Polaris is the closest and brightest Cepheid variable star. Something has changed recently

When you look up into the night sky and toward the North Star, you are looking at Polaris. Not only is it the brightest star in the constellation Ursa Minor (Little Dipper), but its position relative to the north celestial pole (within 1°) makes it useful for orientation and navigation. Since the modern era of astronomy, scientists have understood that the star is a binary system consisting of a yellow F-type supergiant (Polaris Aa) and a smaller main-sequence yellow dwarf (Polaris B). . Further observations revealed that Polaris Aa is a classic variable in the Cepheids, a class of regularly moving stars. During much of the 20th century, records indicate that pulse period increased while pulse amplitude decreased. But recently this has changed as the pulse period started to shorten while the amplitude of the rate variations stopped increasing. According to a new study published on the preprint server arXiv by astronomer Guillermo Torres at the Harvard & Smithsonian Center for Astrophysics (CfA), these behaviors may be due to long-term changes associated to the binary nature of the system, where the two stars come closer. , and the secondary disrupts the atmosphere of the primary.

Cepheid variables are stars that move in a radial direction, causing their diameter and temperature to change. These pulses are directly related to changes in their brightness, making them a useful tool for measuring galactic and extragalactic distances. The changing nature of Polaris was confirmed in 1911 by the Danish astronomer Ejnar HertzsDaprung, for whom the Hertzsprung-Russell diagram is partly named. Observations conducted throughout the 20th century have shown that Polaris has a constant pulse period of about four days, increasing steadily each year.

As Dr. Torres explained to Universe Today via email, this has recently begun to change, leaving many astronomers wondering what is driving Polaris’ pulsations. “Over the 150 years up until about 2010, this increased by about four or five seconds per year,” he said. “Modern observations have shown that this trend has now reversed and the pulsation period is getting shorter. This is an unexpected change, showing that there is still much we do not understand about Polaris and the stars.” other similar stars.” To learn more about Polaris’s pulsation period, Torres looked at radial velocity (RV) measurements dating back to 1888. The technique involves measuring the spectrum of a distant star and looking for displacements red and blue shift, indicating when the star moves back and forth. (this technique also gives an accurate estimate of its speed). Torres’ sample includes more than 3,600 RV measurements, including nearly 1,200 spectroscopic observations made by the Lick Observatory over 60 years. This allowed Torres to track the evolution of Polaris’s pulse characteristics, showing the frequency of the pulses as well as their amplitude. Torres said:

“By the early 1990s, the amplitude had become so small that it was thought that the pulses were about to stop. However, Polaris decided otherwise, and by the late 1990s the amplitude had begun to increase again, lasting until about 2015. Recent observations indicate that the amplitude is no longer increasing and may started to decrease again. Additionally, VR has shown that this behavior may be related to the fact that Polaris is orbiting a different star, which gets closer every 30 years and may disrupt the outer layers of the Cepheids, where this occurs. emit impulses. In short, the changes in Polaris’s heart rate cycle could be due to his mate bothering him whenever they come close to each other. When this was taken into account, Torres was able to obtain an improved spectral orbit for the binary system, something astronomers have been trying to solve for generations. This could also lead to more accurate estimates of the dynamic mass of each companion star, which is also subject to uncertainty. As Torres summarizes: “We now know that Polaris behaves erratically and unpredictably. If this is confirmed to be related to the presence of its companion, it could shed light on the behavior of other moving stars with similar properties and help us understand its nature. quality of the oscillations. Therefore, it is important to keep an eye on it, because it can still throw surprises at us. »

source: arXiv (2023). DOI: 10.48550/arxiv.2309.03257