A new weird pulsing star discovered by astronomers. A very small and hot star that shines and dims every few minutes as its exterior layers attempt to maintain equilibrium.
The progenitor of a type of star that is completely new to science could be a star with an interrupted death sequence. Astronomers have just found a sort of very small, very hot star that shines and dims every few minutes as its exterior layers attempt to maintain equilibrium.
The stars have been named hot subdwarf pulsators and could be related to another type of newly discovered, rare and mysterious star: the blue large-amplitude pulsator.
What’s so odd about these pulsating stars?
According to physicist Thomas Kupfer of the Kavli Institute for Theoretical Physics at UC Santa Barbara “Many stars pulsate, even on a very tiny scale our Sun does. Those with the great changes in brightness are generally radial pulsators that breathe in and out as the size of the whole star changes.”
But even though our Sun pulsates, its cycle is 11 years, and over that timeframe, it only differs in brightness by 0.1 percent, so it would not be deemed a pulsator. Due to changes in size and temperature, the brightness of pulsators may differ by up to a huge 10 percent.
In data from the Zwicky Transient Facility survey, the four new stars the team recognized pulsate on time scales between every 200 and 475 seconds, varying by about 5 percent in brightness.
Such a change in brightness can be generated by eclipsing binaries, so this must be excluded before it can be classified as a new type. Once that was achieved by the research team, they realized that they could look at a new class of subdwarf B star.
The Subdwarf B stars are interesting. They are fairly small, size-wise. Around 10% of the Sun’s size. But they are pretty dense. They squeeze between 20 and 50% of the mass of the Sun into that small diameter.
They burn very hotly, between 20,000 and 40,000 Kelvin towards the blue end of the spectrum. So they’re very, very bright as well. It is believed that they form up to eight times the Sun’s mass as it dies along the evolutionary path of a star.
When these stars run out of hydrogen to fuse in their cores, they begin to fuse helium into a red giant. A subdwarf B star is what occurs when a red giant’s outer hydrogen layers are taken off before helium fusion starts -perhaps by a binary companion, but the precise mechanisms are unknown. So, you’ve got a hot, dense, tiny blue star there. And some of them are pulsating.
The V361 Hya class has a mode of pressure oscillation, which implies that the changes in internal pressure in the star produce their pulsations. The V1093 Her class are pulsators of gravity-mode, generated by gravity waves (not to be confused with gravitational waves).
The scientists are still looking into the exact mechanism behind the hot subdwarf pulsator oscillations but believe it may be unstable radial modes generated by something called the iron kappa mechanism, whereby an iron buildup in the star generates an energy layer that results in a pulsation.
They also think that what’s happening in their core might be another difference. Subdwarf B stars are usually regarded as fusing helium, either in their core or a shell around their core. But scientists think hot subdwarf pulsators lost their surface material before the helium was hot and dense enough for fusion.
“By combining them with theoretical models with low mass cores made of comparatively cold helium, we were able to comprehend the fast pulsations,” clarified physicist Evan Bauer of UC Santa Barbara.
The scientists also found that the pulsation resembles that of blue large-amplitude pulsators, a sort of star that was just discovered in 2017. That implies you could relate the two kinds of stars.
The next stage will be to further characterize what is happening within these stars that generate the pulsations and to determine exactly where the stars fit into stellar evolution models.
The study was released in The Astrophysical Journal Letters.