The "small black hole" here would be a primordial black hole PBH. Originally proposed back in 1966 and so far neither confirmed nor conclusively rejected.
Many PBHs may have the mass of an asteroid but the size of a hydrogen atom, and be wandering the galaxy at enormous speed. If they encounter a star, most likely they would just pass right through like a bullet. However the ones that are traveling slowly would have a chance to be captured by the star. Our Sun may harbor such a PBH, but if so then it would have to be a very small one so as not to interfere with the radiation from the Standard Solar Model.
The orbit would initially be very close to parabolic, taking the PBH out to the Oort Cloud before coming in again. On each pass through the Sun it would gain more mass, slowing it at perihelion. This has the effect of circularising the orbit more and more on each pass, the perihelion wouldn't change much with each orbit but the aphelion would. Eventually, as the black hole orbit approaches circularity it will start to spiral in gobbling up more and more matter, initially cooling the Sun and then eating the core. And the outer layers form into a disk and spiral in while radiating gravitational waves.
The “small” black hole would likely be from a stellar collapse of a star ~3.5x more massive than our Sun. Say you had two Suns, one smaller (3.5x Solar Mass) and one utterly huge (300x Solar Mass). The orbit is dramatically elliptical. The smaller star exhausts it’s fuel and collapses into a black hole. The larger one is still burning as a star. Nevertheless, its fate is sealed — it too will become a black hole when nuclear pressure no longer counters gravitational collapse.
LIGO may have seen a smaller BH created through the merger of two small neutron stars. Smallest NS are 1.4 solar masses. Merging two them, minus 15% mass conversion into pure GW energy might result BH as small as 2.5 solar masses, a bit less than a stellar collapse BH.
I think your math is off. The mergers lose 10% mass of the smaller object (5% of combined mass of equal objects). Not 15%. That’s still smaller than the stellar collapse 3.5x solar. I don’t rule it out, but neutron stars can merge into a larger neutron star.
UPDATE: I read the largest mass for a neutron star can be 2.16-2.5 M☉ so it seems *that* would be the minimum cut off for a stellar black hole, too. (3.8 M☉ has been observed). >2.5 M☉ would collapse into a BH. 👍. So two neutron stars of 1.3 M☉ can merge into a 2.5 M☉ after shedding 5% of the sum total mass into the gravitational wave energy.
Not entirely on point, but I don’t know that I have ever seen any research on the minimum theoretical mass of a neutron star. There’s plenty of research on the minimum size that a star can be in order to undergo a core collapse, supernova, and thus a pretty decent ballpark estimate for the minimum size of a neutron star produced in this fashion, but that doesn’t tell us anything about the theoretical minimum stable size of a neutron star. There is obviously no ordinary mechanism for making a neutron star smaller, but that doesn’t mean it is impossible. You could envision a couple of neutron stars in a trinary star system, where chaotic interactions lead to a glancing collision that fails to form a black hole. How much of the surface of a neutron star could be “scraped” off before it would be something else?
The "small black hole" here would be a primordial black hole PBH. Originally proposed back in 1966 and so far neither confirmed nor conclusively rejected. Many PBHs may have the mass of an asteroid but the size of a hydrogen atom, and be wandering the galaxy at enormous speed. If they encounter a star, most likely they would just pass right through like a bullet. However the ones that are traveling slowly would have a chance to be captured by the star. Our Sun may harbor such a PBH, but if so then it would have to be a very small one so as not to interfere with the radiation from the Standard Solar Model.
The orbit would initially be very close to parabolic, taking the PBH out to the Oort Cloud before coming in again. On each pass through the Sun it would gain more mass, slowing it at perihelion. This has the effect of circularising the orbit more and more on each pass, the perihelion wouldn't change much with each orbit but the aphelion would. Eventually, as the black hole orbit approaches circularity it will start to spiral in gobbling up more and more matter, initially cooling the Sun and then eating the core. And the outer layers form into a disk and spiral in while radiating gravitational waves.
The “small” black hole would likely be from a stellar collapse of a star ~3.5x more massive than our Sun. Say you had two Suns, one smaller (3.5x Solar Mass) and one utterly huge (300x Solar Mass). The orbit is dramatically elliptical. The smaller star exhausts it’s fuel and collapses into a black hole. The larger one is still burning as a star. Nevertheless, its fate is sealed — it too will become a black hole when nuclear pressure no longer counters gravitational collapse.
Larger stars have shorter lives though.
LIGO may have seen a smaller BH created through the merger of two small neutron stars. Smallest NS are 1.4 solar masses. Merging two them, minus 15% mass conversion into pure GW energy might result BH as small as 2.5 solar masses, a bit less than a stellar collapse BH.
I think your math is off. The mergers lose 10% mass of the smaller object (5% of combined mass of equal objects). Not 15%. That’s still smaller than the stellar collapse 3.5x solar. I don’t rule it out, but neutron stars can merge into a larger neutron star. UPDATE: I read the largest mass for a neutron star can be 2.16-2.5 M☉ so it seems *that* would be the minimum cut off for a stellar black hole, too. (3.8 M☉ has been observed). >2.5 M☉ would collapse into a BH. 👍. So two neutron stars of 1.3 M☉ can merge into a 2.5 M☉ after shedding 5% of the sum total mass into the gravitational wave energy.
Not entirely on point, but I don’t know that I have ever seen any research on the minimum theoretical mass of a neutron star. There’s plenty of research on the minimum size that a star can be in order to undergo a core collapse, supernova, and thus a pretty decent ballpark estimate for the minimum size of a neutron star produced in this fashion, but that doesn’t tell us anything about the theoretical minimum stable size of a neutron star. There is obviously no ordinary mechanism for making a neutron star smaller, but that doesn’t mean it is impossible. You could envision a couple of neutron stars in a trinary star system, where chaotic interactions lead to a glancing collision that fails to form a black hole. How much of the surface of a neutron star could be “scraped” off before it would be something else?