Long-lived Low-mass Stars Research Articles

Mass segregation and velocity dispersion as evidence for a dark star cluster

ABSTRACT A dark star cluster (DSC) is a system in which the cluster potential is dominated by stellar remnants, such as black holes and neutron stars having larger masses than the long-lived low-mass stars. Due to mass segregation, these remnants are located in the central region of the cluster and form a dark core. We expect that at a few kpc from the Galactic Centre, the efficient evaporation of the lower-mass stars caused by the strong tidal force exposes the dark core, because the dynamical properties of the DSC are dominated by the remnants. Due to the invisibility of the remnants, finding a DSC by observation is challenging. In this project, we use N-body simulations to obtain models of DSCs and try to discern observables that signify a DSC. We consider four observables: the mass spectrum, the observational mass density profile, the observational velocity dispersion profile and the mass segregation. The models show that a DSC typically exhibits two distinct characteristics: for a given mass in stars and a given half-light radius, the expected velocity dispersion is underestimated when only visible stars are considered, and there is a lack of measurable mass segregation among the stars. These properties can be helpful for finding DSCs in observational data, such as the Gaia catalogue.

Exploring the production and depletion of lithium in the Milky Way stellar disk

Despite the recent availability of large samples of stars with high-precision Li abundances, there are many unanswered questions about the evolution of this unique element in the Galaxy and in the stars themselves. It is unclear which parameters and physical mechanisms govern Li depletion in late-type stars and if Galactic enrichment has proceeded differently in different stellar populations. With this study we aim to explore these questions further by mapping the evolution of Li with stellar mass, age, and effective temperature for Milky Way disk stars, linking the metal-poor and metal-rich regimes, and how Li differs in the thin and thick disks. We determine Li abundances for a well-studied sample of 714 F and G dwarf, turn-off, and subgiant stars in the solar neighbourhood. The analysis is based on line synthesis of the 7Li line at 6707 Å in high-resolution and high-signal-to-noise ratio echelle spectra, obtained with the MIKE, FEROS, SOFIN, UVES, and FIES spectrographs. The presented Li abundances are corrected for non-LTE effects. Out of the sample of 714 stars, we are able to determine Li abundances for 394 stars and upper limits on the Li abundance for another 121 stars. Out of 36 stars that are listed as exoplanet host stars, 18 have well-determined Li abundances and 6 have Li upper limits. Our main finding is that there are no signatures of Li production in stars associated with the thick disk. Instead the Li abundance trend is decreasing with metallicity for these thick disk stars. Significant Li production is however seen in the thin disk, with a steady increase towards super-solar metallicities. At the highest metallicities, however, around [Fe/H] ≈ +0.3, we tentatively confirm the recent discovery that the Li abundances level out. Our finding contradicts the other recent studies that found that Li is also produced in the thick disk. We find that this is likely due to the α-enhancement criteria which those studies used to define their thick disk samples. By using the more robust age criteria, we are able to define a thick disk stellar sample that is much less contaminated by thin disk stars. Furthermore, we also tentatively confirm the age-Li correlation for solar twin stars, and we find that there is no correlation between Li abundance and whether the stars have detected exoplanets or not. The major conclusion that can be drawn from this study is that no significant Li production relative to the primordial abundance took place during the first few billion years of the Milky Way, an era coinciding with the formation and evolution of the thick disk. Significant Li enrichment then took place once long-lived low-mass stars (acting on a timescale longer than SNIa) had had time to contribute to the chemical enrichment of the interstellar medium.

Are some CEMP-s stars the daughters of spinstars?

CEMP-s stars are long-lived low-mass stars with a very low iron content as well as overabundances of carbon and s-elements. Their peculiar chemical pattern is often explained by pollution from a AGB star companion. Recent observations have shown that most of the CEMP-s stars are in a binary system, providing support to the AGB companion scenario. A few CEMP-s stars, however, appear to be single. We inspect four apparently single CEMP-s stars and discuss the possibility that they formed from the ejecta of a previous-generation massive star, referred to as the "source" star. In order to investigate this scenario, we computed low-metallicity massive star models with and without rotation and including complete s-process nucleosynthesis. We find that non-rotating source stars cannot explain the observed abundance of any of the four CEMP-s stars. Three out of the four CEMP-s stars can be explained by a $25$ $M_{\odot}$ source star with $v_{\rm ini} \sim 500$ km s$^{-1}$ (spinstar). The fourth CEMP-s star has a high Pb abundance that cannot be explained by any of the models we computed. Since spinstars and AGB predict different ranges of [O/Fe] and [ls/hs], these ratios could be an interesting way to further test these two scenarios.

Predicting the locations of possible long-lived low-mass first stars: importance of satellite dwarf galaxies

The search for metal-free stars has so far been unsuccessful, proving that if there are surviving stars from the first generation, they are rare, they have been polluted, or we have been looking in the wrong place. To predict the likely location of Population~III (Pop~III) survivors, we semi-analytically model early star formation in progenitors of Milky Way-like galaxies and their environments. We base our model on merger trees from the high-resolution dark matter only simulation suite \textit{Caterpillar}. Radiative and chemical feedback are taken into account self-consistently, based on the spatial distribution of the haloes. Our results are consistent with the non-detection of Pop III survivors in the Milky Way today. We find that possible surviving Population III stars are more common in Milky Way satellites than in the main Galaxy. In particular, low mass Milky Way satellites contain a much larger fraction of Pop~III stars than the Milky Way. Such nearby, low mass Milky Way satellites are promising targets for future attempts to find Pop~III survivors, especially for high-resolution, high signal-to-noise spectroscopic observations. We provide the probabilities for finding a Pop~III survivor in the red giant branch phase for all known Milky Way satellites to guide future observations.

Pre-supernova mixing in CEMP-no source stars

Context. CEMP-no stars are long-lived low-mass stars with a very low iron content, overabundances of carbon and no or minor signs for the presence of s- or r-elements. Although their origin is still a matter of debate, they are often considered as being made of a material ejected by a previous stellar generation (source stars). Aims. We place constraints on the source stars from the observed abundance data of CEMP-no stars. Methods. We computed source star models of 20, 32, and 60 M$_{\odot}$ at Z = 10$^{-5}$ with and without fast rotation. For each model we also computed a case with a late mixing event occurring between the hydrogen and helium-burning shell $\sim$ 200 yr before the end of the evolution. This creates a partially CNO-processed zone in the source star. We use the 12C/13C and C/N ratios observed on CEMP-no stars to put constraints on the possible source stars (mass, late mixing or not). Then, we inspect more closely the abundance data of six CEMP-no stars and select their preferred source star(s). Results. Four out of the six CEMP-no stars studied cannot be explained without the late mixing process in the source star. Two of them show nucleosynthetic signatures of a progressive mixing (due e.g. to rotation) in the source star. We also show that a 20 M$_{\odot}$ source star is preferred compared to one of 60 M$_{\odot}$ and that likely only the outer layers of the source stars were expelled to reproduce the observed 12C/13C. Conclusions. The results suggest that (1) a late mixing process could operate in some source stars, (2) a progressive mixing, possibly achieved by fast rotation, is at work in several source stars, (3) $\sim$ 20 M$_{\odot}$ source stars are preferred compared to $\sim$ 60 M$_{\odot}$ ones, and (4) the source star might have preferentially experienced a low energetic supernova with large fallback.

Evolution of long-lived globular cluster stars

Context. Globular clusters host multiple populations of long-lived low-mass stars whose origin remains an open question. Several scenarios have been proposed to explain the associated photometric and spectroscopic peculiarities. They differ, for instance, in the maximum helium enrichment they predict for stars of the second population, which these stars can inherit at birth as the result of the internal pollution of the cluster by different types of stars of the first population. Aims. We present the distribution of helium-rich stars in present-day globular clusters as it is expected in the original framework of the fast-rotating massive stars scenario (FRMS) as first-population polluters. We focus on NGC 6752. (to be continued)

Preserving chemical signatures of primordial star formation in the first low-mass stars

We model early star forming regions and their chemical enrichment by Population III (Pop III) supernovae with nucleosynthetic yields featuring high [C/Fe] ratios and pair-instability supernova (PISN) signatures. We aim to test how well these chemical abundance signatures are preserved in the gas prior to forming the first long-lived low-mass stars (or second-generation stars). Our results show that second-generation stars can retain the nucleosynthetic signature of their Pop III progenitors, even in the presence of nucleosynthetically normal Pop III core-collapse supernovae. We find that carbon-enhanced metal-poor stars are likely second-generation stars that form in minihaloes. Furthermore, it is likely that the majority of Pop III supernovae produce high [C/Fe] yields. In contrast, metals ejected by a PISN are not concentrated in the first star forming haloes, which may explain the absence of observed PISN signatures in metal-poor stars. We also find that unique Pop III abundance signatures in the gas are quickly wiped out by the emergence of Pop II supernovae. We caution that the observed fractions of stars with Pop III signatures cannot be directly interpreted as the fraction of Pop III stars producing that signature. Such interpretations require modelling the metal enrichment process prior to the second-generation stars' formation, including results from simulations of metal mixing. The full potential of stellar archaeology can likely be reached in ultra-faint dwarf galaxies, where the simple formation history may allow for straightforward identification of second-generation stars.

A hint of normality at last?

The chemical diversity of the oldest stars is greater than we thought. The discovery of an extremely iron-poor star with a 'normal' ratio of carbon to iron challenges our perception of early chemical enrichment. See Letter p.67 For theoretical reasons and because of an apparent absence of stars with low metallicities (abundance of elements heavier than helium), it has been suggested that low-mass stars cannot form until the interstellar medium has been enriched above a critical value of metallicity, Z, estimated as lying between 1.5 × 10−8 and 1.5 × 10−6. Caffau et al. now describe a star with a primordial-type composition, suggesting that, in fact, long-lived low-mass stars can form when the concentration of complex nuclei is low. The star is in the Galactic halo, has very low metallicity (Z ≤ 6.9 × 10−7) and no enrichment of carbon, nitrogen or oxygen. Its chemical composition should provide clues as to how the first stars formed.

The role of HD cooling in primordial star formation

The role of HD cooling in the formation of primordial objects is examined by means of a great number of 1-D models of the collapse of halos, exploring a wide range of masses and virialization redshifts. We find that HD has very little effect upon the critical mass separating the objects which are likely to form stars from those which are not. We also find that, once the proto-stellar collapse has started, HD effects are quite negligible. Instead, HD effects can be important during the intermediate stage of gas fragmentation: objects below a certain mass scale (� 3×10 5 M⊙ at zvir = 20 in our “fiducial” case) can be cooled by HD down to T � 50 100 K, whereas H2 cooling never takes the gas below T � 200 K. The lower temperature implies a reduction of a factor � 10 in the Jeans mass of the fragmenting gas, and stars forming in such low-mass halos are probably less massive than their counterparts in larger halos. We estimate the importance of this mode of star formation through a variation of the Press-Schechter formalism, and find that it never exceeds the contribution of halos which are cooled by H2 only. Halos where HD is important account at best for a fraction �0.25 of the total primordial star formation. However, HD cooling might provide a channel through which long-lived low mass stars could be formed in primordial conditions.

Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters

Aims. We propose the Wind of Fast Rotating Massive Stars scenario to explain the origin of the abundance anomalies observed in globular clusters. Methods. We compute and present models of fast rotating stars with initial masses between 20 and 120 M ⊙ for an initial metallicity Z = 0.0005 ([Fe/H] ≃ -1.5). We discuss the nucleosynthesis in the H-burning core of these objects and present the chemical composition of their ejecta. We consider the impact of uncertainties in the relevant nuclear reaction rates. Results. Fast rotating stars reach critical velocity at the beginning of their evolution and remain near the critical limit during the rest of the main sequence and part of the He-burning phase. As a consequence they lose large amounts of material through a mechanical wind which probably leads to the formation of a slow outflowing disk. The material in this slow wind is enriched in H-buming products and presents abundance patterns similar to the chemical anomalies observed in globular cluster stars. In particular, the C, N, O, Na and Li variations are well reproduced by our model. However the rate of the 24 Mg(p, y) has to be increased by a factor 1000 around 50 x 10 6 K in order to reproduce the amplitude of the observed Mg-Al anticorrelation. We discuss how the long-lived low-mass stars currently observed in globular clusters could have formed out of the slow wind material ejected by massive stars.

M82-F: a doomed super star cluster?

We present high dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) `F' in M82, obtained with the 4.2-m William Herschel Telescope (WHT), for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster. We measure a stellar velocity dispersion of 13.4 +/- 0.7 km/s, a projected half-light radius of 2.8 +/- 0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2 +/- 0.1 x 10^6 solar masses, demonstrating that M82-F is a very massive, compact cluster. We determine that the current visual luminosity-to-mass ratio for M82-F is 45 +/- 13. Comparison with spectral synthesis models shows that the visual luminosity-to-mass ratio is a factor of 5 higher than that predicted for a standard Kroupa (2001) initial mass function (IMF) at the well-determined age for M82-F of 60 +/- 20 Myr. This high value of the visual luminosity-to-mass ratio indicates a deficit of low mass stars in M82-F; the current mass function (MF) evidently is `top-heavy'. We find that a lower mass cutoff of 2-3 solar masses is required to match the observations for a MF with a slope of 2.3. Since the cluster apparently lacks long-lived low mass stars, it will not become an old globular cluster. We also derive up-dated luminosity-to-mass ratios for the younger SSCs NGC 1569A and NGC 1705-1. We discuss the implications of our findings in the context of large scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster.

An extraordinary cluster of massive stars near the centre of the Milky Way

The relative numbers of newborn stars of different masses in a galaxy (the initial mass function) determines whether the galaxy's interstellar gas goes mainly into long-lived low-mass stars, as in the disks of normal spiral galaxies, or into short-lived massive stars, as has been proposed for "starburst" galaxies. The centre of the Milky Way is not a fully-fledged starburst region, but its star formation rate per unit volume of space is nevertheless roughly a thousand times that of the disk. It is, however, very difficult to study the initial mass function near the centre, because the dust in the gas clouds obscures the starlight, and the relatively rare young stars are mixed with much more numerous older stars. Here we report high-resolution infrared observations of a compact cluster of stars in the central region of our Galaxy. We find approximately 100 young, massive main-sequence stars, several of which seem to be among the most massive in the Galaxy. This cluster may be a weak analogue of the large star clusters in starburst galaxies, which opens the possibility of studying the starburst phenomenon through a local example.

The Canada-France Redshift Survey: The Luminosity Density and Star Formation History of the Universe to [ITAL]z[/ITAL] ∼ 1

The comoving luminosity density of the universe, (λ), is estimated from the Canada-France Redshift Survey (CFRS) faint galaxy sample in three wave bands (2800 A, 4400 A, and 1 μm) over the redshift range 0 < z < 1. In all three wave bands, increases markedly with redshift. For a (q0 = 0.5, Ω = 1.0) cosmological model, the comoving luminosity density increases as (1 + z)2.1 ± 0.5 at 1 μm, as (1 + z)2.7 ± 0.5 at 4400 A, and as (1 + z)3.9 ± 0.75 at 2800 A, these exponents being reduced by 0.43 and 1.12 for (0.05, 0.1) and (-0.85, 0.1) cosmological models, respectively. The (λ)-τ relation can be reasonably well modeled by an actively evolving stellar population with a Salpeter initial mass function (IMF) extending to 125 M☉, and a star formation rate declining as τ-2.5 with a turn-on of star formation at early epochs. A Scalo IMF extending to the same mass limit produces too many long-lived low-mass stars. This rapid evolution of the star formation rate and comoving luminosity density of the universe is in good agreement with the conclusions of Pei & Fall from their analysis of the evolving metallicity of the universe. One consequence of this evolution is that the physical luminosity density at short wavelengths has probably declined by 2 orders of magnitude since z ~ 1.

Shedding Light on Baryonic Dark Matter

Halo dark matter, if it is baryonic, may plausibly consist of compact stellar remnants. Jeans mass clouds containing 10(6) to 10(8) solar masses could have efficiently formed stars in the early universe and could plausibly have generated, for a suitably top-heavy stellar initial mass function, a high abundance of neutron stars as well as a small admixture of long-lived low mass stars. Within the resulting clusters of dark remnants, which eventually are tidally disrupted when halos eventually form, captures of neutron stars by non-degenerate stars resulted in formation of close binaries. These evolve to produce, by the present epoch, an observable x-ray signal associated with dark matter aggregations in galaxy halos and galaxy cluster cores.

Stochastic self-propagating star formation in three-dimensional disk galaxy simulations

view Abstract Citations (6) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stochastic self-propagating star formation in three-dimensional disk galaxy simulations Statler, T. ; Smith, B. F. ; Comins, N. Abstract The behavior of stochastic self-propagating star formation (SSPSF) in three dimensions is examined in simulation for the first time, emphasizing the effect of the added dimension on the sensitivity of spiral structure to the probability of star formation. The model produces global equilibrium spiral structure over a much more restricted range of star formation probabilities and relaxation times than in two dimensions. Spiral structure also occurs as a transient phenomenon in the runs which eventually fill or evolve to structured nonspiral states. The equilibrium spirals are not as distinctive as those produced by two-dimensional models. However, there are refinements which may modify these results, such as allowing for the depletion of gas in the interstellar medium due to conversion to long-lived low mass stars. Publication: The Astrophysical Journal Pub Date: July 1983 DOI: 10.1086/161099 Bibcode: 1983ApJ...270...79S Keywords: Disk Galaxies; Galactic Evolution; Galactic Rotation; Galactic Structure; Star Formation; Astronomical Models; Computerized Simulation; Spiral Galaxies; Supernovae; Two Dimensional Models; Astrophysics full text sources ADS | Related Materials (1) Erratum: 1984ApJ...280..937S