For the first time, astronomers have observed a star that has entered a state of low, or flat, activity – analogous to the famous Maunder Minimum that gripped the Sun during the latter half of the seventeenth century.
Anna Baum, of Penn State University and Lehigh University, led a survey of 59 approximately Sun-like stars, combining over 50 years’ worth of observations drawn from the likes of the Mount Wilson Observatory HK Project that ran from 1966 until 2003 (though Baum’s team did not have access to data post 1995), and the California Planet Search at Keck Observatory that’s been running since 1996.
Baum’s team was searching for signs of magnetic activity on the stars – the same activity that produces sunspots, flares and coronal mass ejections on our Sun. They did so by observing absorption lines of ionized calcium in each star’s spectra. In particular, the calcium H and K spectral lines are sensitive to the strength of the magnetic field. This is quantified using the so-called S-value, which is a measure of the strength seen in the H and K lines. A higher S-value indicates a more magnetically active star.
Comparing these stars to the Sun enables astronomers to better determine how typical – or not – the Sun is as a star. The Sun’s magnetic activity is defined by its 11-year sunspot cycle. Of the 59 stars that Baum’s team surveyed, 29 appeared to also have starspot cycles, and the period of those cycles could be measured for 14 of them.
“Of these 14 stars, the average length of their cycle is just under 10 years, which is similar to the Sun’s 11-year cycle,” Baum tells Physics World. However, not all the stars adhered to this time frame. One star that was surveyed has a cycle lasting less than four years, while another star, HD 166620, had a cycle 17 years long.
Note the past tense. Sometime between 1995 and 2004, HD 166620’s starspot cycle simply stopped.
The star that switched off
The uncertainty in the timing is because it occurred during an instrument change – the transition from the Mount Wilson survey to the California Planet Search, which uses the HIRES spectrometer, designed not only to search for the radial velocity signals of exoplanets, but also to take more complex spectra of stars. The HIRES instrument received an upgrade in 2004, after which the flat activity of HD 166620 became apparent.
Despite this, “HD 166620 has been shown to have flat activity for over ten years,” says Baum. And although the beginning of this flat activity was missed, “It’s exciting to see exactly when it might return to its cycle.”
This return to activity may be some time coming. The Sun’s Maunder Minimum lasted from 1645 and 1715, during which its solar activity almost entirely fell off a cliff. Between 1672 and 1699, fewer than 50 sunspots were observed. Contrast that with the Sun’s normal magnetic activity: even at the minimum in the Sun’s 11-year cycle there are ordinarily a dozen or so sunspots per year, and at maximum, over 100.
The Maunder Minimum “was as though the Sun’s magnetic cycle mostly turned off for a period of about 70 years, then turned on again,” says Ricky Egeland of NASA’s Johnson Space Center, who was not involved in the Baum study.
Rotational reasons
Exactly what causes Maunder Minimum-type events remains unclear, though Egeland’s previous work found some clues. In a study with Travis Metcalfe of the Space Science Institute in Arizona, and Jennifer van Saders of the University of Hawaii, he showed that the rotation of a star is key to understanding Maunder Minimum events.
Stars begin life spinning fast, and gradually slow down over their lifetime, which in some way affects their dynamo that generates their magnetic field. As they age, Maunder Minimum events may become more regular, until “at some point in a star’s rotational evolution, they enter a grand minimum and never return [to an active cycle],” explains Egeland. Hence studying stars such as HD 166620, which are near this transition point, will help astronomers to better say when the Sun’s magnetic dynamo may shut down permanently. “It might take another billion years or so,” Egeland notes.
What’s going on with the Sun?
Of the other stars in the survey, some displayed chaotic activity, while others had none at all. One star, HD 101501, completely shut down its magnetic activity between 1980 and 1990 before reactivating again. And other surveys have found similarly lethargic stars. These include the star HD 140538, where a transition from flat activity to cycling was observed, and HD 4916, where the magnetic cycle became progressively weaker, but didn’t shut off. However, “HD 166620 is, to my knowledge, the first that we have observed to have clearly just entered its minimum,” says Baum.
Further monitoring to try to catch HD 166620 returning to activity will now take place. The flat activity “might be visible in the overall brightness of the star, which is something we’re looking into,” says Jason Wright, one of Baum’s co-authors from Penn State University.
The team publish the results in The Astronomical Journal.
