Using the VLT and the Hubble Space Telescope to unlock the secret lives of stars in globular clusters

By Anna Marino <anna.marino@unipd.it>

While I was a researcher at the Australian National University's Research School of Astronomy and Astrophysics a couple of years ago, I was able to apply for ESO time with the FLAMES+UVES instrument to get spectra for stars in globular clusters for which I already had HST data. Here I summarise our findings, as recently published in the Astrophysical Journal.

The origin of different populations of stars in globular clusters (GCs) remains a major astrophysical puzzle. Thanks to the high precision photometry from the Hubble Space Telescope (HST) it has been possible to introduce a new and powerful photometric diagram, dubbed a "Chromosome Map" (ChM), which is the most effective tool to isolate the different stellar populations in GCs (Milone et al. 2017). On the ChMs the position of stars is especially sensitive to the abundance of C, N, and O via various spectral features present in different HST filters, to the He content and to the overall metallicity (Marino et al. 2019). Overall, the stars hosted in a GC are separated into two distinct groups: one first-population group (1G) with pristine chemical composition, and a second-population one (2G) enriched in He/N/Na and depleted in C/O (see Figure 1 below).

Figure 1. ChM of the GC NGC3201 constructed by combining the filters F275W, F336W, F438W, and F814W from HST (Milone et al. 2015). The groups of 1G and 2G stars are plotted in green and magenta, respectively. Specifically, 1G stars are located around the origin of the ChM diagram (i.e., ΔC F275W,F336W,F438W = ΔF275W,F814W = 0), while 2G stars have large ΔC F275W,F336W,F438W and low ΔF275W,F814W (see Milone et al. 2015 for a detailed definition of these quantities). The arrows indicate the effect of changing He, N, O, and Fe by the quoted quantities.

One major result based on the ChMs has been the discovery that in many GCs the 1G stars (that are usually associated with the primordial population) are not chemically homogeneous, as they show a large spread along the x axis of the ChMs. Milone et al. (2015) hinted to a variation of helium among 1G stars to account for such a spread. However, as far as we know there is no plausible physical mechanism to account for a helium enrichment not accompanied by a concommitant conversion of C and O into N, that would be consistent with the low values of the 1G stars on the ChM. Thus, either a totally new, unknown process was responsible for the 1G spread in helium, or else helium was not the culprit and some other element was driving it.

Our team, which includes astronomers from the ANU's Research School of Astronomy and Astrophysics and Monash University, has combined HST photometric data with high resolution UVES@VLT spectra collected under the ESO program 0101.D-0113. Thanks to this valuable dataset we have inferred the chemical abundances for many elements and radial velocities of stars distributed along the extended 1G of the GC NGC3201 (Marino et al. 2019).

In Figure 2 below, the abundance ratios [La/Y] and [Ba/Y] have been taken as indicative of the heavy to light n-capture element abundance ratio ([hs/ls]), which is sensitive to the neutron exposure and neutron density. Clearly, the stars with more extreme lower values of ΔF275W,F814W have higher [hs/ls] and lower Fe abundances. The higher [hs/ls] of two stars, that are also enriched in n-capture elements, might suggest direct mass transfer between the star and previous low-mass, low-metallicity AGB stars (e.g. Karakas & Lattanzio 2014). According to the variations in radial velocities, the stars with lowest ΔF275W,F814W are consistent with being binaries. Thus, our results suggest that a combination of two phenomena may account for the 1G spread in this GC: (1) a tiny variation in metals, of the order of 0.10 dex; and (2) binarity, which alleviates the size of chemical inhomogeneity required to account for the photometric spread of 1G stars.

Figure 2. The abundance of [Ba/Y] (left) and [La/Y] (right), representative of the heavy to light element abundances ([hs/ls]), as a function of the ΔF275W,F814W. The dashed lines in both panels highlight the average abundance ratios for the entire sample, neglecting the three (encircled) stars which, based on radial velocity measurements, are binary candidates. The colour of each star is indicative of its inferred Fe, as illustrated in the colour bar at right.

The presence among 1G stars of a small spread in metals suggests that either the molecular cloud out of which 1G stars formed was not chemically homogeneous (i.e., was not fully mixed following its original chemical enrichment), or that some of the 1G stars formed while the first 1G supernovae had already started to enrich the ISM of the nascent GC. These options can provide us with new hints to understand the processes of GC formation at high redshift, its timescale, and help in identifying the class of stars responsible for the further enrichment of the ISM to form 2G stars.