We have recently obtained a set of high-resolution images of Terzan 5 in the K and J bands by using MAD 6 , a Multi-Conjugate Adaptive Optics demonstrator instrument installed at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). MAD operates at near-infrared wavelengths, thus revealing the only component of stellar radiation that can efficiently cross the thick clouds of dust obscuring the Galactic bulge. It is able to perform exceptionally good and uniform adaptive optics correction over its entire field of view (1'x1'), thus compensating for the degradation effects to the astronomical images induced by the Earth's atmosphere. In particular, we have obtained a set of K-band (2.2m) images of Terzan 5 close to the diffraction limit (Fig. 1). The sharpness and uniformity of the images yields very high quality photometry, resulting in accurate (K, JK) colour-magnitude diagram (CMD) even for the very central region of the cluster, and leading to a surprising discovery. We have detected two well-defined red horizontal branch clumps, separated in luminosity: a bright horizontal branch (BHB) at K = 12.85 and a faint horizontal branch (FHB) at K = 13.15, the latter having a bluer (JK) colour (Fig. 2).We have carefully considered whether the double horizontal branch could be spurious. It is neither due to instrumental effects ( Fig. 2), nor to differential reddening 7,8 3 (as the two horizontal branch clumps in the CMD are separated in a direction which is essentially orthogonal to the reddening vector), nor to field contamination (while field stars are expected to be almost uniformly distributed over the MAD field of view, the radial distributions of the stars belonging to the two horizontal branch clumps are significantly concentrated toward the cluster centre and are inconsistent with a uniform distribution at more than 5 level; see Fig. 3a and Supplementary Information). We have also found that the radial distributions of the two horizontal branch populations are different ( Fig. 3a): according to a Kolmogorov-Smirnov test, the BHB population is significantly (at > 3.5level) more centrally concentrated than that of the FHB. The stars belonging to the BHB are also substantially more numerous than those of the FHB near the cluster centre (that is, at distances r < 20''), becoming progressively more rare at larger radii (Fig. 3b).Once alerted to the existence of the double horizontal branch, we have also (Fig. 4a).To date, apart from a significant spread in the abundance patterns of a few light elements (such as Na and O) 1 , the chemical composition of all globular clusters in the Galaxy is known to be extremely uniform in terms of iron content, with the only exception being  Centauri 4,5 in the Galactic halo. Hence, Terzan 5 is the first stellar aggregate discovered in the Galactic bulge that has globular-cluster-like properties but also with the signatures of a much more complex star formation history.To further investigate this issue, we have performed a differential reddening correc...

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.

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 M 30. 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.

We used a proper combination of high-resolution HST observations and wide-field ground based data to derive the radial star density profile of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This is the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that: (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13 and M62 is not confirmed; (2) the majority of clusters in our sample are fitted equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distances; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ∼ 0.3 for about 80% of the clusters, and a secondary peak at ∼ 0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with what expected from simulations of cluster dynamical evolution and the ratio between these two radii well correlates with an empirical dynamical age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.

We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low‐density Galactic globular clusters. By analysing the colour distribution of main‐sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.

We present new optical and near‐infrared (NIR) photometry and spectroscopy of the Type IIP supernova (SN), SN 2004et. In combination with already published data, this provides one of the most complete studies of optical and NIR data for any Type IIP SN from just after explosion to +500 d. The contribution of the NIR flux to the bolometric light curve is estimated to increase from 15 per cent at explosion to around 50 per cent at the end of the plateau and then declines to 40 per cent at 300 d. SN 2004et is one of the most luminous IIP SNe which has been well studied and characterized, and with a luminosity of log L= 42.3 erg s−1 and a 56Ni mass of 0.06 ± 0.04 M⊙, it is two times brighter than SN 1999em. We provide parametrized bolometric corrections as a function of time since explosion for SN 2004et and three other IIP SNe that have extensive optical and NIR data. These can be used as templates for future events in optical and NIR surveys without full wavelength coverage. We compare the physical parameters of SN 2004et with those of other well‐studied IIP SNe and find that the kinetic energies span a range of 1050–1051 erg. We compare the ejected masses calculated from hydrodynamic models with the progenitor masses and limits derived from pre‐discovery images. Some of the ejected mass estimates are significantly higher than the progenitor mass estimates, with SN 2004et showing perhaps the most serious mass discrepancy. With the current models, it appears difficult to reconcile 100 d plateau lengths and high expansion velocities with the low ejected masses of 5–6 M⊙ implied from 7–8 M⊙ progenitors. The nebular phase is studied using very late‐time Hubble Space Telescope photometry, along with optical and NIR spectroscopy. The light curve shows a clear flattening at 600 d in the optical and the NIR, which is likely due to the ejecta impacting on circumstellar material. We further show that the [O i] 6300, 6364 Å line strengths in the nebular spectra of four Type IIP SNe imply ejected oxygen masses of 0.5–1.5 M⊙.

The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses. These initial stellar masses are generally represented by the canonical IMF, which is defined as the result of one star formation event in an embedded cluster. As a consequence, the galaxy-wide IMF (gwIMF) should also be invariant and of the same form as the canonical IMF; gwIMF is defined as the sum of the IMFs of all star-forming regions in which embedded clusters form and spawn the galactic field population of the galaxy. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation, it is useful to relate the observed IMF to the gwIMF. Starting with the IMF determined in resolved star clusters, we apply the IGIMF-theory to calculate a comprehensive grid of gwIMF models for metallicities, [Fe/H] ∈ (−3, 1), and galaxy-wide star formation rates (SFRs), SFR ∈ (10−5, 105) M⊙ yr−1. For a galaxy with metallicity [Fe/H] < 0 and SFR > 1 M⊙ yr−1, which is a common condition in the early Universe, we find that the gwIMF is both bottom light (relatively fewer low-mass stars) and top heavy (more massive stars), when compared to the canonical IMF. For a SFR < 1 M⊙ yr−1 the gwIMF becomes top light regardless of the metallicity. For metallicities [Fe/H] > 0 the gwIMF can become bottom heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top heavy within the first few hundred Myr of the life of the galaxy and that it evolves into a bottom heavy gwIMF in the metal-enriched galactic centre. Using the gwIMF grids, we study the SFR−Hα relation and its dependency on metallicity and the SFR. We also study the correction factors to the Kennicutt SFRK − Hα relation and provide new fitting functions. Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star-forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for gas-consumption timescales and for the main sequence of galaxies. We explicitly discuss Leo P and ultra-faint dwarf galaxies.

Terzan 5 is a globular cluster-like stellar system in the Galactic bulge which has been recently found to harbor two stellar populations with different iron content and probably different ages. This discovery suggests that Terzan 5 may be the relic of a primordial building block that contributed to the formation of the Galactic bulge. Here we present a re-determination of the structural parameters (center of gravity, density and surface brightness profiles, total luminosity, and mass) of Terzan 5, as obtained from the combination of high-resolution (ESO-MAD and Hubble Space Telescope ACS-WFC) and wide-field (ESO-WFI) observations. We find that Terzan 5 is significantly less concentrated and more massive than previously thought. Still it has the largest collision rate of any stellar aggregate in the Galaxy. We discuss the impact of these findings on the exceptional population of millisecond pulsars harbored in this stellar system.