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As has been pointed out by @Envite in the context of a more general discussion (see 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, 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.

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Don't mention it :) –  Envite Nov 25 '13 at 2:17
    
@Envite: So long as it is scientifically valid, why not :) –  Alexey Bobrick Nov 25 '13 at 10:37
    
Don't mention it: I mean thanking me. The question is absolutely valid. –  Envite Nov 25 '13 at 10:39
    
Yes, and I actually like it. But the idea was yours. –  Alexey Bobrick Nov 25 '13 at 10:50
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2 Answers 2

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]. 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:

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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 Nov 26 '13 at 23:44
    
@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 Nov 27 '13 at 13:11
    
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 Nov 27 '13 at 13:26
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@AlexeyBobrick Possibly, but there are already catalogued binary star systems where one companion is a smaller star with different composition. For example, Gilese 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 Nov 27 '13 at 13:40
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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)). 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), Vorobyov et al. (2013), Joos et al. (2013) 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).

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