ALBERT

Description: We interpret the stellar population of Centauri by means of a population synthesis analysis, following the most recent observational guidelines for input metallicities, helium and [(C+N+O)/Fe] contents. We deal at the same time with the main sequences, sub-giant and horizontal branch (HB) data. The reproduction of the observed colour–magnitude features is very satisfying and bears interesting hints concerning the evolutionary history of this peculiar stellar ensemble. Our main results are: (1) no significant spread in age is required to fit the colour–magnitude diagram. Indeed, we can use coeval isochrones for the synthetic populations, and we estimate that the ages fall within a ~0.5 Gyr time interval; in particular the most metal-rich population can be coeval (in the above meaning) with the others, if its stars are very helium-rich ( Y ~ 0.37) and with the observed CNO enhancement ([(C+N+O)/Fe] = +0.7); (2) a satisfactory fit of the whole HB is obtained, consistent with the choice of the populations providing a good reproduction of the main sequence and sub-giant data; (3) the split in magnitude observed in the red HB is well reproduced assuming the presence of two stellar populations in the two different sequences observed: a metal-poor population made of stars evolving from the blue side (luminous branch) and a metal richer one whose stars are in a stage closer to the zero age HB (dimmer branch). This modelization also fits satisfactorily the period and the [Fe/H] distribution of the RR Lyrae stars.

Description: We interpret the stellar population of Centauri by means of a population synthesis analysis, following the most recent observational guidelines for input metallicities, helium and [(C+N+O)/Fe] contents. We deal at the same time with the main sequences, sub-giant and horizontal branch (HB) data. The reproduction of the observed colour–magnitude features is very satisfying and bears interesting hints concerning the evolutionary history of this peculiar stellar ensemble. Our main results are: (1) no significant spread in age is required to fit the colour–magnitude diagram. Indeed, we can use coeval isochrones for the synthetic populations, and we estimate that the ages fall within a ~0.5 Gyr time interval; in particular the most metal-rich population can be coeval (in the above meaning) with the others, if its stars are very helium-rich ( Y ~ 0.37) and with the observed CNO enhancement ([(C+N+O)/Fe] = +0.7); (2) a satisfactory fit of the whole HB is obtained, consistent with the choice of the populations providing a good reproduction of the main sequence and sub-giant data; (3) the split in magnitude observed in the red HB is well reproduced assuming the presence of two stellar populations in the two different sequences observed: a metal-poor population made of stars evolving from the blue side (luminous branch) and a metal richer one whose stars are in a stage closer to the zero age HB (dimmer branch). This modelization also fits satisfactorily the period and the [Fe/H] distribution of the RR Lyrae stars.

Description: We explain the multiple populations recently found in the ‘prototype’ globular cluster (GC) NGC 2808 in the framework of the asymptotic giant branch (AGB) scenario. The chemistry of the five – or more – populations is approximately consistent with a sequence of star formation events, starting after the Type II supernova epoch, lasting approximately until the time when the third dredge-up affects the AGB evolution (age ~90–120 Myr), and ending when the Type Ia supernovae begin exploding in the cluster, eventually clearing it from the gas. The formation of the different populations requires episodes of star formation in AGB gas diluted with different amounts of pristine gas. In the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV Legacy Survey of GCs, the nitrogen increase is due to the third dredge-up in the smallest mass AGB ejecta involved in the star formation of this population. The possibly iron-rich small population in NGC 2808 may be a result of contamination by a single Type Ia supernova. The NGC 2808 case is used to build a general framework to understand the variety of ‘second-generation’ stars observed in GCs. Cluster-to-cluster variations are ascribed to differences in the effects of the many processes and gas sources which may be involved in the formation of the second generation. We discuss an evolutionary scheme, based on pollution by delayed Type II supernovae, which accounts for the properties of s-Fe-anomalous clusters.

Description: We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the horizontal branch (HB) population of the Galactic globular cluster (GC) NGC 2419. A detailed analysis of the composite HB highlights three populations: (1) the blue luminous HB, hosting standard helium stars ( Y  = 0.25) with a very small spread of mass; (2) a small population of stars with intermediate helium content (0.26 〈  Y   0.29); and (3) the well-populated extreme HB. We can fit the last group with models having high helium abundance ( Y  ~ 0.36), half of which (the hottest part, ‘blue hook’ stars) are identified as possible ‘late flash mixed stars’. The initial helium abundance of this extreme population is in nice agreement with the predicted helium abundance in the ejecta of massive asymptotic giant branch (AGB) stars of the same metallicity as NGC 2419. This result further supports the hypothesis that second-generation stars in GCs formed from the ashes of intermediate-mass AGB stars. We find that the distribution in magnitude of the blue hook stars is larger than that predicted by theoretical models. We discuss the possible uncertainties in the magnitude scales and different attempts to model this group of stars. Finally, we suggest that consistency can be better achieved if we assume core masses larger than predicted by our models. This may be possible if the progenitors were fast rotators on the main sequence. If further study confirms this interpretation, a fast initial rotation would be a strong signature of the peculiarity of extreme second-generation stars in GCs.

Description: The Hubble Space Telescope UV Legacy survey of Galactic globular clusters (GC) is characterizing many different aspects of their multiple stellar populations. The ‘Grundahl-jump’ (G-jump) is a discontinuity in ultraviolet brightness of blue horizontal branch (HB) stars, signalling the onset of radiative metal levitation. The HB Legacy data confirmed that the G-jump is located at the same T eff (~=11 500 K) in nearly all clusters. The only exceptions are the metal-rich clusters NGC 6388 and NGC 6441, where the G-jump occurs at T eff ~= 13–14 000 K. We compute synthetic HB models based on new evolutionary tracks including the effect of helium diffusion, and approximately accounting for the effect of metal levitation in a stable atmosphere. Our models show that the G-jump location depends on the interplay between the time-scale of diffusion and the time-scale of the evolution in the T eff range 11 500 K T eff 14 000 K. The G-jump becomes hotter than 11 500 K only for stars that have, in this T eff range, a helium mass fraction Y 0.35. Similarly high Y values are also consistent with the modelling of the HB in NGC 6388 and NGC 6441. In these clusters, we predict that a significant fraction of HB stars show helium in their spectra above 11 500 K, and full helium settling should only be found beyond the hotter G-jump.

Description: All models for the formation of multiple populations in globular clusters (GCs) imply an initial mass of the systems several times greater than the present mass. A recent study of the dwarf spheroidal galaxy Fornax, where the low-metallicity ([Fe/H]  –2) stars contained in GCs appear to account for ~20 per cent of the total number, seems to constrain the initial mass of the four low-metallicity GCs in Fornax to be at most a factor of 5–6 greater than their present mass. We examine the photometric data for Fornax clusters, focusing our attention on their horizontal branch (HB) colour distribution and, when available, on the fraction and period distribution of RR Lyrae variables. Based on our understanding of the HB morphology in terms of varying helium content (and red giant mass-loss rate) in the context of multiple stellar generations, we show that the clusters F2, F3 and F5 must contain substantial fractions of second-generation stars (~54–65 per cent). On the basis of a simple chemical evolution model we show that the helium distribution in these clusters can be reproduced by models with cluster initial masses ranging from values equal to ~4 to ~10 times greater than the current masses. Models with a very short second-generation star formation episode can also reproduce the observed helium distribution but require greater initial masses up to about 20 times the current mass. While the lower limit of this range of possible initial GC masses is consistent with those suggested by observations of the low-metallicity field stars, we also discuss the possibility that the metallicity scale of field stars (based on Ca  ii triplet spectroscopy) and the metallicities derived for the clusters in Fornax may not be consistent with each other. In this case, observational constraints would allow greater initial cluster masses. Two interesting hypotheses are needed in order to reproduce the HB morphology of the clusters F2, F3 and F5. (i) The first-generation HB stars all lie at ‘red’ colours; that is, they populate only the RR Lyraes and the red HB region. According to this interpretation, the low-metallicity stars in the field of Fornax, populating the HB at colours bluer than the blue side [( V – I ) 0 0.3 or ( B – V ) 0 0.2] of the RR Lyraes, should be second-generation stars born in the clusters. A preliminary analysis of available colour surveys of Fornax field provides a fraction ~20 per cent of blue HB stars, in the low-metallicity range. (ii) The mass loss from individual second-generation red giants is a few per cent of a solar mass larger than the mass loss from first-generation stars.

Description: We use the combination of photometric and spectroscopic data of 47 Tuc stars to reconstruct the possible formation of a second generation of stars in the central regions of the cluster, from matter ejected by massive asymptotic giant branch (AGB) stars diluted with pristine gas. The yields from massive AGB stars with the appropriate metallicity ( Z  = 0.004, i.e. [Fe/H] = –0.75) are compatible with the observations in terms of extension and slope of the patterns observed, involving oxygen, nitrogen, sodium and aluminium. Based on the constraints on the maximum helium of 47 Tuc stars provided by photometric investigations and on the helium content of the ejecta, we estimate that the gas out of which second-generation stars formed was composed of about one-third of gas from intermediate-mass stars, with M  ≥ 5 M and about two-thirds of pristine gas. We tentatively identify the few stars whose Na, Al and O abundances resemble the undiluted AGB yields with the small fraction of 47 Tuc stars populating the faint subgiant branch. From the relative fraction of first- and second-generation stars currently observed, we estimate that the initial first-generation population in 47 Tuc was about 7.5 times more massive than the cluster current total mass.

Description: The evolutionary status of the low-mass X-ray binary SAX J1808.4 − 3658 is simulated by following the binary evolution of its possible progenitor system through mass transfer, starting at a period of ∼6.6 h. The evolution includes angular momentum losses via magnetic braking and gravitational radiation. It also takes into account the effects of illumination of the donor by both the X-ray emission and the spin down luminosity of the pulsar. The system goes through stages of mass transfer and stages during which it is detached, where only the rotationally powered pulsar irradiates the donor. We show that the pulsar irradiation is a necessary ingredient to reach SAX J1808.4 − 3658 orbital period when the donor mass is reduced to 0.04–0.06 M⊙. We also show that the models reproduce important properties of the system, including the orbital period derivative, which is shown to be directly linked to the evolution through mass transfer cycles. Moreover, we find that the effects of the irradiation on the internal structure of the donor are non-negligible, causing the companion star to be non-completely convective at the values of mass observed for the system and significantly altering its long term evolution, as the magnetic braking remains active along the whole evolution.