A warped dust disk around Fomalhaut : evidence for a planetary system
A Warped dust disk around Fomalhaut -
Evidence for planetary perturbations?
Wayne Holland & Jane Greaves (UKATC/JAC)
Cold dust around nearby stars
The discovery of large reservoirs of cold dust around nearby stars in
the 1980s provided strong evidence for the existence of planetary systems
other than our own. If no planets exist in such systems then the density
and brightness of the dust should increase smoothly with drag forces towards
the central star. However, if planets do exist interplanetary dust will
interact with these larger bodies whilst spiraling in towards the star,
resulting in irregular variations in both the density and the corresponding
brightness distribution. Although the edge-on disk around b
Pictoris had been studied extensively, it was not until 1998 that significant
new results emerged (Holland et al. 1998; Jayawardhana et al. 1998; Koerner
et al. 1998; Greaves et al. 1998). In particular, submillimetre-wave techniques
can circumvent the difficulties of the starlight that dominates at optical
and near-IR wavelengths, by imaging the faint thermal emission from cold
dust grains. The much hotter star has very little flux at these long wavelengths.
The submillimetre images of Vega, Fomalhaut and e
Eridani showed dust disks similar in size to the Sun's Kuiper Belt of comets
(Holland et al. 1998; Greaves et al. 1998) and revealed evidence of central
cavities in the emission near the star possibly cleared-out by the formation
The need for high resolution imaging?
Although intriguing, these observations lacked sufficient spatial resolution
to investigate smaller scale structure, such as the asymmetries and warps
that are potential signatures of planetary perturbations. New data at short-submillimetre
wavelengths of Fomalhaut shows the presence of a "warp" in the observed
dust torus. At 450m m, where the telescope beam-size
is equivalent to a resolution of 50 AU at the distance of Fomalhaut, the
dust disk appears to have a distinct bend in the connecting emission between
the two offset peaks (see Figure 1). The image confirms that we are looking
at an almost edge-on dust disk or ring, with a cavity devoid of emission
out to approximately 100 AU radius from the star. The two bright peaks
result from looking "down the ends" of the ring where the effective column
density is largest. The new data clearly benefited from not only some of
the best submillimetre weather on Mauna Kea, but are also substantially
improved in both sensitivity and calibration accuracy over the previous
work (due to the new wide-band filter on the short-wave array, and the
calibration work of Archibald et al. paper in prep - also see JCMT
webpage for details).
Figure 1. 5-hr jiggle-map observation
of Fomalhaut at 450m m. North is up and east
is to the left and the star position is marked by the "star" system. The
telescope beam size is shown for comparison, along with the size of Pluto's
orbit for scaling purposes.
Several possible theories for the origin of the asymmetry are possible.
Papers in preparation (Holland et al., Wyatt et al.) will show that no
smooth ring model (e.g. pericentre glow) can fit the data, and the asymmetry
is unlikely to be explicable by dust generation in planetesimal collisions.
The most plausible model is that the dust is concentrated in orbital
resonances with a planet. A comparison with recent numerical simulations
shows that the data can indeed be qualitatively modeled with a Saturn-like
object hidden in a gap in the torus. In figure 2 we show a simple de-projection
of the 450m m image. This was first rotated
for convenience so that the peaks lie in a vertical line, and then stretched
by a factor of three along the horizontal axis. This is roughly equivalent
to converting the image from an inclination of 19.5 degrees to face-on,
although it does not take into account degradation by the finite beam size
or a non-zero disk thickness (we are currently investigating ways of doing
this?). The result shows a long arc below the star and a shorter one to
the upper left. There is a small gap to the left of the star, and a much
larger one covers the remainder of the ring.
A unique interpretation is certainly not possible at this stage, but
we show for comparison (also in figure 2) a model planetary system from
Ozernoy et al. (2000). Here there is a 0.3 M(Jupiter) (roughly Saturn-mass)
planet orbiting at the ring radius, and dust is collected into librating
orbits about the L4 and L5 Lagrangian points. There is a good resemblance
between this model and the Fomalhaut de-projection, after taking into account
the large beam size. In particular, the small and large gaps and short
and long arcs are clearly present.
Figure 2. (left) De-projected (stretched)
450m m image of Fomalhaut with telescope beam,
and (right) Ozernoy model.
There are several possible configurations producing arcs in the models
of Ozernoy et al. (2000), depending on the planet mass and the presence
or absence of dust in outer resonances as well as along the planet's orbit.
Thus this is a hypothesis rather than a solution to the data -
observations at different epochs would be a good test to further investigate
this theory. However, the most plausible model is that a large planet in
reasonably close association with the Fomalhaut dust disk has created very
severe perturbations. As in the case of e Eridani,
this would be evidence for planetary companions at larger radii from the
star, 60- 100 AU, than observed in our own Solar
This work is still on-going with data being collected on other systems
such as Vega and e Eridani at a wavelength of
450m m. Until the ALMA interferometer comes
on-line later this decade, the JCMT and SCUBA offer a unique way to study
such systems with unparalleled sensitivity.
We thank all our collaborators who have contributed to this research:
Bill Dent, Mark Wyatt (UKATC), Ben Zuckerman, Chris McCarthy (UCLA), Rich
Webb (NASA Ames), Iain Coulson, Gerald Moriarty-Schieven, Ian Robson (JAC),
Dolores Walther (Gemini), Walter Gear (Cardiff), Helen Walker
(CCLRC) and Harold Butner (SMTO). This research was supported in part by
PPARC funding, and by NSF and NASA grants to UCLA.
1. Holland et al.1998, Nature 392, 788
2. Jayawardhana et al.1998, ApJ 503, L79
3. Koerner et al. 1998, ApJ 503, L83
4. Greaves et al. 1998, ApJ 506, L133
5. Ozernoy et al. 2000, ApJ 537, L1
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