10,259 research outputs found
Stellar Diameters and Temperatures II. Main Sequence K & M Stars
We present interferometric diameter measurements of 21 K- and M- dwarfs made
with the CHARA Array. This sample is enhanced by literature radii measurements
to form a data set of 33 K-M dwarfs with diameters measured to better than 5%.
For all 33 stars, we compute absolute luminosities, linear radii, and effective
temperatures (Teff). We develop empirical relations for \simK0 to M4 main-
sequence stars between the stellar Teff, radius, and luminosity to broad-band
color indices and metallicity. These relations are valid for metallicities
between [Fe/H] = -0.5 and +0.1 dex, and are accurate to ~2%, ~5%, and ~4% for
Teff, radius, and luminosity, respectively. Our results show that it is
necessary to use metallicity dependent transformations to convert colors into
stellar Teffs, radii, and luminosities. We find no sensitivity to metallicity
on relations between global stellar properties, e.g., Teff-radius and
Teff-luminosity. Robust examinations of single star Teffs and radii compared to
evolutionary model predictions on the luminosity-Teff and luminosity-radius
planes reveals that models overestimate the Teffs of stars with Teff < 5000 K
by ~3%, and underestimate the radii of stars with radii < 0.7 R\odot by ~5%.
These conclusions additionally suggest that the models overestimate the effects
that the stellar metallicity may have on the astrophysical properties of an
object. By comparing the interferometrically measured radii for single stars to
those of eclipsing binaries, we find that single and binary star radii are
consistent. However, the literature Teffs for binary stars are systematically
lower compared to Teffs of single stars by ~ 200 to 300 K. Lastly, we present a
empirically determined HR diagram for a total of 74 nearby, main-sequence, A-
to M-type stars, and define regions of habitability for the potential existence
of sub-stellar mass companions in each system. [abridged]Comment: 73 pages, 12 Tables, 18 Figures. Accepted for publication in The
Astrophysical Journa
Population Parameters of Intermediate-Age Star Clusters in the Large Magellanic Cloud. I. NGC 1846 and its Wide Main Sequence Turnoff
The Advanced Camera for Surveys on board the Hubble Space Telescope has been
used to obtain deep, high-resolution images of the intermediate-age star
cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude
diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the
previously observed broad main sequence turnoff region for "contamination" by
field stars and (evolved) binary star systems. We find that while these impact
the number of objects in this region, none can fully account for the large
color spread. Our results therefore solidify the recent finding that stars in
the main sequence turnoff region of this cluster have a large spread in color
which is unrelated to measurement errors or contamination by field stars, and
likely due to a ~300 Myr range in the ages of cluster stars. An unbiased
estimate of the stellar density distribution across the main sequence turnoff
region shows that the spread is fairly continuous rather than strongly bimodal
as suggested previously. We fit the CMDs with several different sets of
theoretical isochrones, and determine systematic uncertainties for population
parameters when derived using any one set of isochrones. We note a degeneracy
between age and [alpha/Fe], which can be lifted by matching the shape
(curvature) of the full red giant branch in the CMD. We find that stars in the
upper part of the main sequence turnoff region are more centrally concentrated
than those in any other region of the CMD, including more massive red giant
branch and asymptotic giant branch stars. We consider several possible
formation scenarios which account for the unusual features observed in the CMD
of NGC 1846.Comment: 17 pages, 17 figures, in emulateapj format. Figures 1 and 2 have been
downgraded in resolution in this version. Accepted for publication in The
Astronomical Journa
Magnetospheric Cavity Modes Driven by Solar Wind Dynamic Pressure Fluctuations
We present results from Lyon-Fedder-Mobarry (LFM) global, three-dimensional
magnetohydrodynamic (MHD) simulations of the solar wind-magnetosphere
interaction. We use these simulations to investigate the role that solar wind
dynamic pressure fluctuations play in the generation of magnetospheric
ultra-low frequency (ULF) pulsations. The simulations presented in this study
are driven with idealized solar wind input conditions. In four of the
simulations, we introduce monochromatic ULF fluctuations in the upstream solar
wind dynamic pressure. In the fifth simulation, we introduce a continuum of ULF
frequencies in the upstream solar wind dynamic pressure fluctuations. In this
numerical experiment, the idealized nature of the solar wind driving conditions
allows us to study the magnetospheric response to only a fluctuating upstream
dynamic pressure, while holding all other solar wind driving parameters
constant. The simulation results suggest that ULF fluctuations in the solar
wind dynamic pressure can drive magnetospheric ULF pulsations in the electric
and magnetic fields on the dayside. Moreover, the simulation results suggest
that when the driving frequency of the solar wind dynamic pressure fluctuations
matches one of the natural frequencies of the magnetosphere, magnetospheric
cavity modes can be energized.Comment: 2 figure
Discovery and Validation of Kepler-452b: A 1.6-Re Super Earth Exoplanet in the Habitable Zone of a G2 Star
We report on the discovery and validation of Kepler-452b, a transiting planet
identified by a search through the 4 years of data collected by NASA's Kepler
Mission. This possibly rocky 1.63 R planet orbits
its G2 host star every 384.843 days, the longest orbital
period for a small (R < 2 R) transiting exoplanet to date. The
likelihood that this planet has a rocky composition lies between 49% and 62%.
The star has an effective temperature of 575785 K and a log g of
4.320.09. At a mean orbital separation of 1.046 AU,
this small planet is well within the optimistic habitable zone of its star
(recent Venus/early Mars), experiencing only 10% more flux than Earth receives
from the Sun today, and slightly outside the conservative habitable zone
(runaway greenhouse/maximum greenhouse). The star is slightly larger and older
than the Sun, with a present radius of 1.11 R and an
estimated age of 6 Gyr. Thus, Kepler-452b has likely always been in the
habitable zone and should remain there for another 3 Gyr.Comment: 19 pages, 16 figure
Binary Contamination in the SEGUE sample: Effects on SSPP Determinations of Stellar Atmospheric Parameters
Using numerical modeling and a grid of synthetic spectra, we examine the
effects that unresolved binaries have on the determination of various stellar
atmospheric parameters for SEGUE targets measured using the SEGUE Stellar
Parameter Pipeline (SSPP). To model undetected binaries that may be in the
SEGUE sample, we use a variety of mass distributions for the primary and
secondary stars in conjunction with empirically determined relationships for
orbital parameters to determine the fraction of G-K dwarf stars, as defined by
SDSS color cuts, that will be blended with a secondary companion. We focus on
the G-K dwarf sample in SEGUE as it records the history of chemical enrichment
in our galaxy. To determine the effect of the secondary on the spectroscopic
parameters, we synthesize a grid of model spectra from 3275 to 7850 K (~0.1 to
1.0 \msun) and [Fe/H]=-0.5 to -2.5 from MARCS model atmospheres using
TurboSpectrum. We analyze both "infinite" signal-to-noise ratio (S/N) models
and degraded versions, at median S/N of 50, 25 and 10. By running individual
and combined spectra (representing the binaries) through the SSPP, we determine
that ~10% of the blended G-K dwarf pairs with S/N>=25 will have their
atmospheric parameter determinations, in particular temperature and
metallicity, noticeably affected by the presence of an undetected secondary. To
account for the additional uncertainty from binary contamination at a S/N~10,
uncertainties of ~140 K and ~0.17 dex in [Fe/H] must be added in quadrature to
the published uncertainties of the SSPP. (Abridged)Comment: 68 pages, 20 figures, 9 table
- …