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Astron. Astrophys. 347, L43-L46 (1999)

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1. Introduction

Recurrent novae (RN) are a small and diverse subclass of cataclysmic variables, which show multiple outbursts resembling those of classical novae, though of lesser magnitude (see Webbink et al. 1987; Sekiguchi 1995). U Scorpii (U Sco) is one of the six best known members of this class. The source underwent outbursts in 1863, 1906, 1936, 1979, 1987, and most recently on 1999 Feb 25.2 (Schmeer et al. 1999). The last two outbursts were separated by 8 and 12 years respectively. It is the RN with the shortest recurrence period known.

Starrfield et al. (1988) applied thermonuclear runaway (TR) theory to this nova assuming a very massive white dwarf (WD).

The estimate of the distance to U Sco in the literature varies with different assumptions. Kato (1990) obtained a distance range 3.3-8.6 kpc comparing the observed visual light curve with the theoretical one and assuming a high mass ([FORMULA]) WD. If the donor star is a dwarf the distance can be [FORMULA] (Hanes 1985), however most authors recently agree instead on a subgiant nature of the donor, as indicated by the detection of a Mg Ib absorption feature at [FORMULA]5180 in the late 1979 outburst spectrum, consistent with a K2 III spectral type (Pritchet et al. 1977). This is in agreement with a low mass ([FORMULA] 1 [FORMULA]) subgiant secondary with MV=+3.8 which fills the Roche lobe at an orbital period [FORMULA]1.2 days (Portegies Zwart, private communication). From the apparent magnitude V=20.0 in the faint state and the visual extinction AV=0.6 a distance [FORMULA]13 kpc is derived, in rough agreement with d=14.8 kpc derived by Webbink et al. (1987) with the assumption of a G III subgiant, and with d[FORMULA]14 kpc estimated by Warner (1995). In any case U Sco is at a latitude 21o and for any distance d[FORMULA] 3 kpc it belongs to the galactic halo population. It is seen through the full galactic hydrogen column of [FORMULA] (Dickey & Lockman 1990).

U Sco was observed to be an eclipsing system by Schaefer (1990). The orbital period is 1.23 days (Johnston & Kulkarni 1992; Schaefer & Ringwald 1995). [FORMULA] varies from 18.5 to 20. An accretion disk is required from the modeling of the optical continuum during quiescence. The maximum visual magnitude during outburst is [FORMULA]. U Sco shows the fastest visual decline of 0.67 magnitude per day of all known novae (Sekiguchi et al. 1988). Spectroscopically it shows very high ejection velocities of [FORMULA] (Williams et al. 1981; Rosino & Iijima 1988; Niedzielski et al. 1999).

Ejecta abundances have been estimated from optical and UV studies (Williams et al. 1981; Barlow et al. 1981). From the emission lines a depletion in hydrogen relative to helium with He/H[FORMULA]2 has been derived while the CNO abundance was solar with an enhanced N/C ratio. The strongest emission feature at maximum is the HeII [FORMULA] line. Other reported lines are H[FORMULA], HeI , HeII , NII , NIII , CIII , and CIV (Zwitter et al. 1999; Bonifacio et al. 1999). Satellite lines to H[FORMULA], HeII and HeI have also been detected (Bonifacio et al. 1999). The estimated mass of the ejected shell for the 1979 outburst is [FORMULA] (Williams et al. 1981).

It has been suggested that the companion of U Sco may be somewhat evolved (and helium enriched) as the quiescent spectrum shows strong HeII emission lines (cf. Hanes 1985). Hachisu et al. (1999) propose an evolutionary scenario for this system assuming a secondary star which experienced a helium accretion phase. The WD may efficiently grow in mass towards the Chandrasekhar (CH) limit and explode as a SN Ia (Della Valle & Livio 1996). But if U Sco is in the galactic halo then the system belongs to an old stellar population and the evolution may be different. Helium enrichment as observed from U Sco may also be due to helium enriched winds from the WD (cf. Prialnik & Livio, 1995).

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999