A Nearby Stellar Cradle
The Milky Way and other galaxies in the universe harbor many young star
clusters and associations that each contain hundreds to thousands of hot,
massive, young stars known as O and B stars. The star cluster Cygnus OB2
contains more than 60 O-type stars and about a thousand B-type stars. Deep
observations with NASA's Chandra X-ray Observatory have been used to detect the
X-ray emission from the hot outer atmospheres, or coronas, of young stars in the
cluster and to probe how these fascinating star factories form and evolve. About
1,700 X-ray sources were detected, including about 1,450 thought to be stars in
the cluster. In this image, X-rays from Chandra (blue) have been combined with
infrared data from NASA's Spitzer Space Telescope (red) and optical data from
the Isaac Newton Telescope (orange).
Image Credit: NASA
Image Credit: NASA
NASA's Chandra Sees Eclipsing Planet in X-rays for First
Time
For the first time since exoplanets, or planets around stars other than the
sun, were discovered almost 20 years ago, X-ray observations have detected an
exoplanet passing in front of its parent star.
An advantageous alignment of a planet and its parent star in the system HD
189733, which is 63 light-years from Earth, enabled NASA’s Chandra X-ray
Observatory and the European Space Agency’s XMM Newton Observatory to observe a
dip in X-ray intensity as the planet transited the star.
"Thousands of planet candidates have been seen to transit in only optical
light," said Katja Poppenhaeger of Harvard-Smithsonian Center for Astrophysics
(CfA) in Cambridge, Mass., who led a new study to be published in the Aug. 10
edition of The Astrophysical Journal. "Finally being able to study one in X-rays
is important because it reveals new information about the properties of an
exoplanet."
The team used Chandra to observe six transits and data from XMM Newton
observations of one.
The planet, known as HD 189733b, is a hot Jupiter, meaning it is similar in
size to Jupiter in our solar system but in very close orbit around its star. HD
189733b is more than 30 times closer to its star than Earth is to the sun. It
orbits the star once every 2.2 days.
HD 189733b is the closest hot Jupiter to Earth, which makes it a prime target
for astronomers who want to learn more about this type of exoplanet and the
atmosphere around it. They have used NASA's Kepler space telescope to study it
at optical wavelengths, and NASA's Hubble Space Telescope to confirm it is blue
in color as a result of the preferential scattering of blue light by silicate
particles in its atmosphere.
The study with Chandra and XMM Newton has revealed clues to the size of the
planet's atmosphere. The spacecraft saw light decreasing during the transits.
The decrease in X-ray light was three times greater than the corresponding
decrease in optical light.
"The X-ray data suggest there are extended layers of the planet's atmosphere
that are transparent to optical light but opaque to X-rays," said co-author
Jurgen Schmitt of Hamburger Sternwarte in Hamburg, Germany. "However, we need
more data to confirm this idea."
The researchers also are learning about how the planet and the star can
affect one another.
Astronomers have known for about a decade ultraviolet and X-ray radiation
from the main star in HD 189733 are evaporating the atmosphere of HD 189733b
over time. The authors estimate it is losing 100 million to 600 million
kilograms of mass per second. HD 189733b's atmosphere appears to be thinning 25
percent to 65 percent faster than it would be if the planet's atmosphere were
smaller.
"The extended atmosphere of this planet makes it a bigger target for
high-energy radiation from its star, so more evaporation occurs," said co-author
Scott Wolk, also of CfA.
The main star in HD 189733 also has a faint red companion, detected for the
first time in X-rays with Chandra. The stars likely formed at the same time, but
the main star appears to be 3 billion to 3 1/2 billion years younger than its
companion star because it rotates faster, displays higher levels of magnetic
activity and is about 30 times brighter in X-rays than its companion.
"This star is not acting its age, and having a big planet as a companion may
be the explanation," said Poppenhaeger. "It's possible this hot Jupiter is
keeping the star's rotation and magnetic activity high because of tidal forces,
making it behave in some ways like a much younger star."
The paper is available online at:
For Chandra images, multimedia and related materials, visit:
For an additional interactive image, podcast, and video on the finding,
visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
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