COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2607 Konkoly Observatory Budapest 18 October 1984 HU ISSN 0374 - 0676 A SEARCH FOR LOW-HARMONIC PULSATION IN THE Ap STAR HD 184 905 The Ap star HD 184905 (SAO 48604, m_v=6.48, sp=A0p SiCrSrEu) was reported by Panov (1981) to show short-period light variations with a period of about 25-30 minutes and an amplitude of about +-0.01m in U,B,V on several nights. Consequently, this star has been included in an observing program by the author to investigate the presence of short-period variations in Ap stars over long periods of time. Differential photometry was obtained using HD 184875 (HR 7444, SAO 48601, m_v=5.35, sp=A2V) as a comparison star on the Lowell 1.1-m telescope. A dual-channel photometer utilizing cooled S-11 tubes and B and V filters was employed. Alternating sets of three 10-second integrations were obtained on HD 184905 and HD 184875. The observations are summarized in Table I. The differential magnitudes, normalized to zero, are plotted in Fig. 1. Table I, Journal of observations of HD 184905 (V) and HD 184875 (C). Date 1984 t (hours) sigmaB sigmaV HJD (at start) Deltam(V-C) (at start) 2,445,900+ B V -------------------------------------------------------------------------------------- Aug 30/31 0.52 0.m0021 0.m0037 43.7074 1.195 1.288 Sep 27/28 2.86 0. 0014 0. 0024 71.6662 1.205 1.300 Sep 28/29 2.93 0. 0014 0. 0024 72.6659 1.180 1.266 -------------------------------------------------------------------------------------- [FIGURE 1] [FIGURE 1a] [FIGURE 1b] Figure 1. The differential data (HD 184905 - HD 184875), normalized to Deltam=0.0. Each panel is one hour wide; the time series progresses left-to-right and top-down. The height of each vertical division is 0.01m. No evidence is present for low-harmonic pulsation in the period range 25-30 minutes nor in the valid range of periods being investigated (approx. 6 to 90 minutes), as demonstrated in Fig. 2. The maximum semi-amplitude is 0.56 mmag in B and 1.02 mmag in V at the period -12 minutes; the average power lies at about half of these values. While there is a peak in the power spectra at the frequency corresponding to a period of about 12 minutes, it is probably not significant. Rapid oscillations leading to such a period would yield a larger amplitude in B than in V. which is not the case with these data. Consequently, we are most likely dealing with an artifact. A much higher sampling rate (e.g., 20 seconds) would be required to accurately search for such short periods, since the mean sampling rate here is about 2.5 to 3 minutes. [FIGURE 2] Figure 2. Power spectra for the B and V data presented in Fig. 1. Investigations by the author have failed to detect any of the reported rapid light variations in the stars HD 10088 (Kreidl 1984a, 1984c), HD 32633 (Kreidl 1984b), HD 125248 (Kreidl 1984d), and HD 184905 (this paper). As in the other cases, HD 184905 appears to be too hot to lie in the instability strip. TOBIAS J. KREIDL Lowell Observatory P.O. Box 1269 Flagstaff, AZ 86002 References: Panov, K. (1981) Comm. Spec. Astrophys. Obs., No. 32, 73. [BIBCODE 1981SoSAO..32...73P ] Kreidl, T.J. (1984a) Inf. Bull. Var. Stars, No. 2460. Kreidl, T.J. (1984b) Inf. Bull. Var. Stars, No. 2472. Kreidl, T.J. (1984c) Inf. Bull. Var. Stars, No. 2602. Kreidl, T.J. (1984d) Mon. Not. R. astr. Soc., (in press). [BIBCODE 1985MNRAS.212..337K ]