The Milky Way cannot contain all its stars. Some are thrown into intergalactic space and spend their lives on an uncertain journey. A team of astronomers took a closer look at the largest of these runaway stars to see what they could learn about how they were ejected. When astronomers observe the field of stars in the Milky Way, one of the things they measure is the velocity distribution. The overall distribution of velocities of the stellar population reflects the rotation of the galaxy. And when a star doesn’t match the rotation of the galaxy, it attracts astronomers’ attention.

A team of astronomers working with two large star catalogs found a group of stars that moved differently from the galaxy. These are runaway stars leaving the galaxy. The new findings appear in a paper titled “Galactic O and Be stars found with Gaia DR3.” Will be published in the journal Astronomy and Astrophysicsand the lead author is Mar Carretero Castrillo, a PhD student at the Department of Quantum Physics and Astrophysics at the Institute of Space Sciences at the University of Barcelona.

Castrillo and his colleagues based their study on two-star catalogues. These are the Galactic O-Star Catalog (GOSC) and the Be Star Spectra (BeSS). Both are catalogs of different types of massive stars: O-type and Be-type stars and their subtypes.

The researchers also used data from ESA’s powerful star-measuring spacecraft Gaia. It uses astrometry to measure the position, distance and motion of a billion stars. The Gaia mission is changing astronomy by providing accurate and reliable data that other researchers can use in their research. This article is based on a combination of Gaia data and data from two catalogues.

No one knows how many stars have escaped from our galaxy, but astronomers are finding more every day. Some estimates suggest there are 10 million runaway stars escaping the Milky Way, but we don’t know for sure. This may depend on the mechanism that removes them; This is something astrophysicists don’t fully understand.

This study aims to shed light on the phenomenon of runaway stars, especially by looking at massive stars.

“A corresponding fraction of massive stars are runaway stars. These stars move at a significant speed relative to their surroundings,” the authors explain. By examining Gaia data in both catalogues, they aimed to detect and characterize early-type massive stars.

“Massive early-type OB stars are the brightest stars in the Milky Way,” they explain. OB stars are not only massive and young, they are also extremely hot. With each other they form loosely organized groups called OB associations. They don’t live long because they are young and hot. They are important in astronomy because they are very large and energetic, and many explode as supernovae. That’s why there are special catalogs dedicated to them.

The team cross-referenced the Gaia data with the GOSC and BeSS catalogs and found 417 O-type stars and 1,335 Be-type stars in both Gaia and the catalogs, respectively. Of these, they found 106 O-type runaway stars, representing 25.4% of the stars in the GOSC catalogue. Of these, 42 were recently identified.

They found 69 Be runaway stars, accounting for 5.2% of the stars in the Be star catalogue. Forty-seven of these were recently identified. In general, O-type stars move faster than Be-type stars.

Why do massive stars make up such a large portion 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 BSS, a star explodes as a supernova and the explosion ejects another star. If the situation is correct, the surviving star receives enough energy in the right direction to escape its companion, which is now a neutron star or black hole. It can also escape the gravitational pull of the Milky Way. If this happens, it will begin its long journey into intergalactic space.

There is no dramatic supernova explosion in DES. Instead, a star in a compact, dense region undergoes a gravitational interaction with other stars. An encounter between a binary star and a single star can cause escape, as can an encounter between two binary pairs. OB associations, where O-type and B-type stars typically form, are the types of dense environments that can give rise to runaway stars. Since most of these stars are massive, most of the runaway stars are also massive.

Scientists have been interested in and discussing these two scenarios for decades. Both scenarios could produce stars fast enough to escape from the galaxy. Studying a sample of 175 runaway stars, the researchers found that the data favored one explanation over the other.

“The higher percentage and higher velocities found for the O-type compared to the Be-type highlight that a dynamic eruption scenario is more likely than the double supernova scenario,” they write.

The percentage of spectral types represented in runaway stars helps explain their conclusion. 25% of the O-type stars and 5% of the Be-type stars in their samples are runaways. Other studies have produced different figures, but as the authors note, “There is consensus that the percentage of O-type stars is significantly higher than that of B or Be stars.”

Previous studies show that O-type runaway stars have higher velocities than B-type and Be-type stars. Previous studies also show that dynamic ejection leads to faster, larger accelerations than the double supernova scenario. “The overall velocities of GOSC-Gaia DR3 stars are higher than those of BeSS-Gaia DR3,” the authors explain, consistent with previous studies. Source