.238L.149H . The Astrophysical Journal, 238:L149-L153, 1980 June 15 © 1980. The American Astronomical Society. All rights reserved. Printed in U.S.A. OApJ. 198 CN VARIATIONS AMONG MAIN-SEQUENCE 47 TUCANAE STARS James E. Hesser1 Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics AND R. A. Bell Astronomy Program, University of Maryland Received 1979 December 28; accepted 1980 February 26 ABSTRACT Low-resolution CTIO SIT vidicon spectra of seven upper-main-sequence stars in 47 Tucanae (NGC 104) reveal differences in CN X3883 (0, 0) band strengths. Spectrum synthesis calculations suggest that the star-to-star nitrogen abundances differ by factors of ^5, i.e., by amounts com- parable to those required to explain observations of highly evolved stars in the same cluster. If a primordial origin of the variations is to be rejected, our observations require identification of a mixing mechanism or other process capable of affecting the main-sequence atmospheric abundances of CNO group elements independently of heavier ones. Subject headings: clusters : globular — spectrophotometry — stars : abundances — stars : Population II — stars : subdwarfs I. INTRODUCTION found evidence for star-to-star variations in NH and Until a few years ago it was widely believed that CH strengths among subgiants in the more metal-poor globular cluster stars were chemically homogeneous, cluster M92, although Bell, Dickens, and Gustafsson but various recent investigations have provided evi- (1979) have used DDO photometry and spectra to dence that highly evolved globular cluster stars exhibit argue that CH weakening occurs only in M92 and a wide range of line strength differences within small NGC 6397 stars brighter than My ^ — 0.7 mag. Varia- magnitude and color ranges. With the exception of co tions of CN and CH strengths have also been found Centauri and M22, the evidence has involved differ- among metal-poor M22 subgiants (Hesser and Harris ences among features due to CH, NH, CN, and CO. 1979). Such spectral differences among highly evolved stars in Differences among CN (0, 0) band strengths in four clusters with narrow giant branches in their color- fainter 47 Tue stars with Mv ^ 4.3 mag were also magnitude diagrams (CMDs) have been qualitatively noted in the original Tololo vidicon survey (Hesser attributed either to mixing of internally produced ele- 1978). Even though the statistical accuracy (equivalent ments to the observable surface layers or to primordial to ^200-300 counts per pixel) of the spectra was inhomogeneities at the time of star formation (the sufficient to place faith in the result, it was felt that the extensive literature on this subject is reviewed by importance of the conclusion that spectral variations McClure 1979 and Kraft 1979). There is no a priori could occur on the main sequence of a globular cluster reason for excluding the operation of both mechanisms warranted confirmatory observations before interpre- in a single cluster. Since the mixing mechanisms cur- tation. rently known appear to operate for My < +lmag II. OBSERVATIONS (Sweigart and Mengel 1979; Kraft 1979), extension of Additional observations of stars from the CMD study the observations to fainter My’s might provide some of Hesser and Hartwick (1977) were secured during insight into the relative importance of mixing and the dark hours of 1979 August 2-4 (UT), under condi- primordial explanations within a given cluster. tions of ^2''5 seeing and intermittent light cloud. Evidence has been accumulating that differences do Procedures were virtually identical to those used earlier appear at Mv > +0.5 mag. For 47 Tue subgiants DDO (Hesser 1978) with the CTIO 4 m RC spectrograph photometry to Mu ^+2.5 (Hesser, Hartwick, and and the RCA 4804 SIT vidicon (Atwood et al. 1979). McClure 1976, 1977; Hesser 1978) and low-dispersion Grating 09 from the 1.5 m spectrograph (^200 Â mm-1) SIT vidicon spectroscopy to My ~ +3.5 mag (Hesser and a slit width of 590 jum (2''5) were employed. Each 1978) revealed that CN strength variations are present vidicon pixel corresponds to ^6 Â and the resolution is without comparable ^-excess (i.e., heavy element) ~3 pixels. Our aim was to utilize the limited dark time variations. Carbon et al. (1977) and Kraft (1979) have as efficiently as possible to seek evidence of spectral 1 Visiting Astronomer, Cerro Tololo Inter-American Observa- differences over short-wavelength intervals in a stronger tory, which is operated by AURA, Inc., under contract with the and weaker CN star from the earlier survey and to U.S. National Science Foundation. extend the survey to as many stars as possible for L149 © American Astronomical Society • Provided by the NASA Astrophysics Data System .238L.149H . L150 MESSER AND BELL Vol. 238 future selection of the best candidates for higher- to-strong CN. With the increased data sample we feel OApJ. resolution studies. Thus, as before, observations of there is no longer any doubt that CN differences occur 198 only one flux standard star per night were used to among stars located below the knee of the turnoff in place the data on an approximate relative flux scale. 47 Tue. The narrow slit coupled with seeing effects, guiding errors, and differential refraction can result in slope III. CALCULATIONS errors, but comparisons of adjacent spectral features Synthetic spectrum calculations following those used are unaffected. Data for the observed stars are sum- by Dickens, Bell, and Gustafsson (1979, hereafter DBG) marized in Table 1. Reductions used the current version for the analysis of giants in 47 Tue can be used to reach of the software originally described by Schaller et al. some semiquantitative conclusions regarding the be- (1978), and included sky subtraction from star-free havior of CN on the main sequence of 47 Tue. In view frame lines ^lO" distant on opposite sides of each star. of the noise in the data and the similarity of the V and Our final spectra for all stars are shown in Figure 1, B — V values of the stars in Table 1, we computed where it can be seen that stars 3-2195 and 3-2153 show representative spectra which could be compared to the unmistakeably stronger CN (0, 0) features than do group of observed spectra. stars 1-9018 or 1-9004. In addition, during one of our Extrapolating the subgiant branch of Figure 5 of 1979 exposures, star 3-2274 [Fpg = 18.56, (B — F)Pg = DBG we find that the stars in the sample have Te{{ « 0.66, Mv ~ +5.4 mag) also fell directly on the slit. 5750 K and log g ~ 4.0. Adopting the metal abundance Although the resultant spectrum was near the edge of of [m/H] = —0.8 found by DBG, we computed the the vidicon frame, where the standard extraction algo- flux constant model 5750/4.0/—0.8 (T^ff/log g/[A/H]) rithms are not entirely satisfactory, it suggests medium- using the methods and programs of Gustafsson et al. Fig. 1.—CTIO SIT vidicon spectra for the seven faint 47 Tue stars of Table 1. A superposition of the tracings of stars 1-9004 and 3-2153 is also given to facilitate comparison of the differences at the CN (0, 0) X3883 bandhead, although attention is called to star 1-9018 as a CN weak star with a. B — V more like that of 3-2153. The strong feature at ^3825 À in all stars and in the synthesized spectra of Fig. 2 is due to very strong, low-excitation lines of Fe i and an Mg i line, and is common in stars of this spectral type. The resolution and precision of these spectra are such that only strong, broad features spanning several pixels are statistically reliable. © American Astronomical Society • Provided by the NASA Astrophysics Data System .238L.149H . No. 3, 1980 CN VARIATIONS IN 47 TUC STARS L151 TABLE 1 OApJ. Summary of 47 Tucanae Turnoff Region 198 Vidicon Observations* V B-V Date Exp. Hour Star (mag) (mag) (UT) (min) Angle 3-2153 17.40 0.60 1977 Oct 13 30 4h39m 1977 Oct 13 30 5 12 1979 Aug 3 30 0 55 1979 Aug 3 15 1 13 3-2234. 17.48 0.62 1979 Aug 4 30 1 03 1-9004. 17.51 0.54 1977 Oct 13 25 0 47 1977 Oct 13 25 1 40 1979 Aug 2 30 0 17 1979 Aug 2 21 1 18 3-2188. 17.50 0.60 1979 Aug 3 30 1 49 3-2110. 17.53 0.61 1977 Oct 13 30 3 26 1977 Oct 13 30 3 59 1-9018. 17.54 0.62 1977 Oct 13 30 2 19 1977 Oct 13 30 2 52 3-2195. 17.58 0.57 1979 Aug 4 30 0 25 a Identification numbers from Hesser and Hartwick 1977 in the form “figure number—identification number.” For all stars except 1-9004 and 1-9018 the magnitudes are photographic. The hour angle at the end of the exposure (Exp.) is always west. (1975). Since no opacity distribution function (ODF) was available for [^4/H] = —0.8, we used the ODF for [^4/H] = —0.5 and used [^4/H] = —0.8 elsewhere in the model calculations. Two other models (6000/ 4.5/0.0 and 5500/4.5/0.0) were taken from the unpub- lished grid of dwarf models of Eriksson, Gustafsson, and Bell. A synthetic spectrum was computed for the wave- Fig. 2.—Examples of synthesized spectra, convolved with an length interval 3000-7200 Â for the model 5750/4.0/ instrumental profile of 15 Â half-width. The spectrum of the —0.8, with the abundances of all metals being reduced model 5750/4.0/ —0.8 has been computed for two values of the N abundance, [N/^4] = 0.0 {thin line) and +1.0 {thick line).