224 research outputs found
The ACS LCID project VII: the blue stragglers population in the isolated dSph galaxies Cetus and Tucana
We present the first investigation of the Blue Straggler star (BSS)
population in two isolated dwarf spheroidal galaxies of the Local Group, Cetus
and Tucana. Deep HST/ACS photometry allowed us to identify samples of 940 and
1214 candidates, respectively. The analysis of the star formation histories of
the two galaxies suggests that both host a population of BSSs. Specifically, if
the BSS candidates are interpreted as young main sequence stars, they do not
conform to their galaxy's age-metallicity relationship. The analysis of the
luminosity function and the radial distributions support this conclusion, and
suggest a non-collisional mechanism for the BSS formation, from the evolution
of primordial binaries. This scenario is also supported by the results of new
dynamical simulations presented here. Both galaxies coincide with the
relationship between the BSS frequency and the absolute visual magnitude Mv
found by Momany et al (2007). If this relationship is confirmed by larger
sample, then it could be a valuable tool to discriminate between the presence
of BSSs and galaxies hosting truly young populations.Comment: Accepted for publication on ApJ. 15 pages, 3 tables, 13 figures. A
version with high resolution figure can be downloaded from
http://rialto.ll.iac.es/proyecto/LCID/?p=publication
Validation of a multi-scale simulation strategy based on the Pointwise Strain Superposition Method
This paper details the experimental validation of a multi-scale simulation strategy that we developed for predicting the stresses and distortions induced by Powder Bed Fusion processes. The strategy comprises a meso-scale model, a macro-scale model, and a scaling method named Pointwise Strain Superposition. The first model evaluates the temperature, stress, and strain fields produced by a single scan line. The scaling method transfers the meso-scale results to the macro-scale model, which is then able to simulate the entire manufacturing process with a reasonable computational cost. The simulation strategy was validated by comparing its results with the stresses and distortions measured on several specimens made of selective laser melted Inconel 718. Stresses were measured through the blind hole drilling method on a cylindrical specimen printed with two different scanning strategies, while distortions were measured on a hollow cylinder and on a cantilever-shaped specimen after removing its supports. In both cases, the simulation showed first- or higher-order accuracy despite the significant uncertainties regarding the input parameters and material properties. This robustness, coupled with its computational efficiency, leads us to believe that our simulation strategy could enhance the process optimization and provide a better understanding of the underlying physical phenomena along with their effects on the manufactured parts
Variable Stars in the Cetus dSph Galaxy: Population Gradients and Connections with the Star Formation History
We investigate the variable star content of the isolated, Local Group, dwarf
spheroidal galaxy (dSph) Cetus. Multi-epoch, wide-field images collected with
the VLT/VIMOS camera allowed us to detect 638 variable stars (630 RR Lyrae
stars and 8 Anomalous Cepheids), 475 of which are new detections. We present a
full catalogue of periods, amplitudes, and mean magnitudes. Motivated by the
recent discovery that the pulsational properties of the RR Lyrae stars in the
Tucana dSph revealed the presence of a metallicity gradient within the oldest
(>10 Gyr old) stellar populations, we investigated the possibility of an
analogous effect in Cetus. We found that, despite the obvious radial gradient
in the Horizontal Branch (HB) and Red Giant Branch (RGB) morphologies, both
becoming bluer on average for increasing distance from the center of Cetus, the
properties of the RR Lyrae stars are homogeneous within the investigated area
(out to r~15'), with no significant evidence of a radial gradient. We discuss
this in connection with the star formation history (SFH) previously derived for
the two galaxies. The observed differences between these two systems show that
even systems this small show a variety of early evolutionary histories. These
differences could be due to different merger or accretion histories.Comment: Accepted for publication on MNRAS. The complete set of light curves
and finding charts, together with the full table of the pulsational
properties of all variable stars will be available in the on-line edition of
the pape
Clear evidence for the presence of second-generation asymptotic giant branch stars in metal-poor Galactic globular clusters
Galactic globular clusters (GCs) are known to host multiple stellar
populations: a first generation with a chemical pattern typical of halo field
stars and a second generation (SG) enriched in Na and Al and depleted in O and
Mg. Both stellar generations are found at different evolutionary stages (e.g.,
the main-sequence turnoff, the subgiant branch, and the red giant branch). The
non detection of SG asymptotic giant branch (AGB) stars in several metal-poor
([Fe/H] < -1) GCs suggests that not all SG stars ascend the AGB phase, and that
failed AGB stars may be very common in metal-poor GCs. This observation
represents a serious problem for stellar evolution and GC formation/evolution
theories. We report fourteen SG-AGB stars in four metal-poor GCs (M 13, M 5, M
3, and M 2) with different observational properties: horizontal branch (HB)
morphology, metallicity, and age. By combining the H-band Al abundances
obtained by the APOGEE survey with ground-based optical photometry, we identify
SG Al-rich AGB stars in these four GCs and show that Al-rich RGB/AGB GC stars
should be Na-rich. Our observations provide strong support for present,
standard stellar models, i.e., without including a strong mass-loss efficiency,
for low-mass HB stars. In fact, current empirical evidence is in agreement with
the predicted distribution of FG and and SG stars during the He-burning stages
based on these standard stellar models.Comment: Accepted for publication in The Astrophysical Journal Letters (16
pages, 4 figures, and 1 table
Weak Galactic Halo--Fornax dSph Connection from RR Lyrae Stars
For the first time accurate pulsation properties of the ancient variable
stars of the Fornax dwarf spheroidal galaxy (dSph) are discussed in the broad
context of galaxy formation and evolution. Homogeneous multi-band optical
photometry of spanning {\it twenty} years has allowed us to identify and
characterize more than 1400 RR Lyrae stars (RRLs) in this galaxy. Roughly 70\%
are new discoveries. We investigate the period-amplitude distribution and find
that Fornax shows a lack of High Amplitude (A_V\gsim0.75 mag) Short Period
fundamental-mode RRLs (P\lsim0.48 d, HASPs). These objects occur in stellar
populations more metal-rich than [Fe/H]-1.5 and they are common in the
Galactic halo (Halo) and in globulars. This evidence suggests that old (age
older than 10 Gyr) Fornax stars are relatively metal-poor.
A detailed statistical analysis of the role of the present-day Fornax dSph in
reproducing the Halo period distribution shows that it can account for only a
few to 20\% of the Halo when combined with RRLs in massive dwarf galaxies
(Sagittarius dSph, Large Magellanic Cloud). This finding indicates that
Fornax-like systems played a minor role in building up the Halo when compared
with massive dwarfs. We also discuss the occurrence of HASPs in connection with
the luminosity and the early chemical composition of nearby dwarf galaxies. We
find that, independently of their individual star formation histories, bright
(M_V\lsim-13.5 mag) galaxies have HASPs, whereas faint ones (M_V\gsim-11
mag) do not. Interestingly enough, Fornax belongs to a luminosity range
(--11M--13.5 mag) in which the occurrence of HASPs appears to be
correlated with the early star formation and chemical enrichment of the host
galaxy.Comment: 7 pages, 5 figures, A&A, accepte
The ACS LCID project. VI. The SFH of the Tucana dSph and the relative ages of the isolated dSph galaxies
We present a detailed study of the star formation history (SFH) of the Tucana
dwarf spheroidal galaxy. High quality, deep HST/ACS data, allowed us to obtain
the deepest color-magnitude diagram to date, reaching the old main sequence
turnoff (F814 ~ 29) with good photometric accuracy. Our analysis, based on
three different SFH codes, shows that Tucana is an old and metal-poor stellar
system, which experienced a strong initial burst of star formation at a very
early epoch (~ 13 Gyr ago) which lasted a maximum of 1 Gyr (sigma value). We
are not able to unambiguously answer the question of whether most star
formation in Tucana occurred before or after the end of the reionization era,
and we analyze alternative scenarios that may explain the transformation of
Tucana from a gas-rich galaxy into a dSph. Current measurements of its radial
velocity do not preclude that Tucana may have crossed the inner regions of the
Local Group once, and so gas stripping by ram pressure and tides due to a close
interaction cannot be ruled out. On the other hand, the high star formation
rate measured at early times may have injected enough energy into the
interstellar medium to blow out a significant fraction of the initial gas
content. Gas that is heated but not blown out would also be more easily
stripped via ram pressure. We compare the SFH inferred for Tucana with that of
Cetus, the other isolated LG dSph galaxy in the LCID sample. We show that the
formation time of the bulk of star formation in Cetus is clearly delayed with
respect to that of Tucana. This reinforces the conclusion of Monelli et al.
(2010) that Cetus formed the vast majority of its stars after the end of the
reionization era implying, therefore, that small dwarf galaxies are not
necessarily strongly affected by reionization, in agreement with many
state-of-the-art cosmological models. [abridged]Comment: Accepted for publication on ApJ, 19 pages, 10 figures, 2 tables. A
version with full resolution figures is available at
http://www.iac.es/project/LCID/?p=publication
The ACS LCID Project: On the origin of dwarf galaxy types: a manifestation of the halo assembly bias?
We discuss how knowledge of the whole evolutionary history of dwarf galaxies,
including details on the early star formation events, can provide insight on
the origin of the different dwarf galaxy types. We suggest that these types may
be imprinted by the early conditions of formation rather than being only the
result of a recent morphological transformation driven by environmental
effects. We present precise star formation histories of a sample of Local Group
dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest
main-sequence turnoffs. We argue that these galaxies can be assigned to two
basic types: fast dwarfs that started their evolution with a dominant and short
star formation event, and slow dwarfs that formed a small fraction of their
stars early and have continued forming stars until the present time (or
almost). These two different evolutionary paths do not map directly onto the
present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast
dwarfs also differ in their inferred past location relative to the Milky Way
and/or M31, which hints that slow dwarfs were generally assembled in lower
density environments than fast dwarfs. We propose that the distinction between
a fast and slow dwarf galaxy reflects primarily the characteristic density of
the environment where they form. At a later stage, interaction with a large
host galaxy may play a role in the final gas removal and ultimate termination
of star formation.Comment: 7 pages, 3 figures, ApJ Letters, submitted. Comments welcom
The halo+cluster system of the Galactic globular cluster NGC1851
NGC1851 is surrounded by a stellar component that extends more than ten times
beyond the tidal radius. Although the nature of this stellar structure is not
known, it has been suggested to be a sparse halo of stars or associated with a
stellar stream. We analyse the nature of this intriguing stellar component
surrounding NGC1851 by investigating its radial velocities and chemical
composition, in particular in comparison with those of the central cluster
analysed in a homogeneous manner. In total we observed 23 stars in the halo
with radial velocities consistent with NGC1851, and for 15 of them we infer
[Fe/H] abundances. Our results show that: (i) stars dynamically linked to
NGC1851 are present at least up to ~2.5 tidal radii, supporting the presence of
a halo of stars surrounding the cluster; (ii) apart from the NGC1851 radial
velocity-like stars, our observed velocity distribution agrees with that
expected from Galactic models, suggesting that no other sub-structure (such as
a stream) at different radial velocities is present in our field; (iii) the
chemical abundances for the s-process elements Sr and Ba are consistent with
the s-normal stars observed in NGC1851; (iv) all halo stars have metallicities,
and abundances for the other studied elements Ca, Mg and Cr, consistent with
those exhibited by the cluster. The complexity of the whole NGC1851
cluster+halo system may agree with the scenario of a tidally-disrupted dwarf
galaxy in which NGC1851 was originally embedded.Comment: 21 pages, 16 figures, accepted for publication in MNRA
The ACS LCID project. X. The Star Formation History of IC 1613: Revisiting the Over-Cooling Problem
We present an analysis of the star formation history (SFH) of a field near
the half light radius in the Local Group dwarf irregular galaxy IC 1613 based
on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our
observations reach the oldest main sequence turn-off, allowing a time
resolution at the oldest ages of ~1 Gyr. Our analysis shows that the SFH of the
observed field in IC 1613 is consistent with being constant over the entire
lifetime of the galaxy. These observations rule out an early dominant episode
of star formation in IC 1613. We compare the SFH of IC 1613 with expectations
from cosmological models. Since most of the mass is in place at early times for
low mass halos, a naive expectation is that most of the star formation should
have taken place at early times. Models in which star formation follows mass
accretion result in too many stars formed early and gas mass fractions which
are too low today (the "over-cooling problem"). The depth of the present
photometry of IC 1613 shows that, at a resolution of ~1 Gyr, the star formation
rate is consistent with being constant, at even the earliest times, which is
difficult to achieve in models where star formation follows mass assembly.Comment: 13 pages, 12 figures, accepted for publication in the Ap
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