23 research outputs found
The Distance to the Large Magellanic Cloud from the Eclipsing Binary HV2274
The distance to the Large Magellanic Cloud (LMC) is crucial for the
calibration of the Cosmic Distance Scale. We derive a distance to the LMC based
on an analysis of ground-based photometry and HST-based spectroscopy and
spectrophotometry of the LMC eclipsing binary system HV2274. Analysis of the
optical light curve and HST/GHRS radial velocity curve provides the masses and
radii of the binary components. Analysis of the HST/FOS UV/optical
spectrophotometry provides the temperatures of the component stars and the
interstellar extinction of the system. When combined, these data yield a
distance to the binary system. After correcting for the location of HV2274 with
respect to the center of the LMC, we find d(LMC) = 45.7 +/- 1.6 kpc or DM(LMC)
= 18.30 +/- 0.07 mag. This result, which is immune to the metallicity-induced
zero point uncertainties that have plagued other techniques, lends strong
support to the ``short'' LMC distance scale as derived from a number of
independent methods.Comment: 6 pages, including 2 pages of figures. Newly available optical (B and
V) photometry has revealed -- and allowed the elimination of -- a systematic
error in the previously reported determination of E(B-V) for HV2274. The new
result is E(B-V) = 0.12 mag (as compared to the value of 0.083 reported in
the original submission) and produces a DECREASE in the distance modulus of
HV2274 by 0.12 mag. ApJ Letters, in pres
Superflares on Ordinary Solar-Type Stars
Short duration flares are well known to occur on cool main-sequence stars as
well as on many types of `exotic' stars. Ordinary main-sequence stars are
usually pictured as being static on time scales of millions or billions of
years. Our sun has occasional flares involving up to ergs which
produce optical brightenings too small in amplitude to be detected in
disk-integrated brightness. However, we identify nine cases of superflares
involving to ergs on normal solar-type stars. That is,
these stars are on or near the main-sequence, are of spectral class from F8 to
G8, are single (or in very wide binaries), are not rapid rotators, and are not
exceedingly young in age. This class of stars includes many those recently
discovered to have planets as well as our own Sun, and the consequences for any
life on surrounding planets could be profound. For the case of the Sun,
historical records suggest that no superflares have occurred in the last two
millennia.Comment: 16 pages, accepted for publication in Ap
X-Ray, FUV, and UV Observations of alpha Centauri B: Determination of Long-term Magnetic Activity Cycle and Rotation Period
We have been carrying out a study of stellar magnetic activity, dynamos,
atmospheric physics, and spectral irradiances from a sample of solar-type G0-5
V stars with different ages. One of the major goals of this program is to study
the evolution of the Sun's X-ray through NUV spectral irradiances with age. Of
particular interest is the determination of the young Sun's elevated levels of
high-energy fluxes because of the critical roles that X-ray through FUV
emissions play on the photochemical and photoionization evolution of early,
young planetary atmospheres and ionospheres. Motivated by the current
exoplanetary search missions that are hunting for earth-size planets in the
habitable zones of nearby main-sequence G-M stars, we are expanding our program
to cooler, less luminous, but much more numerous main-sequence K-type stars,
such as alpha Centauri B. The long life (2-3x longer than our Sun) and slow
evolution of K stars provide nearly constant energy sources for possible hosted
planets. Presented here are X-ray, UV, and recently acquired FUV observations
of the K1 V star alpha Cen B. These combined high-energy measures provide a
more complete look into the nature of alpha Cen B's magnetic activity and X-UV
radiances. We find that alpha Cen B has exhibited significant long-term
variability in X-ray through NUV emission fluxes, indicating a solar-like
long-term activity cycle of P_cycle = 8.84 years. In addition, analysis of the
short-term rotational modulation of mean light due to the effects of
magnetically active regions has yielded a well-determined rotation period of
P_rotation = 36.2 days. alpha Cen B is the only old main-sequence K star with a
reliably determined age and rotation period, and for early K-stars, is an
important calibrator for stellar age/rotation/activity relations
The LMC eclipsing binary HV 2274: fundamental properties and comparison with evolutionary models
We are carrying out an international, multi-wavelength program to determine
the fundamental properties and independent distance estimates of selected
eclipsing binaries in the LMC and SMC. Eclipsing binaries with well-defined
double-line radial velocity curves and light curves provide valuable
information on orbital and physical properties of their component stars. The
study of stars in the LMC and SMC where the metal abundances are significantly
lower than solar provides an important opportunity to test stellar atmosphere,
interior and evolution models, and opacities. For the first time, we can also
measure direct M-L relations for stars outside our Galaxy. In this paper we
concentrate on the determination of the orbital and physical properties of HV
2274 from analyses of light curves and new radial velocity curves formed from
HST/GHRS observations. From UV/optical spectrophotometry of HV 2274 obtained
with HST/FOS, the temperatures and the metallicity of the stars were found, as
well as the interstellar extinction of the system. The values of mass, absolute
radius, and effective temperature, for the primary and secondary stars are:
12.2(7) Mo, 9.9(2) Ro, 23000(180) K, and 11.4(7) Mo, 9.0(2) Ro, 23110(180) K,
respectively. The age of the system (17(2) Myr), helium abundance (Y=0.26(3))
and a lower limit of the convective core overshooting parameter of 0.2 were
obtained from fitting the stellar data with evolution models. The apsidal
motion analysis corroborates that some amount of convective overshooting
(0.2-0.5) is needed.Comment: 39 pages, 9 figures, accepted for publication in Ap
The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
(abridged:) The XMM-Newton Extended Survey of the Taurus Molecular Cloud
(XEST) surveys the most populated ~5 square degrees of the Taurus star
formation region, using the XMM-Newton X-ray observatory to study the thermal
structure, variability, and long-term evolution of hot plasma, to investigate
the magnetic dynamo, and to search for new potential members of the
association. Many targets are also studied in the optical, and high-resolution
X-ray grating spectroscopy has been obtained for selected bright sources. The
X-ray spectra have been coherently analyzed with two different thermal models
(2-component thermal model, and a continuous emission measure distribution
model). We present overall correlations with fundamental stellar parameters
that were derived from the previous literature. A few detections from Chandra
observations have been added. The present overview paper introduces the project
and provides the basic results from the X-ray analysis of all sources detected
in the XEST survey.Comprehensive tables summarize the stellar properties of all
targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus
by about an order of magnitude and for the first time systematically accesses
very faint and strongly absorbed TMC objects. We find a detection rate of 85%
and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify
about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169
surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass
correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios,
and show evidence for lower X-ray luminosities in CTTS compared to WTTS.
Detailed analysis (e.g., variability, rotation-activity relations, influence of
accretion on X-rays) will be discussed in a series of accompanying papers.Comment: 75 pg, 77 figs. Accepted by A&A, to appear in a special section/issue
dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud
(XEST). V2: ASCII Table 14 added. Version with higher resolution figures at
http://www.issibern.ch/teams/Taurus/papers.html or
http://www.astro.phys.ethz.ch/papers/guedel/guedel_p_nf.htm
The first high-resolution X-ray spectrum of a Herbig Star: The case of AB Aurigae
We present the first high-resolution X-ray spectrum of a prototypical Herbig
star (AB Aurigae), measure and interpret various spectral features, and compare
our results with model predictions. We use X-ray spectroscopy data from
XMM-Newton. The spectra are interpreted using thermal, optically thin emission
models with variable element abundances and a photoelectric absorption
component. We interpret line flux ratios in He-like triplet of O VII as a
function of electron density and the UV radiation field. We use the nearby
co-eval classical T Tauri star SU Aur as a comparison. AB Aurigae reveals a
soft X-ray spectrum, most plasma being concentrated at 1-6 MK. The He-like
triplet reveals no signatures of increased densities and there are no clear
indications for strong abundance anomalies. The light curve displays modulated
variability, with a period of ~ 42 hr. It is unlikely that a nearby, undetected
lower-mass companion is the source of the X-rays. Accretion shocks close to the
star should be irradiated by the photosphere, leading to alteration in the
He-like triplet fluxes of O VII, which we do not measure. Also, no indications
for high densities are found, although the mass accretion rate is presently
unknown. Emission from wind shocks is unlikely, given the weak radiation
pressure. A possible explanation would be a solar-like magnetic corona.
Magnetically confined winds provide a very promising alternative. The X-ray
period is indeed close to periods previously measured in optical lines from the
wind.Comment: 18 pages, 7 Figure
Multisite observations of SU Aurigae
We present results from the 1996 MUSICOS (MUlti-SIte COntinuous Spectroscopy)
campaign on the T Tauri star SU Aurigae. We find a 2.7-d periodicity in the He
I (587.6 nm) line and somewhat longer, less well-pronounced periodicities in
the Balmer lines and in Na D. Our observations support the suggestion that the
wind and infall signatures are out of phase on SU Aur. We present Doppler
images of SU Aur that have been obtained from least-squares deconvolved
profiles. Images taken about one rotation apart show only limited overlap, in
particular at low latitudes. This is in part due to limitations in
signal-to-noise, and in part due to line profile deformations that arise from
short-lived and/or non-surface features. The agreement at high latitudes is
better and suggests that at least some longer-lived features are present. The
analysis of Stokes V profiles yields a marginal magnetic field detection during
one of the phases.Comment: 22 pages, 20 figures, accepted for publication in MNRA