211 research outputs found
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
Capella (alpha Aurigae) revisited: New binary orbit, physical properties, and evolutionary state
Knowledge of the chemical composition and absolute masses of Capella are key
to understanding the evolutionary state of this benchmark binary system
comprising two giant stars. Previous efforts, including our own 2009 study,
have largely failed to reach an acceptable agreement between the observations
and current stellar evolution models, preventing us from assessing the status
of the primary. Here we report a revision of the physical properties of the
components incorporating recently published high-precision radial velocity
measurements, and a new detailed chemical analysis providing abundances for
more than 20 elements in both stars. We obtain highly precise (to about 0.3%)
masses of 2.5687 +/- 0.0074 and 2.4828 +/- 0.0067 solar masses, radii of 11.98
+/- 0.57 and 8.83 +/- 0.33 solar radii, effective temperatures of 4970 +/- 50 K
and 5730 +/- 60 K, and independently measured luminosities based on the orbital
parallax (78.7 +/- 4.2 and 72.7 +/- 3.6 solar luminosities). We find an
excellent match to stellar evolution models at the measured composition of
[Fe/H] = -0.04 +/- 0.06. Three different sets of models place the primary star
firmly at the end of the core helium-burning phase (clump), while the secondary
is known to be evolving rapidly across the Hertzprung gap. The measured lithium
abundance, the C/N ratio, and the 12C/13C isotopic carbon abundance ratio,
which change rapidly in the giant phase, are broadly in agreement with
expectations from models. Predictions from tidal theory for the spin rates,
spin-orbit alignment, and other properties do not fare as well, requiring a
40-fold increase in the efficiency of the dissipation mechanisms in order to
match the observations.Comment: 15 pages in emulateapj format, including figures and tables, accepted
for publication in The Astrophysical Journa
Horizontal Branch Morphology and Mass Loss in Globular Clusters
The connection between mass loss on the red giant branch (RGB) and horizontal
branch (HB) morphology in globular clusters (GCs) has long been acknowledged
but the mechanisms governing mass loss remains poorly understood from a
theoretical perspective. The present study uses synthetic HB models to
demonstrate for the first time that alpha-enhancement and a simple relation
between mass loss and metallicity can explain the entire range of HB morphology
(characterized by the HB type index) observed in old, coeval GCs. The mass
loss-metallicity relation accounts naturally for the fact that the most metal
poor GCs ([Fe/H] < -2) have redder HBs than is typical of GCs with -2 < [Fe/H]
< -1.5 without invoking younger ages. These results may prove useful in
studying the contribution of HB stars to integrated light via stellar
population synthesis.Comment: 13 pages, 5 figures, to appear in ApJ Letters (figure 2 may not
display correctly in some PDF viewers
Optimal Integrated Abundances for Chemical Tagging of Extragalactic Globular Clusters
High resolution integrated light (IL) spectroscopy provides detailed
abundances of distant globular clusters whose stars cannot be resolved.
Abundance comparisons with other systems (e.g. for chemical tagging) require
understanding the systematic offsets that can occur between clusters, such as
those due to uncertainties in the underlying stellar population. This paper
analyses high resolution IL spectra of the Galactic globular clusters 47 Tuc,
M3, M13, NGC 7006, and M15 to (1) quantify potential systematic uncertainties
in Fe, Ca, Ti, Ni, Ba, and Eu and (2) identify the most stable abundance ratios
that will be useful in future analyses of unresolved targets. When stellar
populations are well-modelled, uncertainties are ~0.1-0.2 dex based on
sensitivities to the atmospheric parameters alone; in the worst case scenarios,
uncertainties can rise to 0.2-0.4 dex. The [Ca I/Fe I] ratio is identified as
the optimal integrated [alpha/Fe] indicator (with offsets <0.1 dex), while [Ni
I/Fe I] is also extremely stable to within <0.1 dex. The [Ba II/Eu II] ratios
are also stable when the underlying populations are well modelled and may also
be useful for chemical tagging.Comment: 28 pages, 4 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Deep 2MASS Photometry of M67 and Calibration of the Main Sequence J-Ks Color Difference as an Age Indicator
We present an analysis of Two Micron All Sky Survey (2MASS) calibration
photometry of the old open cluster M67 (NGC 2682). The proper motion-cleaned
color-magnitude diagram (CMD) resulting from these data extends ~3 magnitudes
deeper than one based on data from the point source catalog. The CMD extends
from above the helium-burning red clump to a faint limit that is more than 7
magnitudes below the main sequence turnoff in the Ks band. After adopting a
reddening of E(B-V) = 0.041 +/- 0.004 and a metal abundance of [Fe/H] = -0.009
+/- 0.009 based on a survey of published values, we fit the unevolved main
sequence of M67 to field main sequence stars with 2MASS photometry and
Hipparcos parallaxes. This analysis yields distance moduli of (m-M)Ks = 9.72
+/- 0.05 and (m-M)o = 9.70 +/- 0.05, which are consistent with published
values. We compare the theoretical isochrones of Girardi et al. and Dotter et
al. to the CMD of M67 and comment on the relative merits of each set of models.
These comparisons suggest an age between 3.5 and 4.0 Gyr for M67. The depth of
the M67 data make them ideal for the calibration of a new age indicator that
has recently been devised by Calamida et al.- the difference in (J-Ks) color
between the main sequence turnoff (TO) and the point on the lower main sequence
where it turns down (TD) and becomes nearly vertical [D(J-Ks)]. Coupled with
deep 2MASS photometry for three other open clusters, NGC 2516, M44, and NGC
6791, we calibrate D(J-Ks) in terms of age and find D(J-Ks) = (3.017 +/- 0.347)
- (0.259 +/- 0.037)*Log Age (yrs).Comment: 19 pages, 9 figures, accepted for publication in The Astrophysical
Journa
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