Article

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

Astronomy and Astrophysics (Impact Factor: 5.08). 01/2012; DOI:10.1051/0004-6361/201118491
Source: arXiv

ABSTRACT 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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