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A white dwarf explodes: a supernova fragment accelerates through the Milky Way

Rasstern LP 40−365

Can Escape Gravity: Supernova Fragment Velocities Through the Milky Way

Today 11/15/2021 | 10 o’clock

Astronomers now know more about one of the fastest objects in our galaxy – the musical star LP 40-365. Observations confirm that it is a supernova remnant. A white dwarf exploded after “overeating” on the adsorbed substance from its companion.

Our sun and most of the other stars in the Milky Way are held in place by our galaxy’s gravity and revolve around its center. But there are exceptions: star sprinter They race through space so fast that they can escape the gravitational trough of our galaxy. Some of these “sticky stars” owe their speed to interactions with the central black hole, others were ejected from a binary star system in a supernova.

An unusual type of fast runner

We now have one of these fast-exploding remnants Astronomers have studied JJ Hermes from Boston University in more detail. The star LP 40-365 is very low in mass and resembles a very small white dwarf, but it has an unusual composition. Because its light spectrum reveals that it contains hardly any hydrogen and helium, but it does contain all the heavier elements such as neon, oxygen, magnesium and iron.

Early in 2019, astronomers suspected this high-speed runner is not a healthy white dwarf, but a species cremated remains – the rest Type 1a supernova. It happens when a white dwarf in a binary star system is too greedy: it attracts so much material from its partner that it becomes unstable and explodes. In the case of LP 40-365, the researchers suspected that the white dwarf was not completely destroyed in the process. Instead, a large portion was thrown into space – so the theory goes.

Is it part of a supernova?

The problem, though, is that newer models indicate that about 20 percent of Type 1a supernovae have remnants. So far, however, there is no evidence for this. Now new monitoring data from the LP 40-365 can provide crucial evidence of this. Because Hermes and his team have the tiny blade again using the Hubble Space Telescope and the TESS satellite NASA targeted.

New data shows that LP 40-365 is racing through the Milky Way at a good speed of 850 kilometers per second — which is extremely fast and confirms that this star is not bound by the galactic gravity. “This star is flying so fast that it will almost certainly leave the Milky Way,” Hermes says. Its orbit also indicates that it does not come from the vicinity of the central black hole.

Rotation rate has been determined

But the most important thing is that the light curves of the two telescopes show remarkable fluctuations in brightness, which repeat every 8.9 hours. These periodic changes are visible both in visible light and in the ultraviolet range. “The simplest explanation is that we see the rotation of the body here,” Hermes says. The star’s rotation alternately makes areas of the surface lighter and darker, and thus causes the fluctuations.

With the rate of rotation, astronomers now have information that enables them to draw conclusions about the origin of LP 40-365. With the help of a model, they then reconstructed how fast the remnants of an exploding white dwarf or donor star would spin after being ejected by a supernova.

Who was the predecessor?

The result: a star from which the white dwarf absorbs matter will rotate much faster than LP 40-365 after it has been ejected. “All donor remnants greater than 0.2 solar masses have rotation periods of less than an hour—however, they rotate much faster than 8.9 hours,” Hermes and colleagues say. However, the remnants of an exploding white dwarf should rotate faster than LP 40-365.

However, astronomers believe that the latter is more likely. They assume that LP 40-365 is the remnant of a white dwarf that survived a supernova – at least as a piece. “We cannot fully explain the combination of extremely high speed and slow rotation,” the team said. “But LP 40-365 is unlikely to represent the donor’s star.”

Now astronomers hope that future observations will provide more information. “If we understand what happened to this star, we can also solve the mystery of other similar stars,” says Odelia Pottermann, a colleague of Hermes.

This article was written by Nadja Podbregar