Astron. Astrophys. 356, 585-589 (2000)

1. Introduction

SN 1006 was the brightest supernova witnessed in recorded history. The estimated peak magnitude (V-9.51, Clark & Stephenson 1977), reported visibility for nearly two years, and the lack of a nearby OB association strongly suggests a Type Ia origin (SNIa, Minkowski 1966). Almost all current models of Type Ia supernova involve the nuclear explosion of a white dwarf induced by rapid mass accretion in a binary system. However, no stellar remnant from this supernova explosion has ever been conclusively identified, including a pulsar, or the remains of any companion star.

In 1980, Schweizer & Middleditch searched for just such a stellar remnant from SN 1006 and discovered a faint (V16.7) blue star 2.5' from the projected centre of the supernova remnant (SNR). They identified this object (now known as the Schweizer-Middleditch star, SM star or SM80) as a hot subdwarf sdOB star, and estimated its effective temperature 38,5004500K, and surface gravity log g6.70.6. From an estimate of the absolute magnitude, M_v6.21.8, Schweizer & Middleditch (1980) derived a distance to their subdwarf of 1.1 (+1.4, -0.6) kpc. Since chance projection seemed unlikely, and the distance estimate was in rough agreement with the then exisiting estimates of the distance to the SNR itself, Schweizer & Middleditch (1980) suggested that their subdwarf may in fact be the remnant star, or at least associated with it.

Savedoff & Van Horn (1982) later showed conclusively that the SM star could not be the remnant of the supernova itself, since the time to cool to the observed effective temperature was simply too long, 10⁶ years compared to the SNR age of 10³ years. However, this does not rule out the SM star as a stellar remnant of the donor star in a pre-SNIa interacting binary system.

Subsequent far ultra-violet (far-UV) observations with IUE and HST/FOS revealed the presence of strong Fe II and Si II, III and IV lines superimposed on the continuum of the SM star (Wu et al. 1983, Fesen et al. 1988, Wu et al. 1993). The iron lines have symmetrical velocity profiles, broadened up to 8000 km s^-1 FWHM. The Si features are asymmetric, redshifted and centred at a radial velocity of 5000 km s^-1. These features have been used to estimate the mass of iron in the remnant and to map the positions of various shock regions (e.g. Wu et al. 1997, Hamilton et al. 1997). Importantly, though, the presence of redshifted lines in the supernova ejecta suggests that the SM star must lie behind the SNR, since they are assumed to originate in material moving away from us on the far side of the remnant.

Measurements of the widths of these aborption lines, coupled with the angular size of the remnant, led Wu et al. (1993) to derive a lower limit to the SNR distance of 1.9 kpc. This contrasts strongly with the estimate of Willingale et al. (1995) of 0.70.1 kpc, derived from modelling X-ray emission detected in ROSAT PSPC observations. Therefore, we were motivated to re-observe and re-analyse the SM star in order to place tighter constraints on its distance, and hence on the distance to the SNR itself. Secondly, we learnt of the study by Wellstein et al. (1999) which suggests that the prior donor star in an SN Ia progenitor system (an interacting binary) may appear subsequently as a low mass hot subdwarf star. This new theoretical result re-opens the question first posed by Schweizer & Middleditch (1980) in the conclusion to their discovery paper: "Can one component of a binary system that forms a Type Ia supernova end up being a hot subdwarf or white dwarf ?". In the light of Wellstein et al.'s recent work, we re-address this question.

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
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