PDF (6M) - Isaac Newton Group of Telescopes - Instituto de ...

2005 FY 9
is the third brightest known TNO, after 2003
UB 313
and Pluto. The size of 2005 FY 9
is approximately 0.7
times that of Pluto. The semi-major axis of its orbit is 46
AU, the perihelion distance is 39 AU and the inclination of
the orbit is 29°. These values are typical of the classical
TNO family.
Visible spectroscopy of 2005FY 9
was obtained using the
ISIS spectrograph on the WHT, and near-infrared
spectroscopy was obtained using the NICS spectrograph
on the TNG. The complete visible and near-infrared spectra
were compared with the spectrum of Pluto and that of pure
methane ice and it was found that the spectra of both TNOs
are very similar. They are dominated by strong absorption
bands produced by methane ice. In fact, the absorption
bands in the spectrum of 2005 FY 9
are deeper than in the
spectrum of Pluto, as a result of the larger abundance of
methane ice in 2005 FY 9
. Also the colour of the surface of
the TNO (indicated by the slope of the spectrum) is red,
similar to that of Pluto. This shows the presence of complex
organic compounds in the surface.
Until now only one known TNO, Pluto, showed the
presence of strong methane ice absorption bands in its
spectrum. However, apart from 2005 FY 9
, these bands
were also recently observed in the spectrum of the largest
yet known TNO, 2003UB 313
, as shown in the Figure 5. The
near infrared spectrum of 2003 UB 313
is very similar to that
of 2005 FY 9
.
The visible spectrum of 2003UB 313
shows very prominent
absorption bands from solid CH 4
. The icy-CH 4
bands are
significantly stronger than those of Pluto and slightly
weaker than those observed in the spectrum of 2005 FY 9
.
A shift relative to the position of the bands in the spectrum
of laboratory CH 4
ice is observed in the bands at larger
wavelengths, but not at shorter wavelengths, which can be
explained by a vertical compositional gradient. Purer
methane could have condensed first, while 2003 UB 313
moved towards aphelion during the last 200 years: as the
atmosphere gradually collapsed, the composition became
more nitrogen-rich due to the fact that most volatile
components condensed and CH 4
diluted in N 2
, present in
the outer surface layers.
THE YORP EFFECT DETECTED ON NEAR-
EARTH ASTEROID 2000 PH5
The Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP)
effect is believed to alter the way small bodies in the solar
system rotate. YORP is a torque due to sunlight hitting the
surfaces of asteroids and meteoroids and warming their
Figure 4. Top: The spectrum of 2005 FY 9 compared with the
spectrum of Pluto and that of pure methane ice. Notice the strong
methane ice absorption bands present in the spectrum of both
TNOs. Bottom: Near infrared spectrum of 2005 FY 9 compared
with that of TNO 2003 UB313. The similarity of both spectra shows
that the surface composition of the two objects must be similar
(figure from Licandro et al., 2006, A&A, 445, L35).
surfaces, leading to a gentle recoil effect as the heat is
emitted. By analogy, if one were to shine light on a propeller
over a long enough period, it would start spinning.
Although this is an almost immeasurably weak force,
astronomers believe it may be responsible for spinning
some asteroids up so fast that they break apart, perhaps
leading to the formation of binary asteroids. Others may be
slowed down so that they take many days to rotate once.
The YORP effect also plays an important role in changing
the orbits of asteroids between Mars and Jupiter, including
their delivery to planet-crossing orbits. Despite its
importance, the effect has never been seen acting on a
solar system body, until now.
Using extensive optical and radar imaging from powerful
Earth-based observatories, astronomers have directly
14 • ING BIENNIAL R EPORT 2006–2007