W. A. Lawson, P. L. Cottrell*, A. C. Gilmore & P. Μ. Kilmartin Mount John University Observatory, Department of Physics, Christchurch, New Zealand

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1 J. Astrophys. Astr. (1989) 10, The Reclassification of the Suspected R Coronae Borealis Star SY Hyi as a Semiregular Variable W. A. Lawson, P. L. Cottrell*, A. C. Gilmore & P. Μ. Kilmartin Mount John University Observatory, Department of Physics, Christchurch, New Zealand Received 1988 December 12; accepted 1989 February 9 Abstract. SY Hyi is classified as a suspected R Coronae Borealis-type variable star. Photometric and spectroscopic observations of SY Hyi lead to the star s reclassification as a semiregular variable of spectral type M5-6. Key words: variable stars R Coronae Borealis stars semiregular variables SY Hyi 1. Introduction The variable star SY Hyi is classified in the General Catalogue of Variable Stars (Kholopov et al. 1985) as a suspected R Coronae Borealis-type star (designated as RCB star in this paper). The star was reported by Shapley & Hughes (1934) to have a photographic magnitude range of Unpublished UBV photometry by Sherwood (personal communication to N. K. Rao) give mean magnitudes for SY Hyi of V=11.96, U B = 1.58 and B V =1.53. The seven observations obtained by Sherwood have a spread, in V, of 0.25 mag. The reason for the classification of SY Hyi as a RCB is curious as the amplitude quoted for the star is not representative of RCB stars in general, which show declines in light due to dust obscuration of 5 8 magnitudes (Lawson & Cottrell 1989). Our photometric and spectroscopic observations indicate that SY Hyi is a semiregular (SR) variable. 2. Observations SY Hyi was included on the RCB photometric survey at Mount John University Observatory (MJUO) during November 1986 (JD ). Although Bateson (personal communication) had suggested that SY Hyi was probably a semiregular variable soon after our observations were begun, we continued to observe the star until November 1988 (JD ) in order to determine the nature of the star s variability. ΒV photometry of SY Hyi were obtained with the 0.6-m photometric and 1-m reflectors at MJUO, using automated single channel photometers and either S1l or S20 response photomultiplier tubes. The observations were obtained differentially with respect to the comparison star labelled m on the Finder Chart No. 810 of the Variable Star Section, Royal Astronomical Society of New Zealand (Bateson 1985). * Visiting Fellow (June 1988 to May 1989) at Mount Stromlo and Siding Spring Observatories, Australian National University, Canberra, Australia.

2 152 W. A. Lawson et al. The comparison star magnitudes were defined as V =11.41, (B V)=1.12 The differential magnitudes were then transformed to the standard system using coefficients determined from observations of Εregion standards (Cousins 1983) at MJUO. The V and (B V) observations are listed in Table 1, and are plotted against Julian Date in Fig. 1. The accuracy of the observations, determined from observations of a check star in the SY Hyi field (star labelled n ), is estimated to be better than 0.01 mag. in V and 0.02 mag. in (B V). The resultant light curve is unlike any known RCB star, but is consistent with the light curve of a semiregular variable (see Hoffmeister, Richter & Wenzel 1985). The light curve shows no obvious periodicity, although variations on timescales of 40 to 100 day are seen. The amplitude of the light curve is about 1.0 mag., varying between V = Little structure is seen in the (B V) curve, except at times where the V magnitude varies rapidly, e.g., near JD and Table 1. MJUO BV photometry of SY Hyi.

3 SY Hyi as semiregular variable 153 Figure 1. (Β V) and V photometry of SY Hyi. In order to confirm the identification of SY Hyi as a semiregular variable, and to permit a more accurate classification of the star, several spectra of SY Hyi were obtained. These were taken with the 1.9-m reflector at Mount Stromlo Observatory (MSO) at the coudé focus, and with the Photon Counting Array (PCA) (Rodgers et al. 1988). A 1.5 Å resolution spectrum, obtained on JD in the wavelength range λλ Å, is shown in Fig. 2. A spectrum of S Lep (a semiregular variable, SRb, of spectral type M6 III) was obtained over the same wavelength interval, and is also shown in Fig. 2. We also compared the spectra obtained at MSO with the spectra of cool stars published by Turnshek et al. (1985). Both comparisons suggest a spectral type of M56. The spectrum of SY Hyi is typical of a late type star, dominated by molecular bands of TiO. In contrast, the spectrum of a cool RCB star, e.g., S Aps or HV 5637 is dominated by molecular bands of CN or C 2. (This is consistent with the hydrogen deficient carbon rich composition of RCB stars, see e.g., Cottrell & Lambert 1982.) 3. Discussion The magnitude range given by Shapley & Hughes (1934), mag ph = , can be compared with the Β magnitude range in our photometry. Our observations indicate a typical (B V) index of 1.5, giving Β = This amplitude range (δβ = 1.0) is less than that found by Shapley & Hughes (δ mag ph = 2.1). We also found that the upper magnitude was 0.4 mag. brighter (B =13.0, compared to mag ph = 13.4). These results may indicate that we have not observed the maximum amplitude of SΥ Hyi or that the mean magnitude of SY Hyi may vary over long intervals of time. SY Hyi is listed in the IRAS Point Source Catalogue (Neugebauer et al. 1984). The 12 µm and 25 µm fluxes were 0.77 and 0.43 Janskys (10 26.W.m 2.hz 1 ) respectively. These fluxes have magnitude equivalents of [12] = 3.90, and [25] = 2.98, assuming that a K, mag black body has [12] = 28.0 Jy and [25] = 6.7 Jy respectively (Habing 1987). If we adopt the method of Waters et al. (1987), the star s position in the (V [12])/(B V) plane will reveal the possible presence of an infrared (IR) excess. The (V [12]) colour for SY Hyi has limits of , with (B V) = 1.5. These colours

4 154 W. A. Lawson et al. Figure2. Low resolution (1.5 Å) coudé spectra of SY Hyi (top)and S Lep, an SRb variable star of spectral type M6 III (bottom). The spectra are on the same scale but have been displaced vertically. place SY Hyi slightly above the relationship for stars with no IR excess and indicate the possibility that the star has a weak circumstellar dust shell. 4. Conclusions Out observations of SΥ Hyi have ruled out its classification as a suspected RCB star. The light curve of SY Hyi is characteristic of a semiregular variable, probably of type SRb or SRc. Our magnitude and amplitude range of SY Hyi differs from the values given by Shapley & Hughes (1934), possibly indicating long term changes in the star s luminosity. By comparing our spectra with a low dispersion atlas of spectra of cool stars (Turnshek et al. 1985), and a spectra of S Lep (SRb, M6 III), we obtain a spectral type of M5-6 for SY Hyi. IRAS Point Source Catalogue data of SY Hyi may indicate the presence of a weak IR excess. Acknowledgements PLC would like to thank Professor A. W. Rodgers for the use of the facilities at Mount Stromlo and Siding Spring Observatories during his tenure as a Visiting Fellow (June 1988 to May 1989), while he was on Study Leave from the University of Canterbury. WAL would like to thank the Physics Department, University of Canterbury and the Kingdon-Tomlinson Fund of the Royal Astronomical Society of New Zealand for financial assistance to travel to Mount Stromlo where this paper was written. We would like to thank Dr F. M. Bateson for informing us that SY Hyi was probably a SR star and Dr P. R. Wood for reading an early version of this manuscript.

5 SY Hyi as semiregular variable 155 References Bateson, F. Μ. 1985, Publ. var. Star Section R. astr. Soc. New Zealand, Charts for Southern Variables, 18, no Cottrell, P. L, Lambert, D. L. 1982, Astrophys. J., 261, 595. Cousins, A.W.J.1983, Circ. Sth.Afr. astr.obs. 7, 36. Habing, H. J. 1987, in IAU Symp. 122; Circumstellar Matter, Eds I. Appenzeller & C. Jordan, D. Reidel, Dordrecht, p Hoffmeister, C, Richter, G., Wenzel, W. 1985, Variable Stars, Springer-Verlag, Berlin. Kholopov, P. N., Samus, N. N., Frolov, M. S., Goranskij, V. P., Gorynya, N. A.,Kazarovets, Ε. V., Kireeva, Ν. Ν., Kukarkina, Ν. P., Kurohkin, Ν. Ε., Medvedeva, G. I., Perova, Ν. Β., Rastorguev, A. S., Shugarov, S. Yu. 1985, General Catalogue of Variable Stars, IVth Edn, Moscow. Lawson, W. Α., Cottrell, P. L. 1989, Mon. Not. R. astr. Soc, submitted. Neugebauer, G., et al. 1984, Astrophys. J., 278, L1, 29. Rodgers, A. W., van Harmelen, J., King, D., Conroy, P., Harding, P. 1988, Publ. astr. Soc. Pac, 100, 841. Shapley, H., Hughes, E. M. 1934, Ann. Harvard Obs., 90, no. 4. Turnshek, D. E., Turnshek, D. Α., Craine, E. R., Boeshar, P. C. 1985, An Atlas of Digital Spectra of Cool Stars, Western Research Company, Tuscon. Waters, L. B. F. M., Cote, J., Aumann, H. H. 1987, Astr. Astrophys., 172, 225.