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.
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. 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.
Aims.
The surface brightness–colour relation (SBCR) is a basic tool for establishing precise and accurate distances within the Local Group. Detached eclipsing binary stars with accurately determined ...radii and trigonometric parallaxes allow calibration of the SBCRs with unprecedented accuracy.
Methods.
We analysed four nearby eclipsing binary stars containing late F-type main sequence components: AL Ari, AL Dor, FM Leo, and BN Scl. We determined very precise spectroscopic orbits and combined them with high-precision ground- and space-based photometry. We derived the astrophysical parameters of their components with mean errors of 0.1% for mass and 0.4% for radius. We combined those four systems with another 24 nearby eclipsing binaries with accurately known radii from the literature for which
Gaia
EDR3 parallaxes are available in order to derive the SBCRs.
Results.
The resulting SBCRs cover stellar spectral types from B9 V to G7 V. For calibrations, we used Johnson optical
B
and
V
,
Gaia
G
BP
and
G
, and 2MASS
J
H
K
bands. The most precise relations are calibrated using the infrared
K
band and allow angular diameters of A-, F-, and G-type dwarf and subgiant stars to be predicted with a precision of 1%.
ABSTRACT
The technique of line depth ratios (LDRs) is one of the methods to determine the effective temperature of a star. They are crucial in the spectroscopic studies of variable stars like ...Cepheids since no simultaneous photometry is usually available. A good number of LDR-temperature relations are already available in the optical domain; here we want to expand the number of relations available in the near-infrared (NIR) in order to fully exploit the capabilities of current and upcoming NIR spectrographs. We used 115 simultaneous spectroscopic observations in the optical and the NIR for six Cepheids and optical LDRs to find new pairs of lines sensitive to temperature and to calibrate LDR-temperature relations in the NIR spectral range. We have derived 87 temperature calibrations valid in the 4800–6500 K range of temperatures. The typical uncertainty for a given relation is 60–70 K, and combining many of them provides a final precision within 30–50 K. We found a discrepancy between temperatures derived from optical or NIR LDR for pulsations phases close to ϕ ≈0.0 and we discuss the possible causes for these differences. LDRs in the NIR will allow us to spectroscopically investigate highly reddened Cepheids in the Galactic centre or in the far side of the disc.
Context.
Surface brightness–colour relations (SBCRs) are used to derive the stellar angular diameters from photometric observations. They have various astrophysical applications, such as the distance ...determination of eclipsing binaries or the determination of exoplanet parameters. However, strong discrepancies between the SBCRs still exist in the literature, in particular for early and late-type stars.
Aims.
We aim to calibrate new SBCRs as a function of the spectral type and the luminosity class of the stars. Our goal is also to apply homogeneous criteria to the selection of the reference stars and in view of compiling an exhaustive and up-to-date list of interferometric late-type targets.
Methods.
We implemented criteria to select measurements in the JMMC Measured Diameters Catalog. We then applied additional criteria on the photometric measurements used to build the SBCRs, together with stellar characteristics diagnostics.
Results.
We built SBCRs for F5/K7–II/III, F5/K7–IV/V, M–II/III and M–V stars, with respective rms of
σ
F
V
= 0.0022 mag,
σ
F
V
= 0.0044 mag,
σ
F
V
= 0.0046 mag, and
σ
F
V
= 0.0038 mag. This results in a precision on the angular diameter of 1.0%, 2.0%, 2.1%, and 1.7%, respectively. These relations cover a large
V
−
K
colour range of magnitude, from 1 to 7.5. Our work demonstrates that SBCRs are significantly dependent on the spectral type and the luminosity class of the star. Through a new set of interferometric measurements, we demonstrate the critical importance of the selection criteria proposed for the calibration of SBCR. Finally, using the
Gaia
photometry for our samples, we obtained (
G
−
K
) SBCRs with a precision on the angular diameter between 1.1% and 2.4%.
Conclusions.
By adopting a refined and homogeneous methodology, we show that the spectral type and the class of the star should be considered when applying an SBCR. This is particularly important in the context of PLATO.
Context. The projection factor p is the key quantity used in the Baade-Wesselink (BW) method for distance determination; it converts radial velocities into pulsation velocities. Several methods are ...used to determine p, such as geometrical and hydrodynamical models or the inverse BW approach when the distance is known. Aims. We analyze new HARPS-N spectra of δ Cep to measure its cycle-averaged atmospheric velocity gradient in order to better constrain the projection factor. Methods. We first apply the inverse BW method to derive p directly from observations. The projection factor can be divided into three subconcepts: (1) a geometrical effect (p0); (2) the velocity gradient within the atmosphere (fgrad); and (3) the relative motion of the optical pulsating photosphere with respect to the corresponding mass elements (fo−g). We then measure the fgrad value of δ Cep for the first time. Results. When the HARPS-N mean cross-correlated line-profiles are fitted with a Gaussian profile, the projection factor is pcc−g = 1.239 ± 0.034(stat.) ± 0.023(syst.). When we consider the different amplitudes of the radial velocity curves that are associated with 17 selected spectral lines, we measure projection factors ranging from 1.273 to 1.329. We find a relation between fgrad and the line depth measured when the Cepheid is at minimum radius. This relation is consistent with that obtained from our best hydrodynamical model of δ Cep and with our projection factor decomposition. Using the observational values of p and fgrad found for the 17 spectral lines, we derive a semi-theoretical value of fo−g. We alternatively obtain fo−g = 0.975 ± 0.002 or 1.006 ± 0.002 assuming models using radiative transfer in plane-parallel or spherically symmetric geometries, respectively. Conclusions. The new HARPS-N observations of δ Cep are consistent with our decomposition of the projection factor. The next step will be to measure p0 directly from the next generation of visible interferometers. With these values in hand, it will be possible to derive fo−g directly from observations.
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.
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.
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