1Dynamical Mass Measurements of Pre-Main-Sequence Stars:Fundamental Tests of the Physics of Young StarsR. D. Mathieu University of Wisconsin – MadisonI. Baraffe École Normale Supérieure, LyonM. Simon State University of New York – Stony BrookK. G. Stassun Vanderbilt UniversityR. White University of Alabama in Huntsville_________________________________________________________There are now 23 dynamical mass measurements for PMS stars of less than 2 Mo, with most ofthe measured stars having masses greater than 0.5 Mo. The masses of two PMS brown dwarfshave also been precisely measured. The most important application of these dynamical massmeasurements has been to provide tests of theoretical masses derived from PMS stellarevolution models. On average most models in use today predict stellar masses to within 20%;however, the predictions for individual stars can be in error by 50% or more. Now thatdynamical mass measurements are relatively abundant, and will become more so with theapplication of ground-based optical/infrared interferometers, the primary limitations to suchtests have become systematic errors in the determination of the stellar properties necessary forthe comparison with evolutionary models, such as effective temperature, luminosity, and radii.Additional dynamical mass determinations between 0.5 Mo and 2 Mo will not likely improvethe constraints on evolutionary models until these systematic uncertainties in measurements ofstellar properties are reduced. The nature and origin of these uncertainties, as well as thedominant physical issues in theoretical PMS stellar evolution models, are discussed. There areimmediately realizable possibilities for improving the characterizations of those stars withdynamical mass measurements. Additional dynamical mass measurements for stars below 0.5Mo are also very much needed.1. INTRODUCTIONPrior to the measurement by Popper (1987) ofthe mass of the pre-main-sequence (PMS) secondary ofthe eclipsing binary EK Cep, every mass cited for aPMS star had been derived by comparing the star’slocation in the HR-diagram to the predictions oftheoretical evolutionary models, which track how a starof given mass evolves in luminosity and temperaturewith age. These theoretical assignments of stellarmasses were entirely unconstrained by direct massmeasurements of PMS stars. Furthermore, differentmodels can predict masses that differ by as much as afactor two or more.The masses so assigned to PMS stars are at thevery foundation of our understanding of star and planetformation. These masses define the initial mass functionthat delimits the outcome of the star formation process.They set the energy scale available to explain processesranging from accretion to outflow. They allow us to linkyoung stars with older generations. And these massespermit us to identify stars that can serve as proxies forthe Sun-Earth system at an early age.Equally importantly, stellar masses represent akey observational interface for theoretical stellarevolution models. Such models provide our chronologyof early stellar evolution and thereby touch upon themost basic of questions, including the timescale forcircumstellar disk evolution and planet formation.Thus the importance of the measurement ofaccurate PMS stellar masses in order to test theoreticalmasses can hardly be overstated. As the authors of thispaper, we have the good fortune to be writing one of the2Table 1: Dynamical Masses and Stellar Properties of 23 Pre-Main-Sequence StarsNameMass (Mo)TypeRefRadius (Ro)SpTLog (Teff)Log (L/Lo)RefRS Cha A1.858 ± 0.016EBA912.137±0.055A83.883±0.0101.144±0.044M00RS Cha B1.821 ± 0.018EBA912.338±0.055A83.859±0.0101.126±0.043M00MWC 4801.65 ± 0.07DKSi00...A2-33.948±0.0151.243±0.10HWTY CrA B1.64 ± 0.01EBC982.080±0.140...3.690±0.0350.380±0.145C98045251+3016 A1.45 ± 0.19ASSt01...K53.643±0.015-0.167±0.053St01BP Tau1.32 +0.20/-0.12*DKD03...K73.608±0.012-0.78±0.10J990529.4+0041 A1.25 ± 0.05EBC001.700±0.200K1-23.701±0.0090.243±0.037C00EK Cep B1.124 ± 0.012EBP871.320±0.015...3.755±0.0150.190±0.070P87UZ Tau Ea1.016 ± 0.065*DKSP02...M13.557±0.015-0.201±0.124P02V1174 Ori A1.009 ± 0.015EBS041.339±0.015K4.53.650±0.011-0.193±0.048S04LkCa 150.97 ± 0.03*DKSi00...K53.643±0.015-0.165±0.10HW0529.4+0041 B0.91 ± 0.05EBC001.200±0.200…3.604±0.022-0.469±0.192C00GM Aur0.84 ± 0.05*DKS01...K73.602±0.0150.598±0.10HW045251+3016 B0.81 ± 0.09ASSt01...M23.535±0.015-0.830±0.086St01V1174 Ori B0.731 ± 0.008EBS041.065±0.011…3.558±0.011-0.761±0.058S04DL Tau0.72 ± 0.11*DKSi00...K7-M03.591±0.0150.005±0.10HWHD 98800 Ba0.699 ± 0.064ASB05......3.623 ± 0.0160.330 ± 0.075B05HD 98800 Bb0.582 ± 0.051ASB05......3.602 ± 0.0160.167 ± 0.038B05DM Tau0.55 ± 0.03*DKSi00...M13.557±0.015-0.532±0.10HWCY Tau0.55 ± 0.33*DKSi00...M23.535±0.015-0.491±0.10HWUZ Tau Eb0.294 ± 0.027*DKSP02...M43.491±0.015-0.553±0.124HW2M0535–05 A0.0541 ± 0.0046EBS060.669±0.034M6.53.423±0.016-1.699±0.078S062M0535–05 B0.0340 ± 0.0027EBS060.511±0.026…3.446±0.016-1.848±0.076S06References: A91=Andersen 1991; Si00=Simon et al. 2000; C98=Casey et al. 1998; St01=Steffen et al. 2001;D03=Dutrey et al. 2003; C00=Covino et al. 2000; P87=Popper 1987; P02=Prato et al. 2002; S04=Stassun et al.2004; B05=Boden et al. 2005; S06=Stassun et al. 2006; M00=Mamajek et al. 2000; J99=Johns-Krull et al. 1999;HW=Hillenbrand and White 2004Techniques: EB = Eclipsing Binary, DK = Disk Kinematics, AS = Astrometric and Spectroscopic, DKS = DiskKinematics and Spectroscopic (to divide total mass in double-lined system)* Asterisks indicate that an uncertainty in the distance is not included in the mass uncertainty.first reviews of PMS masses in which much of the textdiscusses actual mass measurements for PMS stars,rather than what needs to be known and how thatknowledge will be gained in the very near future!This paper first reviews in Section 2 thepresent capabilities for dynamical mass measurementsof PMS stars, with an emphasis on random andsystematic uncertainties, and then summarizes thepresent dynamical mass measurements in Table 1. InSection 3 we compare these mass measurements withtheoretical mass values, and note that theory tends tounderpredict masses at a marginally significant
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