Would this ever happen? If it would, what kind of star/supernova would this create? Does it depend on the mass of the main sequence star?
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2$\begingroup$ See answers here astronomy.stackexchange.com/questions/14455/… $\endgroup$– AtmosphericPrisonEscapeCommented Dec 8, 2021 at 12:37
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$\begingroup$ Collisions of any stellar objects are notoriously difficult to predict the outcomes for, but I would imagine after whatever happens happens that a compact object would be left behind of some sorts, be it a WD, NS or BH $\endgroup$– Justin TCommented Dec 8, 2021 at 18:50
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This may seem like a simple question, but it can be quite complicated.
Does it depend on the mass of the main sequence star?
Yes. Assuming the main sequence star approaches the white dwarf star on a parabolic orbit, and if we ignore things like the internal structure of the main sequence star, its spin, general relativistic corrections, etc. then the result of this interaction depends on the tidal radius of the white dwarf star. The tidal radius is,
$$ R_t = R_\star \left( \frac{m_{\rm WD}}{m_{\star}} \right)^{1/3} $$
where $R_\star$ and $m_\star$ are the main sequence star's radius and mass. If the star passes within the tidal radius of the WD, then it will be tidally disrupted, and the tidal streams will create an accretion disk which can drive the WD into further core collapse as a Type Ia supernova.
Since a main sequence star is not as large in radius as a giant star, it is not likely that it can form a Thorne-Zytkow object, wherein a compact object occupies the core of the giant star, as it would be highly unstable. However, if the timing is right, then the WD could be engulfed by the main sequence star after it later evolves into a giant.
answered Dec 8, 2021 at 20:28
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This would result in a Type 1a supernova and formally falls under a scenario known as the single-degenerate scenario (due to collision with a main-sequence star). For comparison, a double-degenerate scenario would involve two white-dwarfs and once again, result in a Type 1a SN.
More info here: https://link.springer.com/referenceworkentry/10.1007%2F978-3-319-21846-5_36
Under section 1.1 and so on...
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$\begingroup$ But for the Single degenerate case, when the primary star had evolved to a white dwarf, the companion star has already evolved into a red giant star, which is not a main-sequence star anymore, so is that still apply? $\endgroup$ Commented Dec 8, 2021 at 13:53
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$\begingroup$ Indeed. Single-degenerate is for any scenario involving only a single white dwarf (this is what the 'degenerate' part is referring to). In the link I attached it is quoted here in section 1.1 as ' In the single-degenerate channel, the companion star is a main sequence, subgiant, or giant star.' I hope that helps. $\endgroup$– r21Commented Dec 8, 2021 at 13:56
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$\begingroup$ Thanks for the document, but in the paper, it feels like they are talking about accreting matter from the MS, subgiant or giant star, not two star collide. Are those two case the same？ $\endgroup$ Commented Dec 8, 2021 at 14:38
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2$\begingroup$ The "single-degenerate" scenario involves the gradual build-up of mass by accretion from a companion star, not an actual collision. (And you're not going to get a Type Ia supernova unless the combined mass of the white dwarf and the accreted material is close to the Chandrasekhar limit, so adding material from, say, a 0.2-solar-mass star to a 0.5-solar-mass white dwarf almost certainly won't get you a Type Ia supernova in any case.) $\endgroup$ Commented Dec 8, 2021 at 15:00
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$\begingroup$ In the same paper, under section 3, they mention various possible progenitor systems. Now as far as I'm concerned, I don't recall (and neither is there mention) of a collisional scenario between a white dwarf and non-degenerate star - most accepted model in this case is via accretion of mass. As pertaining your question, there is a possible theory that involves collision between two white dwarfs i.e. a double degenerate scenario (under section 3 of my link, it mentions all possible progenitor systems and scenarios for both single and double-degenerate. I hope this helps. $\endgroup$– r21Commented Dec 8, 2021 at 15:19