602 research outputs found
Early Results from APOKASC
Asteroseismology and spectroscopy provide complementary constraints on the
fundamental and chemical properties of stars. I describe the first results from
APOKASC, a collaboration between the Kepler asteroseismic science consortium
(KASC) and the SDSS-III APOGEE survey. These include (1) the first test of
asteroseismic scaling relationships in the metal-poor regime using halo and
thick disk stars identified in the APOKASC sample; and (2) the calibration of
spectroscopic parameters using precise asteroseismic measurements of surface
gravity. I also highlight future research avenues that are made possible by
this unique sample of thousands of well-characterized red giant stars.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
The enigmatic pair of dwarf galaxies Leo IV and Leo V: coincidence or common origin?
We have obtained deep photometry in two 1x1 degree fields covering the close
pair of dwarf spheroidal galaxies (dSph) Leo IV and Leo V and part of the area
in between. We find that both systems are significantly larger than indicated
by previous measurements based on shallower data and also significantly
elongated. With half-light radii of r_h=4'.6 +- 0'.8 (206 +- 36 pc) and
r_h=2'.6 +- 0'.6 (133 +- 31 pc), respectively, they are now well within the
physical size bracket of typical Milky Way dSph satellites. Their ellipticities
of epsilon ~0.5 are shared by many faint (M_V>-8) Milky Way dSphs. The large
spatial extent of our survey allows us to search for extra-tidal features with
unprecedented sensitivity. The spatial distribution of candidate red giant
branch and horizontal branch stars is found to be non-uniform at the ~3 sigma
level. This substructure is aligned along the direction connecting the two
systems, indicative of a possible `bridge' of extra-tidal material. Fitting the
stellar distribution with a linear Gaussian model yields a significance of 4
sigma for this overdensity, a most likely FWHM of ~16 arcmin and a central
surface brightness of ~32 mag arcsec^{-2}. We investigate different scenarios
to explain the close proximity of Leo IV and Leo V and the possible tidal
bridge between them. Orbit calculations demonstrate that they are unlikely to
be remnants of a single disrupted progenitor, while a comparison with
cosmological simulations shows that a chance collision between unrelated
subhalos is negligibly small. Leo IV and Leo V could, however, be a bound
`tumbling pair' if their combined mass exceeds 8 +- 4 x 10^9 M_sun. The
scenario of an internally interacting pair appears to be the most viable
explanation for this close celestial companionship. (abridged)Comment: 9 pages, 8 figures, small number of minor textual changes, accepted
for publication in Astrophysical Journa
C3PO: Towards a complete census of co-moving pairs of stars. I. High precision stellar parameters for 250 stars
We conduct a line-by-line differential analysis of a sample of 125 co-moving
pairs of stars (dwarfs and subgiants near solar metallicity). We obtain high
precision stellar parameters with average uncertainties in effective
temperature, surface gravity and metallicity of 16.5 K, 0.033 dex and 0.014
dex, respectively. We classify the co-moving pairs of stars into two groups,
chemically homogeneous (conatal; |Delta[Fe/H]| 0.04 dex) and
inhomogeneous (non-conatal), and examine the fraction of chemically homogeneous
pairs as a function of separation and effective temperature. The four main
conclusions from this study are: (1) A spatial separation of \ds = 10 AU is
an approximate boundary between homogeneous and inhomogeneous pairs of stars,
and we restrict our conclusions to only consider the 91 pairs with \ds
10 AU; (2) There is no trend between velocity separation and the fraction
of chemically homogeneous pairs in the range \dv 4 \kms; (3) We confirm
that the fraction of chemically inhomogeneous pairs increases with increasing
\teff\ and the trend matches a toy model of that expected from planet
ingestion; (4) Atomic diffusion is not the main cause of the chemical
inhomogeneity. A major outcome from this study is a sample of 56 bright
co-moving pairs of stars with chemical abundance differences 0.02 dex
(5\%) which is a level of chemical homogeneity comparable to that of the Hyades
open cluster. These important objects can be used, in conjunction with star
clusters and the \gaia\ ``benchmark'' stars, to calibrate stellar abundances
from large-scale spectroscopic surveys.Comment: MNRAS in press (see source file for full versions of long tables
The peculiar Horizontal Branch of NGC 2808
We present an accurate analysis of the peculiar Horizontal Branch (HB) of the
massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and
optical images of the central region of the cluster obtained with HST. We
confirm the multimodal distribution of stars along the HB: 4 sub-populations
separated by gaps are distinguishable. The detailed comparison with suitable
theoretical models showed that (i) it is not possible to reproduce the
luminosity of the entire HB with a single helium abundance, while an
appropriate modeling is possible for three HB groups by assuming different
helium abundances in the range 0.24 < Y < 0.4 that are consistent with the
multiple populations observed in the Main Sequence; (ii) canonical HB models
are not able to properly match the observational properties of the stars
populating the hottest end of the observed HB distribution, the so called
"blue-hook region". These objects are probably "hot-flashers" , stars that peel
off the red giant branch before reaching the tip and ignite helium at high
effective temperatures. Both of these conclusions are based on the luminosity
of the HB in the optical and UV bands and do not depend on specific assumptions
about mass loss.Comment: Accepted for publication in the MNRAS (12 pages, 11 figures, 1 table
Ruprecht 147: The oldest nearby open cluster as a new benchmark for stellar astrophysics
Ruprecht 147 is a hitherto unappreciated open cluster that holds great
promise as a standard in fundamental stellar astrophysics. We have conducted a
radial velocity survey of astrometric candidates with Lick, Palomar, and MMT
observatories and have identified over 100 members, including 5 blue
stragglers, 11 red giants, and 5 double-lined spectroscopic binaries (SB2s). We
estimate the cluster metallicity from spectroscopic analysis, using
Spectroscopy Made Easy (SME), and find it to be [M/H] = +0.07 \pm 0.03. We have
obtained deep CFHT/MegaCam g'r'i' photometry and fit Padova isochrones to the
(g' - i') and 2MASS (J - K) CMDs using the \tau^2 maximum-likelihood procedure
of Naylor (2009), and an alternative method using 2D cross-correlations
developed in this work. We find best fits for isochrones at age t = 2.5 \pm
0.25 Gyr, m - M = 7.35 \pm 0.1, and A_V = 0.25 \pm 0.05, with additional
uncertainty from the unresolved binary population and possibility of
differential extinction across this large cluster. The inferred age is heavily
dependent by our choice of stellar evolution model: fitting Dartmouth and
PARSEC models yield age parameters of 3 Gyr and 3.25 Gyr respectively. At
approximately 300 pc and 3 Gyr, Ruprecht 147 is by far the oldest nearby star
cluster.Comment: 31 pages, 21 figures, 6 tables. Comments welcom
The Factory and The Beehive II. Activity and Rotation in Praesepe and the Hyades
Open clusters are collections of stars with a single, well-determined age,
and can be used to investigate the connections between angular-momentum
evolution and magnetic activity over a star's lifetime. We present the results
of a comparative study of the relationship between stellar rotation and
activity in two benchmark open clusters: Praesepe and the Hyades. As they have
the same age and roughly solar metallicity, these clusters serve as an ideal
laboratory for testing the agreement between theoretical and empirical
rotation-activity relations at 600 Myr. We have compiled a sample of
720 spectra --- more than half of which are new observations --- for 516
high-confidence members of Praesepe; we have also obtained 139 new spectra for
130 high-confidence Hyads. We have collected rotation periods () for
135 Praesepe members and 87 Hyads. To compare emission, an indicator
of chromospheric activity, as a function of color, mass, and Rossby number
, we first calculate an expanded set of values, with which we can
obtain the to bolometric luminosity ratio, ,
even when spectra are not flux-calibrated and/or stars lack reliable distances.
Our values cover a broader range of stellar masses and colors (roughly
equivalent to spectral types from K0 to M9), and exhibit better agreement
between independent calculations, than existing values. We find no difference
between the two clusters in their equivalent width or
distributions, and therefore take the merged
and data to be representative of 600-Myr-old stars. Our analysis
shows that activity in these stars is saturated for
. Above that value activity declines as a
power-law with slope , before dropping off rapidly
at ...Comment: 17 pages, 15 figures, Accepted by Ap
Variable accretion as a mechanism for brightness variations in T Tau S
(Note: this is a shortened version of the original A&A-style structured
abstract). The physical nature of the strong photometric variability of T Tau
Sa, the more massive member of the Southern "infrared companion" to T Tau, has
long been debated. Intrinsic luminosity variations due to variable accretion
were originally proposed but later challenged in favor of apparent fluctuations
due to time-variable foreground extinction. In this paper we use the timescale
of the variability as a diagnostic for the underlying physical mechanism.
Because the IR emission emerging from Sa is dominantly thermal emission from
circumstellar dust at <=1500K, we can derive a minimum size of the region
responsible for the time-variable emission. In the context of the variable
foreground extinction scenario, this region must be (un-) covered within the
variability timescale, which implies a minimum velocity for the obscuring
foreground material. If this velocity supercedes the local Kepler velocity we
can reject foreground extinction as a valid variability mechanism. The variable
accretion scenario allows for shorter variability timescales since the
variations in luminosity occur on much smaller scales, essentially at the
surface of the star, and the disk surface can react almost instantly on the
changing irradiation with a higher or lower dust temperature and according
brightness. We have detected substantial variations at long wavelengths in T
Tau S: +26% within four days at 12.8 micron. We show that this short-term
variability cannot be due to variable extinction and instead must be due to
variable accretion. Using a radiative transfer model of the Sa disk we show
that variable accretion can in principle also account for the much larger
(several magnitude) variations observed on timescales of several years. For the
long-term variability, however, also variable foreground extinction is a viable
mechanism.Comment: 15 pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
Verifying asteroseismically determined parameters of Kepler stars using hipparcos parallaxes: self-consistent stellar properties and distances
Accurately determining the properties of stars is of prime importance for
characterizing stellar populations in our Galaxy. The field of asteroseismology
has been thought to be particularly successful in such an endeavor for stars in
different evolutionary stages. However, to fully exploit its potential, robust
methods for estimating stellar parameters are required and independent
verification of the results is mandatory. With this purpose, we present a new
technique to obtain stellar properties by coupling asteroseismic analysis with
the InfraRed Flux Method. By using two global seismic observables and
multi-band photometry, the technique allows us to obtain masses, radii,
effective temperatures, bolometric fluxes, and hence distances for field stars
in a self-consistent manner. We apply our method to 22 solar-like oscillators
in the Kepler short-cadence sample, that have accurate Hipparcos parallaxes.
Our distance determinations agree to better than 5%, while measurements of
spectroscopic effective temperatures and interferometric radii also validate
our results. We briefly discuss the potential of our technique for stellar
population analysis and models of Galactic Chemical Evolution.Comment: 28 pages, 5 figures, ApJ, accepte
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