479 research outputs found
Surface Brightness Fluctuations: a theoretical point of view
We present new theoretical evaluations of optical and near-IR Surface
Brightness Fluctuations (SBF) magnitudes for single-burst stellar populations
in the age range t=5-15 Gyr and metallicity from Z_{\sun}/200 to 2Z_{\sun}.
Our theoretical predictions can be successfully used to derive reliable
distance evaluations. They also appear to be a new and valuable tool to trace
the properties of unresolved stellar populations.Comment: 2 pages, incl. 1 figure, uses newpasp.sty, to be published in ``New
Horizons in Globular Cluster Astronomy'', ASP Conference Series, 2002; Eds.:
G. Piotto, G. Meylan, G. Djorgowski and M. Riello, in pres
Predicted HST FOC and broad band colours for young and intermediate Simple Stellar Populations
This paper presents theoretical HST and broad band colours from population
synthesis models based on an homogeneous set of stellar evolutionary tracks as
computed under canonical (no overshooting) assumptions, covering the range of
cluster ages from t=8 Myr to t=5 Gyr for three different metallicities (Z=0.02,
0.006, and 0.001). Statistical fluctuations in the cluster population have been
investigated, assessing the predicted fluctuations of the various colours as a
function of the cluster integrated absolute magnitude. We show that the red
leak in HST UV filters deeply affects the predicted fluxes and colours.
However, we find that for F152M-F307M < 0.5 and for F170M-F278M < 0.5 (which
means ages lower than 1 Gyr) the HST UV colours can still be used to infer
reliable indications on the age of distant clusters. Moreover, one finds that
the age calibration of these colours is scarcely affected by the amount of
original helium or by the assumed IMF. On this basis, we present a calibration
of the HST UV two-colours (F152M-F307M vs F170M-F278M) in terms of cluster ages
for the three above quoted metallicities. We suggest the combined use of HST UV
colours and IR colours (V-K in particular) to disentangle the metallicity-age
effect in integrated colours of young stellar populations (t< 1 Gyr).Comment: Latex 18 pages, 16 encapsulated figures, 6 tables, A&ASS accepte
On the impact of Helium abundance on the Cepheid Period-Luminosity and Wesenheit relations and the Distance Ladder
This work analyses the effect of the Helium content on synthetic
Period-Luminosity Relations (PLRs) and Period-Wesenheit Relations (PWRs) of
Cepheids and the systematic uncertainties on the derived distances that a
hidden population of He-enhanced Cepheids may generate. We use new stellar and
pulsation models to build a homogeneous and consistent framework to derive the
Cepheid features. The Cepheid populations expected in synthetic color-magnitude
diagrams of young stellar systems (from 20 Myr to 250 Myr) are computed in
several photometric bands for Y = 0.25 and Y = 0.35, at a fixed metallicity (Z
= 0.008). The PLRs appear to be very similar in the two cases, with negligible
effects (few %) on distances, while PWRs differ somewhat, with systematic
uncertainties in deriving distances as high as about 7% at log P < 1.5.
Statistical effects due to the number of variables used to determine the
relations contribute to a distance systematic error of the order of few
percent, with values decreasing from optical to near-infrared bands. The
empirical PWRs derived from multi-wavelength datasets for the Large Magellanic
Cloud (LMC) is in a very good agreement with our theoretical PWRs obtained with
a standard He content, supporting the evidence that LMC Cepheids do not show
any He effect
Multipopulation aftereffects on the color-magnitude diagram and Cepheid variables of young stellar systems
Context: The evidence of a multipopulation scenario in Galactic globular
clusters raises several questions about the formation and evolution of the two
(or more) generations of stars. These populations show differences in their age
and chemical composition. These differences are found in old- and intermediate-
age stellar clusters in the Local Group. The observations of young stellar
systems are expected to present footprints of multiple stellar populations.
Aims: This theoretical work intends to be a specific step in exploring the
space of the observational indicators of multipopulations, without covering all
the combinations of parameters that may contribute to the formation of multiple
generations of stars in a cluster or in galaxy. The goal is to shed light on
the possible observational features expected by core He-burning stars that
belong to two stellar populations with different original He content and ages.
Methods: The tool adopted was the stellar population synthesis. We used new
stellar and pulsation models to construct a homogeneous and consistent
framework. Synthetic color-magnitude diagrams (CMDs) of young- and
intermediate-age stellar systems (from 20 Myr up to 1 Gyr) were computed in
several photometric bands to derive possible indicators of double populations
both in the observed CMDs and in the pulsation properties of the Cepheids.
Results: We predict that the morphology of the red/blue clump in VIK bands
can be used to photometrically indicate the two stellar populations in a rich
assembly of stars if there is a significant difference in their original He
content. Moreover, the period distribution of the Cepheids appears to be widely
affected by the coeval multiple generations of stars within stellar systems. We
show that the Wesenheit relations may be affected by the helium content of the
Cepheids.Comment: in press on A&
Simulating CCD images of elliptical galaxies
We introduce a procedure developed by the ``Teramo Stellar Populations
Tools'' group (Teramo-SPoT), specifically optimized to obtain realistic
simulations of CCD images of elliptical galaxies.
Particular attention is devoted to include the Surface Brightness Fluctuation
(SBF) signal observed in ellipticals and to simulate the Globular Cluster (GC)
system in the galaxy, and the distribution of background galaxies present in
real CCD frames. In addition to the physical properties of the simulated
objects - galaxy distance and brightness profile, luminosity function of GC and
background galaxies, etc. - the tool presented allows the user to set some of
the main instrumental properties - FoV, zero point magnitude, exposure time,
etc.Comment: Presented at From Stars to Galaxies: Building the Pieces to Build up
the Universe (StarGal 2006), Venice, Italy, 16-20 Oct 200
Disentangling age and metallicity in distant unresolved stellar systems
We present some results of an observational and theoretical study on
unresolved stellar systems based on the Surface Brightness Fluctuations (SBF)
technique. It is shown that SBF magnitudes are a valuable tracer of stellar
population properties, and a reliable distance indicator. SBF magnitudes,
SBF-colors, and SBF-gradients can help to constrain within relatively narrow
limits the metallicity and age of the dominant stellar component in distant
stellar systems, especially if coupled with other spectro-photometric
indicators.Comment: A contributed paper to the Cefalu' (Italy) "Probing Stellar
Populations out to the Distant Universe", 4 pages. To appear as an AIP
Conference Proceedin
Detection of Radial Surface Brightness Fluctuation and Color Gradients in elliptical galaxies with ACS
We study surface brightness fluctuations (SBF) in a sample of 8 elliptical
galaxies using Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) data
drawn from the Hubble Space Telescope (HST) archive. SBF magnitudes in the
F814W bandpass, and galaxy colors from F814W, F435W, and F606W images -- when
available -- are presented. Galaxy surface brightness profiles are determined
as well. We present the first SBF--broadband color calibration for the ACS/WFC
F814W bandpass, and (relative) distance moduli estimates for 7 of our galaxies.
We detect and study in detail the SBF variations within individual galaxies
as a probe of possible changes in the underlying stellar populations.
Inspecting both the SBF and color gradients in comparison to model predictions,
we argue that SBF, and SBF-gradients, can in principle be used for unraveling
the different evolutionary paths taken by galaxies, though a more comprehensive
study of this issue would be required. We confirm that the radial variation of
galaxy stellar population properties should be mainly connected to the presence
of radial chemical abundance gradients, with the outer galaxy regions being
more metal poor than the inner ones.Comment: 47 pages, 13 figures, ApJ, accepte
GW170817: implications for the local kilonova rate and for surveys from ground-based facilities
We compute the local rate of events similar to GRB 170817A, which has been
recently found to be associated with a kilonova (KN) outburst. Our analysis
finds an observed rate of such events of R
Gpcyr. After comparing at their face values this density of sGRB
outbursts with the much higher density of Binary Neutron Star (BNS) mergers of
1540 Gpcyr, estimated by LIGO-Virgo
collaboration, one can conclude, admittedly with large uncertainty that either
only a minor fraction of BNS mergers produces sGRB/KN events or the sGRBs
associated with BNS mergers are beamed and observable under viewing angles as
large as . Finally we provide preliminary estimates
of the number of sGRB/KN events detected by future surveys carried out with
present/future ground-based/space facilities, such as LSST, VST, ZTF, SKA and
THESEUS.Comment: MNRAS accepted, 6 pages, 1 figur
HST observations of the LMC field around SN87A: distance determinations with Red Clump and Tip of the Red Giant Branch stars
We have used HST-WFPC2 multiband observations of a field around SN 1987A in
the Large Magellanic Cloud to measure its distance from the Sun. The
observations allowed us to carefully determine the interstellar extinction
along the line of sight to a large number of stars and to measure the LMC
distance by using two stellar distance indicators: the Red Clump and the Tip of
the Red Giant Branch. From an application of the Red Clump method we obtain a
distance modulus (m-M)o,rc(LMC)=18.59+-0.04+-0.08 mag (statistical plus
systematic error), in good agreement with the distance derived by using the Tip
of the Red Giant Branch stars, namely (m-M)o,trgb(LMC)=18.69+-0.25+-0.06 mag
(statistical plus systematic error). Both values agree well with the distance
to the SN 1987A as determined from a study of its inner ring fluorescent echo
((m-M)(SN87A)=18.55+-0.05 mag, Panagia 1998), thus excluding distance moduli
lower than 18.43 to a 99.7% significance level. Differences with respect to
previous results obtained using the same distance indicators are discussed.Comment: 7 pages including 3 figures; Astrophysical Journal, accepted for
publicatio
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