NMSU researcher’s discovery nearly triples known highly magnetic stars

https://newscenter.nmsu.edu/Articles/view/13735/nmsu-researcher-s-discovery-nearly-triples-known-highly-magnetic-stars

Scientists have known about the existence of massive, highly magnetic stars like this since at least the 1960s. Because there weren’t enough stars to study back then, the interest in them died.
Thanks to NMSU Department of Astronomy Ph.D. student Drew Chojnowski’s research, there is now much more to discover about the nature of these highly magnetic stars.

“The significance of these findings is that it almost triples the number of these stars that are known,” Chojnowski said. “Previously, we only knew of 84 stars that were like this, but now we have this bigger sample of about 240 stars. This gives a much larger sample that people can use to determine other parameters of the stars that we don't get from APOGEE data, and try to find correlations from that data. So it’s a giant leap after a long period of relative silence in the field.”

Chojnowski, who works as a plate design coordinator for the SDSS, identified these highly magnetic stars after spending more than six months combing through observations of hot, blue stars. SDSS, an international collaboration of hundreds of scientists at dozens of institutions, began in 2000 surveying distant galaxies and quasars, but in 2011, the survey began a more comprehensive mapping of the individual stars within our home galaxy, the Milky Way.

“When the moon is up, we call it 'bright time' because it’s very difficult to see galaxies,” said Chojnowski. “So for the first 10 years, they weren’t really using the time in which the moon was up. But when the moon is up, you can still observe stars. So focus in bright time was quickly shifted to that.”

In order to take advantage of that available time, the Apache Point Observatory Galactic Observatory Experiment was built and deployed on the SDSS telescope. APOGEE is a spectrograph, meaning that instead of taking images of the sky, it gathers the light from objects in the sky, sends that light through a fancy prism to spread it out into its individual colors, and then takes a picture of the resulting pattern using a highly sophisticated camera. From this kind of data, important details about a star such as temperature, motion relative to the Sun, abundances of elements, and most relevant to Chojnowski's work, magnetic field strength, is determined.

“The survey went from looking at galaxies and quasars to looking deeper into the stars in our galaxies,” Chojnowski said. “They had already imaged most of the sky anyway, so they were only really doing spectroscopy by that point. But, in my opinion, a spectrum is worth a thousand images.”

It was in these spectra that Chojnowski was first able to pinpoint these abnormal stars.

“My job is to run code that searches databases of previous surveys and images of the skies,” Chojnowski said. “I run software that chooses targets, and then I personally just look through the data of those targets afterwards, trying to find unique things. I’m interested in weird stars, and since there’s no one else in the survey really covering that, I was able to focus heavily in that area. I was not an expert on this when I started, not by far. But I was just curious, and wanted to dive deeper into the subject.”

Chojnowski was able to find stars in our galaxy with magnetic fields far greater than the norm. Astronomers use a unit called the Gauss to measure the strength of magnetic fields, and to put things into perspective, the Sun's typical magnetic field strength is one Gauss, which peaks at about 3,000 Gauss, when measured from areas such as sunspots. The magnetism of these highly magnetic stars is on a whole new level.

“The magnetic fields of these stars average about 7,000 Gauss, but can reach up to 34,000 Gauss,” Chojnowski said. “These stars don’t have spots of magnetism – their entire surface is basically one big sunspot.”

But it’s not strictly the level of magnetism that makes these stars such an oddity, as the exact reason for these high levels of magnetism remains a mystery. For stars with the mass of the Sun or less, magnetic fields are generated by the Solar dynamo, whereby blobs of gas rise and fall in convection zones on their surfaces. But unlike these traditional, low-mass stars, the highly magnetic stars at the center of Chojnowski’s research have no convection zones on their surfaces. His sample of stars (referred to as Ap/Bp stars, where the “p” stands for “peculiar) are all quite massive, up to 4 or 5 times the mass of the Sun, meaning they have an altogether different internal structure than stars like the Sun.

“There are just photons streaming out, there’s no gas really bubbling or falling or going up and down,” Chojnowski said. “So we have no idea why these stars have the magnetic fields that they do. There’s no physics to explain why they have it.”

As a result of Chojnowski’s research, more can be yielded about the nature of these highly magnetic stars than ever before. And, for Chojnowski, it was exploring unusual phenomenon like this in the first place that sparked his curiosity in the astronomy field.

“For me, it’s all about making discoveries like these,” Chojnowski said. “The process of looking at brand new data fresh out of the telescope that no one has ever seen before, and finding weird things that don’t quite mesh with our theories about how things should work. It’s incredibly exciting.”

The results of Chojnowski’s findings were published this past March in a paper co-authored by other astronomical researchers across the country and internationally. The paper can be found at https://ui.adsabs.harvard.edu/abs/2019ApJ...873L...5C/abstract