85 research outputs found
Microturbulent velocity from stellar spectra: a comparison between different approaches (Research Note)
Context --- The classical method to infer microturbulent velocity in stellar
spectra requires that the abundances of the iron lines are not correlated with
the observed equivalent widths. An alternative method, requiring the use of the
expected line strength, is often used to by-pass the risk of spurious slopes
due to the correlation between the errors in abundance and equivalent width.
Aims --- To compare the two methods and identify pros and cons and
applicability to the typical practical cases. Methods --- I performed a test
with a grid of synthetic spectra, including instrumental broadening and
Poissonian noise. For all these spectra, microturbulent velocity has been
derived by using the two approaches and compared with the original value with
which the synthetic spectra have been generated. Results --- The two methods
provide similar results for spectra with SNR$ > 70, while for lower SNR both
approaches underestimate the true microturbulent velocity, depending of the SNR
and the possible selection of the lines based on the equivalent width errors.
Basically, the values inferred by using the observed equivalent widths better
agree with those of the synthetic spectra. In fact, the method based on the
expected line strength is not totally free from a bias that can heavily affect
the determination of microturbulent velocity. Finally, I recommend to use the
classical approach (based on the observed equivalent widths) to infer this
parameter. In cases of full spectroscopical determination of all the
atmospherical parameters, the difference between the two approaches is reduced,
leading to an absolute difference in the derived iron abundances of less than
0.1 dex.Comment: Accepted for publication in Astronomy and Astrophysic
Photometric properties of stellar populations in Galactic globular clusters: the role of the Mg-Al anticorrelation
We have computed low-mass stellar models and synthetic spectra for an initial
chemical composition that includes the full C-N, O-Na, and Mg-Al abundance
anticorrelations observed in second generation stars belonging to a number of
massive Galactic globular clusters. This investigation extends a previous study
that has addressed the effect of only the C-N and O-Na anticorrelations, seen
in all globulars observed to date. We find that the impact of Mg-Al abundance
variations at fixed [Fe/H] and Helium abundance is negligible on stellar models
and isochrones (from the main sequence to the tip of the red giant branch) and
bolometric corrections, when compared to the effect of C-N and O-Na variations.
We identify a spectral feature at 490-520 nm, for low-mass main sequence stars,
caused by MgH molecular bands. This feature has a vanishingly small effect on
bolometric corrections for Johnson and Stroemgren filters that cover that
spectral range. However, specific narrow-band filters able to target this
wavelength region can be powerful tools to investigate the Mg-poor unevolved
stars and highlight possible splittings of the MS due to variations of Mg
abundances.Comment: 8 pages, 9 figures, Astronomy & Astrophysics in pres
The Horizontal Branch in the UV Colour Magnitude Diagrams. II. The case of M3, M13 and M79
We present a detailed comparison between far-UV/optical colour Magnitude
Diagrams obtained with high-resolution Hubble Space Telescope data and suitable
theoretical models for three Galactic Globular Clusters: M3, M13 and M79. These
systems represents a classical example of clusters in the intermediate
metallicity regime that, even sharing similar metal content and age, show
remarkably different Horizontal Branch morphologies. As a consequence, the
observed differences in the colour distributions of Horizontal Branch stars
cannot be interpreted in terms of either first (metallicity) or a second
parameter such as age. We investigate here the possible role of variations of
initial Helium abundance (Y). Thanks to the use of a proper setup of far-UV
filters, we are able to put strong constraints on the maximum Y (Y_{max})
values compatible with the data. We find differences Delta Y_{max} ~ 0.02-0.04
between the clusters with M13 showing the largest value (Y_{max} ~ 0.30) and M3
the smallest (Y_{max} ~ 0.27). In general we observe that these values are
correlated with the colour extensions of their Horizontal Branches and with the
range of the observed Na-O anti-correlations.Comment: Accepted for publication by MNRAS. 15 pages, 15 figures, 1 tabl
A complete census of emitters in NGC 6397
We used a dataset of archival Hubble Space Telescope images obtained through
the F555W, F814W and F656N filters, to perform a complete search for objects
showing emission in the globular cluster NGC 6397. As photometric
diagnostic, we used the color excess in the
- color-color diagram. In the analysed field of view,
we identified 53 emitters. In particular, we confirmed the optical
counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars
and 11 active binaries) and identified 33 previously unknown sources, thus
significantly enlarging the population of known active binaries in this
cluster. We report the main characteristics for each class of objects.
Photometric estimates of the equivalent width of the emission line,
were derived from the -excess and, for the first time, compared
to the spectroscopic measurements obtained from the analysis of MUSE spectra.
The very good agreement between the spectroscopic and photometric measures
fully confirmed the reliability of the proposed approach to measure the
emission. The search demonstrated the efficiency of this novel
approach to pinpoint and measure -emitters, thus offering a powerful
tool to conduct complete census of objects whose formation and evolution can be
strongly affected by dynamical interactions in star clusters.Comment: Accepted for publication by ApJ; 14 pages, 8 Figures, 1 Tabl
A spectroscopic and photometric study of stellar populations in a sample of clusters in the Magellanic Clouds
This Ph.D. Thesis has been carried out in the framework of a long-term and large project
devoted to describe the main photometric, chemical, evolutionary and integrated properties of
a representative sample of Large and Small Magellanic Cloud (LMC and SMC respectively)
clusters. The globular clusters system of these two Irregular galaxies provides a rich resource for
investigating stellar and chemical evolution and to obtain a detailed view of the star formation
history and chemical enrichment of the Clouds. The results discussed here are based on the
analysis of high-resolution photometric and spectroscopic datasets obtained by using the last
generation of imagers and spectrographs. The principal aims of this project are summarized as
follows:
• The study of the AGB and RGB sequences in a sample of MC clusters, through the analysis
of a wide near-infrared photometric database, including 33 Magellanic globulars obtained
in three observing runs with the near-infrared camera SOFI@NTT (ESO, La Silla).
• The study of the chemical properties of a sample of MCs clusters, by using optical and
near-infrared high-resolution spectra. 3 observing runs have been secured to our group
to observe 9 LMC clusters (with ages between 100 Myr and 13 Gyr) with the optical
high-resolution spectrograph FLAMES@VLT (ESO, Paranal) and 4 very young (<30
Myr) clusters (3 in the LMC and 1 in the SMC) with the near-infrared high-resolution
spectrograph CRIRES@VLT.
• The study of the photometric properties of the main evolutive sequences in optical Color-
Magnitude Diagrams (CMD) obtained by using HST archive data, with the final aim of
dating several clusters via the comparison between the observed CMDs and theoretical
isochrones. The determination of the age of a stellar population requires an accurate
measure of the Main Sequence (MS) Turn-Off (TO) luminosity and the knowledge of the
distance modulus, reddening and overall metallicity. For this purpose, we limited the study
of the age just to the clusters already observed with high-resolution spectroscopy, in order
to date only clusters with accurate estimates of the overall metallicity
Stellar 3-D kinematics in the Draco dwarf spheroidal galaxy
Aims. We present the first three-dimensional internal motions for individual
stars in the Draco dwarf spheroidal galaxy. Methods. By combining first-epoch
observations and second-epoch Data Release
2 positions, we measured the proper motions of sources in the direction
of Draco. We determined the line-of-sight velocities for a sub-sample of
red giant branch stars using medium resolution spectra acquired with the DEIMOS
spectrograph at the Keck II telescope. Altogether, this resulted in a final
sample of Draco members with high-precision and accurate 3D motions, which
we present as a table in this paper. Results. Based on this high-quality
dataset, we determined the velocity dispersions at a projected distance of
pc from the centre of Draco to be
km/s, km/s and
km/s in the projected radial, tangential, and line-of-sight directions. This
results in a velocity anisotropy at
pc. Tighter constraints may be obtained using the spherical Jeans equations and
assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also
based on the assumption that the 3D velocity dispersion should be lower than
of the escape velocity of the system. In this case, we constrain
the maximum circular velocity of Draco to be in the range of
km/s. The corresponding mass range is in good agreement with
previous estimates based on line-of-sight velocities only. Conclusions. Our
Jeans modelling supports the case for a cuspy dark matter profile in this
galaxy. Firmer conclusions may be drawn by applying more sophisticated models
to this dataset and with new datasets from upcoming releases.Comment: 12 pages, 15 figures, 3 tables. Accepted for publication by A&
The structural properties of multiple populations in the dynamically young globular cluster NGC 2419
NGC 2419 is likely the globular cluster (GC) with the lowest dynamical age in
the Galaxy. This makes it an extremely interesting target for studying the
properties of its multiple populations (MPs), as they have been likely affected
only modestly by long-term dynamical evolution effects. Here we present for the
first time a detailed analysis of the structural and morphological properties
of the MPs along the whole extension of this remote and massive GC by combining
high-resolution HST and wide-field ground-based data. In agreement with
formation models predicting that second population (SP) stars form in the inner
regions of the first population (FP) system, we find that the SP is more
centrally concentrated than the FP. This may provide constraints on the
relative concentrations of MPs in the cluster early stages of the evolutionary
phase driven by two-body relaxation. In addition, we find that the fraction of
FP stars is larger than expected from the general trend drawn by Galactic GCs.
If, as suggested by a number of studies, NGC 2419 formed in the Sagittarius
dwarf galaxy and was later accreted by the Milky Way, we show that the observed
FP fraction may be explained as due to the transition of NGC 2419 to a weaker
tidal field (its current Galactocentric distance is d_gc~95 kpc) and
consequently to a reduced loss rate of FP stars.Comment: Accepted for publication in A&
MgAl burning chain in Omega Centauri
In this study, we report the results of Fe, Mg, Al, and Si abundances
analysis for a sample of 439 stars in Omega Centauri, using high-resolution
spectra obtained with the VLT/FLAMES multi-object spectrograph. Our analysis
reveals the presence of four distinct Fe populations, with the main peak
occurring at low metallicity, consistent with previous literature findings. We
observe a discrete and pronounced Mg-Al anti-correlation, which exhibits
variations in shape and extension as a function of metallicity. Specifically,
this anti-correlation is present in stars with metallicities lower than
approximately -1.3 dex, while it becomes less evident or absent for higher
[Fe/H] values. Additionally, we detect (anti-) correlations between Mg and Si,
and between Al and Si, whose extensions also vary with metallicity, similar to
the Mg-Al anti-correlation. These results suggest that the MgAl cycle plays a
crucial role in the formation of multiple populations in Omega Centauri, with
the presence of all (anti-) correlations at metallicities lower than -1.3 dex
providing evidence for the burning of Mg at very high temperatures (> 10^8 K),
at least in the metal-poor regime. Furthermore, we observe a clear trend of
stars with [Al/Fe] > +0.5 dex as a function of metallicity, confirming for the
first time the existence of the two channels of Al production and destruction.
This evidence can help to provide further constraints on the potential nature
of the polluters responsible for the observed chemical anomalies in this
stellar system. Finally, we find that the two most metal-poor populations
identified in our sample are compatible with null or very small metallicity
dispersion and we discuss how this result fit into a scenario where Omega
Centauri is the remnant of a disrupted nucleated dwarf galaxy.Comment: Accepted for pubblication in A&
The updated BaSTI stellar evolution models and isochrones.IV. alpha-depleted calculations
This is the fourth paper of our new release of the BaSTI (a Bag of Stellar
Tracks and Isochrones) stellar model and isochrone library. Following the
updated solar-scaled, alpha-enhanced, and white dwarf model libraries, we
present here alpha-depleted ([alpha/Fe] = -0.2) evolutionary tracks and
isochrones, suitable to study the alpha-depleted stars discovered in Local
Group dwarf galaxies and in the Milky Way. These calculations include all
improvements and updates of the solar-scaled and alpha-enhanced models, and
span a mass range between 0.1 and 15 Msun, 21 metallicities between [Fe/H] =
-3.20 and +0.45 with a helium-to-metal enrichment ratio dY/dZ = 1.31,
homogeneous with the solar-scaled and alpha-enhanced models. The isochrones --
available in several photometric filters -- cover an age range between 20 Myr
and 14.5 Gyr, including the pre-main-sequence phase. We have compared our
isochrones with independent calculations of alpha-depleted stellar models,
available for the same alpha-element depletion adopted in present
investigation. We have also discussed the effect of an alpha-depleted heavy
element distribution on the bolometric corrections in different wavelength
regimes. Our alpha-depleted evolutionary tracks and isochrones are publicly
available at the BaSTI website.Comment: 7 pages, 4 figures, 3 tables. Accepted for publication in MNRA
- …