Abstract
Context. Canopus, the brightest and closest yellow supergiant to our Solar System, offers a unique laboratory for understanding the physics of evolved massive stars. Aims. We aim at quantitatively exploring a large space of fundamental parameters of Canopus based on the combined analysis of its spectral energy distribution (SED) and optical-IR long baseline interferometry. Methods. We use the most recent high resolution near-IR data from the VLTI focal beam combiners PIONIER (H and K bands) and AMBER (K band), together with precise spectrophotometric measures that cover the SED of Canopus, from the UV to the IR, taken from ground and space observatories. Results. The accurate and precise PIONIER data allowed us to simultaneously measure the angular diameter and the limb darkening (LD) profile using different analytical laws. We found that the power-law LD, being also in agreement with predictions from stellar atmosphere models, reproduces the interferometric data well. For this model we measured an angular diameter of 7.184 ± 0.0017 ± 0.029 mas and an LD coefficient of 0.1438 ± 0.0015, which are respectively ≳5 and ~15−25 more precise than in our previous A&A paper on Canopus from 2008. From a dedicated analysis of the interferometric data, we also provide new constraints on the putative presence of weak surface inhomogeneities. Additionally, we analyzed the SED in a innovative way by simultaneously fitting the reddening-related parameters and the stellar effective temperature and gravity. We find that a model based on two effective temperatures is much better at reproducing the whole SED, from which we derived several parameters, including a new bolometric flux estimate: fbol = (59.22 ± 2.45) × 10−6 erg cm−2 s−1. We were also able to estimate the stellar mass from these measurements, and it is shown to be in agreement with additional predictions from evolutionary models, from which we inferred the age of Canopus as well. Conclusions. The Canopus angular diameter and LD measured in this work with PIONIER are the most precise to date, with a direct impact on several related fundamental parameters. Moreover, thanks to our joint analysis, we were able to determine a set of fundamental parameters that simultaneously reproduces both high-precision interferometric data and a good quality SED and, at the same time, agrees with stellar evolution models. This refined set of fundamental parameters constitutes a careful balance between the different methodologies used, providing invaluable observationally based constraints to models of stellar structure and evolution, which still present difficulties in simulating stars such as Canopus in detail.
Highlights
Canopus (α Car, HD 45348) is the brightest star of the southern night sky, and, as such, it has been extensively studied and observed
Limb-darkening models We investigated Canopus with three radial (1D) intensity profile models, which correspond to different limb darkening (LD) laws with zero or one parameter: (i) the classical uniform disk (UD), (ii) the linear LD, which is commonly used, and (iii) the power-law LD, which has been shown to be a realistic model in many cases (e.g., Hestroffer 1997; Kervella et al 2017)
These results exclude the possibility of Canopus being in the H shell burning stage (Hertzsprung gap) because the inferred mass would be larger by 2 M and log g = 1.80 dex, which conflicts with the “Rosseland” parameters in Table 4 obtained from the fit of the spectral energy distribution (SED)
Summary
Canopus (α Car, HD 45348) is the brightest star of the southern night sky, and, as such, it has been extensively studied and observed. In a previous interferometric study of Canopus (Domiciano de Souza et al 2008), we used observations from the near-IR beam combiner AMBER (Petrov et al 2007) of the ESO Very Large Telescope Interferometer (VLTI; Haguenauer et al 2010) to estimate its angular diameter and LD This first work showed the presence of photospheric structures, but no strong constraints could be imposed on these structures because of the limited quantity and quality (S/N and absolute visibility calibration) of the available data. The starting values for Teff and log g are averages of several independent determinations found in the literature (Przybylski & Burnicki 1974; Kovtyukh 2007), corresponding to a spectral MK type A9II-F0Iab (e.g., SIMBAD-CDS, Ayres 2018), which place Canopus in the HR diagram midway between the blue and red supergiants This led us to use the PHOENIX library of stellar local thermodynamic equilibrium (LTE) synthetic models for extended atmospheres with solar metallicity (Husser et al 2013). A more complex model seems necessary for reproducing the entire SED of Canopus
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