The HST Large Programme on NGC 6752 – II. Multiple populations at the bottom of the main sequence probed in NIR

Abstract

Historically, multiple populations in Globular Clusters (GCs) have been mostly studied from ultraviolet and optical filters down to stars that are more massive than ∼0.6M ⊙ . Here we exploit deep near-infrared (NIR) photometry from the Hubble Space Telescope to investigate multiple populations among M-dwarfs in the GC NGC 6752. We discovered that the three main populations (A, B and C), previously observed in the brightest part of the colormagnitude diagram, define three distinct sequences that run from the main-sequence (MS) knee towards the bottom of the MS (∼0.15 M ⊙ ). These results, together with similar findings on NGC 2808, M 4, and ω Centauri, demonstrate that multiple sequences of M-dwarfs are common features of the color-magnitude diagrams of GCs. The three sequences of low-mass stars in NGC 6752 are consistent with stellar populations with different oxygen abundances. The range of [O/Fe] needed to reproduce the NIR CMD of NGC 6752 is similar to the oxygen spread inferred from high-resolution spectroscopy of red-giant branch (RGB) stars. The relative numbers of stars in the three populations of M-dwarfs are similar to those derived among RGB and MS stars more massive than ∼0.6M ⊙ . As a consequence, the evidence that the properties of multiple populations do not depend on stellar mass is a constraint for the formation scenarios.

When Does the Onset of Multiple Stellar Populations in Star Clusters Occur? III. No Evidence of Significant Chemical Variations in Main-sequence Stars of NGC 419

“…Almost all old globular clusters (GCs, older than ∼10 Gyr) are composed of multiple stellar populations (MPs). One signature of MPs is stars in GCs are not chemically homogeneous: they exhibit star-to-star variations in different elemental abundances, such as He, C, N, O, Na, Mg, Al, etc (Piotto et al 2007;Yong et al 2008;Carretta et al 2009;di Criscienzo et al 2010;Lardo et al 2012;Wang et al 2017;Pancino et al 2017;Milone et al 2019). The number of chemically enriched stars is comparable to, or greater than normal stars (Carretta et al 2009;Milone et al 2012b;.…”

Models of Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. I. Bolometric Corrections Derived from New MARCS Synthetic Spectra and Their Implications for Observed Colour-Magnitude Diagrams

“…5, giants with increased C abundances are predicted to have bluer M F110W − M F160W colours. However, oxygen clearly plays a major role in determining the colours of LMS stars in the IR, as already pointed out by Milone et al (2019) in their study of NGC 6752. If the LMS stars in GCs span a wide range in the m F110W − m F160 colour at a given magnitude, the bluest ones are likely to have oxygen abundances that are close to initial abundances (i.e., when they formed), while the reddest ones are likely to be members of the cluster's most oxygen-poor population, though they could also be C-rich stars; see Fig.…”

Self-consistent Analysis of Stellar Clusters: An Application to HST Data of the Halo Globular Cluster NGC 6752

“…In a second run, we assume a helium enhancement by a type of polluter star, changing the amount of helium for each generation, according to values computed by Milone et al (2019): δY 1G,2G = 0.010 and δY 1G,3G = 0.042 for the 2G, and 3G, respectively (Figures 8, 10, and Table 3). We assumed the helium enhancement values from Milone et al (2019) since they were derived using the same DSED stellar evolutionary models employed here, therefore there is compatibility. For the metallicity of NGC 6752, the corresponding canonical helium content in the DSED isochrones is 0.247, which was associated to 1G.…”

“…For the metallicity of NGC 6752, the corresponding canonical helium content in the DSED isochrones is 0.247, which was associated to 1G. The 2G and 3G helium contents were assumed to be of 0.257 and 0.289, adopting the δY values from Milone et al (2019). Table 3 and Figure 11 provide the results of isochrone fitting to the MPs.…”