Stars, BDs, planets and anything in between

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Stars, BDs, planets and anything in between

Post by Bruno on 8th May 2010, 9:35 am

Hello folks,

I am wondering about how to distinguish between low mass stars, brown dwarfs and planets
when masses are at or near the canonical limits more or less established by the
astronomical community.
For example, the HBMM (Hydrogen Burning Minimum Mass) is roughly 0.07-0.072MSun for objects
with solar abundances and would be 0.093MSun for objects with zero metallicity.
Also, I imagine that it is possible to have a star with a mass even lower than 0.07MSun if its
metallicity is higher than Sun's. After all, you have planet-bearing stars with
abundances as high as 2x Sun's.
Then you have a pretty large range of masses, into which you could have stars and BDs mixed
(if, of course, that ages, abundances or lithium presence are not known).
I think the same is true for the BD-Planet boundary, or worse, that this boundary is less
well-defined than the former... I have read elsewhere of the 13-14MJup limit without any
or little further consideration regarding how abundances and other variables change this limit!
What's your opinions?

Bruno
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Re: Stars, BDs, planets and anything in between

Post by Sirius_Alpha on 8th May 2010, 5:44 pm

As you point out, the deuterium fusion limit has a lot of variables. There's problems with using it as a cut-off between the planet-BD bondary as young, higher mass gas giants are able to fuse deuterium for a brief time before they cool down. As a result, some of the objects that we currently call planets may well have been fusing deuterium in their past.

A better discriminate may be the method of formation. Brown dwarfs may form by disk instability, and planets may form by core accretion. A problem with this, however, is that some objects we would rather consider brown dwarfs are in neat, orderly orbits, often with multiple other objects (Ups And, HR 8799?, Iot Aur).

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