This illustration) depicts one of the relatively high mass
stars, which has a large number of flares and spots. This is a sign of
its enhanced X-ray production, which is thinning and destroying the
remnants of its planet-forming disk. Credit: X-ray: NASA/CXC/RIT/J.Kastner et al; Illustration: NASA/CXC/M.Weiss. -
This illustration shows one of the lower mass, fainter
stars. Because it is not as active in X-rays, it has retained a thicker
disk that represents a more suitable environment to form planets. The
planet formation process would cause gaps, not shown in this
illustration, to appear in the disk. The streams near the center show
how matter from the disk is still falling onto the star. These
illustrations are not to scale - the stars are actually miniscule in
size when compared with their surrounding disks. (Credit: NASA/CXC/M.Weiss).
Young stars much less massive than the Sun can unleash a torrent of X-ray radiation
that can significantly shorten the lifetime of planet-forming disks
surrounding these stars. This result comes from a new study of a group
of nearby stars using data from NASA's Chandra X-ray Observatory and other telescopes.
Researchers found evidence that intense X-ray radiation produced by
some of the young stars in the TW Hya association (TWA), which on
average is about 160 light years
from Earth, has destroyed disks of dust and gas surrounding them. These
disks are where planets form. The stars are only about 8 million years
old, compared to the 4.5-billion-year age of the Sun. Astronomers want
to learn more about systems this young because they are at a crucial age
for the birth and early development of planets.
Another key difference between the Sun and the stars in the study involves their mass.
The TWA stars in the new study weigh between about one tenth to one
half the mass of the Sun and also emit less light. Until now, it was
unclear whether X-ray radiation from such small, faint stars could
affect their planet-forming disks of material. These latest findings
suggest that a faint star's X-ray output may play a crucial role in
determining the survival time of its disk. These results mean that
astronomers may have to revisit current ideas on the formation process
and early lives of planets around these faint stars.
Using X-ray data from NASA's Chandra X-ray Observatory, the European Space Agency's XMM-Newton observatory and ROSAT
(the ROentgenSATellite), the team looked at the intensity of X-rays
produced by a group of stars in the TWA, along with how common their
star-forming disks are. They split the stars into two groups to make
this comparison. The first group of stars had masses ranging from about
one third to one half that of the Sun. The second group contained stars
with masses only about one tenth that of the Sun, which included
relatively massive brown dwarfs, objects that do not have sufficient mass to generate self-sustaining nuclear reactions in their cores.
The researchers found that, relative to their total energy output,
the more massive stars in the first group produce more X-rays than the
less massive ones in the second. To find out how common planet-forming
disks in the groups were, the team used data from NASA's Wide-Field
Infrared Survey Explorer (WISE) and, in some cases, ground-based spectroscopy
previously obtained by other teams. They found that all of the stars in
the more massive group had already lost their planet-forming disks, but
only about half of the stars in the less massive group had lost their
disks. This suggests that X-rays from the more massive stars are
speeding up the disappearance of their disks, by heating disk material
and causing it to "evaporate" into deep space.
A typical star and planet-forming disk from each of these two groups
of stars are shown in the illustrations. The illustration above depicts
one of the relatively high mass stars, which has a large number of
flares and spots. This is a sign of its enhanced X-ray production, which
is thinning and destroying the remnants of its planet-forming disk.
Another illustration (below) shows one of the lower mass, fainter
stars. Because it is not as active in X-rays, it has retained a thicker
disk that represents a more suitable environment to form planets. The
planet formation process would cause gaps, not shown in this
illustration, to appear in the disk. The streams near the center show
how matter from the disk is still falling onto the star. These
illustrations, which are not to scale - the stars are actually miniscule
in size when compared with their surrounding disks - are accompanied by
a Chandra image of young binary star system that was included in the new study of the TWA.
In previous studies, astronomers found that 10-million-year-old stars
in the Upper Scorpius region, another star-forming group, displayed a
similar trend of an increase in the lifetime of disks for lower mass
stars.
However, the Upper Scorpius work did not incorporate X-ray data
that might offer an explanation for this trend, which is one reason why
this new study of the 8-million-year-old TWA is important. Another
reason is that theoretical models of the evolution of planet-forming
disks generally predict that the lifetimes of disks should have very
little dependence on the mass of the star. The new results for the
"puny" TWA stars point to the need to revisit disk evolution models to
account for the range in the X-ray outputs of very low-mass stars.
In searching for planets outside of our Solar System,
many astronomers have focused their efforts on observing stars less
massive than the Sun, like those described here. Such stars may offer
some of the best targets for direct imaging of exoplanets in the
so-called habitable zone, the star-to-planet distance range where liquid
water could exist and life may eventually flourish. These low mass
stars are also attractive targets because they are relatively faint and
planets in their habitable zones should be easier to detect and
investigate.
These results appear in The Astronomical Journal and are available online.
The authors of this paper are Joel Kastner (Rochester Institute of
Technology), David Principe (Universidad Diego Portales, Chile),
Kristina Punzi (RIT), Beate Stelzer(INAF Palermo, Italy), Uma Gorti
(SETI Institute), Ilaria Pascucci (University of Arizona), and
CostanzaArgiroffi (INAF).
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for TW Hya Association:
Scale: Inset image is 1.23 arcmin across (about 0.07 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 11h 21m 17.20s | Dec -34° 46’ 45.50"
Constellation: Hydra
Observation Date: 01 Apr 2011
Observation Time: 4 hours 10 min.
Obs. ID: 12389
Instrument: ACIS
References: Kastner, J. et al. 2016, AJ, (accepted); arXiv:1603.09307
Color Code: X-ray (Purple)
Distance Estimate: About 179 light years
Source: NASA’s Chandra X-ray Observatory
