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Mis à jour : 5 mai 2021

Understanding the origin of the circumstellar disks of Be stars is one of the current challenges in astrophysics. Their circumstellar disks are well understood as rotating in a nearly Keplerian fashion and passing by different phases of dissipation and rebuilt due to mass injection from the central star. From using several modeling tools to interpret spectro-interferometric observations with the CHARA/VEGA and VLTI/AMBER instruments, this study draw the most detailed picture of the Be star o Aquarii in both the visible (Hα line) and near-infrared (Brγ line) regions. Interestingly, the extension of o Aquarii’s disk is similar in both Hα and Brγ (see Fig. 10 of de Almeida et al. 2020), which is uncommon since most results for Be stars indicate a larger disk extension in Hα than in Brγ. Furthermore, the disk kinematics were found to be near to the Keplerian rotation in Brγ, but not in Hα. Finally, our results indicate that the disk of o Aquarii is stable over several years and this  can be understood in terms of the high rotational rate that we measured for this star. To further investigate these issues, this study paved the way to future studies on large samples of Be stars observed with VEGA and AMBER, and the newly available VLTI mid-infrared combiner MATISSE and with the near-future CHARA/SPICA visible combiner. 

This study was done by Elisson Saldanha da Gama de Almeida. Link to the paper: https://ui.adsabs.harvard.edu/abs/2020A%26A...636A.110D/abstract

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Mis à jour : 1 décembre 2020

Text pour la Figure: the line-of-sight velocity maps (for an observer located in front of the figure) from radiative hydrodynamics simulations of stellar convection in Red Supergiant stars. Red color corresponds to falling material, and blue color to rising material. The central panel shows the velocity of ascending material as a function of the effective temperature of the star.

Red supergiants are cool massive stars and are the largest and the most luminous stars in the universe. They are characterized by irregular or semi-regular photometric variations, the physics of which is not clearly understood. In this paper, we derived the velocity field in the red supergiant star μ Cep and related it to the photometric variability with the help of the tomographic method. The tomographic method allows to recover the line-of-sight velocity distribution over the stellar disk and within different optical-depth slices. The method is applied to a series of high-resolution spectra of μ Cep, and these results are compared to those obtained from 3D radiative-hydrodynamics CO5BOLD simulations of red supergiants. Fluctuations in the velocity field are compared with photometric and spectroscopic variations, the latter being derived from the TiO band strength and serving (at least partly) a proxy of the variations in effective temperature. We reveals a phase shift between the velocity and spectroscopic/photometric variations. This phase shift results in a hysteresis loop in the temperature – velocity plane (Figure), with a timescale of a few hundred days, similar to the photometric one. The similarity between the hysteresis loop timescale measured in μ Cep and the timescale of acoustic waves disturbing the convective pattern suggests that such waves play an important role in triggering the hysteresis loops.

Work done by Kateryna Kravchenko. Link to the paper: https://ui.adsabs.harvard.edu/abs/2019A%26A...632A..28K/abstract

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Mis à jour : 1 décembre 2020