The Na-O anticorrelation in horizontal branch stars

Abstract

We obtained high-resolution spectra for 94 candidate stars belonging to the HB of M 22 with FLAMES. Previous works have indicated that this cluster has split subgiant (SGB) and red giant branches (RGB) and hosts two different stellar populations, differing in overall metal abundance and both exhibiting a Na-O anti-correlation. The HB stars we observed span a restricted temperature range (7800 < T eff < 11 000 K), where about 60% of the HB stars of M 22 are. Within our sample, we can distinguish three groups of stars segregated (though contiguous) in colours: Group 1 (49 stars) is metal-poor, N-normal, Na-poor, and O-rich: our abundances for this (cooler) group match those determined for the primordial group of RGB stars (a third of the total) from previous studies very well. Group 2 (23 stars) is still metal-poor, but it is N-and Na-rich, though only very mildly depleted in O. We can identify this intermediate group as the progeny of the metal-poor RGB stars that occupy an intermediate location along the Na-O anti-correlation and include about 10% of the RGB stars. The third group (20 stars) is metal-rich, Na-rich, and O-rich. This hotter group most likely corresponds to the most O-rich component of the previously found metal-rich RGB population (a quarter of the total). We did not observe any severely O-depleted stars and we think that the progeny of these stars falls on the hotter part of the HB. Furthermore, we found that the metal-rich population is also over-abundant in Sr, in agreement with results for corresponding RGB and SGB stars. However, we do not find any significant variation in the ratio between the sum of N and O abundances to Fe. We do not have C abundances for our stars. There is some evidence of an enhancement of He content for Groups 2 and 3 stars (Y = 0.338 ± 0.014 ± 0.05); the error bar due to systematics is large, but a consistent analysis of data for several GCs confirms that stars in these groups within M 22 are probably overabundant in He. We conclude that on the whole, our results agree with the proposition that chemical composition drives the location of stars along the HB of a GC. Furthermore, we found a number of fast rotators. They are concentrated in a restricted temperature range along the HB of M 22. Fast rotating stars might be slightly less massive and bluer than slowly rotating ones, but other interpretations are possible.

A possible formation channel for blue hook stars in globular clusters

“…Recently, Na-O anticorrelation is also discovered in HB stars for many GCs (Gratton et al 2013(Gratton et al , 2014. Marino et al (2014) analyzed nearly 100 HB stars with different temperatures in NGC 2808 and confirmed that some blue HB stars in this GC present higher helium abundance than primordial content by △Y = 0.09 ± 0.01.…”

MULTIPLE STELLAR POPULATIONS OF GLOBULAR CLUSTERS FROM HOMOGENEOUS Ca by PHOTOMETRY. I. M22 (NGC 6656)

“…It would be very interesting to point out the recent result by Gratton et al (2014). They carried out the spectroscopic survey of the BHB stars in M22 and found a significant fraction (≈ 18%) of the metal-rich helium-enhanced BHB stars in their sample (Group 3 designated by Gratton et al).…”

“…D' Antona et al 2002;Milone et al 2014) and there also exist direct spectroscopic observational evidences of helium enhancement in GC HB stars (Marino et al 2014a;Gratton et al 2014). …”

“…Marino et al 2013Marino et al , 2014aGratton et al 2014). Marino et al (2013) studied 7 HB stars in M22 and they found that all their HB stars are Ba-poor and Na-poor, equivalent to the s-poor RGB population, leading them to suggest that the position of a HB star is strictly related to the chemical composition.…”

“…Spectroscopic studies of the M22 HB stars also showed that the the true nature of HB populations may be as complex as that of RGB populations (e.g. Marino et al 2014a;Gratton et al 2014). All the available observational evidences up to date may suggest that M22 must have retained not only the material from SNe but also from low-mass and intermediate-mass AGBs 1 , reflecting that the formation history of M22 must have been very complex compared to those of the bulk of normal GCs in our Galaxy.…”