Why did the “runaway star” in the universe suddenly escape?

Produced by: Science Popularization China

Author: Guo Yanjun (Yunnan Astronomical Observatory, Chinese Academy of Sciences)

Producer: China Science Expo

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In the vast universe, stars are like sand, and Early-type Runaway Stars are the most mysterious travelers in this sea of ​​stars. They are like darts thrown suddenly by the giant hand of the universe, piercing the silent void at an astonishing speed, leaving behind dazzling and short tracks. Most of these stars were born in the busy corners of the Milky Way, perhaps in the heart of a dense star cluster or nebula, but driven by some unknown force, they broke free from the original gravitational constraints and embarked on a lonely wandering journey.

What is an “early-type fast-running star”?

When chemical substances burn, different temperatures and elements in the substances will present unique spectra and colors. Therefore, astronomers can know the elemental composition and effective temperature of celestial bodies by analyzing the spectrum of celestial bodies. According to the effective temperature, astronomers divide the star spectrum into O, B, A, F, G, K, M, R, N and S types.

Generally, stars with higher effective temperatures such as O, B, and A types are called "early-type stars", and stars with lower effective temperatures such as K and M types are called "late-type stars".

The rest are called "intermediate stars", about 30% of O-type stars and 5%-10% of B-type stars, which usually move in the Milky Way at a peculiar space velocity greater than 30-40 km/s, which is about 1,000 times the speed of a rocket, far exceeding the normal movement speed of most stars. These stars are so-called "runaway stars". Runaway stars, or "fast escapers in the universe", are a special type of star. Unlike most stars, they do not move steadily around the center of the Milky Way, but quickly leave their birthplace at a very high speed.

Why are early-type runaway stars so important?

The study of the formation mechanism of runaway stars is of great significance to our understanding of supernova explosion models, special binary star systems, binary star evolution, etc. The formation mechanism of runaway stars can be mainly attributed to two types. One is the binary supernova scenario (as shown in Figure 1), when two stars are very close, they will combine to become a binary partner that orbits each other.

Figure 1: The formation pathway of binary supernovae of runaway stars

(Image source: Reference 1)

In this pair, when the more massive star undergoes an asymmetric core collapse supernova explosion, its companion star is ejected to form a runaway star. In addition, in dense star clusters, stars are like participants in a social dance, constantly shuttling and interweaving with each other. In this high-density environment, the complex interaction of multi-body systems can accidentally push some stars out of the original stage and become lone rangers traveling through the Milky Way (see Figure 2 for the dynamical ejection picture).

Figure 2: Dynamic ejection pathway of a runaway star

(Image credit: ESA/Hubble Media)

LAMOST stands for "Large Sky Area Multi-Object Fiber Spectroscopic Telescope", also known as the "Guo Shoujing Telescope". It is an independently innovated telescope developed by the National Astronomical Observatory of the Chinese Academy of Sciences. It is the world's largest telescope with a large field of view, large aperture, and the highest spectral acquisition rate. It can acquire a large amount of optical spectra.

Gaia is the abbreviation of Global Astrometric Interferometer for Astrophysics. It is one of the most important projects of the European Space Agency. Its advent will greatly improve the accuracy of human astronomical measurements and provide the necessary data to solve a series of important problems related to the origin, structure and evolution history of the Milky Way.

Based on the large amount of sample data obtained by the latest observation equipment, the appearance of runaway stars has become clearer in front of astronomers. In previous observational studies, some runaway stars have been discovered, but most samples lacked consistent radial velocity measurement information. The 229 early-type runaway stars discovered this time are the largest number of samples with consistent radial velocity in the known runaway star research based on Gaia astronomical data.

The radial velocity is also often called the radial velocity (Vr) in astronomy. It refers to the speed of an object (such as a celestial body) in the direction of the observer's line of sight. This velocity component reflects the dynamic characteristics of the object moving toward or away from the observer. The radial velocity and the tangential velocity together constitute the space velocity, which is of great significance for studying the motion laws of celestial bodies and exploring the structure and dynamic characteristics of the universe. The projected rotation velocity can be used to infer the true rotation velocity of a celestial body.

Based on the distribution of projected rotation speed and space speed of this sample, the researchers found that most of the runaway stars in this sample have smaller projected rotation speed and space speed, but there are almost no runaway stars with larger projected rotation speed and space speed at the same time (Figure 3). This feature may be closely related to the formation path of runaway stars.

Figure 3: Relationship between the peculiar spatial velocity of a runaway star and its projected rotational velocity

(Image source: Reference 2)

The researchers further explored the spatial distribution of the runaway star samples in the Milky Way and found that most of the runaway stars are likely to be located in the thin disk of the Milky Way. By analyzing the orbits of the runaway stars with multiple radial velocity measurements in the sample, the researchers also found two samples that may be runaway star binaries, with orbital periods of 40 days and 61 days, respectively.

Conclusion

Runaway stars play an important role in studying cutting-edge topics such as the structure of the Milky Way and supernova explosions. Next, researchers will continue to study the origin of runaway stars based on these more statistically significant reference samples. With the continuous advancement of astronomical observation technology and the continuous improvement of data analysis capabilities, we will be able to penetrate interstellar dust, capture the faintest light of distant galaxies, and transform massive astronomical data into golden keys to reveal the mysteries of the universe. One day, the mysteries of runaway stars will be revealed one by one, becoming a key bridge connecting stellar evolution, galaxy dynamics and even the large-scale structure of the universe.

References:

1. Renzo, M., Zapartas, E., de Mink, SE, et al. 2019, A&A, 624, A66.

2.Guo, Y., Wang, L., Liu, C., et al. 2024, ApJS.