29 research outputs found
Stellar substructures in the solar neighbourhood. III. Kinematic group 2 in the Geneva-Copenhagen survey
From correlations between orbital parameters, several new coherent groups of
stars were recently identified in the Galactic disc and suggested to correspond
to remnants of disrupted satellites. To reconstruct their origin at least three
main observational parameters - kinematics, chemical composition and age - must
be known. We determine detailed elemental abundances in stars belonging to the
so-called Group 2 of the Geneva-Copenhagen Survey and compare the chemical
composition with Galactic thin- and thick-disc stars, as well as with the
Arcturus and AF06 streams. The aim is to search for chemical signatures that
might give information about the formation history of this kinematic group of
stars. High-resolution spectra were obtained with the FIES spectrograph at the
Nordic Optical Telescope, La Palma, and were analysed with a differential model
atmosphere method. Comparison stars were observed and analysed with the same
method. The average value of [Fe/H] for the 32 stars of Group 2 is -0.42 +-
0.10 dex. The investigated group consists mainly of two 8- and 12-Gyr-old
stellar populations. Abundances of oxygen, alpha-elements, and
r-process-dominated elements are higher than in Galactic thin-disc dwarfs. This
elemental abundance pattern has similar characteristics as that of the Galactic
thick-disc. The similarity in chemical composition of stars in Group 2 with
that in stars of the thick-disc might suggest that their formation histories
are linked. The chemical composition together with the kinematic properties and
ages of stars in the investigated stars provides evidence of their common
origin and possible relation to an ancient merging event. A gas-rich satellite
merger scenario is proposed as the most likely origin. Groups 2 and 3 of the
Geneva-Copenhagen Survey might have originated in the same merging event.Comment: 17 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics, 201
Stellar substructures in the solar neighbourhood IV. Kinematic Group 1 in the Geneva-Copenhagen survey
We determine detailed elemental abundances in stars belonging to the
so-called Group 1 of the Geneva-Copenhagen survey (GCS) and compare the
chemical composition with the Galactic thin- and thick-disc stars, with the GCS
Group 2 and Group 3 stars, as well as with several kinematic streams of similar
metallicities. The aim is to search for chemical signatures that might give
information about the formation history of this kinematic group of stars.
High-resolution spectra were obtained with the Fibre-fed Echelle Spectrograph
(FIES) spectrograph at the Nordic Optical Telescope, La Palma, and were
analysed with a differential model atmosphere method. Comparison stars were
observed and analysed with the same method. The average value of [Fe/H] for the
37 stars of Group 1 is -0.20 +- 0.14 dex. Investigated Group 1 stars can be
separated into three age subgroups. Along with the main 8- and 12-Gyr-old
populations, a subgroup of stars younger than 5 Gyr can be separated as well.
Abundances of oxygen, alpha-elements, and r-process dominated elements are
higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has
similar characteristics to that of the Galactic thick disc and differs slightly
from those in Hercules, Arcturus, and AF06 stellar streams. The similar
chemical composition of stars in Group 1, as well as in Group 2 and 3, with
that in stars of the thick disc might suggest that their formation histories
are linked. The chemical composition pattern together with the kinematic
properties and ages of stars in the investigated GCS groups provide evidence of
their common origin and possible relation to an ancient merging event. A
gas-rich satellite merger scenario is proposed as the most likely origin.Comment: 17 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics, 201
CNO abundances in giants of the peculiar globular cluster NGC 1851
We provide CNO and Fe abundance investigations for a sample of up to 45 NGC 1851 giants. High-resolution spectra were obtained with the VLT UVES spectrograph in the framework of the Gaia-ESO Survey. The stars in our sample can be separated into two groups with a difference of 0.1 dex in the mean metallicity, 0.3 dex in the mean C/N, and no significant difference in the mean values of C+N+O
The M33 Globular Cluster System with PAndAS Data: The Last Outer Halo Cluster?
We use CFHT/MegaCam data to search for outer halo star clusters in M33 as
part of the Pan-Andromeda Archaeological Survey (PAndAS). This work extends
previous studies out to a projected radius of 50 kpc and covers over 40 square
degrees. We find only one new unambiguous star cluster in addition to the five
previously known in the M33 outer halo (10 kpc <= r <= 50 kpc). Although we
identify 2440 cluster candidates of various degrees of confidence from our
objective image search procedure, almost all of these are likely background
contaminants, mostly faint unresolved galaxies. We measure the luminosity,
color and structural parameters of the new cluster in addition to the five
previously-known outer halo clusters. At a projected radius of 22 kpc, the new
cluster is slightly smaller, fainter and redder than all but one of the other
outer halo clusters, and has g' ~ 19.9, (g'-i') ~ 0.6, concentration parameter
c ~ 1.0, a core radius r_c ~ 3.5 pc, and a half-light radius r_h ~ 5.5 pc. For
M33 to have so few outer halo clusters compared to M31 suggests either tidal
stripping of M33's outer halo clusters by M31, or a very different, much calmer
accretion history of M33.Comment: 37 pages, 9 figures. Accepted by the Astrophysical Journa
Scl-dE1 GC1: An Extended Globular Cluster in a Low-Luminosity Dwarf Elliptical Galaxy
We report the discovery from Hubble Space Telescope ACS images of an extended
globular cluster, denoted by Scl-dE1 GC1, in the Sculptor Group dwarf
Elliptical galaxy Scl-dE1 (Sc22). The distance of the dE is determined as 4.3
+/- 0.25 Mpc from the I magnitude of the tip of the red giant branch in the
color-magnitude diagram. At this distance the half-light radius of Scl-dE1 GC1
is ~22 pc, placing it among the largest clusters known, particularly for
globular clusters associated with dwarf galaxies. The absolute magnitude of
Scl-dE1 GC1 is Mv = -6.7 and, to within the photometric uncertainties of the
data, the cluster stellar population appears indistinguishable from that of the
dE. We suggest that there may be two modes of globular cluster formation in
dwarf galaxies, a "normal" mode with half-light radii of typically 3 pc, and an
"extended" mode with half-light radii of ~10 pc or more.Comment: 18 pages, 5 figures, 1 Table. Accepted for publication in the
Astronomical Journa
The Gaia-ESO Survey: CNO abundances in the open clusters Trumpler 20, NGC 4815, and NGC 6705
Aim of this work is to determine C, N, and O abundances in stars of Galactic
open clusters of the Gaia-ESO survey and to compare the observed abundances
with those predicted by current stellar and Galactic evolution models. In this
pilot paper, we investigate the first three intermediate-age open clusters.
High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO
VLT telescope, were analysed using a differential model atmosphere method.
Abundances of carbon were derived using the C2 band heads at 5135 and 5635.5
{\AA}. The wavelength interval 6470- 6490 {\AA}, with CN features, was analysed
to determine nitrogen abundances. Oxygen abundances were determined from the [O
i] line at 6300 {\AA}. The mean values of the elemental abundances in Trumpler
20 as determined from 42 stars are: [Fe/H] = 0.10 +- 0.08 (s.d.), [C/H] = -0.10
+- 0.07, [N/H] = 0.50 +- 0.07, and consequently C/N = 0.98 +- 0.12. We measure
from five giants in NGC 4815: [Fe/H] = -0.01 +- 0.04, [C/H] = -0.17 +- 0.08,
[N/H] = 0.53 +- 0.07, [O/H] = 0.12 +- 0.09, and C/N = 0.79 +- 0.08. We obtain
from 27 giants in NGC 6705: [Fe/H] = 0.0 +- 0.05, [C/H] = -0.08 +- 0.06, [N/H]
= 0.61 +- 0.07, [O/H] = 0.13 +- 0.05, and C/N = 0.83 +- 0.19. The C/N ratios of
stars in the investigated open clusters were compared with the ratios predicted
by stellar evolutionary models. For the corresponding stellar turn-off masses
from 1.9 to 3.3 solar masses, the observed C/N ratio values are very close to
the predictions of standard first dredge-up models as well as to models of
thermohaline extra-mixing. The average [O/H] abundance ratios of NGC 4815 and
NGC 6705 are compared with the predictions of two Galactic chemical evolution
models. The data are consistent with the evolution at the solar radius within
the errors.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics, 201
Properties of the Hyades, the eclipsing binary HD 27130, and the oscillating red giant ɛ Tauri
Stars and planetary system
The GALAH Survey : Non-LTE departure coefficients for large spectroscopic surveys
19 pages, 25 figures, 2 tables, arXiv abstract abridged; accepted for publication in A&AMassive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 1D MARCS model atmospheres that spans , , and . We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of stars from GALAH DR3. We found that relaxing LTE can change the abundances by between and for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the versus plane by up to , and it can remove spurious differences between the dwarfs and giants by up to . The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy.Peer reviewe
Chemical analysis of ancient relicts in the Milky Way disk
We present detailed analysis of two groups of F- and G- type stars originally found to have similarities in their orbital parameters. The distinct kinematic properties suggest that they might originate from ancient accretion events in the Milky Way. From high resolution spectra taken with the spectrograph FIES at the Nordic Optical Telescope, La Palma, we determined abundances of oxygen, alpha- and r-process elements. Our results indicate that the sample of investigated stars is chemically homogeneous and the abundances of oxygen, alpha and r-process elements are overabundant in comparison with Galactic disk dwarfs. This provides the additional evidence that those stellar groups had the common formation and possible origin from disrupted satellites