Cepheid distances from the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS)

Bigot

Gallenne

et al. 2017

A&A

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The photospheric radius is one of the fundamental parameters governing the radiative equilibrium of a star. We report new observations of the nearest solar-type stars α Centauri A (G2V) and B (K1V) with the VLTI/PIONIER optical interferometer. The combination of four configurations of the VLTI enable us to measure simultaneously the limb darkened angular diameter θ LD and the limb darkening parameters of the two solar-type stars in the near-infrared H band (λ = 1.65 µm). We obtain photospheric angular diameters of θ LD (A) = 8.502 ± 0.038 mas (0.43%) and θ LD (B) = 5.999 ± 0.025 mas (0.42%), through the adjustment of a power law limb darkening model. We find H band power law exponents of α(A) = 0.1404 ± 0.0050 (3.6%) and α(B) = 0.1545 ± 0.0044 (2.8%), which closely bracket the observed solar value (α = 0.15027). Combined with the parallax π = 747.17 ± 0.61 mas previously determined, we derive linear radii of R A = 1.2234 ± 0.0053 R (0.43%) and R B = 0.8632 ± 0.0037 R (0.43%). The power law exponents that we derive for the two stars indicate a significantly weaker limb darkening than predicted by both 1D and 3D stellar atmosphere models. As this discrepancy is also observed on near-infrared limb darkening profile of the Sun, an improvement of the calibration of stellar atmosphere models is clearly needed. The reported PIONIER visibility measurements of α Cen A and B provide a robust basis to validate the future evolutions of these models.

mentioning

confidence: 75%

The radii and limb darkenings of α Centauri A and B

Bigot

Gallenne

et al. 2017

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“…The uncertainty of p was computed from the combination of the error bars of the independent measurements of OGLE-LMC-CEP-0227 (P = 3.80 d, p = 1.21 ± 0.05, Pilecki et al 2013), δ Cep (P = 5.37 d, p = 1.288 ± 0.054, Mérand et al 2015), and the present measurement of RS Pup (P = 41.5 d, p = 1.250 ± 0.064). We did not average the error bars of the different p-factor measurements from the HST/FGS distances as the degree of correlation between them and the possible associated systematics are uncer- Pilecki et al (2013) and Gieren et al (2015), and we selected only the best p-factor of δ Cep derived by Mérand et al (2015). In agreement with the present results, Breitfelder et al (2016) also concluded from a fit to the complete sample of measured p-factors that a constant value of p = 1.324 ± 0.024 (1σ from our value) reproduces the measurements.…”

Section: Discussionmentioning

confidence: 99%

Observational calibration of the projection factor of Cepheids

Trahin

Bond

et al. 2017

A&A

The projection factor (p-factor) is an essential component of the classical Baade-Wesselink (BW) technique, which is commonly used to determine the distances to pulsating stars. It is a multiplicative parameter used to convert radial velocities into pulsational velocities. As the BW distances are linearly proportional to the p-factor, its accurate calibration for Cepheids is of critical importance for the reliability of their distance scale. We focus on the observational determination of the p-factor of the long-period Cepheid RS Pup (P = 41.5 days). This star is particularly important as this is one of the brightest Cepheids in the Galaxy and an analog of the Cepheids used to determine extragalactic distances. An accurate distance of 1910 ± 80 pc (±4.2%) has recently been determined for RS Pup using the light echoes propagating in its circumstellar nebula. We combine this distance with new VLTI/PIONIER interferometric angular diameters, photometry, and radial velocities to derive the p-factor of RS Pup using the code Spectro-Photo-Interferometry of Pulsating Stars (SPIPS). We obtain p = 1.250 ± 0.064 (±5.1%), defined for cross-correlation radial velocities. Together with measurements from the literature, the p-factor of RS Pup confirms the good agreement of a constant p = 1.293 ± 0.039 (±3.0%) model with the observations. We conclude that the p-factor of Cepheids is constant or mildly variable over a broad range of periods (3.7 to 41.5 days).

“…A full description of the code can be found in Mérand et al (2015) 2 . Briefly, this tool is based on the parallax-of-pulsation method (also called the Baade-Wesselink method), i.e.…”

Section: Algorithmmentioning

confidence: 99%

Observational calibration of the projection factor of Cepheids

Gallenne

Mérand

et al. 2017

A&A

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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 3.6µm and 4.5µm, 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 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.