# Can a brown dwarf accrete enough mass from red giant to become a star?

Let's assume a brown dwarf is on orbit around a main sequence star. Than the star becomes a red giant.

Let's assume the brown dwarf has "the right" orbit and can syphon matter from the red giant.

Is it possible for the brown dwarf to accrete enough mass to become a star itself?

By keyword search I could not find anything relevant. So I would not be surprised if the answer is "no". Maybe brown dwarfs have not enough gravity to accrete matter, and the evaporation by the red giant's radiation will prevail.

But still, I could miss some results.

So:

Have anybody modeled such star systems?

Have anybody observed candidates for such star systems?

One scenario that can work is if the wind from the red giant is accreted by the brown dwarf.

The brown dwarf can be in quite a wide orbit and still accrete mass because the wind from the red giant, particularly in the AGB phase, is both dense and slow. This is known as Bondi-Hoyle accretion.

There is no theoretical impediment to a brown dwarf slightly below the hydrogen burning limit accreting a bit of mass and turning the brown dwarf into a star. This would not, at least initially, be a dramatic change in terms of the luminosity of the object and it would be much fainter than the red giant in any case. Over billions of years the difference between an object just above the H-burning limit and one just below would become apparent as a brown dwarf would continue to cool.

An outline of this mechanism and the calculations required to work out how much mass might be transferred can be seen in Jeffries & Stevens (1996) who studied the problem of wind-accretion from red giants onto low-mass stars in wide orbits (although not as low-mass as a brown dwarf). This finds that several tenths of a solar mass could be accreted by $$\sim 0.5 M_{\odot}$$ stars in orbits of $$\sim 20$$ au around an AGB star. The mass accretion will be proportional (via the Bondi-Hoyle radius) to the mass of the secondary squared, so we might expect accretion of only a hundredth or two of a solar mass by a brown dwarf in a similar orbit. But this could still be enough to tip a $$0.07 M_\odot$$ brown dwarf into "starhood".

• Thanks! It's very interesting that a companion can accrete significant mass beyond Roche surface. Commented Nov 28, 2022 at 11:10
• There is no object that is in between a brown dwarf or red dwarf. But it takes billions of years for the verdict to be settled (the brown dwarf will cool slowly but the red dwarf will not). So nothing dramatic happens when an object "crosses this line". Commented Feb 15, 2023 at 8:13

Yes, since white dwarfs can take matter from its neighboring giant star to make a type la supernova or else a micronova. However, we're not talking about white dwarf,

We're talking about a brown dwarf which has less mass, still if the brown dwarf has higher density and the red giant has lower density in the binary star system so that the Roche limit is larger in the brown dwarf and smaller in the red giant so that it shreds the red giant and the matter from the red giant begins to accelerate towards the Brown dwarf,

If the brown dwarf could accrete even 0.055 M☉ then the brown dwarf could become a red dwarf star with a temperature of 2000-3500 K as shown in the Hertzsprung-Russel Diagram assuming that the mass of the brown dwarf is minimum 0.025 M☉.

Roche limit is the point where tidal forces shreds the matter

There has been no such incident happened where a brown dwarf swallowed the red giant but instead a red giant swallowed the brown dwarf.

Moreover this mechanism is plausible because the Roche radius can be more than 800 AU!, so clearly it is above the Brown dwarf desert which is approximately 5 AU

• The question is whether the brown dwarf would become a star before tidal forces led to its engulfment. Commented Nov 25, 2022 at 17:54
• Thanks! Interesting info in the link that accreting brown dwarf can shorten the red giant phase. I didn't know this. Commented Nov 28, 2022 at 5:18
• You're Welcome @Heopps.
– user47732
Commented Nov 28, 2022 at 8:07