98 research outputs found
Calibrating the projection factor for Galactic Cepheids
The projection factor (p), which converts the radial velocity to pulsational
velocity, is an important parameter in the Baade-Wesselink (BW) type analysis
and distance scale work. The p-factor is either adopted as a constant or
linearly depending on the logarithmic of pulsating periods. The aim of this
work is to calibrate the p-factor if a Cepheid has both the BW distance and an
independent distance measurement, and examine the p-factor for delta Cephei --
the prototype of classical Cepheids. We calibrated the p-factor for several
Galactic Cepheids that have both the latest BW distances and independent
distances either from Hipparcos parallaxes or main-sequence fitting distances
to Cepheid-hosted stellar clusters. Based on 25 Cepheids, the calibrated
p-factor relation is consistent with latest p-factor relation in literature.
The calibrated p-factor relation also indicates that this relation may not be
linear and may exhibit an intrinsic scatter. We also examined the discrepancy
of empirical p-factors for delta Cephei, and found that the reasons for this
discrepancy include the disagreement of angular diameters, the treatment of
radial velocity data, and the phase interval adopted during the fitting
procedure. Finally, we investigated the impact of the input p-factor in two BW
methodologies for delta Cephei, and found that different p-factors can be
adopted in these BW methodologies and yet result in the same angular diameters.Comment: 6 pages, 6 figures and 2 tables. A&A accepte
Is There a Difference in Luminosity between Field and Cluster RR Lyrae Variables?
Recent Hipparcos results have lent support to the idea that RR Lyrae
variables in the halo field and in globular clusters differ in luminosity by
~0.2mag. In this Letter, we study the pulsation properties of RR Lyraes in
clusters with distances determined via main-sequence fitting to Hipparcos
parallaxes for field subdwarfs, and compare them with the properties of field
variables also analyzed with Hipparcos. We show that the period-temperature
distributions for field and cluster variables are essentially
indistinguishable, thus suggesting that there is no significant difference in
luminosity between them.Comment: 11 pages, including three embedded figures and one table. ApJ
(Letters), in pres
Cepheid variables in the LMC cluster NGC 1866. I. New BVRI CCD photometry
We report BV(RI)c CCD photometric data for a group of seven Cepheid variables
in the young, rich cluster NGC 1866 in the Large Magellanic Cloud. The
photometry was obtained as part of a program to determine accurate distances to
these Cepheids by means of the infrared surface brightness technique, and to
improve the LMC Cepheid database for constructing Cepheid PL and PLC relations.
Using the new data together with data from the literature, we have determined
improved periods for all variables. For five fundamental mode pulsators, the
light curves are now of excellent quality and will lead to accurate distance
and radius determinations once complete infrared light curves and radial
velocity curves for these variables become available.Comment: To appear in ApJ Supp., AASTeX, 24 pages, 8 tables, 8 figure
Cepheid limb darkening, angular diameter corrections, and projection factor from static spherical model stellar atmospheres
Context. One challenge for measuring the Hubble constant using Classical
Cepheids is the calibration of the Leavitt Law or period-luminosity
relationship. The Baade-Wesselink method for distance determination to Cepheids
relies on the ratio of the measured radial velocity and pulsation velocity, the
so-called projection factor and the ability to measure the stellar angular
diameters. Aims. We use spherically-symmetric model stellar atmospheres to
explore the dependence of the p-factor and angular diameter corrections as a
function of pulsation period. Methods. Intensity profiles are computed from a
grid of plane-parallel and spherically-symmetric model stellar atmospheres
using the SAtlas code. Projection factors and angular diameter corrections are
determined from these intensity profiles and compared to previous results.
Results. Our predicted geometric period-projection factor relation including
previously published state-of-the-art hydrodynamical predictions is not with
recent observational constraints. We suggest a number of potential resolutions
to this discrepancy. The model atmosphere geometry also affects predictions for
angular diameter corrections used to interpret interferometric observations,
suggesting corrections used in the past underestimated Cepheid angular
diameters by 3 - 5%. Conclusions. While spherically-symmetric hydrostatic model
atmospheres cannot resolve differences between projection factors from theory
and observations, they do help constrain underlying physics that must be
included, including chromospheres and mass loss. The models also predict more
physically-based limb-darkening corrections for interferometric observations.Comment: 8 pages, 6 figures, 2 tables, accepted for publication in A&
New Baade-Wesselink distances and radii for four metal-rich Galactic Cepheids
We provided accurate estimates of distances, radii and iron abundances for
four metal-rich Cepheids, namely V340 Ara, UZ Sct, AV Sgr and VY Sgr. The main
aim of this investigation is to constrain their pulsation properties and their
location across the Galactic inner disk. We adopted new accurate NIR (J,H,K)
light curves and new radial velocity measurements for the target Cepheids to
determinate their distances and radii using the Baade-Wesselink technique. In
particular, we adopted the most recent calibration of the IR surface brightness
relation and of the projection factor. Moreover, we also provided accurate
measurements of the iron abundance of the target Cepheids. Current distance
estimates agree within one sigma with similar distances based either on
empirical or on theoretical NIR Period-Luminosity relations. However, the
uncertainties of the Baade-Wesselink distances are on average a factor of 3-4
smaller when compared with errors affecting other distance determinations. Mean
Baade-Wesselink radii also agree at one sigma level with Cepheid radii based
either on empirical or on theoretical Period-Radius relations. Iron abundances
are, within one sigma, similar to the iron contents provided by Andrievsky and
collaborators, thus confirming the super metal-rich nature of the target
Cepheids. We also found that the luminosity amplitudes of classical Cepheids,
at odds with RR Lyrae stars, do not show a clear correlation with the
metal-content. This circumstantial evidence appears to be the consequence of
the Hertzsprung progression together with the dependence of the topology of the
instability strip on metallicity, evolutionary effects and binaries.Comment: 9 pages, 7 figures, A&A accepte
Distances, ages, and epoch of formation of globular clusters
We review the results on distances and absolute ages of galactic globular
clusters (GCs) obtained after the release of the Hipparcos catalogue. Several
methods for the Population II local distance scale are discussed, exploiting
NEW RESULTS for RR Lyraes in the Large Magellanic Cloud (LMC). We find that the
so-called Short and Long Distance Scales may be reconciled whether a consistent
reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC.
Distances and ages for the 9 clusters discussed in Paper I are re-derived using
an enlarged sample of local subdwarfs, which includes about 90% of the
metal-poor dwarfs with accurate parallaxes (Delta p/p < 0.12) in the whole
Hipparcos catalogue. On average, our revised distance moduli are decreased by
0.04 mag with respect to Paper I. The corresponding age of the GCs is
t=11.5+-2.6 Gyr (95% confidence range). The relation between Mv(ZAHB) and
metallicity for the nine programme clusters turns out to be
Mv(ZAHB)=(0.18+-0.09)([Fe/H]+1.5)+(0.53+-0.12).Thanks to Hipparcos the major
contribution to the total error budget associated with the subdwarf fitting
technique has been moved from parallaxes to photometric calibrations, reddening
and metallicity scale. This total uncertainty still amounts to about +-0.12
mag. Comparing the corresponding (true) LMC distance modulus 18.64+-0.12 mag
with other existing determinations, we conclude that at present the best
estimate for the distance of the LMC is: 18.54+-0.03+-0.06, suggesting that
distances from the subdwarf fitting method are 1 sigma too long. Consequently,
our best estimate for the age of the GCs is revised to: Age = 12.9+-2.9 Gyr
(95% confidence range). The best relation between Mv(ZAHB) and [Fe/H] is:
Mv(ZAHB) =(0.18+-0.09)([Fe/H]+1.5)+(0.63+-0.07).Comment: 76 pages, 6 encapsulated figures and 6 tables. Latex, uses
aasms4.sty. Revised and improved version, with new data on field RR Lyraes in
LMC. Accepted in the Astrophysical Journa
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Young and Intermediate-age Distance Indicators
Distance measurements beyond geometrical and semi-geometrical methods, rely
mainly on standard candles. As the name suggests, these objects have known
luminosities by virtue of their intrinsic proprieties and play a major role in
our understanding of modern cosmology. The main caveats associated with
standard candles are their absolute calibration, contamination of the sample
from other sources and systematic uncertainties. The absolute calibration
mainly depends on their chemical composition and age. To understand the impact
of these effects on the distance scale, it is essential to develop methods
based on different sample of standard candles. Here we review the fundamental
properties of young and intermediate-age distance indicators such as Cepheids,
Mira variables and Red Clump stars and the recent developments in their
application as distance indicators.Comment: Review article, 63 pages (28 figures), Accepted for publication in
Space Science Reviews (Chapter 3 of a special collection resulting from the
May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space
Age
Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs
We discuss the impact of Population II and Globular Cluster (GCs) stars on
the derivation of the age of the Universe, and on the study of the formation
and early evolution of galaxies, our own in particular. The long-standing
problem of the actual distance scale to Population II stars and GCs is
addressed, and a variety of different methods commonly used to derive distances
to Population II stars are briefly reviewed. Emphasis is given to the
discussion of distances and ages for GCs derived using Hipparcos parallaxes of
local subdwarfs. Results obtained by different authors are slightly different,
depending on different assumptions about metallicity scale, reddenings, and
corrections for undetected binaries. These and other uncertainties present in
the method are discussed. Finally, we outline progress expected in the near
future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22
pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty
LaTeX style file, enclose
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