861 research outputs found
Observations of classical cepheids
The observations of classical Cepheids are reviewed. The main progress that has been made is summarized and some of the problems yet to be solved are discussed. The problems include color excesses, calibration of color, duplicity, ultraviolet colors, temperature-color relations, mass discrepancies, and radius determination
On the metallicity gradient of the Galactic disk
Aims: The iron abundance gradient in the Galactic stellar disk provides
fundamental constraints on the chemical evolution of this important Galaxy
component. However the spread around the mean slope is, at fixed Galactocentric
distance, larger than estimated uncertainties. Methods: To provide quantitative
constraints on these trends we adopted iron abundances for 265 classical
Cepheids (more than 50% of the currently known sample) based either on
high-resolution spectra or on photometric metallicity indices. Homogeneous
distances were estimated using near-infrared Period-Luminosity relations. The
sample covers the four disk quadrants and their Galactocentric distances range
from ~5 to ~17 kpc. Results: A linear regression over the entire sample
provides an iron gradient of -0.051+/-0.004 dex/kpc. The above slope agrees
quite well, within the errors, with previous estimates based either on Cepheids
or on open clusters covering similar Galactocentric distances. However, once we
split the sample in inner (Rg < 8 kpc) and outer disk Cepheids we found that
the slope (-0.130+/-0.015 dex/kpc) in the former region is ~3 times steeper
than the slope in the latter one (-0.042+/-0.004 dex/kpc). We found that in the
outer disk the radial distribution of metal-poor (MP, [Fe/H]<-0.02 dex) and
metal-rich (MR) Cepheids across the four disk quadrants does not show a clear
trend when moving from the innermost to the external disk regions. We also
found that the relative fractions of MP and MR Cepheids in the 1st and in the
3rd quadrant differ at 8 sigma (MP) and 15 sigma (MR) level.Comment: 6 pages, 6 figures, A&A accepte
CORS Baade-Wesselink method in the Walraven photometric system: the period-radius and the period-luminosity relation of classical Cepheids
We present a new derivation of the CORS Baade-Wesselink method in the
Walraven photometric system. We solved the complete Baade-Wesselink equation by
calibrating the surface brightness function with a recent grid of atmosphere
models. The new approach was adopted to estimate the mean radii of a sample of
Galactic Cepheids for which are available precise light curves in the Walraven
bands. Current radii agree, within the errors, quite well with Cepheid radii
based on recent optical and near-infrared interferometric measurements. We also
tested the impact of the projection factor on the Period-Radius relation using
two different values (p=1.36, p=1.27) that bracket the estimates available in
the literature. We found that the agreement of our Period-Radius relation with
similar empirical and theoretical Period-Radius relations in the recent
literature, improves by changing the projection factor from p=1.36 to p=1.27.
Our Period-Radius relation is log(R)=(0.75\pm 0.03)log(P)+(1.10 \pm 0.03), with
a rms=0.03 dex. Thanks to accurate estimates of the effective temperature of
the selected Cepheids, we also derived the Period-Luminosity relation in the V
band and we found Mv=(-2.78 \pm 0.11)log(P)+(-1.42 \pm 0.11) with rms=0.13 mag,
for p=1.27. It agrees quite well with recent results in the literature, while
the relation for p=1.36 deviates by more than 2sigma. We conclude that, even
taking into account the intrinsic dispersion of the obtained Period-Luminosity
relations, that is roughly of the same order of magnitude as the effect of the
projection factor, the results of this paper seem to favour the value p = 1.27.Comment: 17 pages, 14 figures, accepted for publication in MNRA
Biomechanical Analysis of Reducing Sacroiliac Joint Shear Load by Optimization of Pelvic Muscle and Ligament Forces
Effective stabilization of the sacroiliac joints (SIJ) is essential, since spinal loading is transferred via the SIJ to the coxal bones, and further to the legs. We performed a biomechanical analysis of SIJ stability in terms of reduced SIJ shear force in standing posture using a validated static 3-D simulation model. This model contained 100 muscle elements, 8 ligaments, and 8 joints in trunk, pelvis, and upper legs. Initially, the model was set up to minimize the maximum muscle stress. In this situation, the trunk load was mainly balanced between the coxal bones by vertical SIJ shear force. An imposed reduction of the vertical SIJ shear by 20% resulted in 70% increase of SIJ compression force due to activation of hip flexors and counteracting hip extensors. Another 20% reduction of the vertical SIJ shear force resulted in further increase of SIJ compression force by 400%, due to activation of the transversely oriented M. transversus abdominis and pelvic floor muscles. The M. transversus abdominis crosses the SIJ and clamps the sacrum between the coxal bones. Moreover, the pelvic floor muscles oppose lateral movement of the coxal bones, which stabilizes the position of the sacrum between the coxal bones (the pelvic arc). Our results suggest that training of the M. transversus abdominis and the pelvic floor muscles could help to relieve SI-joint related pelvic pain
Masses for Galactic Beat Cepheids
Accurate mass determinations for Cepheids may be used to determine the degree of excess mixing in the interiors of their main-sequence progenitors : the larger the excess mixing, the larger the luminosity of the Cepheid of a given mass, or the smaller the mass of a Cepheid with given luminosity. Dynamical masses determined recently for a few Cepheid binaries indicate excess mixing somewhat stronger than that corresponding to the convective overshoot models by Schaller et al. Beat Cepheids can be used similarly to test main-sequence mixing in stellar interiors. The period ratios for beat Cepheids depend on luminosity, heavy element abundance, and mass. By comparing pulsational models and the obser- T eff, vationally derived luminosity, metallicities, and period ratios it is possible to obtain masses for these T eff, stars, the so-called beat masses. With the old opacities masses much smaller than the evolutionary masses were obtained. With the new OPAL opacities a beat mass close to the dynamical mass was obtained for the binary beat Cepheid Y Carinae, showing that it is now possible to obtain reliable beat masses. In this paper, we determine beat masses for seven Galactic beat Cepheids for which photometric and spectroscopic data are available. We Ðnd an average mass around 4.2 ^ 0.3 for these stars, M_ though the actual error limits for each star may be larger mainly because of uncertainties in E(B[V ) and the heavy element abundances. (As derived spectroscopically, beat Cepheids are in general metalpoor, with The relation between the derived beat masses and the luminosities [0.4[[Fe/H][ 0.0). again indicates excess mixing that is somewhat larger than that corresponding to the models by Schaller et al
Cepheid Period-Radius and Period-Luminosity Relations and the Distance to the LMC
We have used the infrared Barnes-Evans surface brightness technique to derive
the radii and distances of 34 Galactic Cepheid variables. Radius and distance
results obtained from both versions of the technique are in excellent
agreement. The radii of 28 variables are used to determine the period-radius
relation. This relation is found to have a smaller dispersion than in previous
studies, and is identical to the period-radius relation found by Laney & Stobie
from a completely independent method, a fact which provides persuasive evidence
that the Cepheid period-radius relation is now determined at a very high
confidence level. We use the accurate infrared distances to determine
period-luminosity relations in the V, I, J, H and K passbands from the Galactic
sample of Cepheids. We derive improved slopes of these relations from updated
LMC Cepheid samples and adopt these slopes to obtain accurate absolute
calibrations of the PL relation. By comparing these relations to the ones
defined by the LMC Cepheids, we derive strikingly consistent and precise values
for the LMC distance modulus in each of the passbands which yield a mean value
of DM (LMC) = 18.46 +- 0.02.
Our results show that the infrared Barnes-Evans technique is very insensitive
to both Cepheid metallicity and adopted reddening, and therefore a very
powerful tool to derive accurate distances to nearby galaxies by a direct
application of the technique to their Cepheid variables, rather than by
comparing PL relations of different galaxies, which introduces much more
sensitivity to metallicity and absorption corrections which are usually
difficult to determine.Comment: LaTeX, AASTeX style, 9 Figures, 10 Tables, The Astrophysical Journal
in press (accepted Oct. 14, 1997). Fig. 3 replace
Classical Cepheid Pulsation Models. III. The Predictable Scenario
Within the current uncertainties in the treatment of the coupling between
pulsation and convection, limiting amplitude, nonlinear, convective models
appear the only viable approach for providing theoretical predictions about the
intrinsic properties of radial pulsators. In this paper we present the results
of a comprehensive set of Cepheid models computed within such theoretical
framework for selected assumptions on their original chemical composition.Comment: 24 pages, 1 latex file containing 6 tables, 10 postscript figures,
accepted for publication on Ap
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