X-Ray Detection of a Cepheid Binary
An artist's depiction of the XMM-Newton spacecraft. Astronomers have used this mission to monitor the X-ray light-curve of a peculiar Cepheid variable star, V473 Lyr, and discovered that it has a low mass binary companion star.
A Cepheid variable star is one whose particular mass and age (along with some of its other physical parameters) result in brightness oscillations with a precise period proportional to the star's intrinsic luminosity. This extraordinarily useful property of Cepheid variables, discovered and roughly calibrated at Harvard by Henrietta Leavitt starting in 1908, allows them to be used as cosmic distance calibrators. By comparing the intrinsic brightness as determined from the period (which is easily measured) with the measured brightness, the period-luminosity relationship, a precise distance can in principle be obtained. Cepheids in nearby galaxies that are receding from us provide the basis for the famous distance-velocity relationship of galaxies that underpins the expanding model of the universe (the "big bang" model). Cepheids are so important that they have also become benchmarks for testing our understanding of stellar evolution.
Precise, long-term photometry of Cepheids from new satellites like Kepler and CoRoT has alerted astronomers to the complexity of Cepheid behaviors, and in particular to the subset that have multiple intensity periodicities. The pulsation cycle in these stars results in disturbances both in the stellar photosphere and in the hot layer just above it, the chromosphere, producing intensity fluctuations with more than one period.
The Cepheid variable star V473 Lyr, in addition to having the usual, very regular Cepheid variability (in its case, a period of 1.49 days) also has long-term variations whose approximate period is 3.3 years. Its variations, however, do not conform to the patterns found in other Cepheids. CfA astronomers Nancy Evans, Scott Wolk, Sofia Moschou, Jeremy Drake, and Vinay Kashyap and their colleagues used the XMM-Newton X-ray satellite to monitor V473 Lyr. They find that its X-ray brightness seems to be relatively stable over the cycle. They conclude that the long-term X-ray variations are not from the Cepheid itself, but most likely are caused by an orbiting, low-mass companion star, about one solar mass in size at an estimated separation of 30-300 AU (one AU is about the average distance of the Earth from the Sun). They note that X-ray discoveries of binaries like this one will help provide a more complete statistical accounting of Cepheid binary stars. In the case of V473 Lyr, the result also helps confirm that it is sun-like in its origins, being relatively rich in heavy elements like other stars in the galaxy's spiral arms.
"X-Ray Observations of the Peculiar Cepheid V473 Lyr Identify a Low-Mass Companion," Nancy Remage Evans, Ignazio Pillitteri, Laszlo Molnar, Laszlo Szabados, Emese Plachy, Robert Szabo, Scott Engle, Edward Guinan, Scott Wolk, H. Moritz Gunther, Hilding Neilson, Massimo Marengo, Lynn D. Matthews, Sofia Moschou, Jeremy J. Drake, Vinay Kashyap, Pierre Kervella, Tamas Tordai, Peter Somogyi, and Gilbert Burki, AJ 2020 (in press).