Context.
Surface brightness-color relations (SBCRs) are widely used for estimating angular diameters and deriving stellar properties. They are critical to derive extragalactic distances of early-type ...and late-type eclipsing binaries or, potentially, for extracting planetary parameters of late-type stars hosting planets. Various SBCRs have been implemented so far, but strong discrepancies in terms of precision and accuracy still exist in the literature.
Aims.
We aim to develop a precise SBCR for early-type B and A stars using selection criteria, based on stellar characteristics, and combined with homogeneous interferometric angular diameter measurements. We also improve SBCRs for late-type stars, in particular in the
Gaia
photometric band.
Methods.
We observed 18 early-type stars with the VEGA interferometric instrument, installed on the CHARA array. We then applied additional criteria on the photometric measurements, together with stellar characteristics diagnostics in order to build the SBCRs.
Results.
We calibrated a SBCR for subgiant and dwarf early-type stars. The RMS of the relation is
σ
F
V
0
= 0.0051 mag, leading to an average precision of 2.3% on the estimation of angular diameters, with 3.1% for
V
−
K
< −0.2 mag and 1.8% for
V
−
K
> −0.2 mag. We found that the conversion between Johnson-
K
and 2MASS-
K
s
photometries is a key issue for early-type stars. Following this result, we have revisited our previous SBCRs for late-type stars by calibrating them with either converted Johnson-
K
or 2MASS-
K
s
photometries. We also improve the calibration of these SBCRs based on the
Gaia
photometry. The expected precision on the angular diameter using our SBCRs for late-type stars ranges from 1.0 to 2.7%.
Conclusions.
By reaching a precision of 2.3% on the estimation of angular diameters for early-type stars, significant progress has been made to determine extragalactic distances, such as M31 and M33 galaxies, using early-type eclipsing binaries.
Context.
An inner companion has recently been discovered orbiting the prototype of classical Cepheids,
δ
Cep, whose orbital parameters are still not fully constrained.
Aims.
We collected new precise ...radial velocity measurements of
δ
Cep in 2019 using the HARPS-N spectrograph mounted at the Telescopio Nazionale
Galileo
. Using these radial velocity measurements, we aimed to improve the orbital parameters of the system.
Methods.
We considered a template available in the literature as a reference for the radial velocity curve of the pulsation of the star. We then calculated the residuals between our global dataset (composed of the new 2019 observations plus data from the literature) and the template as a function of the pulsation phase and the barycentric Julian date. This provides the orbital velocity of the Cepheid component. Using a Bayesian tool, we derived the orbital parameters of the system.
Results.
Considering priors based on already published
Gaia
constraints, we find for the orbital period a maximum a posteriori probability of
P
orb
= 9.32
−0.04
+0.03
years (uncertainties correspond to the 95% highest density probability interval), and we obtain an eccentricity
e
= 0.71
−0.02
+0.02
, a semimajor axis
a
= 0.029
−0.003
+0.002
arcsec, and a center-of-mass velocity
V
0
= −17.28
−0.08
+0.08
km s
−1
, among other parameters.
Conclusions.
In this short analysis we derive the orbital parameters of the
δ
Cep inner binary system and provide a cleaned radial velocity curve of the pulsation of the star, which will be used to study its Baade–Wesselink projection factor in a future publication.
Context.
As primary anchors of the distance scale, Cepheid stars play a crucial role in our understanding of the distance scale of the Universe because of their period-luminosity relation. ...Determining precise and consistent parameters (radius, temperature, color excess, and projection factor) of Cepheid pulsating stars is therefore very important.
Aims.
With the high-precision parallaxes delivered by the early third
Gaia
data release (EDR3), we aim to derive various parameters of Cepheid stars in order to calibrate the period-luminosity and period-radius relations and to investigate the relation of period to
p
-factor.
Methods.
We applied an implementation of the parallax-of-pulsation method through the algorithm called spectro-photo-interferometry of pulsating stars (SPIPS), which combines all types of available data for a variable star (multiband and multicolor photometry, radial velocity, effective temperature, and interferometry measurements) in a global modeling of its pulsation.
Results.
We present the SPIPS modeling of a sample of 63 Galactic Cepheids. Adopting
Gaia
EDR3 parallaxes as an input associated with the best available dataset, we derive consistent values of parameters for these stars such as the radius, multiband apparent magnitudes, effective temperatures, color excesses, period changes, Fourier parameters, and the projection factor.
Conclusions.
Using the best set of data and the most precise distances for Milky Way Cepheids, we derive new calibrations of the period-luminosity and period-radius relations:
M
K
S
= −5.529
±0.015
− 3.141
±0.050
(log
P
− 0.9) and log
R
= 1.763
±0.003
+ 0.653
±0.012
(log
P
− 0.9). After investigating the dependences of the projection factor on the parameters of the stars, we find a high dispersion of its values and no evidence of its correlation with the period or with any other parameters such as radial velocity, temperature, or metallicity. Statistically, the
p
-factor has an average value of
p
= 1.26 ± 0.07, but with an unsatisfactory agreement (
σ
= 0.15). In absence of any clear correlation between the
p
-factor and other quantities, the best agreement is obtained under the assumption that the
p
-factor can take any value in a band with a width of 0.15. This result highlights the need for a further examination of the physics behind the
p
-factor.
Context.
Y Ophiuchi (Y Oph) is a classical Cepheid with a pulsation period of
P
= 17.12 days. This star is reported to be as dim as a Cepheid of about half its pulsation period and it exhibits a low ...radial velocity and light-curve amplitude. For these reasons, Y Oph is not used to calibrate period-luminosity (PL) relation and its distance remains uncertain.
Aims.
Our objective is to conduct hydrodynamical pulsation modeling of Y Oph to derive its distance and provide a physical insight into its low amplitude and luminosity, constrained by an extensive set of observations.
Methods.
We first performed a linear analysis on a grid of models using the hydrodynamical pulsation code
MESA-RSP
to find the combinations of mass, metallicity, effective temperature, and luminosity resulting in linear excitation of pulsations with period of about 17 days. Then, we performed non-linear computations to obtain the full-amplitude pulsations of these models. Last, we compare the results to a complete set of observations along the pulsation cycle, including the angular diameter obtained by interferometry, effective temperature, and radial velocity obtained by high-resolution spectroscopy, as well as the light curves in the
VJHK
S
LM
bands. We simultaneously adjusted the distance, the color excess and circumstellar envelope (CSE) model to fit the light curves and the angular diameter.
Results.
We find that all pulsation models at high effective temperatures are in remarkable agreement with the observations along the pulsation cycle. This result suggests that the low amplitude of Y Oph may be explained by proximal location to the blue edge of the instability strip (IS). We also find that a pulsational mass of about 7 − 8
M
⊙
is consistent with a non-canonical evolutionary model with moderate overshooting, PL relation and
Gaia
parallax. However, a much lower mass below 5
M
⊙
is required to match Baade-Wesselink (BW) distance measurements from the literature. We show that the combination of the impact of the CSE on the photometry, together with a projection factor of about 1.5, explains the discrepant distance and luminosity values obtained from BW methods.
Conclusions.
Our findings indicate that the small pulsation amplitude of Y Oph can be attributed to its proximity to the blue edge of the instability strip. Additionally, our analysis reveals that the distances obtained using the BW method are biased compared to
Gaia
, mainly due to the impact of circumstellar envelope on the photometries and a high
p
-factor close to 1.5. Despite these unique characteristics, Y Oph is a long-period classical Cepheid that holds potential for calibration of the PL relation in the Galaxy.
Context.
Estimating the metallicity of classical Cepheids is of prime importance for studying metallicity effects on stellar evolution and the chemical evolution of galaxies, as well as on the ...period–luminosity relation used on the extragalactic distance scale.
Aims.
Our first aim is to establish new empirical relations for estimating the iron content of classical Cepheids for short and long periods based on Fourier parameters from the
V
- and
I
-band light curves. We go on to apply these relations to Cepheids from data on the Milky Way (MW) as well as the Small and Large Magellanic Clouds (SMC and LMC) from the literature.
Methods.
We retrieved the metallicities of 586 fundamental-mode Cepheids from spectroscopic determinations in the literature and we found well-sampled light curves for 545 of them in different
V
-band catalogs. We then described the shape of these light curves by applying a Fourier decomposition and we fit the empirical relations between the Fourier parameters and the spectroscopic metallicities individually, for short-period (2.5 <
P
< 6.3 days) and long-period Cepheids (12 <
P
< 40 days). We verified the accuracy of these relations by applying them to
V
-band light curves of Cepheids from the Small and Large Magellanic Clouds and comparing these derived metallicities to literature values. We calibrated new interrelations of Fourier parameters to convert these empirical relations into the
I
band. We then used these
I
-band relations to derive the metallicity of fundamental-mode Cepheids from OGLE-IV for MW, SMC, and LMC (486, 695, and 1697 stars, respectively). Finally, we mapped the metallicity distribution in these galaxies for the purpose of investigating potential applications in galactic archeology.
Results.
For short-period Cepheids, our best fit is given for a relation based on explicit amplitude terms
A
1
and
A
2
of the first and second harmonic, respectively. In the
V
and
I
bands, these empirical relations are found with an intrinsic scatter (rms) of 0.12 dex. This relation performs well for estimations of Fe/H between about −0.5 and 0.1 dex, but it remains uncertain outside this range because of the lack of a spectroscopic metallicity required for the calibration. For long-period Cepheids, we found a metallicity dependence on the Fourier parameters
A
1
,
ϕ
21
, and
R
41
. We found an intrinsic scatter of 0.25 dex when using this relation. The empirical relations in the
V
and
I
bands allow us to derive the mean metallicity of a sample of MW, SMC, and LMC Cepheids that is in agreement with literature values within 1
σ
. We also show that these relations are precise enough to reconstruct the radial metallicity gradients within the MW from OGLE data.
Conclusions.
The empirical relations in the
V
and
I
bands that are calibrated in this work for short- and long-period Cepheids provide a useful new tool for estimating the metallicity of Cepheids that are not accessible via spectroscopy. The calibration can be improved with further high-resolution spectroscopic observations of metal-poor Cepheids and homogeneous photometries in the
V
and
I
bands.
Context.
The variety of physical processes at play in chemically peculiar stars makes it difficult to determine their fundamental parameters. In particular, for the magnetic ones, called Ap stars, ...the strong magnetic fields and the induced spotted stellar surfaces may lead to biased effective temperatures when these values are derived through spectro-photometry.
Aims.
We propose to benefit from the exquisite angular resolution provided by long-baseline interferometry in the visible to determine the accurate angular diameters of a number of Ap stars, and thus estimate their radii by a method that is as independent as possible of atmospheric models.
Methods.
We used the visible spectrograph VEGA at the CHARA interferometric array to complete the sample of Ap stars currently observable with this technique. We estimated the angular diameter and radius of six new targets. We estimated their bolometric flux based solely on observational spectroscopic and photometric data to derive nearly model-independent luminosities and effective temperatures.
Results.
We extend to 14 the number of Ap stars for which interferometric angular diameters have been measured. The fundamental parameters we derived for the complete Ap sample are compared with those obtained through a self-consistent spectroscopic analysis. Based on a model fitting approach of high-resolution spectra and spectro-photometric observations over a wide wavelength range, this method takes into account the anomalous chemical composition of the atmospheres and the inhomogeneous vertical distribution for different chemical elements. Regarding both the radii and the effective temperatures, the derived values from our interferometric observations and from self-consistent modelling are consistent within better than 2
σ
for nine targets out of ten. We thus benchmark nine Ap stars for effective temperatures ranging from 7200 and 9100 K, and luminosities ranging between 7
L
⊙
and 86
L
⊙
.
Conclusions.
These results will be key for the future derivation of accurate radii and other fundamental parameters of fainter peculiar stars for which both the sensitivity and the angular resolution of the current interferometers are not sufficient. Within the context of the observations of Ap stars with the Transiting Exoplanet Survey Satellite (TESS), these interferometric measurements are crucial for testing the mechanism of pulsation excitation at work in these peculiar stars. In particular, our interferometric measurements provide accurate locations in the Hertzsprung-Russell diagram for hot Ap stars among which pulsations may be searched for with TESS, putting to test the blue edge of the theoretical instability strip. These accurate locations could be used to derive masses and ages of these stars through a specific grid of models, and to test correlations between the properties of these peculiar stars and their evolutionary state.
Aims.
We aim to investigate the infrared excess of 45 Milky Way (MW) Cepheids combining different observables in order to constrain the presence of circumstellar envelopes (CSEs).
Methods.
We used ...the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS) algorithm, a robust implementation of the parallax-of-pulsation method that combines photometry, angular diameter, stellar effective temperature, and radial velocity measurements in a global modelling of the pulsation of the Cepheid. We obtained new photometric measurements at mid-infrared (mid-IR) with the VISIR instrument at the Very Large Telescope complemented with data gathered from the literature. We then compared the mean magnitude of the Cepheids from 0.5 μm to 70 μm with stellar atmosphere models to infer the IR excess, which we attribute to the presence of a circumstellar envelope.
Results.
We report that at least 29% of the Cepheids of our sample have a detected IR excess (> 3
σ
). We estimated a mean excess of 0.08 ± 0.04 mag at 2.2 μm and 0.13 ± 0.06 mag at 10 μm. Other Cepheids possibly also have IR excess, but they were rejected due to their low detection level compared to a single-star model. We do not see any correlation between the IR excess and the pulsation period as previously suspected for MW Cepheids, but a rather constant trend at a given wavelength. We also do not find any correlation between the CO absorption and the presence of a CSE, but rather with the stellar effective temperature, which confirms that the CO features previously reported are mostly photospheric. No bias caused by the presence of the circumstellar material is detected on the average distance estimates from a SPIPS analysis with a fitted colour excess. We also do not find correlation between the presence of IR excess and the evolution stage of the Cepheids.
Conclusions.
We report a fraction of 29% of Cepheids with an IR excess likely produced by the circumstellar envelope surrounding the stars. Longer period Cepheids do not exhibit greater excess than short periods as previously suspected from observations and theoretical dusty-wind models. Other mechanisms such as free-free emission, among others, may be at the origin of the formation of the CSEs. We also show that not fitting the colour excess leads to a bias on the distance estimates in our Galaxy.
Context.
The infrared (IR) excess of classical Cepheids is seldom studied and poorly understood despite observational evidence and the potential for its contribution to induce systematics on the ...period-luminosity (PL) relation used in the calibration of the extragalactic distance scale.
Aims.
This study aims to understand the physical origin of the IR excess found in the spectral energy distribution (SED) of 5 Cepheids: RS Pup (
P
= 41.46d),
ζ
Gem (
P
= 10.15d),
η
Aql (
P
= 7.18d), V Cen (
P
= 5.49d) and SU Cyg (
P
= 3.85d).
Methods.
A time series of atmospheric models along the pulsation cycle were fitted to a compilation of data, including optical and near-IR photometry,
Spitzer
spectra (secured at a specific phase), interferometric angular diameters, effective temperature estimates, and radial velocity measurements.
Herschel
images in two bands were also analyzed qualitatively. In this fitting process, based on the SPIPS algorithm, a residual was found in the SED, whatever the pulsation phase, and for wavelengths larger than about 1.2
μ
m, which corresponds to the so-determined infrared excess of Cepheids. This IR excess was then corrected from interstellar medium absorption in order to infer the presence (or absence) of dust shells and was, ultimately, used in order to fit a model for a shell of ionized gas.
Results.
For all Cepheids, we find a continuum IR excess increasing up to approximately −0.1 magnitudes at 30
μ
m, which cannot be explained by a hot or cold dust model of CircumStellar Environment (CSE). However, a weak but significant dust emission at 9.7
μ
m is found for
ζ
Gem,
η
Aql and RS Pup, while clear interstellar clouds are seen in the
Herschel
images for V Cen and RS Pup. We show, for the first time, that the IR excess of Cepheids can be explained by free–free emission from a thin shell of ionized gas, with a thickness of ≃15% of the star radius, a mass of 10
−9
−10
−7
M
⊙
and a temperature ranging between 3500 and 4500 K.
Conclusions.
The presence of a thin shell of ionized gas around Cepheids must be tested with interferometers operating in the visible or mid-IR, or using radio telescopes. The impact of such CSEs of ionized gas on the PL relation of Cepheids also calls for further investigation.
Context. Accurate radial velocities (vrad) of Cepheids are mandatory within the context of Cepheid distance measurements using the Baade-Wesselink technique. The most common vrad derivation method ...consists in cross-correlating the observed stellar spectra with a binary template and measuring a velocity on the resulting mean profile. Nevertheless, for Cepheids and other pulsating stars, the spectral lines selected within the template as well as the way of fitting the cross-correlation function (CCF) have a direct and significant impact on the measured vrad. Aims. Our first aim is to detail the steps to compute consistent CCFs and vrad of Cepheids. Next, this study aims at characterising the impact of Cepheid spectral properties and vrad computation methods on the resulting line profiles and vrad time series. Methods. We collected more than 3900 high-resolution spectra from seven different spectrographs of 64 Classical Milky Way (MW) Cepheids. These spectra were normalised and standardised using a single custom-made process on pre-defined wavelength ranges. We built six tailored correlation templates selecting unblended spectral lines of different depths based on a synthetic Cepheid spectrum, on three different wavelength ranges from 3900 to 8000 Å. Each observed spectrum was cross-correlated with these templates to build the corresponding CCFs, adopted as the proxy for the spectrum mean line profile. We derived a set of line profile observables as well as three different vrad measurements from each CCF and two custom proxies for the CCF quality and amount of signal. Results. This study presents a large catalogue of consistent Cepheid CCFs and vrad time series. It confirms that each step of the process has a significant impact on the deduced vrad: the wavelength, the template line depth and width, and the vrad computation method. The way towards more robust Cepheid vrad time series seems to go through steps that minimise the asymmetry of the line profile and its impact on the vrad. Centroid or first-moment vrad, that exhibit slightly smaller amplitudes but significantly smaller scatter than Gaussian or biGaussian vrad, should therefore be favoured. Stronger or deeper spectral lines also tend to be less asymmetric and lead to more robust vrad than weaker lines.
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