ARCSAT ID NUMBER: AS05

DESCRIPTIVE TITLE: Time Series photometry of F/G/K + M dwarf eclipsing
binaries

PI: Leslie Hebb

OBSERVER(S): Leslie Hebb

UNCERTIFIED/UNTRAINED OBSERVERS: HWS students, Michelle Gomez and
Betty Vasquez

COLLABORATORS: Yilen Gomez Maqueo Chew
 
CONTACT INFORMATION: Leslie Hebb, hebb@hws.edu, 206-618-9843

TIME REQUESTED:  1 week in April

INSTRUMENT: Flarecam

FILTERS: griz

COMMENTS: no

BRIEF SCIENCE JUSTIFICATION:  

Eclipsing binary (EB) systems provide the most accurate direct
measurements of fundamental stellar properties (mass, radius and
effective temperature) for two stars with the same age and metallicity
[1,2].  Thus EBs are used to calibrate stellar evolutionary models and
to define empirical relationships used to derive the stellar
properties of stars for which direct measurements are not possible.
Generally, the radii of M-dwarf stars in EBs have been found to be
larger than those predicted from stellar models by 5--10\% and their
effective temperatures to be cooler by several hundred degrees Kelvin
(Fig.\ 1; e.g., [2,3,4,13,14]).

Growing observational evidence and theoretical calculations suggest
that magnetic fields on active, rapidly rotating EB components may be
causing the inflated radii and cooler temperatures [4,5,6,7,8].
According to the phenomenological model in [5] designed to explain
this phenomenon, there are two degenerate ways in which the magnetic
fields act to increase the stellar radius.  (1) Magnetic fields
threading the interior of the star limit the efficiency of the heat
flux transfer via convection, and the star compensates by expanding in
radius and cooling [5,6,8].  Alternately, (2) a large fractional
coverage of star spots caused by magnetic fields lowers the overall
surface temperature. In order to maintain the same luminosity, the
star increases in radius.  Efforts to disentangle these degenerate
effects on the stellar temperatures and radii are based on only a
handful of objects (e.g., [7,13,14]).  %a limited sample, a single EB
composed of two fully convective M-dwarfs CM Dra [7].  Recent results
from Kepler have provided precise masses and radii for two M dwarfs
(M$<$0.25M$_\odot$) in a triply eclipsing system [9]. However, their
temperatures are not measurable even from the best optical light
curves from Kepler in which they observe a dip in flux of less than
0.05\%.  To further test these effects requires additional
observations of fully convective M dwarfs with accurately measured
masses, radii, temperatures, and metallicities, and with a range of
magnetic activity.

We have an ongoing program to understand the effects of magnetic
activity and metallicity on the properties of low-mass stars using a
large sample ($\sim$130) of EB systems with F/G/K primaries and late
type M-dwarf secondaries discovered in the SuperWASP survey [10].
These very low-mass M dwarfs are very cool compared to their primary
companions.  Thus, the shallow secondary eclipse is observable in the
near-infrared (NIR), providing the only direct observational
constraint on the temperature of the M-dwarf secondaries.  Here we
propose to obtain g' and i' follow up photometry of approximately 10
of our eclipsing systems using ARCSAT.  These observations will lead
to much better constraints in radii on our low mass stars.

$[1]$ Andersen, J. 1991, A\&AR, 3, 91;
$[2]$ Torres, G. et al. 2010, A\&AR, 18, 67;
$[3]$ Morales, J. C.et al. 2008, A\&A, 478, 507;
$[4]$ L\'opez-Morales, M. 2007, ApJ, 660, 732;
$[5]$ Chabrier, G. et al. 2007, A\&A, 472L, 17;
$[6]$ Mullan, D. J., \& MacDonald, J. 2001, ApJ, 559, 353;
$[7]$ Morales, J. C. et al.\ 2010, ApJ, 718, 502;
$[8]$ Mazeh, T. 2008, EAS Publ. Ser., 29, 1;
$[9]$ Carter et al., 2011, Nature, 331, 6017;
$[10]$ Pollacco, D. et al. 2006, PASP, 118, 1407;
$[11]$ Pr\v{s}a, A. \& Zwitter, T. 2005, ApJ, 628, 426;
$[12]$ Baraffe, I. et al.\ 1998, A\&A, 337, 403;
$[13]$ Irwin et al. 2011, ApJ, 742, 123;
$[14]$ Birkby, J., et al.\ 2012, MNRAS, 426, 1507;