Studying hot Stars with disks

All stars are not created equal. When you look out into the night sky, you are seeing all sorts of unique and interesting objects. Some stars are small and cool (at least, compared to our Sun), and live for many billions of years. Others have evolved and inflated to enormous sizes- even over 1,000 times the size of our sun. There is a class of bright, blue stars called “Classical Be stars” that are between about 5 – 20 times more massive than the sun, and spin so quickly that they are nearly torn apart by the resulting centrifugal force. These stars also have disks that grow and shrink, appear and disappear. Classical Be stars are unique in astronomy, because their disks originate from the stars themselves. Material from the surface of the star is flung outward with so much speed (and angular momentum) that it is launched into orbit, and then settles into a disk in an event called an “outburst”. Lehigh physics professor Joshua Pepper and graduate student Jonathan Labadie-Bartz are studying these objects because there is still much that is unknown, especially regarding the physical mechanisms behind outbursts. The header image shows an artist’s rendition of a Be star and its disk.

Plot of Be star Lehigh University
Figure 2:  Plot of a Be star with two outbursts, as observed by the KELT telescope survey.

As a Classical Be star experiences changes, whether it be an outburst or a shrinking disk, the amount of emitted light will change too. If you look at these stars every night for many years, and record how their brightness is changing, then you can read these signals and interpret what the star is doing. This is the main idea behind the research. The brightness measurements come from the Kilodegree Extremely Little Telescope (KELT), directed by Prof. Pepper. KELT is a survey that has been monitoring large patches of the night sky every clear night for the last ten years, and is being used to discover new planets orbiting these distant stars. So far, KELT has observed over 4.4 million stars. These show lots of interesting behavior, providing many opportunities to explore new science, including the study of Classical Be stars.

Pepper and Labadie-Bartz have analyzed the changes in brightness of over 600 Classical Be stars, and in doing so have uncovered a large variety of variability patterns. They show that many of these stars are pulsating, which means the entire star is vibrating (like a steady heartbeat). Many of the stars show rapid brightening, indicating the creation or building up of a disk. However, the rates of outbursts vary wildly. Some stars had only one outburst every few years (or none at all), while others showed up to 20 outbursts per year. Figure 2 shows a plot of the brightness of one such star over the course of eight years, with the arrows indicating outbursts.  Still others have signals that hint that the Classical Be star is a member of a binary pair: two stars that are orbiting each other.

The results have been submitted for publication in the Astrophysical Journal, with Labadie-Bartz as the lead author, and posted the draft paper online. They are continuing to investigate Classical Be stars by combining their brightness measurements with other types of data (such as spectroscopy). This research is illuminating how strange parts of our universe behave.”

Header image:  Artist’s rendition of a Be star with its disk in the upper right.  Image from here, with credit to Bill Pounds.