Selections from 2015: Mixing in Two Stars that Touch

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Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Discovery of the Massive Overcontact Binary VFTS 352: Evidence for Enhanced Internal Mixing

Published October 2015

 

Main takeaway:

A team led by Leonardo Almeida (Johns Hopkins University and University of São Paulo, Brazil) discovered the binary star system VFTS 352 in the Large Magellanic Cloud. This pair of O-type stars is an “overcontact binary” — the two stars are orbiting each other so closely that they’re actually touching each other.

Why it’s interesting:

Snapshots of VFTS 352 at a few orbital phases, the system’s light curves, and its radial velocity curves. [Almeida et al. 2015]

Snapshots of VFTS 352 at a few orbital phases, the system’s light curves, and its radial velocity curves. [Almeida et al. 2015]

We know little about the overcontact stage that occurs when two massive stars coalesce — primarily because it’s typically short-lived, so we have few observations of stars in this stage. VFTS 352 is the most massive and earliest spectral type overcontact system known to date. It’s especially interesting because the observations suggest that the strong tidal forces in this system may have caused enhanced internal mixing between the stars’ centers and envelopes. These stars’ interiors may therefore be much more homogenous than is typical.

What to expect:

Ultimately, this pair of stars will likely share one of two fates. In the classical scenario, they’ll expand and eventually merge to produce a single rapidly rotating, massive star. If their internal mixing is large enough, however, they could remain compact rather than expanding. In that case, they would progress to the end of their main-sequence lifetimes without ever merging, potentially evolving to become a black-hole binary system.

Citation

L. A. Almeida et al 2015 ApJ 812 102. doi:10.1088/0004-637X/812/2/102

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