832 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&
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
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