Seeing Cepheids in the X-Ray

Cepheid variables — specifically, Classical Cepheid variables (hereafter Cepheids) — hold a special place in astronomy. This is largely thanks to their period–luminosity relations, which were discovered by Henrietta Swan Leavitt over a hundred years ago. In short, a Cepheid’s period is related to its brightness in a well-defined way, and we can use that knowledge to determine how far away a given Cepheid is if we measure its period. This property of Cepheids has made them critical to measuring large distances in space, which are extremely valuable in astronomy. So, it’s important that we understand Cepheid behavior very well.

Cepheid emission at shorter wavelengths (e.g., ultraviolet, X-ray) is strong when Cepheids are at their smallest. However, observations taken over the last few years have shown that Cepheids have an unexpected increase in X-ray emission when they’re at their largest.

Shocks in Cepheid atmospheres could be behind this surprise X-ray emission. As Cepheids pulsate, their layers can bump against each other, sending shock waves outward into the stars’ atmospheres. There, the shocks could heat the plasma, causing emission at X-ray wavelengths. To explore the possibility of shocks causing X-ray emission, a group of researchers led by Sofia-Paraskevi Moschou (Center for Astrophysics | Harvard & Smithsonian) simulated Cepheids along with the spectra that would be observed from those simulated stars.

To account for all these complexities, Moschou and collaborators used modeling software that could simulate Cepheid properties in great detail. They based their simulations on the prototype Cepheid, δ Cephei, which is five times more massive than the Sun. After simulating Cepheids assuming different underlying conditions, Moschou and collaborators then used their results to create corresponding X-ray spectra and light curves.

It turns out that shocks can produce the observed X-ray emission! However, the varying X-ray emission throughout the Cepheid pulsation cycle suggests that the emission at maximum radius is caused by shocks, while the emission at minimum radius is from another, more consistent process.

δ Cephei is a short-period Cepheid as Cepheids go, so the mechanisms in longer-period Cepheids may present different challenges. As is typical with astronomy, more observations will definitely come in handy!

Source: “Seeing Cepheids in the X-Ray” AAS Nova, 11 November 2020

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