Astronomers have discovered a new class of X-ray pulsating variable stars through observations of bright prototype of Classical Cepheids, Delta Cephei.
Located at a distance of 890 light-years from Earth, the Delta Cephei (Delta Cep) is the prototype star after which all Cepheids are named. Cepheids are a famous class of pulsating variable stars and among the most astronomically important objects in the universe.
Cepheids allow astrophysicists to measure distances to other galaxies and calibrate the extragalactic distance scale. Cepheids also play an increasingly vital role in the effort to precisely measure the expansion rate of the universe and to resolve the developing Hubble discrepancy.
Data recently returned for Delta Cep from the Chandra X-ray Observatory, combined with previous X-ray measures secured with the XMM-Newton X-ray satellite, have shown that Delta Cep has X-ray variations occurring in accord with the supergiant star’s 5.4 day pulsation period. X-rays are observed at all phases of the star’s pulsations, but sharply rise by ~400% near the times when the star swells to its maximum diameter of about 45 times that of the Sun.
Delta Cep is a bright star, easily seen without a telescope to the North in the constellation Cepheus. This yellow supergiant star, whose optical brightness variations were discovered in 1784, was one of the first variable stars known. Its light variations are the result of radial pulsations, in which the star contracts and expands with the same 5.4 day period as its brightness variations. The surface of Delta Cep reaches supersonic speeds of about 82,000 miles per hour, while the star shrinks and grows by roughly 2 million miles during each pulsation period. Thousands of Cepheids have been found in our galaxy as well as in other galaxies hundreds of millions of light-years away.
Analyses of the X-ray data indicate the unexpected presence of very hot plasmas in Delta Cep, with temperatures above 10 million degrees Celsius. It is not certain yet whether the X-rays arise from pulsation-induced shock waves in the star’s dynamic atmosphere, or from the generation of a stellar magnetic field that becomes tangled, emitting X-rays. Other Cepheids are being studied to understand the source of the heated, X-ray emitting plasmas. At least two additional Cepheids show potential X-ray variability.
Astronomers involved with the study also used Hubble Space Telescope to study ultraviolet emission lines from Delta Cep and other Cepheids. These emission lines originate in plasmas of up to 300,000 degrees Celsius; cooler than X-ray emitting plasmas but still far hotter than the surfaces of the stars. The ultraviolet emissions also vary in accord with the Cepheids’ pulsation periods but sharply rise after the Cepheid reaches minimum radius, as opposed to the X-ray emissions which peak just after maximum radius. The team is still studying exactly why the ultraviolet and X-ray emissions peak at such different phases of the star’s pulsations.
This discovery of X-rays for Delta Cep and some other Cepheids is the newest in a list of recently discovered Cepheid properties. These include circumstellar gas and dusty environments, infrared excesses, ultraviolet emission lines, and cycle-to-cycle variations in the stars’ periodic light changes.
This combination of discoveries shows that Cepheids, after more than two centuries of study, still have their secrets. Given the astrophysical and cosmological importance of Cepheids, and the high precisions required to test cosmological models, these new discoveries should be better understood. X-ray observations of other bright Cepheids are planned to unravel their X-ray behavior.