22,854 research outputs found
Globular clusters and their contribution to the formation of the Galactic halo
This is a "biased" review because I will show recent evidence on the
contribution of globular clusters (GCs) to the halo of our Galaxy seen through
the lens of the new paradigm of multiple populations in GCs. I will show a few
examples where the chemistry of multiple populations helps to answer hot
questions including whether and how much GCs did contribute to the halo
population, if we have evidence of the GCs-halo link, what are the strengths
and weak points concerning this contribution.Comment: invited review to appear in Proc. IAU Symp. 317, XXIXth IAU General
Assembly, August 2015, Honolulu (HI, USA); eds. A. Bragaglia, M. Arnaboldi,
M. Rejkuba, D. Romano, (7 pages, 4 figures, LaTeX, using iau.cls
The Peculiar Chemical Inventory of NGC 2419: an Extreme Outer Halo "Globular Cluster"
NGC 2419 is a massive outer halo Galactic globular cluster (GC) whose stars have previously been shown to have
somewhat peculiar abundance patterns. We have observed seven luminous giants that are members of NGC 2419
with Keck/HIRES at reasonable signal-to-noise ratio. One of these giants is very peculiar, with an extremely low
[Mg/Fe] and high [K/Fe] but normal abundances of most other elements. The abundance pattern does not match
the nucleosynthetic yields of any supernova model. The other six stars show abundance ratios typical of inner halo
Galactic GCs, represented here by a sample of giants in the nearby GC M30. Although our measurements show
that NGC 2419 is unusual in some respects, its bulk properties do not provide compelling evidence for a difference between inner and outer halo GCs
Elemental Abundances in M31: Alpha and Iron Element Abundances from Low-Resolution Resolved Stellar Spectroscopy in the Stellar Halo
Measurements of [Fe/H] and [/Fe] can probe the minor merging history
of a galaxy, providing a direct way to test the hierarchical assembly paradigm.
While measurements of [/Fe] have been made in the stellar halo of the
Milky Way, little is known about detailed chemical abundances in the stellar
halo of M31. To make progress with existing telescopes, we apply spectral
synthesis to low-resolution DEIMOS spectroscopy (R 2500 at 7000
Angstroms) across a wide spectral range (4500 Angstroms 9100
Angstroms). By applying our technique to low-resolution spectra of 170 giant
stars in 5 MW globular clusters, we demonstrate that our technique reproduces
previous measurements from higher resolution spectroscopy. Based on the
intrinsic dispersion in [Fe/H] and [/Fe] of individual stars in our
combined cluster sample, we estimate systematic uncertainties of 0.11 dex
and 0.09 dex in [Fe/H] and [/Fe], respectively. We apply our
method to deep, low-resolution spectra of 11 red giant branch stars in the
smooth halo of M31, resulting in higher signal-to-noise per spectral resolution
element compared to DEIMOS medium-resolution spectroscopy, given the same
exposure time and conditions. We find [/Fe] = 0.49
0.29 dex and [Fe/H] = 1.59 0.56 dex for our
sample. This implies that---much like the Milky Way---the smooth halo of M31 is
likely composed of disrupted dwarf galaxies with truncated star formation
histories that were accreted early in the halo's formation.Comment: 21 pages, 14 figures, accepted to Ap
Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies
We present carbon abundances of red giants in Milky Way globular clusters and
dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon
abundances for 154 giants in the clusters NGC 2419, M68, and M15 and 398 giants
in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the
number of dSph stars with measurements of [C/Fe]. The [C/Fe] ratio in the
clusters decreases with increasing luminosity above log(L/L_sun) ~= 1.6, which
can be explained by deep mixing in evolved giants. The same decrease is
observed in dSphs, but the initial [C/Fe] of the dSph giants is not uniform.
Stars in dSphs at lower metallicities have larger [C/Fe] ratios. We hypothesize
that [C/Fe] (corrected to the initial carbon abundance) declines with
increasing [Fe/H] due to the metallicity dependence of the carbon yield of
asymptotic giant branch stars and due to the increasing importance of Type Ia
supernovae at higher metallicities. We also identified 11 very carbon-rich
giants (8 previously known) in three dSphs. However, our selection biases
preclude a detailed comparison to the carbon-enhanced fraction of the Milky Way
stellar halo. Nonetheless, the stars with [C/Fe] < +1 in dSphs follow a
different [C/Fe] track with [Fe/H] than the halo stars. Specifically, [C/Fe] in
dSphs begins to decline at lower [Fe/H] than in the halo. The difference in the
metallicity of the [C/Fe] "knee" adds to the evidence from [alpha/Fe]
distributions that the progenitors of the halo had a shorter timescale for
chemical enrichment than the surviving dSphs.Comment: accepted to ApJ; 20 pages, 11 figures, 2 machine-readable table
Spectroscopic Confirmation of the Dwarf Galaxies Hydra II and Pisces II and the Globular Cluster Laevens 1
We present Keck/DEIMOS spectroscopy of stars in the recently discovered Milky
Way satellites Hydra II, Pisces II, and Laevens 1. We measured a velocity
dispersion of 5.4 (+3.6 -2.4) km/s for Pisces II, but we did not resolve the
velocity dispersions of Hydra II or Laevens 1. We marginally resolved the
metallicity dispersions of Hydra II and Pisces II but not Laevens 1.
Furthermore, Hydra II and Pisces II obey the luminosity-metallicity relation
for Milky Way dwarf galaxies ( = -2.02 +/- 0.08 and -2.45 +/- 0.07,
respectively), whereas Laevens 1 does not ( = -1.68 +/- 0.05). The
kinematic and chemical properties suggest that Hydra II and Pisces II are dwarf
galaxies, and Laevens 1 is a globular cluster. We determined that two of the
previously observed blue stars near the center of Laevens 1 are not members of
the cluster. A third blue star has ambiguous membership. Hydra II has a radial
velocity = 303.1 +/- 1.4 km/s, similar to the leading arm of the
Magellanic stream. The mass-to-light ratio for Pisces II is 370 (+310 -240)
M_sun/L_sun. It is not among the most dark matter-dominated dwarf galaxies, but
it is still worthy of inclusion in the search for gamma rays from dark matter
self-annihilation.Comment: Accepted to ApJ. v2 has been revised in response to the referee's
repor
Carbon and nitrogen abundances of individual stars in the Sculptor dwarf spheroidal galaxy
We present [C/Fe] and [N/Fe] abundance ratios and CH({\lambda}4300) and
S({\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the
Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving
power R= 1150 at 4020 {\AA}. This is the first time that [N/Fe] abundances are
derived for a large number of stars in a dwarf spheroidal. We found a trend for
the [C/Fe] abundance to decrease with increasing luminosity on the RGB across
the whole metallicity range, a phenomenon observed in both field and globular
cluster giants, which can be interpreted in the framework of evolutionary
mixing of partially processed CNO material. Both our measurements of [C/Fe] and
[N/Fe] are in good agreement with the theoretical predictions for stars at
similar luminosity and metallicity. We detected a dispersion in the carbon
abundance at a given [Fe/H], which cannot be ascribed to measurement
uncertainties alone. We interpret this observational evidence as the result of
the contribution of different nucleosynthesis sources over time to a not
well-mixed interstellar medium. We report the discovery of two new
carbon-enhanced, metal-poor stars. These are likely the result of pollution
from material enriched by asymptotic giant branch stars, as indicated by our
estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular
clusters in the field of the galaxy by looking for the distinctive C-N pattern
of second population globular clusters stars in a previously detected, very
metal-poor, chemodynamical substructure. We do not detect chemical anomalies
among this group of stars. However, small number statistics and limited spatial
coverage do not allow us to exclude the hypotheses that this substructure forms
part of a tidally shredded globular cluster.Comment: 18 pages, 14 figures, 3 tables. Accepted to A&
NGC 2419 -- Another Remnant of Accretion by the Milky Way
We isolate a sample of 43 upper RGB stars in the extreme outer halo Galactic
globular cluster NGC 2419 from two Keck/DEIMOS slitmasks. The probability that
there is more than one contaminating halo field star in this sample is
extremely low. Analysis of moderate resolution spectra of these cluster
members, as well as of our Keck/HIRES high resolution spectra of a subsample of
them, demonstrates that there is a small but real spread in Ca abundance of ~
0.2 dex within this massive metal-poor globular cluster. This provides
additional support to earlier suggestions that NGC 2419 is the remnant of a
dwarf galaxy accreted long ago by the Milky Way.Comment: Accepted to the Astrophysical Journa
"Galaxy," Defined
A growing number of low luminosity and low surface brightness astronomical
objects challenge traditional notions of both galaxies and star clusters. To
address this, we propose a definition of galaxy independent of the cold dark
matter model of the universe: A galaxy is a gravitationally bound set of stars
whose properties cannot be explained by a combination of baryons and Newton's
laws of gravity. After exploring observational diagnostics of this definition,
we examine the classification of ultra-faint dwarfs, globular clusters,
ultra-compact dwarfs, and tidal dwarfs. While kinematic studies provide an
effective galaxy diagnostic in many regimes, they can be less useful for
compact or faint systems. To explore the use of [Fe/H] spread as a
complementary diagnostic, we use published spectroscopic [Fe/H] measurements of
16 Milky Way dwarfs and 24 globular clusters to calculate their [Fe/H] spreads
and uncertainties. Our principal results are: (i) no old star cluster less
luminous then M_V = -10 has a significant (> 0.1 dex) spread in iron; (ii)
known ultra-faint dwarfs can be classified with a combination of kinematics and
[Fe/H]; (iii) the observed [Fe/H] spreads in massive (> 10^6 M_Sun) globular
clusters do not necessarily imply they are the stripped nuclei of dwarfs, nor a
need for dark matter; and (iv) if ultra-compact dwarfs reside in dark matter
halos akin to those of ultra-faint dwarfs of the same half-light radii, then
they will show no clear dynamical signature of dark matter. We suggest
measurements that may assist future classification of massive globular
clusters, ultra-compact dwarfs, and ultra-faint galaxies. Our galaxy definition
is designed to be independent of the details of current observations and
models, while our proposed diagnostics can be refined or replaced as our
understanding of the universe evolves.Comment: Accepted by AJ; This updated version includes several new references,
as well as improvements throughout the text for clarity - in particular
clarifying our intended distinction between galaxy "definition" and
"diagnostics
Potassium: a new actor on the globular cluster chemical evolution stage. The case of NGC 2808
We derive [K/Fe] abundance ratios for 119 stars in the globular cluster NGC
2808, all of them having O, Na, Mg and Al abundances homogeneously measured in
previous works. We detect an intrinsic star-to-star spread in the Potassium
abundance. Moreover [K/Fe] abundance ratios display statistically significant
correlations with [Na/Fe] and [Al/Fe], and anti-correlations with [O/Fe] and
[Mg/Fe]. All the four Mg deficient stars ([Mg/Fe]<0.0) discovered so far in NGC
2808 are enriched in K by ~0.3 dex with respect to those with normal [Mg/Fe].
NGC 2808 is the second globular cluster, after NGC 2419, where a clear Mg-K
anti-correlation is detected, albeit of weaker amplitude. The simultaneous
correlation/anti-correlation of [K/Fe] with all the light elements usually
involved in the chemical anomalies observed in globular cluster stars, strongly
support the idea that these abundance patterns are due to the same
self-enrichment mechanism that produces Na-O and Mg-Al anti-correlations. This
finding suggests that detectable spreads in K abundances may be typical in the
massive globular clusters where the self-enrichment processes are observed to
produce their most extreme manifestations.Comment: Accepted for publication by ApJ, 5 pages, 3 figure
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