Computer simulation of binary star following the G2 cloud orbit: Core impact less likely

Posted  May 16, 2014
P. LaViolette

In a January 23rd Sphinx Stargate posting I had mentioned that there is an urgent need to do a computer simulation to investigate the trajectory of the G2 cloud stars in the case in which G2 might contain an embedded binary star system.  This was needed to see what the orbit would be of the separated companion; i.e., whether or not a stripped off companion would strike the Galactic core. Well, early last month a group of Czech and German astronomers, Zajacek, Karas, and Eckart, posted a paper  which is to appear in the journal Astronomy and Astrophysics which investigated this situation.  It discussed computer simulation results of the G2 cloud for three scenarios, the case where the cloud: a) contained no star, b) contained a solitary star, and c) contained a binary star.  This third simulation, which is of particular interest to us, is discussed at the very end of their paper.  I had written to all three on January 12th and 13th noting that if the G2 cloud contained an embedded binary star, there would be an increased threat for a core outburst, as in the case where a companion star or planet might be tidally stripped away and ultimately consumed by the core.  I had also pointed out to them that, as of then, no simulation study had been performed of this case.  Interestingly, they were among the majority that did not respond to my email.  So, I will not know if they already were investigating the binary star scenario, or whether they had gotten the idea from my email and added this scenario in to their study.

Their paper investigates the case in which the primary star has a mass of either 3 or 4 solar masses and the companion star has a mass of 1.4 solar masses.  In both cases the binary system is assumed to follow the trajectory of the G2 cloud and to have a pericenter velocity equal to that estimated for the cloud (6000 - 7000 kilometers per second).  Their simulation results showed that once the stars reached pericenter (~150 AU), the two began to separate from one another due to the action of the Galactic core's tidal forces, and thereafter to continue away from the core following slightly different elliptical orbits.  Consequently, the simulation showed that the companion would not follow a path that would take it spiraling in toward the core, as I had surmised in the January 23rd posting.  In fact, according to Michal Zajacek, for this outcome to occur, a binary star would have to follow an orbit that would take it almost on a collision course with the core event horizon, or bring it within the critical radius where stars begin to break up due to core tidal forces (personal communication, May 10, 2014).  This break-up distance would likely be closer than 80 AU to the core, since stars currently orbiting the Galactic center are not seen to have pericenters closer than this distance.

So based on the simulation of Zajacek, et al., I believe that the risk of a G2 cloud binary star triggering a core outburst is highly unlikely if in fact the companion star has a mass of about one solar mass.  However, as discussed in the previous posting, it is likely that the primary star is a white dwarf and if a companion is present that it is either a brown dwarf or planet.  Zajacek, et al. did not model such cases of a low mass companion where "hydrodrag" forces exerted by the core's wind become more important.  That is, for cases where the companion is of low mass, the effects of the galactic core ionized gas wind and cosmic ray wind play a more important role and could possibly decelerate a rapidly moving body sufficiently to cause it to spiral in toward the core.

Their simulation did show that gas and small dust particles making up the G2 cloud could be  captured by the core if the core's gas wind were sufficiently low.  But they found that if the core's gas wind was higher than 2800 km/s, G2's gas and dust would be blown outward away from the core and would never make an entry.  Since observation indicates that the core's wind could indeed be this high, this would explain why the Swift telescope has seen no X-ray emission during the current pericenter passage period.  As I had reported earlier, dust from the G2 cloud would likely be blown away from the core by the core's wind and would not generate any X-ray emission.

But larger sized debris would be too massive to be blown away, and it is possible that comet sized stellar companion bodies of 1 to 100 kilometers in diameter, and maybe even companions the size of the Earth, would be sufficiently small in mass as to have their orbital momentum overpowered by the decelerating effects of the core's hydro drag effects.  Until simulations of these smaller sized bodies are carried out, we must consider the possibility that such bodies could spiral inward and impact the core.  If such material followed an inward spiral trajectory, the earliest possible arrival would be perhaps mid August provided that it followed a spiral similar to that depicted in the sketch below.  If the spiral involved many orbitings of the core along a descending spiral path, then there would be a greater delay before energetic activity would be detected.

G2 cloud trajectory showing hypothetical path for both the primary star and smaller debris accreted due to hydrodynamic drag

G2 cloud trajectory showing hypothetical path for both the primary star and smaller debris accreted due to hydrodynamic drag

So, these new simulation results dramatically reduce the possibility of a superwave occurrence.  There is still a chance that small diameter companion debris could impact the core, but the resulting energy release would be far smaller, raising the question of whether the released energy would be sufficiently large to unleash even a low intensity superwave.  The chance of occurrence of such a low level event is still mired in uncertainty until further orbital simulations are done for such cases.  Unfortunately, I know of none that are being planned.

The bottom line is that the threat for a possible near term superwave event is drastically reduced.

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