From astrobites: “All spun up”

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Title: Constraints on the spin evolution of young planetary-mass companions
Authors: M.L. Bryan, B. Benneke, H.A. Knutson, K. Batygin, B.P. Bowler
First Author’s Institution: California Institute of Technology

Status: Published in Nature Astronomy, open access

Every star has its own spin. Surprisingly, the rates at which stars spin are not completely random. This is because a star’s spin rate contains the imprint of the star’s evolution and interactions with its environment. For example, if a star were a perfect, isolated sphere, conservation of angular momentum would cause a contracting star to spin faster. There are complicating factors, like magnetic fields which thread through circumstellar material, and cause angular momentum to be transferred from the star to the disk. Stellar winds can blow away material, which carries angular momentum away with it. Angular momentum can also be transferred between different layers of a star— for example, a star that appears to be rotating slowly could be hiding a reservoir of angular momentum in a rapidly-spinning core.

Knowledge of the spin evolution of stars can reveal a lot about the physics of all those processes, and astronomers have accordingly scrutinized the spins of stars in young open clusters, which provide an entire sample of stars of similar age. These studies have provided thousands of measurements of stellar spin, and there are models that go at least some way to explaining the spin evolution over stellar lifetimes.

But for the spin of young, massive planets, there is… almost no data at all. Our own solar system has planets that are old and small enough so that the primordial physics is difficult to disentangle (though the fast spins of the gas giants undoubtedly harbor a trace of their primordial states). To probe the spin states of young planets, we must turn to exoplanets.

Before today’s paper, only two bona fide exoplanets had had their spins measured: Beta pic b and 2M1207 b. The authors of today’s paper elected to use spectroscopy to measure the rotation rates of three more planetary-mass objects on wide orbits. To link the physics of the planetary mass regime with the brown-dwarf-mass regime, they also looked at some brown dwarfs of similar age and spectral type.

The authors’ tool of choice was the NIRSPEC spectrograph at the Keck II telescope.

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NIRSPEC spectrograph at the Keck II telescope schematic. www2.keck.hawaii.edu

Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft) above sea level

NIRSPEC allowed them to take spectra of sufficiently high resolution of water and CO absorption bandheads around 2.3 microns, and Keck’s 10-m diameter mirror allowed them to scoop up just about as many photons as they could from the ground.

See the full article here .

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