Astronomers offer theory about mysterious location of massive stars
Researchers found the secret seems to lie in the merging of medium-mass
stars

Date:
February 2, 2022
Source:
Georgia State University
Summary:
Astronomers have found an explanation for the strange occurrence
of massive stars located far from their birthplace in the disk of
our Milky Way Galaxy.



FULL STORY ========================================================================== Astronomers from Georgia State University have found an explanation for
the strange occurrence of massive stars located far from their birthplace
in the disk of our Milky Way Galaxy.


========================================================================== Stars more massive than the Sun have very hot cores that drive nuclear
energy generation at very high rates. They are among the brightest
objects in our galaxy. But because they burn through their hydrogen fuel
so quickly, their lifetimes are relatively short, perhaps 10 million
years compared to 10 billion years for the Sun.

Their short lifetime means that there is little time for them to stray
too far from their birthplace. Most massive stars are found in the flat
disk part of our galaxy, where gas clouds are dense enough to promote
star birth and where astronomers find young clusters of massive stars.

So, when a massive star is found far away from the galaxy's disk, how did
it get there? "Astronomers are finding massive stars far away from their
place of origin, so far, in fact, that it takes longer than the star's
lifetime to get there," said Georgia State astronomer Douglas Gies. "How
this could happen is a topic of active debate among scientists." This is
the problem presented by the massive star known as HD93521 that lies
about 3,600 light years above the galaxy's disk. A new study by Gies
and other astronomers from Georgia State reveals a profound discrepancy:
The flight time to reach this location far exceeds the predicted age of
this massive star.



==========================================================================
The astronomers used a new distance estimate from the European Space
Agency's Gaia spacecraft together with an investigation of the star's
spectrum to determine the star's mass and age as well as its motion
through space. They find that HD93521 has a mass about 17 times larger
than the Sun's, and this leads to a predicted age of about 5 million
years. On the other hand, the motion of the star indicates that its
journey from the disk has taken much longer, about 39 million years.

The Georgia State astronomers explain this strange difference between
the star's lifetime and travel time by suggesting that HD93521 left the
disk as two lower-mass and longer-lived stars, rather than the single
massive star we see today. Their findings have been published in The Astronomical Journal.

The clue to the mystery is that HD93521 is one of the fastest rotating
stars in the galaxy. Stars can spin up through stellar mergers where
two close orbiting stars can grow over time and collide to form one star.

"HD93521 probably began life as a close pair of medium-mass stars that
were fated to engulf each other and create the single, fast-spinning
star we see today," Gies said.

Such intermediate mass stars live long enough to match the long flight
time of HD93521.

HD93521 is not the only case of a massive star found so far away
from its birthplace. Georgia State graduate student Peter Wysocki
is investigating an example of a distant massive binary pair that is
probably representative of the stage just before a merger. This star
is known as IT Librae, and it has an orientation that creates mutual
eclipses as the two stars pass in front of each other. An investigation
of the variations in the light output and motions detected in the spectra
leads to estimates of the stellar masses.

Wysocki finds a similar conundrum from the mass results -- the predicted
age is much less than IT Librae's travel time from the disk. But the study
also reveals that the lower-mass star in the pair has already begun to
transfer much of its mass to the higher-mass star, initiating the process
that may eventually lead to a merger. This means that the higher-mass star
is actually older than it appears, having begun life as a lower-mass star.

These distant massive stars provide striking evidence that close pairs
of stars can merge to make even larger stars, Gies said, and they are
key clues about how rapidly rotating massive stars are able to create
black holes with large spins.

This work was supported through a grant from the National Science
Foundation.

========================================================================== Story Source: Materials provided by Georgia_State_University. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Douglas R. Gies, Katherine Shepard, Peter Wysocki, Robert
Klement. The
Transformative Journey of HD 93521. The Astronomical Journal,
2022; 163 (2): 100 DOI: 10.3847/1538-3881/ac43be ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220202153849.htm

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