A careful study of observations spanning decades has revealed that a star in Gemini is regularly eclipsed by a disk-shrouded companion.

A huge disk of dust likely surrounds the close companion of the naked-eye star Propus, in the constellation Gemini, astronomers report in a new study. The discovery of the disk, which resembles a similar structure orbiting the star Epsilon (ε) Aurigae, was made possible in part by amateur variable star observers.

Propus (Eta Geminorum or η Gem, formerly also known as Tejat Prior) is a third-magnitude red giant that marks the foot of Castor, one of the heavenly Twins. Due to slow pulsations, the star shows semiregular brightness variations with a period of 230 days or so. In 1887, Sherburne Burnham discovered a 6th-magnitude companion, orbiting the giant once every 470 years; at present, the separation is 1.8 arcseconds.

However, the newly discovered dust disk sits around a second companion star that’s much closer in. This star can’t be seen; its existence was first deduced in 1901 from periodic shifts in the spectral lines of the wobbling primary, due to the Doppler effect.

Guillermo Torres and Kristy Sakano (Center for Astrophysics, Harvard & Smithsonian) have now analyzed 11.5 years of spectroscopic data on Propus, obtained with 1.5-meter telescopes at Oak Ridge Observatory in Massachusetts and Whipple Observatory in Arizona. Combined with data from the 1930s and 1940s, these observations yield a precise orbital period for Eta Gem and its companion of 2,979 days, or 8.2 years. The orbit is very oblong, with an eccentricity of 0.55.

Torres and Sakano use this and other information to derive the main properties of the stars, concluding that the primary is a bloated star 275 times as large, 10,000 times as luminous, and 5.1 times as massive as the Sun. The secondary is significantly smaller, about 2 solar masses and only 1.65 times as wide as the Sun. The results have been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

“I really enjoyed the paper,” comments Steve Howell (NASA). “It is back to fundamental astrophysics on an interesting and unusual binary system – about as good as it gets.”

But there’s an interesting twist to the story. Apart from the semiregular variations mentioned above, amateurs have long reported periodic dimmings of the star by about half a magnitude, once every 8.2 years. It looks like the companion eclipses Propus once every orbit, although some professional astronomers remained unconvinced.

Armed with the new, precise orbit, which exactly predicts when eclipses might occur, Torres and Sakano analyzed tens of thousands of brightness estimates over the past decades from members of the American Association of Variable Star Observers (AAVSO) and similar organizations in France, the United Kingdom, and Japan. “[We] identified many instances where light minima were close to the predicted times of eclipse,” they write. “This leaves no doubt as to the eclipsing nature of Eta Gem.”

However, the observed eclipses last for something like five months. Given the physical size and orbital speed of Propus’s companion star, it’s impossible that the star alone can produce such long-lasting obscurations. The inevitable conclusion is that the companion must be surrounded by a huge disk of light-absorbing material, measuring at least 225 million kilometers across (1.5 times the Sun-Earth distance).

Astronomers have found a handful of other examples of giant stars orbited by relatively long-period companions that are also surrounded by huge dust disks, the most famous example being Epsilon  Aurigae, which has an orbital period of 27.1 years. Besides Epsilon Aur and Eta Gem, Torres and Sakano list nine others, including EE Cephei and TYC-2505-672-1, with periods of 5.6 and 69.1 years, respectively. Howell, who took part in a study of ε Aur, knows of two more: XX Ophiuchi and AS 325 in Sagittarius.

“I believe that Torres and Sakano have done a great job, and the idea of a large disk orbiting the secondary star seems right to me,” Howell says. Probably, as in the other systems, the disk consists of material that has been ejected by the rapidly evolving and pulsating red giant, and captured by the companion star. “Thus, I'd believe these systems are likely very common, but require detective work such as this paper along with the tremendous long-term efforts of variable star observers to uncover,” he says.

There’s no doubt that future eclipses of Propus will be meticulously observed. If you want to help solve the star’s secrets, mark your calendar: the next event will play itself out between late October 2028 and early March 2029, with maximum eclipse predicted for January 1, 2029.

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