Substellar object

A substellar object, sometimes called a substar, is an astronomical object whose mass is smaller than the smallest mass, approximately 0.08 solar masses, at which a star can sustain hydrogen fusion. This definition includes brown dwarfs; it also includes objects of planetary mass, regardless of their formation mechanism and whether or not they are associated with a primary star. [§3, [http://adsabs.harvard.edu/abs/2006AJ....132.2513S What Is a Planet?] , Steven Soter, "Astronomical Journal", 132, #6 (December 2006), pp. 2513–2519.] [pp. 337–338, [http://adsabs.harvard.edu/abs/2000ARA&A..38..337C Theory of Low-Mass Stars and Substellar Objects] , Gilles Chabrier and Isabelle Baraffe, "Annual Review of Astronomy and Astrophysics" 38 (2000), p. 337–377.] [ [http://www.astro.uiuc.edu/~kaler/sow/alulaaus.html Alula Australis] , Jim Kaler, in "Stars", a collection of web pages. Accessed on line September 17, 2007.] [ [http://adsabs.harvard.edu/abs/2006A%26A...460..799G A search for substellar members in the Praesepe and σ Orionis clusters] , B. M. González-García, M. R. Zapatero Osorio, V. J. S. Béjar, G. Bihain, D. Barrado Y Navascués, J. A. Caballero, and M. Morales-Calderón, "Astronomy and Astrophysics" 460, #3 (December 2006), pp. 799–810.]

Assuming that a substellar object has composition similar to the Sun's and at least the mass of Jupiter (approximately 10^-3 solar masses), its radius will be comparable to that of Jupiter (approximately 0.1 solar radii). This is because the center of such a substellar object just below the hydrogen-burning limit is quite degenerate, with a density of ~10³ g/cm³, but this degeneracy lessens with decreasing mass until, at the mass of Jupiter, a substellar object has central density less than 10 g/cm³. The density decrease balances the mass decrease, keeping the radius approximately constant. [Chabrier and Baraffe, §2.1.1, 3.1.]

A substellar object with mass just below the hydrogen-fusing limit may ignite hydrogen fusion temporarily at its center, but although this will provide some energy, it will not be enough to overcome the object's ongoing gravitational contraction; likewise, although an object with mass above approximately 0.013 solar masses will be able to fuse deuterium for a time, this source of energy will be exhausted in approximately 10⁶ to 10⁸ years. Apart from these sources, the radiation of an isolated substellar object comes only from the release of its gravitational potential energy, which causes it to gradually cool and shrink. A substellar object in orbit about a star will shrink more slowly as it is kept warm by the star, evolving towards an equilibrium state where it emits as much energy as it receives from the star. [Chabrier and Baraffe, §4.1, Figures 6–8.]

References

ee also

* Brown dwarf
* Planemo
* Sub-brown dwarf