COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2647 Konkoly Observatory Budapest 17 December 1984 HU ISSN 0374 - 0676 PHOTOELECTRIC MONITORING OF BUTLER'S FLARE STAR Butler's Flare Star (finding chart: Arp (1958), Plate 1, Star 'R') was discovered (Andrews, 1966) at Boyden Observatory during photo-electric observations of a "standard" star in the field of the Small Magellanic Cloud previously observed by Arp (1958), with further monitoring (Andrews et al., 1966) confirming UV Ceti-type flaring activity. This activity was considered as extremely unusual as objective prism spectroscopy gave the spectral type as an early G-type star, later revised (Andrews, 1967) by slit spectroscopy to dK0 with no trace of emission lines in the spectrum. As this classification is earlier than any solar neighbourhood flare star listed by Pettersen (1977), and no report of monitoring since 1966 has been published, it was decided to investigate the variability of Butler's Flare Star to determine the rate and photometric characteristics of any flaring activity. Butler's Flare Star was photoelectrically monitored in the Johnson 'B' band with the Nishimura 0.41 m reflector at Boyden Observatory employing an EMI 6256 photomultiplier tube and pulse-counting electronics. As a compromise between photometric precision and the resolution of any rapid variation, 10 second integrations were used. The dates and times of observing hours are given in Table I. Table I ---------------------------------------------------------------------------- Date Times of observations(SAST) Duration ---------------------------------------------------------------------------- 25/26 October 1984 01.30 - 03.05 1 Hour 35 minutes 30/31 October 1984 21.05 - 23.40 2 Hours 35 minutes 01/02 November 1984 20.40 - 23.00 and 23.20- 02.20 5 Hours 20 minutes 03/04 November 1984 20.35 - 23.25 and 23.40- 02.15 5 Hours 25 minutes 21/22 November 1984 20.30 - 21.30 and 21.50- 01.00 4 Hours 10 minutes 22/23 November 1984 20.30 - 21.30 and 21.50- 01.00 4 Hours 10 minutes Total monitoring time: 23 Hours 15 minutes ---------------------------------------------------------------------------- The typical photometric precision of any 10 second integration was estimated by evaluating the statistical fluctuations of short periods of (sky subtracted) data selected at random from the observing runs; the mean of the standard deviations was found to be 0.015m. Flare activity of more than 0.090m would thus be detectable at a high level of confidence (6 sigma) even if of short duration, quite adequate for UV Ceti-type flares which should typically produce a 'B' band flux increase if several tenths of a magnitude persisting for several hundred seconds. No statistically significant flare activity greater than 0.09m persisting for longer than 10 seconds was observed during the 23 hours of monitoring. Smaller amplitude flare activity cannot be completely discounted due to the photon noise statistics, but these are considered unlikely above 0.06m (4 sigma) as any flares persisting for longer than 10 seconds should be detected at this lower threshold of activity. Flare activity below 0.06m cannot be discounted by the observations. In addition to monitoring, differential photometry was also obtained on the night of 01/02 November 1984 employing HD 6172 = SAO 225729 as the comparison star. No evidence of variability in excess of 0.03m was detected, with the mean differential 'B' magnitude determined as 2.31m, within 0.01m of the value determined by Arp (1958). The complete absence of flares of the sort reported by Andrews et al. (1966) is disconcerting and suggests that Butler's flare Star has quite long quiescent periods with very low levels of activity. An alternate possibility is that reported flare activity is erroneous, explaining the unusually early spectral type and the lack of emission lines in the spectrum. In order to investigate these possibilities it is intended to resume photoelectric monitoring of Butler's Flare Star after an interval of a few months and if possible to obtain a contemporary spectral classification. G.J. MALCOLM Boyden Observatory Institute of Astronomy University of the Orange Free State Bloemfontein 9300 Republic of South Africa References: Andrews, A.D.(1966) I.A.U. Inform. Bull. Var. Stars, No. 156 Andrews, A.D., Butler, C.J., and Eksteen, J.P. (1966) I.A.U. Inform. Bull. Var. Stars, No. 157 Andrews, A.D. (1967) P.A.S.P. 79, No. 469, 368. [BIBCODE 1967PASP...79..368A ] Arp, H.C. (1958) A.J. 63, 118. [BIBCODE 1958AJ.....63..118A ] Pettersen, B.R. (1977) Inst. of Theor. Astrophysics Blindern-Oslo Report No. 46.