477 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 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
R-process enrichment from a single event in an ancient dwarf galaxy
Elements heavier than zinc are synthesized through the (r)apid and (s)low
neutron-capture processes. The main site of production of the r-process
elements (such as europium) has been debated for nearly 60 years. Initial
studies of chemical abundance trends in old Milky Way halo stars suggested
continual r-process production, in sites like core-collapse supernovae. But
evidence from the local Universe favors r-process production mainly during rare
events, such as neutron star mergers. The appearance of a europium abundance
plateau in some dwarf spheroidal galaxies has been suggested as evidence for
rare r-process enrichment in the early Universe, but only under the assumption
of no gas accretion into the dwarf galaxies. Cosmologically motivated gas
accretion favors continual r-process enrichment in these systems. Furthermore,
the universal r-process pattern has not been cleanly identified in dwarf
spheroidals. The smaller, chemically simpler, and more ancient ultra-faint
dwarf galaxies assembled shortly after the first stars formed, and are ideal
systems with which to study nucleosynthesis events such as the r-process.
Reticulum II is one such galaxy. The abundances of non-neutron-capture elements
in this galaxy (and others like it) are similar to those of other old stars.
Here, we report that seven of nine stars in Reticulum II observed with
high-resolution spectroscopy show strong enhancements in heavy neutron-capture
elements, with abundances that follow the universal r-process pattern above
barium. The enhancement in this "r-process galaxy" is 2-3 orders of magnitude
higher than that detected in any other ultra-faint dwarf galaxy. This implies
that a single rare event produced the r-process material in Reticulum II. The
r-process yield and event rate are incompatible with ordinary core-collapse
supernovae, but consistent with other possible sites, such as neutron star
mergers.Comment: Published in Nature, 21 Mar 2016:
http://dx.doi.org/10.1038/nature1742
Recent Advances in Modeling Stellar Interiors
Advances in stellar interior modeling are being driven by new data from
large-scale surveys and high-precision photometric and spectroscopic
observations. Here we focus on single stars in normal evolutionary phases; we
will not discuss the many advances in modeling star formation, interacting
binaries, supernovae, or neutron stars. We review briefly: 1) updates to input
physics of stellar models; 2) progress in two and three-dimensional evolution
and hydrodynamic models; 3) insights from oscillation data used to infer
stellar interior structure and validate model predictions (asteroseismology).
We close by highlighting a few outstanding problems, e.g., the driving
mechanisms for hybrid gamma Dor/delta Sct star pulsations, the cause of giant
eruptions seen in luminous blue variables such as eta Car and P Cyg, and the
solar abundance problem.Comment: Proceedings for invited talk at conference High Energy Density
Laboratory Astrophysics 2010, Caltech, March 2010, submitted for special
issue of Astrophysics and Space Science; 7 pages; 5 figure
High-precision photometry by telescope defocussing. III. The transiting planetary system WASP-2
We present high-precision photometry of three transits of the extrasolar
planetary system WASP-2, obtained by defocussing the telescope, and achieving
point-to-point scatters of between 0.42 and 0.73 mmag. These data are modelled
using the JKTEBOP code, and taking into account the light from the
recently-discovered faint star close to the system. The physical properties of
the WASP-2 system are derived using tabulated predictions from five different
sets of stellar evolutionary models, allowing both statistical and systematic
errorbars to be specified. We find the mass and radius of the planet to be M_b
= 0.847 +/- 0.038 +/- 0.024 Mjup and R_b = 1.044 +/- 0.029 +/- 0.015 Rjup. It
has a low equilibrium temperature of 1280 +/- 21 K, in agreement with a recent
finding that it does not have an atmospheric temperature inversion. The first
of our transit datasets has a scatter of only 0.42 mmag with respect to the
best-fitting light curve model, which to our knowledge is a record for
ground-based observations of a transiting extrasolar planet.Comment: Accepted for publication in MNRAS. 9 pages, 3 figures, 10 table
A uniform asteroseismic analysis of 22 solar-type stars observed by Kepler
Asteroseismology with the Kepler space telescope is providing not only an
improved characterization of exoplanets and their host stars, but also a new
window on stellar structure and evolution for the large sample of solar-type
stars in the field. We perform a uniform analysis of 22 of the brightest
asteroseismic targets with the highest signal-to-noise ratio observed for 1
month each during the first year of the mission, and we quantify the precision
and relative accuracy of asteroseismic determinations of the stellar radius,
mass, and age that are possible using various methods. We present the
properties of each star in the sample derived from an automated analysis of the
individual oscillation frequencies and other observational constraints using
the Asteroseismic Modeling Portal (AMP), and we compare them to the results of
model-grid-based methods that fit the global oscillation properties. We find
that fitting the individual frequencies typically yields asteroseismic radii
and masses to \sim1% precision, and ages to \sim2.5% precision (respectively 2,
5, and 8 times better than fitting the global oscillation properties). The
absolute level of agreement between the results from different approaches is
also encouraging, with model-grid-based methods yielding slightly smaller
estimates of the radius and mass and slightly older values for the stellar age
relative to AMP, which computes a large number of dedicated models for each
star. The sample of targets for which this type of analysis is possible will
grow as longer data sets are obtained during the remainder of the mission.Comment: 13 pages, 5 figures in the main text, 22 figures in Appendix.
Accepted for publication in Ap
Magnetic inflation and Stellar Mass. II. On the radii of wingle, rapidly rotating, fully convective M-dwarf stars
Main-sequence, fully convective M dwarfs in eclipsing binaries are observed to be larger than stellar evolutionary models predict by as much as 10%–15%. A proposed explanation for this discrepancy involves effects from strong magnetic fields, induced by rapid rotation via the dynamo process. Although, a handful of single, slowly rotating M dwarfs with radius measurements from interferometry also appear to be larger than models predict, suggesting that rotation or binarity specifically may not be the sole cause of the discrepancy. We test whether single, rapidly rotating, fully convective stars are also larger than expected by measuring their distribution. We combine photometric rotation periods from the literature with rotational broadening () measurements reported in this work for a sample of 88 rapidly rotating M dwarf stars. Using a Bayesian framework, we find that stellar evolutionary models underestimate the radii by 10 \% \mbox{--}15{ \% }_{-2.5}^{+3}, but that at higher masses (0.18 < M < 0.4 M Sun), the discrepancy is only about 6% and comparable to results from interferometry and eclipsing binaries. At the lowest masses (0.08 < M < 0.18 M Sun), we find that the discrepancy between observations and theory is 13%–18%, and we argue that the discrepancy is unlikely to be due to effects from age. Furthermore, we find no statistically significant radius discrepancy between our sample and the handful of M dwarfs with interferometric radii. We conclude that neither rotation nor binarity are responsible for the inflated radii of fully convective M dwarfs, and that all fully convective M dwarfs are larger than models predict.The authors would like to thank the referee for the thoughtful report, which greatly improved the manuscript. The authors would also like to thank Lisa Prato and Larissa Nofi for IGRINS training, and Heidi Larson, Jason Sanborn, and Andrew Hayslip for operating the DCT during our observations. We would also like to thank Jen Winters, Jonathan Irwin, Paul Dalba, Mark Veyette, Eunkyu Han, and Andrew Vanderburg for useful discussions and helpful comments on this work. Some of this work was supported by the NASA Exoplanet Research Program (XRP) under grant No. NNX15AG08G issued through the Science Mission Directorate.These results made use of the Lowell Observatory's Discovery Channel Telescope, supported by Discovery Communications, Inc., Boston University, the University of Maryland, the University of Toledo and Northern Arizona University; the Immersion Grating Infrared Spectrograph (IGRINS) that was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of the US National Science Foundation under grant AST-1229522, of the University of Texas at Austin, and of the Korean GMT Project of KASI; data taken at The McDonald Observatory of The University of Texas at Austin; and data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF. (NNX15AG08G - NASA Exoplanet Research Program (XRP); Discovery Communications, Inc.; Boston University; University of Maryland; University of Toledo; Northern Arizona University; AST-1229522 - US National Science Foundation; University of Texas at Austin; Korean GMT Project of KASI; NASA; NSF
Searching for transits in the Wide Field Camera Transit Survey with difference-imaging light curves
The Wide Field Camera Transit Survey is a pioneer program aiming at for searching extra-solar planets in the near-infrared. The images from the survey are processed by a data reduction pipeline, which uses aperture photometry to construct the light curves. We produce an alternative set of light curves using the difference-imaging method for the most complete field in the survey and carry out a quantitative comparison between the photometric precision achieved with both methods. The results show that differencephotometry light curves present an important improvement for stars with J > 16. We report an implementation on the box-fitting transit detection algorithm, which performs a trapezoid-fit to the folded light curve, providing more accurate results than the boxfitting model. We describe and optimize a set of selection criteria to search for transit candidates, including the V-shape parameter calculated by our detection algorithm. The optimized selection criteria are applied to the aperture photometry and difference-imaging light curves, resulting in the automatic detection of the best 200 transit candidates from a sample of ~475 000 sources. We carry out a detailed analysis in the 18 best detections and classify them as transiting planet and eclipsing binary candidates. We present one planet candidate orbiting a late G-type star. No planet candidate around M-stars has been found, confirming the null detection hypothesis and upper limits on the occurrence rate of short-period giant planets around M-dwarfs presented in a prior study. We extend the search for transiting planets to stars with J ≤ 18, which enables us to set a stricter upper limit of 1.1%. Furthermore, we present the detection of five faint extremely-short period eclipsing binaries and three M-dwarf/M-dwarf binary candidates. The detections demonstrate the benefits of using the difference-imaging light curves, especially when going to fainter magnitudes.Peer reviewe
Dynamical age differences among coeval star clusters as revealed by blue stragglers
Globular star clusters that formed at the same cosmic time may have evolved
rather differently from a dynamical point of view (because that evolution
depends on the internal environment) through a variety of processes that tend
progressively to segregate stars more massive than the average towards the
cluster centre. Therefore clusters with the same chronological age may have
reached quite different stages of their dynamical history (that is, they may
have different dynamical ages). Blue straggler stars have masses greater than
those at the turn-off point on the main sequence and therefore must be the
result of either a collision or a mass-transfer event. Because they are among
the most massive and luminous objects in old clusters, they can be used as test
particles with which to probe dynamical evolution. Here we report that globular
clusters can be grouped into a few distinct families on the basis of the radial
distribution of blue stragglers. This grouping corresponds well to an effective
ranking of the dynamical stage reached by stellar systems, thereby permitting a
direct measure of the cluster dynamical age purely from observed properties.Comment: Published on the 20 December 2012 issue of Natur
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