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Astron. Astrophys. 321, L21-L24 (1997)

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1. Introduction

The class of Luminous Blue Variable (LBV) stars is thought to represent a short-lived and violent post-main-sequence phase in the evolution of very massive ([FORMULA] [FORMULA]) stars. During this phase they are characterized by a high continuous mass-loss rate (10-6 to 10 [FORMULA] [FORMULA] yr-1), a high luminosity (log L/ [FORMULA] 5.5 to 6.5) and typical brightness and spectral variations of 1 to 2 magnitudes in V in about 1 to 10 years (Humphreys & Davidson 1994). HR Car is known to change its spectral type from B2 I to B9 I (Bateson 1987-1996), which corresponds to a change in [FORMULA] from roughly 18,000 to 10,000 K.

Almost all known LBVs are surrounded by a nebula, varying in shape from nearly circular (Wray 751) to strongly bipolar ([FORMULA] Car). The origin of these nebulae is still uncertain: are they due to giant eruptions (Hutsemékers 1994) or to the high mass loss rate over a long period of time (e.g. García-Segura et al. 1996)? In only two galactic LBVs, P Cyg and [FORMULA] Car, giant eruptions have been observed. From statistical arguments it is estimated that giant eruptions occur with a frequency of about 10-3 to 10-4 yr-1 (Lamers 1987).

The nebula of HR Car was discovered (Hutsemékers & van Drom 1991) by narrow band filter imaging. Clampin et al. (1995) and Nota et al. (1997), report coronographic imaging, long-slit spectroscopy and spectropolarimetry of the HR Car nebula. They conclude that the nebula is of filamentary, bipolar and nearly point-symmetric structure and that the large-scale morphology is in agreement with the asymmetries on a smaller scale as detected by spectropolarimetry. From IRAS photometry McGregor et al. (1988) estimate a dust temperature of 165 K and a dust mass of 2.6 10 [FORMULA] [FORMULA], assuming a distance of 2.5 kpc. This low temperature suggests that the dust is situated far from the central star, but is considerably warmer than that in e.g. AG Car.

In this Letter we present mid-infrared images of HR Car and its nebula, in both the broad N-band and the narrow [Ne II ] 12.8 [FORMULA] m-band. For the first time this allows us to get a view of the surroundings close to the star, which in the optical region cannot be observed due to the relative brightness of the central star (Clampin et al. 1995).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998