A well-studied binary system called Alpha Draconis exhibits primary and secondary eclipses, according to an analysis of data from NASA’s Transiting Exoplanet Survey Satellite (TESS).
Alpha Draconis (circled). Image credit: NASA / MIT / TESS.
Alpha Draconis, also known as Thuban, HR 5291, HD 123299, and HIP 68756, is located about 270 light-years away in the northern constellation of Draco.
“Despite its ‘alpha’ designation, it shines as Draco’s fourth-brightest star,” said Dr. Timothy Bedding from the University of Sydney and Aarhus University and his colleagues.
“Thuban’s fame arises from a historical role it played some 4,700 years ago, back when the earliest pyramids were being built in Egypt.”
“At that time, it appeared as the North Star, the one closest to the northern pole of Earth’s spin axis, the point around which all of the other stars appear to turn in their nightly motion.”
“Today, this role is played by Polaris, a brighter star in the constellation Ursa Minor.”
“The change happened because Earth’s spin axis performs a cyclic 26,000-year wobble, called precession, that slowly alters the sky position of the rotational pole.”
While inspecting light curves of Alpha Draconis obtained with TESS, the team noticed that the system shows clear and well-separated primary and secondary.
“Alpha Draconis ranks among the brightest-known eclipsing binaries where the two stars are widely separated, or detached, and only interact gravitationally,” the researchers said.
“Such systems are important because astronomers can measure the masses and sizes of both stars with unrivaled accuracy.”
The team’s paper was published in October 2019 in the Research Notes of the AAS.
In a separate study, Villanova University postdoctoral researcher Angela Kochoska and her colleagues attempted to understand the Alpha Draconis system in greater detail.
“As known from earlier studies, the stars orbit every 51.4 days at an average distance of about 61 million km (38 million miles), slightly more than Mercury’s distance from the Sun,” they explained.
“The current preliminary model shows that we view the system about three degrees above the stars’ orbital plane, which means neither star completely covers the other during the eclipses.”
“The primary star is 4.3 times bigger than the Sun and has a surface temperature around 9,700 degrees Celsius (17,500 degrees Fahrenheit), making it 70% hotter than our Sun. Its companion, which is five times fainter, is most likely half the primary’s size and 40% hotter than the Sun.”
“Discovering eclipses in a well-known, bright, historically important star highlights how TESS impacts the broader astronomical community,” said TESS project scientist Dr. Padi Boyd, from NASA’s Goddard Space Flight Center.
“In this case, the high precision, uninterrupted TESS data can be used to help constrain fundamental stellar parameters at a level we’ve never before achieved.”
The team presented the findings January 6 at the 235th Meeting of the American Astronomical Society in Honolulu, Hawai’i.
_____
Timothy R. Bedding et al. 2019. A Dance with Dragons: TESS Reveals α Draconis is a Detached Eclipsing Binary. Res. Notes AAS 3, 163; doi: 10.3847/2515-5172/ab5112
A. Kochoska et al. 2020. Fitting in the wild: exploration of new approaches and methods for estimating binary system parameters from light curve data. 235th AAS Meeting, abstract # 114.03
K. Conroy et al. 2020. New Physics and Features in the 2.2 Release of the PHOEBE Eclipsing Binary Modeling Code. 235th AAS Meeting, abstract # 114.05
