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An optical and near-IR spectroscopic study of the extreme P Cygni-type supergiant HDE 316285
D.J. Hillier 1,
P.A. Crowther * 2,
F. Najarro 3 and
A.W. Fullerton; ** 3
Received 29 May 1998 / Accepted 12 June 1998
A detailed study of the Galactic P Cygni-type supergiant HDE 316285, based on high quality optical (AAT, MSO, CTIO) and near-IR (UKIRT, CFHT, CTIO) spectroscopy, is presented. As has been noted previously, its spectrum is dominated by H, He i, and Fe ii P Cygni profiles. Emission lines due to N i, N ii, [N ii], O i, Na i, Mg ii, Al ii, Ca ii, Si ii, Si iii, Fe ii and [Fe ii] can also be readily identified. Many of the metal lines are produced by continuum fluorescence. The rich N spectrum, the paucity of the O spectrum (only 2 O lines can be identified), and the apparent absence of emission due to C, strongly suggest that the atmosphere of the star is contaminated by CNO processed material. A comparison of the spectrum of HDE 316285 with P Cygni and He 3-519 is presented.
From a spectral analysis using the non-LTE atmosphere code of Hillier (1991), and assuming a distance of 1.85 kpc, our preferred model for HDE 316285 has the following parameters: , , , km s-1, =1.81 mag, and H/He1.5 by number. Due to the low degree of He ionization the derived H/He abundance ratio and mass-loss rate are strongly coupled. Models with H/He=10 to 0.5 are equally capable of explaining the H and He i spectrum provided the mass-loss rate is scaled according to the approximate formula . Preliminary work, however, indicates that a solar H/He ratio can be ruled out on the basis of line strengths of other species - particularly N, Mg, Al.
The stellar wind from HDE 316285 is more extreme than P Cygni with a performance number (= ratio of wind momentum to radiative momentum) 30 times greater. The low H/He abundance ratio and high N/He abundance ratio confirms that HDE 316285 is evolved.
Although we find no evidence in the literature for photometric variability, we find strong evidence for significant spectral variability. Because of the spectral variability, and because the stellar properties and chemical content of HDE 316285 are similar to known luminous blue variables (LBVs), we suggest that it is a LBV. Support for this contention comes from the detection by McGregor et al. (1988) of a cold circumstellar dust shell associated with HDE 316285. However HDE 316285, like P Cygni, could currently be in a relatively quiescent phase of its LBV life, exhibiting significant spectral variations but not undergoing major photometric outbursts similar to AG Car.
The mass loss of HDE 316285 is prodigious. In less than 105 years it will lose over 20 . Even if HDE 316285 is not an LBV, it is obviously in an evolutionary phase of short duration.
Key words: stars: early-type stars: individual: HDE 316285 stars: mass-loss stars: supergiants
* Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.
Present address: CSIC, Instituto de Estructura de la Materia, Dpto. Fisica Molecular C/Serrano 121, 28006 Madrid, Spain
Present address: CAS, Department of Phys. and Astr., The Johns Hopkins University
Send offprint requests to: D. John Hillier
Online publication: November 9, 1998