Ultracool dwarf binary stars break records
Astrophysicists discover the closest and oldest ultracool dwarf binary
ever observed

Date:
March 1, 2023
Source:
Northwestern University
Summary:
Astrophysicists have discovered the tightest ultracool dwarf binary
system ever observed. The two stars are so close that it takes
them less than one Earth day to revolve around each other. In
other words, each star's 'year' lasts just 17 hours.


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FULL STORY ========================================================================== Northwestern University and the University of California San Diego (UC
San Diego) astrophysicists have discovered the tightest ultracool dwarf
binary system ever observed.


==========================================================================
The two stars are so close that it takes them less than one Earth day
to revolve around each other. In other words, each star's "year" lasts
just 17 hours.

The newly discovered system, named LP 413-53AB, is composed of a pair of ultracool dwarfs, a class of very low-mass stars that are so cool that
they emit their light primarily in the infrared, making them completely invisible to the human eye. They are nonetheless one of the most common
types of stars in the universe.

Previously, astronomers had only detected three short-period ultracool
dwarf binary systems, all of which are relatively young -- up to 40
million years old. LP 413-53AB is estimated to be billions of years old -- similar age to our sun -- but has an orbital period that is at least three times shorter than the all ultracool dwarf binaries discovered so far.

The research was published on March 1 in the Astrophysical Journal
Letters.

"It's exciting to discover such an extreme system," said Chih-Chun "Dino"
Hsu, a Northwestern astrophysicist who led the study. "In principle, we
knew these systems should exist, but no such systems had been identified
yet." Hsu is a postdoctoral researcher in Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics(CIERA). He
began this study while a Ph.D. student at UC San Diego, where he was
advised by Professor Adam Burgasser.

The team first discovered the strange binary system while exploring
archival data. Hsu developed an algorithm that can model a star based
on its spectral data. By analyzing the spectrum of light emitted from
a star, astrophysicists can determine the star's chemical composition, temperature, gravity and rotation. This analysis also shows the star's
motion as it moves toward and away from the observer, known as radial
velocity.

When examining the spectral data of LP 413-53AB, Hsu noticed something
strange.

Early observations caught the system when the stars were roughly aligned
and their spectral lines overlapped, leading Hsu to believe it was just
one star.

But as the stars moved in their orbit, the spectral lines shifted in
opposite directions, splitting into pairs in later spectral data. Hsu
realized there were actually two stars locked into an incredibly tight
binary.

Using powerful telescopes at the W.M. Keck Observatory, Hsu decided to
observe the phenomenon for himself. On March 13, 2022, the team turned
the telescopes toward the constellation Taurus, where the binary system
is located, and observed it for two hours. Then, they followed up with
more observations in July, October and December as well as January 2023.

"When we were making this measurement, we could see things changing over
a couple of minutes of observation," Burgasser said. "Most binaries we
follow have orbit periods of years. So, you get a measurement every few
months. Then, after a while, you can piece together the puzzle. With this system, we could see the spectral lines moving apart in real time. It's
amazing to see something happen in the universe on a human time scale."
The observations confirmed what Hsu's model predicted. The distance
between the two stars is about 1% of the distance between the Earth and
the sun. "This is remarkable, because when they were young, something
like 1 million years old, these stars would have been on top of each
other," said Burgasser.

The team speculates that the stars either migrated toward each other
as they evolved, or they could have come together after the ejection of
a third -- now lost -- stellar member. More observations are needed to
test these ideas.

Hsu also said that by studying similar star systems researchers can learn
more about potentially habitable planets beyond Earth. Ultracool dwarfs
are much fainter and dimmer than the sun, so any worlds with liquid water
on their surfaces -- a crucial ingredient to form and sustain life --
would need to be much closer to the star. However, for LP 413-53AB,
the habitable zone distance happens to be the same as the stellar orbit,
making it impossible to form habitable planets in this system.

"These ultracool dwarfs are neighbors of our sun," Hsu said. "To
identify potentially habitable hosts, it's helpful to start with our
nearby neighbors.

But if close binaries are common among ultracool dwarfs, there may be
few habitable worlds to be found." To fully explore these scenarios,
Hsu, Burgasser and their collaborators hope to pinpoint more ultracool
dwarf binary systems to create a full data sample.

New observational data could help strengthen theoretical models for
binary-star formation and evolution. Until now, however, finding ultracool binary stars has remained a rare feat.

"These systems are rare," said Chris Theissen, study co-author and a Chancellor's Postdoctoral Fellow at UC San Diego. "But we don't know
whether they are rare because they rarely exist or because we just don't
find them.

That's an open-ended question. Now we have one data point that we can
start building on. This data had been sitting in the archive for a long
time. Dino's tool will enable us to look for more binaries like this."
* RELATED_TOPICS
o Space_&_Time
# Stars # Galaxies # Astrophysics # Black_Holes #
Extrasolar_Planets # Astronomy # Solar_Flare #
Solar_System
* RELATED_TERMS
o Gravitational_wave o Light-year o Extrasolar_planet o Planet
o Neptune's_natural_satellites o Pluto o Globular_cluster
o Open_cluster

========================================================================== Story Source: Materials provided by Northwestern_University. Original
written by Amanda Morris. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Chih-Chun Hsu, Adam J. Burgasser, Christopher A. Theissen. Discovery
of
the Exceptionally Short Period Ultracool Dwarf Binary LP
413-53AB. The Astrophysical Journal Letters, 2023; 945 (1): L6 DOI:
10.3847/2041-8213/ acba8c ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/03/230301162706.htm

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