HE 0437–5439: A star reborn?
As of 2018, we know of around two dozen stars speeding through space at between 500 and 1500 km/s relative to a local rest frame — faster than the average star by a factor of five or ten. These stars are known as hypervelocity stars, and their existence has only been known with certainty for a little over a decade. These stars are often challenging to study; as their place of birth is not well-known, it can be hard to determine their evolution. Clues like composition, spectral type and their motion today are needed to reconstruct the stories of their lives.
A basic method for generating hypervelocity stars is well-known. Take a binary system close to a supermassive black hole — in the case of the Milky Way, that supermassive black hole corresponds to the radio source Sagittarius A*. Let the stars come close to the black hole. If they wander too close, the pair may be disrupted, with one staying in orbit near the black hole and the other ejected from the area. Under certain conditions, the ejected star can be given an extremely high speed — perhaps high enough to leave the galaxy.
If we observe a hypervelocity star, and determine the kinematics behind its motion, we can determine how long it would have taken it to reach the supermassive black hole that sent it on its way. In most cases, this time is consistent with models of stellar evolution, and the timescales on which stars form and evolve. In the case of one star — HE 0437–5439 — there was a problem: Models showed that the star had only been around for one fifth the time of its putative journey from Sagittarius A*!
A question of age
HE 0437–5439 was originally discovered in 2001, in a survey for B-type stars. B-type main sequence stars are hot and massive, and are very short-lived. It was first studied as a target of interest by Edelmann et al. (2005), who observed it in 2001 and 2002, at La Silla Observatory and Paranal Observatory, respectively, and confirmed it to be the second-known hypervelocity star. Spectroscopic observations at ultraviolet and visual wavelengths determined the star’s surface temperature, surface gravity and metallicity by looking at the presence of certain spectral lines. All matched expectations of normal B-type stars, and models indicated a mass 8 to 8.4 times that of the Sun, with an age of 25 to 35 million years.
Problems arose after the motion of the star was determined. Simulations indicated that the star must have traveled for 99 +/- 19 million years if it originated in the galactic center. Even assuming the largest uncertainties, this was inconsistent with the spectrographic data. Other initial positions in the galactic disk — though less likely — were also simulated. All came up short of matching the stellar models.
The authors came up with two possibilities to avoid the conundrum. The first was that HE 0437–5439 is a blue straggler — a star that has either siphoned off a lot of material from a companion, creating the illusion of a young star, or merged completely with another star, creating a new, more massive, star. Two similar stars, each with masses of approximately 4 solar masses, could survive long enough after an interaction with the supermassive black hole to travel so far before merging. They could have lived for 165 million years — more than enough.
However, this option wasn’t appealing, as it required very specific initial conditions that seemed unlikely. A more promising alternative was that the star was a normal B-type star that originated in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way. HE 0437–5439 is much closer to the LMC than the Milky Way, and could easily have formed there and been ejected by some unknown mechanism.
The abundance puzzle
To verify the LMC origin hypothesis, it would be key to determine how the star was ejected in the first place. There is no supermassive black hole known to exist in the Large Magellanic Cloud, and it seems likely that if there was one, if would already have been detected through its effects on neighboring stars and gas. However, there was still the possibility that a slightly less massive black hole — a hypothetical intermediate-mass black hole — could be responsible. While Sagittarius A* is 4 million times the mass of the Sun, an intermediate-mass black hole could be less than 1% as massive as that — a mere 1,000 or 10,000 solar masses.
Gualandris & Zwart (2007) attempted to determine the feasibility of this model. They found that a black hole on the order of 1,000 solar masses would be sufficient to eject a hypervelocity star — and, moreover, that several star clusters in the LMC could hold one. Masses in the range 1,600 to 2,200 solar masses were possible. This would have ejected HE 0437–5439 some 20 million years ago, consistent with the age measured through spectroscopy.
More evidence for an origin in the Large Magellanic Cloud was found by measuring key elemental abundances in the star. Pryzbilla et al. (2008) compared HE 0437–5439 with two other B-type stars, NGC 2004-D15 in the LMC and HR 3468 near the Sun. The hypervelocity star appeared to fall between the two in measurements of certain heavier elements (including isotopes of carbon, oxygen, and nitrogen). However, when compared to a selection of stars from the galactic center, it became clear that HE 0437–5439 was much different — having, on the whole, lower abundances, even taking into account uncertainties. It matched NGC 2004-D15 much better, and was even somewhat similar to the star in the solar neighborhood. The authors concluded that some unknown mechanism must have ejected it from the LMC.
Kinematics investigations, and a strange straggler
A couple years later, however, another group appeared to have ruled out the LMC hypothesis entirely. Brown et al. (2010). They analyzed images from the Hubble Space Telescope taken in 2006 and 2009, and found that the star was moving directly away from the galactic center — not the LMC. The supermassive black hole hypothesis suddenly seemed a lot more plausible. The abundance problem remained, but the authors stated that blue straggler abundances can vary, as material is dredged up from the inner layers of the progenitors. They also ruled out a distinct possibility — that HE 0437–5439 is a post-main sequence giant star of some sort — because such phases are extremely short, and because its rotation was inconsistent with models of these particular stars.
The narrative Brown et al. pieced together is a slightly more complicated one than the traditional tale of hypervelocity star evolution. Instead of a binary system, a three-star system must have interacted with Sagittarius A*. Two of the components were ejected at a high speed; it was these two stars that then merged to form HE 0437–5439. The resulting blue straggler appears to be quite young, but in reality, it is the result of two older stars merging.
I’m reluctant to say that these observations have, with certainty, settled the dispute. After all, earlier groups were certain that the abundance discrepancies ruled out an origin in the galactic center — and, indeed, the abundance issue has yet to be conclusively solved. However, the observations by Brown et al. provide strong support for the blue straggler hypothesis, and, for the time being, appear to be the best explanation for the strange saga of HE 0437–5439.
