A comprehensive chemical abundance study of the outer halo globular cluster M 75

Abstract

Context. M 75 is a relatively young globular cluster (GC) found at 15 kpc from the Galactic centre at the transition region between the inner and outer Milky Way halos. Aims. Our aims are to perform a comprehensive abundance study of a variety of chemical elements in this GC such as to investigate its chemical enrichment history in terms of early star formation, and to search for any multiple populations. Methods. We have obtained high resolution spectroscopy with the MIKE instrument at the Magellan telescope for 16 red giant stars. Their membership within the GC is confirmed from radial velocity measurements. Our chemical abundance analysis is performed via equivalent width measurements and spectral synthesis, assuming local thermodynamic equilibrium. Additionally the two most Na-rich stars are also Ba-enhanced by 0.4 and 0.6 dex, respectively, indicative of pollution by lower mass (M ∼ 4−5 M ) asymptotic giant branch stars. The overall n-capture element pattern is compatible with predominant r-process enrichment, which is rarely the case in GCs of such a high metallicity.

TheGaia-ESO Survey: Detailed abundances in the metal-poor globular cluster NGC 4372

“…Only a handful of metal-poor globular clusters show a potential star-to-star dispersion in neutron-capture elements (Roederer 2011;Kacharov et al 2013). Marino et al (2009) find a wide range of abundances values for s-process elements, Y, Zr, and Ba, in M22.…”

“…This scenario has the distinct advantage of not requiring that a very high percentage of the original cluster population has been lost. In addition, variations in heavier elements have also been found in some massive GCs, such as ω Centauri (Marino et al 2011), M54 (Carretta et al 2010b), M22 (Marino et al 2009), NGC 1851 (Carretta et al 2011), Terzan5 ), NGC 2419 (Cohen et al 2010), M2 (Lardo et al 2013;Yong et al 2014), and M75 (Kacharov et al 2013). They are generally thought to be the vestige of more massive primitive dwarf galaxies that merged with the Galaxy.…”

A COMPARATIVE STUDY OF TWO 47 Tuc GIANT STARS WITH DIFFERENT s-PROCESS ENRICHMENT

“…Although Omega Cen is known to have a large population of stars highly enriched in s-process elements, (Yong et al 2014), and NGC 6864 (Kacharov et al 2013). From published samples of s-process elements (∼200 stars) we find that nine stars are s-process enriched in 47 Tuc, i.e., ∼4%±2%.…”

Light and Heavy Element Abundance Variations in the Outer Halo Globular Cluster NGC 6229

“…The origin of the O-Na gap is not clear and may simply reflect the small (11 star) sample size analyzed here; however, if confirmed with a larger sample, NGC 6229 would possess one of the largest O-Na discontinuities of any cluster. Photometric studies have shown that nearly all globular clusters contain stellar populations with unique light element chemistry (e.g., Piotto et al 2015;Milone et al 2017), but the difference in [O/Fe] between adjacent populations is typically ∼0.3 dex or less (e.g., Yong et al 2005;Marino et al 2008;Gratton et al 2012b;Carretta 2013;Kacharov et al 2013;Carretta 2015;Johnson et al 2017b). In contrast, Figure 6 shows that the difference in [O/Fe] between O-rich and O-poor stars for NGC 6229 may be as large as ∼0.5 dex.…”

“…These "iron-complex" clusters share a common set of chemical and physical properties, including: (1) intrinsic [Fe/H] spreads that are larger than the measurement errors and range from ∼12-18% for clusters such as NGC 1851 and M 75 (e.g., Carretta et al 2011;Kacharov et al 2013) to about a factor of 100 for ω Cen (e.g., Johnson & Pilachowski 2010;Marino et al 2011a); (2) significant enhancements in elements produced by the slow neutron-capture process (s-process) that are correlated with [Fe/H]; (3) high cluster masses (M V -8); (4) very blue and extended horizontal branch (HB) morphologies that frequently include significant numbers of extreme HB and blue hook stars; and (5) the simultaneous presence of first (O/Mg-rich; Na/Al-poor) and second (O/Mg-poor; Na/Al-rich) generation stars in populations with different metallicities. Although ironcomplex cluster formation is not yet understood, the consistent signature of strong s-process enhancements in the more metal-rich populations is an indication that these systems experienced prolonged ( 100 Myr) star formation.…”