Whats The Reason Behind Betelguese Mysterious Dimming? (Planetary Science)

Astronomers finally revealed the secret of betelguese dimming with the help of tomography

Red supergiant (RSG) stars represent the late stage in the evolution of stars with initial masses larger than 8 M before they end their lives in spectacular supernova explosions. Despite the fact that RSGs were extensively studied during the last decades, important properties such as photometric variability and mass loss are still poorly constrained. Understanding these properties is crucial for a broad range of astrophysical questions including the chemical enrichment of the Galaxy, supernova progenitors, and the extragalactic distance scale.

Betelgeuse is one of the closest (222 pc), brightest (0.0 – 1.6 mag in V-band), and best studied RSGs in our Galaxy. Between October 2019 and February 2020, Betelgeuse has experienced an unusual decrease in its visual brightness by about 1 mag (relative to the most recent maximum in September 2019), which is referred to as the Great Dimming event. Different hypotheses have been proposed to interpret the nature of this unprecedented fading. They include the formation of a dust cloud due to a recent mass-loss episode, thermal changes in the photosphere accompanying a temperature drop of ∼200 K or the presence of a dark spot covering ∼50% of the stellar surface, or a critical transition in Betelgeuse seen as a complex dynamical system undergoing a regime shift.

Now, K. Kravchenko and colleagues aim to relate the atmospheric dynamics of Betelgeuse to its photometric variability, with the main focus on the dimming event. They used the tomographic method which allowed them to probe different depths in the stellar atmosphere and to recover the corresponding disk-averaged velocity field. The method was applied to a series of high-resolution HERMES observations of Betelgeuse. Variations in the velocity field were then compared with photometric and spectroscopic variations.

Fig. 1. Top panel: AAVSO visual light curve (gray crosses). Black squares correspond to TiO-band temperatures derived from the HERMES spectra. Gray shaded areas in all panels define four photometric cycles, three of those being represented as hysteresis loops. Second panel from the top: Radial velocities (RVs) in all masks derived from fitting Betelgeuse cross-correlation functions (CCFs) with single- or multi-component Gaussian functions. RVs from the main CCF component are connected with lines. Line-doubling events are shown with a star symbol. The different colors correspond to the different masks. The same color coding is applied in the second to fifth panels. The zero point of the velocity scale corresponds to Betelgeuse center-of-mass velocity, represented by a horizontal dashed line. The uncertainty on HERMES individual RVs is on the order of 0.1 km s–¹. Third panel: M1 velocities of CCFs in masks C2–C5. Fourth panel: Atmospheric velocity gradients between masks C5 and C2 (orange), C5 and C3 (green), and C5 and C4 (blue). Bottom panel: The FWHM of CCFs in masks C2–C5. © Kravchenko et al.

The tomographic method revealed the succession of two shocks along their line-of-sight (in February 2018 and January 2019), the second one amplifying the effect of the first one, combined with underlying convection and/or outward motion present at this phase of the 400 d pulsation cycle, produced a rapid expansion of a portion of the atmosphere of Betelgeuse and an outflow between October 2019 and February 2020. This resulted in a sudden increase in molecular opacity in the cooler upper atmosphere of Betelgeuse and, thus, in the observed unusual decrease of the star’s brightness.

“We interpret the Great Dimming event as a consequence of this strong outflow which has caused a cooling of the outer layers, resulting in a sudden increase in molecular opacity. This phenomenon has been described in the literature as “molecular plumes” rising from the photosphere of supergiants, or “molecular reservoirs”, a non-spherical version of the “molsphere” observed around AGB stars”.

— told Kravchenko, lead author of the study

This scenario is supported by the VLT/SPHERE measurements, which revealed an obscuration in the southern half of Betelgeuse disk in the optical in January 2020. Whether or not this obscuration was caused by dust is a crucial point. Contrary to expectations, when new dust is produced, no significant flux increase has been recorded in the IR. Moreover this obscuration event has been recorded (20% dimming) in the submillimeter range; however, new dust is not expected to significantly change the spectrum at wavelengths larger than 100 µm. Therefore, they concluded that dust does not appear to be a crucial ingredient to account for the Great Dimming.

Featured image: An artist’s illustration of the red supergiant Betelgeuse. (Image credit: Graphics Department/MPIA)

Reference: K. Kravchenko, A. Jorissen, S. Van Eck, T. Merle, A. Chiavassa, C. Paladini, B. Freytag, B. Plez, M. Montargès, H. Van Winckel, “Atmosphere of Betelgeuse before and during the Great Dimming event revealed by tomography”, A&A, pp. 1-12, 2021. https://arxiv.org/abs/2104.08105

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