A re-examination of the white dwarf star WD0810-353, located just 36 light years from Earth, has revealed it to be quite normal. According to a team of astronomers led by John Landstreet of the Armagh Observatory and Planetarium in the UK, the star's only claim to strangeness is its strong magnetic field.

It was this magnetic field that led scientists to misinterpret the star's trajectory through the Milky Way galaxy, the researchers say. The findings, accepted for publication in The Astrophysical Journal, are available on the preprint server arXiv.

However, WD0810-353 is interesting in its own right and could help scientists better understand how white dwarf magnetic fields evolve as they cool.

The Solar System does not exist in isolation. It belongs to a vibrant and fascinating stellar community in which each object is on its own orbit or path around the galactic centre. Some of these paths can result in encounters with other stars, including our Sun.

If another star were to have a close encounter with our Solar System, it could have quite devastating consequences, so astronomers are keen to find out which stars, if any, are in such an orbit.

A paper published earlier this year by astronomers Vadim Bobylev and Anisa Bajkova of the Russian Academy of Sciences in Russia details a handful of candidates for a future close encounter.

The most interesting of these was a white dwarf called WD0810-353. Using data from the Gaia mission, Bobylev and Bajkova calculated that WD0810-353 was moving at 373.7 (232 miles) kilometres per second.

They also calculated that in just 29,000 years, WD0810-353 will pass within 0.49 light years of the Sun. That's a distance of just 31,000 astronomical units, and would bring the white dwarf into the Oort Cloud (a population of small, icy bodies estimated to orbit near the edge of the Solar System) and scatter the frozen blocks as comets towards the inner Solar System.

This is certainly an intriguing possibility and has a significant impact on the future of the system.

But there are some problems with this finding. The Gaia data have given us the most accurate, three-dimensional map of the Milky Way to date, but there are significant limitations when it comes to using this data to calculate the velocities of white dwarfs.

In fact, a follow-up study led by astronomers Raúl de la Fuente Marcos and Carlos de la Fuente Marcos of the Complutense University of Madrid in Spain revealed that although the white dwarf appears to be heading towards the Solar System, its speed is too far away. Their recalculation, based on the hydrogen alpha line in its spectrum, found that WD0810-353 was moving at more than 4,200 kilometres (just over 2,600 miles) per second.

Events in space are often violent, especially when stars reach the bottom of their fusion fuel reserves, resulting in explosions that hurl relic cores at high speed across the galaxy.

White dwarfs are such remnant cores of stars less than about 8 times the mass of the Sun (more massive stars turn into neutron stars and black holes), and are often hurled across the galaxy at high speed in irregular explosions; however, we have never seen a runaway star close to this speed.

However, Spanish astronomers have indicated that there is another possibility. Using non-Gaia data, they compared WD0810-353 with other white dwarfs with similar spectra and found a much more sedate speed of about 60 to 70 kilometres per second.

The new speeds, both fast and slow, would rule out a close encounter between our Solar System and WD0810-353 - but which one?

Landstreet and his colleagues, Eva Villaver of the Centre for Astrobiology in Spain and Stefano Bagnulo of the Armagh Observatory and Planetarium, are on the trail. Noting that it is possible for a strong magnetic field to distort the spectrum of a white dwarf, the researchers obtained new spectra and performed new analyses to derive its magnetic field.

They found that the hydrogen alpha line is strongly shifted towards the bluer part of the spectrum by the white dwarf's strong magnetic field. This can make the star appear to be moving towards us, as this movement can cause light wavelengths to compress and shorten towards the blue range, a phenomenon known as blueshifting.

In this case, Landstreet and colleagues found that blueshift leads to an illusion of motion. They found that the star's speed was close to about 83 kilometres (52 miles) per second. This means that it is not a hyper-fast or runaway star and will not hit the Oort cloud in 29,000 years.

"Nevertheless, WD0810-353 is intrinsically a very interesting star," the researchers write.

"It is one of the closest strongly magnetic white dwarfs to Earth. With an age of almost 3 billion years, it is entering the phase of its cooling lifetime when very strong magnetic fields appear on the surface of middle-aged white dwarfs. It appears to have a rather complex local field strength distribution over the visible surface. Further spectropolarimetric monitoring and more detailed modelling of this object would certainly be useful."

 

Source: https://www.sciencealert.com/