PRELIMINARY ORBITS AND SYSTEM MASSES FOR FIVE BINARY T TAURI STARS Vakhtang S. Tamazian, 1 José A. Docobo, 1 Russel J. White, 2,3 and Jens Woitas 4

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1 The Astrophysical Journal, 578: , 2002 October 20 # The American Astronomical Society. All rights reserved. Printed in U.S.A. PRELIMINARY ORBITS AND SYSTEM MASSES FOR FIVE BINARY T TAURI STARS Vakhtang S. Tamazian, 1 José A. Docobo, 1 Russel J. White, 2,3 and Jens Woitas 4 Received 2002 May 14; accepted 2002 July 1 ABSTRACT First visual orbits for the binary T Tauri stars V773 Tau, FO Tau, and FS Tau and revised orbits for DF Tau and GG Tau are presented. The orbits are determined from a compilation of previous high spatial resolution imaging measurements and new astrometric measurements from the Calar Alto 3.5 m telescope. The orbital solutions, which must be considered preliminary because of the short orbital arcs observed, are used in combination with distance estimates to determine the dynamical masses for all five systems. The total mass of the triple system V773 Tau (whose primary is a K2+K5 spectroscopic binary) is 3:20 0:71 M, and the masses of the four binary pairs range from 0.77 to 1.22 M. The orbital solutions for these systems predict projected relative orbital velocities that range from 0.1 to 13.4 km s 1 (2002 epoch); the majority are spectroscopically measurable if spatially resolved. Of particular interest is the GG Tau binary, which has a mass that is consistent with the value determined from circumbinary disk kinematics and an orbit that is coplanar with this disk. The stellar properties of the components of these five systems are determined from spatially resolved optical measurements, assuming dwarflike colors and a dwarflike temperature scale. With luminosity and temperature estimates, theoretical masses are determined from the predictions of four pre main-sequence evolutionary models and compared with the dynamically determined values as a test of these models. Of the models tested, those of Palla & Stahler and Siess, Dufour, & Forestini predict masses that are the most consistent (within 1 ) with the dynamical values over the component mass range of M. All models support coeval formation of these binaries. Subject headings: astrometry binaries: general celestial mechanics stars: individual (V773 Tauri, FO Tauri, FS Tauri, DF Tauri, GG Tauri) stars: pre main-sequence 1. INTRODUCTION The study of low-mass, pre main-sequence (PMS) binaries in star-forming regions registered a spectacular advance in the recent years as a result of the application of adaptive optics and speckle interferometry techniques, especially in the near-infrared (NIR), where young late-type stars are most luminous (Mathieu 1994; Mathieu et al. 2000). Of particular importance are the multiplicity surveys of low-mass stars in different star-forming regions and young clusters (see, e.g., Ghez, Neugebauer, & Matthews 1993; Leinert et al. 1993; Simon et al. 1995; Ghez et al. 1997a; Bouvier et al. 1997; Petr et al. 1998). These surveys show that the binary frequency in all these regions is at least as high as for G dwarfs in the field (56%; Duquennoy & Mayor 1991). In the Taurus-Auriga T association there is even a strong binary excess (see, e.g., Köhler & Leinert 1998). Therefore, studies of these binary systems are important both for understanding how the majority of stars form and, as is well known, because binary stars offer unique tests of stellar models. Of primary importance in the study of young stars are accurate mass and age estimates. The mass and age of a 1 Observatorio Astronómico Ramón María Aller, Universidade de Santiago de Compostela, Avenida das Ciencias s/n, Santiago de Compostela, Spain; oatamaz@usc.es, oadoco@usc.es. 2 McDonald Observatory, University of Texas at Austin, Austin, TX Current address: Department of Astronomy, MS , California Institute of Technology, 1201 East California Boulevard, Pasadena, CA 91125; rjw@astro.caltech.edu. 4 Thüringer Landessternwarte Tautenburg, Sternwarte 5, D Tautenburg, Germany; woitas@tls-tautenburg.de. 925 young star are usually inferred from the comparison of its temperature and luminosity with those predicted by a given set of PMS evolutionary models (see, e.g., D Antona & Mazzitelli 1994, 1997; Swenson et al. 1994; Baraffe et al. 1998; Palla & Stahler 1999; Siess, Dufour, & Forestini 2000). Unfortunately, there are still large uncertainties in these models because of different input physics, continually improving molecular line lists, convection details, and initial conditions (see Baraffe et al. 2002). These uncertainties lead to variations by factors as large as 10 in age and 2 in mass. Comparing the predictions of the PMS models with purely empirical results is thus highly desirable. Young binary stars offer a unique possibility for this, because dynamical masses can be obtained from their orbital motion and their components can be assumed to be coeval. The most reliable empirical PMS masses have been derived for the eclipsing, double-lined, spectroscopic binaries TY CrA (Casey et al. 1998) and RX J (Covino et al. 2000). However, eclipsing systems are very rare and thus will unlikely yield constraints for the whole range of masses and ages covered by the PMS models. To achieve this, the orbital motion of a large number of visual pairs spanning a range of masses must be studied. As an example, Steffen et al. (2001) have derived a visual orbit for NTT by combining measurements with the Hubble Space Telescope (HST) fine guidance sensor and radial velocity data. To date, all binaries with orbital solutions have system masses 2 M and component masses 0.8 M. No dynamical mass constraints exist for young stars at masses below 0.5 M. In this work, we calculate preliminary orbits for five young binary stars with system masses of 0:77 3:20 M and

2 926 TAMAZIAN ET AL. component masses ranging from 0.3 to 1.5 M. The observational data set is sufficient for the method of binary star orbit calculation proposed by Docobo (1985) to be applied. The orbits for V773 Tau, FO Tau, and FS Tau are new, while orbits for DF Tau and GG Tau are revised from previous studies (Thiébaut et al. 1995; Roddier et al. 1996; McCabe, Duchêne, & Ghez 2002). The dynamical mass estimates from the orbital solutions are then used to test the predictions of PMS evolutionary models. 2. ORBITS AND DYNAMICAL MASSES 2.1. Sample Selection In a recent paper by Seymour et al. (2002), two criteria are used to establish the sample of binary stars for which the first preliminary orbits could be calculated. Binary stars were selected that have at least eight spatially resolved observations (of any kind) and exhibit at least a 30 change in position angle or a 30% change in separation. In selecting our young binary sample, we use these two criteria and impose one additional criterion: the binary must have at least one spatially resolved optical measurement using the HST. Selection criteria on the number of measurements and h and variations are chosen to insure sufficient orbital motion and sampling. Our additional criterion is chosen because of (1) the high accuracy of HST relative astrometry (to serve as a base point; see x 2.3) and (2) the improvement in stellar property estimates that optical measurements provide. After a careful inspection of all low-mass, PMS binary astrometric measurements in the Taurus region, five binaries are identified that satisfy these criteria. These are V773 Tau A and B, DF Tau A and B, FO Tau A and B, FS Tau A and B, and GG Tau Aa and Ab. The primary of the V773 Tau system, V773 Tau A, is actually a spectroscopic binary with an accurately established mass ratio (Welty 1995). The components of this spectroscopic binary are subsequently referred to as V773 Tau SB1 and SB2. We note that Duchêne, Ghez, & McCabe (2001) have identified a fourth, and much fainter, component in this system at wider separations. Similarly, the binary GG Tau Aa & Ab has a binary companion at a much wider separation (White et al. 1999). These wider companions should not significantly affect the orbital motion. The application of Docobo s method in these five systems should lead to a preliminary, but rather reliable, orbit. The orbits presented in this work generally correspond to the grade 4, according to the orbits grading method suggested by Hartkopf, Mason, & Worley (2001a), where grade 1 is ascribed to the definitive and grade 5 to indeterminate orbits Observational Database Most of the binary data obtained before 1998 have been published in the articles of Ghez et al. (1995) and Woitas, Köhler, & Leinert (2001), who have firmly demonstrated the presence of measurable orbital motion in a large number of T Tauri binary systems. The time basis of these studies is now significantly enlarged by adding results from White & Ghez (2001) and new observations obtained with the NIR camera OMEGA Cass at the 3.5 m telescope on Calar Alto (Spain) in 2001 February and November. The new observations were conducted using infrared speckle interferometry to obtain diffraction-limited spatial resolution. For the process of data acquisition and reduction of the new measurements, we refer to Köhler et al. (2000). Woitas et al. (2001, x 2) have discussed in detail how the analysis of the speckle data leads to a highly precise determination of the relative position of the companion with respect to the primary. The uncertainty is usually only a few milliarcseconds, even close to the diffraction limit (0>13 for a 3.5 m telescope at ¼ 2:2 lm). In Table 1 are listed all of the published HST, speckle imaging, and (multiple-epoch) lunar occultation measurements for these five systems. The first six columns give the star name (along with the Washington Double-Star Catalog number and visual magnitude), measurement date, and the measured position angle and separation with their corresponding errors; the original references are listed in the last column. The Fourth Catalog of Interferometric Measurements of Binary Stars, 5 maintained at US Naval Observatory (Hartkopf et al. 2002; see also Hartkopf, McAlister, & Mason 2001b), was also consulted for completeness. In column (4) of Table 1, certain techniques and filters are identified. Measurements marked A and B correspond to the filters with throughput maxima at 6580 and 6670 Å, respectively (Thiébaut et al. 1995). Letter H is used to indicate HST measurements and O to indicate a lunar occultation observation. Finally, letters K and L indicate speckle imaging in these two NIR photometric bands, while an asterisk indicates that the position angle was rotated 180 ; in some systems the primary is occasionally the fainter component, because of source variability Orbital Calculations The method of orbit calculation, described extensively by Docobo (1985; see also Docobo et al. 2000) and briefly summarized here, is applicable even when only a relatively short arc of the orbit has been measured. The advantage of this method over more formal solutions is that it does not require knowledge of the areal constant. Initially, each measurement included in the orbital fit is weighted according to observational criteria (telescope aperture, technique, number of combined measurements, etc.). A family of Keplerian orbits are then generated whose apparent orbits pass through three base points. The three base points are assumed to be either the most reliable measurements (for example, the HST measurements) or positions belonging to the areas with maximum observational likelihood (see Docobo 1985). Simultaneously, O C (observed minus calculated) residuals in both and h are determined for these orbits. The next step is to select the orbit with smallest weighted rms values for both observed parameters (h and ). Usually, the orbit with the smallest rms values in h is not exactly the same as the orbit with the smallest rms values in. Therefore, a number of orbital solutions between these two minimal solutions are determined. The orbital solution with the smallest O C residuals is selected as the best fit, and the range of orbits is used to characterize the uncertainties in the orbital elements. Over this range of orbital solutions, all orbital elements are computed simultaneously and inde- 5 See

3 TABLE 1 Measurements and O C Residuals Star (1) Catalog Number a (2) Visual Magnitude a (mag) (3) Date b (4) (5) (6) ðo C (7) Þ ðo C (8) Þ Reference c (9) V773 Tau... WDS H FO Tau... WDS K L H FS Tau... WDS O H H K L DF Tau... WDS O * * H H H H A* B* H H * GG Tau... WDS H

4 928 TAMAZIAN ET AL. Vol. 578 TABLE 1 Continued Star (1) Catalog Number a (2) Visual Magnitude a (mag) (3) Date b (4) (5) (6) ðo C (7) Þ ðo C (8) Þ Reference c (9) K L H H H a From the Washington Double-Star Catalog. b H: HST; O: occultation; A: at 6580 Å; B: at 6670 Å; K: K photometric band; L: L photometric band; *: position angle is rotated 180. c References are for h and measurements. References. (1) Ghez et al. 1993; (2) Leinert et al. 1993; (3) Ghez et al. 1995; (4) Woitas et al. 2001; (5) Ghez, White, & Simon 1997b; (6) This work; (7) White & Ghez 2001; (8) Chen et al. 1990; (9) Simon et al. 1992; (10) Krist et al. 1998; (11) Thiébaut et al. 1995; (12) Simon, Holfeltz, & Taff 1996; (13) Balega et al. 2002; (14) Leinert et al. 1991; (15) Roddier et al. 1996; (16) McCabe et al pendently, to insure a proper error estimation for each. The range of orbital solutions typically corresponds to O C residuals at 1 2. Although this technique works well at identifying the best-fit orbital solution and characterizing the uncertainties, the orbits determined for the five binary stars studied here must be considered preliminary because of the short orbital arcs observed Orbital Solutions The orbits determined from these calculations are shown in Figure 1. In columns (7) and (8) of Table 1 are listed O C residuals of the best-fit orbits, as described above. In Table 2, the orbital elements, with their corresponding standard errors, are given, and Table 3 contains the ephemerides up to 2010 in steps of 2 yr, sufficient time to detect orbital motion. All orbits but GG Tau s were initially and briefly announced in IAU Commission 26 Information Circular 146 (Docobo & Tamazian 2002; Docobo, Tamazian, & Woitas 2002). We introduce only minor refinements to these orbits here. The orbit for DF Tau is revised from the first orbit determined by Thiébaut et al. (1995), who obtained a period of yr, a semimajor axis a ¼ mas, and a mass estimate of 2:8 1:5 M, adopting a distance of 140 pc. The number of measurements for DF Tau has quadrupled since this initial orbital solution was presented. The orbit for GG Tau is revised from the first orbit by Roddier et al. (1996) and the recent calculation by McCabe et al. (2002). Although our solution is consistent with the careful analysis of McCabe et al. (2002), who adopt orbital assumptions to account for the minimal sampling, we find that their solution predicts much larger ðo CÞ residuals than ours. The mean and standard deviation of the ðo CÞ residuals from our fit to the GG Tau measurements are +0.6 and 1.9, respectively (see Table 1), while those for the McCabe et al. best fit are 6.3 and 1.9, respectively [the ðo CÞ values for the two orbits are indistinguishable]. This error could be accounted for by an offset in the line of nodes, perhaps imposed by the adopted assumptions. Regardless, this highlights the usefulness of minimizing O C residuals in orbital calculations. Using our new orbital solutions and distance estimates, we determine the projected relative orbital velocities for each pair. Effectively, these are radial velocities of the secondary components, under the assumption that primaries are at rest. Values at each ephemeris epoch are listed in Table 4 and orbital maximum and minimum values in Table 5. The maximum absolute projected relative velocities range from 1.27 to km s 1, with a mean of 5.5 km s 1.We present these values because facilities now exist that can obtain spectroscopy of sufficient spatial resolution (0>1) and radial velocity precision (0.1 km s 1 ) to measure the relative velocities of the majority of these binaries (e.g., NIRSPEC with adaptive optics at Keck II). Radial velocity measurements, in combination with the visual orbits presented here, could be used to directly determine the distances and stellar mass ratios Dynamical es With an orbital solution and a known distance, it is then possible to determine the total stellar mass. Fortunately, for the radio-bright young star V773 Tau, a distance of pc has been determined, using VLBI astrometry (Lestrade TABLE 2 Orbital Elements Star P (yr) T e a (arcsec) i! V773 Tau FO Tau FS Tau DF Tau GG Tau

5 No. 2, 2002 ORBITS AND MASSES FOR BINARY T TAURI STARS 929 Fig. 1. Apparent orbits of five T Tauri stars. Each measurement (cross) is connected to its predicted position by an O C line. The dashed line passing through the primary star is the line of nodes. North is up and east is to the left. et al. 1999). For the remaining four systems, we adopted the more commonly assumed distance to Tau-Aur of pc based on geometric parallax measurements (Bertout, Robichon, & Arenou 1999; Wichmann et al. 1998) and indirect methods (Kenyon, Dobrzycka, & Hartmann 1994; Preibisch & Smith 1997). Using the orbital solutions and these distances, the masses of the binary systems are calculated; they are listed in the second column of Table 6. For comparison, we note that for the GG Tau system a dynamical mass has been determined from the kinematics

6 TABLE 3 Ephemerides V773 Tau FO Tau FS Tau DF Tau GG Tau h h h h h TABLE 4 Ephemerides for Projected Radial Velocity V773 Tau FO Tau FS Tau DF Tau GG Tau TABLE 5 Maximum and Minimum Projected Radial Velocities V773 Tau FO Tau FS Tau DF Tau GG Tau Event Value Value Value Value Value V max V min TABLE 6 Comparison of Dynamical and Theoretical es DM Model BCAH Model PS Model SDF Model Star Dynamical Total (M ) Ratio (M p /M s ) Total (M ) Ratio (M p /M s ) Total (M ) Ratio (M p /M s ) Total (M ) Ratio (M p /M s ) References a V773 Tau > DF Tau FO Tau FS Tau GG Tau V773 Tau A > a References are for dynamical masses. References. (1) This work; (2) Guilloteau, Dutrey, & Simon 1999; (3) Welty 1995.

7 ORBITS AND MASSES FOR BINARY T TAURI STARS 931 of its circumbinary disk (Dutrey, Guilloteau, & Simon 1994; Guilloteau et al. 1999). Orbital velocity measurements of the circumbinary disk suggest that the pair have a combined mass of 1:28 0:08 M (Guilloteau et al. 1999), using the same distance we adopt here. This value agrees well with the orbital mass of 1:22 0:28 M and supports the method used to determine the orbital solution. Interestingly, this orbit has an inclination of 37 7 relative to face-on, which is identical with the inclination determined for the circumbinary disk (i ¼ 37 1 ). The binary orbit and disk appear to be coplanar. 3. STELLAR PROPERTIES AND THEORETICAL MASSES 3.1. Temperature, Luminosity, and Extinction Estimates The stellar properties of this sample are determined from optical spectroscopic and photometric observations. GG Tau has been spatially resolved both photometrically and spectroscopically (White et al. 1999); both components have a determined spectral type. For the remaining systems, all have been spatially resolved optically, but have only a combined spectral type for the system (White & Ghez 2001). In these cases, the optically inferred spectral type of the unresolved system is assigned to the optically brighter component. These spectral types are converted to temperatures using the dwarf temperature scale of Schmidt-Kaler (1982) for K7 and hotter stars and that of Leggett et al. (1996) for M0 and cooler stars. The line-of-sight extinctions are determined by comparing the observed colors (V R C, R I C, V I C ) with the colors of the adopted spectral types, using the extinction law of Rieke & Lebofsky (1985). Three colors are used to minimize color uncertainties; the adopted extinctions (A v ) are determined from the averages of these three, and the uncertainties are determined from the standard deviations. Standard dwarf colors are taken from the compilation of Hartigan, Strom, & Strom (1994) for spectral types of K7 and hotter and from Leggett (1992; young disk) for M0 and cooler stars. The luminosities are then derived from the reddening-corrected I C -band measurements and a bolometric correction (Bessell 1991), assuming a distance of pc (148 5 pc for V773 Tau). The uncertainties in the luminosities are determined from the uncertainties in the photometry, extinction, and distance. The inferred properties are listed in Table 7. For the companion stars without spectroscopic measurements of their spectral types, we assume that the extinctions to the secondaries are the same as to the primaries. This assumption is supported by observations of binary stars in which the extinction to each component has been determined separately (White & Ghez 2001). The extinction estimates are used to deredden the observed optical colors of the secondaries. The spectral types and corresponding temperatures of these stars are then estimated from their reddening-corrected colors. The uncertainties in the spectral types of the secondaries are estimated by changing the spectral types of the primaries by their uncertainties and then rederiving the extinctions and the colors of the secondaries; a minimum uncertainty of 1 spectral subclass is assumed. The luminosities of the secondaries are derived from the dereddened I C -band magnitudes as described above. The stellar properties for these companion stars are also listed in Table 7. For the spatially unresolved spectroscopic binary V773 Tau A, Welty (1995) successfully fitted the system s spectrum by combining a K2 V and a K5 V with a flux ratio at 0.65 lm of Using these values and assuming that both components experience the same extinction, the extinction and the luminosity of each component are determined following the above prescription Theoretical es and Ages With stellar luminosity and temperature estimates for all components, these values can then be compared with the predictions of PMS evolutionary models to determine theoretical stellar masses. The four models we use for this are D Antona & Mazzitelli (1997, hereafter DM), Baraffe et al. (1998; with mixing length parameter ¼ 1:9, hereafter BCAH), Palla & Stahler (1999, hereafter PS), and Siess et al. TABLE 7 Stellar Properties of Binary Stars Star Spectral Type a log T A v (mag) log (L/L ) Reference b V773 Tau Aa... K V773 Tau Ab... K V773 Tau B... M DF Tau A... M DF Tau B... M FO Tau A... M FO Tau B... M FS Tau A c... M FS Tau B c... M GG Tau Aa... K GG Tau Ab... M a Spectral type uncertainties are one spectral subclass unless otherwise noted. b References are for spectral type. c The properties of FS Tau, especially luminosity, likely have unaccounted-for errors (see x 3). References. (1) Welty 1995; (2) This work (x 3); (3) Basri & Bertout 1989; (4) Cohen & Kuhi 1979; (5) White et al

8 932 TAMAZIAN ET AL. Vol. 578 Fig. 2. Components of V773 Tau ( filled circles), DF Tau ( pentagons), FO Tau (squares), and GG Tau (triangles) on H-R diagrams, along with four evolutionary models (see text). Three isochrones are shown for each model (10 6,10 7, and 10 8 yr), and the mass tracks are labeled. The low luminosities inferred for the components of FS Tau are illustrated with asterisks, while the luminosities adopted for masses estimates are shown as open circles (see x 3.2). (2000, hereafter SDF); they are illustrated in Figure 2. These models differ primarily in the choice of opacities, treatment of the atmosphere (gray vs. nongray), initial conditions, and convection model. Previous studies compare the differences between these models (see, e.g., SDF; Simon, Dutrey, & Guilloteau 2000; Steffen et al. 2001; Baraffe et al. 2002), and we do not repeat this discussion here. The total stellar mass for each system, as determined from each evolutionary model, is listed in Table 6 for comparison with the dynamically inferred value. For stars that lie above the youngest isochrone, their mass is determined using the youngest isochrone and their temperature. Table 6 also lists the mass ratios predicted by each model for the spectroscopic binary V773 Tau A. The components of FS Tau are distinctly underluminous relative to the other Taurus stars. As noted by White & Ghez (2001), the components of this system appear to have a combination of large continuum excesses (from accretion) and extinctions that compromise stellar properties estimates. However, the stellar temperatures, which are determined from a system spectral type, are less likely to be affected by uncertain excesses and extinctions. Therefore, the luminosity of each component of FS Tau is corrected to have a luminosity that makes it coeval with the other stars in this study (typically 1 Myr). These corrected values are plotted in Figure 2, assuming twice the uncertainty, and are used to determine the theoretical mass estimates for FS Tau. The ages for each component can also be estimated from these theoretical evolutionary models. Although both V773 Tau and DF Tau have secondary stars that are somewhat younger than their primaries, this difference is less than 2. All models support coeval formation for the components of these systems. 4. A COMPARISON OF DYNAMICAL AND THEORETICAL MASSES The dynamical masses determined from the orbital solutions are compared with the theoretical masses to test the validity of evolutionary models (see Table 6). For all five

9 No. 2, 2002 ORBITS AND MASSES FOR BINARY T TAURI STARS 933 systems, the masses predicted by the DM models are systematically too small relative to the dynamically determined values. The average difference between the dynamical masses and the DM theoretical masses p is 1.1, giving a combined difference of 2.5 (1.1 ffiffiffiffi N ). In contrast, the masses predicted by the BCAH models are systematically too high relative to the dynamical values. These models give a combined difference of 1.9 (note that this does not include the V773 Tau system, for which the BCAH models give only a mass lower limit). We note that the version of BCAH models computed with a smaller mixing length ( ¼ 1:0; see BCAH), which yields larger masses above 0.6 M, is even more inconsistent with the dynamical mass values. Both the PS and SDF models predict masses that are in better agreement with dynamically determined values. These models yield a combined difference of only 0.5 (too low) and 0.1 (too low), respectively. The additional dynamical constraint provided by the mass ratio of the spectroscopic binary V773 Tau A supports these results. For this pair, the mass ratio as predicted by the DM models is inconsistent with the dynamical value by 1.4. Only a lower limit to the mass ratio is available using the BCAH models, but it is nevertheless consistent with the dynamical value. The PS and SDF models predict a mass ratio that agrees with the dynamical value to within 0.2. Since the majority of these stars are in the fully convective portion of their evolution, it is important to consider the effect of the adopted temperature scale. T Tauri stars commonly show surface gravity features with strengths intermediate between those of dwarfs and giants, so a slightly hotter-than-dwarf temperature scale might be more appropriate (see White et al. 1999; Luhman 1999). For simplicity of comparison, we use the proposed scale of Luhman (1999), 6 which slowly diverges from the dwarf scale for spectral types cooler than M0. For the DM models, this hotter temperature scale increases the average theoretical mass and thereby provides a slightly better match to the dynamical values. However, the combined difference is still too low at the 2.3 level. For the BCAH models, the hotter temperature scale similarly leads to larger masses, giving an even larger discrepancy with dynamical mass values. Since both the PS and SDF models predict masses that are consistent with the dynamical values, there is no need to introduce an hotter temperature scale in these cases. However, it is worth realizing that for the three lowest-mass systems (DF Tau, FO Tau, FS Tau), with component masses of M, both models predict masses that are slightly too low, by 0.9 (PS models) and 0.4 (SDF models). These slight offsets to low masses would be reduced with the hotter temperature scale, but again, the need for this is not statistically significant. These results are generally consistent with other dynamical tests of evolutionary models. In combination with a dwarf temperature scale, the PS and SDF models usually predict consistent stellar masses (see, e.g., Simon et al. 2000; Palla & Stahler 2001), but they do not always predict the most consistent masses (see, e.g., Steffen et al. 2001). 6 Although the scale proposed by Luhman (1999) is chosen to make the components of the GG Tau quadruple and the members of IC 348 coeval using the BCAH models, it nevertheless represents a plausible temperature scale, intermediate between that of dwarfs and giants. Although the PS and SDF models are very similar at masses above 0.4 M, the SDF models predict mass tracks that are cooler for the lowest masses (Fig. 2). This, in part, leads to the lower masses determined for the PS models relative to the SDF models. Dynamical mass estimates below 0.3 M are needed to determine if either of these models, or perhaps more sophisticated nongray atmospheric models (e.g., BCAH), are more appropriate at these low temperatures and low masses. Nevertheless, from both our new results and the results of previous studies, we conclude that the PS and SDF models predict masses that are the most consistent with dynamically determined values using a dwarf temperature scale over the mass range of M. We emphasize, however, that these results do not imply that the PS and SDF models are correct, but simply that they yield consistent masses. 5. SUMMARY High spatial resolution imaging studies of young binary stars are producing a rapidly growing database of astrometric measurements. Using these and new measurements from the 3.5 m telescope on Calar Alto, we calculate the first visual orbits for the T Tauri stars V773 Tau, FO Tau, and FS Tau and revised orbits for GG Tau and DF Tau. The orbital solutions, in combination with distance estimates (148 5 pc for V773 Tau; pc for the other four), are used to determine dynamical masses for all five systems. The most massive system is the triple system V773 Tau (whose primary is a K2+K5 spectroscopic binary), with a combined mass of 3:20 0:71 M. The other four systems have total masses that range from 0.77 to 1.22 M. Of particular interest is the GG Tau binary; its dynamical orbital mass (1:22 0:28 M ) agrees well with the dynamical mass determined from the disk kinematics of its circumbinary disk (1:28 0:07; Guilloteau et al. 1999), and its orbit appears to be coplanar with this disk. The orbital solutions for these five systems also predict projected relative orbital velocities that, at the current epoch, range from 0.1 to 13.4 km s 1.The majority of these velocities are spectroscopically measurable with high spatial resolution spectroscopic techniques. The stellar properties of the components of these five systems are determined from spatially resolved optical measurements, assuming dwarflike colors and temperature scale. The luminosities and temperatures are used to determine theoretical masses and ages from the predictions of four PMS evolutionary models. These models include those by DM, BCAH ( = 1.9), PS, and SDF. All models are consistent with the components of these systems being coeval. The theoretical masses are compared with the dynamically determined values as a test of PMS evolutionary models. Using all five systems, the DM models predict masses that are too low, at a combined significance of 2.5. Although a hotter temperature scale would yield larger masses, the discrepancy would still remain at the 2.3 level. The BCAH models predict masses that are too large, at a combined significance of 1.9. For these models, a hotter temperature scale or a smaller mixing (e.g., ¼ 1:0; see BCAH) would only increase the mass discrepancy. The PS and SDF models predict masses that are the most consistent with the dynamically determined values. They predict masses that are consistent (but slightly low) at the 0.5 and 0.1 levels, respectively. A slightly hotter-than-dwarf temperature scale

10 934 TAMAZIAN ET AL. could correct this slight offset, but the need for such is not statistically significant. The PS and SDF models also predict a mass ratio for the spectroscopic binary V773 Tau A that agrees with its dynamically determined value. These findings are generally consistent with other dynamical mass tests of PMS evolutionary models (Simon et al. 2000; Palla & Stahler 2001; Steffen et al. 2001). We conclude that the PS and SDF models predict masses that are the most consistent with dynamically determined values over the mass range M. Although the observed orbital motion of young binary systems can currently determine masses to only 30% in the best cases, the constraints from many systems, as is demonstrated here, can be combined to provide a meaningful test of PMS evolutionary models. The authors thank the referee, M. Simon, for many useful comments. This work was supported by the Project Grant AYA of the Ministerio de Ciencia y Tecnologia (Spain). Balega, I. 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