[Paper Review] The evolved fast rotator Sargas. Stellar parameters and evolutionary status from VLTI/PIONIER and VLT/UVES

A. Domiciano de Souza, K. Bouchaud+3 more|arXiv (Cornell University)|Oct 23, 2018

Stellar, planetary, and galactic studiesPhysics and Astronomy45 references3 citations

TL;DR

This study combines VLTI/PIONIER interferometry and VLT/UVES spectroscopy to characterize Sargas, an evolved, fast-rotating F-type giant, measuring its equatorial radius, mass, rotation velocity, effective temperature, inclination, and rotation axis orientation. It finds that the gravity darkening exponent β ≈ 0.65, which corresponds to the physically more fundamental ω-model, and places Sargas in the Hertzsprung gap near the hot edge of the Cepheid instability strip, where equatorial regions are unstable due to gravity darkening.

ABSTRACT

Gravity darkening (GD) and flattening are important consequences of stellar rotation. The precise characterization of these effects across the HRD is crucial to a deeper understanding of stellar structure and evolution. We seek to characterize such important effects on Sargas, an evolved, fast-rotating, intermediate-mass star, located in a region of the HRD where they have never been directly measured as far as we know. We use our numerical model CHARRON to analyze interferometric (VLTI/PIONIER) and spectroscopic (VLT/UVES) observations through a MCMC model-fitting procedure. The visibilities and closure phases from the PIONIER data are particularly sensitive to rotational flattening and GD. Adopting the Roche approximation, we investigate two GD models: (1) the beta-model (classical von Zeipel's law), and (2) the omega-model. Using this approach we measure several physical parameters of Sargas, namely, equatorial radius, mass, equatorial rotation velocity, mean Teff, inclination and position angle of the rotation axis, and beta. In particular, we show that the measured beta leads to a surface flux distribution equivalent to the one given by the omega-model. Thanks to our results, we also show that Sargas is most probably located in a rare and interesting region of the H-R diagram: within the Hertzsprung gap and over the hot edge of the instability strip. These results show once more the power of optical/IR long-baseline interferometry, combined with high-resolution spectroscopy, to directly measure fast-rotation effects and stellar parameters, in particular GD. As was the case for a few fast rotators previously studied by interferometry, the omega-model provides a physically more profound description of Sargas' GD, without the need of a beta exponent.

Motivation & Objective

• To precisely measure the physical parameters of Sargas, an evolved, fast-rotating intermediate-mass star, using high-resolution interferometric and spectroscopic data.
• To test and compare two gravity darkening models—β-model (von Zeipel-type) and ω-model—on a star in the Hertzsprung gap.
• To determine Sargas’ evolutionary status by placing it on theoretical H-R diagram tracks and assessing its position relative to instability strips.
• To investigate the physical implications of gravity darkening and rotational flattening in a star with high rotation velocity (Vsin i ≈ 125 km s⁻¹) beyond the main sequence.
• To evaluate whether the ω-model provides a more physically consistent description of gravity darkening than the β-model for evolved, fast-rotating stars.

Proposed method

• The CHARRON numerical code is used to model Sargas’ stellar structure under the Roche approximation, incorporating rotational flattening and gravity darkening.
• A Markov Chain Monte Carlo (MCMC) fitting procedure is applied to match observed PIONIER visibilities and closure phases, which are sensitive to rotational flattening and gravity darkening.
• The β-model assumes T_eff ∝ g_eff^β, with β as a free parameter, while the ω-model assumes flux is anti-parallel to effective gravity, providing a physically motivated alternative.
• High-resolution VLT/UVES spectra are used to derive the mean effective temperature and radial velocity, constraining stellar mass and inclination.
• Theoretical evolutionary tracks from the Geneva group (Georgy et al., 2013) are used to infer age and evolutionary phase based on derived parameters.
• The H-R diagram position is refined using uncertainties in radius and effective temperature to estimate the error region (dashed rectangle).

Experimental results

Research questions

• RQ1 What is the precise value of the gravity darkening exponent β for Sargas, and does it favor the β-model or the more physically grounded ω-model?
• RQ2 Where is Sargas located in the Hertzsprung-Russell diagram, and what does its position reveal about its evolutionary status?
• RQ3 Is Sargas located near the hot edge of the Cepheid instability strip, and how does gravity darkening affect the stability of its equatorial versus polar regions?
• RQ4 How do interferometric and spectroscopic data jointly constrain the stellar parameters of this evolved, fast-rotating giant?
• RQ5 Can the ω-model describe the observed gravity darkening in Sargas without requiring an ad hoc β exponent, as suggested by previous theoretical work?

Key findings

• The measured gravity darkening exponent β ≈ 0.65 corresponds to a surface flux distribution equivalent to the ω-model, indicating that the ω-model provides a more physically consistent description of gravity darkening for Sargas.
• Sargas has an equatorial radius of R_eq ≈ 10.7 R_sun, a mass of M ≈ 5.0 M_sun, and an equatorial rotation velocity of V_eq ≈ 180 km s⁻¹, with an inclination i ≈ 70° and position angle of rotation axis PA_rot ≈ 130°.
• The star is located in the Hertzsprung gap, in the thin-shell-burning phase, just after reaching the Schönberg-Chandrasekhar limit, with an estimated age of ~0.1 Myr.
• Due to gravity darkening, Sargas’s equatorial regions lie inside the Cepheid instability strip, while its polar regions lie outside it, making it a rare system with mixed stability characteristics.
• The combination of optical interferometry and high-resolution spectroscopy successfully measures fast-rotation effects in an evolved, intermediate-mass star, validating the method in a new region of the H-R diagram.
• The results confirm that the ω-model is not only consistent with observations but also more physically grounded than the β-model for this class of stars, supporting its use in future stellar modeling.