Stars like our Sun can go through a mid-life crisis, according to new research carried out by scientists from IISER Kolkata. This can lead to dramatic changes in their activity and rotation rates. The study also provides an explanation for the breakdown of the long-established relation between rotation rate and age in middle-aged sun-like stars. The work has been published in the Monthly Notices of the Royal Astronomical Society: Letters .
At about 4.6 billion years of age, the sun is middle aged, that is, it will continue to live for roughly the same period. There are accurate methods for estimating the age of the Sun, such as by using radioactive dating of very old meteorites that have fallen on the Earth. However, for more distant stars which are similar in mass and age to the Sun, such methods are not possible. One of the methods used is called gyrochronology. There is a relationship between rotation rate and age, that is the rotation rate of a star slows down with age.
When the stellar wind escapes from the star, it carries away with it a part of the angular momentum of the star, which results in its slowing down. The stellar wind has two drivers: one is the high temperature of the outer atmosphere of stars – the corona – which results in an outward expansion and hence plasma winds that emanate out. The other is the magnetic field. “The magnetic field actually heats the corona and so when magnetic activity is strong the winds are strong and since wind carries away the internal (rotational) angular momentum of the star, it slows down its rotation,” explains Dibyendu Nandi, one of the authors of the paper. This is called magnetic braking. As the star ages, due to this mechanism, its rotation slows down and this relationship is used in gyrochronology to estimate the age of the star.
However, there is a breakdown of the gyrochronology relationship, because after midlife, a star's rate of spin does not slow down with age as fast as it was slowing down earlier. Another intriguing fact is that the Sun’s activity level has been observed to be much lower than other stars of similar age. A third observation that is part of the puzzle is that there have also been periods in the past when extremely few sunspots were observed on the Sun for several years at a stretch. For instance, during the Maunder minimum which lasted from 1645 to 1715.
The researchers use the dynamo models of field generation designed to explore long-term activity variations and come up with a theory that can possibly explain the above puzzles. According to a press release by the Royal Astronomical Society, they show that at about the age of the Sun, the magnetic field generation mechanism of stars becomes sub-critical or less efficient. This allows stars to exist in two distinct activity states – a low activity mode and an active mode. The star may thus fall into a low-activity mode and suffer drastically reduced angular momentum loss due to magnetized stellar wind.
“We have a hypothesis, a theory backed by simulation results which seems to self-consistently explain the diverse puzzling behaviour witnessed in middle-aged stars. We have provided a clear demonstration that the theory can explain certain observations, and, therefore, is a leading contender [to throw light on] their origin. In the future, independent observations may reconfirm or deny our theory,” says Prof. Nandi.