DISCOVERY OF VERY RED GIANTS IN THE FORNAX GALAXY

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1 Publications of the Astronomical Society of the Pacific 91: , December 1979 DISCOVERY OF VERY RED GIANTS IN THE FORNAX GALAXY SERGE DEMERS 0 Université de Montréal, Observatoire Astronomique du mont Mégantic, Montréal, Quebec, Canada AND W. E. KUNKEL 0 Observatorio Nacional, Rio de Janeiro, Brazil Received 1979 July 31 Sixty-six very red stars were discovered in the Fornax galaxy by blinking a pair of B,V plates. Their mean apparent magnitude is V = 18 r M and their (B V) color indices range from 1U6 to 2" 1 9. These stars form on the color-magnitude diagram a natural extension of the giant branch of Fornax. Even though no spectroscopic observations are available to establish the nature of the stars, it is reasonable to suspect that a certain number of carbon stars must be present in Fornax. Key words: galaxies: individual galaxies: stellar content I. Introduction A photometric study of the Fornax dwarf elliptical galaxy by Demers, Kunkel, and Hardy (1979, hereafter Paper I) has revealed that Fornax has a wide giant branch similar to or more pronounced than co Centauri. The results of Paper I suggest that the stars of Fornax have a range of abundances and possibly ages. One very red giant ((B V) = 2T4) was discovered in the region sampled. We report the results of a search of the whole galaxy which has yielded over 60 very red stars. The magnitudes and colors of these stars enable us to extend the giant branch of Fornax to (B V)~ 2 9. II. Observations and Results The photographic material used in this investigation consists of two 103a-D + Schott GG 14 and three 103a-O -f Schott GG 13 plates obtained with the CTIO 1.5-m reflector (scale mm -1 ), the three B plates were taken during three consecutive nights, namely 1970 September 8, 9, and 10. The two V observations are separated by an interval of 132 days and were obtained on 1970 September 9 and 1971 January 19. The red stars were discovered by blinking a B,V pair. They are identified on Figures la and lb. the candidates were measured with the Astro Mechanics iris photometer of the Université de Montréal. The plates were calibrated with the potoelectric sequence of Paper I, and the iris measures were reduced as described in Paper I. The limiting magnitude of the Visiting Astronomer, Cerro Tololo Inter-American Observatory, supported by the National Science Foundation under contract No. AST plates material is approximately V ~ 20 5 and B ~ 21*75. The average magnitudes and color indices of the stars discovered are listed in Table I. The difference between the first and second V observations is listed as AV. It is obvious that several stars are variable. The three B observations which have been obtained on three consecutive nights show as expected very little dispersion. Only a lower limit of the color index of four stars can be given, two stars do not have blue images while two other stars have only one faint blue image visible on one of the three B plates. These stars have an apparent visual magnitude fainter than the majority of the stars discovered in the survey. A comparison between the red stars iris-measured in two regions of Fornax (Paper I) and the stars discovered in this survey shows that nearly all stars with (B V)> 1 8 and brighter than V ~ 19 5 have been detected. Fornax does not seem to have extremely red stars ((B V) > 3 0) at the magnitude level of the tip of the giant branch. Our blinking technique is not so effective in detecting bluer stars. We estimate that 50% of the stars in the interval and very few ~5% of those in the interval have been identified. Stars of Table I are plotted on the color-magnitude diagram in Figure 2. The lines represent the core and the upper envelope of the giant branch of Fornax (Paper I). We note that the very red stars form a natural extension of the core of the giant branch. Their mean apparent V = ls^ ± 0*M. The distance of Fornax has not yet been well established but it is believed to be between 150 and 180 kpc (Hodge 1971). We favor, based on our unpublished Fornax data, the smaller distance. Adopting (V M^0 ~ 21T0, corresponding to 158 kpc, without reddening, the mean absolute magni-

2 762 DEMERS AND KUNKEL FIG. la Identification of the red stars in Fornax, 70 minutes, Ila-D + GG 14, CTIO 4-m

3 VERY RED STARS IN FORNAX 763. lb Identification of the red stars in Fornax, 70 minutes, lla-d + GG 14, CTIO 4-m.

4 764 DEMERS AND KUNKEL TABLE I RED STARS IN FORNAX Stars V B-V AV star V B-V AV : > > > > Fig. 2 Color-magnitude diagram of the very red giants in Fornax. The core and the upper envelope of the giant branch identified in Paper I are sketched. Four stars with very faint or invisible blue images are marked with a bar. tude of the very red giants becomes M v ^ 2T6. III. Discussion It has been known for some time that the stellar population of dwarf elliptical galaxies differs from that in galactic globular clusters. Van de Bergh (1975) pointed out that their peculiarities are reminiscent of those observed among the Population II stars in the Magellanic Clouds. The discovery of very red giants in Fornax gives support to this observation. None of the galactic globular clusters, with the possible exception of one star in NGC 362 (Menzies 1967) contain very red giants ((B V) > l 1^). On the other hand, very red giants have been observed in and near a certain number of SMC and LMC clusters (Arp 1958; van den Bergh 1975). Carbon stars have been identified among these very red stars in at least four SMC clusters by Feast and Lloyd Evans (1973) and Lloyd Evans (1978). The reddest stars in NGC 362 do not show carbon features (Catchpole and Feast 1973). co Cen is the only known galactic globular cluster to contain carbon stars. Its carbon stars are similar to the halo-type carbon stars defined by Eggen (1972) and have a (B V) color index in the ltô-l^ range, much bluer than those observed in the SMC clusters.

5 VERY RED STARS IN FORNAX 765 Because a few intermediate-age galactic open clusters are known to contain very red carbon stars (Catchpole and Feast 1973; Hartwick and Hesser 1971; Gaustad and Conti 1971) Lloyd Evans (1978) has suggested that the presence of very red carbon stars in the SMC clusters may be indicative of abundance and/or age similarities. The Fornax photometry does not permit us to distinguish a carbon or an S-type star from an M giant. Without spectroscopic observations we cannot at the present time establish the true nature of the very red giants in Fornax. If, however, we assume, following Lloyd Evans (1978), that the only stars redder than (B V) 0 = 2 5 are carbon stars, Fornax would then have a few of them. The estimated absolute magnitude of the very red giants of Fornax falls within the range of the absolute magnitudes of the Clouds carbon stars. Lloyd Evans (1978) estimates that the mean absolute magnitudes of the carbon stars associated with NGC371 and NGC 419 are, respectively, M v 2^4 and M v The bright, presumably more massive, carbon stars in the LMC discussed by Crabtree, Richer, and Westerlund (1976) are near M v ~ 3 m. The Fornax galaxy being by far the most massive known dwarf elliptical galaxy (Hodge 1971) may be the only one to have a population of very red giants. Very few data are available for the other galaxies. The Sculptor system may also contain very red giants, the two reddest stars in the color-magnitude diagram of the giant branch published by Hodge (1965) have (B V) > 2 r?0. On the other hand, no very red giants were observed in the sample studied by Kunkel and Demers (1977). Blinking of a B,V plate-pair of the Draco system by one of the authors (S.D.) fails to detect very red giants. The discovery of very red giants in Fornax may give clues to the evolutionary history of this galaxy. In this context it will be of interest to extend the survey of Blanco, Blanco, and McCarthy (1978) to Fornax and determine its ratio of carbon stars to M giants. This will establish if part of the stellar population of Fornax is indeed similar to the SMC clusters. This work was supported in part (S.D.) by an operating grant from the Natural Sciences and Engineering Research Council Canada. REFERENCES Arp, H. C. 1958, A.J. 63, 273. Blanco, B. M., Blanco, V. M., and McCarthy, M. F. 1978, Nature 270, 638. Catchpole, R. M., and Feast, M. W. 1973, M.N.R.A.S. 164, lip. Crabtree, D. R., Richer, H. B., and Westerlund, B. E. 1976, Ap. J. (Letters) 203, L81. Demers, S., Kunkel, W. E., and Hardy, E. 1979, Ap. J. 232, 84. Eggen, O. J. 1972, Ap. J. 174, 45. Feast, M. W., and Lloyd Evans, T. 1973, M.N.R.A.S. 164, 15p. Gaustad, J. E.,and Conti, P. S. 1971, Pub. A.S.P. 83, 351. Hartwick, F. D. A., and Hesser, J. E. 1971, Pub. A.S.P. 83, 53. Hodge, P. W. 1965, Ap. J. 142, , Ann. Rev. Astr. and Ap. 9, 35. Kunkel, W. E., and Demers, S. 1977, Ap. J. 214, 21 Lloyd Evans, T. 1978, M.N.R.A.S. 183, 319. Menzies, J. W. 1967, Dissertation, Australian National University, van den Bergh, S. 1975, Ann. Rev. Astr. and Ap. 13, 217.

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