650 research outputs found
Synthetic properties of bright metal-poor variables. II. BL Her stars
We investigate the properties of the so-called BL Her stars, i.e., Population
II Cepheids with periods shorter than 8 days, using updated pulsation models
and evolutionary tracks computed adopting a metal abundance in the range of
Z=0.0001 to Z=0.004. We derive the predicted Period-Magnitude (PM) and
Period-Wesenheit (PW) relations at the various photometric bands and we show
that the slopes of these relations are in good agreement with the slopes
determined by observed variables in Galactic globular clusters, independently
of the adopted M_V(RR)-[Fe/H] relation to get the cluster RR Lyrae-based
distance. Moreover, we show that also the distances provided by the predicted
PM and PW relations for BL Her stars agree within the errors with the RR Lyrae
based values. The use of the predicted relations with W Vir stars, which are
Population II Cepheids with periods longer than 8 days, provides no clear
evidence for or against a change in the PM and PW slopes around P~10 days.Comment: 12 pages, 6 figures, 6 tables, accepted for publication in A&A on
date 16/05/200
Predicted properties of RR Lyrae stars in the SDSS photometric system
The luminosities and effective temperatures, as well as the whole bolometric
lightcurves of nonlinear convective RR Lyrae models with 0.0001<Z<0.006 are
transformed into the SDSS photometric system. The obtained ugriz lightcurves,
mean magnitudes and colors, pulsation amplitudes and color-color loops are
shown and analytical relations connecting pulsational to intrinsic stellar
parameters, similarly to the ones currently used in the Johnson-Cousins
filters, are derived. Finally the behaviour in the color-color planes is
compared with available observations in the literature and possible systematic
uncertainties affecting this comparison are discussed.Comment: accepted for publication in MNRA
Synthetic properties of bright metal-poor variables. I. "Anomalous" Cepheids
We present new grids of evolutionary models for the so-colled ``Anomalous''
Cepheids (ACs), adopting Z=0.0001 and various assumptions on the progenitor
mass and mass-loss efficiency. These computations are combined with the results
of our previous set of pulsation models and used to build synthetic populations
of the predicted pulsators as well as to provide a Mass-Luminosity relation in
the absence of mass-loss. We investigate the effect of mass-loss on the
predicted boundaries of the instability strip and we find that the only
significant dependence occurs in the Period-Magnitude plane, where the
synthetic distribution of the pulsators is, on average, brighter by about 0.1
mag than the one in absence of mass-loss. Tight Period-Magnitude relations are
derived in the K band for both fundamental and first overtone pulsators,
providing a useful tool for distance evaluations with an intrinsic uncertainty
of about 0.15 mag, which decreases to about 0.04 mag if the mass term is taken
into account. The constraints provided by the evolutionary models are used to
derive evolutionary (i.e, mass-independent) Period-Magnitude-Color relations
which provide distance determinations with a formal uncertainty of the order of
about 0.1 mag, once the intrinsic colors are well known. We also use model
computations from the literature to investigate the effect of metal content
both on the instability strip and on the evolutionary Period-Magnitude-Color
relations. Finally, we compare our theoretical predictions with observed
variables and we confirm that a secure identification of actual ACs requires
the simultaneous information on period, magnitude and color, that also provide
constraints on the pulsation mode.Comment: accepte
Updated pulsation models for BL Herculis stars
Population II pulsatingvariables play a relevant role both as distance
indicators and as tracers of the properties of old stellar populations. In this
paper we present an update and homogeneous pulsational scenario for a wide
range of stellar parameters typical of BL Her stars i.e., Population II
Cepheids with periods shorter than 8 days. To this purpose, we adopt a
nonlinear convective hydrodinamical code evaluate the stability and full
amplitude behaviour of an extensive set of BL Her pulsation models. Various
assumptions of mass, luminosity and metallicity consistent with the most recent
evolutionary prescriptions, are adopted. We obtain the theoretical instability
strip for both fundamental and first overtone pulsators and present a detailed
atlas of light/radial-velocity curves. Some relations for the boundaries of the
instability strip and for the dependence of the absolute magnitude on period,
mass and color, as well as the fundamental period-amplitude relations are
derived.Finally, we provide the theoretical period-radius relation for BL Her
and find that it is in excellent agreement with the empirical relation by Burki
& Meylan and consistent with the one holding for shorter periods for RR Lyrae
stars.Comment: 7 pages, accepted for publication in A&
Classical Cepheid Pulsation Models. XI. Effects of convection and chemical composition on the Period-Luminosity and Period-Wesenheit relations
In spite of the relevance of Classical Cepheids as primary distance
indicators, a general consensus on the dependence of the Period-Luminosity (PL)
relation on the Cepheid chemical composition has not been achieved yet. From
the theoretical point of view, our previous investigations were able to
reproduce some empirical tests for suitable assumptions on the helium to metal
relative enrichment, but those results relied on specific assumptions
concerning the Mass-Luminosity relation and the efficiency of the convective
transfer in the pulsating envelopes. In this paper, we investigate the effects
of the assumed value of the mixing length parameter l/Hp on the pulsation
properties and we release the assumption of a fixed Mass-Luminosity relation.
As a whole, we show that our pulsation relations appear fully consistent with
the observed properties of Galactic and Magellanic Cloud Cepheids, supporting
the predicted steepening and brightening of the PL relations when moving from
metal-rich to metal-poor variables. Moreover, we show that the distances
inferred by the predicted PW relations agree with recently measured
trigonometric parallaxes, whereas they suggest a correction to the values based
on the Infrared Surface Brightness technique, as already found from an
independent method. Finally, also the pulsation metal contents suggested by the
predicted PW relations appear in statistical agreement with spectroscopic
[Fe/H] measurements.Comment: 20 pages, 15 figure
The distance to the Galactic Centre based on Population-II Cepheids and RR Lyrae stars
Context: The distance to the Galactic Centre (GC) is of importance for the
distance scale in the Universe. The value derived by Eisenhauer et al. (2005)
of 7.62 +- 0.32 kpc based on the orbit of one star around the central black
hole is shorter than most other distance estimates based on a variety of
different methods. Aim: To establish an independent distance to the GC with
high accuracy. To this end Population-II Cepheids are used that have been
discovered in the OGLE-II and III surveys. Method: Thirty-nine Pop-II Cepheids
have been monitored on 4 nights spanning 14 days. Light curves have been fitted
using the known periods from the OGLE data to determine the mean K-band
magnitude. It so happens that 37 RR Lyrae stars are in the field-of-views and
mean K-band magnitudes are derived for this sample as well. Results: The
period-luminosity relation of Pop-II Cepheids in the K-band is determined, and
the derived slope of -2.24 +- 0.14 is consistent with the value derived by
Matsunaga et al. (2006). Fixing the slope to their more accurate value results
in a zero point, and implies a distance modulus to the GC of 14.51 +- 0.12,
with an additional systematic uncertainty of 0.07 mag. Similarly, from the RR
Lyrae K-band PL-relation we derive a value of 14.48 +- 0.17 (random) +- 0.07
(syst.). The two independent determinations are averaged to find 14.50 +- 0.10
(random) +- 0.07 (syst.), or 7.94 +- 0.37 +- 0.26 kpc.Comment: A&A accepte
RR Lyrae stars in Galactic globular clusters. VI. The Period-Amplitude relation
We compare theory and observations for fundamental RR Lyrae in the solar
neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic
globular clusters (GGCs). The distribution of cluster RR_ab in the PA_V plane
depends not only on the metal abundance, but also on the cluster Horizontal
Branch (HB) morphology. On average the observed k_puls parameter, connecting
the period to the visual amplitude, increases when moving from metal-poor to
metal-rich GGCs. However, this parameter shows marginal changes among OoI
clusters with intermediate to red HB types and iron abundances -1.8<= [Fe/H]
<=-1.1, whereas its value decreases in OoII clusters with the bluer HB
morphology. Moreover, at [Fe/H]=-1.7+-0.1 the OoI clusters present redder HB
types and larger values than the OoII clusters. The RR_ab variables in
Omega Cen and in the solar neighborhood further support the evidence that the
spread in [Fe/H], at fixed k_puls, is of the order of +-0.5 dex. Synthetic HB
simulations show that the PA_V plane can provide accurate cluster distance
estimates. The RR_ab variables in OoI and in OoII clusters with very blue HB
types obey a well-defined M_V(RR)-k_puls relation, while those in OoII clusters
with moderately blue HB types present a zero-point that is ~0.05 mag brighter.
Regarding field variables, we show that with [Fe/H]=> -1.0 a unique
M_V(RR)-k_puls relation can be adopted, independently of the parent HB
morphology. Current findings suggest that the PA_V distribution does not seem
to be a robust diagnostic for the metal abundance of RR_ab variables. However,
the same observables can be used to estimate the absolute magnitude of globular
cluster and field RR_ab variables. We show that over the metallicity range
-2.4<= [Fe/H] <= 0.0 the M_V(RR)-[Fe/H] relation shows a parabolic behavior.Comment: Paper accepted on A&A, 13 pages, 18 figure
Electroencephalographic functional connectivity in extreme prematurity: a pilot study based on graph theory
Background: Connectivity studies based on functional magnetic resonance imaging (MRI) provided new insights in neonatal brain development but cannot be performed at bedside in the clinical setting. The electroencephalogram (EEG) connectivity has been less studied, particularly using the new approach based on graph theory. This study aimed to explore the functional EEG connectivity using graph theory analysis at an early post-conception age in extremely premature and late-preterm babies free of medical complications and overt brain damage. Methods: Sixteen neonates (8 extremely low gestational age (ELGA) and 8 late-preterm infants), both groups having performed multichannel EEG recordings at 35 weeks’ post-conception, were recruited in a single tertiary-level neonatal intensive care unit and well-baby nursery, respectively. Global (i.e., small-worldness) and local (i.e., clustering and strength) connectivity measures were calculated on a single-subject connectivity matrix of EEG data. Results: Both ELGA and late-preterm infants showed small-worldness organization at 35 weeks’ post-conception. The ELGA group had the strength parameter of the theta frequency band lower in the right than in the left hemisphere. This asymmetry did not emerge in the late-preterm group. Moreover, the mean strength parameter was significantly greater in the right hemisphere in the late preterms than in the ELGA group. Conclusion: EEG connectivity measures could represent an index of left-to-right maturation and developmental disadvantage in extremely preterm infants
Constraints on the Formation of the Globular Cluster IC 4499 from Multi-Wavelength Photometry
We present new multiband photometry for the Galactic globular cluster IC 4499
extending well past the main sequence turn-off in the U, B, V, R, I, and DDO51
bands. This photometry is used to determine that IC4499 has an age of 12 pm 1
Gyr and a cluster reddening of E(B-V) = 0.22 pm 0.02. Hence, IC 4499 is coeval
with the majority of Galactic GCs, in contrast to suggestions of a younger age.
The density profile of the cluster is observed to not flatten out to at least
r~800 arcsec, implying that either the tidal radius of this cluster is larger
than previously estimated, or that IC 4499 is surrounded by a halo. Unlike the
situation in some other, more massive, globular clusters, no anomalous color
spreads in the UV are detected among the red giant branch stars. The small
uncertainties in our photometry should allow the detection of such signatures
apparently associated with variations of light elements within the cluster,
suggesting that IC 4499 consists of a single stellar population.Comment: accepted to MNRA
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