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Testing Theoretical Evolutionary Models with AB Doradus C and the Initial Mass Function
We assess the constraints on the evolutionary models of young low-massobjects that are provided by the measurements of the companion AB Dor Cby Close and coworkers and by a new comparison of model-derived IMFs ofstar-forming regions to the well-calibrated IMF of the solarneighborhood. After performing an independent analysis of all of theimaging and spectroscopic data for AB Dor C that were obtained by Close,we find that AB Dor C (which has no methane) is not detected at asignificant level (S/N~1.2) in the SDI data when one narrowband image issubtracted from another but that it does appear in the individual SDIframes, as well as the images at J, H, and Ks. Although ourbroadband photometry for AB Dor C is consistent with that of Close, theuncertainties that we measure are larger. Using the age of τ=75-150Myr recently estimated for AB Dor by Luhman and coworkers, theluminosity predicted by the models of Chabrier and Baraffe is consistentwith the value that we estimate from the photometry for AB Dor C. Wemeasure a spectral type of M6+/-1 from the K-band spectrum of AB Dor C,which is earlier than the value of M8+/-1 reported by Close and isconsistent with the model predictions when a dwarf temperature scale isadopted. In a test of these evolutionary models at much younger ages, weshow that the low-mass IMFs that they produce for star-forming regionsare similar to the IMF of the solar neighborhood. If the masses of thelow-mass stars and brown dwarfs in these IMFs of star-forming regionswere underestimated by a factor of 2 as suggested by Close, then the IMFcharacterizing the current generation of Galactic star formation wouldhave to be radically different from the IMF of the solar neighborhood.Based on observations made with ESO telescopes at the ParanalObservatories under program ID 60.A-9026. This publication makes use ofdata products from the Two Micron All Sky Survey, which is a jointproject of the University of Massachusetts and the Infrared Processingand Analysis Center/California Institute of Technology, funded by theNational Aeronautics and Space Administration and the National ScienceFoundation.
| AB Doradus C: age, spectral type, orbit, and comparison to evolutionary models
We expand upon the results of Close et al. (2005) regarding the young,low-mass object AB Dor C and its role as a calibration point fortheoretical tracks. We present an improved spectral reduction and a neworbital solution with two additional epochs. Our improved analysisconfirms our spectral type of M8 (±1) and mass of 0.090±0.003 M\sun for AB Dor C. Comparing the results forAB Dor C with other young, low-mass objects with dynamical masses wefind a general trend where current evolutionary models tend toover-predict the temperature (or under-predict the mass) for low-massstars and brown dwarfs. Given our precision, there is a ˜99%chance that the mass of AB Dor C is underestimated by the DUSTY tracksin the HR diagram.
| A dynamical calibration of the mass-luminosity relation at very low stellar masses and young ages
Mass is the most fundamental parameter of a star, yet it is also one ofthe most difficult to measure directly. In general, astronomers estimatestellar masses by determining the luminosity and using the`mass-luminosity' relationship, but this relationship has never beenaccurately calibrated for young, low-mass stars and brown dwarfs. Massesfor these low-mass objects are therefore constrained only by theoreticalmodels. A new high-contrast adaptive optics camera enabled the discoveryof a young (50million years) companion only 0.156arcseconds (2.3AU) fromthe more luminous (> 120 times brighter) star AB Doradus A. Here wereport a dynamical determination of the mass of the newly resolvedlow-mass companion AB Dor C, whose mass is 0.090 +/- 0.005 solar masses.Given its measured 1-2-micrometre luminosity, we have found that thestandard mass-luminosity relations overestimate the near-infraredluminosity of such objects by about a factor of ~2.5 at young ages. Theyoung, cool objects hitherto thought to be substellar in mass aretherefore about twice as massive, which means that the frequency ofbrown dwarfs and planetary mass objects in young stellar clusters hasbeen overestimated.
| Long-period variables in the Large Magellanic Cloud - IV. A compendium of northern variables
We present new data for an extensive sample of candidate long-periodvariable stars in a 16-deg^2 field in the northern part of the LMC. Ourdata are derived from COSMOS scans of I-band UK Schmidt plates, and wehave used two techniques to search for variables: first, an extension ofthe interactive methods employed by Reid, Glass & Catchpole;secondly, the semi-automatic methods of variable star detection used toidentify long-period variables in the southern and central parts of theLMC by Hughes. We detect a total of 302 periodic variables, of which 258(and 29 non-periodic variables) are identified through the formertechnique and 186 by the latter method. We compare the results from thetwo methods, and present positions and finding charts for all of thestars identified in our survey. We have also obtained near-infrared(JHK) data for 276 stars, most of which are AGB stars and at least 190of which are LMC Miras. A further 34 variables are also likely to beMiras but currently lack JHK photometry. We discuss theperiod-luminosity relations defined by our observations of the shorterperiod Miras.
| UBV photometry of galactic foreground and LMC member stars. I - Galactic foreground stars
UBV photometry of 955 galactic foreground stars in the direction to theLarge Magellanic Cloud is presented. The stars have been chosen fromforeground star catalogs and have been measured to complete a new database containing entries of more than 5000 stars in the direction of theLMC. First and second order extinction coefficients at La Silla/Chileare given, which differ from the standard values because of the 1991eruption of the volcano Mt. Pinatubo.
| Positional reference stars in the Magellanic Clouds
The equatorial coordinates are determined of 926 stars (mainly ofgalactic origin) in the direction of the Magellanic Clouds at the meanepoch T = 1978.4 with an overall accuracy characterized by the meanvalues of the O-C coordinates, Sa = 0.35 arcsec and Sd = 0.38 arcsec,calculated from the coordinates of the Perth reference stars. Thesevalues are larger than the accuracy expected for primary standard stars.They allow the new positions to be considered as those of reliablesecondary standard stars. The published positions correspond to anunquestionable improvement of the quality of the coordinates provided inthe current catalogs. This study represents an 'astrometric step' in thestarting of a 'Durchmusterung' of the Magellanic Clouds organized by deBoer (1988, 1989).
| Large Magellanic Cloud. 2nd list of L. M. C. members and list of galactic stars
Abstract image available at:http://adsabs.harvard.edu/abs/1973A&AS...10..231F
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