COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1580


                                 Konkoly Observatory
                                 Budapest
                                 1979 April 2



            TWO IUE SPECTRA OF RY SCUTI


     RY Sct (HD 169515,BD-12d5045) has been studied several times
but the spectroscopic evidence for this large mass system has been most
recently summarized by Cowley and Hutchings (1976).
      O'Connell (1949) has described a complex light curve derived from
photographic estimates. Woodsworth and Hughes (1977) observed an apparently
thermal radio spectrum for the binary; Geisel (1970) a (K-N) index greater
than 0.5 mag; and Allen and Swings (1976) an (H-K)-excess of 0.40 mag.
      Under program PG2SS, originated by S. Sobieski, RY Sct was
observed in the low dispersion, large aperture mode with the spectrograph 
aboard IUE. The instrument is adequately described in the 5 October,
1978 issue of Nature. For a variety of reasons only two spectra were
obtained but, because of the intrinsic interest of the binary, they are
briefly described here. The circumstances of the spectra appear in
Table I; the ephemeris by Cowley and Hutchings was used to calculate



                        Table I
          Low Dispersion Spectra of RY Sct
---------------------------------------------------------------
Image     Hel.J.D.      Phase Exposure        Remarks
---------------------------------------------------------------
LWR1493   2443643.229   0.004  45 min.   Weakly exposed for
                                         lambda<2400A and saturated
                                         for 2750A<lambda<2950a.
SWP1543   2443643.258   0.005  19 min.   Very weak exposure
---------------------------------------------------------------


phase. The counts from the Fine Error Sensor on board the spacecraft
give V = +9.59 and +9.68 for the LWR and SWP images, respectively. Comparison 
with the V magnitudes by Hilditch and Hill (1975) shows that
RY Sct was deeply in its primary minimum for both exposures. For both
LWR1493 and SWP1543, the weak exposures are partly due to the large 
reddening while the interval of saturation on LWR1493 is due to the change
in slope of the reddening law and to the dependence of the cathode
sensitivity upon wavelength. In fact, for RY Sct it is impossible to
obtain a properly exposed spectrum with a unique exposure over
1900A < lambda < 3200A.
      Stromgren indices for RY Sct have been observed by Hilditch
and Hill. Their c_1-index shows considerable intrinsic scatter and the
(b-y) reddening was derived from the (b-v, u-b) plane calibrated by
Crawford (1975) and Crawford and Mandwewala (1976). The (b-y) reddening
was transformed to E(B-V) = 1.29 through the relation given by Crawford
and Mandwewala, the interstellar extinction law due to Jamar, et al.
(1976) was assumed, and the cathode spectral responses were taken from
IUE Newsletter No. 2. The dereddened spectra were faced against model
atmospheres by Carbon and Gingerich (1969) and Auer and Mihalas (1972).
In general, an atmosphere with T_e = 40,000K, 3.5 < log g < 5 describes
the spectrum over the interval 1350A < lambda < 3300A within the precision
of the observations. A temperature less than 35000K will not represent
the observations adequately, and RY Sct would, therefore, appear to be
an 06 to 07 object by the temperature scale of Conti (1973). Such a
classification is consistent with the dereddened Stromgren indices.
     The temperature and inferred spectral type could be in error
if a substantial part of the reddening is circumstellar, rather than
interstellar. The small (H-K)-index noted by Allen and Swings, however,
favors an interpretation based on free-free emission and not on thermal
emission due to dust. Therefore, a difficulty remains: the IUE spectra
were taken during the eclipse of the less massive, and presumably hotter,
component, and it is remarkable that a temperature as hot as 40,000K
would be found for an object usually classified as B0.
     Although SWP1543 is a very weak exposure, it appears that the
C IV lambda1550 feature shows a P Cyg-type profile. It is also possible
that shell features are present for Si IV lambdalambda1394,1402, OV lambda1371, and
N IV lambda1719. However, N IV lambdalambda1239, 1243 and O IV 
lambdalambda1338, 1343 could
not be detected. Except for lambdalambda2307, 2733 which are obliterated by
cathode reseaux, the 3-5 through 3-13 lines. of He II may be weakly
present in absorption. Other weak absorptions and emissions are also
suspected, but the spectra are not really suitable for detailed study.
     Obviously more spectra for RY Sct would be useful but no
more observing time remains for PG2SS. For other interested observers,
exposure times with the large aperture in the low dispersion mode at
the same orbital phase would be of the order of 100 minutes for an SWP
image, 25 minutes for 2750A < lambda < 2950A, and 50 minutes for an LWR
image to either side of the wavelength interval just noted.
     The assistance by the IUE team is appreciated. This research
was supported by NASA grant NSG-5296 which is gratefully acknowledged.



                          ROBERT H. KOCH
                          Department of Astronomy
                          University of Pennsylvania
                          Philadelphia, PA 19104
                          U.S.A.



References:

Allen, D.A., and Swings, J.P. 1976, A. Ap. 47, 293. [BIBCODE 1976A&A....47..293A ]
Auer, L.H., and Mihalas, D. 1972, Ap. J. Supp. 24, 193. [BIBCODE 1972ApJS...24..193A ]
Carbon, D.F., and Gingerich, O. 1969, in Proc. Third Harvard-Smithsonian
     Conference on Stellar Atmospheres, O. Gingerich, ed.
     (Cambridge: M.I.T. Press), p. 377. [BIBCODE 1969tons.conf..377C ]
Conti, P.S. 1973, Ap. J. 179, 181. [BIBCODE 1973ApJ...179..181C ]
Cowley, A.P., and Hutchings, J.B. 1976, Pub. A. S. P. 88, 456. [BIBCODE 1976PASP...88..456C ]
Crawford, D.L. 1975, Pub. A. S. P. 87, 481. [BIBCODE 1975PASP...87..481C ]
Crawford, D.L., and Mandwewala, R. 1976, Pub. A. S. P. 88, 917. [BIBCODE 1976PASP...88..917C ]
Geisel, S.L. 1970, Ap. J. Lett. 161, L105. [BIBCODE 1970ApJ...161L.105G ]
Hilditch, R.W., and Hill, G. 1975, Mem. R. A. S. 79, 101. [BIBCODE 1975MmRAS..79..101H ]
Jamar, C., Macau-Hercot, D., Monfils, A., Thompson, G.I., Houziaux, L. and
      Wilson, R. 1976, Ultraviolet Bright Star Spectrophotometric Catalogue. [BIBCODE 1976ubss.book.....J ]
O'Connell, D.J.K. 1949, A. J. 54, 134. [BIBCODE 1949AJ.....54..134O ]
Woodsworth, A.W., and Hughes, V.A. 1977, A. Ap. 58, 105. [BIBCODE 1977A&A....58..105W ]