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Register a new userA chemical signature from fast-rotating low-metallicity massive stars: ROA 276 in omega Centauri
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We present a chemical abundance analysis of a metal-poor star, ROA 276, in the stellar system omega Centauri. We confirm that this star has an unusually high [Sr/Ba] abundance ratio. Additionally, ROA 276 exhibits remarkably high abundance ratios, [X/Fe], for all elements from Cu to Mo along with normal abundance ratios for the elements from Ba to Pb. The chemical abundance pattern of ROA 276, relative to a primordial omega Cen star ROA 46, is best fit by a fast-rotating low-metallicity massive stellar model of 20 Msun, [Fe/H] = -1.8, and an initial rotation 0.4 times the critical value; no other nucleosynthetic source can match the neutron-capture element distribution. ROA 276 arguably offers the most definitive proof to date that fast-rotating massive stars contributed to the production of heavy elements in the early Universe.
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We present a detailed spectroscopic analysis of RR Lyrae (RRL) variables in the globular cluster NGC 5139 (omega Cen). We collected optical (4580-5330 A), high resolution (R = 34,000), high signal-to-noise ratio (200) spectra for 113 RRLs with the multi-fiber spectrograph M2FS at the Magellan/Clay Telescope at Las Campanas Observatory. We also analysed high resolution (R = 26,000) spectra for 122 RRLs collected with FLAMES/GIRAFFE at the VLT, available in the ESO archive. The current sample doubles the literature abundances of cluster and field RRLs in the Milky Way based on high resolution spectra. Equivalent width measurements were used to estimate atmospheric parameters, iron, and abundance ratios for alpha (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn), and s-process (Y) elements. We confirm that omega Cen is a complex cluster, characterised by a large spread in the iron content: -2.58 < [Fe/H] < -0.85. We estimated the average cluster abundance as [Fe/H] = -1.80 +- 0.03, with sigma = 0.33 dex. Our findings also suggest that two different RRL populations coexist in the cluster. The former is more metal-poor ([Fe/H] < -1.5), with almost solar abundance of Y. The latter is less numerous, more metal-rich, and yttrium enhanced ([Y/Fe] > 0.4). This peculiar bimodal enrichment only shows up in the s-process element, and it is not observed among lighter elements, whose [X/Fe] ratios are typical for Galactic globular clusters.
By using high-resolution spectra acquired with FLAMES-GIRAFFE at the ESO/VLT, we measured radial and rotational velocities for 110 Blue Straggler stars (BSSs) in Omega Centauri, the globular cluster-like stellar system harboring the largest known BSS population. According to their radial velocities, 109 BSSs are members of the system. The rotational velocity distribution is very broad, with the bulk of BSSs spinning at less than ~40 km/s (in agreement with the majority of such stars observed in other globular clusters) and a long tail reaching ~200 km/s. About 40% of the sample has vsini >40 km/s and about 20% has vsini >70 km/s. Such a large fraction is very similar to the percentage of of fast rotating BSSs observed in M4. Thus, Omega Centauri is the second stellar cluster, beyond M4, with a surprisingly high population of fast spinning BSSs. We found a hint of a radial behaviour of the fraction of fast rotating BSSs, with a mild peak within one core radius, and a possibile rise in the external regions (beyond four core radii). This may suggest that recent formation episodes of mass transfer BSSs occurred preferentially in the outskirts of Omega Centauri, or that braking mechanisms able to slow down these stars are least efficient in lowest density environments.
Context. Rotation is known to affect the nucleosynthesis of light elements in massive stars, mainly by rotation-induced mixing. In particular, rotation boosts the primary nitrogen production. Models of rotating stars are able to reproduce the nitrogen observed in low-Z halo stars. Aims. Here we present the first grid of stellar models for rotating massive stars at low Z, where a full s-process network is used to study the impact of rotation-induced mixing on the nucleosynthesis of heavy elements. Methods. We used the Geneva stellar evolution code that includes an enlarged reaction network with nuclear species up to bismuth to calculate 25 M$_odot$ models at three different Z and with different initial rotation rates. Results. First, we confirm that rotation-induced mixing leads to a production of primary $^{22}$Ne, which is the main neutron source for the s process in massive stars. Therefore rotation boosts the s process in massive stars at all Z. Second, the neutron-to-seed ratio increases with decreasing Z in models including rotation, which leads to the complete consumption of all iron seeds at Z < 1e-3 by the end of core He-burning. Thus at low Z, the iron seeds are the main limitation for this boosted s process. Third, as Z decreases, the production of elements up to the Ba peak increases at the expense of the elements of the Sr peak. We studied the impact of the initial rotation rate and of the uncertain $^{17}$O$(alpha,gamma)$ rate (which strongly affects the neutron poison strength of $^{16}$O) on our results. This study shows that rotating models can produce significant amounts of elements up to Ba over a wide range of Z. Fourth, compared to the He-core, the primary $^{22}$Ne production in the He-shell is even higher (> 1% in mass fraction at all Z), which could open the door for an explosive neutron capture nucleosynthesis in the He-shell, with a primary neutron source.
The most massive and complex globular clusters in the Galaxy are thought to have originated as the nuclear cores of now tidally disrupted dwarf galaxies, but the connection between globular clusters and dwarf galaxies is tenuous with the M54/Sagittarius system representing the only unambiguous link. The globular cluster Omega Centauri (w Cen) is more massive and chemically diverse than M 54, and is thought to have been the nuclear star cluster of either the Sequoia or Gaia-Enceladus galaxy. Local Group dwarf galaxies with masses equivalent to these systems often host significant populations of very metal-poor stars ([Fe/H] < -2.5), and one might expect to find such objects in w Cen. Using high resolution spectra from Magellan-M2FS, we detected 11 stars in a targeted sample of 395 that have [Fe/H] ranging from -2.30 to -2.52. These are the most metal-poor stars discovered in the cluster, and are 5x more metal-poor than w Cens dominant population. However, these stars are not so metal-poor as to be unambiguously linked to a dwarf galaxy origin. The clusters metal-poor tail appears to contain two populations near [Fe/H] ~ -2.1 and -2.4, which are very centrally concentrated but do not exhibit any peculiar kinematic signatures. Several possible origins for these stars are discussed.
We present Li, Na, Al and Fe abundances of 199 lower red giant branch stars members of the stellar system Omega Centauri, using high-resolution spectra acquired with FLAMES at the Very Large Telescope. The A(Li) distribution is peaked at A(Li) ~ 1 dex with a prominent tail toward lower values. The peak of the distribution well agrees with the lithium abundances measured in lower red giant branch stars in globular clusters and Galactic field stars. Stars with A(Li) ~ 1 dex are found at metallicities lower than [Fe/H] ~ -1.3 dex but they disappear at higher metallicities. On the other hand, Li-poor stars are found at all the metallicities. The most metal-poor stars exhibit a clear Li-Na anticorrelation, with about 30% of the sample with A(Li) lower than ~ 0.8 dex, while in normal globular clusters these stars represent a small fraction. Most of the stars with [Fe/H] > -1.6 dex are Li-poor and Na-rich. The Li depletion measured in these stars is not observed in globular clusters with similar metallicities and we demonstrate that it is not caused by the proposed helium enhancements and/or young ages. Hence, these stars formed from a gas already depleted in lithium. Finally, we note that Omega Centauri includes all the populations (Li-normal/Na-normal, Li-normal/Na-rich and Li-poor/Na-rich stars) observed, to a lesser extent, in mono-metallic GCs.
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