Are Brown and Sub-Brown Dwarfs secretly more common than stars?

Question

I recently heard that Red Dwarfs are the most common type of star, and low-mass Red Dwarfs are the most common type of Red Dwarf. This seems to imply a generic trend that the lower the mass, the more common the star. I have 2 questions.

1. I know that Brown Dwarfs and Sub-Brown Dwarfs are much harder to detect because they are not very luminous. Is it possible that Brown Dwarfs and Sub-Brown Dwarfs are actually even more common than Red Dwarfs? Are there any theories or papers on this?

2. Is there a lower limit on the mass of an object that can be formed at the center of a gas/dust nebula collapse (as opposed to being formed in a protoplanetary disk)?

Answer 1

The answer to your first question is (now) fairly simple: No, brown dwarfs are not more common than red dwarfs. A crude approximation is that stars (which are indeed mostly red dwarfs) outnumber brown dwarfs 4 or 5 to 1; see, for example, the review article by Chabrier et al. (2014). This is supported by extensive surveys done in the 2000s and 2010s, including observations with the WISE telescope; these are sensitive enough to uncover quite faint brown dwarfs.

Empirically, attempts to count stars and brown dwarfs in local volumes strongly suggest that stars outnumber brown dwarfs. For example, Reylé et al. (2021) combine Gaia observations (sensitive to hydrogen-burning stars and the more massive brown dwarfs) with literature observations of brown dwarfs using IR telescopes (Spitzer, WISE) to make a census of different objects within 10 pc (33 light years) of the Sun. They find a total of 355 (hydrogen-burning) stars -- including 276 M dwarfs --, 20 white dwarfs (formerly hydrogen-burning), and 85 brown dwarfs (plus 3 more candidates). They do note that there are probably more brown dwarfs waiting to be identified, though these would have to be at the very low-mass (and thus low-luminosity) end. One can still argue that stars outnumber brown dwarfs in our local volume by about three or four to one.

It is true that lower mass stars tend to be more common than higher mass stars. However, this trend becomes weaker for lower masses. The figure below (from the Wikipedia article on the initial mass function (IMF) of stars, which tells you how many stars of a given mass are born, as a function of mass) shows several different versions of the IMF. The best current measurements are encoded in either the "Kroupa01" or "Chabrier03" curves, all of which show that the curves flatten -- or even turn over -- for very low masses. This means that if you add up all the brown dwarfs and add up all the stars, you end up with more stars than brown dwarfs.

Edited to add: Empirical counts of stars and brown dwarfs in the local volume.

Answer 2

This is an important question to ask about the initial mass function of objects in the Galaxy - and the final answer hasn't been cast as it is a matter of research.

Yet, observational data (e.g. see the mass functions for various clusters in this talk), and simulations in more or less good agreement with that (e.g. here or here), seem to indicate that this so-called brown dwarf desert is real and that we talk about fundamentally different formation mechanisms between stars and planets. Yet this doesn't mean that no such objects exist nor do we completely understand the causes for these observational findings. One thing playing a role here could be that there is a lower limit to the Jeans mass (or rather density) which you will not pass for a single star / brown dwarf by fluctuations in a collapsing gas cloud due to thermodynamics - which seems to be around the Brown dwarf mass.

Just recently there have been attempts to get a better handle on some Brown Dwarfs by help of transit detection (e.g. see here) which allows to constrain its parameter much better than before.

Answer 3

According to the wiki article on sub-brown dwarfs, the lowest mass object that can form from collapse of gas cloud is about 1 Jupiter mass.

Original source: Boss, Alan P.; Basri, Gibor; Kumar, Shiv S.; Liebert, James; Martín, Eduardo L.; Reipurth, Bo; Zinnecker, Hans (2003), "Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?"