At What Angular Speed Brown Dwarf Will Not Exist? (Planetary Science)

Using an analytical brown dwarf model, Tapobrata Sarkar and colleagues studied the effects of rapid rotation of brown dwarfs on the minimum mass of hydrogen burning. They have obtained an upper bound of angular speed beyond which a brown dwarf will not exist. Their study recently appeared in Arxiv.

Brown dwarfs (BDs) are substellar objects whose masses range between those of Jupiter (∼ 10¯3 M). Unlike main sequence stars, Brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen into helium in their cores, as their masses are less than a certain minimum value, dubbed as the minimum mass of hydrogen burning (Mmmhb) or sometimes as the minimum main sequence mass.

Now, Tapobrata Sarkar and colleagues adapted the polytropic model of the brown dwarfs via a suitable numerical scheme in order to study the effects of rapid rotation.

“We have used a toy model with a number of assumptions. First, all the thermodynamic relations have been assumed to remain unaltered in the presence of rotation. Second, we have considered the effect of uniform stellar rotation on brown dwarfs evolution.”

They have obtained an analytical formula of the minimum mass of hydrogen burning as a function of the angular speed and found that the Brown dwarf will not exist beyond angular speed of 0.0045s¯1.

Schematic diagram of the results obtained in this paper. © Tapobrata Sarkar et al.

For this angular speed, they have also obtained the mass range Mmmhb ≤ M < Mmax for brown dwarfs to evolve into main sequence stars.

Finally, they obtained the luminosity of a brown dwarf at the point where it reaches the main sequence, as a function mass and angular speed.

“Our toy model has been able to decode the underlying physics of a rotating brown dwarf and has revealed several important limits.”

— concluded authors of the study

Featured image: This artist’s concept shows the dimmest star-like bodies currently known, twin brown dwarfs referred to as 2M 0939. © Getty Images

Reference: Shaswata Chowdhury, Pritam Banerjee, Debojyoti Garain, Tapobrata Sarkar, “Stable Hydrogen burning limits in rapidly rotating brown dwarfs”, Arxiv, pp. 1-9, 2021.

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