COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3353 Konkoly Observatory Budapest 12 July 1989 HU ISSN 0374 - 0676 THE PHOTOMETRIC VARIABILITY (?) OF PHI CASSIOPEIAE Phi Cassiopeiae (HR 382, HD 7927, F0Ia, V=5.00) is a bright, luminous supergiant with a long history of suspected velocity variability. Adams et al. (1924) reported a total scatter of 10 km/s based on 13 spectra. Abt (1970) lists 20 velocities obtained at Mount Wilson in late 1929 which suggest that this star varies in velocity by 10 km/s on a time scale of about 40 days. There are, however, some large changes in velocity from night to night, which cast some doubt on this result. Abt (1957) himself observed a monotonic increase of 4 km/s in the velocity of this star over 10 nights, which suggests a time scale of a month or two. Arellano Ferro et al. (1988) recently reported variations in velocity of 6 km/s. There was no indication of a strict period longer than 15 days, though irregular variability on a longer time scale was not ruled out. No detailed study of the photometric variability of this star has been carried out. It is not listed as a confirmed or suspected variable in the Yale Catalogue of Bright Stars. The 14 photometric observations by Moffett and Barnes (1979) show no unusual scatter. Arellano Ferro et al. (1988) obtained multi-colour photometry over 20 days, and found scatter of up to 0.06m, but they found similar scatter in the comparison stars. Rosenzweig (1987, 1988) has carried out a detailed study of the energy distribution of this star, and has deduced the following parameters: T_e=7200+-100 K; log g=0.4+-0.1; R=263+-34 R_sun. These parameters imply a mass of 6.3+-3.6 m_sun and a bolometric magnitude of -8.3+-0.3, which is consistent with the value derived by assuming that the star is a member of NGC 457. This membership, however, is in some doubt (Sowell, 1987). The part of the H-R diagram occupied by phi Cas is characterized by average peak-to-peak photometric variations of 0.05m and quasi-periods of 50 to 100 days (Maeder, 1980; Lovy et al., 1984). We have therefore been making sporadic photometric observations of phi Cas for many years, at Toronto (Percy and Welch, 1981) and Kitt Peak. Inspired by the report of Arellano Ferro et al. (1988), we used the Automatic Photoelectric Telescope (APT) Service (Genet et al., 1987) to make intensive UBV observations of this star for several months in 1988-89. The sporadic observations are listed in Table I, which includes the Toronto observations (Percy and Welch, 1981) for completeness. Observations were corrected for extinction and transformation in the usual way. In particular, the procedures used by the APT Service are described in detail by Genet et al. (1987). The comparison and check star were HR 326 (HD 6676, B8V, V=5.79) and HR 442 (HD 9408, G9III, V=4.71) respectively; these were the comparison stars used by Arellano Ferro et al. (1988). Table I. Photometric Observations of Phi Cassiopeiae Julian Date Magnitude V/y Observatory 2440000 + 4088.734 4.994 V Toronto 4094.804 5.022 V Toronto 4101.734 5.006 V Toronto 4116.731 5.014 V Toronto 4796.828 5.009 V Kitt Peak 4918.733 4.982 y Kitt Peak 4919.653 4.986 y Kitt Peak 4920.863 4.989 y Kitt Peak 4921.650 4.989 y Kitt Peak 4924.640 4.985 y Kitt Peak 4925.639 4.991 y Kitt Peak 5715.667 5.001 V Kitt Peak 5716.583 5.019 V Kitt Peak The APT observations are shown in Figure 1. There is no evidence for any significant variability on a time scale of a few days, either in Table I or Figure 1. Some of the possible variability in Table I is due to the fact that some observations were made through a V filter and others through a (Stromgren) y filter. There are, however, apparent variations in Figure 1 on a time scale of 50 to greater than 100 days, with amplitudes in V, B and U of 0.015m, 0.020m and 0.030m respectively. It is not possible to be specific about the time scale because the variations are small and irregular, and the time span of the observations is only a hundred days. Some of the variability could be instrumental in nature. The differential magnitude of the check star shows no long-term variations larger than 0.005m in V and 0.010m in B. There is, however, a systematic variation of 0.030m in U, which is similar to that seen in phi Cas. Expressed differently: if the [FIGURE 1] Figure 1. Photometric observations of Phi Cassiopeiae obtained in UBV with the Automatic Photoelectric Telescope Service telescope on Mt. Hopkins in Arizona. The comparison star is HR 326 (HD 6676, B8V, V=5.79). The variations in V and B are probably real, but the variations in U are probably instrumental. magnitude of the variable is expressed relative to the check star (which is more similar in colour), then the small variations in V and B persist, but those in U are lost in the noise. Whether or not the variations are real, it is remarkable that they are so small. Other stars as luminous as phi Cas show more pronounced variations. Furthermore: if phi Cas were re a Cepheid-like variable, then one might expect the velocity and light amplitudes to be related by: 2K=54 DeltaV=35 DeltaB km/s (Allen, 1973; Fernie, private communication). A velocity amplitude of 6 km/s (Arellano Ferro et al., 1988) would correspond to light amplitudes of DeltaV=0.11m and DeltaB=0.17m, which are much larger than those observed. One must conclude that, either the velocity variations are not real, or phi Cas is not a Cepheid-like variable. Indeed, velocity variations have been found in other supergiants, such as Deneb, and ascribed to non-radial pulsation. Any future photometric monitoring of phi Cas should be done carefully, on a long time scale, using comparison stars more suitable than the ones used in this paper. Acknowledgements: This work was supported by the Natural Sciences and Engineering Research Council of Canada, and by Erindale College, University of Toronto. I thank the staff of the APT for their "user-friendly" service. JOHN R. PERCY Department of Astronomy University of Toronto Toronto, Ontario Canada M5S 1A1 References: Abt, H. A. 1957, Astrophys. J. 126, 138. [BIBCODE 1957ApJ...126..138A ] Abt, H. A. 1970, Astrophys. J. Suppl. 19, 387. [BIBCODE 1970ApJS...19..387A ] Adams, W. S., Joy, A. H. and Sanford, R. F. 1924, Publ. Astron. Soc. Pacific 36, 137. [BIBCODE 1924PASP...36..137A ] Allen, C. W. 1973, "Astrophysical Quantities: Third Edition", U. of London Press. [BIBCODE 1973asqu.book.....A ] Arellano Ferro, A., Parrao, L. and Giridhar, S. 1988, Publ. Astron. Soc. Pacific 100, 993. [BIBCODE 1988PASP..100..993A ] Genet, R. M., Boyd, L. J., Kissell, K. E., Crawford, D. L., Hall, D. S., Hayes, D. S. and Baliunas, S. L. 1987, Publ. Astron. Soc. Pacific 99, 660. [BIBCODE 1987PASP...99..660G ] Lovy, D., Maeder, A., Noels, A. and Gabriel, M. 1984, Astron. Astrophys. 133, 307. [BIBCODE 1984A&A...133..307L ] Maeder A. 1980, Astron. Astrophys. 90, 311. [BIBCODE 1980A&A....90..311M ] Moffett, T. and Barnes, T. G. 1979, Publ. Astron. Soc. Pacific 91, 180. [BIBCODE 1979PASP...91..180M ] Percy, J. R. and Welch, D. L. 1981, Publ. Astron. Soc. Pacific 93, 367. [BIBCODE 1981PASP...93..367P ] Rosenzweig, P. 1987, PhD Thesis, University of Toledo, U.S.A. [BIBCODE 1987PhDT.........5R ] Rosenzweig, P. 1988, Publ. Astron. Soc. Pacific 100, 1025. [BIBCODE 1988PASP..100.1025R ] Sowell, J. R. 1987, Astrophys. J. Suppl. 64, 241. [BIBCODE 1987ApJS...64..241S ]