Removing the hydrogen-rich layers from a main sequence star exposes the helium-rich core. Such stripped helium stars are known at high and low masses, but not at intermediate masses, despite theoretical predictions that they should be common. In a new study, astronomers from the University of Toronto and elsewhere used ultraviolet photometry to identify candidate stripped helium stars in two nearby dwarf galaxies — the Large and Small Magellanic Clouds — and then observed 25 such candidate stars with optical spectroscopy. Most of these systems were shown to be binary systems, in which the companion stars probably stripped the outer hydrogen-rich layers off the helium stars.
An artist’s impression of a massive binary system. Image credit: ESO / M. Kornmesser / S.E. de Mink.
The hydrogen-rich outer layers of massive stars can be removed by interactions with a binary companion.
Theoretical models predict that this stripping produces a population of hot helium stars of 2 to 8 solar masses, however, only one such system has been identified until now.
“This was such a big, glaring hole. If it turned out that these stars are rare, then our whole theoretical framework for all these different phenomena is wrong, with implications for supernovae, gravitational waves, and the light from distant galaxies,” said Dr. Maria Drout, an astronomer at the University of Toronto.
“This finding shows these stars really do exist.”
“Going forward, we are going to be able to do much more detailed physics with these stars.”
“For example, predictions for how many neutron star mergers we should see are dependent on the properties of these stars, such as how much material comes off of them in stellar winds.”
“Now, for the first time, we’ll be able to measure that, whereas people have been extrapolating it before.”
Dr. Drout and her colleagues designed a new survey to look in the ultraviolet part of the spectrum where extremely hot stars emit most of their light.
Using data from the Swift Ultra-Violet/Optical Telescope, the astronomers collected brightnesses for millions of stars in the Large and Small Magellanic Clouds, two of the closest galaxies to Earth.
They developed the first wide-field UV catalog of the Magellanic Clouds and used UV photometry to detect systems with unusual UV emissions, signaling the possible presence of a stripped star.
They carried out a pilot study of 25 objects, obtaining optical spectroscopy with the Magellan Telescopes at Las Campanas Observatory between 2018 and 2022.
These stripped stars had high temperatures (60,000 to 100,000 K), high surface gravities, and hydrogen-depleted surfaces; 16 stars also showed binary motion.
Dr. Drout and co-authors propose that these stars will eventually explode as hydrogen-poor supernovae.
These objects are also thought to be necessary to form neutron star mergers, like those that emit gravitational waves detected from Earth by the LIGO experiment.
In fact, the researchers believe that a few objects in their current sample are stripped stars with neutron star or blackhole companions.
These objects are at the stage immediately before they become double neutron star or neutron star plus blackhole systems that could eventually merge.
“Many stars are part of a cosmic dance with a partner, orbiting each other in a binary system,” said Bethany Ludwig, a Ph.D. student at the University Toronto.
“They’re not solitary giants but part of dynamic duos, interacting and influencing each other throughout their lifetimes.”
“Our work sheds light on these fascinating relationships, revealing a universe that is far more interconnected and active than we previously imagined.”
“Just as humans are social beings, stars too, especially the massive ones, are rarely alone.”
The results appear in the journal Science.
M.R. Drout et al. 2023. An observed population of intermediate-mass helium stars that have been stripped in binaries. Science 382 (6676): 1287-1291; doi: 10.1126/science.ade4970
Source : Breaking Science News