<p>One scenario that can work is if the wind from the red giant is accreted by the brown dwarf.</p>
<p>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 <a href="https://arxiv.org/abs/astro-ph/0406166" rel="nofollow noreferrer">Bondi-Hoyle accretion</a>.</p>
<p>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.</p>
<p>An outline of this mechanism and the calculations required to work out how much mass might be transferred can be seen in <a href="https://ui.adsabs.harvard.edu/abs/1996MNRAS.279..180J/abstract" rel="nofollow noreferrer">Jeffries &amp; Stevens (1996)</a> 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 <span class="math-container">$\sim 0.5 M_{\odot}$</span> stars in orbits of <span class="math-container">$\sim 20$</span> 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 <span class="math-container">$0.07 M_\odot$</span> brown dwarf into &quot;starhood&quot;.</p>

CC BY-SA 4.0
2023-02-15T08:13:36.633Z
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".
Kevin Kostlan
CC BY-SA 4.0
2022-11-28T11:10:30.840Z
Thanks! It's very interesting that a companion can accrete significant mass beyond Roche surface.
Heopps
CC BY-SA 4.0
2022-11-25T17:59:40.613Z
2023-02-15T08:34:49.780Z
2023-02-15T08:34:49.780Z
ProfRob
<p>Let's assume a brown dwarf is on orbit around a main sequence star. Than the star becomes a red giant.</p>
<p>Let's assume the brown dwarf has &quot;the right&quot; orbit and can syphon matter from the red giant.</p>
<p><strong>Is it possible for the brown dwarf to accrete enough mass to become a star itself?</strong></p>
<p>By keyword search I could not find anything relevant. So I would not be surprised if the answer is &quot;no&quot;. Maybe brown dwarfs have not enough gravity to accrete matter, and the evaporation by the red giant's radiation will prevail.</p>
<p>But still, I could miss some results.</p>
<p>So:</p>
<p><strong>Have anybody modeled such star systems?</strong></p>
<p><strong>Have anybody observed candidates for such star systems?</strong></p>

<stellar-evolution><brown-dwarf><red-giant><accretion>
Can a brown dwarf accrete enough mass from red giant to become a star?
Heopps
# Can a brown dwarf accrete enough mass from red giant to become a star?

(Asked by: username_0 on 2022-11-24)

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?**

**Tags:** stellar-evolution, brown-dwarf, red-giant, accretion

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## 1 answers

### Answer by username_1 on 2022-11-25. Score: 9

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](https://arxiv.org/abs/astro-ph/0406166).

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)](https://ui.adsabs.harvard.edu/abs/1996MNRAS.279..180J/abstract) 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".

> **Comments:**
> **username_0**: Thanks! It's very interesting that a companion can accrete significant mass beyond Roche surface.  
> **username_2**: 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".  

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