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Dynamical Masses of Pre-Main Sequence Visual Binary Stars
Q.M. Konopacky , A.M. Ghez , F. Altenbach , C. McCabe The most fundamental parameter in determining the evolution of a star is its mass; unfortunately, it is also difficult to measure directly. Typically one instead uses theoretical models to calculate the mass of a star. However, the most popular models [1-6] give masses and ages are systematically discrepant by up to factors of two and ten, respectively, in the pre-main sequence (PMS). This obvious disparity stems from differences in the treatment of convection, opacity, equations of state, and atmospheres [7]. Thus, in spite of continual improvement in the calculation of these quantities over the past 15 years, the ”correct” model remains a debatable topic. In order to calibrate these models, it is essential to have well-determined dynamical masses for comparison. The current number of PMS stars with dynamical mass estimates is small, and these measurements are riddled with uncertainities. Worse, there are no definitive mass estimates for stars 0.3 M (spectral types later than M2 at 2 Myrs), which encompasses the critical stellar/substellar boundary. Thus, it is imperative that the number of well-determined dynamical masses in the PMS be increased, particularly on the very low mass end. We present here the most recent results of our ongoing monitoring campaign of young, low-mass, visual binary stars using high angular resolution techniques. The goal of this project is to derive astrometric orbits for these stars and thereby enable further calibration of PMS evolutionary tracks. Our observations have been obtained using speckle interferometry on the Keck I 10 m telescope and adaptive optics on the Keck II 10 m telescope. The requirements constraining the observed sample are that the binaries are resolvable with Keck and that they have periods that allow for the determination of their orbital solutions within a reasonable period of time. These requirements lead to a sample that consists of bi naries with 0 02 (D / 140 pc) separation 0 1 (D / 140 pc). Our sample can be broken down into three major component. The first component of the sample is composed of 11 stars in the Taurus-Auriga star forming region and 10 stars the Ophiuchus and Scorpius-Centaurus star forming regions that where identified more than a decade ago. The advantage 1 UCLA Division of Astronomy and Astrophysics, Los Angeles, CA 90095-1562; quinn,
[email protected] 2 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1565 3 Department of Astronomy, California Institute of Technology, Pasadena, CA 91125;
[email protected],
[email protected] 4 current address: NASA Jet Propulsion Laboratory, Pasadena, CA 91109-8099;
[email protected] 5 current address: Laboratoire d’Astrophysique, Observatoire de Grenoble, Universite Joseph Fourier, Grenoble Cedex 9, France;
[email protected] 6 Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Avenue L-413, Livermore, CA 94551;
[email protected]
, G. Duchene
, R.J. White , B.A. Macintosh
X
DF Tau
X
MHO 8
TWA 5A
Fig. 1.— Orbital solution for three stars in our sample (north is up, east is to the left). In the top panel is the star DF Tau, a member of the PMS solar analog component of our sample. The middle panel shows the PMS M dwarf binary MHO 8, a recently identified binary in Taurus. Finally, the bottom panel shows the orbital solution for TWA 5A, a PMS star in the nearby TW Hydrae association. The lines from the data points to the ellipse indicate where the fit believes the point should lie on the orbit. The filled circles are points from our Keck data set, while the unfilled points are taken from the literature [8,12-22].
Protostars and Planets V 2005
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of this sample is the significant time baseline over which observations have been obtained [8-9], allowing us to calculate orbital solutions for binaries with periods of up to 80 years. This assumes that only one-quarter of the orbit needs to be observed for adequate orbital parameter estimation. However, stars in this subsample are also typically solar analogs, meaning that while their orbital solutions will be useful, they will not allow us to probe currently untested regions of the evolutionary tracks. More recently, we have added a new component to our survey, consisting of 10 M dwarf close binaries in Taurus discovered by our group [10-11]. While the time baseline is shorter than that of the above sample component, these low mass stars are the most interesting for testing the evolutionary tracks, as they will fill in a region of parameter space currently completely unconstrained observationally. Finally, we have begun tracking binaries in newly identified, nearby young stellar associations, such as TW Hydrae. So far this component of our sample consists of 16 binary stars, but this number will continue to increase as new young, nearby stars and their companions are identified. These stars are typically K and later spectral types, meaning again that this sample spans the most interesting regime of the evolutionary tracks. The greatest advantage of stars in this sample is their proximity to the Earth (D 60 pc), which allows for the resolution of companions with smaller physical separations than those of binaries in regions such as Taurus (D = 140 pc). This permits orbital solutions to be calculable with much shorter time baselines. Many of these systems are now beginning to yield orbital solutions. Figure 1 show an example of an orbit from a star in each of the three samples: DF Tau from the solar analog sample in Taurus, MHO 8 from the lower mass sample in Taurus, and TWA 5A, a PMS star from our nearby, young sample. Based upon our orbital solutions, we derive a total system mass of 1.14 0.5 M for DF Tau, 0.32 1.12 M for MHO 8, and 0.70 0.23 M for TWA 5A. Using these mass estimates, we have begun to investigate the predictions of four theoretical PMS models: Baraffe et al. 1998 (BCAH98), D’Antona Mazzitelli 1997 (DM97), Palla Stahler 1999 (PS99), and Swenson et al. 1994 (SW94). We find that the predictions of BCAH98 are the most consistent with our dynamical values, but that all other models are doing a reasonable job at reproducing the correct masses. Over the next few years, with more observations aimed at tightening orbital parameters, obtaining spatially resolved spectral types, and measuring distances, we will be able to distinguish between these models at the 5 level.
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