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Getting to Know Our Nearest Neighbors with ALMA

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Image of two bright blue stars side by side and one dimmer circled star below, against a background of more distant stars.
Alpha centauri is the nearest star system to us. In the image above, the binary system Alpha centauri AB is the bright source on the left; Beta Centauri is the bright star on the right. Proxima Centauri is the faint star circled in red below. [Skatebiker]

Don’t plan to knock on a door at Alpha Centauri asking to borrow a cup of sugar just yet — but with a new look at this star system using a powerful telescope, we now know a bit more about our neighbors.

Secrets Among Nearby Stars

Proxima Centauri b

This artist’s interpretation shows the planet Proxima Centauri b around its host star. You can see the binary α Cen AB in between the planet and star, as two faint white dots in the background. [ESO]

When it comes to exploring Sun-like stars that might host planets, the Alpha Centauri (α Cen) star system is an ideal target. At just over 4 light-years away, this triple system — the binary pair α Cen AB and an additional companion, Proxima Centauri — contains the closest stars to the Sun.

Recent news has hyped the discovery of two exoplanets around the red dwarf Proxima Centauri — but what other surprises might the larger stellar system harbor? Given that α Cen A and B are both very similar to the Sun, it would be particularly valuable if we could find Earth-like, potentially habitable worlds around these near neighbors.

Choosing a Method

But how to detect them? Searching for transits works only for very specific orbit orientations. Direct imaging might be an option, since α Cen is so close. But even these nearby stars are challenging: α Cen A’s habitable zone lies at about 1.2 au, or just 0.9” in angular separation, from our point of view. It’s hard to confidently detect a small, dim object at that separation!

Four images of a single source in the center of a frame.

Calibrated images of α Cen A and B taken with ALMA in October 2018 (left two panels) and in August 2019 (right two panels). [Adapted from Akeson et al. 2021]

Another method may prove useful in this case, however: astrometry. In a new study, a team of scientists led by Rachel Akeson (NASA Exoplanet Science Institute, Caltech-IPAC) has used the high resolving power of the Atacama Large Millimeter/submillimeter Array (ALMA) to make some of the most precise astrometric measurements of α Cen AB yet.

Hints of Influence

Astrometry relies on the idea that the slight gravitational tug of an orbiting planet causes a star to “wobble” in place. If this effect is large enough, we can detect it via meticulous imaging that very precisely tracks the location and motion of the star on the sky over time.

Taking advantage of the high-resolution observations provided by ALMA’s long baseline, Akeson and collaborators captured measurements of α Cen A and B during 2018 and 2019. Their results provide the first high-accuracy absolute measurements of the stars’ positions on the sky since 1991, as well as the highest-accuracy differential astrometry yet, comparing their relative separation and searching for the tiny influence of planets around the two stars. 

Four-panel plot showing the best-fit orbit for the binary stars, a closeup of the data on the model, and the residuals.

Top left: astrometric measurements (red: Hipparcos and ALMA data, blue: archival data) and best-fit orbit of α Cen B relative to α Cen A. Top right: enlargement of the 2019 ALMA measurements (the total orbit takes ~80 years). Bottom: residuals of the fit as a function of time. [Akeson et al. 2021]

The authors then combine these results with archival data to better constrain α Cen AB’s orbit and properties.

A Promising Future

Akeson and collaborators show that ALMA can produce remarkably precise astrometric measurements for the α Cen system, demonstrating the exciting potential of using ALMA for this technique. Though the observations don’t reveal signs of a planet yet, continued monitoring should allow us to ultimately be able to detect planets of a few tens of Earth masses in stable orbits between 1 and 3 au around α Cen A.

But these results go beyond our search for planets — they also refine our measurements of α Cen’s motions. This allows us to make more accurate estimates of the physical properties of α Cen A and B, filling in our understanding of our nearest neighbors.

Citation

“Precision Millimeter Astrometry of the α Centauri AB System,” Rachel Akeson et al 2021 AJ 162 14. doi:10.3847/1538-3881/abfaff