Photometric and Spectroscopic Studies of Massive Binaries in the Large Magellanic Cloud
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The Astrophysical Journal,748:96(22pp),2012April1 doi:10.1088/0004-637X/748/2/96 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. ⃝ PHOTOMETRIC AND SPECTROSCOPIC STUDIES OF MASSIVE BINARIES IN THE LARGE MAGELLANIC CLOUD. I. INTRODUCTION AND ORBITS FOR TWO DETACHED SYSTEMS: EVIDENCE FOR A MASS DISCREPANCY?∗ Philip Massey1,NidiaI.Morrell2,KathrynF.Neugent1,LauraR.Penny3, Kathleen DeGioia-Eastwood4,andDouglasR.Gies5 1 Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001, USA; [email protected], [email protected] 2 Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile; [email protected] 3 Department of Physics and Astronomy, The College of Charleston, Charleston, SC 29424, USA; [email protected] 4 Department of Physics and Astronomy, Northern Arizona University, P.O. Box 6010, Flagstaff, AZ 86011-6010, USA; [email protected] 5 Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 4106, Atlanta, GA 30302, USA; [email protected] Received 2011 December 2; accepted 2012 January 16; published 2012 March 13 ABSTRACT The stellar mass–luminosity relation is poorly constrained by observations for high-mass stars. We describe our program to find eclipsing massive binaries in the Magellanic Clouds using photometry of regions rich in massive stars, and our spectroscopic follow-up to obtain radial velocities and orbits. Our photometric campaign identified 48 early-type periodic variables, of which only 15 (31%) were found as part of the microlensing surveys. Spectroscopy is now complete for 17 of these systems, and in this paper we present analysis of the first two, LMC 172231 and ST2-28, simple detached systems of late-type O dwarfs of relatively modest masses. Our orbit analysis yields very precise masses ( 2%), and we use tomography to separate the components and determine effective temperatures by model fitting,∼ necessary for determining accurate (0.05–0.07 dex) bolometric luminosities in combination with the light-curve analysis. Our approach allows more precise comparisons with evolutionary theory than previously possible. To our considerable surprise, we find a small, but significant, systematic discrepancy: all of the stars are slightly undermassive, by typically 11% (or overluminous by 0.2 dex) compared with that predicted by the evolutionary models. We examine our approach for systematic problems, but find no satisfactory explanation. The discrepancy is in the same sense as the long-discussed and elusive discrepancy between the masses measured from stellar atmosphere analysis with the stellar evolutionary models, and might suggest that either increased rotation or convective overshooting is needed in the models. Additional systems will be discussed in future papers of this series, and will hopefully confirm or refute this trend. Key words: binaries: eclipsing – binaries: spectroscopic – stars: early-type – stars: fundamental parameters Online-only material: color figures, machine-readable tables 1. INTRODUCTION However, for high-mass stars there are two additional com- plications that make applying the MLR difficult. First, main- 1.1. Motivation sequence massive stars (O- and early B-type stars) are quite hot (Teff 25,000–50,000 K) and because of this only a tiny frac- The mass of a star is arguably its most fundamental quantity; tion of= their light leaks out in the visible. In order to apply the according to the Russell–Vogt theorem, it is the mass of MLR, one needs to know not only the distance and reddening astar(alongwiththechemicalcomposition)thatuniquely of a star (in order to get the absolute visual magnitude, MV ), determines a star’s evolution. We now know that the initial but also an accurate value for the effective temperature, as the angular momentum also plays an important role in determining correction to MV needed to obtain the bolometric luminosity is the evolution of a star (Maeder & Meynet 2000;Meynet& asteepfunctionofeffectivetemperature.Thesecondcomplica- Maeder 2000). tion is that the MLR is really a function of age. This is true for For most stars, the simple way to estimate the mass (m)of all stars, as their luminosities increase slightly as they evolve, a star is by measuring the star’s luminosity (L), as L mx, ∼ but massive stars also lose mass as they evolve, due to radia- where the exponent x is approximately 4 for solar-type stars. tively driven stellar winds. For massive stars these issues can be For both lower mass (<0.5 M )andhighermass(>10 M ) ⊙ ⊙ solved by first modeling the optical spectra using non-LTE stel- stars, the exponent becomes smaller, due to the importance of lar atmosphere codes, such as CMFGEN (Hillier & Miller 1998) convection in lower mass stars, and radiation pressure in higher or FASTWIND (Puls et al. 2005). This then provides both the mass stars. For most stars, the exponent in this mass–luminosity effective temperature and the bolometric luminosity, allowing relationship (MLR) is well established both by evolutionary 6 placement of the stars on the H-R diagram (HRD). Reference to theory and empirical measurements. stellar evolutionary models then allows the determination of age and current mass (often referred to as the “evolutionary mass”) ∗ This paper includes data gathered with the 6.5 m Magellan and 1.0 m Swope Telescopes located at Las Campanas Observatory, Chile, as well as data without the use of an MLR per se. obtained with the SMARTS Consortium 1.3 and 1.0 m telescopes located at However, this method is no better than the stellar atmosphere Cerro Tololo Inter-American Observatory, Chile. and evolutionary models on which they are based. A worrisome 6 Indeed, one of the great vindications of stellar evolutionary theory was the issue that remains is that of the so-called mass discrepancy, a fact that Eddington (1924) was able to derive the exponent for solar-type stars purely from the physics of radiative diffusion. systematic difference between the masses one obtains from a 1 The Astrophysical Journal,748:96(22pp),2012April1 Massey et al. stellar atmosphere analysis and that inferred from evolutionary the system, as the comparison between the modeled absolute theory. Modeling the star’s spectra with a stellar atmosphere visual magnitude and the observed absolute visual magnitudes code produces a measurement of the mass via the surface gravity provides assurance that the physical parameters are correct.9 g,sinceg m/R2,andtheradiusR is known once the effective Since eclipsing systems are necessary for the full analysis, we temperature∼ is known if the distance and hence the luminosity L decided to instead start by searching for stars whose light curves 2 4 are known, since R L/Teff by the Stefan–Boltzmann suggested they might be eclipsing massive stars. We chose to equation. Herrero et al. (1992∼ )calledattentiontothefactthatthe concentrate on massive stars in the Magellanic Clouds since masses derived from spectroscopic analysis are systematically their distances are well known, and they are bright enough lower than those found from evolutionary models. This mass for spectroscopic follow-up, albeit with larger apertures. In discrepancy has never been fully resolved, despite significant addition, since their metallicities are relatively low, mass loss improvements in both the evolution and stellar atmosphere on the main sequence should be relatively modest, and thus models (see, for example, Massey et al. 2005). But it would the connection to the initial masses should be less dependent be of great interest to measure masses in some more direct way upon the assumed mass-loss rates of the evolutionary models. to test the validity of the models. Such an opportunity is granted By precisely refining the periods and times of primary eclipses to us by binaries, where Newtonian physics and Kepler’s third using frequent photometric observations prior to spectroscopic law provide us (in principle) with direct mass measurements. observations, we should know exactly when to observe these This is the first of a series of papers presenting dynamical stars at maximum velocity separation (orbital quadrature). masses and bolometric luminosities for massive stars in the This allowed us to measure the orbital semi-amplitudes very Large Magellanic Cloud (LMC). The title of our series is efficiently for two reasons. First, O-type stars have very broad intended to pay homage to the “Spectroscopic Studies of spectral lines due to rotational broadening (v sin i typically 1 O-type Binaries” series by Peter S. Conti and collaborators, greater than 100 km s− )andmost spectral observations more which appeared 30–35 years ago (e.g., Bohannan & Conti 1976; than 0.1 phase away from quadrature are unlikely to show re- Conti & Walborn 1976; Massey & Conti 1977;Morrison& solvable double lines and hence do not provide useful measure- Conti 1978, 1980;Contietal.1980). Although telescopes have ments. Second, it is the velocities around quadrature that best gotten larger since those days, and instruments and data analysis define the orbital semi-amplitudes. And, since the phases would methods have improved, the basic need to test theory with be known exactly, determining the amplitude of the orbital mo- fundamental mass determinations remains. Work over the past tion could be done with very few observations, as we could fix several decades has helped improve the situation7 but the paucity the phases in the orbital solutions to those determined from the of systems with well determined parameters is underscored by light curve. Thus, we minimized the amount of large telescope our continuing poor knowledge of the MLR and the persistence time needed for the spectroscopy by utilizing small aperture of the mass discrepancy. telescopes to determine the light curves.10 One of the downsides to this approach is that it is easier to 1.2. Our Approach find short-period systems as their light variabilities are more Historically, studies of massive binaries have usually begun pronounced; for a given orbital inclination, eclipses will be by the accidental discovery of double lines in the spectrum as a deeper for shorter period systems.