2009A&A...499..755V


C.D.S. - SIMBAD4 rel 1.7 - 2019.11.17CET12:22:10

2009A&A...499..755V - Astronomy and Astrophysics, volume 499, 755-763 (2009/6-1)

The helium content of globular clusters: light element abundance correlations and HB morphology. I. NGC 6752.

VILLANOVA S., PIOTTO G. and GRATTON R.G.

Abstract (from CDS):

In the context of the multiple stellar population scenario in globular clusters (GC), helium (He) has been proposed as the key element to interpret the observed multiple main sequences (MS), subgiant branches (SGB) and red giant branches (RGB), as well as the complex horizontal branch (HB) morphology. However, up to now, He has never been directly measured in GC stars (8500<Teff<11500K) to verify this hypothesis. We studied hot blue horizontal branch (BHB) stars in the GC NGC 6752 to measure their He content. Our goal is to verify the feasibility of He measurement from high resolution spectra in stars cooler than 11500 K, where chemical abundances are not altered by sedimentation or levitation. We observed 7 BHB stars using the UVESLT2 spectroscopic facility. Spectra of S/N∼200 were obtained and the very weak He line at 5875Å measured. We compared this feature with synthetic spectra to obtain He abundances. In addition, iron peak (Fe,Cr), α (Si,Ti), light (O,Na), and s-element (Ba) abundances were measured. We could measure He abundance only for stars warmer than Teff=8500K. All our targets with measurable He are zero age HB (ZAHB) objects and turned out to have a homogeneous He content with a mean value of Y=0.245±0.012, compatible with the most recent measurements of the primordial He content of the Universe (Y∼0.25). The whole sample of stars has a metallicity of [Fe/H]=-1.56±0.03 and [α/Fe]=+0.21±0.03, in agreement with other studies available in the literature. Our HB targets show the same Na-O anticorrelation identified among the TO-SGB-RGB stars. This is the first direct measurement of the He abundance for a significative sample of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flasher HB stars.

Abstract Copyright:

Journal keyword(s): Galaxy: globular clusters: individual: NGC 6752 - stars: abundances

Nomenclature: Table 1: [VPG2009] NNNNN N=7 among (Nos 8745-24938).

Simbad objects: 13

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Number of rows : 13

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2020
#notes
1 IRAS 09110-6439 GlC 09 12 03.10 -64 51 48.6   7.77 5.69     ~ 1204 0
2 NGC 5139 GlC 13 26 47.28 -47 28 46.1   6.12 5.33     ~ 2920 0
3 M 4 GlC 16 23 35.22 -26 31 32.7           ~ 1636 0
4 M 22 GlC 18 36 23.94 -23 54 17.1   7.16 6.17     ~ 1177 0
5 [VPG2009] 13415 * 19 10 40.310 -59 59 04.70 13.937 13.858 13.800     ~ 1 0
6 [VPG2009] 24938 * 19 10 40.870 -59 52 18.90 14.018 14.001 13.983     ~ 1 0
7 [VPG2009] 8745 * 19 10 42.340 -60 00 34.70 14.031 13.972 13.931     ~ 1 0
8 NGC 6752 GlC 19 10 52.11 -59 59 04.4   6.96 6.28     ~ 1766 0
9 [VPG2009] 11583 * 19 10 53.850 -59 59 38.00 13.736 13.661 13.584     ~ 1 0
10 GES J19105510-5957229 * 19 10 55.100 -59 57 22.92 14.004 13.950 13.920     ~ 2 0
11 [VPG2009] 12404 * 19 10 57.120 -59 59 22.90 13.984 13.927 13.903     ~ 1 0
12 [VPG2009] 16149 * 19 11 10.490 -59 58 15.20 13.968 13.906 13.844     ~ 1 0
13 M 15 GlC 21 29 58.33 +12 10 01.2   3       ~ 2851 0

    Equat.    Gal    SGal    Ecl

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2019.11.17-12:22:10

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