63 research outputs found
The Blue Straggler Population in Dwarf Galaxies
In this chapter I review the recent developments regarding the study of Blue Stragglers (BSS) in dwarf galaxies. The loose density environment of dwarf galaxies resembles that of the Galactic Halo, hence it is natural to compare their common BSS properties. At the same time, it is unescapable to compare with the BSS properties in Galactic globular clusters, which constitute the reference point for BSS studies. Admittedly, the literature on BSS in dwarf galaxies is not plentiful. The limitation is mostly due to the large distance to even the closest dwarf galaxies. Nevertheless, recent studies have allowed a deeper insight on the BSS photometric properties that are worth examining
A new red giant-based distance modulus of 13.3 Mpc to the Antennae galaxies and its consequences
The Antennae galaxies are the closest example of an ongoing major galaxy
merger, and thereby represent a unique laboratory for furthering the
understanding of the formation of exotic objects (e.g., tidal dwarf galaxies,
ultra-luminous X-ray sources, super-stellar clusters, etc). In a previous paper
HST/WFPC2 observations were used to demonstrate that the Antennae system might
be at a distance considerably less than that conventionally assumed in the
literature. Here we report new, much deeper HST/ACS imaging that resolves the
composite stellar populations, and most importantly, reveals a well-defined red
giant branch. The tip of this red giant branch (TRGB) is unambiguously detected
at Io(TRGB)=26.65 +/- 0.09 mag. Adopting the most recent calibration of the
luminosity of the TRGB then yields a distance modulus for the Antennae of
(m-M)o= 30.62 +/- 0.17 corresponding to a distance of 13.3 +/- 1.0 Mpc. This is
consistent with our earlier result, once the different calibrations for the
standard candle are considered. We briefly discuss the implications of this now
well determined shorter distance.Comment: 11 pages, 3 figures, accepted for publication in the Ap
Multiple stellar populations in Galactic globular clusters: observational evidence
An increasing number of both photometric and spectroscopic observations over
the last years have shown the existence of distinct sub-populations in many
Galactic globular clusters and shattered the paradigm of globulars hosting
single, simple stellar populations.
These multiple populations manifest themselves in a split of different
evolutionary sequences in the cluster color-magnitude diagrams and in
star-to-star abundance variations. In this paper we will summarize the
observational scenario.Comment: 6 pages, 3 figures, Proceedings SF2A 201
Lithium abundances in globular cluster giants: NGC 6218 (M12) and NGC 5904 (M5)
Convergent lines of evidence suggest that globular clusters host multiple
stellar populations. It appears that they experience at least two episodes of
star formation whereby a fraction of first-generation stars contribute astrated
ejecta to form the second generation(s). To identify the polluting progenitors
we require distinguishing chemical signatures such as that provided by lithium.
Theoretical models predict that lithium can be synthesised in AGB stars,
whereas no net Li production is expected from other candidates. It has been
shown that in order to reproduce the abundance pattern found in M4, Li
production must occur within the polluters, favouring the AGB scenario. Here we
present Li and Al abundances for a large sample of RGB stars in M12 and M5.
These clusters have a very similar metallicity, whilst demonstrating
differences in several cluster properties. Our results indicate that the
first-generation and second-generation stars share the same Li content in M12;
we recover an abundance pattern similar to that observed in M4. In M5 we find a
higher degree of complexity and a simple dilution model fails in reproducing
the majority of the stellar population. In both clusters we require Li
production across the different stellar generations, but production seems to
have occurred to different extents. We suggest that such a difference might be
related to the cluster mass with the Li production being more efficient in
less-massive clusters. This is the first time a statistically significant
correlation between the Li spread within a GC and its luminosity has been
demonstrated. Finally, although Li-producing polluters are required to account
for the observed pattern, other mechanisms, such as MS depletion, might have
played a role in contributing to the Li internal variation, though at
relatively low level.Comment: Accepted for publication in The Astrophysical Journal. 15 pages, 14
figure
On the serendipitous discovery of a Li-rich giant in the globular cluster NGC 362
We have serendipitously identified the first lithium-rich giant star located
close to the red giant branch bump in a globular cluster. Through
intermediate-resolution FLAMES spectra we derived a lithium abundance of
A(Li)=2.55 (assuming local thermodynamical equilibrium), which is extremely
high considering the star's evolutionary stage. Kinematic and photometric
analysis confirm the object as a member of the globular cluster NGC 362. This
is the fourth Li-rich giant discovered in a globular cluster but the only one
known to exist at a luminosity close to the bump magnitude. The three previous
detections are clearly more evolved, located close to, or beyond the tip of
their red giant branch. Our observations are able to discard the accretion of
planets/brown dwarfs, as well as an enhanced mass-loss mechanism as a formation
channel for this rare object. Whilst the star sits just above the cluster bump
luminosity, its temperature places it towards the blue side of the giant branch
in the colour-magnitude diagram. We require further dedicated observations to
unambiguously identify the star as a red giant: we are currently unable to
confirm whether Li production has occurred at the bump of the luminosity
function or if the star is on the pre zero-age horizontal branch. The latter
scenario provides the opportunity for the star to have synthesised Li rapidly
during the core helium flash or gradually during its red giant branch ascent
via some extra mixing process.Comment: Accepted for publication in The Astrophysical Journal Letter
The near-IR luminosity-metallicity relation of dwarf irregular galaxies
We report on the recent developments of our long-term investigation of the
near-IR luminosity-metallicity relation for dwarf irregular galaxies in nearby
groups. A very well-defined relation is emerging from our observational
database, and a preliminary discussion of its implications is given.Comment: 5 pages, 3 figures, to be published in the IAU Coll. 198, "Near-Field
Cosmology With Dwarf Elliptical Galaxies", ed. B. Binggeli and H. Jerje
Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement
Having shown in a recent paper that the main sequence of omega Centauri is
split into two distinct branches, we now present spectroscopic results showing
that the bluer sequence is_less_ metal-poor. We have carefully combined
GIRAFFE@VLT spectra of 17 stars on each side of the split into a single
spectrum for each branch, with adequate S/N to show clearly that the stars of
the blue main sequence are less metal poor by 0.3 dex than those of the
dominant red one. From an analysis of the individual spectra, we could not
detect any abundance spread among the blue main sequence stars, while the red
main sequence stars show a 0.2 dex spread in metallicity. We use
stellar-structure models to show that only greatly enhanced helium can explain
the color difference between the two main sequences, and we discuss ways in
which this enhancement could have arisen.Comment: 24 pages, 7 figures, The Astrophysical Journal, accepted 30 Nov. 200
Transforming observational data and theoretical isochrones into the ACS/WFC Vega-mag system
We propose a zero-point photometric calibration of the data from the ACS/WFC
on board the Hubble Space Telescope, based on a spectrum of Vega and the most
up to date in-flight transmission curves of the camera. This calibration is
accurate at the level of a few hundredths of a magnitude. The main purpose of
this effort is to transform the entire set of evolutionary models by
Pietrinferni et al. (2004) into a simple observational photometric system for
ACS/WFC data, and make them available to the astronomical community. We provide
the zero points for the most used ACS/WFC bands, and give basic recipes for
calibrating both the observed data and the models.
We also present the Colour Magnitude Diagram (CMD) from ACS data of 5
Galactic globular clusters, spanning the metallicity range -2.2<[Fe/H]<-0.04,
and provide fiducial points representing their sequences from several
magnitudes below the turnoff to the red giant branch tip. The observed
sequences are compared with the models in the newly defined photometric system.Comment: 10 pages, 3 figures, 7 Tables. Accepted (2004 Dec 14) on M.N.R.A.
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