149 research outputs found
M Dwarfs in SDSS Stripe 82: Photometric Light Curves and Flare Rate Analysis
We present a flare rate analysis of 50,130 M dwarf light curves in SDSS
Stripe 82. We identified 271 flares using a customized variability index to
search ~2.5 million photometric observations for flux increases in the u- and
g-bands. Every image of a flaring observation was examined by eye and with a
PSF-matching and image subtraction tool to guard against false positives.
Flaring is found to be strongly correlated with the appearance of H-alpha in
emission in the quiet spectrum. Of the 99 flare stars that have spectra, we
classify 8 as relatively inactive. The flaring fraction is found to increase
strongly in stars with redder colors during quiescence, which can be attributed
to the increasing flare visibility and increasing active fraction for redder
stars. The flaring fraction is strongly correlated with |Z| distance such that
most stars that flare are within 300 pc of the Galactic plane. We derive flare
u-band luminosities and find that the most luminous flares occur on the
earlier-type M dwarfs. Our best estimate of the lower limit on the flaring rate
(averaged over Stripe 82) for flares with \Delta u \ge 0.7 magnitudes on stars
with u < 22 is 1.3 flares hour^-1 square degree^-1 but can vary significantly
with the line-of-sight.Comment: 44 pages, 13 figure
The Factory and the Beehive III: PTFEB132.707+19.810, a Low-Mass Eclipsing Binary in Praesepe Observed by PTF and K2
Theoretical models of stars constitute a fundamental bedrock upon which much
of astrophysics is built, but large swaths of model parameter space remain
uncalibrated by observations. The best calibrators are eclipsing binaries in
clusters, allowing measurement of masses, radii, luminosities, and
temperatures, for stars of known metallicity and age. We present the discovery
and detailed characterization of PTFEB132.707+19.810, a P=6.0 day eclipsing
binary in the Praesepe cluster (~600--800 Myr; [Fe/H]=0.140.04). The
system contains two late-type stars (SpT=M3.50.2;
SpT=M4.30.7) with precise masses (~;
~) and radii (~;
~). Neither star meets the predictions of stellar
evolutionary models. The primary has the expected radius, but is cooler and
less luminous, while the secondary has the expected luminosity, but is cooler
and substantially larger (by 20%). The system is not tidally locked or
circularized. Exploiting a fortuitous 4:5 commensurability between
and , we demonstrate that fitting errors from the unknown spot
configuration only change the inferred radii by <1--2%. We also analyze subsets
of data to test the robustness of radius measurements; the radius sum is more
robust to systematic errors and preferable for model comparisons. We also test
plausible changes in limb darkening, and find corresponding uncertainties of
~1%. Finally, we validate our pipeline using extant data for GU Boo, finding
that our independent results match previous radii to within the mutual
uncertainties (2--3%). We therefore suggest that the substantial discrepancies
are astrophysical; since they are larger than for old field stars, they may be
tied to the intermediate age of PTFEB132.707+19.810.Comment: Accepted to ApJ; 36 pages, 19 figures, 8 tables in two-column AASTEX6
forma
The Distance to NGC 2264
We determine the distance to the open cluster NGC 2264 using a statistical
analysis of cluster member inclinations. We derive distance-dependent values of
sin i (where i is the inclination angle) for 97 stars in NGC 2264 from the
rotation periods, luminosities, effective temperatures, and projected
equatorial rotation velocities, v sin i, measured for these stars. We have
measured 96 of the v sin i values in our sample by analyzing high-resolution
spectra with a cross-correlation technique. We model the observed distribution
of sin i for the cluster by assuming that member stars have random axial
orientations and by adopting prescriptions for the measurement errors in our
sample. By adjusting the distance assumed in the observed sin i distribution
until it matches the modeled distribution, we obtain a best-fit distance for
the cluster. We find the data to be consistent with a distance to NGC 2264 of
913 pc. Quantitative tests of our analysis reveals uncertainties of 40 and 110
pc due to sampling and systematic effects, respectively. This distance estimate
suggests a revised age for the cluster of 1.5 Myrs, although more detailed
investigations of the full cluster membership are required to draw strong
conclusions.Comment: 12 pages, 11 figure
The GALEX View of "Boyajian's Star" (KIC 8462852)
The enigmatic star KIC 8462852, informally known as "Boyajian's Star", has
exhibited unexplained variability from both short timescale (days) dimming
events, and years-long fading in the Kepler mission. No single physical
mechanism has successfully explained these observations to date. Here we
investigate the ultraviolet variability of KIC 8462852 on a range of timescales
using data from the GALEX mission that occurred contemporaneously with the
Kepler mission. The wide wavelength baseline between the Kepler and GALEX data
provides a unique constraint on the nature of the variability. Using 1600
seconds of photon-counting data from four GALEX visits spread over 70 days in
2011, we find no coherent NUV variability in the system on 10-100 second or
months timescales. Comparing the integrated flux from these 2011 visits to the
2012 NUV flux published in the GALEX-CAUSE Kepler survey, we find a 3% decrease
in brightness for KIC 8462852. We find this level of variability is
significant, but not necessarily unusual for stars of similar spectral type in
the GALEX data. This decrease coincides with the secular optical fading
reported by Montet & Simon (2016). We find the multi-wavelength variability is
somewhat inconsistent with typical interstellar dust absorption, but instead
favors a R = 5.0 0.9 reddening law potentially from circumstellar
dust.Comment: 8 pages, 4 figures, ApJ Accepte
Our Nearest 15 Million Neighbors: The Field Low-Mass Stellar Luminosity and Mass Functions
We report on a new measurement of the luminosity function (LF) and mass
function (MF) of field low-mass dwarfs using Sloan Digital Sky Survey (SDSS)
photometry. The final catalog is composed of ~15 million low-mass stars (0.1
Msun < M < 0.8 Msun), spread over 8,400 square degrees. Distances to the stars
are estimated using new photometric parallax relations, constructed from ugriz
photometry of nearby low-mass stars with trigonometric parallaxes. The LF is
measured with a novel technique, which simultaneously measures Galactic
structure and the stellar LF. The resulting LF is compared to previous studies
and converted to a MF. The MF is well-described by a log-normal distribution,
with Mo = 0.27 Msun.Comment: 4 pages, 3 figures. Included in the proceedings of Cool Stars 1
The APOGEE-2 Survey of the Orion Star Forming Complex: I. Target Selection and Validation with early observations
The Orion Star Forming Complex (OSFC) is a central target for the APOGEE-2
Young Cluster Survey. Existing membership catalogs span limited portions of the
OSFC, reflecting the difficulty of selecting targets homogeneously across this
extended, highly structured region. We have used data from wide field
photometric surveys to produce a less biased parent sample of young stellar
objects (YSOs) with infrared (IR) excesses indicative of warm circumstellar
material or photometric variability at optical wavelengths across the full 420
square degrees extent of the OSFC. When restricted to YSO candidates with H <
12.4, to ensure S/N ~100 for a six visit source, this uniformly selected sample
includes 1307 IR excess sources selected using criteria vetted by Koenig &
Liesawitz and 990 optical variables identified in the Pan-STARRS1 3
survey: 319 sources exhibit both optical variability and evidence of
circumstellar disks through IR excess. Objects from this uniformly selected
sample received the highest priority for targeting, but required fewer than
half of the fibers on each APOGEE-2 plate. We fill the remaining fibers with
previously confirmed and new color-magnitude selected candidate OSFC members.
Radial velocity measurements from APOGEE-1 and new APOGEE-2 observations taken
in the survey's first year indicate that ~90% of the uniformly selected targets
have radial velocities consistent with Orion membership.The APOGEE-2 Orion
survey will include >1100 bona fide YSOs whose uniform selection function will
provide a robust sample for comparative analyses of the stellar populations and
properties across all sub-regions of Orion.Comment: Accepted for publication in ApJ
The Very Short Period M Dwarf Binary SDSS J001641-000925
We present follow-up observations and analysis of the recently discovered
short period low-mass eclipsing binary, SDSS J001641-000925. With an orbital
period of 0.19856 days, this system has one of the shortest known periods for
an M dwarf binary system. Medium-resolution spectroscopy and multi-band
photometry for the system are presented. Markov chain Monte Carlo modeling of
the light curves and radial velocities yields estimated masses for the stars of
M1 = 0.54 +/- 0.07 Msun and M2 = 0.34 +/- 0.04 Msun, and radii of R1 = 0.68 +/-
0.03 Rsun and R2 = 0.58 +/- 0.03 Rsun respectively. This solution places both
components above the critical Roche overfill limit, providing strong evidence
that SDSS J001641-000925 is the first verified M-dwarf contact binary system.
Within the follow-up spectroscopy we find signatures of non-solid body rotation
velocities, which we interpret as evidence for mass transfer or loss within the
system. In addition, our photometry samples the system over 9 years, and we
find strong evidence for period decay at the rate of dP/dt ~8 s/yr. Both of
these signatures raise the intriguing possibility that the system is in
over-contact, and actively losing angular momentum, likely through mass loss.
This places SDSS J001641-000925 as not just the first M-dwarf over-contact
binary, but one of the few systems of any spectral type known to be actively
undergoing coalescence. Further study SDSS J001641-000925 is on-going to verify
the nature of the system, which may prove to be a unique astrophysical
laboratory.Comment: 11 figures, ApJ Accepte
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