COMMISSIONS 27 AND 42 OF THE IAU 
             INFORMATION BULLETIN ON VARIABLE STARS 
                          Number 3768 
 
                                        Konkoly Observatory 
                                        Budapest 
                                        11 September 1992 

                                        HU ISSN 0374 - 0676 
 
 
       Theta^2 Sgr: SERENDIPITOUS DISCOVERY OF VARIABILITY 
                         IN AN Am STAR? 
 
 The bright star HR 7631 was recently found to be a strong X-ray 
source by both the X-ray telescope (XRT) and wide-field camera (WFC) 
aboard the ROSAT X-ray satellite. This activity, along with the large 
rotational velocity of approx. 90 km/s (Dr. R.D. Jeffries^1, private 
communication) suggested that the star may show evidence of 
variability due to photospheric spots, and in July 1992 it was 
included in the observing program at the Monash Observatory. 
 
 Comparison stars chosen were HR 7585 (HD 188158) and HR 7624 (HD 
189118, also known as Theta^2 Sgr), both of which are within 1deg of 
the target star. Preliminary observations on July 16th yielded 28 
V-band measurements of HR 7631, and while reducing these data it was 
immediately noticed that the RMS deviation in comparison star 
differences was very large. The data are shown in Figures 1 (a) and 
(b) as magnitude difference between target and comparison stars. The 
minimal scatter in the points indicates the very high quality of the 
night. It can be seen from Figure 1 (a) that the difference between 
HR 7631 and comparison star 1 remained reasonably constant over the 
seven hours of observations, with an RMS deviation of only 0.007 
magnitude. Assuming then that neither of these stars were varying, 
Figure 1 (b) indicates that the second comparison star (Theta^2 Sgr) 
experienced a steady 0.25 magnitude increase in brightness. In 
addition, a flare-like event from the star was seen at about HJD 
2448820.11. This showed as a rapid increase in brightness of 0.07 
magnitude (corresponding to a luminosity increase in the star of 
about 7%), followed by an approximately exponential decrease over the 
next 25 minutes or so. 
 
 Theta^2 Sgr is not listed in the Bright Star Catalog as a variable 
star, and in fact a literature search showed it to be an Am star 
(Henry & Hesser,1971). Wolf (1983) gives a detailed summary of 
research on peculiar A stars, saying "several extensive searches for 
photometric variability in the Am stars, have yielded, with few 
exceptions, negative results", and concludes that there is no evidence 
for either pulsation or surface inhomogeneities (starspots) for these 
objects. This is clearly at odds with the data shown in Figure 1 (b) 
Future observations will be made as soon as possible to verify these 
findings, although the evidence presented here strongly suggests 
photometric variability - probably due to starspots - and flaring of 
the Am star Theta^2 Sgr. The ROSAT target HR 7631, although listed 
in the Bright Star Catalog as a variable of amplitude 0.07 magnitude, 
showed little change over the duration of these observations. The 
star may simply have been in a quiescent state of photospheric 
activity, and further observations will examine this. 
 
1) Dept. of Physics and Space Research, University of Birmingham 



                             [FIGURE 1]

 Figure 1: Magnitude differences between HR 7631 and comparison stars 
 (a) HR 7578 and (b) HR 7624. Measurements were made in the V filter and 
 span approximately 7 hours.

  
                      G. J. ANDERS 
                      Department of Physics 
                      Monash University 
                      Clayton, Victoria, Australia, 3168 
 
 
References: 
 
Henry R.C. and Hesser J.E.: 1971, Ap. J., 23, 421.  [BIBCODE 1971ApJS...23..421H ]
 
Wolff S.C.:  1983, "The A-stars:  Problems and Perspectives", NASA 
  monograph series on non-thermal phenomena in stellar atmospheres.  [BIBCODE 1983spp..book.....W ]
 

                      [From IBVS 3820]


Erratum:
In I.B.V.S. 3768, first ref., for Ap.J. read Ap.J. Supp.