Characterizing the Parents: Exoplanets Around Cool Stars
Kaspar von Braun, Tabetha S. Boyajian, Gerard T. van Belle, Andrew Mann, Stephen R. Kane
aa r X i v : . [ a s t r o - ph . S R ] A ug Characterizing the Parents: Exoplanets Around Cool Stars
Kaspar von Braun , Tabetha S. Boyajian , Gerard T. van Belle ,Andrew Mann , Stephen R. Kane Max-Planck-Institute for Astronomy (MPIA), K¨onigstuhl 17, 69117Heidelberg, Germany; [email protected] Yale University, P.O. Box 208101, New Haven, CT 06520-8101, USA Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA University of Texas, 2515 Speedway, Stop C1400, Austin, TX 78712-1205,USA San Francisco State University, 1600 Holloway Ave., San Francisco, CA94132, USA
Abstract.
The large majority of stars in the Milky Way are late-typedwarfs, and the frequency of low-mass exoplanets in orbits around theselate-type dwarfs appears to be high. In order to characterize the radiationenvironments and habitable zones of the cool exoplanet host stars, stellarradius and effective temperature, and thus luminosity, are required. It is inthe stellar low-mass regime, however, where the predictive power of stellarmodels is often limited by sparse data volume with which to calibrate themethods. We show results from our CHARA survey that provides directlydetermined stellar parameters based on interferometric diameter measure-ments, trigonometric parallax, and spectral energy distribution fitting.
1. “Why?” and “How?”: Introduction and Methods
Essentially every astrophysical parameter of any exoplanet is a function of itsequivalent host star parameters (radius, surface temperature, mass, etc.).
Youonly understand any exoplanet as well as you understand its respec-tive parent star.
The main purpose of the presented research is to directlycharacterize exoplanets in orbits around their hosts and to produce empiricalconstraints to stellar models. We use infrared and optical interferometry, cou-pled with spectral energy distribution fitting and trigonometric parallax values,to get estimates of stellar radii and effective temperatures that are as model-independent as possible. For more details, see, e. g., Boyajian et al. (2013) andvon Braun et al. (2014). For transiting planets, using literature photometry andspectroscopy time-series data allows for the determination of model-independentplanetary and stellar masses, radii, and bulk densities (e. g., von Braun et al.,2012). 1 von Braun et al.
Figure 1.:
Habitable zones are calculated based on our empirical values of stellarradii and effective temperatures. This plot shows the system architecture of theGJ 876 system. The HZ is shown in grey. Planets b and c spend most or all oftheir orbital durations in the HZ. For scale: the size of the box is 0.8 AU ×
2. “So What?”: Results
Our results provide empirically determined values for stellar radii, effective tem-peratures, and luminosities. They confirm the well-documented discrepancy be-tween predicted and empirical radii and temperatures (e.g., Torres et al., 2010;Boyajian et al., 2012, 2013) and can thus provide constraints to improvementsto stellar models. They can furthermore be used to establish relations to pre-dict stellar sizes based on observable quantities, like stellar broad-band colors,for stars too faint and/or small to be studied interferometrically (Boyajian et al.,2014). In addition, any individual system’s circumstellar habitable zone (HZ) isa function of stellar radius and effective temperature (Fig. 1).
3. “And what have you done for me lately?”: Status
Over the course of the last 5+ years, we have been using the CHARA inter-ferometric arrray to directly determine the stellar parameters of over 100 main-sequence stars and of around 30 exoplanet host stars, with a particular emphasison cool stars (Fig. 2).
Acknowledgements.
We would like to sincerely thank the organizers for a phantasticconference. We furthermore express our gratitude to the poster judges for their thumbsup on our work. This research has made use of the Habitable Zone Gallery at hzgallery.org(Kane & Gelino, 2012). xoplanets Around Cool Stars eff (K)−3−2−1012345 l o g L ( L ⊙ ) Figure 2.:
Empirical H-R Diagram for all stars with interferometrically de-termined stellar radii whose random uncertainties are smaller than 5%. Thediameter of each data point is representative of the logarithm of the cor-responding stellar radius. Error bars in effective temperature and luminos-ity are smaller than the size of the data points. Exoplanet host stars areshown in red; stars that are not currently known to host any exoplanets areshown in grey. Stellar radii data are taken from Baines et al. (2008, 2012,2013); Bigot et al. (2006); Boyajian et al. (2008, 2012, 2013); di Folco et al.(2004, 2007); Henry et al. (2013); Kervella et al. (2003); Ligi et al. (2012);Richichi et al. (2005); van Belle et al. (1999); van Belle & von Braun (2009);von Braun et al. (2011a,b, 2012, 2014); White et al. (2013). von Braun et al.
Figure 3.: