Observational calibration of the projection factor of Cepheids. IV. Periodprojection factor relation of Galactic and Magellanic Cloud Cepheids
Abstract
Context. The BaadeWesselink (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, pfactor, 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 510% uncertainties on the derived distances.
Aims: We focus on empirically measuring the pfactor 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, multiband 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 pfactor of several Magellanic Cloud Cepheids. Combined with our previous Galactic results, we find the following Pp relation: 0.08_{± 0.04}(log P1.18) + 1.24_{± 0.02}. We find no evidence of a metallicity dependent pfactor. We also derive a new calibration of the periodradius relation, log R = 0.684_{± 0.007}(log P0.517) + 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 ∆m_{3.6} = 0.057 ± 0.006 mag and ∆m_{4.5} = 0.065 ± 0.008 mag.
Conclusions: We provide a new Pp relation based on a multiwavelength 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 PR 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 midIR wavelengths to derive periodluminosity 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 midIR PL relations.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 November 2017
 DOI:
 10.1051/00046361/201731589
 arXiv:
 arXiv:1708.09851
 Bibcode:
 2017A&A...608A..18G
 Keywords:

 techniques: photometric;
 techniques: radial velocities;
 stars: variables: Cepheids;
 stars: fundamental parameters;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 13 pages, 11 figures, Astronomy &