and less active (unsaturated) stars. This could be the result ofdifferent star-disk decoupling times. Future monitoring of a larger andunbiased sample of PTTS will be important to confirm the significance ofthese results.Based on observations collected at Cerro Tololo Inter-AmericanObservatory and the European Southern Observatory, La Silla, underproject 66.C-0119(B).
| Binary systems with post-T Tauri secondaries
The identification of post-T Tauri (pTT) stars selected throughspectroscopic criteria by Pallavicini et al. (\cite{Pallavicini92}) andby Martín et al. (1992) among the candidates belonging to visualbinary systems is revisited in the present paper by studying theirposition in the HR diagram. These stars belong to the so-called Lindroosbinary sample (Lindroos \cite{Lindroos85}), i.e. to systems withearly-type primaries and late-type companions. If these binaries arephysical and not simply optical pairs, similar ages must be found forthe early-type primary and the late-type component of each system. Theages of these systems have been derived by Lindroos in 1986, by usingcalibrations of the uvbyβ indices. In this paper, we revisit theseages through the position of these stars among new evolutionary tracksin the HR diagram for pre- and post-main sequence stars. We derive newestimations of the ages of each system component, as well as theirmasses, using parallaxes of the early-type component derived fromHipparcos data and by forcing the late-type companion to be at the samedistance. Teff and log g of the early-type components havebeen computed using the calibrations of two independent photometricsystems: the uvbyβ photometry and the Geneva system. TheTeff of the late-type stars have been determined by usingvarious calibrations of several photometric systems: uvbyβ, UBV andVRI, in order to determine the uncertainties and systematic errors onthese parameters and consequently on the ages. Differences in the agesand masses obtained by using various sources of recent evolutionarymodels are considered and discussed. The consistency of the age of thelate type component with that of its early type primary is examined; thevalidity of this criterion for a selection of physical pairs isdiscussed. The accuracy of the observational and theoretical data arenot sufficient to assign stringent values to the age for several of theexamined systems. Nevertheless, in spite of the large error bars, wehave established that we could select a number of systems which,according to their position in the HR diagram, may be physicallyassociated. The selection of possible physically bounded systemsobtained with the present approach and that made by Pallavicini et al.(\cite{Pallavicini92}) or Martín et al. (1992) on the basis ofspectroscopic criteria are not always coincident. Spectroscopiccriteria, for example the presence of a strong Li feature, are morestringent conditions than that of coherent ages of primaries andsecondaries; however the Li I 6708 doublet is expected to fade in thelatest stages of the pre-main sequence life of a star, so that the``oldest" pTTs may not be detected by spectroscopy only. The differentresults so obtained are discussed for each system and we conclude thatthe present approach may be used as a powerful criterion to select newpTT candidates in visual binaries to be observed and analyzed with highresolution spectrographs and to select candidates that have almostreached the main sequence. Partly based on data from the ESA Hipparcosastrometric satellite. Tables 2, 5, 7 and 9 are only available inelectronic form at the 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/379/162
| Spectra of binaries classified as lambda Bootis stars
High angular resolution observations have shown that some starsclassified as lambda Boo are binaries with low values of angularseparation and magnitude difference of the components; therefore theobserved spectrum of these objects is a combination of those twocomponents. These composite spectra have been used to definespectroscopic criteria able to detect other binaries among starsclassified as lambda Boo. The application of this method to HD 111786 ispresented: the contribution of 5 components to the observed spectrum isdemonstrated by the shape of the O I 7774 Å feature. This resultmakes unreliable any attempt to perform an abundance analysis of thisobject which therefore must be definitely rejected from the class of thepeculiar lambda Boo stars. This approach allowed us also to recognizethat the SB2 star HD 153808 is in reality a triple system. Based on datafrom ESO, Observatoire du Pic du Midi, IUE Final Archive and onobservations obtained with the Hipparcos satellite.
| X-ray emission from Lindroos binary systems
We present a study of the X-ray emission from binary systems extractedfrom the Lindroos catalogue (Lindroos 1986) based on the ROSAT All-Skysurvey as well as ROSAT PSPC and HRI pointings. The studied sampleconsists of visual binary systems comprised of early-type primaries andlate-type secondaries. The ages of the systems were determined byLindroos (1985) from uvbybeta photometry of the primaries. These agesrange between 33 and 135 Myr, so if the late-type secondaries arephysically bound to the early-type primaries, they could be Post-T Tauristars (PTTS). We have found strong X-ray emission from severalsecondaries. This fact together with their optical and IR data, makethem bona fide PTTS candidates. We have also detected X-ray emissionfrom several early-type primaries and, in particular, from most of thelate-B type stars. Because their HRI hardness ratios are similar tothose from resolved late-type stars, the presence of an unresolvedlate-type companion seems to be the cause of this emission.
| Evidence for dust around POST T Tauri stars
Post T Tauri stars are young stars which, although older than classicalT Tauri stars, have not yet reached the main sequence. We report thedetection of a far-infrared excess in binary systems consisting of apost T-Tauri secondary and a B-type primary. Assuming this excess arisesfrom dust surrounding the secondary, it is not clear whether the primaryor the secondary, i.e., the post T Tauri star, is the dominant heatingsource. We searched for continuum 1.1 mm emission in a small sample ofcandidate (or confirmed) post T Tauri and post Herbig Ae/Be stars withthe caltech Submillimeter Observatory. In no case did we detect anyemission. The inferred upper limits to the amount of dust present istypically one or two orders of magnitude lower than that found aroundclassical T Tauri/Herbig Ae/Be stars suggesting rapid evolution of thedisk before a star reaches the post T Tauri/Herbig Ae/Be phase.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Visual measurements of southern double stars. II
A group of 248 micrometric observations of 104 double stars arepresented, forming the second series of a program carried out at Cordobawith a 30 cm refractor. Usually neglected IDS pair south of -60 deg areselected for observation, most of them having been measured only one ortwo times previously. The measurements are compared with all the earlierobservations to derive estimates for the personal equation in P.A. andseparation, and the nature of many pairs is discussed by making use ofcatalog proper motions.
| A study of visual double stars with early-type primaries. V - Post-T Tauri secondaries
In an ongoing study of companions of O and B type primaries, themajority of the secondaries are found to be of spectral type F, G, or K.Their properties are investigated in relation to their evolutionarystatus. The ages of the systems have been determined from uvby-betaphotometry of the primaries and it is found that all are younger than150 million years and half are less than 30 million years old.Thirty-seven secondaries have ages less than the expected contractiontime to the zero-age-main sequence (ZAMS). More than 50 percent of theF, G, and K secondaries exhibit spectroscopic features (Ca II H, K andH-alpha emission and strong Li absorption) typical of young stars andreminiscent of T Tauri stars although not as conspicuous. It isappropriate to classify them as post-T Tauri stars. The lifetime of somefeatures characteristic of early stellar evolution is discussed.
| A study of visual double stars with early type primaries. IV Astrophysical data
Astrophysical parameters (MK class, color excess, absolute magnitude,distance, effective temperature, mass, and age) are derived fromcalibrations of the uvby-beta indices for the members of 253 doublestars with O or B type primaries and faint secondaries. The photometricspectral classification is compared to the MK classes, and the agreementis very good. The derived data together with spectroscopic and JHKL dataare used for deciding which pairs are likely to be physical and whichare optical, and it is shown that 98 (34 percent) of the secondaries arelikely to be members of physical systems. For 90 percent of the physicalpairs the projected separation between the components is less than25,000 AU. A majority of the physical secondaries are late-type stars,and 50 percent of them are contracting towards the zero-agemain-sequence. Also presented are new uvby-beta data for 43 secondariesand a computer program for determining astrophysical parameters fromuvby-beta data.
| A study of visual double stars with early type primaries. II - Photometric results
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1983A&AS...51..161L&db_key=AST
| A study of visual double stars with early type primaries. I - Spectroscopic results
Attention is given to spectral peculiarities found in data on thespectral classes of 486 stars in 254 visual doublet or multiplet systemswith O or B type primaries, in order to isolate a group of very youngstars that may serve for the study of early stellar evolutioncharacteristics. It is noted that the material contains a substantialfraction of secondaries that are likely to be physical, and that severalof these may be in the premain-sequence phase of stellar evolution, orhave reached the zero-age main sequence.
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