325 research outputs found
High resolution spectroscopy for Cepheids distance determination. V. Impact of the cross-correlation method on the p-factor and the gamma-velocities
The cross correlation method (hereafter CC) is widely used to derive the
radial velocity curve of Cepheids when the signal to noise of the spectra is
low. However, if it is used with the wrong projection factor, it might
introduce some biases in the Baade-Wesselink (hereafter BW) methods of
determining the distance of Cepheids. In addition, it might affect the average
value of the radial velocity curve (or gamma-velocity) important for Galactic
structure studies. We aim to derive a period-projection factor relation
(hereafter Pp) appropriate to be used together with the CC method. Moreover, we
investigate whether the CC method can explain the misunderstood previous
calculation of the K-term of Cepheids. We observed eight galactic Cepheids with
the HARPS spectrograph. For each star, we derive an interpolated CC radial
velocity curve using the HARPS pipeline. The amplitudes of these curves are
used to determine the correction to be applied to the semi-theoretical
projection factor derived in Nardetto et al. (2007). Their average value (or
gamma-velocity) are also compared to the center-of-mass velocities derived in
Nardetto et al. (2008). The correction in amplitudes allows us to derive a new
Pp relation: p = [-0.08+-0.05] log P +[1.31+-0.06]. We also find a negligible
wavelength dependence (over the optical range) of the Pp relation. We finally
show that the gamma-velocity derived from the CC method is systematically
blue-shifted by about 1.0 +- 0.2km/s compared to the center-of-mass velocity of
the star. An additional blue-shift of 1.0km/s is thus needed to totally explain
the previous calculation of the K-term of Cepheids (around 2km/s). The new Pp
relation we derived is a solid tool for the distance scale calibration
(abridged).Comment: Comments : 9 pages, 3 Postscript figures, 5 Tables, accepted for
publication in A&
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
The Baade-Wesselink p-factor applicable to LMC Cepheids
Context. Recent observations of LMC Cepheids bring new constraints on the
slope of the period-projection factor relation (hereafter Pp relation) that is
currently used in the Baade-Wesselink (hereafter BW) method of distance
determination. The discrepancy between observations and theoretical analysis is
particularly significant for short period Cepheids Aims. We investigate three
physical effects that might possibly explain this discrepancy: (1) the
spectroscopic S/N that is systematically lower for LMC Cepheids (around 10)
compared to Galactic ones (up to 300), (2) the impact of the metallicity on the
dynamical structure of LMC Cepheids, and (3) the combination of infrared
photometry/interferometry with optical spectroscopy. Methods. To study the S/N
we use a very simple toy model of Cepheids. The impact of metallicity on the
projection factor is based on the hydrodynamical model of delta Cep already
described in previous studies. This model is also used to derive the position
of the optical versus infrared photospheric layers. Results. We find no
significant effect of S/N, metallicity, and optical-versus-infrared
observations on the Pp relation. Conclusions. The Pp relation of Cepheids in
the LMC does not differ from the Galactic relation. This allows its universal
application to determine distances to extragalactic Cepheids via BW analysis.Comment: accepted in A&A LETTER
The operation of VEGA/CHARA : from the scientific idea to the final products
We describe the data flow in the operation of the VEGA/CHARA instrument.
After a brief summary of the main characteristics and scientific objectives of
the VEGA instrument, we explain the standard procedure from the scientific idea
up to the execution of the observation. Then, we describe the different steps
done after the observation, from the raw data to the archives and the final
products. Many tools are used and we show how the Virtual Observatory
principles have been implemented for the interoperability of these software and
databases.Comment: 9 pages, 3 figure
High resolution spectroscopy for Cepheids distance determination. II. A period- projection factor relation
The projection factor is a key quantity for the interferometric
Baade-Wesselink (hereafter IBW) and surface-brightness (hereafter SB) methods
of determining the distance of Cepheids. Indeed, it allows a consistent
combination of angular and linear diameters of the star. We aim to determine
consistent projection factors that include the dynamical structure of the
Cepheids' atmosphere. Methods. Hydrodynamical models of delta Cep and l Car
have been used to validate a spectroscopic method of determining the projection
factor. This method, based on the amplitude of the radial velocity curve, is
applied to eight stars observed with the HARPS spectrometer. The projection
factor is divided into three sub-concepts : (1) a geometrical effect, (2) the
velocity gradient within the atmosphere, and (3) the relative motion of the
"optical" pulsating photosphere compared to the corresponding mass elements
(hereafter fo-g). Both, (1) and (3) are deduced from geometrical and
hydrodynamical models, respectively, while (2) is derived directly from
observations. The Fe I 4896.439 A line is found to be the best one to use in
the context of IBW and SB methods. A coherent and consistent period-projection
factor relation (hereafter Pp relation) is derived for this specific spectral
line: p = [0.064 +- 0.020] log P + [1.376 +- 0.023]. This procedure is then
extended to derive dynamic projection factors for any spectral line of any
Cepheid. This Pp relation is an important tool for removing bias in the
calibration of the period-luminosity relation of Cepheids. Moreover, it reveals
a new physical quantity fo-g to investigate in the near future
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&
Notes on disentangling of spectra II. Intrinsic line-profile variability due to Cepheid pulsations
The determination of pulsation velocities from observed spectra of Cepheids
is needed for the Baade-Wesselink calibration of these primary distance
markers. The applicability of the Fourier-disentangling technique for the
determination of pulsation velocities of Cepheids and other pulsating stars is
studied. The KOREL-code was modified to enable fitting of free parameters of a
prescribed line-profile broadening function corresponding to the radial
pulsations of the stellar atmosphere. It was applied to spectra of delta Cep in
the H-alpha region observed with the Ondrejov 2-m telescope. The telluric lines
were removed using template-constrained disentangling, phase-locked variations
of line-strengths were measured and the curves of pulsational velocities
obtained for several spectral lines. It is shown that the amplitude and phase
of the velocities and line-strength variations depend on the depth of line
formation and the excitation potential. The disentangling of pulsations in the
Cepheid spectra may be used for distance determination
Dynamical phasing of Type II Cepheids
In this paper we examine the problems of phasing using light curves and offer
an alternate technique using the changes in acceleration to establish the zero
point. We give astrophysical justification as to why this technique is useful
and apply the technique to a selection of Type II Cepheids. We then examine
some limitations of the technique which qualify its use.Comment: 6 pages, including 4 figures, accepted in MNRA
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