The object, identified earlier this year as the closest black hole to Earth, has just been demoted. After re-analyzing the data, separate teams of scientists concluded that the system in question, called HR 6819, does not include a black hole after all.
Instead, they found they were two stars with slightly unusual double orbits, making interpretation difficult.
The HR 6819 system, located about 1,120 light years away, has been something of a mystery for a while. It was originally thought to be a single star of spectral type Be.
It is a hot blue-white main-sequence star whose spectrum contains a strong hydrogen emission line, interpreted as evidence of a disk of circumstellar gas ejected by the star as it rotates at an equatorial speed of about 200 kilometers per second.
In the 1980s, astronomers noticed that the object also appears to display the light signature of a second type B-type star, B3 III. It was discovered in 2003 that this means HR 6819 is not one but two stars, although they could not be identified individually.
Further analysis showed that the star B3 III, which has about 6 solar masses, was in about a 40-day orbit, but the star Be, also estimated at about 6 solar masses, appeared to be stationary. If two stars form a binary system of equal masses, they must revolve around a common center of gravity, not one around the other.
After careful calculations, a team of astronomers came to the conclusion that the star B3 III may have orbited another, third object that could not be seen. Around the black hole.
But, as other astronomers have argued, this is far from the only possibility. What if we miscalculated the masses of the stars?
“The presence of the stellar component Be in the spectrum of HR 6819 offers a different interpretation of the system,” wrote astronomers Douglas Gies and Lucian Wang of Georgia State University in their paper.
It is possible that the stellar component B3 III is actually a low mass star that is still relatively young and bright. In this case, the Be star will be a companion in a 40-day binary system, not a black hole. '
In other words, a much lower mass B3 III star will orbit Be. If that were the case, then this orbital motion could be detected in the hydrogen gas surrounding the Be star – it would move almost imperceptibly when pulled by a smaller star. This is what Gis and Wang were looking for.
They carefully studied the emission of hydrogen in the spectrum of the system and found that the hydrogen disk around the star Be does show a 40-day periodicity both in the Doppler shift and in the form of the emission line. This is consistent with the orbit of B3 III – which is to be expected if the system were binary.
'This indicates that HR 6819 is a binary system composed of a massive Be star and a low-mass companion that is the remnant of a former donor star in a mass transfer binary.'
In other words, Be has absorbed a whole bunch of material from B3 III, making it much smaller. As noted by the scientists, recent data indicate that many Be stars are the product of this process. According to their calculations, the Be star has a mass of about 6 solar masses, as previously established; and the star B3 III has a mass between 0.4 and 0.8 solar masses.
So the future of the black hole interpretation looks grim, although it has yet to be decided. Further observation may help resolve any remaining issues.
The study is published in The Astrophysical Journal Letters.
Sources: Photo: HR 6819. (DSS / SIMBAD / CDS)
