166 research outputs found
Gemini spectroscopy of the outer disk star cluster BH176
BH176 is an old metal-rich star cluster. It is spatially and kinematically
consistent with belonging to the Monoceros Ring. It is larger in size and more
distant from the Galactic plane than typical open clusters, and it does not
belong to the Galactic bulge. Our aim is to determine the origin of this unique
object by accurately determining its distance, metallicity, and age. The best
way to reach this goal is to combine spectroscopic and photometric methods. We
present medium-resolution observations of red clump and red giant branch stars
in BH176 obtained with the Gemini South Multi-Object Spectrograph.We derive
radial velocities, metallicities, effective temperatures, and surface gravities
of the observed stars and use these parameters to distinguish member stars from
field objects. We determine the following parameters for BH176:
km/s, , age Gyr, , distance
kpc, -element abundance dex (the
mean of [Mg/Fe], and [Ca/Fe]). BH176 is a member of old Galactic open clusters
that presumably belong to the thick disk. It may have originated as a massive
star cluster after the encounter of the forming thin disk with a high-velocity
gas cloud or as a satellite dwarf galaxy.Comment: 15 pages, 7 fufures, Accepted for publication in Astronomy &
Astrophysic
The environment of formation as a second parameter for globular cluster classification
We perform an evolutionary multivariate analysis of a sample of 54 Galactic globular clusters with high-quality colour-magnitude diagrams and well-determined ages. The four parameters adopted for the analysis are: metallicity, age, maximum temperature on the horizontal branch and absolute V magnitude. Our cladistic analysis breaks the sample into three novel groups. An a posteriori kinematical analysis puts groups 1 and 2 in the halo, and group 3 in the thick disc. The halo and disc clusters separately follow a luminosity-metallicity relation of much weaker slope than galaxies. This property is used to propose a new criterion for distinguishing halo and disc clusters. A comparison of the distinct properties of the two halo groups with those of Galactic halo field stars indicates that the clusters of group 1 originated in the inner halo, while those of group 2 formed in the outer halo of the Galaxy. The inner halo clusters were presumably initially the most massive one, which allowed the formation of more strongly helium-enriched second generation stars, thus explaining the presence of Cepheids and of very hot horizontal-branch stars exclusively in this group. We thus conclude that the ‘second parameter' is linked to the environment in which globular clusters form, the inner halo favouring the formation of the most massive clusters which subsequently become more strongly self-enriched than their counterparts of the galactic outer halo and dis
The construction of non-spherical models of quasi-relaxed stellar systems
Spherical models of collisionless but quasi-relaxed stellar systems have long
been studied as a natural framework for the description of globular clusters.
Here we consider the construction of self-consistent models under the same
physical conditions, but including explicitly the ingredients that lead to
departures from spherical symmetry. In particular, we focus on the effects of
the tidal field associated with the hosting galaxy. We then take a stellar
system on a circular orbit inside a galaxy represented as a "frozen" external
field. The equilibrium distribution function is obtained from the one
describing the spherical case by replacing the energy integral with the
relevant Jacobi integral in the presence of the external tidal field. Then the
construction of the model requires the investigation of a singular perturbation
problem for an elliptic partial differential equation with a free boundary, for
which we provide a method of solution to any desired order, with explicit
solutions to two orders. We outline the relevant parameter space, thus opening
the way to a systematic study of the properties of a two-parameter family of
physically justified non-spherical models of quasi-relaxed stellar systems. The
general method developed here can also be used to construct models for which
the non-spherical shape is due to internal rotation. Eventually, the models
will be a useful tool to investigate whether the shapes of globular clusters
are primarily determined by internal rotation, by external tides, or by
pressure anisotropy.Comment: AASTeX v5.2, 37 pages with 2 figures, accepted for publication in The
Astrophysical Journa
A G1-like globular cluster in NGC 1023
The structure of a very bright (MV = -10.9) globular cluster in NGC 1023 is
analyzed on two sets of images taken with the Hubble Space Telescope. From
careful modeling of King profile fits to the cluster image, a core radius of
0.55+/-0.1 pc, effective radius 3.7+/-0.3 pc and a central V-band surface
brightness of 12.9+/-0.5 mag / square arcsec are derived. This makes the
cluster much more compact than Omega Cen, but very similar to the brightest
globular cluster in M31, G1 = Mayall II. The cluster in NGC 1023 appears to be
very highly flattened with an ellipticity of about 0.37, even higher than for
Omega Cen and G1, and similar to the most flattened clusters in the Large
Magellanic Cloud.Comment: 14 pages, 3 figures, 1 table. Accepted for AJ, Oct 200
A CH star in the globular cluster NGC 6426
We report on the serendipitous discovery of a carbon star near the centre of
the low-metallicity globular cluster NGC 6426. We determined its membership and
chemical properties using medium-resolution spectra. The radial velocity of
-159 km/s makes it a member of the cluster. We used photometric data from the
literature and the COMARCS stellar atmospheric models to derive its luminosity,
effective temperature, surface gravity, metallicity, and approximate C, N, and
O abundance ratios. According to these properties, we suggest that this star is
a genuine carbon rich low-metallicity AGB star.Comment: 5 pages, accepted for publication in MNRA
Gemini spectroscopy of the outer disk star cluster BH176
© ESO, 2014. Context. BH176 is an old metal-rich star cluster. It is spatially and kinematically consistent with belonging to the Monoceros Ring. It is larger in size and more distant from the Galactic plane than typical open clusters, and it does not belong to the Galactic bulge. Aims. Our aim is to determine the origin of this unique object by accurately determining its distance, metallicity, and age. The best way to reach this goal is to combine spectroscopic and photometric methods. Methods. We present medium-resolution observations of red clump and red giant branch stars in BH176 obtained with the Gemini South Multi-Object Spectrograph. We derive radial velocities, metallicities, effective temperatures, and surface gravities of the observed stars and use these parameters to distinguish member stars from field objects. Results. We determine the following parameters for BH176: Vh = 0 ± 15 km s-1, [Fe/H] = -0.1 ± 0.1, age 7 ± 0.5 Gyr, E(V - I) = 0.79 ± 0.03, distance 15.2 ± 0.2 kpc, α-element abundance [α/Fe] ~ 0.25 dex (the mean of [Mg/Fe], and [Ca/Fe]). Conclusions. BH176 is a member of old Galactic open clusters that presumably belong to the thick disk. It may have originated as a massive star cluster after the encounter of the forming thin disk with a high-velocity gas cloud or as a satellite dwarf galaxy
Clues to Nuclear Star Cluster Formation from Edge-on Spirals
We find 9 nuclear cluster candidates in a sample of 14 edge-on, late-type
galaxies observed with HST/ACS. These clusters have magnitudes (M_I ~ -11) and
sizes (r_eff ~ 3pc) similar to those found in previous studies of face-on,
late-type spirals and dE galaxies. However, three of the nuclear clusters are
significantly flattened and show evidence for multiple, coincident structural
components. The elongations of these three clusters are aligned to within 10
degrees of the galaxies' major axes. Structurally, the flattened clusters are
well fit by a combination of a spheroid and a disk or ring. The nuclear cluster
disks/rings have F606W-F814W (~V-I) colors 0.3-0.6 magnitudes bluer than the
spheroid components, suggesting that the stars in these components have ages <
1 Gyr. In NGC 4244, the nearest of the nuclear clusters, we further constrain
the stellar populations and provide a lower limit on the dynamical mass via
spectroscopy. We also present tentative evidence that another of the nuclear
clusters (in NGC 4206) may also host a supermassive black hole. Based on our
observational results we propose an in situ formation mechanism for nuclear
clusters in which stars form episodically in compact nuclear disks, and then
lose angular momentum or heat vertically to form an older spheroidal structure.
We estimate the period between star formation episodes to be 0.5 Gyr and
discuss possible mechanisms for tranforming the disk-like components into
spheroids. We also note the connection between our objects and massive globular
clusters (e.g. Cen), UCDs, and SMBHs. (Abridged)Comment: Accepted for publication in the A
Emergence of Anaplasma Species Related to A. phagocytophilum and A. platys in Senegal
The genus Anaplasma (Anaplasmataceae, Rickettsiales) includes tick-transmitted bacterial species of importance to both veterinary and human medicine. Apart from the traditionally recognized six Anaplasma species (A. phagocytophilum, A. platys, A. bovis, A. ovis, A. centrale, A. marginale), novel strains and candidate species, also of relevance to veterinary and human medicine, are emerging worldwide. Although species related to the zoonotic A. platys and A. phagocytophilum have been reported in several African and European Mediterranean countries, data on the presence of these species in sub-Saharan countries are still lacking. This manuscript reports the investigation of Anaplasma strains related to zoonotic species in ruminants in Senegal by combining different molecular tests and phylogenetic approaches. The results demonstrated a recent introduction of Candidatus (Ca) Anaplasma turritanum, a species related to the pathogenic A. platys, possibly originating by founder effect. Further, novel undetected strains related to Candidatus (Ca) Anaplasma cinensis were detected in cattle. Based on groEL and gltA molecular comparisons, we propose including these latter strains into the Candidatus (Ca) Anaplasma africanum species. Finally, we also report the emergence of Candidatus (Ca) A. boleense in Senegal. Collectively, results confirm that Anaplasma species diversity is greater than expected and should be further investigated, and that Anaplasma routine diagnostic procedures and epidemiological surveillance should take into account specificity issues raised by the presence of these novel strains, suggesting the use of a One Health approach for the management of Anaplasmataceae in sub-Saharan Africa
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