17 research outputs found
A planetary system as the origin of structure in Fomalhaut's dust belt
The Sun and >15 percent of nearby stars are surrounded by dusty debris disks
that must be collisionally replenished by asteroids and comets, as the dust
would otherwise be depleted on <10 Myr timescales (ref. 1). Theoretical studies
show that disk structure can be modified by the gravitational influence of
planets (ref. 2-4), but the observational evidence is incomplete, at least in
part because maps of the thermal infrared emission from disks have low linear
resolution (35 AU in the best case; ref. 5). Optical images provide higher
resolution, but the closest examples (AU Mic and Beta Pic) are edge-on (ref.
6,7), preventing the direct measurement of azimuthal and radial disk structure
that is required for fitting theoretical models of planetary perturbations.
Here we report the detection of optical light reflected from the dust grains
orbiting Fomalhaut (HD 216956). The system is inclined 24 degrees away from
edge-on, enabling the measurement of disk structure around its entire
circumference, at a linear resolution of 0.5 AU. The dust is distributed in a
belt 25 AU wide, with a very sharp inner edge at a radial distance of 133 AU,
and we measure an offset of 15 AU between the belt's geometric centre and
Fomalhaut. Taken together, the sharp inner edge and offset demonstrate the
presence of planet-mass objects orbiting Fomalhaut.Comment: 8 pages, 3 figures, 1 tabl
A gas cloud on its way towards the super-massive black hole in the Galactic Centre
Measurements of stellar orbits provide compelling evidence that the compact
radio source Sagittarius A* at the Galactic Centre is a black hole four million
times the mass of the Sun. With the exception of modest X-ray and infrared
flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and
radiation efficiency near the event horizon are currently very low. Here we
report the presence of a dense gas cloud approximately three times the mass of
Earth that is falling into the accretion zone of Sgr A*. Our observations
tightly constrain the cloud's orbit to be highly eccentric, with an innermost
radius of approach of only ~3,100 times the event horizon that will be reached
in 2013. Over the past three years the cloud has begun to disrupt, probably
mainly through tidal shearing arising from the black hole's gravitational
force. The cloud's dynamic evolution and radiation in the next few years will
probe the properties of the accretion flow and the feeding processes of the
super-massive black hole. The kilo-electronvolt X-ray emission of Sgr A* may
brighten significantly when the cloud reaches pericentre. There may also be a
giant radiation flare several years from now if the cloud breaks up and its
fragments feed gas into the central accretion zone.Comment: in press at Natur
The Lyman Alpha Forest in the Spectra of QSOs
Observations of redshifted Lyman alpha forest absorption in the spectra of
quasistellar objects (QSOs) provide a highly sensitive probe of the
distribution of gaseous matter in the universe. Over the past two decades
optical spectroscopy with large ground-based telescopes, and more recently
ultraviolet spectroscopy from space have yielded a wealth of information on
what appears to be a gaseous, photoionized intergalactic medium, partly
enriched by the products of stellar nucleosynthesis, residing in coherent
structures over many hundreds of kiloparsecs. Recent progress with cosmological
hydro-simulations based on hierarchical structure formation models has led to
important insights into the physical structures giving rise to the forest. If
these ideas are correct, a truely inter- and proto-galactic medium [at high
redshift (z ~ 3), the main repository of baryons] collapses under the influence
of dark matter gravity into flattened or filamentary structures, which are seen
in absorption against background QSOs. With decreasing redshift, galaxies
forming in the denser regions, may contribute an increasing part of the Lyman
alpha absorption cross-section. Comparisons between large data samples from the
new generation of telescopes and artificial Lyman alpha forest spectra from
cosmological simulations promise to become a useful cosmological tool.Comment: latex plus three postscript figures, uses psfig,sty; Annual Review of
Astronomy and Astrophysics 1998, vol. 36 (in press