402 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
The origin of the LMC stellar bar: clues from the SFH of the bar and inner disk
We discuss the origin of the LMC stellar bar by comparing the star formation
histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar
and in a number of fields in different directions within the inner disk. The
CMDs, reaching the oldest main sequence turnoffs in these very crowded fields,
have been obtained with VIMOS on the VLT in service mode, under very good
seeing conditions. We show that the SFHs of all fields share the same patterns,
with consistent variations of the star formation rate as a function of time in
all of them. We therefore conclude that no specific event of star formation can
be identified with the formation of the LMC bar, which instead likely formed
from a redistribution of disk material that occurred when the LMC disk became
bar unstable, and shared a common SFH with the inner disk thereafter. The
strong similarity between the SFH of the center and edge of the bar rules out
significant spatial variations of the SFH across the bar, which are predicted
by scenarios of classic bar formation through buckling mechanisms.Comment: MNRAS Letters, accepte
On the central helium-burning variable stars of the LeoI dwarf spheroidal galaxy
We present a study of short period, central helium-burning variable stars in
the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and
51 Cepheids. So far, this is the largest sample of Cepheids and the largest
Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with
other Local Group dwarf spheroidals, Carina and Fornax, shows that the period
distribution of RR Lyrae stars is quite similar, suggesting similar properties
of the parent populations, whereas the Cepheid period distribution in LeoI
peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a
broader range, from 0.5 to 1.78d.
Evolutionary and pulsation predictions indicate, assuming a mean metallicity
peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids
traces a unique mix of Anomalous Cepheids (blue extent of the red--clump,
partially electron degenerate central helium-burning stars) and short-period
classical Cepheids (blue-loop, quiescent central helium-burning stars). Current
evolutionary prescriptions also indicate that the transition mass between the
two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars
metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different
implications of the current findings on the star formation history of LeoI.Comment: 5 Pages, 4 Figures, ApJ letter, accepte
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
Spatially resolved LMC star formation history: I. Outside in evolution of the outer LMC disk
We study the evolution of three fields in the outer LMC disk Rgc=3.5-6.2 Kpc.
Their star formation history indicates a stellar populations gradient such that
younger stellar populations are more centrally concentrated. We identify two
main star forming epochs, separated by a period of lower activity between ~7
and ~4 Gyr ago. Their relative importance varies from a similar amount of stars
formed in the two epochs in the innermost field, to only 40% of the stars
formed in the more recent epoch in the outermost field. The young star forming
epoch continues to the present time in the innermost field, but lasted only
till ~0.8 and 1.3 Gyr ago at Rgc=5.5 degrees and 7.1 degrees, respectively.
This gradient is correlated with the measured HI column density and implies an
outside-in quenching of the star formation, possibly related to a variation of
the size of the HI disk. This could either result from gas depletion due to
star formation or ram-pressure stripping, or from to the compression of the gas
disk as ram-pressure from the Milky Way halo acted on the LMC interstellar
medium. The latter two situations may have occurred when the LMC first
approached the Milky Way.Comment: 15 pages, 13 figures, 4 tables. MNRAS, in pres
Homogeneous Photometry VI: Variable Stars in the Leo I Dwarf Spheroidal Galaxy
We have characterized the pulsation properties of 164 candidate RR Lyrae
variables (RRLs) and 55 candidate Anomalous and/or short-period Cepheids in Leo
I dwarf spheroidal galaxy. On the basis of its RRLs Leo I is confirmed to be an
Oosterhoff-intermediate type galaxy, like several other dwarfs. We show that in
their pulsation properties, the RRLs representing the oldest stellar population
in the galaxy are not significantly different from those of five other nearby,
isolated dwarf spheroidal galaxies. A similar result is obtained when comparing
them to RR Lyrae stars in recently discovered ultra-faint dwarf galaxies. We
are able to compare the period distributions and period-amplitude relations for
a statistically significant sample of ab type RR Lyrae stars in dwarf galaxies
(~1300stars) with those in the Galactic halo field (~14,000stars) and globular
clusters (~1000stars). Field RRLs show a significant change in their period
distribution when moving from the inner (dG14kpc)
halo regions. This suggests that the halo formed from (at least) two dissimilar
progenitors or types of progenitor. Considered together, the RRLs in classical
dwarf spheroidal and ultra-faint dwarf galaxies-as observed today-do not appear
to follow the well defined pulsation properties shown by those in either the
inner or the outer Galactic halo, nor do they have the same properties as RRLs
in globular clusters. In particular, the samples of fundamental-mode RRLs in
dwarfs seem to lack High Amplitudes and Short Periods ("HASP":AV>1.0mag and P
<0.48d) when compared with those observed in the Galactic halo field and
globular clusters. The observed properties of RRLs do not support the idea that
currently existing classical dwarf spheroidal and ultra-faint dwarf galaxies
are surviving representative examples of the original building blocks of the
Galactic halo.Comment: 49 pages in referee format, 12 figure
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
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
Homogeneous photometry VII. Globular clusters in the Gaia era
We present wide-field, ground-based Johnson-Cousins UBVRI photometry for 48
Galactic globular clusters based on almost 90000 public and proprietary images.
The photometry is calibrated with the latest transformations obtained in the
framework of our secondary standard project, with typical internal and external
uncertainties of order a few millimagnitudes. These data provide a bridge
between existing small-area, high-precision HST photometry and all
sky-catalogues from large surveys like Gaia, SDSS, or LSST. For many clusters,
we present the first publicly available photometry in some of the five bands
(typically U and R). We illustrate the scientific potential of the photometry
with examples of surface density and brightness profiles and of
colour-magnitude diagrams, with the following highlights: (i) we study the
morphology of NGC 5904, finding a varying ellipticity and position angle as a
function of radial distance; (ii) we show U-based colour-magnitude diagrams and
demonstrate that no cluster in our sample is free from multiple stellar
populations, with the possible exception of a few clusters with high and
differential reddening or field contamination, for which more sophisticated
investigations are required. This is true even for NGC 5694 and Terzan 8, that
were previously considered as (mostly) single-population candidates.Comment: 24 pages, 12 figures, accepted for publication by MNRA
Variable Stars in Local Group Galaxies. I: Tracing the Early Chemical Enrichment and Radial Gradients in the Sculptor dSph with RR Lyrae Stars
We identified and characterized the largest (536) RR Lyrae (RRL) sample in a
Milky Way dSph satellite (Sculptor) based on optical photometry data collected
over 24 years.
The RRLs display a spread in V-magnitude (0.35 mag) which appears
larger than photometric errors and the horizontal branch (HB) luminosity
evolution of a mono-metallic population. Using several calibrations of two
different reddening free and metal independent Period-Wesenheit relations we
provide a new distance estimate =19.62 mag (=0.04 mag) that
agrees well with literature estimates. We constrained the metallicity
distribution of the old population, using the Period-Luminosity relation,
and we found that it ranges from -2.3 to -1.5 dex. The current estimate is
narrower than suggested by low and intermediate spectroscopy of RGBs
([Fe/H] 1.5).
We also investigated the HB morphology as a function of the galactocentric
distance. The HB in the innermost regions is dominated by red HB stars and by
RRLs, consistent with a more metal-rich population, while in the outermost
regions it is dominated by blue HB stars and RRLs typical of a metal-poor
population. Our results suggest that fast chemical evolution occurred in
Sculptor, and that the radial gradients were in place at an early epoch.Comment: 10 pages, 5 figures, MNRAS accepte
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