81 research outputs found
Eclipsing binary systems as tests of low-mass stellar evolution theory
Stellar fundamental properties (masses, radii, effective temperatures) can be
extracted from observations of eclipsing binary systems with remarkable
precision, often better than 2%. Such precise measurements afford us the
opportunity to confront the validity of basic predictions of stellar evolution
theory, such as the mass-radius relationship. A brief historical overview of
confrontations between stellar models and data from eclipsing binaries is
given, highlighting key results and physical insight that have led directly to
our present understanding. The current paradigm that standard stellar evolution
theory is insufficient to describe the most basic relation, that of a star's
mass to its radius, along the main sequence is then described. Departures of
theoretical expectations from empirical data, however, provide a rich
opportunity to explore various physical solutions, improving our understanding
of important stellar astrophysical processes.Comment: 9 pages, 2 figures. To appear in proceedings of "Living Together:
Planets, Host Stars, and Binaries" convened in memory of Zdenek Kopa
Magnetic Inhibition of Convection and the Fundamental Properties of Low-Mass Stars. III. A Consistent 10 Myr Age for the Upper Scorpius OB Association
When determining absolute ages of identifiably young stellar populations,
results strongly depend on which stars are studied. Cooler (K, M) stars
typically yield ages that are systematically younger than warmer (A, F, G)
stars by a factor of two. I explore the possibility that these age
discrepancies are the result of magnetic inhibition of convection in cool young
stars by using magnetic stellar evolution isochrones to determine the age of
the Upper Scorpius subgroup of the Scorpius-Centaurus OB Association. A median
age of 10 Myr consistent across spectral types A through M is found, except for
a subset of F-type stars that appear significantly older. Agreement is shown
for ages derived from the Hertzsprung-Russell diagram and from the empirical
mass-radius relationship defined by eclipsing multiple-star systems. Surface
magnetic field strengths required to produce agreement are of order 2.5 kG and
are predicted from a priori estimates of equipartition values. A region in the
HR diagram is identified that plausibly connects stars whose structures are
weakly influenced by the presence of magnetic fields with those whose
structures are strongly influenced by magnetic fields. The models suggest this
region is characterized by stars with rapidly thinning outer convective
envelopes where the radiative core mass is greater than 75% of the total
stellar mass. Furthermore, depletion of lithium predicted from magnetic models
appears in better agreement with observed lithium equivalent widths than
predictions from non-magnetic models. These results suggest that magnetic
inhibition of convection plays an important role in the early evolution of
low-mass stars and that it may be responsible for noted age discrepancies in
young stellar populations.Comment: 11 pages, 6 figures, 2 tables. Accepted to A&A. Models available
online: https://github.com/gfeiden/MagneticUpperSco
The Interior Structure Constants as an Age Diagnostic for Low-Mass, Pre-Main Sequence Detached Eclipsing Binary Stars
We propose a novel method for determining the ages of low-mass, pre-main
sequence stellar systems using the apsidal motion of low-mass detached
eclipsing binaries. The apsidal motion of a binary system with an eccentric
orbit provides information regarding the interior structure constants of the
individual stars. These constants are related to the normalized stellar
interior density distribution and can be extracted from the predictions of
stellar evolution models. We demonstrate that low-mass, pre-main sequence stars
undergoing radiative core contraction display rapidly changing interior
structure constants (greater than 5% per 10 Myr) that, when combined with
observational determinations of the interior structure constants (with 5 -- 10%
precision), allow for a robust age estimate. This age estimate, unlike those
based on surface quantities, is largely insensitive to the surface layer where
effects of magnetic activity are likely to be most pronounced. On the main
sequence, where age sensitivity is minimal, the interior structure constants
provide a valuable test of the physics used in stellar structure models of
low-mass stars. There are currently no known systems where this technique is
applicable. Nevertheless, the emphasis on time domain astronomy with current
missions, such as Kepler, and future missions, such as LSST, has the potential
to discover systems where the proposed method will be observationally feasible.Comment: Accepted for publication in ApJ, 8 pages, 3 figure
Self-Consistent Magnetic Stellar Evolution Models of the Detached, Solar-Type Eclipsing Binary EF Aquarii
We introduce a new one-dimensional stellar evolution code, based on the
existing Dartmouth code, that self-consistently accounts for the presence of a
globally pervasive magnetic field. The methods involved in perturbing the
equations of stellar structure, the equation of state, and the mixing-length
theory of convection are presented and discussed. As a first test of the code's
viability, stellar evolution models are computed for the components of a
solar-type, detached eclipsing binary (DEB) system, EF Aquarii, shown to
exhibit large disagreements with stellar models. The addition of the magnetic
perturbation corrects the radius and effective temperature discrepancies
observed in EF Aquarii. Furthermore, the required magnetic field strength at
the model photosphere is within a factor of two of the magnetic field strengths
estimated from the stellar X-ray luminosities measured by ROSAT and those
predicted from Ca II K line core emission. These models provide firm evidence
that the suppression of thermal convection arising from the presence of a
magnetic field is sufficient to significantly alter the structure of solar-type
stars, producing noticeably inflated radii and cooler effective temperatures.
The inclusion of magnetic effects within a stellar evolution model has a wide
range of applications, from DEBs and exoplanet host stars to the donor stars of
cataclysmic variables.Comment: Accepted for publication in ApJ, 15 pages, 3 figures; Misprints are
corrected in version
Magnetic Inhibition of Convection and the Fundamental Properties of Low-Mass Stars. II. Fully Convective Main Sequence Stars
We examine the hypothesis that magnetic fields are inflating the radii of
fully convective main sequence stars in detached eclipsing binaries (DEBs). The
magnetic Dartmouth stellar evolution code is used to analyze two systems in
particular: Kepler-16 and CM Draconis. Magneto-convection is treated assuming
stabilization of convection and also by assuming reductions in convective
efficiency due to a turbulent dynamo. We find that magnetic stellar models are
unable to reproduce the properties of inflated fully convective main sequence
stars, unless strong interior magnetic fields in excess of 10 MG are present.
Validation of the magnetic field hypothesis given the current generation of
magnetic stellar evolution models therefore depends critically on whether the
generation and maintenance of strong interior magnetic fields is physically
possible. An examination of this requirement is provided. Additionally, an
analysis of previous studies invoking the influence of star spots is presented
to assess the suggestion that star spots are inflating stars and biasing light
curve analyses toward larger radii. From our analysis, we find that there is
not yet sufficient evidence to definitively support the hypothesis that
magnetic fields are responsible for the observed inflation among fully
convective main sequence stars in DEBs.Comment: Accepted for publication in ApJ, 17 pages, 11 figures, 2 table
Revised age for CM Draconis and WD 1633+572: Toward a resolution of model-observation radius discrepancies
We report an age revision for the low-mass detached eclipsing binary CM
Draconis and its common proper motion companion, WD 1633+572. An age of 8.5
3.5 Gyr is found by combining an age estimate for the lifetime of WD
1633+572 and an estimate from galactic space motions. The revised age is
greater than a factor of two older than previous estimates. Our results provide
consistency between the white dwarf age and the system's galactic kinematics,
which reveal the system is a highly probable member of the galactic thick disk.
We find the probability that CM Draconis and WD 1633+572 are members of the
thick disk is 8500 times greater than the probability that they are members of
the thin disk and 170 times greater than the probability they are halo
interlopers. If CM Draconis is a member of the thick disk, it is likely
enriched in -elements compared to iron by at least 0.2 dex relative to
the Sun. This leads to the possibility that previous studies under-estimate the
[Fe/H] value, suggesting the system has a near-solar [Fe/H]. Implications for
the long-standing discrepancies between the radii of CM Draconis and
predictions from stellar evolution theory are discussed. We conclude that CM
Draconis is only inflated by about 2% compared to stellar evolution
predictions.Comment: Accepted to A&A, 7 pages, 3 figures, 1 tabl
Revised Age for CM Draconis and WD 1633+ 572-Toward a Resolution of Model-Observation Radius Discrepancies
We report an age revision for the low-mass detached eclipsing binary CM Draconis and its common proper motion companion, WD 1633+572. An age of 8.5 +/- 3.5 Gyr is found by combining an age estimate for the lifetime of WD 1633+572 and an estimate from galactic space motions. The revised age is greater than a factor of two older than previous estimates. Our results provide consistency between the white dwarf age and the system\u27s galactic kinematics, which reveal the system is a highly probable me
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