1,102 research outputs found
Cepheid theoretical models and observations in HST/WFC3 filters: the effect on the Hubble constant Ho
We present a complete theoretical scenario for classical Cepheids in the most
commonly used HST/WFC3 filters, going from optical (F555W, F606W and F814W) to
near-infrared (F160W) bands. The importance of such a study is related to the
recent release of new classical Cepheids observed with HST/WFC3 in 8 distant
galaxies where SNIa are hosted. These observations have posed sound constraints
to the current distance scale with uncertainties on the Hubble constant Ho
smaller than 3%. Our models explore a large range of metallicity and Helium
content, thus providing a robust and unique theoretical tool for describing
these new and future HST/WFC3 observations. As expected, the Period-Luminosity
(PL) relation in F160W filter is linear and slightly dependent on the
metallicity when compared with optical bands, thus it seems the most accurate
tool to constrain extragalactic distances with Cepheids. We compare the
pulsation properties of Cepheids observed with HST/WFC3-IR with our theoretical
scenario and we discuss the agreement with the predicted Instability Strip for
all the investigated galaxy samples including the case of NGC4258. Finally,
adopting our theoretical F160W PL relation for Z=0.02 and log P>1.0, we derive
new distance moduli. In particular, for NGC 4258, we derive a distance modulus
mu0 = 29.345 +- 0.004 mag with a sigma = 0.34 mag, which is in very good
agreement with the geometrical maser value. Moreover, using the obtained
distance moduli, we estimate the Hubble constant value, Ho=76.0 +- 1.9 km s-1
Mpc-1 in excellent agreement with the most recent literature values.Comment: 13 pages, 9 figures, 4 tables, MNRAS, accepte
Massive stellar systems: observational challenges and perspectives in the E-ELT era
We introduce the empirical framework concerning optical and near-infrared (NIR) photometry of crowded stellar fields. In particular, we address the impact that linear detectors and analytical PSF played in improving the accuracy and the precision of multi-band color-magnitude diagrams (CMDs). We focus our attention on recent findings based on deep NIR images collected with Adaptive Optics (AO) systems at the 8-10m class telescopes and discuss pros and cons of the different approaches. We also discuss the estimate of the absolute age of globular clusters using a well defined knee along the lower main sequence. We mention the role which the current AO-assisted instruments will have in addressing longstanding astrophysical problems of the Galactic center. Finally, we outline the role of first generation of E-ELT instruments upon photometry and spectroscopy of crowded stellar fields
Ultra Long Period Cepheids: a primary standard candle out to the Hubble flow
The cosmological distance ladder crucially depends on classical Cepheids
(with P=3-80 days), which are primary distance indicators up to 33 Mpc. Within
this volume, very few SNe Ia have been calibrated through classical Cepheids,
with uncertainty related to the non-linearity and the metallicity dependence of
their period-luminosity (PL) relation. Although a general consensus on these
effects is still not achieved, classical Cepheids remain the most used primary
distance indicators. A possible extension of these standard candles to further
distances would be important. In this context, a very promising new tool is
represented by the ultra-long period (ULP) Cepheids (P \geq 80 days), recently
identified in star-forming galaxies. Only a small number of ULP Cepheids have
been discovered so far. Here we present and analyse the properties of an
updated sample of 37 ULP Cepheids observed in galaxies within a very large
metallicity range of 12+log(O/H) from ~7.2 to 9.2 dex. We find that their
location in the colour(V-I)-magnitude diagram as well as their Wesenheit (V-I)
index-period (WP) relation suggests that they are the counterparts at high
luminosity of the shorter-period (P \leq 80 days) classical Cepheids. However,
a complete pulsation and evolutionary theoretical scenario is needed to
properly interpret the true nature of these objects. We do not confirm the
flattening in the studied WP relation suggested by Bird et al. (2009). Using
the whole sample, we find that ULP Cepheids lie around a relation similar to
that of the LMC, although with a large spread (~0.4 mag).Comment: 8 pages, 4 figures, accepted for publication in Astrophysics & Space
Scienc
The Carina Project. VI. The helium burning variable stars
We present new optical (BVI) time-series data for the evolved variable stars
in the Carina dwarf spheroidal galaxy. The quality of the data and the
observing strategy allowed us to identify 14 new variable stars. Eight out of
the 14 are RR Lyrae (RRL) stars, four are Anomalous Cepheids (ACs) and two are
geometrical variables. Comparison of the period distribution for the entire
sample of RRLs with similar distributions in nearby dSphs and in the Large
Magellanic Cloud indicates that the old stellar populations in these systems
share similar properties. This finding is also supported by the RRL
distribution in the Bailey diagram. On the other hand, the period distribution
and the Bailey diagram of ACs display significant differences among the above
stellar systems. This evidence suggests that the properties of intermediate-age
stellar populations might be affected both by environmental effects and
structural parameters. We use the BV Period--Wesenheit (PW) relation of RRLs
together with evolutionary prescriptions and find a true distance modulus of
20.09+/-0.07(intrinsic)+/-0.1(statistical) mag that agrees quite well with
similar estimates available in the literature. We identified four peculiar
variables. Taking into account their position in the Bailey diagram and in the
BV PW relation, two of them (V14, V149) appear to be candidate ACs, while two
(V158, V182) might be peculiar RRLs. In particular, the variable V158 has a
period and a V-band amplitude very similar to the low-mass RRL
---RRLR-02792---recently identified by Pietrzynski at al. (2012) in the
Galactic bulge.Comment: Accepted for publication in ApJ, publication date September 20, 2013.
31 pages, 7 figure, 3 tables. Figs. 3 and 4 corrected, references added,
minor correction
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
Multi-Epoch HST Observations of IZw18: Characterization of Variable Stars at Ultra-Low Metallicities
Variable stars have been identified for the first time in the very metal-poor
Blue Compact dwarf galaxy IZw18, using deep multi-band (F606W,
F814W)time-series photometry obtained with the Advanced Camera for Surveys
(ACS) on board the Hubble Space Telescope (HST). We detected 34 candidate
variable stars in the galaxy. We classify three of them as Classical Cepheids,
with periods of 8.71, 125.0 and 130.3 days, respectively, and other two as long
period variables with periodicities longer than a hundred days. These are the
lowest metallicity Classical Cepheids known so far, thus providing the
opportunity to explore and fit models of stellar pulsation fo Classical
Cepheids at previously inaccessible metallicities. The period distribution of
the confirmed Cepheids is markedly different from what is seen in other nearby
galaxies, which is likely related to the star bursting nature of IZw18. By
applying to the 8.71 days Cepheid theoretical Wesenheit (V,I) relations based
on new pulsation models of Classical Cepheids specifically computed for the
extremely low metallicity of this galaxy (Z=0.0004, Y=0.24), we estimate the
distance modulus of IZw18 to be mu_0= 31.4pm0.2 D=19.0^{+1.8}_{-1.7}Mpc) for
canonical models of Classical Cepheids, and of 31.2pm0.2 mag
(D=17.4^{+1.6}_{-1.6}Mpc) using over luminous models. The theoretical modeling
of the star's light curves provides mu_0=31.4pm0.1 mag, D=19.0^{+0.9}_{-0.9}
Mpc, in good agreement with the results from the theoretical Wesenheit
relations. These pulsation distances bracket the distance of 18.2pm1.5Mpc
inferred by Aloisi et al. (2007) using the galaxy's Red Giant Branch Tip.Comment: 13 Pages, 6 Figures, accepted, Ap
MyDi application: Towards automatic activity annotation of young patients with Type 1 diabetes
Type I diabetes mellitus (T1DM) is a widespread metabolic disorder characterized by pancreatic insufficiency. People with T1DM require: a lifelong insulin injection, to constantly monitor glycemia and to take note of their activities. This continuous follow-up, especially at a very young age, may be challenging. Adolescents with T1DM may develop anxiety symptoms and depression which can lead to the loss of glycemic control. An assistive technology that automatizes the activity monitoring process could support these young patient in managing T1DM. The aim of this work is to present the MyDi framework which integrates a smart glycemic diary (for Android users), to automatically record and store patient's activity via pictures and a deep-learning (DL)-based technology able to classify the activity performed by the patients (i.e., meal and sport) via picture analysis. The proposed approach was tested on two different datasets, the Insta-Dataset with 3498 pictures (also used for training and validating the DL model) and the MyDi-Dataset with 126 pictures, achieving very encouraging results in both cases (Preci= 1.0, Reci= 1.0, f1i= 1.0 with i E C:[meal, sport]) prompting the possibility of translating this application in the T1DM monitoring process
Probing the early chemical evolution of the Sculptor dSph with purely old stellar tracers
We present the metallicity distribution of a sample of 471 RR Lyrae (RRL)
stars in the Sculptor dSph, obtained from the -band Period-Luminosity
relation. It is the first time that the early chemical evolution of a dwarf
galaxy is characterized in such a detailed and quantitative way, using
photometric data alone. We find a broad metallicity distribution (FWHM=0.8 dex)
that is peaked at [Fe/H]-1.90 dex, in excellent agreement with
literature values obtained from spectroscopic data. Moreover, we are able to
directly trace the metallicity gradient out to a radius of 55 arcmin. We
find that in the outer regions (r32 arcmin) the slope of the metallicity
gradient from the RRLs (-0.025 dex arcmin) is comparable to the
literature values based on red giant (RG) stars. However, in the central part
of Sculptor we do not observe the latter gradients. This suggests that there is
a more metal-rich and/or younger population in Sculptor that does not produce
RRLs. This scenario is strengthened by the observation of a metal-rich peak in
the metallicity distribution of RG stars by other authors, which is not present
in the metallicity distribution of the RRLs within the same central area.Comment: 5 pages, 4 figures. Accepted for publication on MNRAS Letter
Variable stars in Local Group Galaxies - II. Sculptor dSph
We present the identification of 634 variable stars in the Milky Way dSph
satellite Sculptor based on archival ground-based optical observations spanning
24 years and covering 2.5 deg. We employed the same
methodologies as the "Homogeneous Photometry" series published by Stetson. In
particular, we have identified and characterized one of the largest (536) RR
Lyrae samples so far in a Milky Way dSph satellite. We have also detected four
Anomalous Cepheids, 23 SX Phoenicis stars, five eclipsing binaries, three field
variable stars, three peculiar variable stars located above the horizontal
branch - near to the locus of BL Herculis - that we are unable to classify
properly. Additionally we identify 37 Long Period Variables plus 23 probable
variable stars, for which the current data do not allow us to determine the
period. We report positions and finding charts for all the variable stars, and
basic properties (period, amplitude, mean magnitude) and light curves for 574
of them. We discuss the properties of the RR Lyrae stars in the Bailey diagram,
which supports the coexistence of subpopulations with different chemical
compositions. We estimate the mean mass of Anomalous Cepheids
(1.5M) and SX Phoenicis stars (1M). We discuss
in detail the nature of the former. The connections between the properties of
the different families of variable stars are discussed in the context of the
star formation history of the Sculptor dSph galaxy.Comment: 22 pages, 17 figures, 13 tables. Accepted for publication on MNRA
The Carina Project IX: on Hydrogen and helium burning variables
We present new multi-band (UBVI) time-series data of helium burning variables
in the Carina dwarf spheroidal galaxy. The current sample includes 92 RR
Lyrae-six of them are new identifications-and 20 Anomalous Cepheids, one of
which is new identification. The analysis of the Bailey diagram shows that the
luminosity amplitude of the first overtone component in double-mode variables
is located along the long-period tail of regular first overtone variables,
while the fundamental component is located along the short-period tale of
regular fundamental variables. This evidence further supports the transitional
nature of these objects. Moreover, the distribution of Carina double-mode
variables in the Petersen diagram (P_1/P_0 vs P_0) is similar to metal-poor
globulars (M15, M68), to the dwarf spheroidal Draco and to the Galactic Halo.
This suggests that the Carina old stellar population is metal-poor and affected
by a small spread in metallicity. We use trigonometric parallaxes for five
field RR Lyrae stars to provide an independent estimate of the Carina distance
using the observed reddening free Period--Wesenheit [PW, (BV)] relation. Theory
and observations indicate that this diagnostic is independent of metallicity.
We found a true distance modulus of \mu=20.01\pm0.02 (standard error of the
mean) \pm0.05 (standard deviation) mag. We also provided independent estimates
of the Carina true distance modulus using four predicted PW relations (BV, BI,
VI, BVI) and we found: \mu=(20.08\pm0.007\pm0.07) mag,
\mu=(20.06\pm0.006\pm0.06) mag, \mu=(20.07\pm0.008\pm0.08) mag and
\mu=(20.06\pm0.006\pm0.06) mag. Finally, we identified more than 100 new SX
Phoenicis stars that together with those already known in the literature (340)
make Carina a fundamental laboratory to constrain the evolutionary and
pulsation properties of these transitional variables.Comment: 44 pages, 13 tables, 13 figures, Accepted for publication in Ap
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