498 research outputs found
Fundamental parameters of Cepheids. V. Additional photometry and radial velocity for southern Cepheids
I present photometric and radial velocity data for Galactic Cepheids, most of
them being in the southern hemisphere. There are 1250 Geneva 7-color
photometric measurements for 62 Cepheids, the average uncertainty per
measurement is better than 0.01 mag. A total of 832 velocity measurements have
been obtained with the CORAVEL radial velocity spectrograph for 46 Cepheids.
The average accuracy of the radial velocity data is 0.38 km/s. There are 33
stars with both photometry and radial velocity data. I discuss the possible
binarity or period change that these new data reveal. I also present reddenings
for all Cepheids with photometry. The data are available electronically.Comment: To appear in ApJS. Data available electronically at
ftp://cfa-ftp.harvard.edu/pub/dbersier
An Error Analysis of the Geometric Baade-Wesselink Method
We derive an analytic solution for the minimization problem in the geometric
Baade-Wesselink method. This solution allows deriving the distance and mean
radius of a pulsating star by fitting its velocity curve and angular diameter
measured interferometrically. The method also provide analytic solutions for
the confidence levels of the best fit parameters, and accurate error estimates
for the Baade-Wesselink solution. Special care is taken in the analysis of the
various error sources in the final solution, among which the uncertainties due
to the projection factor, the limb darkening and the velocity curve. We also
discuss the importance of the phase shift between the stellar lightcurve and
the velocity curve as a potential error source in the geometric Baade-Wesselink
method. We finally discuss the case of the Classical Cepheid zeta Gem, applying
our method to the measurements derived with the Palomar Testbed Interferometer.
We show how a careful treatment of the measurement errors can be potentially
used to discriminate between different models of limb darkening using
interferometric techniques.Comment: 24 pages, to be published on the Astrophysical Journal, vol. 603
March 200
Theoretical Limb Darkening for Classical Cepheids: II. Corrections for the Geometric Baade-Wesselink Method
The geometric Baade-Wesselink method is one of the most promising techniques
for obtaining a better calibration of the Cepheid period-luminosity relation by
means of interferometric measurements of accurate diameters. In this paper we
present new wavelength- and phase-dependent limb darkening corrections based on
our time-dependent hydrodynamic models of the classical Cepheid zeta Gem. We
show that a model simulation of a Cepheid atmosphere, taking into account the
hydrodynamic effects associated with the pulsation, shows strong departures
from the limb darkening otherwise predicted by a static model. For most of its
pulsational cycle the hydrodynamic model predicts a larger limb darkening then
the equivalent static model. The hydrodynamics affects the limb darkening
mainly at UV and optical wavelengths. Most of these effects evolve slowly as
the star pulsates, but there are phases, associated with shocks propagating
into the photosphere, in which significant changes in the limb darkening take
place on time-scales of the order of less than a day. We assess the implication
of our model LD corrections fitting the geometric Baade-Wesselink distance of
zeta Gem for the available near-IR PTI data. We discuss the effects of our
model limb darkening on the best fit result, and analyze the requirements
needed to test the time-dependence of the limb darkening with future
interferometric measurements.Comment: 22 pages, 5 figures, to be published on the Astrophysical Journal,
June 1 2003 issu
Cepheid distances from infrared long-baseline interferometry - I. VINCI/VLTI observations of seven Galactic Cepheids
We report the angular diameter measurements of seven classical Cepheids (X
Sgr, eta Aql, W Sgr, zeta Gem, beta Dor, Y Oph and L Car) that we have obtained
with the VINCI instrument, installed at ESO's VLT Interferometer (VLTI). We
also present reprocessed archive data obtained with the FLUOR/IOTA instrument
on zeta Gem, in order to improve the phase coverage of our observations. We
obtain average limb darkened angular diameter values of LD(X Sgr) = 1.471 +/-
0.033 mas, LD(eta Aql) = 1.839 +/- 0.028 mas, LD(W Sgr) = 1.312 +/- 0.029 mas,
LD(beta Dor) = 1.891 +/- 0.024 mas, LD(zeta Gem) =1.747 +/- 0.061 mas, LD(Y
Oph) = 1.437 +/- 0.040 mas and LD(L Car) = 2.988 +/- 0.012 mas. For four of
these stars (eta Aql, W Sgr, beta Dor, and L Car) we detect the pulsational
variation of their angular diameter. This enables us to compute directly their
distances, using a modified version of the Baade-Wesselink method: d(eta Aql) =
276 [+55 -38] pc, d(W Sgr) = 379 [+216 -130] pc, d(beta Dor) = 345 [+175 -80]
pc, d(L Car) = 603 [+24 -19] pc. The stated error bars are statistical in
nature. Applying a hybrid method, that makes use of the Gieren et al. (1998)
Period-Radius relation to estimate the linear diameters, we obtain the
following distances (statistical and systematic error bars are mentioned): d(X
Sgr) = 324 +/- 7 +/- 17 pc, d(eta Aql) = 264 +/- 4 +/- 14 pc, d(W Sgr) = 386
+/- 9 +/- 21 pc, d(beta Dor) = 326 +/- 4 +/- 19 pc, d(zeta Gem) = 360 +/- 13
+/- 22 pc, d(Y Oph) = 648 +/- 17 +/- 47 pc and d(L Car) = 542 +/- 2 +/- 49 pc.Comment: 16 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
A new Cepheid distance to the maser-host galaxy NGC 4258 and its implications for the Hubble Constant
We present initial results from a time-series BVI survey of two fields in NGC
4258 using the Advanced Camera for Surveys onboard the Hubble Space Telescope.
This galaxy was selected because of its accurate maser-based distance, which is
anticipated to have a total uncertainty of ~3%. The goal of the HST
observations is to provide an absolute calibration of the Cepheid Distance
Scale and to measure its dependence on chemical abundance (the so-called
"metallicity effect").
We carried out observations of two fields at different galactocentric
distances with a mean abundance difference of 0.5 dex. We discovered a total of
281 Cepheids with periods ranging from 4 to 45 days (the duration of our
observing window). We determine a Cepheid distance modulus for NGC 4258
(relative to the LMC) of 10.88 +- 0.04 (random) +- 0.05 (systematic) mag. Given
the published maser distance to the galaxy, this implies \mu (LMC)=18.41 +-
0.10 (r) +- 0.13 (s) mag or D(LMC)= 48.1 +- 2.3 (r) +- 2.9 (s) kpc. We measure
a metallicity effect of \gamma=-0.29 +- 0.09 (r) +- 0.05 (s) mag/dex. We see no
evidence for a variation in the slope of the Period-Luminosity relation as a
function of abundance.
We estimate a Hubble Constant of H_0= 74 +- 3 (r) +- 6 (s) km/s Mpc using a
recent sample of 4 well-observed type Ia SNe and our new calibration of the
Cepheid Distance Scale. It may soon be possible to measure the value of H_0
with a total uncertainty of 5%, with consequent improvement in the
determination of the equation of state of dark energy.Comment: 39 pages, 24 figures. Accepted for publication in the Astrophysical
Journal. Full-resolution version available in PS and PDF formats at
http://www.noao.edu/staff/lmacri/0608211-full.ps.gz and
http://www.noao.edu/staff/lmacri/0608211-full.pd
Theoretical fits of the \delta Cephei light, radius and radial velocity curves
We present a theoretical investigation of the light, radius and radial
velocity variations of the prototype Cephei. We find that the best fit
model accounts for luminosity and velocity amplitudes with an accuracy better
than , and for the radius amplitude with an accuracy of .
The chemical composition of this model suggests a decrease in both helium (0.26
vs 0.28) and metal (0.01 vs 0.02) content in the solar neighborhood. Moreover,
distance determinations based on the fit of light curves agree at the
level with the trigonometric parallax measured by the Hubble Space
Telescope (HST). On the other hand, distance determinations based on angular
diameter variations, that are independent of interstellar extinction and of the
-factor value, indicate an increase of the order of 5% in the HST parallax.Comment: accepted for publication on ApJ Letter
The distance to the Fornax dwarf galaxy using red clump stars, and the discrepancy between red clump and tip of the red giant branch distances
I determine a distance to the Fornax dwarf galaxy using stars in the red
clump and at the tip of the red giant branch. They are in very good agreement,
with . Comparing the magnitudes of the tip of the red giant
branch and of the red clump in Fornax, Carina and the Magellanic Clouds, I
propose a possible solution to the problem of the discrepancy between these two
types of distance measurements.Comment: To appear in ApJ
Radii and Distances of Cepheids, I., Method and Measurement Errors
We develop a formulation of the Baade-Wesselink method which uses the Fourier
coefficients of the observables. We derive an explicit, analytic expression to
determine the mean radius from each Fourier order. The simplicity of this
method allows us to derive the uncertainty in the mean radius due to
measurement errors.
Using simulations and a recent dataset we demonstrate that the precision of
the radius measurement with optical magnitudes is in most cases limited by the
accuracy of the measurement of the phase difference between the light and the
color index curve. In this case it is advantageous to determine the inverse
radius, because it has normal errors.Comment: 18 pages, postscript, accepted for publication in Ap
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