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Session 44 - Circumstellar Material.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
Long-slit optical spectroscopy of H\alpha emission nebulae around stars in M31 has led to the discovery of 5 stars with high mass loss rates, and the re-discovery of a Hubble-Sandage variable. The objects were selected from the catalog of emission nebulae in M31 compiled by us several years ago. Selection was based on the properties of the emission nebula; we had no information on the star inside it. We obtained KPNO 4-m spectra of a bright, barely resolved, nebula with an anomalous low upper limit to the [SII] over H\alpha flux ratio. The spectrum of this object shows broad wings on the Balmer lines and various emission lines from the star, mostly due to iron multiplets. Only later did we discover that this object is centered on Hubble's Variable 15, a Luminous Blue Variable (LBV). We subsequently obtained spectra with the ARC 3.5-m telescope of a sample of similar compact nebulae, all characterized by the absence of [SII] emission, implying anomalously low [SII]/H\alpha line ratios. The angular sizes of the nebulae, or their upper limits, imply radii less than a few pc. All objects have narrow Balmer lines with broad wings at low levels, indicating (apparent) velocity widths up to 1000 or 2000 km/s. Several objects show P-Cygni profiles in the Balmer lines, providing further evidence for high mass loss rates. None of the nebulae show [SII], [NII], or [OIII] emission. If these lines are absent because of collisional de-excitation, the nebulae must be quite dense. The 5 newly discovered stars are all fainter than Variable 15, and we did not detect a rich emission-line spectrum such as found in Variable 15. Since it is not known if any of the stars has experienced eruptive mass loss, they may not be LBV's. Nevertheless, the presence of a compact nebula and the high mass loss indicated by the Balmer lines make this interesting objects for further study. Since only 4 LBV's are known in M31, discovery of more LBV's or related objects may lead to better understanding of an important phase in the evolution of massive stars. We will discuss the spectra and broadband colors of the objects, and the physical parameters of the nebulae.
Program listing for Tuesday