Astronomers have discovered a type of very small, hot star which brightens and dims very often as the outer layers try to maintain an equilibrium. These stars are named as hot subdwarf pulsators and they might be related to a recently discovered rare star known as blue large-amplitude pulsator. The paper has been published in The Astrophysical Journal Letters.
Physicist Thomas Kupfer, Kavli Institute for Theoretical Physics, UC Santa Barbara said that there are many stars that pulsate including the Sun on a small scale. The ones having the largest brightness changes are known as radial pulsators which breath in and out as the size of the star changes. Although our Sun pulsates, its cycle is 11 years and the variation in brightness is only by 0.1 percent in the entire timeframe, hence it is not a pulsator in the truest sense.
The brightness of the pulsators often vary by as much as 10 percent due to size and temperature change.
Researchers identified four new stars from Zwicky Transient Facility survey that pulsate on timescales between 200 and 475 seconds with variation in brightness upto 5 percent. This change in brightness occurs in eclipsing binaries thus it was ruled out before being classified as a new type. Researchers then realised it to be subdwarf B stars.
Subdwarf B stars are pretty small, about 10 percent of the sun’s size although very dense. In their small diameter, they pack in almost 20 to 50 percent of the mass of Sun. They are very bright as they burn at 20000 to 40000 Kelvin. It is considered that they form along a star’s evolutionary path upto eight times the mass of Sun when it dies.
As they run out of hydrogen to fuse in cores, they fuse helium turning to a red giant. subdwarf B star is what happens when the outer hydrogen layers of a red giant are stripped away before helium fusion begins – possibly by a binary companion, but the exact mechanisms are unknown.
The V361 Hya class have a mode of pressure oscillation meaning that their pulsations are produced by the star’s internal pressure fluctuations. The V1093 Her class are gravity-mode pulsators that are produced by gravity waves. (not to be confused with gravitational waves).
Researchers are trying to find the exact mechanism of the oscillations of hot subdwarf pulsators although some believe it might be due to the unstable radial modes that are produced by iron kappa mechanism where iron buildup in star creates an energy layer resulting in pulsation. Subdwarf B stars are normally considered to be fusing helium, in their core or shell around the core. However, scientists believe that hot subdwarf pulsators lost the outer material before the helium was hot and dense.
Physicist Evan Bauer explained that rapid pulsations are understood by matching them to theoretical models where the low mass cores are composed of cold helium. The next step is to understand the actual mechanism behind pulsation and where the stars fit in the stellar evolution.
Journal Reference:The Astrophysical Journal Letters