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1983Apj. . .269. .580B the Astrophysical Journal, 269:580-591 .580B .269. The Astrophysical Journal, 269:580-591, 1983 June 15 . © 1983. The American Astronomical Society. All rights reserved. Printed in U.S.A. 1983ApJ. SPECTROSCOPIC ANALYSIS OF DWARF AND SUBGIANT STARS IN 47 TUCANAE R. A. Bell1 National Science Foundation James E. Hesser1 Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics AND R. D. Cannon1 Royal Observatory, Edinburgh Received 1982 August 10; accepted 1982 December 6 ABSTRACT Spectra at ~ 4 À resolution have been obtained with the SIT vidicon detector on the CTIO 4 m telescope’s Ritchey-Chretien spectrograph for 11 dwarfs (B -18.2 mag) and five brighter stars (2?~16 mag) in NGC 104 (47 Tue, C0021-723). A range of CN strengths is observed among subgiant and dwarf stars of otherwise very similar spectra. The spectroscopic results are combined with photometric data to argue that some, if not all, of the dwarfs have the same temperature. Spectrum synthesis calculations are then used to infer that the observed range of CN strengths could be produced by a star-to-star range of nitrogen abundances of ~ 5. This result is in accord with a previous analysis based upon much lower resolution data. The range inferred is similar to that required to explain observations of highly evolved stars in 47 Tue. Subject headings: clusters: globular — spectrophotometry— stars: abundances — stars: Population II I. INTRODUCTION unreddened, and located at high galactic latitude. Fur- It is now quite clear that the evolved stars in the thermore, its main-sequence turnoff region stars are cool metal-rich globular cluster 47 Tucanae ( = NGC 104 = enough that CN bands are visible in their spectra. C0021-723) exhibit a range of CN band strengths. This The problem of the overall metal abundance has conclusion is based on low-dispersion spectroscopy and been addressed by DBG using photometry, and by DDO photometry carried out by Hesser, Hartwick, and Pilachowski, Cantema, and Wallerstein (1980), using McClure (1976, 1977), Hesser (1978), Mallia (1978), spectroscopy. This abundance is not yet as well known Dickens, Bell, and Gustafsson (1979, hereafter DBG) as might be hoped. Recent work has shown that the and Norris and Cottrell (1979, hereafter NC). Norris cluster is probably more metal-poor than [M/H] = - 0.5, and Freeman (1979) have suggested that the distribution the value used for many years. DBG obtained [M/H] = of CN strengths is, in fact, bimodal. Analyses of these -0.8 from synthetic color analysis of uvby colors of red observational data, with the aim of obtaining carbon horizontal branch stars observed by Gustafsson and and nitrogen abundances, have been carried out by Ardeberg (1978). Pilachowski, Cantema, and Wallerstein DBG and by NC. (1980) found [Fe/H]=-1.2 from analysis of echelle The observational data available for the fainter clus- spectra of two of the brighter giant stars. In view of the ter stars are, first, DDO photometry for stars with assumptions made in the different analyses, the dif- Mv< 2.5 mag, and, second, low-dispersion spectroscopy ferences may not be too surprising, but their existence for stars down to —b 4.3 mag. These spectroscopic causes uncertainty in the analysis of a number of prob- data do suggest that CN strength variations occur in the lems. A similar problem holds for other metal-rich subgiants and in stars which are only slightly evolved clusters such as M71, but the discrepancy between pho- (Hesser 1978; Hesser and Bell 1980, hereafter HB). The tometric and spectroscopic abundances is even greater cluster 47 Tue is, of course, ideal for such studies in that cluster (Bell and Gustafsson 1982). inasmuch as it is populous, relatively nearby, nearly The spectroscopic data available to DBG were not of sufficient caliber for them to draw firm conclusions on Visiting Astronomer, Cerro Tololo Inter-American Observa- the carbon abundances, except for the AGB stars where tory, which is supported by the U.S. National Science Foundation they concluded that [C/A] = -0.5. DDO photometry is under contract AST 78-27879. very valuable for studies of carbon and cyanogen abun- 580 © American Astronomical Society • Provided by the NASA Astrophysics Data System .580B .269. DWARF AND SUBGIANT STARS IN 47 TUCANAE 581 . dances, and DBG showed that, if the carbon abundance Since instrumental developments at Cerro Tololo In- in the cluster stars was “normal,” the N abundance in ter-American Observatory have made it possible to ob- tain spectra with much higher resolution than those 1983ApJ. some stars was considerably enhanced, by more than a factor of 3 in some cases. Any reduction in the carbon available to HB, we undertook a further program of abundance found for a particular object does, of course, observations of faint 47 Tue stars. The objectives of this lead to a higher N abundance being deduced. Unfor- work were to see if (a) the new data confirmed the tunately, DBG were unable to deduce carbon abun- results of HB; (b) more extreme examples of CN dances of high precision from the photometry. The strengths could be found by an extension of the HB carbon abundances found for the AGB stars were de- survey; and (c) it were possible to obtain information duced under the assumption that the oxygen abundance on the abundance of carbon, using the G band, and of was “normal” since no data were available for the [O i] calcium, using the H and K Unes. We also obtained Unes or the CO bands. The DBG photometric interpre- spectra of the star HH 1-9038, a subgiant suspected of tation was checked by comparison of the values found having very strong CN as judged from DDO photome- for Arcturus with those found from high-dispersion try, and four other faint subgiants. spectroscopy. A different result on the CN abundances was ob- tained in the NC analysis of the spectra of two AGB stars. Assuming an overall metal abundance of [M/H] II. OBSERVATIONS = -0.5, NC found one star to have slightly enhanced Spectra with - 4 À resolution were obtained using carbon and a very low nitrogen abundance. This latter the SIT vidicon camera on the spectrograph at the overdeficiency would be reduced if lower overall metal Ritchey-Chrêtien focus of the CTIO 4 m telescope dur- abundance had been used, but it is unclear whether their ing the nights of 1980 September 8-11 (UT), in a results might still be systematically different from the manner identical to that described by Hesser and Harris DBG values. (1981). The instrumental configuration consisted of 300 Using ~ 16 Á resolution spectra of seven stars and /im (270) slit; KPNO Grating Lab grating number 1 spectrum synthesis techniques, HB showed that some (632 Unes mm-1) in the first order blue (angle 59.°20); slightly evolved stars (Mv - 4.3 mag) in 47 Tue were and an RCA 4804 UV transmitting SIT vidicon (Atwood overabundant in N (assuming that carbon and oxygen etal. 1979) with a 250 mm focal length, f/1.4 camera. were normal) by a factor comparable to that found by With the exception of the first night, when most of the DBG for the giants, i.e., nitrogen overabundances of observations of the five brighter subgiant stars were about a factor of 5. This result is rather surprising. It secured, the seeing was better than 275 and the skies had been widely assumed that the cyanogen variations were clear for most of the observing time. in the giant stars were the result of mixing of material We wished to reexamine, at higher resolution, several from the center of the star to the surface, but invocation of HB’s stars to see if the deduced range in CN strengths of that mechanism on the upper main sequence repre- was in whole or part due to noise in their lower resolu- sents a radical departure from long-held (but not neces- tion data. On our first night of good seeing we conse- sarily correct) beliefs. Although a number of other quently reobserved stars 1-9004, 3-2153, and 3-2195 mechanisms can be postulated to explain the CN en- by taking two 45 minute exposures of each. “Quick hancement in the dwarfs (cf. Bell et al. 1981 and § IV), look” facilities at the telescope indicated that these none of them are completely satisfactory. The hypothe- spectra, particularly in the crucial CN (0,0) band region sis that the enhanced nitrogen abundances are primor- near 3883 A, did appear to be similar to HB’s. We also dial in nature seems to be supported by stellar evolution wished to search for stars exhibiting more extreme CN calculations showing that mixing is unable to produce differences than those of 1-9004 and 3-2153. For this such large N overabundances (Da Costa and Demarque survey aspect, carried out on the final two nights of the 1982) as well as by the observation that the CN-strong run, only a single spectrum, sometimes at larger hour stars with Mv < -0.6 mag possess sodium abundances angles and/or briefer exposure times, was obtained per which are about twice those of the CN-weak stars star. (Cottrell and Da Costa 1981; Da Costa 1981; Lloyd The total sample of stars observed, selected from Evans, Smith, and Menzies 1982). There appears to be Hesser and Hartwick’s (1977) study of the color-magni- no evidence for a range in the overall metal abundance, tude (C-M) diagram, is listed in Table 1, together with nor in the abundances of other light elements such as relevant photometric data, the journal of spectroscopic magnesium, although it is possible that enhancements of observations, and the radial velocities derived from them.
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