arXiv subjects

I. Hunter

Publications and source records attributed to I. Hunter.

23 records · Page 2Linked to original sources

arXiv2006

The VLT-FLAMES survey of massive stars: Surface chemical compositions of B-type stars in the Magellanic Clouds

We present an analysis of high-resolution FLAMES spectra of approximately 50 early B-type stars in three young clusters at different metallicities, NGC6611 in the Galaxy, N11 in the Large Magellanic Cloud (LMC) and NGC346 in the Small Magellanic Cloud (SMC). Using the TLUSTY non-LTE model atmospheres code, atmospheric parameters and photospheric abundances (C, N, O, Mg and Si) of each star have been determined. These results represent a significant improvement on the number of Magellanic Cloud B-type stars with detailed and homogeneous estimates of their atmospheric parameters and chemical compositions. The relationships between effective temperature and spectral type are discussed for all three metallicity regimes, with the effective temperature for a given spectral type increasing as one moves to a lower metallicity regime. Additionally the difficulties in estimating the microturbulent velocity and the anomalous values obtained, particularly in the lowest metallicity regime, are discussed. Our chemical composition estimates are compared with previous studies, both stellar and interstellar with, in general, encouraging agreement being found. Abundances in the Magellanic Clouds relative to the Galaxy are discussed and we also present our best estimates of the base-line chemical composition of the LMC and SMC as derived from B-type stars. Additionally we discuss the use of nitrogen as a probe of the evolutionary history of stars, investigating the roles of rotational mixing, mass-loss, blue loops and binarity on the observed nitrogen abundances and making comparisons with stellar evolutionary models where possible.

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arXiv2006

The VLT-FLAMES Survey of Massive Stars: Stellar parameters and rotational velocities in NGC3293, NGC4755 and NGC6611

An analysis is presented of VLT-FLAMES spectroscopy for three Galactic clusters, NGC3293, NGC4755 and NGC6611. Non-LTE model atmosphere calculations have been used to estimate effective temperatures (from either the helium spectrum or the silicon ionization equilibrium) and gravities (from the hydrogen spectrum). Projected rotational velocities have been deduced from the helium spectrum (for fast and moderate rotators) or the metal line spectrum (for slow rotators). The observed Hertzsprung-Russell diagrams are compared with theoretical predictions and some discrepancies including differences in the main sequence luminosities are discussed. Cluster ages have been deduced and evidence for non-coeval star formation is found for all three of the clusters. Projected rotational velocities for targets in the older clusters, NGC3293 and NGC4755, have been found to be systematically larger than those for the field, confirming recent results in other similar age clusters. The distribution of projected rotational velocities are consistent with a Gaussian distribution of intrinsic rotational velocities. For the relatively unevolved targets in the older clusters, NGC3293 and NGC4755, the peak of the velocity distribution would be 250km/s with a full-width-half-maximum of approximately 180km/s. For NGC6611, the sample size is relatively small but implies a lower mean rotational velocity. This may be evidence for the spin-down effect due to angular momentum loss through stellar winds, although our results are consistent with those found for very young high mass stars. For all three clusters we deduce present day mass functions with Gamma-values in the range of -1.5 to -1.8, which are similar to other young stellar clusters in the Milky Way (Abridged).

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arXiv2006

Early-type stars observed in the ESO UVES Paranal Observatory Project: I -- Interstellar Na I UV, Ti II and Ca II K observations

We present an analysis of interstellar Na I (lambda=3302.37Å, 3302.98Å), Ti II (lambda=3383.76Å) and Ca II K (lambda=3933.66Å) absorption features for 74 sightlines towards O- and B-type stars in the Galactic disc. The data were obtained from the UVES Paranal Observatory Project, at a spectral resolution of 3.75km/s and with mean signal to noise ratios per pixel of 260, 300 and 430 for the Na I, Ti II and Ca II observations, respectively. Interstellar features were detected in all but one of the Ti II sightlines and all of the Ca II sightlines. The dependence of the column density of these three species with distance, height relative to the Galactic plane, H I column density, reddening and depletion relative to the solar abundance has been investigated. We also examine the accuracy of using the Na I column density as an indicator of that for H I. In general we find similar strong correlations for both Ti and Ca, and weaker correlations for Na. Our results confirm the general belief that Ti and Ca occur in the same regions of the interstellar medium and also that the Ti II/Ca II ratio is constant over all parameters. We hence conclude that the absorption properties of Ti and Ca are essentially constant under the general interstellar medium conditions of the Galactic disc.

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arXiv2005

CaII K observations of QSOs in the line-of-sight to the Magellanic Bridge

We describe medium-resolution spectroscopic observations taken with the ESO Multi-Mode Instrument in the CaII K line (3933.661 Angstroms) towards 7 QSOs located in the line-of-sight to the Magellanic Bridge. At a spectral resolution R = 6,000, five of the sightlines have a signal-to-noise ratio of 20 or higher. Definite Ca absorption due to Bridge material is detected towards 3 objects, with probable detection towards two other sightlines. Gas-phase CaII K Bridge and Milky Way abundances or lower limits for the all sightlines are estimated by the use of Parkes 21-cm HI emission line data. These data only have a spatial resolution of 14 arcminutes compared with the optical observations which have milli-arcsecond resolution. With this caveat, for the three objects with sound CaII K detections, we find that the ionic abundance of CaII K relative to HI, A=log(N(CaK)/N(HI)) for low-velocity Galactic gas ranges from -8.3 to -8.8 dex, with HI column densities varying from 3-6x10^20 cm^-2. For Magellanic Bridge gas, the values of A are 0.5 dex higher, ranging from -7.8 to -8.2 dex, with N(HI)=1-5x10^20 cm^-2. Higher values of A correspond to lower values of N(HI), although numbers are small. For the sightline towards B0251--675, the Bridge gas has two different velocities, and in only one of these is CaII K tentatively detected, perhaps indicating gas of a different origin or present-day characteristics (such as dust content), although this conclusion is uncertain and there is the possibility that one of the components could be related to the Magellanic Stream. Higher signal-to-noise CaII K data and higher resolution HI data are required to determine whether A changes with N(HI) over the Bridge and if the implied difference in the metalicity of the two Bridge components towards B0251-675 is real.

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arXiv2005

A non-LTE analysis of the spectra of two narrow lined main sequence stars in the SMC

An analysis of high-resolution VLT/UVES spectra of two B-type main sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud (SMC), has been undertaken, using the non-LTE TLUSTY model atmospheres to derive the stellar parameters and chemical compositions of each star. The chemical compositions of the two stars are in reasonable agreement. Moreover, our stellar analysis agrees well with earlier analyses of H II regions. The results derived here should be representative of the current base-line chemical composition of the SMC interstellar medium as derived from B-type stars.

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