In the era of precision cosmology, it is essential to determine the Hubble constant empirically with an accuracy of one per cent or better
. At present, the uncertainty on this constant is dominated ...by the uncertainty in the calibration of the Cepheid period-luminosity relationship
(also known as the Leavitt law). The Large Magellanic Cloud has traditionally served as the best galaxy with which to calibrate Cepheid period-luminosity relations, and as a result has become the best anchor point for the cosmic distance scale
. Eclipsing binary systems composed of late-type stars offer the most precise and accurate way to measure the distance to the Large Magellanic Cloud. Currently the limit of the precision attainable with this technique is about two per cent, and is set by the precision of the existing calibrations of the surface brightness-colour relation
. Here we report a calibration of the surface brightness-colour relation with a precision of 0.8 per cent. We use this calibration to determine a geometrical distance to the Large Magellanic Cloud that is precise to 1 per cent based on 20 eclipsing binary systems. The final distance is 49.59 ± 0.09 (statistical) ± 0.54 (systematic) kiloparsecs.
Abstract We present the results from a complex study of an eclipsing O-type binary (Aa+Ab) with the orbital period of P A = 3.2254367 days that forms part of a higher-order multiple system in a ...configuration of (A+B)+C. We derived masses of the Aa+Ab binary of M 1 = 19.02 ± 0.12 and M 2 = 17.50 ± 0.13 M ⊙ , the radii of R 1 = 7.70 ± 0.05 and R 2 = 6.64 ± 0.06 R ⊙ , and temperatures of T 1 = 34,250 ± 500 K and T 2 = 33,750 ± 500 K. From the analysis of the radial velocities, we found a spectroscopic orbit of A in the outer A+B system with P A+B = 195.8 days ( P A+B / P A ≈ 61). In the O − C analysis, we confirmed this orbit and found another component orbiting the A+B system with P AB+C = 2550 days ( P AB+C / P A+B ≈ 13). From the total mass of the inner binary and its outer orbit, we estimated the mass of the third object, M B ≳ 10.7 M ⊙ . From the light travel time effect fit to the O − C data, we obtained the limit for the mass of the fourth component, M C ≳ 7.3 M ⊙ . These extra components contribute about 20%–30% (increasing with wavelength) to the total system light. From the comparison of model spectra with the multiband photometry, we derived a distance modulus of 18.59 ± 0.06 mag, a reddening of 0.16 ± 0.02 mag, and an R V of 3.2. This work is part of our ongoing project, which aims to calibrate the surface brightness–color relation for early-type stars.
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.
The instability strip (IS) of classical Cepheids has been extensively studied theoretically. Comparison of the theoretical IS edges with those obtained empirically, using the most recent Cepheids ...catalogs available, can provide us with insights into the physical processes that determine the position of the IS boundaries. We investigate the empirical positions of the IS of the classical Cepheids in the Large Magellanic Cloud (LMC) using data of classical fundamental-mode and first-overtone LMC Cepheids from the OGLE-IV variable star catalog, together with a recent high-resolution reddening map from the literature. We studied their position on the Hertzsprung-Russell diagram and determined the IS borders by tracing the edges of the color distribution along the strip. We obtain the blue and red edges of the IS in V- and I-photometric bands, in addition to Teff and log L⊙. The results obtained show a break located at the Cepheids’ period of about 3 days, which was not reported before. This phenomenon is most likely explained by the depopulation of second and third crossing classical Cepheids in the faint part of the IS, since blue loops of evolutionary tracks in this mass range do not extend blueward enough to cross the IS at the LMC metallicity. Furthermore, our empirical borders show good agreement with theoretical ones published in the literature. This proves that our empirical IS is a useful tool to put constraints on theoretical models.
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.
Stellar pulsation theory provides a means of determining the masses of pulsating classical Cepheid supergiants-it is the pulsation that causes their luminosity to vary. Such pulsational masses are ...found to be smaller than the masses derived from stellar evolution theory: this is the Cepheid mass discrepancy problem, for which a solution is missing. An independent, accurate dynamical mass determination for a classical Cepheid variable star (as opposed to type-II Cepheids, low-mass stars with a very different evolutionary history) in a binary system is needed in order to determine which is correct. The accuracy of previous efforts to establish a dynamical Cepheid mass from Galactic single-lined non-eclipsing binaries was typically about 15-30% (refs 6, 7), which is not good enough to resolve the mass discrepancy problem. In spite of many observational efforts, no firm detection of a classical Cepheid in an eclipsing double-lined binary has hitherto been reported. Here we report the discovery of a classical Cepheid in a well detached, double-lined eclipsing binary in the Large Magellanic Cloud. We determine the mass to a precision of 1% and show that it agrees with its pulsation mass, providing strong evidence that pulsation theory correctly and precisely predicts the masses of classical Cepheids.
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.
Aims.
In this work we study 35 stellar clusters in the Small Magellanic Cloud (SMC) in order to provide their mean metallicities and ages. We also provide mean metallicities of the fields surrounding ...the clusters.
Methods.
We used Strömgren photometry obtained with the 4.1 m SOAR telescope and take advantage of (
b
−
y
) and
m
1 colors for which there is a metallicity calibration presented in the literature.
Results.
The spatial metallicity and age distributions of clusters across the SMC are investigated using the results obtained by Strömgren photometry. We confirm earlier observations that younger, more metal-rich star clusters are concentrated in the central regions of the galaxy, while older, more metal-poor clusters are located farther from the SMC center. We construct the age–metallicity relation for the studied clusters and find good agreement with theoretical models of chemical enrichment, and with other literature age and metallicity values for those clusters. We also provide the mean metallicities for old and young populations of the field stars surrounding the clusters, and find the latter to be in good agreement with recent studies of the SMC Cepheid population. Finally, the Strömgren photometry obtained for this study is made publicly available.
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.
PDF
