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 alpha 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 theta sub(LD) and the limb darkening parameters of the two solar-type stars in the near-infrared H band (lambda= 1.65 mu m). We obtain photospheric angular diameters of theta sub(LD)(A) = 8.502 + or - 0.038mas (0.43%) and theta sub(LD)(B) = 5.999 + or - 0.025mas (0.42%), through the adjustment of a power law limb darkening model. We find H band power law exponents of alpha (A) = 0.1404 + or - 0.0050 (3.6%) and alpha (B) = 0.1545 + or - 0.0044 (2.8%), which closely bracket the observed solar value ( alpha sub(middot in circle)= 0.15027). Combined with the parallax pi= 747.17 + or - 0.61mas previously determined, we derive linear radii of R sub(A)= 1.2234 + or - 0.0053 R sub(middot in circle)(0.43%) and R sub(B)= 0.8632 + or - 0.0037 R sub(middot in circle)(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 the 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 alpha Cen A and B provide a robust basis to validate the future evolutions of these models.
The interaction of the light from astronomical objects with the constituents of the Earth's atmosphere leads to the formation of telluric absorption lines in ground-based collected spectra. We ...present molecfit, a tool to correct for telluric absorption lines based on synthetic modeling of the Earth's atmospheric transmission. Molecfit is versatile and can be used with data obtained with various ground-based telescopes and instruments. The atmospheric profiles are created by merging a standard atmospheric profile representative of a given observatory's climate, of local meteorological data, and of dynamically retrieved altitude profiles for temperature, pressure, and humidity. We discuss the various ingredients of the method, its applicability, and its limitations. Molecfit is able to accurately model and correct for telluric lines over a broad range of wavelengths and spectral resolutions. The accuracy reached is comparable to or better than the typical accuracy achieved using a telluric standard star observation. The availability of such a general tool for telluric absorption correction may improve future observational and analyzing strategies, as well as empower users of archival data.
In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the ...distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.
Context. While planet formation is thought to occur early in the history of a protoplanetary disk, the presence of planets embedded in disks, or of other processes driving disk evolution, might be ...traced from their imprints on the disk structure. Aims. We study the morphology of the disk around the T Tauri star HD 143006, located in the ~5–11 Myr-old Upper Sco region, and we look for signatures of the mechanisms driving its evolution. Methods. We observed HD 143006 in polarized scattered light with VLT/SPHERE at near-infrared (J-band, 1.2 μm) wavelengths, reaching an angular resolution of ~0.037′′ (~6 au). We obtained two datasets, one with a 145 mas diameter coronagraph, and the other without, enabling us to probe the disk structure down to an angular separation of ~0.06′′ (~10 au). Results. In our observations, the disk of HD 143006 is clearly resolved up to ~0.5′′ and shows a clear large-scale asymmetry with the eastern side brighter than the western side. We detect a number of additional features, including two gaps and a ring. The ring shows an overbrightness at a position angle (PA) of ~140°, extending over a range in position angle of ~60°, and two narrow dark regions. The two narrow dark lanes and the overall large-scale asymmetry are indicative of shadowing effects, likely due to a misaligned inner disk. We demonstrate the remarkable resemblance between the scattered light image of HD 143006 and a model prediction of a warped disk due to an inclined binary companion. The warped disk model, based on the hydrodynamic simulations combined with three-dimensional radiative transfer calculations, reproduces all major morphological features. However, it does not account for the observed overbrightness at PA ~ 140°. Conclusions. Shadows have been detected in several protoplanetary disks, suggesting that misalignment in disks is not uncommon. However, the origin of the misalignment is not clear. As-yet-undetected stellar or massive planetary companions could be responsible for them, and naturally account for the presence of depleted inner cavities.
Context. The extragalactic distance scale builds on the Cepheid period-luminosity (PL) relation. Decades of work have not yet convincingly established the sensitivity of the PL relation to ...metallicity. This currently prevents a determination of the Hubble constant accurate to 1% from the classical Cepheid-SN Ia method. Aims. In this paper we carry out a strictly differential comparison of the absolute PL relations obeyed by classical Cepheids in the Milky Way (MW), LMC, and SMC galaxies. Taking advantage of the substantial metallicity difference among the Cepheid populations in these three galaxies, we want to establish a possible systematic trend of the PL relation absolute zero point as a function of metallicity, and to determine the size of such an effect in the optical and near-infrared photometric bands. Methods. We used a IRSB Baade-Wesselink-type method to determine individual distances to the Cepheids in our samples in the MW, LMC, and SMC. For our analysis, we used a greatly enhanced sample of Cepheids in the SMC (31 stars) compared to the small sample (5 stars) available in our previous work. We used the distances to determine absolute Cepheid PL relations in the optical and near-infrared bands in each of the three galaxies. Results. Our distance analysis of 31 SMC Cepheids with periods of 4–69 days yields tight PL relations in all studied bands, with slopes consistent with the corresponding LMC and MW relations. Adopting the very accurately determined LMC slopes for the optical and near-infrared bands, we determine the zero point offsets between the corresponding absolute PL relations in the three galaxies. Conclusions. We find that in all bands the metal-poor SMC Cepheids are intrinsically fainter than their more metal-rich counterparts in the LMC and MW. In the K band the metallicity effect is −0.23 ± 0.06 mag dex−1, while in the V, (V − I) Wesenheit index it is slightly stronger, −0.34 ± 0.06 mag dex−1. We find suggestive evidence that the metallicity sensitivity of the PL relation might be nonlinear, being small in the range between solar and LMC Cepheid metallicity, and becoming steeper towards the lower-metallicity regime.
Long-baseline interferometry is an important technique to spatially resolve binary or multiple systems in close orbits. By combining several telescopes together and spectrally dispersing the light, ...it is possible to detect faint components around bright stars in a few hours of observations. We provide a rigorous and detailed method to search for high-contrast companions around stars, determine the detection level, and estimate the dynamic range from interferometric observations. We developed the code CANDID (Companion Analysis and Non-Detection in Interferometric Data), a set of Python tools that allows us to search systematically for point-source, high-contrast companions and estimate the detection limit using all interferometric observables, i.e., the squared visibilities, closure phases and bi-spectrum amplitudes. We used CANDID to search for the companions around the binary Cepheids V1334 Cyg, AX Cir, RT Aur, AW Per, SU Cas, and T Vul. We found that there is no companion with a spectral type earlier than B7V, A5V, F0V, B9V, A0V, and B9V orbiting the Cepheids V1334 Cyg, AX Cir, RT Aur, AW Per, SU Cas, and T Vul, respectively.
Context. The precise determinations of stellar mass at ≲1% provide important constraints on stellar evolution models. Accurate parallax measurements can also serve as independent benchmarks for the ...next Gaia data release. Aims. We measured the masses and distance of binary systems with a precision level better than 1% using a fully geometrical and empirical method. Methods. We obtained the first interferometric observations for the eclipsing systems AI Phe, AL Dor, KW Hya, NN Del, ψ Cen and V4090 Sgr with the VLTI/PIONIER combiner, which we combined with radial velocity measurements to derive their three-dimensional orbit, masses, and distance. Results. We determined very precise stellar masses for all systems, ranging in precision from 0.04% to 3.3%. We combined these measurements with the stellar effective temperature and linear radius to fit stellar isochrones models and determined the age of the systems. We also derived the distance to the systems with a precision level of 0.4%. Conclusions. The comparison of theoretical models with stellar parameters shows that stellar models are still deficient in simultaneously fitting the stellar parameters (Teff, R and M) with this level of precision on individual masses. This stresses the importance of precisely measuring the stellar parameters to better calibrate stellar evolution models. The precision of our model-independent orbital parallaxes varies from 24 μas as to 70 μas and the parallaxes provide a unique opportunity to verify whether the future Gaia measurements have systematic errors.
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.
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