# How do we know that 2MASS J0523-1403 is a red dwarf?

• The spectral type is L2.5
• The luminosity class is V
• The mass is 67.54±12.79 Jupiter masses, or 0.0644±0.0122 solar masses.

The first bullet point states that the spectral type is L, a brown dwarf class. Sure, some red dwarfs have spectral type L0-L2, but L2.5 seems a bit too much. Also, the luminosity class is V (main sequence dwarf), not VI or sd (subdwarf). The mass is also way below the red dwarf limit (~85 Jupiter masses). Even if the uncertainty was at its maximum, the mass would fall short by about 3 Jupiter masses.

So, how do we determine/prove that a potential red/brown dwarf is truly a red or brown dwarf? Also, has this 2MASS object been proved to be one or the other?

• Can't we somehow determine whether a star is fusing hydrogen or deuterium and/or also lithium? – Greenhorn Dec 3 '20 at 17:30

The brown dwarf "limit" is about $$0.072 M_{\odot}$$ at solar metallicity (e.g. Chabrier et al. 2000) and is composition dependent. It gets a little higher in metal-poor gas and a little lower in metal-rich gas. $$0.064 \pm 0.012 M_\odot$$ (the third significant figure is superfluous) is within one error bar of that limit, which in itself is only a 68% confidence limit, not an absolute limit of what is possible. Therefore this object could be a star or more likely, it is a brown dwarf.
To establish whether something really was a brown dwarf I think you would have to detect (or not) neutrino emission from its core. A sort-of proxy is to look for lithium. (Old) objects with lithium must have a mass lower than about $$0.06 M_\odot$$ and are almost certainly brown dwarfs. Objects that have depleted their lithium, which fuses at lower temperatures than hydrogen, are most likely stars, but could just be brown dwarfs. For example, I imagine that this object would have depleted its lithium, so it doesn't really tell you one way or another.