COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1738 Konkoly Observatory Budapest 1980 February 5 NOTES ON THE SPECTRUM OF U SCORPII AND ON ITS POSITION AMONG THE RECURRENT NOVAE A spectrogram (dispersion 132 A mm^1, emulsion IIa-O) of the recurrent nova U Sco during its recent outburst was taken on 1979 June 28.95 (UT) with the Cassegrain spectrograph of the 1.06 m telescope of the Hoher List Observatory. While the blue part of the spectrum is underexposed due to the faintness of the object and the strong atmospheric extinction, some features in the region between H_delta and 5050 A can be studied. The nova had at that time a magnitude of about 11.5, and was nearly 3^m below maximum. Simultaneous high speed photometry has been reported by Warner (1979). Spectroscopic observations on 1979 July 2 and 3 are briefly described by Hill et al. (1979). At the time of our observations, the most prominent feature is a broad blend of emission lines of Fe II, N III, C III, and He II at 4580 - 4690 A, which is much stronger than the Balmer lines H_beta and H_gamma. The tracing of the spectrum with some (often tentative) identifications is shown in Fig. 1. As already suggested by Hill et al. (1979) the spectrum bears some resemblance to that of T CrB 3m below maximum (see, e.g., Herbig and Neubauer 1946). However, it is also similar to that of RS Oph 3m below maximum (see, e.g., Dufay et al. 1964). While the lines of RS Oph are much more diffuse than those of T CrB, the lines of U Sco are by far the most diffuse at this stage, making line identifications very uncertain. Fast recurrent novae show a tendency of decreasing line width with time: T CrB: 570 - 270 kms^-1 in 7 days (Herbig and Neubauer 1946) RS Oph: 1600 - 950 kms^-1 in 10 days (Folkart et al. 1964) U Sco, however, seems to show very broad emission lines also in later stages of the outburst. We derive half-widths of +-2800+-200 kms^-1 (m.e.) 5.5 days after outburst. Hill et al. report 10000 kms^-1 zero-intensity line widths 9 days after outburst. An attempt is made to estimate the interstellar extinction in the direction of U Sco. Unlike ordinary novae, recurrent novae do not show dramatic changes in the B-V index during decline from maximum. T CrB had B-V=0.1 around maximum (Gordon and Kron 1979) and suffers no noticeable reddening. RS Oph had B-V=0.74 (Connelley and Sandage 1958); Svolopoulos (1966) derived (B-V)_0=-0.02. For U Sco, the visual and photographic observations of Narumi and Kuwano (Kosai 1979) indicate m_pg-m_vis=0.1 at maximum, and Whitney (1979) observed B-V=-0.13 2.5 days after maximum. It appears that U Sco is very little reddened. Unfortunately, there is no clear distance - reddening relationship in this region (see, e.g., Neckel's (1967) field No. 194). E_B-V=0.3 should be present if U Sco is not closer than 250 pc, but the extinction does not increase up to a distance of 3 kpc and likely more, since the line of vision at that distance has left the Galactic plane. [FIGURE 1] Fig. 1. The spectrum of U Sco, 1979 June 28.95 (UT) [FIGURE 2] Fig. 2. A complete light curve of the outbursts of U Sco. The 1979 outburst appears to have a more rapid decline Let us assume that U Sco has reached an absolute magnitude at maximum equal to that of T CrB and RS Oph. T CrB has shown some nebular whisps that may have been ejected during the 1946 outburst (Williams 1977). They yield an expansion rate of 0.33" year^-1. Due to the ambiguity of our knowledge of the principal expansion velocity (radial velocities of 4000 ... 1000 km^s-1 have been observed), the absolute magnitude is only coarsely determined: M_V=-8.5+-1.5. Svolopoulos' study of the reddening and distance of RS Oph yields M_V=-8.7. With an apparent maximum brightness of U Sco of m_v=8.7, an estimated A_V of 1m, and an absolute magnitude M_V=-8.5, a distance of the order of 17 kpc is derived. This brings U Sco 6 kpc above the Galactic plane! If we assume that the minimum V magnitude is only due to the late type companion, then M_V=-0.5+-1.5 for T CrB, and -2.5 (variable; Tempesti 1975) for RS Oph indicate that the secondaries are in the range of M giants and semiregular variables. The estimated brightness and colour of U Sco at minimum, as derived from the POSS charts (Webbink 1978) make it more likely that the secondary is an F or G giant with M_V=+1.5, if the distance of 17 kpc is correct. Spectroscopic observations of U Sco at minimum would be highly desirable, since they could settle the question whether all fast recurrent novae have indeed giant secondaries. For the slow recurrent nova T Pyx, the colour at minimum light (Webbink 1978) and the nebular expansion parallax (Duerbeck and Seitter 1979) put severe restrictions on the size of the secondary. It is very likely a dwarf star. The knowledge of the colour of U Sco also allows a construction of the combined light curve for all well-observed outbursts. While the visual light curve of Pogson et al. (1908) and the photographic light curve of Thomas (1940) appear to be very similar, the new observations (Bortle 1979, Kosai and Mattei 1979, Whitney 1979) indicate a more rapid decline. H. W. DUERBECK Observatorium Hoher List der Universitats-Sternwarte Bonn 5568 Daun, F.R. Germany W. C. SEITTER Astronomisches Institut der Universitat Munster 4400 Munster, F.R. Germany References : Bortle, J., 1979, IAU Circ. 3378 Connelley, M., Sandage, A., 1958, Publ. astr. Soc. Pacific 70, 600 [BIBCODE 1958PASP...70..600C ] Duerbeck, H.W., Seitter, W.C., 1979, ESO Messenger 17, 1 [BIBCODE 1979Msngr..17....1D ] Dufay, J., Bloch, M., Bertaut, C., Dufay, M., 1964, Ann. Astrophys. 27, 555 [BIBCODE 1964AnAp...27..555D ] Folkart, B., Pecker, J.C., Pottasch, S.R., 1964, Ann. Astrophys. 27, 249 [BIBCODE 1964AnAp...27..249F ] Gordon, K.C., Kron, G.E., 1979, Inf. Bull. var. Stars 1610 Herbig, G.H., Neubauer, F.J., 1946, Publ. astr. Soc. Pacific 58, 196 [BIBCODE 1946PASP...58..196H ] Hill, P.W., Pringle, J.E., Whelan, J.A.J., 1979, IAU Circ. 3378 [BIBCODE 1979IAUC.3378....3H ] Kosai, Y., Mattei, J., 1979, IAU Circ. 3373 [BIBCODE 1979IAUC.3373....2K ] Pogson, N.R., Brook, C.L., Turner, H.H., 1908, Mem. R. astr. Soc. 58, 90 Svolopoulos, S.N., 1966, Publ. astr. Soc. Pacific 78, 159 [BIBCODE 1966PASP...78..157S ] Tempesti, P., 1975, Inf. Bull. var. Stars 974 Thomas, H.L., 1940, Harvard Bull. 912 Warner, B., 1979, Inf. Bull. var. Stars 1707 Webbink, R.F., Publ. astr. Soc. Pacific 90, 57 [BIBCODE 1978PASP...90...57W ] Whitney, C.A., 1979, IAU Circ. 3373 Williams, R.E., 1977, IAU Coll. 42 "The Interaction of Variable Stars with Their Environment", Veroff. Remeis-Sternw. Bamberg 9, No. 121