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Here, we report on the first results from the e-MERLIN Cyg OB2 Radio Survey (COBRaS), which is designed to exploit e-MERLIN’s enhanced capabilities to conduct deep-field mapping of the tremendously rich Cyg OB2 association. The project aims to deliver the most detailed radio census of the most massive OB association in the northern hemisphere. There exists considerable evidence for clumping in the winds of hot stars, which has hugely important consequences for mass-loss determinations. The amount of mass lost from a massive star is a crucial parameter required for stellar and cluster evolution models that are paramount to our understanding of the formation and evolution of massive star clusters. Presenting some of the first 20cm (L band) detections of massive stars in Cyg OB2, both accurate mass-loss determinations and constraints upon clumping within their winds are made. These data substantially increase the observational detections of the outer wind of massive stars and in combination with other observations at different wavelengths, COBRaS will greatly advance our knowledge of clumping as a function of radial distance around massive stars.
Using new, high signal-to-noise CFHT ESPaDOnS visual spectrograms, and archive IUE and FUSE UV spectrograms, together with state-of-the-art non-LTE hydrodynamical model atmospheres, we have obtained accurate He, C, N, O photospheric abundance determinations in the central stars of NGC 2392, IC 4593, and NGC 6826. We compare with the corresponding nebular abundances, taken from the literature. The central star of NGC 2392 shows high He, N, and very low C, O abundances. We propose that these peculiar abundances must have originated in a common-envelope phase of interaction with a close binary companion. If we assume that the companion is more evolved than the visible central star, this offers a way of solving the old mystery of the discrepant He II Zanstra temperature of NGC 2392.
The science of extra-solar planets is one of the most rapidly changing areas of astrophysics and since 1995 the number of planets known has increased by almost two orders of magnitude. A combination of ground-based surveys and dedicated space missions has resulted in 560-plus planets being detected, and over 1200 that await confirmation. NASA's Kepler mission has opened up the possibility of discovering Earth-like planets in the habitable zone around some of the 100,000 stars it is surveying during its 3 to 4-year lifetime. The new ESA's Gaia mission is expected to discover thousands of new planets around stars within 200 parsecs of the Sun. The key challenge now is moving on from discovery, important though that remains, to characterisation: what are these planets actually like, and why are they as they are?
In the past ten years, we have learned how to obtain the first spectra of exoplanets using transit transmission and emission spectroscopy. With the high stability of Spitzer, Hubble, and large ground-based telescopes the spectra of bright close-in massive planets can be obtained and species like water vapour, methane, carbon monoxide and dioxide have been detected. With transit science came the first tangible remote sensing of these planetary bodies and so one can start to extrapolate from what has been learnt from Solar System probes to what one might plan to learn about their faraway siblings. As we learn more about the atmospheres, surfaces and near-surfaces of these remote bodies, we will begin to build up a clearer picture of their construction, history and suitability for life.
The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. By characterising spectroscopically more bodies in different environments we will take detailed planetology out of the Solar System and into the Galaxy as a whole.
EChO has now been selected by the European Space Agency to be assessed as one of four M3 mission candidates.
There are currently two optical interferometry recombiners that can provide spectral resolutions better than 10000, AMBER/VLTI operating in the H-K bands, and VEGA/CHARA, recently commissioned, operating in the visible. These instruments are well suited to study the wind activity of the brightest AB supergiants in our vicinity, in lines such as Hα or Brγ. We present here the first observations of this kind, performed on Rigel (B8Ia) and Deneb (A2Ia). Rigel was monitored by AMBER in two campaigns, in 2006-2007 and 2009-2010, and observed in 2009 by VEGA; whereas Deneb was monitored in 2008-2009 by VEGA. The extension of the Hα and Brγ line forming regions were accurately measured and compared with CMFGEN models of both stars. Moreover, clear signs of activity were observed in the differential visibility and phases. These pioneer observations are still limited, but show the path for a better understanding of the spatial structure and temporal evolution of localized ejections using optical interferometry.
We use a combination of VJHK and Spitzer [3.6], [5.8] and [8.0] photometry, to determine IR excesses in a sample of LMC and SMC O stars. This sample is ideal for determining excesses because: 1) the distances to the stars, and hence their luminosities, are well-determined, and; 2) the very small line of sight reddenings minimize the uncertainties introduced by extinction corrections. We find IR excesses much larger than expected from Vink et al. (2001) mass loss rates. This is in contrast to previous wind line analyses for many of the LMC stars which suggest mass loss rates much less than the Vink et al. predictions. Together, these results indicate that the winds of the LMC and SMC O stars are strongly structured (clumped).
Some late-type O stars display anomalously weak winds, possibly due to decoupling of the main driving ions from the bulk plasma. This issue and the uncertainty about the nature of wind clumping are a challenge to line-driven wind theory and need resolving in order to fully understand hot stars. We describe the results from the computation of ion fractions for the various elements in O star winds using non-LTE code CMFGEN, including parameterisation of microclumping and X-rays.
The e-MERLIN Cyg OB2 Radio Survey (COBRaS) is designed to exploit e-MERLIN's enhanced capabilities to conduct uniquely probing, targeted deep-field mapping of the massive Cyg OB2 association in our Galaxy. The project aims to deliver (between 2010 to 2013) the most detailed radio census for the most massive OB association in the northern hemisphere, offering direct comparison to not only massive clusters in general, but also young globular clusters and super star clusters. With the COBRaS Legacy project we will assemble a uniform dataset of lasting value that is critical for advancing our understanding of current astrophysical problems in the inter-related core themes of (i) mass loss and evolution of massive stars, (ii) the formation, dynamics and content of massive OB associations, and (iii) the frequency of massive binaries and the incidence of non-thermal radiation.
FUSE1 observations of stellar wind variability in the LMC supergiant Sk–67°166 (O4 If+)
We discuss why B supergiant winds are particularly well suited for wind studies, and present or refer to dynamic spectra which suggest the presence of disks, bifurcated winds, shock formation, rotationally modulated winds and the spontaneous generation of wind enhancements. They underscore the strength and richness of wind variability in B supergiants and the challenges these phenomena present to theoretical studies of stellar winds.
High resolution optical spectroscopy of V795 Herculis shows complex time- and phase-dependent behaviour of the disk emission. Separate low- and high-velocity fluctuations phased on the 2.6 hr orbital period are observed in the wings of the Balmer lines, prompting a gas stream overflow model.
Spectroscopic observations are discussed which indicate that the winds of luminous OB stars are highly structured in space and variable in time. Blueward migrating discrete optical depth enhancements in the absorption troughs of P Cygni profiles are the characteristic signatures of the evolving structure. Constraints on physical mechanisms are discussed, provided by the observed accelerations of the migrating features, the evidence for rotationally modulated variability, and the behaviour of the winds at very low velocities.
We present high resolution IUE ultraviolet observations of multiple narrow absorption components seen in the SiIII, NV, SiIV, and CIV profiles of the B1 IIIe star, HD110432. Spectra taken during March 1986, spanning ∼ 11 days, are modelled using line profile fits. Central velocities and column densities of the discrete features are derived.
The 11 observations obtained during March 1986 (SWP24923–28031) exhibit only minimal line profile variability, but are substantially different from the only other available IUE spectrum of HD110432, taken in April 1981 (fig. 1). This previous image reveals just a single extremely narrow (FWHM ∼ 50 km/s) feature at ∼ 1360 km/s.
The most sensitive indicators of mass-loss for stars in the upper left part of the HR diagram are the UV P Cygni profiles observed in the resonance lines of common ions such as N V, Si IV, and C IV. We present here some results from a study of these lines in the high resolution IUE spectra of 197 Ï stars. Profile fits were carried out in the manner described by Prinja & Howarth (1986) for all unsaturated P Cygni resonance doublets. The parameterisations adopted enable the product of mass-loss rate (Ṁ) and ion fraction (qi) to be determined at a given velocity, such that Ṁ qi°C Ni R* v∞, where Ni is the column density of the observed ion i, v∞ is the terminal velocity, and R⋆ is the stellar radius. The accompanying figures illustrate the behaviour of Ṁ qi (evaluated at 0.5 v∞) for N V and C IV.
The inclusion of rotation as an ingredient in radiation pressure driven stellar wind models is a nontrivial undertaking. Those bold enough to attempt an investigation of its likely importance include Castor (1979), Abbott (1980), and Marlborough & Zamir (1984), whose work shows that the critical point is expected to move away from the stellar surface with increasing rotation. Observationally, the surface mass flux is predicted to be insensitive to rotation, but the wind acceleration is expected to be less than in a nonrotating star and the terminal velocity smaller.
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