Absolute Dimensions of the M-Type Eclipsing Binary YY Geminorum (Castor C): A Challenge to Evolutionary Models in the Lower Main Sequence

By G. Torres, I. Ribas. 2001

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We present new spectroscopic observations of the detached late-type double-lined eclipsing binary YY Geminorum (P = 0.814 day), a member of the Castor sextuple system and one of the benchmarks for the comparison between observations and stellar evolution theory in the lower main sequence. In addition, we have reanalyzed existing light curves in several passbands using modern techniques that account for the conspicuous presence of spots. This, combined with the spectroscopy, has yielded a very precise determination of the absolute dimensions of the components, which are virtually identical to each other. We obtain for the mean mass, radius, and effective temperature the values M = 0.5992 ± 0.0047 M☉, R = 0.6191 ± 0.0057 R☉, and Teff = 3820 ± 100 K, respectively. Both the mass and the radius determinations are good to better than 1%, which in the case of the radius represents a fourfold improvement over previous results and significantly enhances the value of this quantity for testing the models. We discuss the importance of systematic effects in these measurements by comparison with another high-precision determination of the mass by Ségransan and coworkers. A reanalysis of the Hipparcos transit data for Castor AB that accounts for the relative motion of the pair in its 467 yr period orbit has yielded an improved parallax for the system of 66.90 ± 0.63 mas. With this, we have estimated the age (~370 Myr) and metal abundance ([Fe/H] ~ 0.0) of YY Gem from isochrone fits to Castor A and B under the assumption of a common origin. This, along with the other physical properties, allow for an unusually stringent test of the models for low-mass stars. We have compared the observations of YY Gem with a large number of recent theoretical calculations, and we show that all models underestimate the radius by up to 20% and that most overestimate the effective temperature by 150 K or more. Both of these trends are confirmed by observations of another similar system in the Hyades (V818 Tau). Consequently, theoretical ages for relatively low mass objects such as T Tauri stars derived by placing them on the H-R diagram may be considerably biased. If the radius is used directly as a measure of evolution, ages could be underestimated by as much as a factor of 10 in this mass regime. In view of these discrepancies, absolute ages from essentially all current models for the lower main sequence must be viewed with at least some measure of skepticism. Finally, we derive a new and very accurate ephemeris based on all available times of eclipse, and we lay to rest previous claims of sudden changes in the orbital period of the binary, which we show to be spurious.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/d6b5441cc3cb40617ea2cc424bed4b098b67ad56

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Citations in the corpus

• The star formation history and accretion-disc fraction among the K-type members of the Scorpius–Centaurus OB association (2016, background): “The most obvious reason is that evolutionary models have difficulty predicting the radii of main-sequence M-type stars, systematically underestimating their radii by ∼5- 20% (Torres & Ribas 2002; Boyajian et al. 2012; see also Kraus et al. 2011).”
• PLANET HUNTERS: A TRANSITING CIRCUMBINARY PLANET IN A QUADRUPLE STAR SYSTEM (2012, background): “…and these stars have shown significant disagreements with theory in the sense that they tend to be larger and cooler than predicted (Ribas 2003; Torres & Ribas 2002; López-Morales – 21 – & Ribas 2005; Morales et al. 2009; Torres 2012) Figure 13 compares the measured properties of the Aa star…”
• On the Correlation between the Magnetic Activity Levels, Metallicities, and Radii of Low-Mass Stars (2007, methodology): “For the binaries, rotational velocity measurements are only available for some, that is, YYGemAB (Torres & Ribas 2002), GU Boo AB (López-Morales & Ribas 2005), FL Lyr B, V1061 Cyg Ab-B, and RW Lac B (Torres et al. 2006).”
• The Habitability of Planets Orbiting M-dwarf Stars (2016, background): “There exists a tension between stellar models and empirically-derived properties for low-mass stars at the level of 5-10% (Torres & Ribas, 2002; López-Morales, 2007; Morales et al., 2008, 2009; Vida et al., 2009; Carter et al., 2011; Kraus et al., 2011).”
• Masses and Radii of Low-Mass Stars: Theory Versus Observations (2005, unknown): “Also listed in the table are the components of two eclipsing systems with masses below 0.8 M ⊙ : The Hyades EB V818 Tau (Torres & Ribas 2002) and RXJ0239.”
• KEPLER ECLIPSING BINARY STARS. VII. THE CATALOG OF ECLIPSING BINARIES FOUND IN THE ENTIRE KEPLER DATA SET (2015, methodology): “These mass and radius measurements in turn allow for accurate tests of stellar evolution models (e.g., Pols et al. 1997; Schroder et al. 1997; Guinan et al. 2000; Torres & Ribas 2002).”
• AN EMPIRICAL UBV RI JHK COLOR–TEMPERATURE CALIBRATION FOR STARS (2006, methodology): “Eclipsing binaries YY Geminorum (Torres & Ribas 2002) and CM Draconis (Viti et al. 1997) were supplemented with interferometrically derived temperatures from Berger et al. (2006) and, in the case of Barnard’s star, from Dawson & de Robertis (2004).”
• First radius measurements of very low mass stars with the VLTI (2002, background): “The discrepancy is highly significant for the eclipsing binaries, as extensively discussed by Torres & Ribas (2002) and is more marginal for long baseline interferometry data.”
• HOW TO CONSTRAIN YOUR M DWARF: MEASURING EFFECTIVE TEMPERATURE, BOLOMETRIC LUMINOSITY, MASS, AND RADIUS (2015, unknown): “LMEB parameters are taken from Torres & Ribas (2002), Ribas (2003), LópezMorales & Ribas (2005), López-Morales et al. (2006), LópezMorales & Shaw (2007), Irwin et al. (2009, 2011), Morales et al. (2009a, 2009b), Carter et al. (2011), Doyle et al. (2011), Kraus et al. (2011), Orosz et al. (2012a,…”
• A PHOTOMETRIC VARIABILITY SURVEY OF FIELD K AND M DWARF STARS WITH HATNet (2009, background): “…K and M dwarf components have led to the realization that the radii of early-M dwarfs and late-K dwarfs are somewhat larger than predicted by theory (the number typically stated is 10%; Torres & Ribas 2002; Ribas 2003; López-Morales 2005; Ribas 2006; Beatty et al. 2007; Fernandez et al. 2009).”
• Evolution of low-mass star and brown dwarf eclipsing binaries (2007, background): “%) from these predictions (Torres & Ribas 2002; Chabrier et al. 2005).”
• S4N: A spectroscopic survey of stars in the solar neighborhood: The nearest 15 pc (2004, background): “Interestingly, comparison between precise stellar radii for stars in eclipsing binaries with evolutionary isochrones has shown that G and K-type stars in detached systems are brighter than predicted (see, e.g., Clausen et al. 1999; Allende Prieto 2001; Torres & Ribas 2002; Ribas 2003).”
• M dwarfs: effective temperatures, radii and metallicities (2008, background): “The discrepancy between the predicted and observed radii in M dwarfs could be related to the activity level of the stars (e.g. Torres & Ribas 2002; López-Morales & Ribas 2005).”
• High-precision photometry by telescope defocussing. I. The transiting planetary system WASP-5 (2009, background): “Unfortunately, stellar models are persistently unable to match the directly measured physical properties of low-mass eclipsing binary star systems, although this effect is likely due to magnetic activity (Hoxie 1973; Clausen 1998; Torres & Ribas 2002; Ribas et al. 2008).”
• GU Bootis: A New 0.6 M☉ Detached Eclipsing Binary (2005, methodology): “The absolute magnitudes of YY Gem were computed by Torres & Ribas (2002) using the Hipparcos parallax of stars in the Castor group (YY Gem is thought to be part of it), while the absolute magnitudes of GU Boo were derived using bolometric corrections from models (see x 5.1).”