R. T. Rood

A Helium Spread among the Main-Sequence Stars in NGC 2808

We have made a detailed study of the color distribution of the main sequence of the globular cluster (GC) NGC 2808, based on new deep HST WFPC2 photometry of a field in the uncrowded outskirts of the cluster. The observed color distribution of main-sequence stars is not Gaussian and is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. Using synthetic color-magnitude diagrams based on new stellar models, we estimate that the helium mass fraction of these stars is Y ~ 0.4. The newly found anomaly on the main sequence gives credence to the idea that GCs like NGC 2808 have undergone self-enrichment and that different stellar populations were born from the ejecta of the intermediate-mass asymptotic giant branch (AGB) stars of the first generation. The enhancement and spread of helium among the stars in NGC 2808 have recently been suggested as a simple way to explain the very peculiar morphology of its horizontal branch. We find that if in addition to the Y = 0.40 stars, roughly 30% of the stars have Y distributed between 0.26-0.29, while 50% have primordial Y, this leads to a horizontal-branch morphology similar to that observed. In this framework, three main stages of star formation are identified, the first with primordial helium content Y 0.24, the second born from the winds of the most massive AGBs of the first stellar generation (~6-7 M☉), with Y ~ 0.4, and a third born from the matter ejected from less massive AGBs (~3.5-4.5 M☉), with Y ~ 0.26-0.29. There could have been a long hiatus (several times 107 yr), between the second and third generation in which no star formed in the protocluster. We suggest that during this period, star formation has been inhibited by the explosion of late Type II supernovae deriving from binary evolution.

The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge

Globular star clusters are compact and massive stellar systems old enough to have witnessed the entire history of our Galaxy, the Milky Way. Although recent results suggest that their formation may have been more complex than previously thought, they still are the best approximation to a stellar population formed over a relatively short timescale (less than 1u2009Gyr) and with virtually no dispersion in the iron content. Indeed, only one cluster-like system (ω Centauri) in the Galactic halo is known to have multiple stellar populations with a significant spread in iron abundance and age. Similar findings in the Galactic bulge have been hampered by the obscuration arising from thick and varying layers of interstellar dust. Here we report that Terzan 5, a globular-cluster-like system in the Galactic bulge, has two stellar populations with different iron contents and ages. Terzan 5 could be the surviving remnant of one of the primordial building blocks that are thought to merge and form galaxy bulges.

Two distinct sequences of blue straggler stars in the globular cluster M 30

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy’s history. ‘Blue stragglers’ are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in Mu200930. This globular cluster is thought to have undergone ‘core collapse’, during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.

Dynamical age differences among coeval star clusters as revealed by blue stragglers

Globular star clusters that formed at the same cosmic time may have evolved rather differently from the dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different ‘dynamical ages’). Blue straggler stars have masses greater than those at the turn-off point on the main sequence and therefore must be the result of either a collision or a mass-transfer event. Because they are among the most massive and luminous objects in old clusters, they can be used as test particles with which to probe dynamical evolution. Here we report that globular clusters can be grouped into a few distinct families on the basis of the radial distribution of blue stragglers. This grouping corresponds well to an effective ranking of the dynamical stage reached by stellar systems, thereby permitting a direct measure of the cluster dynamical age purely from observed properties.

The peculiar horizontal branch of NGC 2808

We present an accurate analysis of the peculiar horizontal branch (HB) of the massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and optical images of the central region of the cluster obtained with the Hubble Space Telescope. We confirm the multimodal distribution of stars along the HB: four sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modelling is possible for three HB groups by assuming different helium abundances in the range 0.24 <� Y < 0.4 that are consistent with the multiple populations observed in the main sequence; and (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so-called blue hook region. These objects are probably ‘hot flashers’, stars that peel off the red giant branch before reaching the tip and ignite helium at high T eff. Both of these conclusions are based on the luminosity of the HB in the optical and ultraviolet bands and do not depend on specific assumptions about mass loss.

The Pure Noncollisional Blue Straggler Population in the Giant Stellar System ω Centauri

We have used high spatial resolution data from the Hubble Space Telescope (HST) and wide-field ground-based observations to search for blue straggler stars (BSSs) over the entire radial extent of the large stellar system ? Centauri. We have detected the largest population of BSSs ever observed in any stellar system. Even though the sample is restricted to the brightest portion of the BSS sequence, more than 300 candidates have been identified. BSSs are thought to be produced by the evolution of binary systems (formed either by stellar collisions or mass exchange in binary stars). Since systems like Galactic globular clusters (GGCs) and ? Cen evolve dynamically on timescales significantly shorter than their ages, binaries should have settled toward the center, showing a more concentrated radial distribution than the ordinary, less massive single stars. Indeed, in all GGCs that have been surveyed for BSSs, the BSS distribution is peaked at the center. Conversely, in ? Cen we find that the BSSs share the same radial distribution as the adopted reference populations. This is the cleanest evidence ever found that such a stellar system is not fully relaxed even in the central region. We further argue that the absence of central concentration in the BSS distribution rules out a collisional origin. Thus, the ? Cen BSSs are the purest and largest population of noncollisional BSSs ever observed. Our results allow the first empirical quantitative estimate of the production rate of BSSs via this channel. BSSs in ? Cen may represent the best local template for modeling the BSS populations in distant galaxies where they cannot be individually observed.

Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process*

We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 blue straggler stars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs shows a significant depletion of carbon and oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.

Another Nonsegregated Blue Straggler Population in a Globular Cluster: the Case of NGC 2419*

We have used a combination of ACS HST high-resolution and wide-field Subaru data in order to study the blue straggler star (BSS) population over the entire extension of the remote Galactic globular cluster NGC 2419. The BSS population presented here is among the largest ever observed in any stellar system, with more than 230 BSSs in the brightest portion of the sequence. The radial distribution of the selected BSSs is essentially the same as that of the other cluster stars. In this sense the BSS radial distribution is similar to that of ω Centauri and unlike that of all Galactic globular clusters studied to date, which have highly centrally segregated distributions and, in most cases, a pronounced upturn in the external regions. As in the case of ω Centauri, this evidence indicates that NGC 2419 is not yet relaxed even in the central regions. This observational fact is in agreement with estimated half-mass relaxation time, which is of the order of the cluster age.

The Puzzling Dynamical Status of the Core of the Globular Cluster NGC 6752

We have used high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and ground-based wide-field images to determine the center of gravity and construct an extended radial density and brightness profile of the cluster NGC 6752 including, for the first time, detailed star counts in the very inner region. The barycenter of the nine innermost X-ray sources detected by Chandra is located only 19 off the new center of gravity. Both the density and the brightness profile of the central region are best fitted by a double King model, suggesting that NGC 6752 is experiencing a post-core-collapse bounce. Taking advantage of our new optical data, we discuss the puzzling nature of the accelerations displayed by the innermost millisecond pulsars detected in this cluster. We discuss two possible origins to the accelerations: (1) the overall cluster gravitational potential, which would require a central projected mass-to-light ratio of the order of 6-7 and the existence of a few thousand solar masses of low-luminosity matter within the inner 0.08 pc of NGC 6752, and (2) the existence of a local perturber(s) of the pulsar dynamics, such as a recently proposed binary black hole of intermediate (100-200 M☉) mass.

Blue Straggler Stars in the Unusual Globular Cluster NGC 6388

We have used multiband 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 BSSs or their binary progenitors to settle to the cluster center. Instead, in NGC 6388, BSSs 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 a 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 subpopulations: 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.