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Detailed Abundances of 15 Stars in the Metal-Poor Globular Cluster NGC 4833

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Detailed Abundances of 15 Stars in the Metal-Poor Globular Cluster NGC 4833 Mon. Not. R. Astron. Soc. 000, 1–24 (2014) Printed 10 September 2021 (MN LATEX style file v2.2) Detailed Abundances of 15 Stars in the Metal-Poor Globular Cluster NGC 4833⋆ Ian U. Roederer1† and Ian B. Thompson2‡ 1Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA 2Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA 10 September 2021 ABSTRACT We have observed 15 red giant stars in the relatively massive, metal-poor globular cluster NGC 4833 using the MIKE spectrograph at Magellan. We calculate stellar parameters for each star and perform a standard abundance analysis to derive abun- dances of 43 species of 39 elements, including 20 elements heavier than the iron group. We derive h[Fe/H]i = −2.25 ± 0.02 from Fe i lines and h[Fe/H]i = −2.19 ± 0.013 from Fe ii lines. We confirm earlier results that found no internal metallicity spread in NGC 4833, and there are no significant star-to-star abundance dispersions among any elements in the iron group (19 6 Z 6 30). We recover the usual abundance variations among the light elements C, N, O, Na, Mg, Al, and possibly Si. The heavy-element dis- tribution reflects enrichment by r-process nucleosynthesis ([Eu/Fe] = +0.36 ± 0.03), as found in many other metal-poor globular clusters. We investigate small star-to-star variations found among the neutron-capture elements, and we conclude that these are probably not real variations. Upper limits on the Th abundance, log ǫ (Th/Eu) < −0.47 ± 0.09, indicate that NGC 4833, like other globular clusters where Th has been studied, did not experience a so-called “actinide boost.” Key words: globular clusters: individual (NGC 4833) — nuclear reactions, nucle- osynthesis, abundances — stars: abundances — stars: Population II 1 INTRODUCTION Mar´ın-Franch et al. 2009), and it ranks in the top quartile of most luminous clusters around the Milky Way (Harris 1996). Globular clusters are the remnants of some of the most vio- The proper motion measurements and orbit calculations of lent epochs of star formation in the history of the Universe. Casetti-Dinescu et al. (2007) suggest that NGC 4833 and The compositions of the present-day stars provide a detailed NGC 5986, another old and metal-poor cluster, share similar arXiv:1503.03079v2 [astro-ph.SR] 11 Apr 2015 chemical inventory to probe those ancient nucleosynthesis orbital characteristics. Casetti-Dinescu et al. suggested that and enrichment events. Nevertheless, detailed chemical in- these two clusters are dynamically associated and could be ventories for some Galactic globular clusters are still rela- an accreted pair with a common origin in a now-disrupted tively unknown. satellite galaxy. Only recently were the chemical compositions of a large sample of stars examined in one such cluster, NGC 4833 Until last year, the only high-resolution spectroscopic (Carretta et al. 2014). The properties and orbital param- observations of stars in NGC 4833 had been conducted by eters of NGC 4833 are listed in Table 1. NGC 4833 has Pilachowski, Sneden, & Wallerstein (1983), who observed one of the more extended, richly-populated blue hori- two stars, Gratton & Ortolani (1989), who observed two zontal branches among Galactic globular clusters (e.g., stars, and Minniti et al. (1993, 1996), who observed one. Menzies 1972; Samus et al. 1995; Melbourne et al. 2000; These studies derived abundances of ≈ 15–20 species in indi- Piotto et al. 2002). This metal-poor cluster is a member of vidual stars in NGC 4833, including O and Na abundances. the “old” globular cluster population (e.g., Melbourne et al.; They recognized that [O/Fe] in some stars was depleted rel- ative to the maximum [O/Fe] ratios found in other clusters and field stars of similar metallicity. The small sample sizes limited these authors’ abilities to characterize the global ⋆ This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. chemical properties of NGC 4833. Carretta et al. (2014) ob- † Email: [email protected] served 78 stars in NGC 4833 with the UVES and GIRAFFE ‡ Email: [email protected] spectrometers in the FLAMES instrument on the VLT. c 2014 RAS 2 Ian U. Roederer and Ian B. Thompson Table 1. Basic properties and orbital parameters of NGC 4833 Quantity Symbol Value Reference Right ascension α (J2000) 12:59:34 1 Declination δ (J2000) −70:52:35 1 Galactic longitude ℓ 303.6◦ 1 Galactic latitude b −8.0◦ 1 Luminosity MV −8.17 1 Mass-to-light ratio M/L 0.84 ± 0.45 2 5 Mass M∗ 1.2×10 M⊙ 1 Central concentration c 1.25 1 Distance modulus m − M 15.05 ± 0.06 3 Distance from Sun R⊙ 6.5 kpc 4 Distance from Galactic Center RG.C. 7.0 kpc 4 Perigalactic distance Rperi 0.7 ± 0.2 kpc 4 Apogalactic distance Rapo 7.7 ± 0.7 kpc 4 Maximum distance above Galactic plane Zmax 1.8 ± 0.4 kpc 4 Orbital eccentricity e 0.84 ± 0.03 4 Orbital period Torbit 91 ± 9 Myr 4 References: (1) Harris 1996; (2) Carretta et al. 2014; (3) Melbourne et al. 2000; (4) Casetti-Dinescu et al. 2007 This dataset enabled Carretta et al. to characterize the pat- in NGC 4833, those produced by neutron (n) capture reac- tern of light-element (O, Na, Mg, Al, and Si) variations in tions. NGC 4833. That study also characterized the abundances Throughout this work we adopt the standard definitions of 18 species of 15 heavier elements. of elemental abundances and ratios. For element X, the log- arithmic abundance is defined as the number of atoms of X Carretta et al. (2014) presented several forms of evi- 12 per 10 hydrogen atoms, log ǫ(X) ≡ log (NX/NH)+ 12.0. dence that suggest that NGC 4833 may have been tidally 10 For elements X and Y, [X/Y] is the logarithmic abundance stripped more than the average cluster. First, NGC 4833 ratio relative to the solar ratio, defined as log (NX/NY) − has a wide span of [Mg/Fe] ratios. This property has only 10 log (NX/NY)⊙, using like ionization states; i.e., neutrals been identified in a few massive or metal-poor clusters 10 with neutrals and ions with ions. We adopt the solar abun- (Yong et al. 2005; Carretta et al. 2009b; Cohen & Kirby dances listed in Asplund et al. (2009). Abundances or ratios 2012), yet NGC 4833 is only the 36th most luminous Galac- denoted with the ionization state indicate the total elemen- tic globular cluster today. Second, NGC 4833 has an ec- tal abundance as derived from transitions of that particular centric orbit, small perigalactic distance, and modest cen- state. tral concentration. These values predict a high destruction rate relative to other clusters (Casetti-Dinescu et al. 2007; Allen, Moreno, & Pichardo 2008). Third, the inner-quartile range of the [O/Na] ratios (IQR[O/Na]) in NGC 4833 lies on 2 OBSERVATIONS the upper envelope of clusters in the IQR[O/Na] versus lu- We have observed 15 probable members of NGC 4833 with minosity correlation. Evidence discussed by Carretta et al. the Magellan Inamori Kyocera Echelle (MIKE) spectro- (2014) indicates that less-concentrated clusters of a given lu- graph (Bernstein et al. 2003) on the 6.5 m Landon Clay minosity may have larger values of IQR[O/Na] and have lost (Magellan II) Telescope at Las Campanas Observatory. Ta- more mass than more-concentrated ones. Finally, the low ble 2 presents a log of these observations, and Figure 1 mass-to-light ratio of NGC 4833 hints that significant mass highlights these 15 stars on a color-magnitude diagram of loss may have occurred as low-mass stars have been pref- NGC 4833. ThAr comparison lamp spectra have been taken erentially lost from the cluster. Carretta et al. (2014) note immediately preceding or following each observation. The that no attempt has been made to detect these escaped stars red and blue arms of MIKE are split by a dichroic at as tidal tails, and potential investigators may have been in- ≈ 4950A.˚ This setup provides complete wavelength cover- timidated by the stellar crowding and high reddening at the age from 3350–9150A.˚ Data reduction, extraction, and wave- low Galactic latitude of NGC 4833. The preponderance of length calibration have been performed using the CarPy evidence suggests that NGC 4833 has lost a larger fraction MIKE data reduction pipeline written by D. Kelson (see also of its initial stellar mass than the average Galactic globular Kelson 2003). Continuum normalization and order stitching cluster. have been performed within the iraf environment. Our sample includes only 15 stars in NGC 4833, but Six stars were observed with the 0′′. 7×5′′. 0 slit. This our data cover most of the optical spectral range. We de- setup yields a resolving power of R ∼ 41,400 in the blue and tect 43 species of 39 elements heavier than He, and this R ∼ 36,300 in the red, as measured from isolated ThAr lines complements the more limited chemical inventory derived in the extracted comparison lamp spectra. Nine stars were from a larger sample of stars studied by Carretta et al. observed during poorer seeing conditions with the 1′′. 0×5′′. 0 (2014). These data enable us to focus new attention on slit, which yields a resolving power of R ∼ 30,500 in the blue the abundance pattern exhibited by the heaviest elements and R ∼ 25,900 in the red. Table 3 presents signal-to-noise c 2014 RAS, MNRAS 000, 1–24 Abundances in Globular Cluster NGC 4833 3 Table 3.
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