The period-luminosity relation for Cepheid variable stars

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).

Section: Discussionmentioning

confidence: 99%

Observational calibration of the projection factor of Cepheids

Trahin

Bond

et al. 2017

A&A

“…The conversion from magnitude to flux takes into account the photometric system and the filter bandpass. During the fitting procedure, all flux densities <3 µm were corrected for interstellar extinction A λ = R λ E(B − V) using the total-to-selective absorption ratios R λ from Fouqué et al (2003) and Hindsley & Bell (1989). The mid-IR data were not corrected for the interstellar extinction, which we assumed to be negligible.…”

Section: Single Dust Shell Modelmentioning

confidence: 99%

Extended envelopes around Galactic Cepheids

Gallenne

Mérand

et al. 2013

A&A

Aims. We study the close environment of nearby Cepheids using high spatial resolution observations in the mid-infrared with the VLTI/MIDI instrument, a two-beam interferometric recombiner. Methods. We obtained spectra and visibilities for the classical Cepheids X Sgr and T Mon. We fitted the MIDI measurements, supplemented by B, V, J, H, K literature photometry, with the numerical transfer code DUSTY to determine the dust shell parameters. We used a typical dust composition for circumstellar environments. Results. We detect an extended dusty environment in the spectra and visibilities for both stars, although T Mon might suffer from thermal background contamination. We attribute this to the presence of a circumstellar envelope (CSE) surrounding the Cepheids. This is optically thin for X Sgr (τ 0.55 µm = 0.008), while it appears to be thicker for T Mon (τ 0.55 µm = 0.15). They are located at about 15−20 stellar radii. Following our previous work, we derived a likely period-excess relation in the VISIR PAH1 filter, f 8.6 µm [%]= 0.81(±0.04)P[day]. We argue that the impact of CSEs on the mid-IR period-luminosity (P−L) relation cannot be negligible because they can bias the Cepheid brightness by up to about 30%. For the K-band P−L relation, the CSE contribution seems to be lower (<5%), but the sample needs to be enlarged to firmly conclude that the impact of the CSEs is negligible in this band.

“…All flux densities <3 μm are corrected for interstellar extinction A λ = R λ E(B − V) using both the total-to-selective absorption ratios R λ from Fouqué et al (2003) and Hindsley & Bell (1989), and the color excess E(B − V) from Fouqué et al (2007).…”

Section: Spectral Energy Distribution Of Classical Cepheidsmentioning

confidence: 99%

Thermal infrared properties of classical and type II Cepheids

Gallenne

Mérand

2012

A&A

We present new thermal infrared (IR) photometry and spectral energy distributions (SEDs) of eight classical Cepheids (type I) and three type II Cepheids, using VISIR thermal IR photometric measurements, supplemented with literature data. We used the BURST mode of the instrument to get diffraction-limited images at 8.59, 11.25, and 11.85 μm. The SEDs show a IR excess at wavelengths longer than 10 μm in ten of the eleven stars. We tentatively attribute these excesses to circumstellar emission created by mass loss from the Cepheids. On the basis of some hypotheses for the dust composition, we estimate a total mass of the envelope ranging from 10 −10 to 10 −8 M . We also detect a spatially extended emission around AX Cir, X Sgr, W Sgr, Y Oph, and U Car, while we do not resolve the circumstellar envelope (CSE) of the other stars. The averaged circumstellar envelope brightnesses relative to the stellar photosphere are α(AX Cir) = 13.8 ± 2.5%, α(X Sgr) = 7.9 ± 1.4%, α(W Sgr) = 3.8 ± 0.6%, α(Y Oph) = 15.1 ± 1.4%, and α(U Car) = 16.3 ± 1.4% at 8.59 μm. With this study, we extend the number of classical Cepheids with detected CSEs from 9 to 14, confirming that at least a large fraction of all Cepheids are experiencing significant mass loss. The presence of these CSEs may also impact the future use of Cepheids as standard candles at near and thermal infrared wavelengths.