Stellar black holes typically have masses of about ten times that of the Sun. But a Chinese team of astronomers recently claimed to have discovered a black hole with 70 solar masses, which, if proven, would revolutionise the current view of stellar evolution.
The news spurred other astrophysicists to take a closer look at the object.
At first glance, the spectrum did indeed look like one from a young massive star
Dr Andreas Irrgang
Now, astronomers from the Universities of Erlangen-Nürnberg and Potsdam believe the object may not necessarily be a black hole at all, but could potentially be a massive neutron star or even an ordinary star.
The putative black hole was detected indirectly from the motion of a bright companion star, orbiting an invisible compact object over a period of about 80 days.
From new observations, a Belgian team revealed the original measurements were misinterpreted and the black hole’s mass is, in fact, very uncertain.
A crucial aspect is the mass of the visible companion, the hot star LS V+22 25.
The more massive this star is, the more massive the black hole has to be to induce the observed motion of the bright star.
The latter was considered to be a normal star, eight times larger than our solar system’s Sun.
The star has become famous for possibly having a massive black hole companion.
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However, a closer look at the star itself reveals it is a fascinating object in its own right, because only very few stripped helium stars have so far been discovered.
Astronomers from Friedrich-Alexander-Universität Erlangen-Nürnberg and the University of Potsdam made a closer investigation of the archival spectrum of LS V+22 25, taken by Hawaii’s Keck telescope.
In particular, they were interested in studying the abundances of the chemical elements on the stellar surface.
The team detected deviations in the abundances of helium, carbon, nitrogen, and oxygen compared to the standard composition of a young massive star.
The observed pattern on the surface showed ashes resulting from the nuclear fusion of hydrogen, a process only associated with the deep core of young stars.
Dr Andreas Irrgang, the leading scientist of this study and a member of the Dr. Karl Remeis-Observatory, said: “At first glance, the spectrum did indeed look like one from a young massive star.
“However, several properties appeared rather suspicious. This motivated us to have a fresh look at the archival data.”
The authors conclude LS V+22 25 must have interacted with its compact companion in the past.
During this episode of mass-transfer, the outer layers of the star were removed and now the stripped helium core is visible, enriched with the ashes from the burning of hydrogen.
However, stripped helium stars are much lighter than their normal counterparts.
Combining their results with recent distance measurements from the European Space Agency’s Gaia space telescope, the authors determined a most likely stellar mass of only 1.1 times that of the Sun.
This yields a minimum mass of only two or three solar masses for the compac
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