Observational Consequences of the Hypothesized Helium‐rich Stellar Population in ω Centauri (original) (raw)
Related papers
Metallicities on the Double Main Sequence of ω Centauri Imply Large Helium Enhancement
The Astrophysical Journal, 2005
Having shown in a recent paper that the main sequence of ω Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is less metal-poor. We have carefully combined GIRAFFE@VLT spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate S/N to show clearly that the stars of the blue main sequence are less metal poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main sequence stars, while the red main sequence stars show a 0.2 dex spread in metallicity. We use stellar-structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.
On the origin of the helium-rich population in ω Centauri
Monthly Notices of the Royal Astronomical Society, 2010
To study the possible origin of the huge helium enrichment attributed to the stars on the blue main sequence of ω Centauri, we make use of a chemical evolution model that has proven able to reproduce other major observed properties of the cluster, namely, its stellar metallicity distribution function, age-metallicity relation and trends of several abundance ratios with metallicity. In this framework, the key condition to satisfy all the available observational constraints is that a galactic-scale outflow develops in a much more massive parent system, as a consequence of multiple supernova explosions in a shallow potential well. This galactic wind must carry out preferentially the metals produced by explosive nucleosynthesis in supernovae, whereas elements restored to the interstellar medium through low-energy stellar winds by both asymptotic giant branch (AGB) stars and massive stars must be mostly retained. Assuming that helium is ejected through slow winds by both AGB stars and fast rotating massive stars (FRMSs), the interstellar medium of ω Centauri's parent galaxy gets naturally enriched in helium in the course of its evolution. throughout this paper are by number, except for Y and Z, that indicate the mass fraction of helium and total metals, respectively. As usual, log ε(X) ≡ 12 + log (X/H), c 2009 RAS
Monthly Notices of the Royal Astronomical Society, 2007
We present a spectral atlas of the post-main-sequence population of the most massive Galactic globular cluster, ω Centauri. Spectra were obtained of more than 1500 stars selected as uniformly as possible from across the (B, B-V) colour-magnitude diagram of the proper motion cluster member candidates of van Leeuwen et al. (2000). The spectra were obtained with the 2dF multi-fibre spectrograph at the Anglo Australian Telescope, and cover the approximate range λ ∼ 3840-4940Å at a resolving power of λ/∆λ ≃ 2000. This constitutes the most comprehensible spectroscopic survey of a globular cluster. We measure the radial velocities, effective temperatures, metallicities and surface gravities by fitting atlas9 stellar atmosphere models. We analyse the cluster membership and stellar kinematics, interstellar absorption in the Ca ii K line at 3933Å, the RR Lyrae instability strip and the extreme horizontal branch, the metallicity spread and bimodal CN abundance distribution of red giants, nitrogen and s-process enrichment, carbon stars, pulsation-induced Balmer line emission on the asymptotic giant branch (AGB), and the nature of the post-AGB and UV-bright stars. Membership is confirmed for the vast majority of stars, and the radial velocities clearly show the rotation of the cluster core. We identify long-period RR Lyrae-type variables with low gravity, and low-amplitude variables coinciding with warm RR Lyrae stars. A barium enhancement in the coolest red giants indicates that 3 rd dredge-up operates in AGB stars in ω Cen. This is distinguished from the pre-enrichment by more massive AGB stars, which is also seen in our data. The properties of the AGB, post-AGB and UV-bright stars suggest that RGB mass loss may be less efficient at very low metallicity, [Fe/H]≪ −1, increasing the importance of mass loss on the AGB. The catalogue and spectra are made available via CDS.
The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation
The Astrophysical Journal, 2007
We present metallicity measurements based on GIRAFFE@VLT spectra of 80 subgiant-branch stars of the Galactic globular cluster ω Centauri. The VLT spectroscopic data are complemented by color-magnitude diagrams from high-accuracy photometry on a ∼ 10 × 10 arcmin 2 mosaic of ACS/HST images centered on the cluster center, and on multicolor images of a ∼ 34 × 33 arcmin 2 field, taken with the WFI@ESO2.2m camera. Our main purpose was to combine photometric data with spectroscopic data, in the hope of teasing apart some of the population mysteries that neither data set can answer on its own. We have obtained the [Fe/H] abundance for each of the 80 target stars, and the abundances of C, N, Ca, Ti, and Ba for a subset of them, by comparison with synthetic spectra. We show that stars with [Fe/H] < −1.25 have a large magnitude spread on the flat part of the SGB. We interpret this as empirical evidence for an age spread. A relative age has been obtained for each star, from theoretical isochrones for its metallicity, α-enhancement, and presumed He content. We have identified four distinct stellar groups within the SGB region: (i) an old, metal-poor group ([Fe/H] ∼ −1.7); (ii) an old, metal-rich group ([Fe/H] ∼ −1.1); (iii) a young (up to 4-5 Gyr younger than the old component) metal-poor group ([Fe/H] ∼ −1.7); (iv) a young, intermediate-metallicity ([Fe/H] ∼ −1. group, on average 1-2 Gyr younger than the old metal-poor population, and with an age spread that we cannot properly quantify with the present sample. In addition, a group of SGB stars are spread between the intermediate-metallicity and metal-rich branches of the SGB. We tentatively propose connections between the SGB stars and both the multiple main sequence and the red giant branch. Finally, we discuss the implications of the multiple stellar populations on the formation and evolution of ω Cen. The spread in age within each population establishes that the original system must have had a composite nature.
The Abundance Spread Among Giants and Subgiants in the Globular Cluster Omega Centauri
The Astronomical Journal, 1996
We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster ω Centauri based on the Ca II infrared triplet. Two samples of stars were observed: 234 stars at M V ∼ 1.25 on the lower giant branch at radial distances between 8 ′ and 23 ′ , and 145 stars at M V ∼ −1.3 at radial distances between 3 ′ and 22 ′. We found 199 and 144 radial velocity members, respectively, in the two samples. These samples were corrected for evolutionary effects to provide an unbiased distribution of the underlying stellar metallicity. We find < v r >= 234.7 ± 1.3, σ obs = 11.3 km s −1 (bright sample), and < v r >= 232.9 ± 1.2, σ obs = 10.6 km s −1 (faint sample). The statistical errors of the dispersions are less than 1 km s −1. Previous metallicity studies found a non-gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are narrower. They contain the following key features: (1) No very metal-poor stars (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of-1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity
The hottest horizontal-branch stars in ω Centauri. Late hot flasher vs. helium enrichment
Astronomy and Astrophysics, 2007
Context. UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as results of the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ω Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C, N enrichment could not be verified. Aims. We want to compare effective temperatures, surface gravities, and abundances of He, C, and N of blue hook and canonical extreme horizontal branch (EHB) star candidates to the predictions of the two scenarios. Methods. Moderately high resolution spectra of stars at the hot end of the blue HB in the globular cluster ω Cen were analysed for atmospheric parameters and abundances using LTE and Non-LTE model atmospheres.
Monthly Notices of The Royal Astronomical Society, 2007
We present a spectral atlas of the post-main-sequence population of the most massive Galactic globular cluster, omega Centauri. Spectra were obtained of more than 1500 stars selected as uniformly as possible from across the (B, B-V) colour-magnitude diagram of the proper motion cluster member candidates of van Leeuwen et al. (2000). The spectra were obtained with the 2dF multi-fibre spectrograph at the Anglo Australian Telescope, and cover the approximate range lambda~3840-4940 Angstroem. We measure the radial velocities, effective temperatures, metallicities and surface gravities by fitting ATLAS9 stellar atmosphere models. We analyse the cluster membership and stellar kinematics, interstellar absorption in the Ca II K line at 3933 Angstroem, the RR Lyrae instability strip and the extreme horizontal branch, the metallicity spread and bimodal CN abundance distribution of red giants, nitrogen and s-process enrichment, carbon stars, pulsation-induced Balmer line emission on the asymptotic giant branch (AGB), and the nature of the post-AGB and UV-bright stars. Membership is confirmed for the vast majority of stars, and the radial velocities clearly show the rotation of the cluster core. We identify long-period RR Lyrae-type variables with low gravity, and low-amplitude variables coinciding with warm RR Lyrae stars. A barium enhancement in the coolest red giants indicates that 3rd dredge-up operates in AGB stars in omega Cen. This is distinguished from the pre-enrichment by more massive AGB stars, which is also seen in our data. The properties of the AGB, post-AGB and UV-bright stars suggest that RGB mass loss may be less efficient at very low metallicity, [Fe/H]<<-1, increasing the importance of mass loss on the AGB. The catalogue and spectra are made available via CDS.
Spectroscopy of horizontal branch stars in ω Centauri
Astronomy & Astrophysics, 2012
Aims. We analyze the reddening, surface helium abundance and spectroscopic mass of 115 blue horizontal branch (HB) and blue hook (BH) stars in ω Centauri, spanning the cluster HB from the blue edge of the instability strip (T eff =8 000 K) to BH objects with T eff ≈50 000 K. Methods. The temperatures, gravities, and surface helium abundances were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra. From these parameters, the mass and reddening were estimated. Results. The mean cluster reddening is E(B − V)=0.115±0.004, in good agreement with previous estimates, but we evidence a pattern of differential reddening in the cluster area. The stars in the western half are more reddened than in the southwest quadrant by 0.03-0.04 magnitudes. We find that the helium abundances measured on low-resolution spectra are systematically lower by 0.20-0.25 dex than the measurements based on higher resolution. No difference in surface helium abundance is detected between HB stars in ω Centauri and in three comparison clusters, and the stars in the range 11 500-20 000 K follow a trend with temperature, which probably reflects a variable efficiency of the diffusion processes. There is mild evidence that two families of extreme HB (EHB) cluster stars (T eff ≥20 000 K) could exist, as observed in the field, with ∼15% of the objects being helium depleted by a factor of ten with respect to the main population. The distribution of helium abundance above 30 000 K is bimodal, but we detect a fraction of He-poor objects lower than previous investigations. The observations are consistent with these being stars evolving off the HB. Their spatial distribution is not uniform across the cluster, but this asymmetric distribution is only marginally significative. We also find that EHB stars with anomalously high spectroscopic mass could be present in ω Centauri, as previously found in other clusters. The derived temperature-color relation reveals that the HB stars hotter than ∼11 000 K are fainter than the expectations of the canonical models in the U band, while no anomaly is detected in B and V. This behavior, not observed in NGC 6752, is a new peculiarity of ω Centauri HB stars. More investigation is needed to reach a full comprehension of this complex observational picture.
A Large Sample Study of Red Giants in the Globular Cluster Omega Centauri (NGC 5139)
The Astrophysical Journal, 2009
We present abundances of several light, α, Fe-peak, and neutron-capture elements for 66 red giant branch (RGB) stars in the Galactic globular cluster Omega Centauri (ω Cen). Our observations lie in the range 12.0<V<13.5 and focus on the intermediate and metal-rich RGBs. Abundances were determined using equivalent width measurements and spectrum synthesis analyses of moderate resolution (R≈18,000) spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. Combining these data with previous work, we find that there are at least four peaks in the metallicity distribution function at [Fe/H]=-1.75, -1.45, -1.05, and -0.75, which correspond to about 55%, 30%, 10%, and 5% of our sample, respectively. Additionally, the most metal-rich stars are the most centrally located. Na and Al are correlated despite exhibiting starto-star dispersions of more than a factor of 10, but the distribution of those elements appears to be metallicity dependent and are divided at [Fe/H]≈-1.2. About 40-50% of stars with [Fe/H]<-1.2 have Na and Al abundances consistent with production solely in Type II supernovae and match observations of disk and halo stars at comparable metallicity. The remaining metal-poor stars are enhanced in Na and Al compared to their disk and halo counterparts and are mostly consistent with predicted yields from >5 M ⊙ asymptotic giant branch (AGB) stars. At [Fe/H]>-1.2, more than 75% of the stars are Na/Al enhanced and may have formed almost exclusively from AGB ejecta. Most of these stars -2are enhanced in Na by at least 0.2 dex for a given Al abundance than would be expected based on "normal" globular cluster values. All stars in our sample are α-rich with [Ca/Fe] =+0.36 (σ=0.09) and [Ti/Fe] =+0.23 (σ=0.14). The Fepeak elements give solar-scaled abundances and similarly small dispersions with [Sc/Fe] =+0. 09 (σ=0.15) and [Ni/Fe] =-0.04 (σ=0.09). Europium does not vary extensively as a function of metallicity and has [Eu/Fe] =+0.19 (σ=0.23). However, [La/Fe] varies from about -0.4 to +2 and stars with [Fe/H] -1.5 have [La/Eu] values indicating domination by the s-process. A quarter of our sample have [La/Eu]≥+1