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تسجيل مستخدم جديدUsing HII region spectra to probe the ionizing radiation from massive stars
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S. Simon-Diaz
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We present some results of an on-going project aimed at studying a sample of Galactic HII regions ionized by a single massive star to test the predictions of modern generation stellar atmosphere codes in the H Lyman continuum. The observations collected for this study comprise the optical spectra of the corresponding ionizing stars, along with imaging and long-slit spatially resolved nebular observations. The analysis of the stellar spectra allows to obtain the stellar parameters of the ionizing star, while the nebular observations provide constraints on the nebular abundances and gas distribution. All this information is then used to construct tailored photoionization models of the HII regions. The reliability of the stellar ionizing fluxes is hence tested by comparing the photoionization model results with the observations in terms of the spatial variation across the nebula of an appropriate set of nebular line ratios.
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We present a detailed comparison of the ionizing spectral energy distributions (SEDs) predicted by four modern stellar atmosphere codes, TLUSTY, CMFGEN, WMbasic, and FASTWIND. We consider three sets of stellar parameters representing a late O-type dw
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The part played by stars in the ionization of the intergalactic medium remains an open question. A key issue is the proportion of the stellar ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst afterg
lows can be used to determine the neutral hydrogen column-density in their host galaxies and hence the opacity to extreme ultra-violet radiation along the lines-of-sight to the bursts. Thus, making the reasonable assumption that long-duration GRB locations are representative of the sites of massive stars that dominate EUV production, one can calculate an average escape fraction of ionizing radiation in a way that is independent of galaxy size, luminosity or underlying spectrum. Here we present a sample of NH measures for 138 GRBs in the range 1.6<z<6.7 and use it to establish an average escape fraction at the Lyman limit of <fesc>~0.005, with a 98% confidence upper limit of ~0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget of the IGM at z<5, where the bulk of the bursts lie. At higher redshifts, z>5, firm conclusions are limited by the small size of the GRB sample, but any decline in average HI column-density seems to be modest. We also find no indication of a significant correlation of NH with galaxy UV luminosity or host stellar mass, for the subset of events for which these are available. We discuss in some detail a number of selection effects and potential biases. Drawing on a range of evidence we argue that such effects, while not negligible, are unlikely to produce systematic errors of more than a factor ~2, and so would not affect the primary conclusions. Given that many GRB hosts are low metallicity, high specific star-formation rate, dwarf galaxies, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the universe.
S106 is one of the best known bipolar HII regions, thoroughly studied and modelled at infrared, submillimeter and millimeter wavelengths, and it is one of the nearest examples of the late stages of massive star formation in which the newly formed sta
r that ionizes it is still surrounded by vast amounts of gas and dust. However, little is known about its heavily obscured central source, S106IR. The possible binarity of the central source is investigated, which is considered to be likely given the high binarity fraction among massive stars. We have carried out visible and near-infrared photometric monitoring looking for short-term variability, with special interest in that related to the presence of a close binary companion to S106IR that may produce periodic eclipses or tidal distortion of the shape of the members of the system. A periodic variability of S106IR in the J band is found with a period of 5.0 days and an amplitude of about 0.1 mag. The light curve displays a slow rise from minimum to maximum followed by a steep decrease, and can be well reproduced by a close binary system composed of two stars with different luminosity orbiting each other in an elliptical orbit of moderate eccentricity. S106IR also shows hints of short-term variability possibly related to accretion. We also report variability of four other stars previously classified as members of the S106 cluster, all of which are strong X-ray emitters. The newly discovered close binarity of S106IR adds a new element to the modeling of the nebula and to the understanding of the dynamics of the gas around the ionizing source, which suggests that the components of the binary are accreting via a circumbinary disk. Binarity also helps to explain the apparent mismatch between the spectral type of the ionizing source inferred from the nebular spectrum and its high brightness at near-infrared wavelengths.
HII regions are particularly interesting because they can generate dense layers of gas and dust, elongated columns or pillars of gas pointing towards the ionizing sources, and cometary globules of dense gas, where triggered star formation can occur.
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