Observational calibration of the projection factor of Cepheids I. The Type II Cepheid kappa Pavonis

Abstract

The distances 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 quasi-geometrical way. However, the linear diameter amplitude is directly proportional to the projection factor (hereafter p-factor), which is used to convert spectroscopic radial velocities (i.e., disk integrated) into pulsating (i.e., photospheric) velocities. The value of the p-factor and its possible dependence on the pulsation period are still widely debated. Our goal is to measure an observational value of the p-factor of the type-II Cepheid kappa Pavonis, whose parallax was measured with an accuracy of 5% using HST/FGS. We used this parallax as a starting point to derive the p-factor of kappa Pav, using the SPIPS technique, which is a robust version of the parallax-of-pulsation 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 multi-colour photometry and radial velocities from the literature. We obtain a value of p = 1.26 +/- 0.07 for the p-factor of kappa Pav. This result agrees with several of the recently derived Period-p-factor relationships from the literature, as well as previous observational determinations for Cepheids. Individual estimates of the p-factor 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 p-factor on period.

8 pages, 6 figures, accepted in A&A

References in corpus (6)

• The Luminosities and Distance Scales of Type II Cepheid and RR Lyrae variables
• High resolution spectroscopy for Cepheids distance determination. II. A period- projection factor relation
• High resolution spectroscopy for Cepheids distance determination. III. A relation between gamma-velocities and gamma-asymmetries
• Tuning in on Cepheids: Radial velocity amplitude modulations. A source of systematic uncertainty for Baade-Wesselink distances
• Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes
• The long-period Galactic Cepheid RS Puppis - III. A geometric distance from HST polarimetric imaging of its light echoes

Cited by in corpus (7)

• Astrometry with Hubble Space Telescope Fine Guidance Sensors - A Review
• The period-luminosity and period-radius relations of Type II and anomalous Cepheids
• Observational calibration of the projection factor of Cepheids IV. Period-projection factor relation of Galactic and Magellanic Cloud Cepheids
• VEGA/CHARA interferometric observations of Cepheids. I. A resolved structure around the prototype classical Cepheid delta Cep in the visible spectral range
• An absolute calibration of the near-infrared Period-Luminosity Relations of Type II Cepheids in the Milky Way and in the Large Magellanic Cloud
• Spectroscopic properties of a two-dimensional time-dependent Cepheid model I. Description and validation of the model
• A new method of measuring center-of-mass velocities of radially pulsating stars from high-resolution spectroscopy