423 research outputs found
The Yale-Potsdam Stellar Isochrones (YaPSI)
We introduce the Yale-Potsdam Stellar Isochrones (YaPSI), a new grid of
stellar evolution tracks and isochrones of solar-scaled composition. In an
effort to improve the Yonsei-Yale database, special emphasis is placed on the
construction of accurate low-mass models (Mstar < 0.6 Msun), and in particular
of their mass-luminosity and mass-radius relations, both crucial in
characterizing exoplanet-host stars and, in turn, their planetary systems. The
YaPSI models cover the mass range 0.15 to 5.0 Msun, densely enough to permit
detailed interpolation in mass, and the metallicity and helium abundance ranges
[Fe/H] = -1.5 to +0.3, and Y = 0.25 to 0.37, specified independently of each
other (i.e., no fixed Delta Y/Delta Z relation is assumed). The evolutionary
tracks are calculated from the pre-main sequence up to the tip of the red giant
branch. The isochrones, with ages between 1 Myr and 20 Gyr, provide UBVRI
colors in the Johnson-Cousins system, and JHK colors in the homogeneized
Bessell & Brett system, derived from two different semi-empirical Teff-color
calibrations from the literature. We also provide utility codes, such as an
isochrone interpolator in age, metallicity, and helium content, and an
interface of the tracks with an open-source Monte Carlo Markov-Chain tool for
the analysis of individual stars. Finally, we present comparisons of the YaPSI
models with the best empirical mass- luminosity and mass-radius relations
available to date, as well as isochrone fitting of well-studied steComment: 17 pages, 14 figures; accepted for publication in the Astrophysical
Journa
Modelling the KIC8462852 light curves : compatibility of the dips and secular dimming with an exocomet interpretation
This paper shows how the dips and secular dimming in the KIC8462852 light curve can originate in circumstellar material distributed around a single elliptical orbit (e.g. exocomets). The expected thermal emission and wavelength dependent dimming is derived for different orbital parameters and geometries, including dust that is optically thick to stellar radiation, and for a size distribution of dust with realistic optical properties. We first consider dust distributed evenly around the orbit, then show how to derive its uneven distribution from the optical light curve and to predict light curves at different wavelengths. The fractional luminosity of an even distribution is approximately the level of dimming times stellar radius divided by distance from the star at transit. Non-detection of dust thermal emission for KIC8462852 thus provides a lower limit on the transit distance to complement the 0.6 au upper limit imposed by 0.4 d dips. Unless the dust distribution is optically thick, the putative 16 per cent century-long secular dimming must have disappeared before the WISE 12 mum measurement in 2010, and subsequent 4.5 mum observations require transits at >0.05 au. However, self-absorption of thermal emission removes these constraints for opaque dust distributions. The passage of dust clumps through pericentre is predicted to cause infrared brightening lasting tens of days and dimming during transit, such that total flux received decreases at wavelengths <5 mum, but increases to potentially detectable levels at longer wavelengths. We suggest that lower dimming levels than seen for KIC8462852 are more common in the Galactic population and may be detected in future transit surveys
Modelling the KIC8462852 light curves : compatibility of the dips and secular dimming with an exocomet interpretation
This paper shows how the dips and secular dimming in the KIC8462852 light curve can originate in circumstellar material distributed around a single elliptical orbit (e.g. exocomets). The expected thermal emission and wavelength dependent dimming is derived for different orbital parameters and geometries, including dust that is optically thick to stellar radiation, and for a size distribution of dust with realistic optical properties. We first consider dust distributed evenly around the orbit, then show how to derive its uneven distribution from the optical light curve and to predict light curves at different wavelengths. The fractional luminosity of an even distribution is approximately the level of dimming times stellar radius divided by distance from the star at transit. Non-detection of dust thermal emission for KIC8462852 thus provides a lower limit on the transit distance to complement the 0.6 au upper limit imposed by 0.4 d dips. Unless the dust distribution is optically thick, the putative 16 per cent century-long secular dimming must have disappeared before the WISE 12 mum measurement in 2010, and subsequent 4.5 mum observations require transits at >0.05 au. However, self-absorption of thermal emission removes these constraints for opaque dust distributions. The passage of dust clumps through pericentre is predicted to cause infrared brightening lasting tens of days and dimming during transit, such that total flux received decreases at wavelengths <5 mum, but increases to potentially detectable levels at longer wavelengths. We suggest that lower dimming levels than seen for KIC8462852 are more common in the Galactic population and may be detected in future transit surveys
Direct imaging of planet transit events
Exoplanet transit events are attractive targets for the ultrahigh-resolution capabilities afforded by optical interferometers. The intersection of two developments in astronomy enable direct imaging of exoplanet transits: first, improvements in sensitivity and precision of interferometric instrumentation; and second, identification of ever-brighter host stars. Efforts are underway for the first direct high-precision detection of closure phase signatures with the CHARA Array and Navy Precision Optical Interferometer. When successful, these measurements will enable recovery of the transit position angle on the sky, along with characterization of other system parameters, such as stellar radius, planet radius, and other parameters of the transit event. This technique can directly determine the planet\u27s radius independent of any outside observations, and appears able to improve substantially upon other determinations of that radius; it will be possible to extract wavelength dependence of that radius determination, for connection to characterization of planetary atmospheric composition & structure. Additional directly observed parameters - also not dependent on transit photometry or spectroscopy - include impact parameter, transit ingress time, and transit velocity. Copyright © International Astronomical Union 2014
Measurement of Source Star Colors with the K2C9-CFHT Multi-color Microlensing Survey
K2 Campaign 9 (K2C9) was the first space-based microlensing parallax survey
capable of measuring microlensing parallaxes of free-floating planet candidate
microlensing events. Simultaneous to K2C9 observations we conducted the K2C9
Canada-France-Hawaii Telescope Multi-Color Microlensing Survey (K2C9-CFHT MCMS)
in order to measure the colors of microlensing source stars to improve the
accuracy of K2C9's parallax measurements. We describe the difference imaging
photometry analysis of the K2C9-CFHT MCMS observations, and present the
project's first data release. This includes instrumental difference flux
lightcurves of 217 microlensing events identified by other microlensing
surveys, reference image photometry calibrated to PanSTARRS data release 1
photometry, and tools to convert between instrumental and calibrated flux
scales. We derive accurate analytic transformations between the PanSTARRS
bandpasses and the Kepler bandpass, as well as angular diameter-color relations
in the PanSTARRS bandpasses. To demonstrate the use of our data set, we analyze
ground-based and K2 data of a short timescale microlensing event,
OGLE-2016-BLG-0795. We find the event has a timescale ~days and microlens parallax or , subject to the standard satellite parallax degeneracy. We argue that the
smaller value of the parallax is more likely, which implies that the lens is
likely a stellar-mass object in the Galactic bulge as opposed to a
super-Jupiter mass object in the Galactic disk.Comment: Submitted to PAS
Young "Dipper" Stars in Upper Sco and Oph Observed by K2
We present ten young (10 Myr) late-K and M dwarf stars observed in
K2 Campaign 2 that host protoplanetary disks and exhibit quasi-periodic or
aperiodic dimming events. Their optical light curves show 10-20 dips in
flux over the 80-day observing campaign with durations of 0.5-2 days and
depths of up to 40%. These stars are all members of the Ophiuchus
(1 Myr) or Upper Scorpius (10 Myr) star-forming regions. To
investigate the nature of these "dippers" we obtained: optical and
near-infrared spectra to determine stellar properties and identify accretion
signatures; adaptive optics imaging to search for close companions that could
cause optical variations and/or influence disk evolution; and
millimeter-wavelength observations to constrain disk dust and gas masses. The
spectra reveal Li I absorption and H emission consistent with stellar
youth (<50 Myr), but also accretion rates spanning those of classical and
weak-line T Tauri stars. Infrared excesses are consistent with protoplanetary
disks extending to within 10 stellar radii in most cases; however, the
sub-mm observations imply disk masses that are an order of magnitude below
those of typical protoplanetary disks. We find a positive correlation between
dip depth and WISE-2 excess, which we interpret as evidence that the dipper
phenomenon is related to occulting structures in the inner disk, although this
is difficult to reconcile with the weakly accreting aperiodic dippers. We
consider three mechanisms to explain the dipper phenomenon: inner disk warps
near the co-rotation radius related to accretion; vortices at the inner disk
edge produced by the Rossby Wave Instability; and clumps of circumstellar
material related to planetesimal formation.Comment: Accepted to ApJ, 19 pages, 10 figure
The Ages of A-Stars I: Interferometric Observations and Age Estimates for Stars in the Ursa Major Moving Group
We have observed and spatially resolved a set of seven A-type stars in the
nearby Ursa Major moving group with the Classic, CLIMB, and PAVO beam combiners
on the CHARA Array. At least four of these stars have large rotational
velocities ( 170 ) and are expected to
be oblate. These interferometric measurements, the stars' observed photometric
energy distributions, and values are used to computationally
construct model oblate stars from which stellar properties (inclination,
rotational velocity, and the radius and effective temperature as a function of
latitude, etc.) are determined. The results are compared with MESA stellar
evolution models (Paxton et al. 2011, 2013) to determine masses and ages. The
value of this new technique is that it enables the estimation of the
fundamental properties of rapidly rotating stars without the need to fully
image the star. It can thus be applied to stars with sizes comparable to the
interferometric resolution limit as opposed to those that are several times
larger than the limit. Under the assumption of coevality, the spread in ages
can be used as a test of both the prescription presented here and the MESA
evolutionary code for rapidly rotating stars. With our validated technique, we
combine these age estimates and determine the age of the moving group to be 414
23 Myr, which is consistent with, but much more precise than previous
estimates.Comment: Accepted by Ap
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