Astronomers probe layer-cake structure of brown dwarf's atmosphere


Date:
July 30, 2021
Source:
W. M. Keck Observatory
Summary:
Astronomers have developed a new way to capture all the exquisite
'layer- cake' details of a brown dwarf's cloud structure. Because
brown dwarfs are similar to super-Jupiters, this innovative
technique can help deepen scientists' understanding of the
atmospheres of giant alien worlds that are more massive than
Jupiter.



FULL STORY ========================================================================== Maunakea, Hawaii --Jupiter may be the bully planet of our solar system
because it's the most massive planet, but it's actually a runt compared
to many of the giant planets found around other stars.


========================================================================== These alien worlds, called super-Jupiters, weigh up to 13 times Jupiter's
mass.

Astronomers have analyzed the composition of some of these monsters,
but it has been difficult to study their atmospheres in detail because
these gas giants get lost in the glare of their parent stars.

Researchers, however, have a substitute: the atmospheres of brown dwarfs,
so- called failed stars that are up to 80 times Jupiter's mass. These
hefty objects form out of a collapsing cloud of gas, as stars do, but
lack the mass to become hot enough to sustain nuclear fusion in their
cores, which powers stars.

Instead, brown dwarfs share a kinship with super-Jupiters. Both types of objects have similar temperatures and are extremely massive. They also
have complex, varied atmospheres. The only difference, astronomers think,
is their pedigree. Super-Jupiters form around stars; brown dwarfs often
form in isolation.

A team of astronomers, led by Elena Manjavacas of the Space Telescope
Science Institute in Baltimore, Maryland, has tested a new way to peer
through the cloud layers of these nomadic objects. The researchers used
an instrument at W.

M. Keck Observatory on Maunakea in Hawaii to study in near-infrared light
the colors and brightness variations of the layer-cake cloud structure
in the nearby, free-floating brown dwarf known as 2MASS J22081363+2921215.

The Keck Observatory instrument, called the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), also analyzed the spectral fingerprints
of various chemical elements contained in the clouds and how they change
with time. This is the first time astronomers have used MOSFIRE in this
type of study.



========================================================================== These measurements offered Manjavacas a holistic view of the brown dwarf's atmospheric clouds, providing more detail than previous observations of
this object. Pioneered by Hubble observations, this technique is difficult
for ground-based telescopes to do because of contamination from Earth's atmosphere, which absorbs certain infrared wavelengths. This absorption
rate changes due to the weather.

"The only way to do this from the ground is by using Keck's
high-resolution MOSFIRE instrument because it allows us to observe
multiple stars simultaneously with our brown dwarf," said Manjavacas,
a former staff astronomer at Keck Observatory and the lead author of
the study. "This allows us to correct for the contamination introduced
by the Earth's atmosphere and measure the true signal from the brown
dwarf with good precision. So, these observations are a proof-of-concept
that MOSFIRE can do these types of studies of brown dwarf atmospheres."
She decided to study this particular brown dwarf because it is very
young and therefore extremely bright. It has not cooled off yet. Its
mass and temperature are similar to those of the nearby giant exoplanet
Beta Pictoris b, discovered in 2008 near-infrared images taken by the
European Southern Observatory's Very Large Telescope in northern Chile.

"We don't have the ability yet with current technology to analyze in
detail the atmosphere of Beta Pictoris b," Manjavacas said. "So, we're
using our study of this brown dwarf's atmosphere as a proxy to get an idea
of what the exoplanet's clouds might look like at different heights of
its atmosphere." Both the brown dwarf and Beta Pictoris b are young, so
they radiate heat strongly in the near-infrared. They are both members of
a flock of stars and sub-stellar objects called the Beta Pictoris moving
group, which shares the same origin and a common motion through space. The group, which is about 33 million years old, is the closest grouping of
young stars to Earth. It is located roughly 115 light-years away.



========================================================================== While they're cooler than bona fide stars, brown dwarfs are still
extremely hot. The brown dwarf in Manjavacas' study is a sizzling 2,780
degrees Fahrenheit (1,527 degrees Celsius).

The giant object is about 12 times heavier than Jupiter. As a young body,
it is spinning incredibly fast, completing a rotation every 3.5 hours,
compared to Jupiter's 10-hour rotation period. So, clouds are whipping
around the planet, creating a dynamic, turbulent atmosphere.

Keck Observatory's MOSFIRE instrument stared at the brown dwarf for 2.5
hours, watching how the light filtering up through the atmosphere from
the dwarf's hot interior brightens and dims over time. Bright spots that appeared on the rotating object indicate regions where researchers can
see deeper into the atmosphere, where it is hotter. Infrared wavelengths
allow astronomers to peer deeper into the atmosphere. The observations
suggest the brown dwarf has a mottled atmosphere with scattered clouds. If viewed close-up, the planet might resemble a carved Halloween pumpkin,
with light escaping from the hot interior.

Its spectrum reveals clouds of hot sand grains and other exotic elements.

Potassium iodide traces the object's upper atmosphere, which also includes magnesium silicate clouds. Moving down in the atmosphere is a layer of
sodium iodide and magnesium silicate clouds. The final layer consists
of aluminum oxide clouds. The atmosphere's total depth is 446 miles
(718 kilometers). The elements detected represent a typical part of the composition of brown dwarf atmospheres, Manjavacas said.

She and her team used computer models of brown dwarf atmospheres to
determine the location of the chemical compounds in each cloud layer.

The study will be published in The Astronomical Journal.

Manjavacas' plan is to use Keck Observatory's MOSFIRE to study
other atmospheres of brown dwarfs and compare them to those of gas
giants. Future telescopes such as NASA's James Webb Space Telescope, an infrared observatory scheduled to launch later this year, will provide
even more information about a brown dwarf's atmosphere.

"JWST will give us the structure of the entire atmosphere, providing
more coverage than any other telescope," Manjavacas said.

She hopes that MOSFIRE can be used in tandem with JWST to sample a wide
range of brown dwarfs and gain a better understanding of brown dwarfs
and giant planets.

"Exoplanets are so much more diverse than what we see locally in the
solar system," said Keck Observatory Chief Scientist John O'Meara. "It's
work like this, and future work with Keck and JWST, that will give
us a fuller picture of the diversity of planets orbiting other stars." ========================================================================== Story Source: Materials provided by W._M._Keck_Observatory. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Elena Manjavacas, Theodora Karalidi, Johanna Vos, Beth Biller,
Ben W. P.

Lew. Revealing the Vertical Cloud Structure of a young low-mass
Brown Dwarf, an analog to the beta-Pictoris b directly-imaged
exoplanet, through Keck I/MOSFIRE spectro-photometric
variability. The Astronomical Journal, 2021 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210730165453.htm

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