Multiple Generations Of Stars Research Articles

Lithium in Turnoff Stars in the Globular Cluster M5: A Quest for Primordial Lithium

The light element lithium is formed by nucleosynthesis during the Big Bang. Its abundance can help to define the parameters of the early Universe. To find this primordial value, it is necessary to determine Li abundances in the oldest stars because it is readily destroyed by nuclear reactions in stellar interiors. We have made high-resolution (∼45,000) spectroscopic observations of five identical unevolved turnoff stars in the 13 Gyr old globular cluster M5. In our analysis we find a range in Li abundance of a factor of 2; the spread is 5 times the individual error. The comparison of these results with those for turnoff stars from five other globular clusters reveals a similarly large range in Li. Lithium in M5 and the other clusters all have stars above the field star Li plateau, but none are as high as the predictions for primordial Li. The maximum values for Li are the same in all six clusters. Multiple generations of stars are found in many globular clusters; those later generations are expected to have formed from Li-depleted gas. Such second- and later-generation stars would have no Li. However, only one of the six clusters has a few unevolved stars with upper limits on the Li abundance.

Modelling the formation of two stellar generations in massive star clusters: the case of 30 Doradus

ABSTRACT We study the evolution of embedded star clusters with the goal to reproduce 30 Doradus, specifically the compact star cluster known as R136 and its surrounding stellar envelope, which is believed to be part of an earlier star formation event. We employ the high-precision stellar dynamics code Nbody6+ + GPU to calculate the dynamics of the stars embedded in different evolving molecular clouds modelled with the 1D cloud/clusters code warpfield. We explore clouds with initial masses of Mcloud = 3.16 × 105 M⊙ that (re)-collapse allowing for the birth of a second generation. We explore different star formation efficiencies to find the best set of parameters that can reproduce the observations. Our best-fit models correspond to a first generation of stars with a total mass M in the range $1.26 \!-\! 2.85\times \,\,10^4\,$ M⊙. As the initial stellar feedback is insufficient to unbind the parental cloud, the gas re-collapses after about 2–4 million years and builds up a second generation of stars with M ≈ 6.32 × 104 M⊙. We can match the observed stellar ages, the radius of the shell of swept up cloud material, and the fact that the second generation of stars is more concentrated than the first one. This is independent of the cluster starting out with mass segregation or without. By comparing with recent measurements of mass segregation and density profile in the central region of the cluster we again find close agreement, providing further evidence for a re-collapse scenario building up multiple generations of stars in 30 Doradus.

Proton partial widths evaluation through the 30Si(3He,d)31P transfer reaction for understanding abundance anomalies in Globular Clusters

Some observed abundances in globular clusters have suggested the existence of multiple generations of stars within the clusters as the observations require temperature ranges higher than current stars. The 30Si(p,γ)31P reaction plays a key role in the synthesis of the observed abundances. The study of the 30Si(3He,d)31P transfer reaction is a tool for constraining the strengths of low-lying resonances, and the proton partial widths are the main ingredients for calculating those strengths. We present the method used for estimating the proton partial widths and their associated uncertainties.

No evidence for significant age spreads in young massive LMC clusters

Recent discoveries have put the picture of stellar clusters being simple stellar populations into question. In particular, the color-magnitude diagrams of intermediate age (1-2 Gyr) massive clusters in the Large Magellanic Cloud (LMC) show features that could be interpreted as age spreads of 100-500 Myr. If multiple generations of stars are present in these clusters then, as a consequence, young (<1 Gyr) clusters with similar properties should have age spreads of the same order. In this paper we use archival Hubble Space Telescope (HST) data of eight young massive LMC clusters (NGC 1831, NGC 1847, NGC 1850, NGC 2004, NGC 2100, NGC 2136, NGC 2157 and NGC 2249) to test this hypothesis. We analyzed the color-magnitude diagrams of these clusters and fitted their star formation history to derive upper limits of potential age spreads. We find that none of the clusters analyzed in this work shows evidence for an extended star formation history that would be consistent with the age spreads proposed for intermediate age LMC clusters. Tests with artificial single age clusters show that the fitted age dispersion of the youngest clusters is consistent with spreads that are purely induced by photometric errors. As an additional result we determined a new age of NGC 1850 of ~100 Myr, significantly higher than the commonly used value of about 30 Myr, although consistent with early HST estimates.

Updated properties of the old open cluster Melotte 66: Searching for multiple stellar populations

Multiple generations of stars are routinely encountered in globular clusters but no convincing evidence has been found in Galactic open clusters to date. In this paper we use new photometric and spectroscopic data to search for multiple stellar population signatures in the old, massive open cluster, Melotte~66. The cluster is known to have a red giant branch wide in color, which could be an indication of metallicity spread. Also the main sequence is wider than what is expected from photometric errors only. This evidence might be associated with either differential reddening or binaries. Both hypothesis have, however, to be evaluated in detail before recurring to the presence of multiple stellar populations. New, high-quality, CCD UBVI photometry have been acquired to this aim with high-resolution spectroscopy of seven clump stars, that are complemented with literature data. Our photometric study confirms that the width of the main sequence close to the turn off point is entirely accounted for by binary stars and differential reddening, with no need to advocate more sofisticated scenarios, such as metallicity spread or multiple main sequences. By constructing synthetic color-magnitude diagrams, we infer that the binary fraction has to be as large as 30$%$ and their mass ratio in the range 0.6-1.0. As a by-product of our simulations, we provide new estimates of the cluster fundamental parameters. We measure a reddening E(B-V)=0.15$\pm$0.02, and confirm the presence of a marginal differential reddening. The distance to the cluster is $4.7^{+0.2}_{-0.1} $kpc and the age is 3.4$\pm$0.3 Gyr, which is somewhat younger and better constrained than previous estimates. Our detailed abundance analysis reveals that, overall, Melotte~66 looks like a typical object of the old thin disk population.

Andromeda and the Seven Dwarfs

With the new generation of wide-field surveys, our understanding of the Andromeda satellite system has dramatically improved in recent years. Since 2004, 12 new dwarf galaxies have been discovered around Andromeda, doubling the number of previously known companions. What are the properties of these newly discovered dwarfs and what do they tell us about galaxy formation? Are these systems bound? Do they show evidence of multiple star formation epochs? To better understand the stellar populations of the faintest dwarfs around M 31, we have used the Large Binocular Camera (LBT) and Suprime-Cam (Subaru) to obtain photometric observations of And X, And XVII, And XVIII, And XIX, And XX, And XXI and And XXII. Reaching below the horizontal branch, these observations have allowed for accurate distance determinations to be made, together with metallicity estimates based on the red giant branch stars. Our analysis shows our 7 dwarfs to be metal poor and with large spreads in [Fe/H], it strongly suggests multiple generations of stars are present in And X, And XVII, And XVIII, and And XIX, And XXI.

Star Formation and Feedback in Dwarf Galaxies

We examine the star formation history and stellar feedback effects of dwarf galaxies under the influence of extragalactic ultraviolet radiation. Previous work has indicated that the background UV flux can easily ionize the gas within typical dwarf galaxies, delaying or even preventing cooling and star formation within them. Many dwarf galaxies within the Local Group are, however, observed to contain multiple generations of stars, the oldest of which formed in the early epochs of cosmic evolution, when the background UV flux was intense. In order to address this paradox, we consider the dynamical evolution of gas in dwarf galaxies using a one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute the effects of radiative transfer and photoionization. We include a physically motivated star formation recipe and consider the effects of feedback. This scheme allows us to follow the history of the gas and of star formation within dwarf galaxies, as influenced by both external and internal UV radiation. Our results indicate that star formation in the severe environment of dwarf galaxies is a difficult and inefficient process. In potentials with total mass less than a few times 106 M☉ and velocity dispersion less than a few kilometers per second, residual gas is efficiently photoionized by cosmic background UV radiation. Since the density scale height of the gas within these galaxies is comparable to their size, gas may be tidally removed from them, leaving behind starless residual dark matter clumps. For intermediate-mass systems, such as the dSphs around the Galaxy, star formation can proceed within early cosmic epochs despite the intense background UV flux. Triggering processes such as merger events, collisions, and tidal disturbance can lead to density enhancements, reducing the recombination timescale, allowing gas to cool and star formation to proceed. However, the star formation and gas retention efficiency may vary widely in galaxies with similar dark matter potentials because they depend on many factors, such as the baryonic fraction, external perturbation, initial mass function, and background UV intensity. We suggest that the presence of very old stars in these dwarf galaxies indicates that their initial baryonic-to-dark matter content was comparable to the cosmic value. This constraint suggests that the initial density fluctuation of baryonic matter may be correlated with that of the dark matter. For the more massive dwarf elliptical galaxies, the star formation efficiency and gas retention rate are much higher. Their mass-to-light ratio is regulated by star formation feedback and is expected to be nearly independent of their absolute luminosity. The results of our theoretical models reproduce the observed (M/L)-Mv correlation.

Supernova Enrichment of Dwarf Spheroidal Galaxies

Many dwarf galaxies exhibit subsolar metallicities, with some star-to-star variation, despite often containing multiple generations of stars. The total metal content in these systems is much less than expected from the heavy-element production of massive stars in each episode of star formation. Such a deficiency implies that a substantial fraction of the enriched material has been lost from these small galaxies. Mass ejection from dwarf galaxies may have important consequences for the evolution of the intergalactic medium and for the evolution of massive galaxies, which themselves may have formed via the merger of smaller systems. We report here the results of three-dimensional simulations of the evolution of supernova-enriched gas within dwarf spheroidal galaxies (dSphs), with the aim of determining the retention efficiency of supernova ejecta. We consider two galaxy models, selected to represent opposite ends of the dSph sequence. One contains 106 M☉ of gas, and the other 5.5 × 106 M☉. In both, the baryonic-to-dark matter ratio is assumed to be 0.1. The total binding energies of the gas in the two models are 9.8 × 1050 and 1.6 × 1052 ergs. For each model galaxy we investigate a number of scenarios. The simplest is a single supernova within a smooth gas distribution. We also investigate the evolution of 10 supernovae, within initially smooth gas distributions, occurring over time spans of either 10 or 100 Myr. Finally, we investigate the effects of 10 supernovae occurring over 10 Myr in a medium filled with hot bubbles, such as would be expected in the presence of an initial generation of hot stars. For models with only a single supernova, no enriched material is lost from the galaxies. When multiple supernovae occur within an initially smooth gas distribution, less than one-half the enriched gas is lost from the galaxy (fractional losses range from 0% to 47%). Most of the enriched gas is lost, however, from the cores of the galaxies. In the presence of an initially disturbed gas distribution, 6% or less of the enriched gas remains in the core, and much is lost from the galaxies as a whole (47% and 71% for the larger and smaller galaxy models, respectively). If subsequent star formation occurs predominantly within the core where most of the residual gas is concentrated, then these results could explain the poor self-enrichment efficiency observed in dwarf galaxies.