The Na-O anticorrelation in horizontal branch stars. II. NGC1851

Astronomy and Astrophysics (Impact Factor: 4.38). 01/2012; 539(0004-6361). DOI: 10.1051/0004-6361/201118491
Source: arXiv


We studied the Na-O anti-correlation from moderately high resolution spectra
for 35 stars on the blue HB (BHB), one RR Lyrae, and 55 stars are on the red HB
(RHB) of NGC1851. We also derived abundances for He and N in BHB stars, and Ba
and upper limits for N in RHB stars. The RHB stars clearly separate into two
groups: the vast majority are O-rich and Na-poor, while about 10-15% are
Na-rich and moderately O-poor. Most Na-rich RHB stars are also Ba-rich and
there is an overall correlation between Na and Ba abundances within the RHB.
The group of Ba-rich RHB stars resides on the warmer edge and includes ~10% of
the RHB stars. We propose that they are the descendant of the stars on the RGB
sequence with very red v-y colour. This sequence is known also to consist of Ba
and perhaps CNO-rich stars. However, the upper limit we obtain for N
([N/Fe]<1.55) for one of the Ba-rich stars coupled with the low C-abundances
for RGB Ba-rich stars from the literature suggests that the total CNO might not
be particularly high ([(C+N+O)/Fe]<=0.15). The other Na-rich RHB stars are also
at the warm edge of the RHB and the only RR Lyrae is Na-rich and moderately
O-poor. We also find a Na-O anticorrelation among BHB stars, partially
overlapping that found among RHB stars, though generally BHB stars are more
Na-rich and O-poor. However, there is no clear correlation between temperature
and Na and O abundances within the BHB. The average He abundance in BHB stars
is Y=0.29+/-0.05. N abundances are quite uniform at [N/Fe]=1.16+/-0.14 among
BHB stars, with a small trend with temperature. This value is consistent with
normal CNO abundance and excludes that BHB stars are very CNO-rich: this leaves
an age spread of ~1.5 Gyr as the only viable explanation for the split of the
SGB. [Abridged]

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Available from: Valentina D'Orazi
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    ABSTRACT: Star formation from matter including the hot CNO processed ejecta of asymptotic giant branch (AGB) winds is regarded as a plausible scenario to account for the chemical composition of a stellar second generation (SG) in globular clusters. The chemical evolution models, based on this hypothesis, so far included only the yields available for the massive AGB stars, while the possible role of super-AGB ejecta was either extrapolated or not considered. In this work, we explore in detail the role of super-AGB ejecta on the formation of the SG abundance patterns using yields recently calculated by Ventura and D’Antona. An application of the model to clusters showing extended Na–O anticorrelations, like NGC 2808, indicates that an SG formation history similar to that outlined in our previous work is required: formation of an extreme population with very large helium content from the pure ejecta of super-AGB stars, followed by formation of an intermediate population by dilution of massive AGB ejecta with pristine gas. The present models are able to account for the very O-poor, Na-rich extreme stars once deep-mixing is assumed in SG giants forming in a gas with helium abundance Y > 0.34, which significantly reduces the atmospheric oxygen content, while preserving the sodium abundance. On the other hand, for clusters showing a mild O–Na anticorrelation, like M 4, the use of the new yields broadens the range of SG formation routes leading to abundance patterns consistent with observations. Specifically, our study shows that a model in which SG stars form only from super-AGB ejecta promptly diluted with pristine gas can reproduce the observed patterns. We briefly discuss the variety of (small) helium variations occurring in this model and its relevance for the horizontal branch morphology. In some of these models, the duration of the SG formation episode can be as short as ∼10 Myr; the formation time of the SG is therefore compatible with the survival of a cooling flow in the cluster core, previous to the explosion of the SG core collapse supernovae. We also explore models characterized by the formation of multiple populations in individual bursts, each lasting no longer than ∼10 Myr each.
    Preview · Article · Mar 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: NGC1851 possibly shows a spread in [Fe/H], but the relation between this spread and the division in the SGB is unknown. We obtained blue (3950-4600 A) intermediate resolution (R~8,000) spectra for 47 stars on the bright and 30 on the faint SGB of NGC 1851 (b-SGB and f-SGB, respectively). The determination of the atmospheric parameters to extremely high internal accuracy leads to small errors when comparing different stars in the cluster. We found that the b-SGB is slightly more metal-poor than the f-SGB, with [Fe/H]=-1.227+/-0.009 and [Fe/H]=-1.162+/- 0.012, respectively. This implies that the f-SGB is only slightly older by ~0.6 Gyr than the b-SGB if the total CNO abundance is constant. There are more C-normal stars in the b-SGB than in the f-SGB. This is consistent with what is found for HB stars, if b-SGB are the progenitors of red HB stars, and f-SGB those of blue HB ones. The abundances of the n-capture elements Sr and Ba have a bimodal distribution, reflecting the separation between f-SGB (Sr and Ba-rich) and b-SGB stars (Sr and Ba-poor). In both groups, there is a clear correlation between [Sr/Fe] and [Ba/Fe], suggesting that there is a real spread in the abundances of n-capture elements. There is some correlation between C and Ba abundances, while the same correlation for Sr is much more dubious. We identified six C-rich stars, which have a moderate overabundance of Sr and Ba and rather low N abundances. This group of stars might be the progenitors of these on the anomalous RGB in the (v, v-y) diagram. These results are discussed within different scenarios for the formation of NGC1851. It is possible that the two populations originated in different regions of an inhomogeneous parent object. However, the striking similarity with M22 calls for a similar evolution for these two clusters. Deriving reliable CNO abundances for the two sequences would be crucial.
    Preview · Article · May 2012 · Astronomy and Astrophysics
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    ABSTRACT: We study the distribution of aluminum abundances among red giants in the peculiar globular cluster NGC 1851. Aluminum abundances were derived from the strong doublet Al I 8772-8773 A measured on intermediate resolution FLAMES spectra of 50 cluster stars acquired under the Gaia-ESO public survey. We coupled these abundances with previously derived abundance of O, Na, Mg to fully characterize the interplay of the NeNa and MgAl cycles of H-burning at high temperature in the early stellar generation in NGC 1851. The stars in our sample show well defined correlations between Al,Na and Si; Al is anticorrelated with O and Mg. The average value of the [Al/Fe] ratio steadily increases going from the first generation stars to the second generation populations with intermediate and extremely modified composition. We confirm on a larger database the results recently obtained by us (Carretta et al. 2011a): the pattern of abundances of proton-capture elements implies a moderate production of Al in NGC 1851. We find evidence of a statistically significant positive correlation between Al and Ba abundances in the more metal-rich component of red giants in NGC 1851.
    Full-text · Article · May 2012 · Astronomy and Astrophysics
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