Draco C-1 (Munari)

(vsnet-chat 60)

I-band image taken at Ouda Station

To the best of my knowledge, there are several SSXS (Super Soft X-ray Sources) in the northern emisphere, not only that one mentioned by T.Kato. One that could be be an excellent target for CCD-equipped amateurs is Draco C-1, a symbiotic star in the Draco dwarf galaxy. It shines at V=17 and I=15. The donor star to the accreting white dwarf is a carbon bright giant, at the very tip of the AGB (to have a carbon giant in a dwarf galaxy means that star formation was still active not much long ago). Is the Carbon star in Draco C-1 pulsating ? If yes, the mass transfert to the WD companion would be larger. And if yes, what is the pulsating period ? The carbon stars in the Draco dwarfs seems unsually blue, and this could trigger quite short pulsation periods (lass than 100 days).

          Ulisse Munari
          Padova-Asiago Astronomical Observatories

Draco C-1 and the SSXS (Munari)

(vsnet-chat 81)

A few additional words about Draco C-1 for those interested to have a look at it:

        RA (2000.) =  17 19 57.5 
        DEC        = +57 50 05

An I-band excellent finding chart is published by Aaronson et al. (1982) ApJ 254, 507. Inside 4 arcmin from C-1 there are also C-2 and C-3, the only other known carbon stars is the Draco dwarf galaxy (satellite of our own Galaxy). They all shine around V=17 and I=15 mag. The ROSAT observations on the Super-Soft X-ray Source nature of Draco C-1 have been discussed at the June 1996 Colloquium on Symbiotic Stars (the proceedings due to appear in press in these weeks).

The ultraviolet properties as inferred from IUE observations have been discussed by Munari & Buson (1994, Astron.Astrophys. 287, 87). The WD in C-1 is so hot and luminous that no better explanation exists that it is stably burning the accreted material from the carbon giant companion. A bright outburst (no one so far recorded) could onset any moment. From application of standard mass-luminosity relations it appears that the WD in Draco C-1 is closing on the Chadrasekhar mass, opening bright prospects for a SN Ia display. Because the distance module and reddening of the Draco galaxy is

                         m - M = 19.6         E(B-V)= 0.03       mag

a SN Ia exploding there could rival in apparent magnitude with Vega !

A last word about finding charts of ROSAT sources. Apart from those in the Grainer et al. (ed) proceedings, finding charts for 384 and 169 respectively extreme ultraviolet sources (EUV) observed by ROSAT can be found in:

    Shara et al. (1993) PASP 105, 387

    Mason et al. (1995) MNRAS 274, 1194


              Ulisse Munari
              Padova-Asiago Astronomical Observatories