86 research outputs found
Multiwavelength photometry in the Globular Cluster M2
We present a multiwavelength photometric analysis of the globular cluster M2.
The data-set has been obtained by combining high-resolution (HST/WFPC2 and ACS)
and wide-field (GALEX) space observations and ground based (MEGACAM-CFHT,
EMMI-NTT) images. The photometric sample covers the entire cluster extension
from the very central regions up to the tidal radius and beyond. It allows an
accurate determination of the cluster center of gravity and other structural
parameters derived from the star count density profile. Moreover we study the
BSS population and its radial distribution. A total of 123 BSS has been
selected, and their radial distribution has been found to be bimodal (highly
peaked in the center, decreasing at intermediate radii and rising outward), as
already found in a number of other clusters. The radial position of the minimum
of the BSS distribution is consistent with the radius of avoidance caused by
the dynamical friction of massive objects over the cluster age. We also
searched for gradients in the red giant branch (RGB) and the asymptotic giant
branch (AGB) populations. At the level we found an overabundance of
AGB stars within the core radius and confirmed the result of Sohn et al.(1996)
that the central region of M2 is bluer than the outer part. We show that the
latter is due to a deficit of very luminous RGB stars in the central region.Comment: 35 pages, 18 figures; Accepted for publication by ApJ
High-resolution Extinction Map in the Direction of the Bulge Globular Cluster NGC 6440
We used optical images acquired with the UVIS channel of the Wide Field Camera 3 on board the Hubble Space Telescope to construct the first high-resolution extinction map in the direction of NGC 6440, a globular cluster located in the bulge of our Galaxy. The map has a spatial resolution of 0.\u20335 over a rectangular region of about 160\u2033
7 240\u2033 around the cluster center, with the long side in the northwest/southeast direction. We found that the absorption clouds show patchy and filamentary substructures with extinction variations as large as \u3b4E(B-V) 3c0.5 mag. We also performed a first-order proper motion analysis to distinguish cluster members from field interlopers. After the field decontamination and the differential reddening correction, the cluster sequences in the color-magnitude diagram appear much better defined, providing the best optical color-magnitude diagram so far available for this cluster
Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars
One of the most dramatic events in the life of a low-mass star is the He
flash, which takes place at the tip of the red giant branch (RGB) and is
followed by a series of secondary flashes before the star settles into the
zero-age horizontal branch (ZAHB). Yet, no stars have been positively
identified in this key evolutionary phase, mainly for two reasons: first, this
pre-ZAHB phase is very short compared to other major evolutionary phases in the
life of a star; and second, these pre-ZAHB stars are expected to overlap the
loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in
the color-magnitude diagram (CMD). We investigate the possibility of detecting
these stars through stellar pulsations, since some of them are expected to
rapidly cross the Cepheid/RR Lyrae instability strip in their route from the
RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a
consequence of their very high evolutionary speed, some of these stars may
present anomalously large period change rates. We constructed an extensive grid
of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for
the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR
Lyrae stars with high period change rates are found. Our results suggest that
some -- but certainly not all -- of the RR Lyrae stars in M3 with large period
change rates are in fact pre-ZAHB pulsators.Comment: Conference Proceedings HELAS Workshop on 'Synergies between solar and
stellar modelling', Rome, June 2009, Astrophys. Space Sci., in the pres
Star-density Profiles of Six Old Star Clusters in the Large Magellanic Cloud
We used resolved star counts from Hubble Space Telescope (HST) images to determine the center of gravity and the projected density profiles of six old globular clusters (GCs) in the Large Magellanic Cloud (LMC), namely NGC 1466, NGC 1841, NGC 1898, NGC 2210, NGC 2257, and Hodge 11. For each system, the LMC field contribution was properly taken into account by making use, when needed, of parallel HST observations. The derived values of the center of gravity may differ by several arcseconds (corresponding to more dal 1 pc at the distance of the LMC) from previous determinations. The cluster density profiles are all well fit by King models, with structural parameters that may differ from the literature ones by even factors of two. Similar to what was observed for Galactic GCs, the ratio between the effective and the core radius has been found to anticorrelate with the cluster dynamical age
Radio Timing and Optical Photometry of the Black Widow System PSR J1953+1846A in the Globular Cluster M71
The distance to the Sgr dwarf spheroidal galaxy from the Red Giant Branch Tip
We derived the distance to the central region of the Sagittarius dwarf
spheroidal galaxy from the Red Giant Branch Tip. The obtained distance modulus
is , corresponding to a heliocentric distance
Kpc. This estimate is in good agreement with the distance
obtained from RR Lyrae stars of the globular cluster M~54, located in the core
of the Sgr galaxy, once the most accurate estimate of the cluster metallicity
and the most recent calibration of the relation are
adopted.Comment: 6 pages, 5 figure, Accepted for publication in MNRA
Multiple stellar population in the Sextans dwarf spheroidal galaxy?
We present wide field (33 X 34 arcmin^2) multiband (BVI) CCD photometry (down
to I <= 20.5) of the very low surface brightness dwarf Spheroidal (dSph) galaxy
Sextans. In the derived Color Magnitude Diagrams we have found evidences
suggesting the presence of multiple stellar populations in this dSph. In
particular we discovered: {\it (i)} a Blue Horizontal Branch (HB) tail that
appears to lie on a brighter sequence with respect to the prominent Red HB and
the RR Lyrae stars, very similar to what found by Majewski et al. (1999) for
the Sculptor dSph; {\it (ii)} hints of a bimodal distribution in color of the
RGB stars; {\it (iii)} a double RGB-bump. All these features suggest that (at
least) two components are present in the old stellar population of this galaxy:
a main one with [Fe/H]~ -1.8 and a minor component around [Fe/H]<~ -2.3. The
similarity with the Sculptor case may indicate that multiple star formation
episodes are common also in the most nearby dSphs that ceased their star
formation activity at very early epochs.Comment: accepted by MNRAS Letters - 6 pages, 4 .eps figures. Standard LaTeX,
mn.st
QCD ghost f(T)-gravity model
Within the framework of modified teleparallel gravity, we reconstruct a f(T)
model corresponding to the QCD ghost dark energy scenario. For a spatially flat
FRW universe containing only the pressureless matter, we obtain the time
evolution of the torsion scalar T (or the Hubble parameter). Then, we calculate
the effective torsion equation of state parameter of the QCD ghost f(T)-gravity
model as well as the deceleration parameter of the universe. Furthermore, we
fit the model parameters by using the latest observational data including
SNeIa, CMB and BAO data. We also check the viability of our model using a
cosmographic analysis approach. Moreover, we investigate the validity of the
generalized second law (GSL) of gravitational thermodynamics for our model.
Finally, we point out the growth rate of matter density perturbation. We
conclude that in QCD ghost f(T)-gravity model, the universe begins a matter
dominated phase and approaches a de Sitter regime at late times, as expected.
Also this model is consistent with current data, passes the cosmographic test,
satisfies the GSL and fits the data of the growth factor well as the LCDM
model.Comment: 19 pages, 9 figures, 2 tables. arXiv admin note: substantial text
overlap with arXiv:1111.726
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
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