Title: The spectra of extragalactic Wolf-Rayet stars
Abstract: We are studying Wolf-Rayet (WR) stars in nearby galaxies in order to measure the massive-star content of these systems and to see what differences might exist in the evolution and properties of their most massive stars. WR candidates have previously been identified in M31, M33, NGC 6822, and IC 1613, and in this paper we discuss spectrophotometry of 93 WR stars in these Local Group galaxies, including 27 newly confirmed ones. The spectra were obtained with the IIDS and Cryogenic Camera on the KPNO 4 m telescope, and with the photon-counting Reticon system on the MMT. Line parameters (equivalent widths (ew), line widths (FWHM) were then measured with IRAF and compared to our extensive collection of data on Galactic and Magellanic cloud WR stars, both for the WN and WC classes. In general, stars with the broadest lines also have the strongest features. The weakest-lined stars are those of the lowest excitation classes (WN9, WN8,..., WC9, WC8,...), while the earliest-type stars (the highest excitation) have the broadest and strongest lines. The exceptions are always binaries, in which we expect the ew to be too weak but the FWHM unaltered. We find from our measures that the M33 WN and WC stars follow the same line width-line-strength relation as do the Galactic and LMC stars and thus appear "normal." The WR stars in NGC 6822 are all quite weak-lined, and as such are reminiscent of the SMC stars. The M31 WR stars do not quite fall in the same region of these diagrams, being relatively weak lined for their line widths Possibly, this is due to small-number statistics---only a few M31 stars have been observed so far. However, possibly this is saying that the stellar wind laws are different from those of other systems. We note the curious fact that in none of these galaxies, including the LMC or SMC, have late-type WC stars yet been found: the latest type seen is WC7. We are readily detecting WN stars with lines maker than that of a WC8 or WC9 Galactic star, so this is not a selection effect. The WC8's and WC9's in the solar neighborhood are found generally towards the Galactic center, where the metallicity is higher. But if this were the explanation elsewhere, why have we not found these stars in M31?
Publication Year: 1987
Publication Date: 1987-12-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 29
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