7 research outputs found
Sc and neutron-capture abundances in Galactic low- and high-α field halo stars
We determine relative abundance ratios for the neutron-capture elements Zr, La, Ce, Nd and Eu for a sample of 27 Galactic dwarf stars with â1.5 < [Fe/H] < â0.8. We also measure the iron-peak element Sc. These stars separate into three populations (low- and high-α halo and thick-disc stars) based on the [α/Fe] abundance ratio and their kinematics as discovered by Nissen & Schuster. We find differences between the low- and high-α groups in the abundance ratios of [Sc/Fe], [Zr/Fe], [La/Zr], [Y/Eu] and [Ba/Eu] when including Y and Ba from Nissen & Schuster. For all ratios except [La/Zr], the low-α stars have a lower abundance compared to the high-α stars. The low-α stars display the same abundance patterns of high [Ba/Y] and low [Y/Eu] as observed in present-day dwarf spheroidal galaxies, although with smaller abundance differences, when compared to the high-α stars. These distinct chemical patterns have been attributed to differences in the star formation rate between the two populations and the contribution of low-metallicity, low-mass asymptotic giant branch (AGB) stars to the low-α population. By comparing the low-α population with AGB stellar models, we place constraints on the mass range of the AGB stars
Chemical abundances in bright giants of the globular cluster M62 (NGC 6266)
With the exception of Terzan 5, all the Galactic globular clusters that
possess significant metallicity spreads, such as omega Cen and M22, are
preferentially the more luminous clusters with extended horizontal branches.
Here we present radial velocities and chemical abundances for seven bright
giants in the globular cluster M62, a previously little-studied cluster. With
M_V = -9.18, M62 is the ninth most luminous Galactic globular cluster and has
an extended horizontal branch. Within our sample, we find (i) no evidence for a
dispersion in metallicity, [Fe/H], beyond the measurement uncertainties, (ii)
star-to-star abundance variations for C, O, Na and Al with the usual
correlations between these elements as seen in other globular clusters, and
(iii) a global enrichment for the elements Zr, Ba and La at the level [X/Fe] =
+0.4 dex. For elements heavier than La, the abundance ratios are consistent
with the scaled-solar -process distribution. Below La, the abundances are
anomalous when compared to the scaled-solar s-process or r-process
distributions. For these elements, the abundance signature in M62 is in
agreement with predictions of the s-process from fast-rotating massive stars,
although the high [Rb/Y] ratio we measure may be a challenge to this scenario.Comment: Accepted for publication in MNRA
The architecture of Abell 1386 and its relationship to the Sloan Great Wall
We present new radial velocities from AAOmega on the Anglo-Australian
Telescope for 307 galaxies (b_J < 19.5) in the region of the rich cluster Abell
1386. Consistent with other studies of galaxy clusters that constitute
sub-units of superstructures, we find that the velocity distribution of A1386
is very broad (21,000--42,000 kms^-1, or z=0.08--0.14) and complex. The mean
redshift of the cluster that Abell designated as number 1386 is found to be
~0.104. However, we find that it consists of various superpositions of
line-of-sight components. We investigate the reality of each component by
testing for substructure and searching for giant elliptical galaxies in each
and show that A1386 is made up of at least four significant clusters or groups
along the line of sight whose global parameters we detail. Peculiar velocities
of brightest galaxies for each of the groups are computed and found to be
different from previous works, largely due to the complexity of the sky area
and the depth of analysis performed in the present work. We also analyse A1386
in the context of its parent superclusters: Leo A, and especially the Sloan
Great Wall. Although the new clusters may be moving toward mass concentrations
in the Sloan Great Wall or beyond, many are most likely not yet physically
bound to it.Comment: 21 pages, 9 figures, includes the full appendix table. Accepted for
publication in MNRA