Light element discontinuities suggest an early termination of star formation in the globular cluster NGC 6402 (M14)

Abstract

NGC 6402 is among the most massive globular clusters in the Galaxy, but little is known about its detailed chemical composition. Therefore, we obtained radial velocities and/or chemical abundances of 11 elements for 41 red giant branch cluster members using high resolution spectra obtained with the Magellan-M2FS instrument. We find NGC 6402 to be only moderately metal-poor with [Fe/H] = −1.13 dex (σ = 0.05 dex) and to have a mean heliocentric radial velocity of −61.1 km s −1 (σ = 8.5 km s −1 ). In general, NGC 6402 exhibits mean composition properties that are similar to other inner Galaxy clusters, such as [α/Fe] ∼ +0.3 dex, [Cr,Ni/Fe] ∼ 0.0 dex, and [La/Eu] = −0.08 dex. Similarly, we find large star-to-star abundance variations for O, Na, Mg, Al, and Si that are indicative of gas that experienced high temperature proton-capture burning. Interestingly, we detect three distinct populations, but also find large gaps in the [O/Fe], [Na/Fe], and [Al/Fe] distributions that may provide the first direct evidence of delayed formation for intermediate composition stars. A qualitative enrichment model is discussed where clusters form stars through an early ( < ∼ 5−10 Myr) phase, which results in first generation and "extreme" composition stars, and a delayed phase ( > ∼ 40 Myr), which results in the dilution of processed and pristine gas and the formation of intermediate composition stars. For NGC 6402, the missing intermediate composition stars suggest the delayed phase terminated prematurely, and as a result the cluster may uniquely preserve details of the chemical enrichment process.

Light element discontinuities in the globular cluster NGC 6402 (M14)

“…Therefore, Johnson et al (2019) proposed a modified version of previous models and suggested that the P1 and E populations form within the first few Myr while the P2/I1/I2 stars form via pollution and mixing after the SN era. In order to match observations in other clusters, this model would require that a high temperature burning source produced E composition gas within a small region around the cluster core.…”

“…Although NGC 6402 is a more typical monometallic cluster, Johnson et al (2019) found a surprising result when investigating the cluster's light element abundance pattern. As summarized in Figure 1, NGC 6402 exhibits the usual O-Na anti-correlation that is prevalent in nearly all old globular clusters.…”

“…The data ranged in wavelength from ∼6140-6700 Angstroms in order to target key absorption lines of light, α, Fe-peak, and neutron-capture lines ranging from oxygen to europium. Johnson et al (2019) found that NGC 6402 is a moderately metal-poor cluster with [Fe/H] = −1.13 dex. However, the [Fe/H] dispersion is only 0.05 dex, which indicates that although NGC 6402 is among the most massive clusters in the Galaxy it does not possess an intrinsic metallicity spread.…”

“…A comparison between the second parameter pair clusters NGC 6402 and NGC 2808 by Johnson et al (2019) indicated that the pollution sources between the two clusters were likely similar, but that NGC 2808 experienced pollution from stars that reached temperatures at least 5-10 MK higher than for NGC 6402. Although NGC 6402 is unique in that it is the only monometallic cluster to exhibit such a clear paucity of intermediate composition stars, Figure 1 shows that the [Fe/H] ∼ −1 stars in ω Cen exhibit a similar composition gap.…”

“…These observations have motivated a search for additional clusters that exhibit clear metallicity spreads, and a particular emphasis has been placed on clusters with M V < −8. As a consequence, Johnson et al (2019) investigated the chemical composition of NGC 6402 (M14), which is both massive and exhibits some ancillary evidence that it may have an extragalactic origin (e.g., peculiar RR Lyrae properties and the existence of a CH star).…”