Astronomers discover dozens of massive stars fleeing the Milky Way

The Milky Way cannot keep all of its stars. Some of them are ejected into intergalactic space and spend their lives on an uncertain journey. A team of astronomers took a closer look at the most massive of these fleeing stars to see how they were ejected.

When astronomers observe a star field in the Milky Way, one of the things they measure is the distribution of velocities. The overall distribution of stellar population velocities reflects the rotation of the galaxy. And when a star is out of alignment with the rotation of the galaxy, it attracts the attention of astronomers.

A team of astronomers working with two catalogs of massive stars has discovered a whole bunch of stars moving differently than the galaxy. These are runaway stars that are about to leave the galaxy.

The new findings appear in a paper titled “Galactic runaway stars O and Be found using Gaia DR3.” It is to be published in the newspaper Astronomy and Astrophysicsand the main author is Mar Carretero Castrillo, postgraduate researcher at the Department of Quantum Physics and Astrophysics at the Institute of Cosmos Sciences at the University of Barcelona.

Castrillo and his colleagues based their work on two stellar catalogs. These are the Galactic O-Star Catalog (GOSC) and the Be Star Spectra (BeSS). They are both catalogs of different types of massive stars: O-type stars and Be-type stars, as well as their subtypes.

The researchers also used data from Gaia, ESA’s powerful star-measuring spacecraft. It uses astrometry to measure the positions, distances and movements of a billion stars. Gaia’s mission is changing astronomy by providing accurate, robust data that other researchers can use in their own research. This article is based on a combination of Gaia data and data from both catalogs.

No one knows how many runaway stars are about to leave our galaxy, but astronomers continue to discover more and more. Some estimates say there are 10 million runaway stars fleeing the Milky Way, but we don’t know for sure. It may depend on the mechanism that pushes them apart, and that’s something astrophysicists don’t completely understand.

This study aims to shed light on the phenomenon of runaway stars by focusing specifically on massive stars.

“A significant fraction of massive stars are runaway stars. These stars move with a particular and significant speed relative to their environment,” the authors explain. They set out to discover and characterize the runaway massive and early-type stars in both catalogs by examining data from Gaia.

“Early-type massive OB stars are the most luminous stars in the Milky Way,” they explain. OB stars are not only massive and young, they are extremely sexy. They form loosely organized groups with each other, called OB associations. Because they are young and hot, they don’t last long. They are important in astronomy because they are very massive and energetic and because many of them explode as supernovae. This is why specific catalogs are dedicated to them.

The team cross-referenced Gaia data with the GOSC and BeSS catalogs and came up with 417 O-type stars and 1,335 Be-type stars present in Gaia and the catalogs, respectively. Among these, they found 106 runaway O-type stars, or 25.4% of the stars in the GOSC catalog. Forty-two of them are newly identified.

They found 69 runaway Be stars, representing 5.2% of the stars in the Be-type star catalog. Forty-seven of them are newly identified. Overall, O-type stars move faster than Be-type stars.

Why do massive stars make up such a high proportion of runaway stars? There are two competing theories that attempt to explain runaway stars, and both involve massive stars. One is the dynamic ejection scenario (DES) and the other is the binary supernova scenario (BSS).

OB stars often form in binary pairs. In the BSS, one star explodes like a supernova and the explosion hits the other star. If the situation is correct, the surviving star receives enough energy in the right direction that it can escape its bond with its partner, which is now a neutron star or black hole. It can also escape the gravitational pull of the Milky Way. If this happens, it begins its long journey into intergalactic space.

In the DES, there is no dramatic supernova explosion. Instead, a star located in a compact, densely populated region experiences gravitational interactions with other stars. Encounters between binary and single stars can produce runaways, as can encounters between two binary pairs. The OB associations in which type O and type B stars tend to form are the types of dense environments that can trigger runaway stars. Since most of these stars are massive, most of the runaway stars are massive as well.

Scientists have wondered about and debated these two scenarios for decades. Both scenarios can produce stars fast enough to escape the galaxy. Studying their sample of 175 runaway stars, the researchers found that their data leaned toward one explanation over another.

“The higher percentages and velocities found for O-type fugitives compared to Be-type ones highlight that the dynamic ejection scenario is more likely than the binary supernova scenario,” they write.

The percentages of spectral types represented in the fleeing stars help explain their conclusion. 25% of the O-type stars in their sample are runaways compared to 5% of the Be-type stars. Other studies have come up with different numbers, but as the authors point out, “there is agreement in the sense that the percentage of runaway O stars is significantly higher than that of B or Be stars.”

Previous research shows that runaway O-type stars have higher velocities than B- and Be-type stars. Previous research also shows that dynamic ejection often results in runaways that are faster and more massive than the binary supernova scenario. “GOSC-Gaia DR3 stars in general have higher velocities than BeSS-Gaia DR3,” the authors explain, consistent with previous research.

“This reinforces the dominance of the DES scenario compared to that of the BSS,” they conclude.