COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3347 Konkoly Observatory Budapest 3 July 1989 HU ISSN 0374 - 0676 IS THE MK STANDARD ETA UMA AN INCIPIENT Be STAR? Eta UMa (HR 5191, HD 120315) is a well-studied B3 V star with measured v sin i of 226 km/s (Bernacca and Perinotto 1971). The star is suspected to be a low amplitude photometric variable, and is listed as number 6450 in the New Catalog of Suspected Variable Stars. Eta UMa is frequently used as a standard star for spectral classification. Morgan and Keenan (1973) establish eta UMa as an "MK dagger type", a star whose spectrum has been carefully examined and is used to define the MK system. Over the past four years we have obtained a number of observations of the Halpha region of eta UMa using the echelle spectrograph and Reticon or CCD detectors at Ritter Observatory and Kitt Peak National Observatory. These spectral observations had resolving power of 20,000 or 10,000, and the signal to noise ratio was generally 50-100:1. A log of these observations is given in Table 1. Our initial observations of eta UMa were obtained to test our detector and acquire a series of observations of MK standards, and the first observations from April and May of 1986 showed the expected symmetric, rotationally broadened profile (Figure 1). However, observations the following year showed an H_alpha profile that appeared to be slightly distorted and asymmetric (Figure 2). Though some of these observations were complicated by the presence [FIGURE 1] Figure 1: The Halpha profile of eta UMa in May 1986, obtained with an intensified Reticon detector at Ritter Observatory. [FIGURE 2] Figure 2: The Halpha profile of eta UMa in May 1987, showing a distorted and shallower line as compared with a year earlier. of occasionally strong telluric water vapor lines, the asymmetry was present in scans from both Ritter and KPNO, bolstering our confidence that the effect was real. Additionally, the central residual intensity of the H_alpha line, [FIGURE 3] Figure 3: The Halpha profile in May 1989, obtained with a CCD detector at Ritter Observatory. The line shows a shallow, flat-bottom profile and is presumably partly filled by emission. R(H_alpha), (Table 1) showed a slight increase from 1986 to 1987, as if the line were becoming slightly filled in by emission. This asymmetric profile persisted throughout 1988. Our most recent observation from May 1989 was obtained with our new CCD detector system at Ritter (Bopp et al. 1989) and showed an H_alpha line with a flat bottom (Figure 3), quite unlike the profile seen two years before. The residual intensity has increased again, suggesting a greater filling of the line by emission. The profile variations that we have observed in the H_alpha line of eta UMa are not unexpected if the star has weak and/or transient Be characteristics. Certainly the temperature and v sin i of eta UMa make it a credible candidate for such activity. The behavior of eta UMa may be reminiscent of that of Zeta Oph, an "ordinary" O9.5 star until 1973, when strong H_alpha emission unexpectedly appeared. Perhaps eta UMa will surprise observers as well, and perhaps some small caution is appropriate before using it as a standard star. Table 1 H_alpha Observations of eta UMa Date UT Observatory/Detector R(H_alpha)^1 27 April 1986 Ritter/Reticon 0.628 3 May 1986 " 0.638 4 May 1986 " 0.640 3 June 1986 " 0.643 7 May 1987 " 0.663 23 May 1987 " 0.666 28 May 1987 KPNO/CCD 0.650 26 May 1988 Ritter/Reticon 0.646 8 June 1988 " 0.652 17 May 1989 Ritter/CCD 0.703 ^1 Defined as the ratio of the central intensity of Halpha to the continuum. B. W. BOPP J. R. CAPLINGER Ritter Observatory and N. A. CROSBY Dept. Of Physics and Astronomy D. F. MAHONEY University of Toledo S. P. O'BRIEN Toledo, OH 43606, USA R. C. DEMPSEY References: Bernacca, P. L., and Perinotto, M. 1971, Contr. Oss. Astro. Univ. Padova in Asiago, No. 250. [BIBCODE 1971CoAsi.250....1B ] Bopp, B. W., Dempsey, R. C., Morrison, N. D., Burmeister, R. J., Jones, R. A. 1989, Bull. Amer. Astron. Soc., 21, 759. [BIBCODE 1989BAAS...21R.759B ] Morgan, W. W., and Keenan, P. 1973, Ann. Rev. Astron. Astrophys., 11, 29. [BIBCODE 1973ARA&A..11...29M ]