133 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
Stellar substructures in the solar neighbourhood. II. Abundances of neutron-capture elements in the kinematic Group 3 of the Geneva-Copenhagen survey
The evolution of chemical elements in a galaxy is linked to its star
formation history. Variations in star formation history are imprinted in the
relative abundances of chemical elements produced in different supernova events
and asymptotic giant branch stars. We determine detailed elemental abundances
of s- and r-process elements in stars belonging to Group3 of the
Geneva-Copenhagen survey and compare their chemical composition with Galactic
disc stars. The aim is to look for possible chemical signatures that might give
information about the formation history of this kinematic group of stars, which
is suggested to correspond to remnants of disrupted satellites. 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.
Abundances of chemical elements produced mainly by the s-process are similar to
those in the Galactic thin-disc dwarfs of the same metallicity, while
abundances of chemical elements produced predominantly by the r-process are
overabundant. The similar elemental abundances are observed in Galactic
thick-disc stars. The chemical composition together with the kinematic
properties and ages of stars in Group3 of the Geneva-Copenhagen survey support
a gas-rich satellite merger scenario as the most likely explanation for the
origin. The similar chemical composition of stars in Group3 and the thick-disc
stars might suggest that their formation histories are linked.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics, 2013. arXiv admin note: text overlap with arXiv:1203.619
Discovery of the most isolated globular cluster in the local universe
We report the discovery of two new globular clusters in the remote halos of
M81 and M82 in the M81 Group based on Hubble Space Telescope archive images.
They are brighter than typical globular clusters (MV = -9.34 mag for GC-1 and
M_V = -10.51 mag for GC-2), and much larger than known globular clusters with
similar luminosity in the MilkyWay Galaxy and M81. Radial surface brightness
profiles for GC-1 and GC-2 do not show any features of tidal truncation in the
outer part. They are located much farther from both M81 and M82 in the sky,
compared with previously known star clusters in these galaxies. Color-magnitude
diagrams of resolved stars in each cluster show a well-defined red giant branch
(RGB), indicating that they are metal-poor and old. We derive a low metallicity
with [Fe/H] and an old age ~14 Gyr for GC-2 from the analysis of
the absorption lines in its spectrum in the Sloan Digital Sky Survey in
comparison with the simple stellar population models. The I-band magnitude of
the tip of the RGB for GC-2 is 0.26 mag fainter than that for the halo stars in
the same field, showing that GC-2 is ~400 kpc behind the M81 halo along our
line of sight. The deprojected distance to GC-2 from M81 is much larger than
any other known globular clusters in the local universe. This shows that GC-2
is the most isolated globular cluster in the local universe.Comment: 6 pages with 5 figure
Compact Star Clusters in the M31 Disk
We have carried out a survey of compact star clusters (apparent size <3
arcsec) in the southwest part of the M31 galaxy, based on the high-resolution
Suprime-Cam images (17.5 arcmin x 28.5 arcmin), covering ~15% of the
deprojected galaxy disk area. The UBVRI photometry of 285 cluster candidates (V
< 20.5 mag) was performed using frames of the Local Group Galaxies Survey. The
final sample, containing 238 high probability star cluster candidates (typical
half-light radius r_h ~ 1.5 pc), was selected by specifying a lower limit of
r_h > 0.15 arcsec (>0.6 pc). We derived cluster parameters based on the
photometric data and multiband images by employing simple stellar population
models. The clusters have a wide range of ages from ~5 Myr (young objects
associated with 24 um and/or Ha emission) to ~10 Gyr (globular cluster
candidates), and possess mass in a range of 3.0 < log(m/M_sol) < 4.3 peaking at
m ~ 4000 M_sol. Typical age of these intermediate-mass clusters is in the range
of 30 Myr < t < 3 Gyr, with a prominent peak at ~70 Myr. These findings suggest
a rich intermediate-mass star cluster population in M31, which appears to be
scarce in the Milky Way galaxy.Comment: 16 pages, 8 figures, 1 table, accepted for publication in Ap
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