<p><strong>Yes</strong>, unless you want to get really particular with the "protoplanetary" part. For example, there are stars forming in the circumstellar disks around Wolf-Rayet stars [<a href="http://arxiv.org/abs/1203.4659">reference</a>]. If we were picky, we might not call the circumstellar disk around this Wolf-Rayet star a protoplanetary disk (and instead refer to the planet-forming circumstellar disks of the newly formed stars as such), but there exists no clear line between the disks of gas within and the larger disk of gas around the Wolf-Rayet star of which they are a part.</p>

<p><strong>Source:</strong></p>

<ul>
<li><a href="http://arxiv.org/abs/1203.4659">Triggered star formation surrounding Wolf-Rayet star HD 211853 - Liu, Wu, Zhang, and Qin</a></li>
</ul>

CC BY-SA 3.0
2013-11-26T23:44:24.720Z
Thank you very much for your answer and the reference! Can you speculate on whether a planet turning into a MS star could affect the dynamics of the whole planetary system? (perhaps, in a less violent case than that of WR stars) And also, I presume, there are no existing estimates on the occurence rate of such events in general?
Alexey Bobrick
CC BY-SA 3.0
2013-11-27T13:11:39.780Z
@AlexeyBobrick I don't really feel qualified to speculate in my answer based on the data I have, but I would guess that such a system would not be entirely different from a binary system which formed the conventional way.
called2voyage
CC BY-SA 3.0
2013-11-27T13:26:55.420Z
Thanks a lot! One point to add here is that the stars, formed in PPDs are most likely less massive and possibly a bit different in composition from the normal binary companions formed through gravitational collapse.
Alexey Bobrick
CC BY-SA 3.0
2013-11-27T13:40:50.920Z
@AlexeyBobrick Possibly, but there are already catalogued binary star systems where one companion is a smaller star with different composition. For example, [Gilese 777](http://en.wikipedia.org/wiki/Gliese_777) has two (possibly three) stars of varying sizes and compositions. I would actually consider it a fairly good candidate for a system where the companions of the largest star formed from the circumstellar disk, not from traditional gravitational collapse.
called2voyage
CC BY-SA 3.0
2013-11-26T17:50:12.657Z
2013-11-26T17:50:12.657Z
called2voyage
<p>No, not really.</p>

<p>Stars can form in circumstellar disks, that are, in general, disks surrounding forming stars, but not in protoplanetary disks. Protoplanetary disks are, by definition, flat, rotationing disks composed of gas and dust, found around newly born low-mass stars (see the review by <a href="http://adsabs.harvard.edu/abs/2011ARA&amp;A..49...67W">Williams &amp; Cieza (2011)</a>). There are two important point in this definition: protoplanetary disks orbit around <strong>low-mass</strong> stars, and, in particular, <strong>newly-born</strong> low-mass stars. These two points are important, because at this stage of the life of the disk and for this range of stellar mass, its mass it clearly too small to form any star.</p>

<p>That being said, it is not impossible to form stars in the protostellar disk of a forming star (that is one of the possibility to form multiple systems, that are still hard to explain), as discussed by various authors (see, for example  <a href="http://adsabs.harvard.edu/abs/2009MNRAS.392..413S">Stamatellos et al. (2009)</a>, <a href="http://adsabs.harvard.edu/abs/2013MNRAS.433.3256V">Vorobyov et al. (2013)</a>, <a href="http://adsabs.harvard.edu/abs/2013A%26A...554A..17J">Joos et al. (2013)</a> and many others), but it is more likely to form brown dwarfs of very low-mass stars in these disks than main sequence stars (their mass is simply too small, once again).</p>

<p>That being said (<em>bis repetita</em>), you could, as pointed out by called2voyage, you could form a main sequence star in the circumstellar disk of a high-mass star (as a Wolf-Rayet type star).</p>

CC BY-SA 3.0
2013-12-12T08:50:45.913Z
2013-12-12T08:50:45.913Z
MBR
<p>As has been pointed out by @Envite in the context of a more general discussion (see <a href="https://astronomy.stackexchange.com/questions/987/generalised-planets">Generalised planets?</a>), there seems to be a moderate possibility for protoplanetary discs forming main-sequence stars (MS stars).</p>

<p>Main argument here is that Jupiter has mass of about $10^{-3}M_\odot$, while in principle for an object to be a MS star, it would need the mass of at least about $0.07M_\odot$. Also, there is a handful of systems, such as <a href="http://en.wikipedia.org/wiki/HD_29587" rel="nofollow noreferrer">HD 29587</a>, which contain a star and a brown dwarf, possibly forming from a protoplanetary disk.</p>

<p>Hence, a few questions. Can low-mass MS stars form in protoplanetary discs? If no, why? If yes, how often does it happen? If yes, which implications would it have for the subsequent dynamics of the planetary system, if one of the objects became a MS star, as opposed to it being a brown dwarf?</p>

<p>Again, thanks to @Envite for the idea.</p>

CC BY-SA 3.0
2013-11-25T02:17:02.523Z
Don't mention it :)
Envite
CC BY-SA 3.0
2013-11-25T10:37:36.330Z
@Envite: So long as it is scientifically valid, why not :)
Alexey Bobrick
CC BY-SA 3.0
2013-11-25T10:39:12.450Z
Don't mention it: I mean thanking me. The question is absolutely valid.
Envite
CC BY-SA 3.0
2013-11-25T10:50:13.130Z
Yes, and I actually like it. But the idea was yours.
Alexey Bobrick
<planetary-formation><star-formation>
Can protoplanetary disks form main-sequence stars?
Alexey Bobrick
# Can protoplanetary disks form main-sequence stars?

(Asked by: username_0 on 2013-11-24)

As has been pointed out by @Envite in the context of a more general discussion (see [Generalised planets?](https://astronomy.stackexchange.com/questions/987/generalised-planets)), there seems to be a moderate possibility for protoplanetary discs forming main-sequence stars (MS stars).

Main argument here is that Jupiter has mass of about $10^{-3}M\_\odot$, while in principle for an object to be a MS star, it would need the mass of at least about $0.07M\_\odot$. Also, there is a handful of systems, such as [HD 29587](http://en.wikipedia.org/wiki/HD_29587), which contain a star and a brown dwarf, possibly forming from a protoplanetary disk.

Hence, a few questions. Can low-mass MS stars form in protoplanetary discs? If no, why? If yes, how often does it happen? If yes, which implications would it have for the subsequent dynamics of the planetary system, if one of the objects became a MS star, as opposed to it being a brown dwarf?

Again, thanks to @Envite for the idea.

**Tags:** planetary-formation, star-formation
> **Comments:**
> **username_1**: Don't mention it :)  
> **username_0**: @Envite: So long as it is scientifically valid, why not :)  
> **username_1**: Don't mention it: I mean thanking me. The question is absolutely valid.  
> **username_0**: Yes, and I actually like it. But the idea was yours.  

---

## 2 answers

### Answer by username_2 on 2013-11-26. Score: 9

**Yes**, unless you want to get really particular with the "protoplanetary" part. For example, there are stars forming in the circumstellar disks around Wolf-Rayet stars [[reference](http://arxiv.org/abs/1203.4659)]. If we were picky, we might not call the circumstellar disk around this Wolf-Rayet star a protoplanetary disk (and instead refer to the planet-forming circumstellar disks of the newly formed stars as such), but there exists no clear line between the disks of gas within and the larger disk of gas around the Wolf-Rayet star of which they are a part.

**Source:**

* [Triggered star formation surrounding Wolf-Rayet star HD 211853 - Liu, Wu, Zhang, and Qin](http://arxiv.org/abs/1203.4659)

> **Comments:**
> **username_0**: Thank you very much for your answer and the reference! Can you speculate on whether a planet turning into a MS star could affect the dynamics of the whole planetary system? (perhaps, in a less violent case than that of WR stars) And also, I presume, there are no existing estimates on the occurence rate of such events in general?  
> **username_2**: @AlexeyBobrick I don't really feel qualified to speculate in my answer based on the data I have, but I would guess that such a system would not be entirely different from a binary system which formed the conventional way.  
> **username_0**: Thanks a lot! One point to add here is that the stars, formed in PPDs are most likely less massive and possibly a bit different in composition from the normal binary companions formed through gravitational collapse.  
> **username_2**: @AlexeyBobrick Possibly, but there are already catalogued binary star systems where one companion is a smaller star with different composition. For example, [Gilese 777](http://en.wikipedia.org/wiki/Gliese_777) has two (possibly three) stars of varying sizes and compositions. I would actually consider it a fairly good candidate for a system where the companions of the largest star formed from the circumstellar disk, not from traditional gravitational collapse.  

---

### Answer by username_3 on 2013-12-12. Score: 8

No, not really.

Stars can form in circumstellar disks, that are, in general, disks surrounding forming stars, but not in protoplanetary disks. Protoplanetary disks are, by definition, flat, rotationing disks composed of gas and dust, found around newly born low-mass stars (see the review by [Williams & Cieza (2011)](http://adsabs.harvard.edu/abs/2011ARA&A..49...67W)). There are two important point in this definition: protoplanetary disks orbit around **low-mass** stars, and, in particular, **newly-born** low-mass stars. These two points are important, because at this stage of the life of the disk and for this range of stellar mass, its mass it clearly too small to form any star.

That being said, it is not impossible to form stars in the protostellar disk of a forming star (that is one of the possibility to form multiple systems, that are still hard to explain), as discussed by various authors (see, for example [Stamatellos et al. (2009)](http://adsabs.harvard.edu/abs/2009MNRAS.392..413S), [Vorobyov et al. (2013)](http://adsabs.harvard.edu/abs/2013MNRAS.433.3256V), [Joos et al. (2013)](http://adsabs.harvard.edu/abs/2013A%26A...554A..17J) and many others), but it is more likely to form brown dwarfs of very low-mass stars in these disks than main sequence stars (their mass is simply too small, once again).

That being said (*bis repetita*), you could, as pointed out by called2voyage, you could form a main sequence star in the circumstellar disk of a high-mass star (as a Wolf-Rayet type star).


---

