Observational calibration of the projection factor of Cepheids

IV. Period-projection factor relation of Galactic and Magellanic Cloud Cepheids

¹ European Southern Observatory, Alonso de Córdova 3107, 19001 Casilla, Santiago 19, Chile
e-mail: agallenne@astro-udec.cl
² Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU, UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, 1058 Santiago, Chile
³ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
⁶ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
⁷ Millenium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 1058 Santiago, Chile
⁸ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France

Received: 17 July 2017
Accepted: 30 August 2017

Abstract

Context. The Baade-Wesselink (BW) method, which combines linear and angular diameter variations, is the most common method to determine the distances to pulsating stars. However, the projection factor, p-factor, used to convert radial velocities into pulsation velocities, is still poorly calibrated. This parameter is critical on the use of this technique, and often leads to 5–10% uncertainties on the derived distances.

Aims. We focus on empirically measuring the p-factor of a homogeneous sample of 29 LMC and 10 SMC Cepheids for which an accurate average distances were estimated from eclipsing binary systems.

Methods. We used the SPIPS algorithm, which is an implementation of the BW technique. Unlike other conventional methods, SPIPS combines all observables, i.e. radial velocities, multi-band photometry and interferometry into a consistent physical modelling to estimate the parameters of the stars. The large number and their redundancy insure its robustness and improves the statistical precision.

Results. We successfully estimated the p-factor of several Magellanic Cloud Cepheids. Combined with our previous Galactic results, we find the following P−p relation: −0.08_{± 0.04}(log P−1.18) + 1.24_{± 0.02}. We find no evidence of a metallicity dependent p-factor. We also derive a new calibration of the period-radius relation, log R = 0.684_{± 0.007}(log P−0.517) + 1.489_{± 0.002}, with an intrinsic dispersion of 0.020. We detect an infrared excess for all stars at 3.6 μm and 4.5 μm, which might be the signature of circumstellar dust. We measure a mean offset of Δm_3.6 = 0.057 ± 0.006 mag and Δm_4.5 = 0.065 ± 0.008 mag.

Conclusions. We provide a new P−p relation based on a multi-wavelength fit that can be used for the distance scale calibration from the BW method. The dispersion is due to the LMC and SMC width we took into account because individual Cepheids distances are unknown. The new P−R relation has a small intrinsic dispersion: 4.5% in radius. This precision will allow us to accurately apply the BW method to nearby galaxies. Finally, the infrared excesses we detect again raise the issue of using mid-IR wavelengths to derive period-luminosity relation and to calibrate the Hubble constant. These IR excesses might be the signature of circumstellar dust, and are never taken into account when applying the BW method at those wavelengths. Our measured offsets may give an average bias of ~ 2.8% on the distances derived through mid-IR P−L relations.