Brown Dwarf Solar System Entering
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While spectroscopic features can help to distinguish between low mass stars and brown dwarfs, it is often necessary to estimate the mass to come to a conclusion. The theory behind the mass estimate is that brown dwarfs with a similar mass form in a similar way and are hot when they form. Some have spectral types that are similar to low-mass stars, such as 2M1101AB. As they cool down the brown dwarfs should retain a range of luminosities depending on the mass. Without the age and luminosity a mass estimate is difficult; for example, an L-type brown dwarf could be an old brown dwarf with a high mass (possibly a low-mass star) or a young brown dwarf with a very low mass. For Y-dwarfs this is less of a problem as they remain low mass objects near the sub-brown dwarf limit, even for relative high age estimates. For L- and T-dwarfs it is still useful to have an accurate age estimate. The luminosity is here the less concerning property, as this can be estimated from the spectral energy distribution.  The age estimate can be done in two ways. Either the brown dwarf is young and still has spectral features that are associated with youth or the brown dwarf co-moves with a star or stellar group (star cluster or association), which have easier to obtain age estimates. A very young brown dwarf that was further studied with this method is 2M1207 and the companion 2M1207b. Based on the location, proper motion and spectral signature, this object was determined to belong to the ~8 million year old TW Hydrae association and the mass of the secondary was determined to be below the deuterium burning limit with 8 ± 2 MJ.  A very old example of an age estimate that makes use of co-movement is the brown dwarf + white dwarf binary COCONUTS-1, with the white dwarf having a total age of 7. 3+2. 8−1. 6 billion years. In this case the mass was not estimated with the derived age, but the co-movement provided an accurate distance estimate, using Gaia parallax. Using this measurement the authors estimated the radius, which was then used to estimate the mass for the brown dwarf as 15. 4+0. 9−0. 8 MJ.