NGC 6388
Credit: X-ray: NASA/CXC/IASF Palermo/M.Del Santo et al; Optical: NASA/STScI
The destruction of a planet may sound like the stuff of science fiction, but a team of astronomers has found evidence that this may have happened in an ancient cluster of stars at the edge of the Milky Way galaxy.
Using several telescopes, including NASA's Chandra X-ray Observatory,
researchers have found evidence that a white dwarf star - the dense
core of a star like the Sun that has run out of nuclear fuel - may have
ripped apart a planet as it came too close.
How could a white dwarf star, which is only about the size of the
Earth, be responsible for such an extreme act? The answer is gravity.
When a star reaches its white dwarf stage, nearly all of the material
from the star is packed inside a radius one hundredth that of the
original star. This means that, for close encounters, the gravitational
pull of the star and the associated tides, caused by the difference in
gravity's pull on the near and far side of the planet, are greatly
enhanced. For example, the gravity at the surface of a white dwarf is
over ten thousand times higher than the gravity at the surface of the
Sun.
Researchers used the European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) to discover a new X-ray source near the center of the globular cluster
NGC 6388. Optical observations had hinted that an intermediate-mass
black hole with mass equal to several hundred Suns or more resides at
the center of NGC 6388. The X-ray detection by INTEGRAL then raised the
intriguing possibility that the X-rays were produced by hot gas swirling
towards an intermediate-mass black hole.
In a follow-up X-ray observation, Chandra's excellent X-ray vision
enabled the astronomers to determine that the X-rays from NGC 6388 were
not coming from the putative black hole at the center of the cluster,
but instead from a location slightly off to one side. A new composite
image shows NGC 6388 with X-rays detected by Chandra in pink and visible
light from the Hubble Space Telescope in red, green, and blue, with
many of the stars appearing to be orange or white. Overlapping X-ray
sources and stars near the center of the cluster also causes the image
to appear white.
With the central black hole ruled out as the potential X-ray source,
the hunt continued for clues about the actual source in NGC 6388. The
source was monitored with the X-ray telescope on board NASA's Swift
Gamma Ray Burst mission for about 200 days after the discovery by
INTEGRAL.
The source became dimmer during the period of Swift observations. The
rate at which the X-ray brightness dropped agrees with theoretical
models of a disruption of a planet by the gravitational tidal forces of a
white dwarf. In these models, a planet is first pulled away from its
parent star by the gravity of the dense concentration of stars in a
globular cluster. When such a planet passes too close to a white dwarf,
it can be torn apart by the intense tidal forces of the white dwarf.
The planetary debris is then heated and glows in X-rays as it falls
onto the white dwarf. The observed amount of X-rays emitted at different
energies agrees with expectations for a tidal disruption event.
The researchers estimate that the destroyed planet would have
contained about a third of the mass of Earth, while the white dwarf has
about 1.4 times the Sun's mass.
While the case for the tidal disruption of a planet is not iron-clad,
the argument for it was strengthened when astronomers used data from
the multiple telescopes to help eliminate other possible explanations
for the detected X-rays. For example, the source does not show some of
the distinctive features of a binary containing a neutron star, such as
pulsations or rapid X-ray bursts. Also, the source is much too faint in
radio waves to be part of a binary system with a stellar-mass black hole.
A paper
describing these results was published in an October 2014 issue of the
Monthly Notices of the Royal Astronomical Society. The first author is
Melania Del Santo of the National Institute for Astrophysics (INAF),
IASF-Palermo, Italy, and the co-authors are Achille Nucita of the
Universitá del Salento in Lecce, Italy; Giuseppe Lodato of the
Universitá Degli Studi di Milano in Milan, Italy; Luigi Manni and
Francesco De Paolis of the Universitá del Salento in Lecce, Italy;
Jay Farihi of University College London in London, UK; Giovanni De
Cesare of the National Institute for Astrophysics in IAPS-Rome, Italy
and Alberto Segreto of the National Institute for Astrophysics (INAF),
IASF-Palermo, Italy.
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 NGC 6388:
Release Date: April 16, 2015
Scale Image: is 3 arcmin across (about 38 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 17h 36m 17.46s | Dec -44° 44' 08.34"
Constellation: Scorpius
Observation Date: 2 pointings on 21 Apr 2005 and 29 Aug 2011
Observation Time: 13 hours.
Obs. ID: 5505, 12453
Instrument: ACIS
References: Del Santo, M. et al, 2014, MNRAS, 444, 93; arXiv:1407.5081
Color Code: X-ray (Pink); Optical (Red, Green, Blue)
Distance Estimate: About 43,000 light years
Source: NASA’s Chandra X-ray Observatory
