1,349 research outputs found
Fundamental stellar parameters of zeta Pup and gamma^2 Vel from HIPPARCOS data
We report parallax measurements by the HIPPARCOS satellite of zeta Puppis and
gamma^2 Velorum. The distance of zeta Pup is d=429 (+120/ -77) pc, in agreement
with the commonly adopted value to Vela OB2. However, a significantly smaller
distance is found for the gamma^2 Vel system: d=258 (+41/-31) pc. The total
mass of gamma^2 Vel derived from its parallax, the angular size of the
semi-major axis as measured with intensity interferometry, and the period is
M(WR+O)=29.5 (+/-15.9) Msun. This result favors the orbital solution of Pike et
al. (1983) over that of Moffat et al. (1986). The stellar parameters for the O
star companion derived from line blanketed non-LTE atmosphere models are:
Teff=34000 (+/-1500) K, log L/Lsun=5.3 (+/-0.15) from which an evolutionary
mass of M=29 (+/-4) Msun and an age of 4.0 (+0.8/-0.5) Myr is obtained from
single star evolutionary models. With non-LTE model calculations including He
and C we derive a luminosity log L/Lsun~4.7 (+/-0.2) for the WR star. The
mass-luminosity relation of hydrogen-free WR stars implies a mass of M(WR)~5
(+/-1.5) Msun. From our data we favor an age of ~10 Myr for the bulk of the
Vela OB2 stars. Evolutionary scenarios for zeta Pup and gamma^2 Vel are
discussed in the light of our results.Comment: Submitted to ApJ Letters (misprints corrected
Resolution of Corneal Fibrosis After Descemet's Stripping Automated Endothelial Keratoplasty: A Case Report.
To report the resolution of anterior corneal fibrosis after Descemet's stripping automated endothelial keratoplasty (DSAEK), in a patient with chronic corneal edema and anterior stromal scarring.
A 63-year-old woman, with a history of Fuchs endothelial dystrophy, presented with increasing discomfort and gradual visual loss in her right eye. Clinical examination revealed long-standing bullous keratopathy accompanied by marked subepithelial fibrosis (SEF). Based on the low postoperative visual potential due to glaucomatous optic neuropathy, we decided to proceed with DSAEK.
During the follow-up period, SEF was found to gradually resolve. Corneal clarity was restored and an improvement in visual acuity was observed up to 12 months after surgery.
DSAEK alone may represent an effective therapeutic option for the restoration of impaired corneal clarity in patients with long-standing corneal edema and concomitant anterior subepithelial scarring
A simple proof of the Markoff conjecture for prime powers
We give a simple and independent proof of the result of Jack Button and Paul
Schmutz that the Markoff conjecture on the uniqueness of the Markoff triples
(a,b,c), where a, b, and c are in increasing order, holds whenever is a
prime power.Comment: 5 pages, no figure
An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss rates and
terminal velocities during their evolution from O-star through the Luminous
Blue Variable phase to the Wolf-Rayet phase. The luminosity remains
approximately unchanged during these phases. These large changes in wind
properties are explained in the context of the radiation driven wind theory, of
which we consider four different models. They are due to the evolutionary
changes in radius, gravity and surface composition and to the change from
optically thin (in continuum) line driven winds to optically thick radiation
driven winds.Comment: Accepted for publication in Astronomy and Astrophysics (Letter to the
Editor
Time delays in quasi-periodic pulsations observed during the X2.2 solar flare on 2011 February 15
We report observations of quasi-periodic pulsations (QPPs) during the X2.2
flare of 2011 February 15, observed simultaneously in several wavebands. We
focus on fluctuations on time scale 1-30 s and find different time lags between
different wavebands. During the impulsive phase, the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) channels in the range 25-100 keV lead all
the other channels. They are followed by the Nobeyama RadioPolarimeters at 9
and 17 GHz and the Extreme Ultra-Violet (EUV) channels of the Euv
SpectroPhotometer (ESP) onboard the Solar Dynamic Observatory (SDO). The
Zirconium and Aluminum filter channels of the Large Yield Radiometer (LYRA)
onboard the Project for On-Board Autonomy (PROBA2) satellite and the SXR
channel of ESP follow. The largest lags occur in observations from the
Geostationary Operational Environmental Satellite (GOES), where the channel at
1-8 {\AA} leads the 0.5-4 {\AA} channel by several seconds. The time lags
between the first and last channels is up to 9 s. We identified at least two
distinct time intervals during the flare impulsive phase, during which the QPPs
were associated with two different sources in the Nobeyama RadioHeliograph at
17 GHz. The radio as well as the hard X-ray channels showed different lags
during these two intervals. To our knowledge, this is the first time that time
lags are reported between EUV and SXR fluctuations on these time scales. We
discuss possible emission mechanisms and interpretations, including flare
electron trapping
Neon Abundances from a Spitzer/IRS Survey of Wolf-Rayet Stars
We report on neon abundances derived from {\it Spitzer} high resolution
spectral data of eight Wolf-Rayet (WR) stars using the forbidden line of
[\ion{Ne}{3}] 15.56 microns. Our targets include four WN stars of subtypes
4--7, and four WC stars of subtypes 4--7. We derive ion fraction abundances
of Ne^{2+} for the winds of each star. The ion fraction abundance is a
product of the ionization fraction in stage i and the abundance by
number of element E relative to all nuclei. Values generally
consistent with solar are obtained for the WN stars, and values in excess of
solar are obtained for the WC stars.Comment: to appear in Astrophysical Journa
Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-UV Spectroscopic Analysis
We observed seven central stars of planetary nebulae (CSPN) in the Large
Magellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE),
and performed a model-based analysis of these spectra in conjunction with
Hubble Space Telescope (HST) spectra in the UV and optical range to determine
the stellar and nebular parameters. Most of the objects show wind features, and
they have effective temperatures ranging from 38 to 60 kK with mass-loss rates
of ~= 5x10^-8 Msun/yr. Five of the objects have typical LMC abundances. One
object (SMP LMC 61) is a [WC4] star, and we fit its spectra with He/C/O-rich
abundances typical of the [WC] class, and find its atmosphere to be
iron-deficient. Most objects have very hot (T ~> 2000 K) molecular hydrogen in
their nebulae, which may indicate a shocked environment. One of these (SMP LMC
62) also displays OVI 1032-38 nebular emission lines, rarely observed in PN.Comment: 53 pages, 15 figures (11 color). Accepted for publication in Ap
XMM-Newton Detection of Hard X-ray Emission in the Nitrogen-Type Wolf-Rayet Star WR110
We have used the excellent sensitivity of XMM-Newton to obtain the first
high-quality X-ray spectrum of a Wolf-Rayet (WR) star which is not known to be
a member of a binary system. Our target, the nitrogen-type star WR 110 (= HD
165688) was also observed and detected with the VLA at four different
frequencies. The radio data are in excellent agreement with that expected for
free-free wind emission. and the ionized mass-loss rate is derived. The X-ray
emission measure distribution shows a dominant contribution from cool plasma at
kT = 0.5 keV (6 MK) which is only weakly absorbed. We argue that this
cool emission originates at hundreds of radii if the wind is spherical and
homogeneous and derive shock velocities and the X-ray filling factor using
radiative shock models. A surprising result is the unambiguous detection of a
hard X-ray component clearly seen in the hard-band images and the spectra. This
hard component accounts for about half of the observed flux and can be
acceptably fitted by a hot optically thin thermal plasma or a power-law model.
If the emission is thermal, then a temperature kT 3 keV is
derived. Such high temperatures are not predicted by current instability-driven
wind shock models. We examine several alternatives and show that the hard
emission could be accounted for by the WR wind shocking onto a close stellar
companion which has so far escaped detection. However, until persuasive
evidence for binarity is found we are left with the intriguing possibility that
the hard X-ray emission is produced entirely by the Wolf-Rayet star.Comment: 2 tables, 7 figure
Semi-Static Hedging Based on a Generalized Reflection Principle on a Multi Dimensional Brownian Motion
On a multi-assets Black-Scholes economy, we introduce a class of barrier
options. In this model we apply a generalized reflection principle in a context
of the finite reflection group acting on a Euclidean space to give a valuation
formula and the semi-static hedge.Comment: Asia-Pacific Financial Markets, online firs
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