Two of the fastest runaway stars discovered in the Milky Way

The two white dwarfs might explain the genesis of some types of supernovae. 
Mrigakshi Dixit
Milky Way
Milky Way


Two stars moving at breakneck speeds have been spotted in our galaxy.

According to the Harvard-Smithsonian Center for Astrophysics, two white dwarfs are the fastest runaway stars and might explain the genesis of some types of supernovae. 

The observation of the stars

White dwarfs are stars that have depleted their nuclear fuel. Our own Sun may become one a billion years from now. 

The latest data was gathered using the European Space Agency's Gaia survey, an ongoing endeavor to map the Milky Way with great precision. The survey also collects information on the movements of stars in the Milky Way galaxy.

Six free-moving stars were discovered by astronomers in total. As per the study, Type 1a supernovas are likely responsible for kicking out these runaway stars from their original positions.

Two of them break the record for the fastest radial velocity (the motion of a star with respect to an observer's line of sight) ever seen. 

The new fastest star is J0927, followed by J1235 in the galaxy. The fastest J0927 clocks at a massive speed of 1,420 miles (2,285 kilometers) per second, while J1235 clocks in at 1,053 miles (1,694 kilometers per second.  

What are runaway stars?

The Sun, like all other stars, moves in a particular orbit around the center of our galaxy. However, within this, there are local movements of some stars, known as runaway stars, that move at higher speeds than expected. 

The strongest evidence shows that free-moving stars are usually ejected from their initial star clusters by pairs of binary stars or a supernova explosion.

To understand the origin of these runaway stars, astronomers study various parameters, including differences in velocity and spectroscopic signatures. This data also helps them to classify the type of star. 

This is not the first time that astronomers have spotted runaway stars, but in terms of speed, these two are said to be the fastest known to date. According to the authors of the new study, there might be even faster stars than the currently known ones. They are, however, difficult to locate since scientists tend only to notice the brightest ones. But they will continue to explore and understand the dynamics behind runaway stars.

Their findings are available on the preprint server arXiv and the study has been submitted for publication in the Open Journal of Astrophysics.

Study abstract:

We report a spectroscopic search for hypervelocity white dwarfs (WDs) that are runaways from Type Ia supernovae (SNe Ia) and related thermonuclear explosions. Candidates are selected from Gaia data with high tangential velocities and blue colors. We find six new runaways, including four stars with radial velocities (RVs) >1000kms−1 and total space velocities ≳1300kms−1. These are most likely the surviving donors from double-degenerate binaries in which the other WD exploded. The other two objects have lower minimum velocities, ≳600kms−1, and may have formed through a different mechanism, such as pure deflagration of a WD in a Type Iax supernova. The four fastest stars are hotter and smaller than the previously known "D6 stars," with effective temperatures ranging from ∼20,000 to ∼130,000 K and radii of ∼0.02−0.10R⊙. Three of these have carbon-dominated atmospheres, and one has a helium-dominated atmosphere. Two stars have RVs of −1694 and −2285kms−1 -- the fastest systemic stellar RVs ever measured. Their inferred birth velocities, ∼2200−2500kms−1, imply that both WDs in the progenitor binary had masses >1.0M⊙. The high observed velocities suggest that a dominant fraction of the observed hypervelocity WD population comes from double-degenerate binaries whose total mass significantly exceeds the Chandrasekhar limit. However, the two nearest and faintest D6 stars have the lowest velocities and masses, suggesting that observational selection effects favor rarer, higher-mass stars. A significant population of fainter low-mass runaways may still await discovery. We infer a birth rate of D6 stars that is consistent with the SN Ia rate. The birth rate is poorly constrained, however, because the luminosities and lifetimes of D6 stars are uncertain.

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