COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 4070 Konkoly Observatory Budapest 16 August 1994 HU ISSN 0374 - 0676 HD 183324, A PULSATING LAMBDA Boo STAR? Lambda Bootis stars are a class of chemically peculiar A-type (CP2) stars. They have a typical Pop I kinematics, but their metallicity is up to a factor of 100 too low for this type of population. HD 183324 (m_V=5.8, HR 7400, BD-1d4010) was selected as a candidate for a southern hemisphere survey for variability among lambda Boo stars. The spectral classification by Gray and Corbally (1993) for this object is A0 Vb lambda Boo. Our observations were obtained in May 1994 at SAAO with the 0.5m telescope and a single channel photometer. A Stromgren v filter, and a 10 s integration time was chosen. Two comparison stars were used: HD 180482 (C1, m_V=5.6, HR 7303, BD-4d4045, A3IV), and HD 178596 (C2, m_V=5.2, HR 7266, F0III). More information on the observations is listed in Table 1. Table 1: Observing log. night hours number of observations May '94 HD183324 HD180482 HD178596 1/2 2.99 410 47 53 2/3 3.66 1073 57 56 3/4 3.76 1205 41 - 4/5 3.54 1169 25 - 5/6 1.55 478 28 - 7/8 4.13 1360 45 - 9/10 4.32 1098 94 57 total 23.95 6802 307 166 Figure 1 shows the residuals to the mean instrumental magnitudes for the night of May 9/10. The light curve of both comparison stars is basically constant, only smooth sky transparency variations are visible. The parabolic trend in the light curve of C2 comes from a slightly incorrect extinction coefficient, which is inadequate for the late spectral type of C2 (F0) - compared to C1 (A3). The measurements of HD 183324, however, give clear evidence for amplitude modulated variability. After subtraction of long term trends caused by sky transparency variations we computed an amplitude spectrum for all merged data of the program star (Figure 2). The maximum semi- amplitude for HD 183324 with 2.08 mmag(v) appears at the frequency f=47.371/d (30.39min). The bulk of frequencies around this highest peak is broader than a single period would produce according to the spectral window (inset in Figure 2). This property may be taken as evidence for at least one other frequency in this lambda Boo star. Furthermore, all light curves show a significant amplitude modulation in each night (Figure 3). [FIGURE 1] Figure 1: Instrumental Stromgren v data for HD 183324 and both comparison stars [FIGURE 2] Figure 2: Amplitude spectrum (v) for HD 183324 [FIGURE 3] Figure 3: Light curves of HD 183324 Because of the period of about 30 minutes, the shortest period ever found in this group of peculiar stars, HD 183324 is an outstanding lambda Boo star. Variations in this frequency domain come already close to the well known rapid oscillations in some cool Ap (roAp) stars. Our discovery increases the number of known variables in the group of lambda Boo stars to 10. It is the fourth variable we found in our survey which started at ESO in 1993. The suspected presence of multi-mode pulsation similar to delta Sct stars definitely requires the organisation of photometric multi-site observing campaigns in order to derive reliable pulsation frequency spectra (Weiss et al. 1994). Acknowledgement: This research was supported partly by the Osterreichische Akademie der Wissenschaften (project Asteroseismologie), and by the Hochschuljubilaumsstiftung der Stadt Wien. Rainer KUSCHNIG Ernst PAUNZEN Werner W. WEISS Institut fur Astronomie Turkenschanzstr. 17 1180 Wien Austria e-mail:familyname@astro.ast.univie.ac.at References: Gray R.O., Corbally C.J., 1993, AJ 106, 632 [BIBCODE 1993AJ....106..632G ] Weiss W.W., Paunzen E., Kuschnig R., Schneider H., 1994, A&A, 281, 797 [BIBCODE 1994A&A...281..797W ]