Observational calibration of the projection factor of Cepheids. I. The type II Cepheid κ Pavonis
Abstract
Context. The distance of pulsating stars, in particular Cepheids, are commonly measured using the parallax of pulsation technique. The different versions of this technique combine measurements of the linear diameter variation (from spectroscopy) and the angular diameter variation (from photometry or interferometry) amplitudes, to retrieve the distance in a quasigeometrical way. However, the linear diameter amplitude is directly proportional to the projection factor (hereafter pfactor), which is used to convert spectroscopic radial velocities (i.e., disk integrated) into pulsating (i.e., photospheric) velocities. The value of the pfactor and its possible dependence on the pulsation period are still widely debated.
Aims: Our goal is to measure an observational value of the pfactor of the typeII Cepheid κ Pavonis.
Methods: The parallax of the typeII Cepheid κ Pav was measured with an accuracy of 5% using HST/FGS. We used this parallax as a starting point to derive the pfactor of κ Pav, using the SPIPS technique (SpectroPhotoInterferometry of Pulsating Stars), which is a robust version of the parallaxofpulsation method that employs radial velocity, interferometric and photometric data. We applied this technique to a combination of new VLTI/PIONIER optical interferometric angular diameters, new CORALIE and HARPS radial velocities, as well as multicolour photometry and radial velocities from the literature.
Results: We obtain a value of p = 1.26 ± 0.07 for the pfactor of κ Pav. This result agrees with several of the recently derived Periodpfactor relationships from the literature, as well as previous observational determinations for Cepheids.
Conclusions: Individual estimates of the pfactor are fundamental to calibrating the parallax of pulsation distances of Cepheids. Together with previous observational estimates, the projection factor we obtain points to a weak dependence of the pfactor on period.
Based on observations realized with ESO facilities at Paranal Observatory under program IDs 091.D0020 and 093.D0316.Based on observations collected at ESO La Silla Observatory using the Coralie spectrograph mounted to the Swiss 1.2 m Euler telescope, under program CNTAC2014A5.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 April 2015
 DOI:
 10.1051/00046361/201425171
 arXiv:
 arXiv:1503.05176
 Bibcode:
 2015A&A...576A..64B
 Keywords:

 stars: individual: k Pavonis;
 techniques: interferometric;
 methods: observational;
 stars: variables: Cepheids;
 stars: distances;
 Astrophysics  Instrumentation and Methods for Astrophysics;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 8 pages, 6 figures, accepted in A&