On the orbital and physical parameters of the HDE 226868/Cygnus X-1 binary system

In this paper we explore the consequences of the recent determination of the mass m=(8.7 +/- 0.8)M_Sun of Cygnus X-1, obtained from the Quasi-Periodic Oscillation (QPO)-photon index correlation scaling, on the orbital and physical properties of the binary system HDE 226868/Cygnus X-1. By using such a result and the latest spectroscopic optical data of the HDE 226868 supergiant star we get M=(24 +/- 5)M_Sun for its mass. It turns out that deviations from the third Kepler law significant at more than 1-sigma level would occur if the inclination i of the system's orbital plane to the plane of the sky falls outside the range 41-56 deg: such deviations cannot be due to the first post-Newtonian (1PN) correction to the orbital period because of its smallness; interpreted in the framework of the Newtonian theory of gravitation as due to the stellar quadrupole mass moment Q, they are unphysical because Q would take unreasonably large values. By conservatively assuming that the third Kepler law is an adequate model for the orbital period we obtain i=(48 +/- 7) deg which yields for the relative semimajor axis a=(42 +/- 9)R_Sun. Our estimate for the Roche's lobe of HDE 226868 is r_M = (21 +/- 6)R_Sun.Comment: Latex2e, 7 pages, 1 table, 4 figures. To appear in ApSS (Astrophysics and Space Science

The impact of the oblateness of Regulus on the motion of its companion

The fast spinning B-star Regulus has recently been found to be orbited by a fainter companion in a close circular path with orbital period P_b = 40.11(2) d. Being its equatorial radius R_e 32% larger than the polar one R_p, Regulus possesses a remarkable quadrupole mass moment Q. We investigate the effects of Q on the orbital period P_b of its companion in order to see if they are measurable, given the present-day level of accuracy in measuring P_b. Conversely, we will look for deviations from the third Kepler law, attributed to the quadrupole mass moment Q of Regulus, to constrain the ratio \gamma=m/M of the system's masses.Comment: LaTex, 6 pages, 5 figures, 1 table. Accepted by Astrophysics and Space Scienc

Horsterwold - Veluwe - Maaswoud: een quick scan van robuuste ecologische verbindingen van het ambitieniveau 'edelhert'

De natuur in Nederland raakt steeds meer versnipperd. De gevolgen worden goed zichtbaar wanneer wordt gekeken naar de situatie van grote zoogdieren als het edelhert. Teruggedrongen achter rasters in een sub-optimaal habitat leven zij op twee plaatsen: de Veluwe en de Oostvaardersplassen. Het voorkomen van edelherten in andere, grootschalige natuurgebieden wordt van belang geacht voor de compleetheid van deze ecosystemen. Realisatie van de EHS biedt in dit opzicht onvoldoende kansen. Daarom zijn door het Rijk en de provincies een aantal zogenaamde robuuste verbindingen indicatief op kaart gezet. Doel daarbij kan zijn uitbreiding van het huidige leefgebied van het edelhert en het bieden van de mogelijkheid van migratie. Dit rapport is de weerslag van een ecologisch onderzoek naar mogelijke aanvullingen op de reeds verkende robuuste verbindingen voor het edelhert. Daartoe werden elf potentiële tracés verkend. Onderzocht is de mogelijkheid om de Veluwe te verbinden met het Horsterwold, de Utrechtse Heuvelrug en Duitsland via de Gelderse Poort. Tevens is onderzocht hoe vanuit dit laatste gebied de verbinding met het Maaswoud zou kunnen worden gerealiseerd. Ecologische en economische argumenten worden gepresenteerd om te helpen een keuze te bepalen tussen de mogelijke verbindingen

Stellar dynamics in young clusters: the formation of massive runaways and very massive runaway mergers

In the present paper we combine an N-body code that simulates the dynamics of young dense stellar systems with a massive star evolution handler that accounts in a realistic way for the effects of stellar wind mass loss. We discuss two topics: 1. The formation and the evolution of very massive stars (with a mass >120 Mo) is followed in detail. These very massive stars are formed in the cluster core as a consequence of the successive (physical) collison of 10-20 most massive stars of the cluster (the process is known as runaway merging). The further evolution is governed by stellar wind mass loss during core hydrogen burning and during core helium burning (the WR phase of very massive stars). Our simulations reveal that as a consequence of runaway merging in clusters with solar and supersolar values, massive black holes can be formed but with a maximum mass of 70 Mo. In small metallicity clusters however, it cannot be excluded that the runaway merging process is responsible for pair instability supernovae or for the formation of intermediate mass black holes with a mass of several 100 Mo. 2. Massive runaways can be formed via the supernova explosion of one of the components in a binary (the Blaauw scenario) or via dynamical interaction of a single star and a binary or between two binaries in a star cluster. We explore the possibility that the most massive runaways (e.g., zeta Pup, lambda Cep, BD+433654) are the product of the collision and merger of 2 or 3 massive stars.Comment: Updated and final versio

Setting a new standard in the analysis of binary stars: progress and challenges

This is an era of enormous opportunity in binary star research. The Leuven meeting provided an assessment of the broad scope of observational work, merits of the analysis methods, and connections with theory. Here I outline some of the issues related to spectroscopic, photometric, and high angular resolution observations of binary stars

Colliding Stellar Winds in O-type Close Binary Systems

In close binary systems of O-type stars, the individual stellar winds will collide between the stars to form shock fronts (Stevens et al. 1992). Binaries with equally luminous stars will have winds of comparable strength, and the shock will occur near the mid-plane between the stars, but in binaries of unequal luminosity, the interaction will occur along a bow shock wrapped around the star with the weaker wind. The presence of the shock region can be detected through excess X-ray emission (Chlebowski &amp; Garmany 1990), and orbital phase-related variations in the UV P Cygni lines (Shore &amp; Brown 1988) and optical emission lines (formed in high density regions of circumstellar gas).We have begun a search for colliding winds through a study of the optical emission lines and UV P Cygni lines in four massive binaries, AO Cas (Gies &amp; Wiggs 1991), Plaskett’s star = HD 47129 (Wiggs &amp; Gies 1992), 29 UW CMa and ι Ori. The optical observations consist of high S/N spectra of the Hα and He I λ6678 region obtained with the University of Texas McDonald Observatory 2.1-m telescope and coudé Reticon system. The UV observations were culled from archival IUE high dispersion spectra of several P Cygni features (N V λ1240, Si IV λ1400, C IV λ1550).</jats:p

Tomographic Separation of Composite Spectra of O-Type Stars

AbstractWe have analyzed UV photospheric lines of three O-type binaries (AO Cas, Plaskett’s Star, and 29 UW CMa) by means of tomographic and cross-correlation analyses, with the goal of estimating the physical properties of the individual stars. The tomographic techniques allowed us to estimate Teff and log g the luminosity ratio, and rotational velocities. The mass ratios were obtained from a cross-correlation technique, applied to spectral regions in which preliminary tomography shows the secondary has strong photospheric line features.</jats:p

Spectral Transients in the Line Profiles of λ Eridani

Recently we have monitored the absorption line profile variations of a number of rapidly rotating B stars. These variations are observed as alternating quasi-absorption and -emission bumps that travel blue-to-red across the profile with a local period defined by the time to move one bump spacing. This type of pattern can be modelled successfully by assuming the presence of one or two active nonradial pulsation (NRP) modes in the photosphere of the star. Many observations show significant departures from the simple expectations of the models: successive bumps can have different amplitudes (which may vary on timescales of hours), and individual bumps can occasionally appear just ahead or behind the position predicted from a mean period. Nevertheless, we believe these anomalies can be treated as perturbations from a time-averaged pulsation model.</jats:p

Spectroscopic Line Modeling of the Fastest Rotating O-type Stars

We present a spectroscopic analysis of the most rapidly rotating stars currently known, VFTS 102 (ve sin i = 649 ± 52 km s-1; O9: Vnnne+) and VFTS 285 (ve sini = 610 ± 41 km s-1; O7.5: Vnnn), both members of the 30 Dor complex in the Large Magellanic Cloud. This study is based on high-resolution ultraviolet spectra from Hubble Space Telescope/Cosmic Origins Spectrograph and optical spectra from the Very Large Telescope (VLT) X-shooter plus archival VLT GIRAFFE spectra. We utilize numerical simulations of their photospheres, rotationally distorted shape, and gravity darkening to calculate model spectral line profiles and predicted monochromatic absolute fluxes. We use a guided grid search to investigate parameters that yield best fits for the observed features and fluxes. These fits produce estimates of the physical parameters for these stars (plus a Galactic counterpart, ζ Oph) including the equatorial rotational velocity, inclination, radius, mass, gravity, temperature, and reddening. We find that both stars appear to be radial-velocity constant. VFTS 102 is rotating at critical velocity, has a modest He enrichment, and appears to share the motion of the nearby OB-association LH 99. These properties suggest that the star was spun up through a close binary merger. VFTS 285 is rotating at 95% of critical velocity, has a strong He enrichment, and is moving away from the R136 cluster at the center of 30 Dor. It is mostly likely a runaway star ejected by a supernova explosion that released the components of the natal binary system