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Register a new userIs Betelgeuse the Outcome of a Past Merger?
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Emmanouil Chatzopoulos Dr
Publication date
2020
fields
Physics
and research's language is
English
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We explore the possibility that the star alpha Orionis (Betelgeuse) is the outcome of a merger that occurred in a low mass ratio (q = M2/M1 = 0.07 - 0.25) binary system some time in the past hundreds of thousands of years. To that goal, we present a simple analytical model to approximate the perturbed internal structure of a post-merger object following the coalescence of a secondary in the mass range 1-4 Msun into the envelope of a 15-17 Msun primary. We then compute the long-term evolution of post-merger objects for a grid of initial conditions and make predictions about their surface properties for evolutionary stages that are consistent with the observed location of Betelgeuse in the Hertzsprung-Russell diagram. We find that if a merger occurred after the end of the primarys main-sequence phase, while it was expanding toward becoming a red supergiant star and typically with radius ~200 - 300 Rsun, then its envelope is spun-up to values which remain in a range consistent with the Betelgeuse observations for thousands of years of evolution. We argue that the best scenario that can explain both the fast rotation of Betelgeuse and its observed large space velocity is one where a binary was dynamically ejected by its parent cluster a few million years ago and then subsequently merged. An alternative scenario in which the progenitor of Betelgeuse was spun up by accretion in a binary and released by the supernova explosion of the companion requires a finely tuned set of conditions but cannot be ruled out.
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We previously proposed that Betelgeuse might have been spun up by accreting a companion of about 1 solar mass. Here we explore in more detail the possible systematics of such a merger and a larger range of accreted masses. We use the stellar evolutionary code MESA to add angular momentum to a primary star in core helium burning, core carbon burning, or shell carbon burning. Our models provide a reasonable natural explanation for why Betelgeuse has a large, but sub-Keplerian equatorial velocity. They eject sufficient mass and angular momentum in rotationally-induced mass loss to reproduce the observed ratio of the equatorial velocity to escape velocity of Betelgeuse, ~0.23, within a factor of three nearly independent of the primary mass, the secondary mass, and the epoch at which merger occurs. Our models suggest that merger of a primary of somewhat less than 15 solar masses with secondaries of from 1 to 10 solar masses during core helium burning or core carbon burning could yield the equatorial rotational velocity of ~15 km/s attributed to Betelgeuse. For accreting models, a wave of angular momentum is halted at the composition boundary at the edge of the helium core. The inner core is thus not affected by the accretion of the companion in these simulations. Accretion has relatively little effect on the production of magnetic fields in the inner core. Our results do not prove, but do not negate that Betelgeuse might have ingested a companion of several solar masses.
The dynamics of the surface and inner atmosphere of the red supergiant star Betelgeuse are the subject of numerous high angular resolution and spectroscopic studies. Here, we present three-telescope interferometric data obtained at 11.15 microns wavelength with the Berkeley Infrared Spatial Interferometer (ISI), that probe the stellar surface continuum. We find striking variability in the size, effective temperature, and degree of asymmetry of the star over the years 2006-2009. These results may indicate an evolving shell of optically thick material close to the stellar photosphere.
Observations of the Galactic Center (GC) have accumulated a multitude of forensic evidence indicating that several million years ago the center of the Milky Way galaxy was teaming with starforming and accretion-powered activity -- this paints a rather different picture from the GC as we understand it today. We examine a possibility that this epoch of activity could have been triggered by the infall of a satellite galaxy into the Milky Way which began at the redshift of 10 and ended few million years ago with a merger of the Galactic supermassive black hole with an intermediate mass black hole brought in by the inspiralling satellite.
Betelgeuse is one of the most magnificent stars in the sky, and one of the nearest red supergiants. Astronomers gathered in Paris in the Autumn of 2012 to decide what we know about its structure, behaviour, and past and future evolution, and how to place this in the general context of the class of red supergiants. Here I reflect on the discussions and propose a synthesis of the presented evidence. I believe that, in those four days, we have achieved to solve a few riddles.
We present optical spectrophotometry of the red supergiant Betelgeuse from 2020 February 15, during its recent unprecedented dimming episode. By comparing this spectrum to stellar atmosphere models for cool supergiants, as well as spectrophotometry of other Milky Way red supergiants, we conclude that Betelgeuse has a current effective temperature of 3600 +/- 25 K. While this is slightly cooler than previous measurements taken prior to Betelgeuses recent lightcurve evolution, this drop in effective temperature is insufficient to explain Betelgeuses recent optical dimming. We propose that episodic mass loss and an increase in the amount of large-grain circumstellar dust along our sightline to Betelgeuse is the most likely explanation for its recent photometric evolution.
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