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 $c$ 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 $\gamma$ of Ne^{2+} for the winds of each star. The ion fraction abundance is a product of the ionization fraction $Q_{\rm i}$ in stage i and the abundance by number ${\cal A}_E$ 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$_{cool}$ = 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$_{hot}$ $\geq$ 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