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Register a new userOn the Origin of Bimodal Horizontal-Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441
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Despite the efforts of the past decade, the origin of the bimodal horizontal-branch (HB) found in some globular clusters (GCs) remains a conundrum. Inspired by the discovery of multiple stellar populations in the {it most massive} Galactic GC, $omega$ Centauri, we investigate the possibility that two distinct populations may coexist and are responsible for the bimodal HBs in the {it third} and {it fifth} brightest GCs, NGC 6388 and NGC 6441. Using the population synthesis technique, we examine two different chemical ``self-enrichment hypotheses in which a primordial GC was sufficiently massive to contain two or more distinct populations as suggested by the populations found in $omega$ Cen: (1) the age-metallicity relation scenario in which two populations with different metallicity and age coexist, following an internal age-metallicity relation, and (2) the super-helium-rich scenario in which GCs contain a certain fraction of helium-enhanced stars, for instance, the second generation stars formed from the helium-enriched ejecta of the first. The comparative study indicates that the detailed color-magnitude diagram morphologies and the properties of the RR Lyrae variables in NGC 6388 and NGC 6441 support the latter scenario; i.e., the model which assumes a minor fraction ($sim$ 15 %) of helium-excess (Y $simeq$ 0.3) stars. The results suggest that helium content is the main driver behind the HB bimodality found most often in massive GCs. If confirmed, the GC-to-GC variation of helium abundance should be considered a {it local} effect, further supporting the argument that age is the {it global} second parameter of HB morphology.
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NGC 6388 and NGC 6441 are two massive Galactic bulge globular clusters which share many properties, including the presence of an extended horizontal branch (HB), quite unexpected because of their high metal content. In this paper we use HSTs WFPC2, ACS, and WFC3 images and present a broad multicolor study of their stellar content, covering all main evolutionary branches. The color-magnitude diagrams (CMDs) give compelling evidence that both clusters host at least two stellar populations, which manifest themselves in different ways. NGC 6388 has a broadened main sequence (MS), a split sub-giant branch (SGB), and a split red giant branch (RGB) that becomes evident above the HB in our data set; its red HB is also split into two branches. NGC 6441 has a split MS, but only an indication of two SGB populations, while the RGB clearly splits in two from the SGB level upward, and no red HB structure. The multicolor analysis of the CMDs confirms that the He difference between the two main stellar populations in the two clusters must be similar. This is observationally supported by the HB morphology, but also confirmed by the color distribution of the stars in the MS optical band CMDs. However, a MS split becomes evident in NGC 6441 using UV colors, but not in NGC 6388, indicating that the chemical patterns of the different populations are different in the two clusters, with C, N, O abundance differences likely playing a major role. We also analyze the radial distribution of the two populations.
We present an analysis of FLAMES-Giraffe spectra for several bright giants in NGC 6441, to investigate the presence and extent of the Na-O anticorrelation in this anomalous globular cluster. The field of NGC 6441 is very crowded, with severe contamination by foreground (mainly bulge) field stars. Appropriate membership criteria were devised to identify a group of 25 likely cluster members among the about 130 stars observed. Combined with the UVES data obtained with the same observations, high dispersion abundance analyses are now available for a total of 30 stars in NGC 6441, 29 of them having data for both O and Na. The spectra were analyzed by a standard line analysis procedure; care was taken to minimize the impact of the differential interstellar reddening throughout the cluster, and to extract reliable information from crowded, and moderately high S/N (30-70), moderately high resolution (R ~ 23,000) spectra. NGC 6441 is very metal-rich ([Fe/H]=$-0.34pm 0.02pm0.04$ dex). There is no clear sign of star-to-star scatter in the Fe-peak elements. The alpha-elements Mg, Si, Ca, and Ti are overabundant by rather large factors, suggesting that the cluster formed from material enriched by massive core collapse SNe. The O-Na anticorrelation is well defined, with about 1/4 of the stars being Na-rich and O-poor. One of the stars is a Ba-rich and moderately C-rich star. The distribution of [Na/O] ratios among RGB stars in NGC 6441 appears similar to the distribution of colors of stars along the horizontal branch. The fraction of Na-poor, O-rich stars found in NGC 6441 agrees well with that of stars on the red horizontal branch of this cluster (in both cases about 80%), with a sloping distribution toward lower values of [O/Na] (among RGB stars and bluer colors (among HB stars).
The metal-rich and old bulge globular cluster (GC) NGC 6388 is one of the most massive Galactic GCs (M ~ 10^6 Msun). However, the spectroscopic properties of its multiple stellar populations rested only on 32 red giants (only seven of which observed with UVES, the remaining with GIRAFFE), given the difficulties in observing a rather distant cluster, heavily contaminated by bulge and disc field stars. We bypassed the problem using the largest telescope facility ever: the European Southern Observatory (ESO) archive. By selecting member stars identified by other programmes, we derive atmospheric parameters and the full set of abundances for 15 species from high resolution UVES spectra of another 17 red giant branch stars in NGC 6388. We confirm that no metallicity dispersion is appreciable in this GC. About 30% of stars show the primordial composition of first generation stars, about 20% present an extremely modified second generation composition, and half of the stars has an intermediate composition. The stars clearly distribute in the Al-O and Na-O planes into three discrete groups. We find substantial hints that more than a single class of polluters is required to reproduce the composition of the intermediate component in NGC 6388. In the heavily polluted component the sum Mg+Al increases as Al increases. The sum Mg+Al+Si is constant, and is the fossil record of hot H-burning at temperatures higher than about 70 MK in the first generation polluters that contributed to form multiple populations in this cluster.
We have used multi-band high resolution HST WFPC2 and ACS observations combined with wide field ground-based observations to study the blue straggler star (BSS) population in the galactic globular cluster NGC 6388. As in several other clusters we have studied, the BSS distribution is found to be bimodal: highly peaked in the cluster center, rapidly decreasing at intermediate radii, and rising again at larger radii. In other clusters the sparsely populated intermediate-radius region (or ``zone of avoidance) corresponds well to that part of the cluster where dynamical friction would have caused the more massive BSS or their binary progenitors to settle to the cluster center. Instead, in NGC 6388, BSS still populate a region that should have been cleaned out by dynamical friction effects, thus suggesting that dynamical friction is somehow less efficient than expected. As by-product of these observations, the peculiar morphology of the horizontal branch (HB) is also confirmed. In particular, within the (very extended) blue portion of the HB we are able to clearly characterize three sub-populations: ordinary blue HB stars, extreme HB stars, and blue hook stars. Each of these populations has a radial distribution which is indistinguishable from normal cluster stars.
Observations of the globular clusters NGC 6388 and M 15 were carried out by the H.E.S.S. array of Cherenkov telescopes for a live time of 27.2 and 15.2 hours respectively. No gamma-ray signal is found at the nominal target position of NGC 6388 and M 15. In the primordial formation scenario, globular clusters are formed in a dark matter halo and dark matter could still be present in the baryon-dominated environment of globular clusters. This opens the possibility of observing a dark matter self-annihilation signal. The dark matter content of the globular clusters NGC 6388 and M 15 is modelled taking into account the astrophysical processes that can be expected to influence the dark matter distribution during the evolution of the globular cluster: the adiabatic contraction of dark matter by baryons, the adiabatic growth of a black hole in the dark matter halo and the kinetic heating of dark matter by stars. 95% confidence level exclusion limits on the dark matter particle velocity-weighted annihilation cross section are derived for these dark matter haloes. In the TeV range, the limits on the velocity-weighted annihilation cross section are derived at the 10-25 cm3 s-1 level and a few 10-24 cm3 s-1 for NGC 6388 and M 15 respectively.
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