The Giant Branches of Centauri: Multiwavelength Observations of Evolved Stars

Hughes, Joanne; Wallerstein, George; Leeuwen, F. van; Hilker, M.

doi:10.1086/379852

Cited by 26 publications

(37 citation statements)

References 46 publications

(43 reference statements)

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“…The true age spread in ω Cen and within each of its subpopulations remains a matter of lively debate. Several studies (Hughes et al 2004;Sollima et al 2005b;Stanford et al 2006) concluded that ω Cen enriched itself on a timescale <2-5 Gyr, with each subpopulation being essentially coeval within the uncertainties implied in the analysis (∼1.5 Gyr, see, e.g., Sollima et al 2005b). On the other hand, Johnson et al (2009) presented evidence of different degrees of s-process enrichment among stars in the metal-poor population, which is the one sampled in the present analysis.…”

Section: Discussionmentioning

confidence: 92%

The lithium content ofω Centauri

Monaco

Bonifacio

Sbordone

et al. 2010

A&A

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Context.A discrepancy has emerged between the cosmic lithium abundance inferred by the WMAP satellite measurement coupled with the prediction of the standard big-bang nucleosynthesis theory, and the constant Li abundance measured in metal-poor halo dwarf stars (the so-called Spite plateau). Several models are being proposed to explain this discrepancy, involving either new physics, in situ depletion, or the efficient depletion of Li in the pristine Galaxy by a generation of massive first stars. The realm of possibilities may be narrowed considerably by observing stellar populations in different galaxies, which have experienced different evolutionary histories. Aims. The ω Centauri stellar system is commonly considered as the remnant of a dwarf galaxy accreted by the Milky Way. We investigate the lithium content of a conspicuous sample of unevolved stars in this object. Methods. We obtained moderate resolution (R = 17 000) spectra for 91 main-sequence/early sub-giant branch (MS/SGB) ω Cen stars using the FLAMES-GIRAFFE/VLT spectrograph. Lithium abundances were derived by matching the equivalent width of the Li i resonance doublet at 6708 Å to the prediction of synthetic spectra computed with different Li abundances. Synthetic spectra were computed using the SYNTHE code along with ATLAS-9 model atmospheres. The stars effective temperatures are derived by fitting the wings of the H α line with synthetic profiles. Results. We obtain a mean content of A(Li) = 2.19 ± 0.14 dex for ω Centauri MS/SGB stars. This is comparable to what is observed in Galactic halo field stars of similar metallicities and temperatures. Conclusions. The Spite plateau seems to be an ubiquitous feature of old, warm metal-poor stars. It exists also in external galaxies, if we accept the current view about the origin of ω Cen. This implies that the mechanism(s) that causes the "cosmological lithium problem" may be the same in the Milky Way and other galaxies.

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Section: Discussionmentioning

confidence: 92%

The lithium content ofω Centauri

Monaco

Bonifacio

Sbordone

et al. 2010

A&A

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“…An age spread of 2-5 Gyr, among the various sub-populations was first found by Hughes & Wallerstein (2000) and Hilker & Richtler (2000), based on the TO region morphology of their high-quality Strömgren photometries and colour-metallicty calibrations. Later several other papers came out based on combinations of high quality photometry and lowresolution spectroscopy (Hughes et al 2004;Hilker et al 2004;Rey et al 2004;Sollima et al 2005b;Stanford et al 2006;Villanova et al 2007), finding again various age dispersions all around 2-5 Gyr. The only studies reporting on age difference below 2 Gyrs are those by Ferraro et al (2004), who find that the SGB-a cannot be fitted with any isochrone younger than the SGB-MP population, and Sollima et al (2005b), who similarly found that the overall age spread of the SGB sub-populations cannot amount to more than 2 Gyr.…”

Section: Towards a Solution Of The Age Spread Problemmentioning

confidence: 96%

“…A large number of photometric and low-resolution spectroscopic studies (Hughes & Wallerstein 2000;Hilker & Richtler 2000;Pancino 2003;Hughes et al 2004;Hilker et al 2004;Rey et al 2004;Ferraro et al 2004;Stanford et al 2006) found age spreads ranging from 2 to 6 Gyr, with a few exceptions and puzzles (see Sect. 5.3, for more details).…”

Section: Introductionmentioning

confidence: 99%

The subgiant branch ofω Centauri seen through high-resolution spectroscopy

Pancino¹,

Mucciarelli²,

Sbordone³

et al. 2011

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We analysed high-resolution UVES spectra of six stars belonging to the subgiant branch of ω Centauri, and derived abundance ratios of 19 chemical elements (namely Al, Ba, C, Ca, Co, Cr, Cu, Fe, La, Mg, Mn, N, Na, Ni, Sc, Si, Sr, Ti, and Y). A comparison with previous abundance determinations for red giants provided remarkable agreement and allowed us to identify the sub-populations to which our targets belong. We found that three targets belong to a low-metallicity population at [Fe/H] -2.0 dex, [α/Fe] +0.4 dex and [s/Fe] 0 dex. Stars with similar characteristics were found in small amounts by past surveys of red giants. We discuss the possibility that they belong to a separate sub-population that we name VMP (very metal-poor, at most 5% of the total cluster population), which -in the self-enrichment hypothesis -is the best-candidate first stellar generation in ω Cen. Two of the remaining targets belong to the dominant metal-poor population (MP) at [Fe/H] -1.7 dex, and the last one to the metal-intermediate (MInt) one at [Fe/H] -1.2 dex. The existence of the newly defined VMP population could help to understand some puzzling results based on low-resolution spectroscopy for age differences determinations, because the metallicity resolution of these studies was probably not enough to detect the VMP population. The VMP could also correspond to some of the additional substructures of the subgiant-branch region found in the latest HST photometry. After trying to correlate chemical abundances with substructures in the subgiant branch of ω Cen, we found that the age difference between the VMP and MP populations should be small (0 ± 2 Gyr), while the difference between the MP and MInt populations could be slightly larger (2 ± 2 Gyr).

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“…The first studies employed photometric metallicity indicators (Hilker & Richtler 2000;Hughes & Wallerstein 2000), soon followed by low-resolution spectroscopic samples (Hughes et al 2004;Hilker et al 2004;Rey et al 2004;Sollima et al 2005b;Stanford et al 2006;Villanova et al 2007), coupled with exquisite new photometric catalogues Bedin et al 2004;Bellini et al 2010), but the puzzle got deeper, Based on observations with the ESO GIRAFFE@VLT, under programme 079.D-0021(A). Also based on literature data obtained with WFI@VLT (programmes 62.L-0345 and 63.L-0439) and FORS@VLT (programme 68.D-0332).…”

Section: Introductionmentioning

confidence: 99%

The subgiant branch ofω Centauri seen through high-resolution spectroscopy

Pancino

Mucciarelli

Bonifacio

et al. 2011

A&A

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We analyze spectra of 18 stars belonging to the faintest subgiant branch in ω Centauri (the SGB-a), obtained with GIRAFFE@VLT at a resolution of R 17 000 and a S /N ratio between 25 and 50. We measure abundances of Al, Ba, Ca, Fe, Ni, Si, and Ti and we find that these stars have [Fe/H] = −0.73 ± 0.14 dex, similarly to the corresponding red giant branch population (the RGB-a). We also measure [α/Fe] = +0.40 ± 0.16 dex, and [Ba/Fe] = +0.87 ± 0.23 dex, in general agreement with past studies. It is very interesting to note that we found a uniform Al abundance, [Al/Fe] = +0.32 ± 0.14 dex, for all the 18 SGB-a stars analysed here, thus supporting past evidence that the usual (anti-)correlations are not present in this population, and suggesting a non globular cluster-like origin of this particular population. In the dwarf galaxy hypothesis for the formation of ω Cen, this population might be the best candidate for the field population of its putative parent galaxy, although some of its properties appear contradictory. It has also been suggested that the most metal-rich population in ω Cen is significantly enriched in helium. If this is true, the traditional abundance analysis techniques, based on model atmospheres with normal helium content, might lead to errors. We have computed helium enhanced atmospheres for three stars in our sample and verified that the abundance errors due to the use of non-enhanced atmospheres are negligible. Additional, indirect support to the enhanced helium content of the SGB-a population comes from our Li upper limits.

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The Giant Branches of Centauri: Multiwavelength Observations of Evolved Stars

Cited by 26 publications

References 46 publications

The lithium content ofω Centauri

The lithium content ofω Centauri

The subgiant branch ofω Centauri seen through high-resolution spectroscopy

The subgiant branch ofω Centauri seen through high-resolution spectroscopy

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