49 research outputs found
X-ray Spectroscopy of the Contact Binary VW Cephei
Short-period binaries represent extreme cases in the generation of stellar
coronae via a rotational dynamo. Such stars are important for probing the
origin and nature of coronae in the regimes of rapid rotation and activity
saturation. VW Cep (P=0.28 d) is a relatively bright, partially eclipsing, and
very active object. Light curves made from Chandra/HETGS data show flaring and
rotational modulation, but no eclipses. Velocity modulation of emission lines
indicates that one component dominates the X-ray emission. The emission measure
is highly structured, having three peaks. Helium-like triplet lines give
electron densities of about 3.0E+10 - 18.0E+10 /cm^3. We conclude that the
corona is predominantly on the polar regions of the primary star and compact.Comment: Accepted for publication in the Astropysical Journal, 23 June 2006;
22 pages, 15 figure
Stellar Coronal Spectroscopy with the Chandra HETGS
Spectroscopy with the Chandra High Energy Transmission Grating Spectrometer
provides details on X-ray emission and activity from young and cool stars
through resolution of emission lines from a variety of ions. We are beginning
to see trends in activity regarding abundances, emission measures, and
variability. Here we contrast spectra of TV Crt, a weak-lined T Tauri star,
with TW Hya, a Classical T Tauri star. TV Crt has a spectrum more like magnetic
activity driven coronae, relative to the TW Hya spectrum, which we have
interpreted as due to accretion-produced X-rays. We have also observed the long
period system, IM Pegasi to search for rotational modulation, and to compare
activity in a long period active binary to shorter period systems and to the
pre-main sequence stars. We detected no rotational modulation, but did see
long-duration flares.Comment: 5 pages, 2 figures; to be published in IAU Symposium 219: "Stars as
Suns: Activity, Evolution, Planets" (Ed. A. Dupree and A. O. Benz
A Tight Upper Limit on Oscillations in the Ap star Epsilon Ursae Majoris from WIRE Photometry
Observations of Epsilon UMa obtained with the star tracker on the Wide Field
Infrared Explorer (WIRE) satellite during a month in mid-2000 are analyzed.
This is one of the most precise photometry of an Ap star. The amplitude
spectrum is used to set an upper limit of 75 parts per million for the
amplitude of stellar pulsations in this star unless it accidentally oscillates
with a single mode at the satellite orbit, its harmonics or their one day
aliases. This is the tightest limit put on the amplitude of oscillations in an
Ap star. As the rotation period of Epsilon UMa is relatively short (5.1 d), it
cannot be argued that the observations were made at a wrong rotational phase.
Our results thus support the idea that some Ap stars do not pulsate at all.Comment: 4 pages, 4 figures, 2 style files, accepted for publication in ApJ
Asteroseismology of massive stars with the TESS mission: the runaway Beta Cep pulsator PHL 346 = HN Aqr
We report an analysis of the first known Beta Cep pulsator observed by the
TESS mission, the runaway star PHL 346 = HN Aqr. The star, previously known as
a singly-periodic pulsator, has at least 34 oscillation modes excited, 12 of
those in the g-mode domain and 22 p modes. Analysis of archival data implies
that the amplitude and frequency of the dominant mode and the stellar radial
velocity were variable over time. A binary nature would be inconsistent with
the inferred ejection velocity from the Galactic disc of 420 km/s, which is too
large to be survivable by a runaway binary system. A kinematic analysis of the
star results in an age constraint (23 +- 1 Myr) that can be imposed on
asteroseismic modelling and that can be used to remove degeneracies in the
modelling process. Our attempts to match the excitation of the observed
frequency spectrum resulted in pulsation models that were too young. Hence,
asteroseismic studies of runaway pulsators can become vital not only in tracing
the evolutionary history of such objects, but to understand the interior
structure of massive stars in general. TESS is now opening up these stars for
detailed asteroseismic investigation.Comment: accepted for ApJ
Kepler Mission Stellar and Instrument Noise Properties
Kepler Mission results are rapidly contributing to fundamentally new
discoveries in both the exoplanet and asteroseismology fields. The data
returned from Kepler are unique in terms of the number of stars observed,
precision of photometry for time series observations, and the temporal extent
of high duty cycle observations. As the first mission to provide extensive time
series measurements on thousands of stars over months to years at a level
hitherto possible only for the Sun, the results from Kepler will vastly
increase our knowledge of stellar variability for quiet solar-type stars. Here
we report on the stellar noise inferred on the timescale of a few hours of most
interest for detection of exoplanets via transits. By design the data from
moderately bright Kepler stars are expected to have roughly comparable levels
of noise intrinsic to the stars and arising from a combination of fundamental
limitations such as Poisson statistics and any instrument noise. The noise
levels attained by Kepler on-orbit exceed by some 50% the target levels for
solar-type, quiet stars. We provide a decomposition of observed noise for an
ensemble of 12th magnitude stars arising from fundamental terms (Poisson and
readout noise), added noise due to the instrument and that intrinsic to the
stars. The largest factor in the modestly higher than anticipated noise follows
from intrinsic stellar noise. We show that using stellar parameters from
galactic stellar synthesis models, and projections to stellar rotation,
activity and hence noise levels reproduces the primary intrinsic stellar noise
features.Comment: Accepted by ApJ; 26 pages, 20 figure