468 research outputs found
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
Kinematical & Chemical Characteristics of the Thin and Thick Disks
I discuss how the chemical abundance distributions, kinematics and age
distributions of stars in the thin and thick disks of the Galaxy can be used to
decipher the merger history of the Milky Way, a typical large galaxy. The
observational evidence points to a rather quiescent past merging history,
unusual in the context of the `consensus' cold-dark-matter cosmology favoured
from observations of structure on scales larger than individual galaxies.Comment: 12 pages, 4 figures; review at IAU Symposium 254, `The Galaxy Disk in
Cosmological Context', Copenhagen, June 2008, eds J. Andersen, J.
Bland-Hawthorn & B. Nordstro
The ubiquitous nature of the Horizontal Branch second U-jump: A link with the Blue Hook scenario?
In a previous paper we reported on a discontinuity in the extreme horizontal
branch (EHB) of the Galactic globular cluster NGC6752, which we called the
second U-jump. This feature was attributed to a combination of post zero-age
horizontal branch evolution and diffusion effects. In this follow-up study we
analyze other EHB clusters and show that the second U-jump is a common feature
among EHB clusters reaching T_{eff}\ge 23,000K, and that its onset in different
clusters converges around T_{eff}\sim 21,000\pm3,000K. We also present
near-ultraviolet diagrams of \omega Cen and NGC2808, the only two objects with
spectroscopically confirmed ``blue hook'' stars (T_{eff}\ge 35,000K). We
confirm predictions of a photometric discontinuity separating late from
early-helium flashers. Moreover, we present empirical evidence that the second
U-jump population might be mainly composed by early-helium flashers. Lastly, we
revisit the discussion on the ubiquitous nature of the gaps and jumps so far
identified in the blue HB tails, suggesting a possible discrete nature of the
distribution in temperature of the HB stars.Comment: 15 pages, 8 figures. Higher resolution version available via ftp at
ftp://ftp.pd.astro.it/pub/momany/momany.tar A&A accepte
Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
(Abridged) We derive the structure of the Galactic stellar Warp and Flare
using 2MASS RC and RGB stars, selected at mean heliocentric distances of 3, 7
and 17 kpc.
Our results are: (i) a clear stellar warp signature is derived for the 3
selected rings; (ii) the derived stellar warp is consistent (both in amplitude
and phase-angle) with that for the Galactic interstellar dust and HI gas; (iii)
the Sun seems not to fall on the line of nodes. The stellar warp phase-angle
orientation (+15 degrees) is close to the orientation angle of the Galactic bar
and this produces an asymmetric warp for the inner rings; (iv) a
Northern/Southern warp symmetry is observed only for the ring at 17 kpc; (v)
treating a mixture of thin and thick disk populations we trace the disk flaring
and derive a constant scale-height (~0.65 kpc) within R(GC)~15 kpc. Further
out, the disk flaring increase gradually reaching a mean scale-height of ~1.5
kpc at R(GC)~23 kpc; and (vi) these results provide further robust evidence
that there is no disk radial truncation at R(GC)~14 kpc.
In the particular case of the Canis Major over-density we confirm its
coincidence with the Southern stellar maximum warp occurring near l=240. We
present evidence to conclude that all observed parameters (e.g. number density,
radial velocities, proper motion etc) of CMa are consistent with it being a
normal Milky Way outer-disk population, thereby leaving no justification for a
more complex interpretations of its origin. The present analysis does not
provide a conclusive test of the structure or origin of the Monoceros Ring.
Nevertheless, we show that a warped flared Milky Way contributes significantly
at the locations of the Monoceros Ring.Comment: 25 pages, 22 figures, accepted for publication in A&A. A higher
resolution pdf file is available at
http://wwwuser.oat.ts.astro.it/zaggia/public_html/warp
The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)
We present the abundance analysis of 82 red giant branch stars in the dense,
metal-poor globular cluster NGC 6093 (M 80), the largest sample of stars
analyzed in this way for this cluster. From high resolution UVES spectra of 14
stars and intermediate resolution GIRAFFE spectra for the other stars we
derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu. On our UVES metallicity scale the mean
metal abundance of M 80 is [Fe/H]=-1.791+/-0.006+/-0.076 (+/-statistical
+/-systematic error) with rms=0.023 (14 stars). M 80 shows star to star
variations in proton-capture elements, and the extension of the Na-O
anticorrelation perfectly fit the relations with (i) total cluster mass, (ii)
horizontal branch morphology, and (iii) cluster concentration previously found
by our group. The chemistry of multiple stellar populations in M 80 does not
look extreme. The cluster is also a typical representative of halo globular
clusters for what concerns the pattern of alpha-capture and Fe-group elements.
However we found that a significant contribution from the s-process is required
to account for the distribution of neutron-capture elements. A minority of
stars in M 80 seem to exhibit slightly enhanced abundances of s-process
species, compatible with those observed in M 22 and NGC 1851, although further
confirmation from larger samples is required.Comment: 18 pages, 21 figures, 10 tables; accepted for publication on
Astronomy and Astrophysic
Mapping low-latitude stellar substructure with SEGUE photometry
Encircling the Milky Way at low latitudes, the Low Latitude Stream is a large
stellar structure, the origin of which is as yet unknown. As part of the SEGUE
survey, several photometric scans have been obtained that cross the Galactic
plane, spread over a longitude range of 50 to 203 degrees. These data allow a
systematic study of the structure of the Galaxy at low latitudes, where the Low
Latitude Stream resides. We apply colour-magnitude diagram fitting techniques
to map the stellar (sub)structure in these regions, enabling the detection of
overdensities with respect to smooth models. These detections can be used to
distinguish between different models of the Low Latitude Stream, and help to
shed light on the nature of the system.Comment: To appear in the proceedings of IAU Symposium 254 "The Galaxy disk in
a cosmological context", Copenhagen, June 200
NGC 362: another globular cluster with a split red giant branch
We obtained FLAMES GIRAFFE+UVES spectra for both first and second-generation
red giant branch (RGB) stars in the globular cluster (GC) NGC 362 and used them
to derive abundances of 21 atomic species for a sample of 92 stars. The
surveyed elements include proton-capture (O, Na, Mg, Al, Si), alpha-capture
(Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr,
Ba, La, Ce, Nd, Eu, Dy). The analysis is fully consistent with that presented
for twenty GCs in previous papers of this series. Stars in NGC 362 seem to be
clustered into two discrete groups along the Na-O anti-correlation, with a gap
at [O/Na] 0 dex. Na-rich, second generation stars show a trend to be more
centrally concentrated, although the level of confidence is not very high. When
compared to the classical second-parameter twin NGC 288, with similar
metallicity, but different horizontal branch type and much lower total mass,
the proton-capture processing in stars of NGC 362 seems to be more extreme,
confirming previous analysis. We discovered the presence of a secondary RGB
sequence, redder than the bulk of the RGB: a preliminary estimate shows that
this sequence comprises about 6% of RGB stars. Our spectroscopic data and
literature photometry indicate that this sequence is populated almost
exclusively by giants rich in Ba, and probably rich in all s-process elements,
as found in other clusters. In this regards, NGC 362 joins previously studied
GCs like NGC 1851, NGC 6656 (M 22), and NGC 7089 (M 2).Comment: 16 pages, 23 figures, 11 tables, accepted for publication on
Astronomy and Astrophysic
The extreme chemistry of multiple stellar populations in the metal-poor globular cluster NGC 4833
Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is
extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal
branch (HB). We present the abundance analysis for a large sample of 78 red
giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived
abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y,
Ba, La, Nd. This is the first extensive study of this cluster from high
resolution spectroscopy. On the scale of our survey, the metallicity of NGC
4833 is [Fe/H]=-2.015+/-0.004+/-0.084 dex (rms=0.014 dex) from 12 stars
observed with UVES, where the first error is from statistics and the second one
refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous
at better than 6%. On the other hand, the light elements involved in
proton-capture reactions at high temperature show the large star-to-star
variations observed in almost all GCs studied so far. The Na-O anticorrelation
in NGC 4833 is quite extended, as expected from the high temperatures reached
by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with
extreme [O/Na] ratios. More striking is the finding that large star-to-star
variations are seen also for Mg, which spans a range of more than 0.5 dex in
this GC. Depletions in Mg are correlated to the abundances of O and
anti-correlated with Na, Al, and Si abundances. This pattern suggests the
action of nuclear processing at unusually high temperatures, producing the
extreme chemistry observed in the stellar generations of NGC 4833. This extreme
changes are also seen in giants of the much more massive GCs M 54 and omega
Cen, and our conclusion is that NGC 4833 has probably lost a conpicuous
fraction of its original mass due to bulge shocking, as also indicated by its
orbit.Comment: 18 pages, 16 figures, 11 tables; accepted for publication on
Astronomy and Astrophysic
Probing the Canis Major stellar over-density as due to the Galactic warp
Proper-motion, star counts and photometric catalog simulations are used to
explain the detected stellar over-density in the region of Canis Major (CMa),
claimed to be the core of a disrupted dwarf galaxy (Martin et al. 2004,
Bellazzini et al. 2003), as due to the Galactic warp and flare in the external
disk. We compare the kinematics of CMa M-giant selected sample with surrounding
Galactic disk stars in the UCAC2 catalog and find no peculiar proper motion
signature: CMa stars mimic thick disk kinematics. Moreover, when taking into
account the Galactic warp and flare of the disk, 2MASS star count profiles
reproduce the CMa stellar over-density. This star count analysis is confirmed
by direct comparison with synthetic color-magnitude diagrams simulated with the
Besancon models (Robin et al. 2003) that include the warp and flare of the
disk. The presented evidence casts doubt on the identification of the CMa
over-density as the core of a disrupted Milky Way satellite. This however does
not make clear the origin of over-densities responsible for the ring structure
in the anticenter direction of the Galactic halo (Newberg et al. 2002; Yanny et
al. 2003; Zaggia et al. 2004, in preparation).Comment: Accepted for publication in A&A Letters, 4 page
Multiple stellar populations in the Galactic globular cluster NGC 6752
We have carried out high-precision photometry on a large number of archival
HST images of the Galactic globular cluster NGC 6752, to search for signs of
multiple stellar populations. We find a broadened main sequence, and
demonstrate that this broadening cannot be attributed either to binaries or to
photometric errors. There is also some indication of a main-sequence split. No
significant spread could be found along the subgiant branch, however.
Ground-based photometry reveals that in the U vs. (U-B) color-magnitude
diagram the red-giant branch exhibits a clear color spread, which we have been
able to correlate with variations in Na and O abundances. In particular the
Na-rich, O-poor stars identified by Carretta et al. (2007) define a sequence on
the red side of the red-giant branch, while Na-poor, O-rich stars populate a
bluer, more dispersed portion of the red-giant branch.Comment: 31 pages, 12 figures; Accepted for Publication in the Astrophysical
Journa
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