MWC 297, B1.5Ve: a zero-age main-sequence star in the Aquila Rift

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Abstract
Moderate-resolution optical spectra of the significantly reddened Herbig Be star MWC 297 are presented. The data are of sufficient quality that it has been possible to obtain a spectral type for this star by comparing heavy element absorption features with those present in early B field stars of known spectral type. The best fit is to B1.5V, with an uncertainty of half a sub-type. It is estimated that v sin i for MWC 297 is about 350 ± 50 km −1, suggesting that the star's rotation axis is almost in the plane of the sky. The reddening towards this object is reviewed and is derived afresh from spectra at wavelengths spanning the B–R optical bands. Using Av ≃ 8 and the absolute magnitude corresponding to B1.5V, it is argued that the distance to MWC 297 is 250 ± 50 pc rather than ∼500 pc as has recently been quoted in the literature. At this distance the star can be located very plausibly in the Aquila Rift. We go on to present a MERLIN 5-GHz radio map of the source which shows it to be markedly elongated in the north-south direction (to a dimension of 125 au at 250 pc). Existing data on the optical linear polarization of MWC 297 suggest a complicated picture that leaves open the question of whether the radio axis might trace an ionized electron-scattering equatorial disc. We also present and briefly discuss high-resolution Bra and Hei 1.083-μmline profiles obtained within a week of the optical spectra. The He i profile is very complex, but confirms that MWC 297 is an outflow source. It is noted that the Hi emission lines are undoubtedly highly variable, although the time-scales for this are not yet clear.

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