A team of researchers affiliated with the Swarthmore physics and astronomy department has discovered a companion star to Beta Crucis, the nineteenth brightest star in the sky.
Led by Associate Professor of Astronomy David Cohen, the research team includes fellow astronomy professor Eric Jensen, Michael Kuhn ’07 and West Chester University Assistant Professor of Astronomy Marc Gagné. Jensen and Cohen launched the project two years ago, “but it really got off the ground last summer when Mike Kuhn started working on it,” Cohen wrote in an e-mail. The team will submit a paper on its findings to The Monthly Notices of the Royal Astronomical Society for publication.
According to the Jan. 9 press release, the discovery was made largely by accident. The project undertaken by Cohen, Jensen and Gagné originally aimed “to study how the X-ray emission properties of very luminous, massive stars change as one looks from the absolutely most luminous ones … to somewhat less luminous ones,” Cohen wrote.
The emission data of Beta Crucis was especially pertinent to Cohen’s field of expertise. “[Cohen] had originally been interested in [Beta Crucis] because he studies X-rays from hot stars,” Kuhn said. “More massive stars,” like Beta Crucis, “also tend to be hotter and more luminous.”
When they noticed the significantly less luminous star in the vicinity of the Beta Crucis complex, the research team developed “the hypothesis that this was a companion star, first of all, and then that it’s a pre-main-sequence star,” Kuhn said.
Pre-main-sequence stars are still developing, unlike Beta Crucis, which is nearing the end of its life as a main-sequence star.
Using instruments such as the Advanced CCD Imaging Spectrometer and the High Energy Transmission Grating Spectrometer in conjunction with NASA-designed X-ray data analysis software, the team downloaded X-ray data from the Chandra space telescope to computers in the Science Center astronomy lab.
Companion stars are often observable using less complex data collection techniques than those involved in “characterizing X-ray properties” of main-sequence star beta Crucis, which was the principal goal of data analysis, according to Cohen.
At least one other companion star to Beta Crucis had already been identified prior to this project’s undertaking. Over the course of data collection, the research team “noticed this other object, which happened to not have been noticed before … This was really odd that it hadn’t been discovered before,” Kuhn added.
The “other object” is most likely a companion star to Beta Crucis, which means that the two stars are gravitationally bound and both are around eight to 11 million years old. “We think they’re gravitationally bound, but we need further observations to actually prove that,” Kuhn said. “If they’re moving in the same direction then they’re probably bound. If they’re not, then they can’t be bound,” Kuhn said. Future experiments are currently in the planning stages.
The project was jointly sponsored by the college and the federal government. In addition to a Eugene M. Lang Summer Research Fellowship granted by Swarthmore’s administration, Cohen’s research team received funding from the National Aeronautics and Space Administration.
Cohen submitted a four-page proposal to NASA after an annual call for applications by the Chandra X-Ray Observatory Center based in Cambridge, Massachusetts. According to Cohen, proposals are evaluated and ranked by a panel of scientists, and the highest-ranking applications are selected to receive funding.
Companion stars are studied because more information about their properties can lend significant insight into the nature of star formation.
According to Kuhn, pre-main-sequence stars, such as the newly discovered companion to Beta Crucis, “are kind of interesting and popular, because they have to do with star formation.”
Scientists are intrigued by the process of star formation, which mimics planetary formation. In addition, studying the emissions of luminous stars like Beta Crucis can illuminate patterns produced by interaction of matter with strong radiation, such as that transmitted by X-rays.
According to Cohen, the Beta Crucis complex has garnered a level of “cultural interest.” Visible only from the southern hemisphere, the complex is featured prominently on the national flags of Australia, Brazil, New Zealand, Papua New Guinea and Samoa.
The identification of a previously unmapped companion star “is a reminder that there are still many things left to discover in nature,” Cohen said.
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