There were no new TNO/Centaur/SDO discoveries announced since the previous issue of Distant EKOs.
Objects recently assigned numbers:
2003 AZ84 = (208996)
Current number of TNOs: 1093 (including Pluto)
Current number of Centaurs/SDOs: 242
Current number of Neptune Trojans: 6
Out of a total of 1341 objects:
556 have measurements from only one opposition
543 of those have had no measurements for more than a year
288 of those have arcs shorter than 10 days
(for more details, see:
http://www.boulder.swri.edu/ekonews/objects/recov_stats.gif)
Context. Pluto possesses a thin atmosphere, primarily composed of
nitrogen, in which the detection of methane has been reported.
Aims. The goal is to constrain essential but so far unknown parameters of Pluto's atmosphere such as the surface pressure, lower atmosphere thermal stucture, and methane mixing ratio.
Methods. We use high-resolution spectroscopic observations of gaseous methane, and a novel analysis of occultation light-curves.
Results. We show that (i) Pluto's surface pressure is currently in the
6.5-24 
bar range (ii) the methane mixing ratio is 0.5
0.1 %,
adequate to explain Pluto's inverted thermal structure and 
100 K
upper atmosphere temperature (iii) if Pluto has a troposphere, its depth
is at most 17 km, i.e. less than one pressure scale height, and methane
is supersaturated in most of it. The atmospheric and bulk surface
abundance of methane are strikingly similar, a possible consequence of
the presence of a CH4-rich top surface layer.
To appear in: Astronomy and Astrophysics Letters, 495, L17 (2009 March)
For preprints, contact emmanuel.lellouch@obspm.fr
or on the web at http://fr.arxiv.org/abs/0901.4882
See also http://www.eso.org/public/outreach/press-rel/pr-2009/pr-08-09.html
We present new photometric and spectroscopic data of the dwarf planet
Eris obtained on 2006 October and 2007 December with the Very Large
Telescopes at ESO, Chile. We use three different instruments (FORS,
ISAAC, and SINFONI) covering the 0.4-2.4 m wavelength range. We
show that N2 ice is not directly detected, but the wavelength
positions of the bands of CH4 measured on the complete wavelength
range seem to indicate that, as already suggested by Brown et al. and
Licandro et al., a part of CH4 ice is diluted in N2. Spectral
modeling using the Hapke theory reveals that a segregation of small and
large particles of methane ice could exist on the surface. The presence
of water ice and nitrogen is not completely excluded even if the
respective absorption bands of these ices have not been directly
detected. We present in this paper our methods to determine the
wavelength shifts of the methane bands and the chemical composition from
spectral modeling.
Published in: The Astronomical Journal, 137, 315 (2009 January)
For preprints, contact frederic.merlin@obspm.fr
The Centaurs are recent escapees from the Kuiper belt that are destined either to meet fiery oblivion in the hot inner regions of the Solar system or to be ejected to the interstellar medium by gravitational scattering from the giant planets. Dynamically evolved Centaurs, when captured by Jupiter and close enough to the Sun for near-surface water ice to sublimate, are conventionally labeled as ``short-period'' (specifically, Jupiter-family) comets. Remarkably, some Centaurs show comet-like activity even when far beyond the orbit of Jupiter, suggesting mass-loss driven by a process other than the sublimation of water ice. We observed a sample of 23 Centaurs and found nine to be active, with mass-loss rates measured from several kg s-1 to several tonnes s-1. Considered as a group, we find that the ``active Centaurs'' in our sample have perihelia smaller than the inactive Centaurs (median 5.9 AU vs. 8.7 AU), and smaller than the median perihelion distance computed for all known Centaurs (12.4 AU). This suggests that their activity is thermally driven. We consider several possibilities for the origin of the mass-loss from the active Centaurs. Most are too cold for activity at the observed levels to originate via the sublimation of crystalline water ice. Solid carbon monoxide and carbon dioxide have the opposite problem: they are so volatile that they should drive activity in Centaurs at much larger distances than observed. We consider the possibility that activity in the Centaurs is triggered by the conversion of amorphous ice into the crystalline form accompanied by the release of trapped gases, including carbon monoxide. By imposing the condition that crystallization should occur when the crystallization time is shorter than the orbital period we find a qualitative match to the perihelion distribution of the active Centaurs and conclude that the data are consistent with the hypothesis that the Centaurs contain amorphous ice.
To appear in: The Astronomical Journal
Preprints available on the web at
http://www.ifa.hawaii.edu/faculty/jewitt/papers/2009/J09.pdf
It has long been assumed that the planet Jupiter acts as a giant shield, significantly lowering the impact rate of minor bodies upon the Earth, and thus enabling the development and evolution of life in a collisional environment which is not overly hostile. In other words, it is thought that thanks to Jupiter, mass extinctions have been sufficiently infrequent that the biosphere has been able to diversify and prosper. However, in the past, little work has been carried out to examine the validity of this idea. In the second of a series of papers, we examine the degree to which the impact risk resulting from objects on Centaur-like orbits is affected by the presence of a giant planet, in an attempt to fully understand the impact regime under which life on Earth has developed. The Centaurs are a population of ice-rich bodies which move on dynamically unstable orbits in the outer Solar system. The largest Centaurs known are several hundred kilometres in diameter, and it is certain that a great number of kilometre or sub-kilometre sized Centaurs still await discovery. These objects move on orbits which bring them closer to the Sun than Neptune, although they remain beyond the orbit of Jupiter at all times, and have their origins in the vast reservoir of debris known as the Edgeworth-Kuiper belt that extends beyond Neptune. Over time, the giant planets perturb the Centaurs, sending a significant fraction into the inner Solar System where they become visible as short-period comets. In this work, we obtain results which show that the presence of a giant planet can act to significantly change the impact rate of short-period comets on the Earth, and that such planets often actually increase the impact flux greatly over that which would be expected were a giant planet not present.
To appear in: International Journal of Astrobiology
For preprints, contact j.a.horner@open.ac.uk
or on the web at http://arxiv.org/abs/0903.3305
Aims. The aim of this work is to estimate the size distribution of
small Trans-Neptunian objects.
Methods. We simulate the irradiation and collisional processes affecting the surface of a Trans-Neptunian object using for the first time as constraint the peculiar crystalline to amorphous water ice ratio observed on (136108) 2003 EL61.
Results. We find that the size distribution changes its exponent
from q0=4.2 at larger sizes to
at the smaller ones,
with a break radius of 
km. If this size distribution is
applied to study the collisional surface evolution of (136108) 2003 EL61,
we find that the object must be covered by a thin ice crust of
cm, while the original composition of the object is
still present at 1.61 cm or more below the surface. This result is
not affected by a collision with a large projectile that occurred by
chance more than 109 yr ago since after a short time the mean value
obtained for the crystalline to amorphous water ice ratio is
indistinguishable from that obtained without a collision with a large
projectile. Since the simulations are not sensitive to the effects of
very small projectiles (rp<10-30 m), it is possible that the
exponent of the size distribution for these very small objects change
again approaching a Donhanyi's size distribution.
To appear in: Astronomy & Astrophysics
For preprints, contact rgilhutton@casleo.gov.ar
By examining the absolute magnitude (H) distributions (hereafter HD)
of the cold and hot populations in the Kuiper belt and of the Trojans
of Jupiter, we find evidence that the Trojans have been captured from
the outer part of the primordial trans-Neptunian planetesimal disk. We
develop a sketch model of the HDs in the inner and outer parts of the
disk that is consistent with the observed distributions and with the
dynamical evolution scenario known as the `Nice model'. This leads us
to predict that the HD of hot population should have the same slope of
the HD of the cold population for 6.5<H<9, both as steep as the slope of
the Trojans' HD. Current data partially support this prediction, but
future observations are needed to clarify this issue. Because the HD
of the Trojans rolls over at 
to a collisional equilibrium
slope that should have been acquired when the Trojans were still
embedded in the primordial trans-Neptunian disk, our model implies
that the same roll-over should characterize the HDs of the Kuiper belt
populations, in agreement with the results of Bernstein et al. (2004)
and Fuentes and Holman (2008). Finally, we show that the constraint on
the total mass of the primordial trans-Neptunian disk imposed by the
Nice model implies that it is unlikely that
the cold population formed beyond 35 AU.
To appear in: Icarus
For preprints, contact morby@oca.eu
or on the web at http://www.oca.eu/morby/papers/Note-SFD3.pdf
We present new color results of cometary nuclei obtained with the Hubble Space Telescope (HST) whose superior resolution enables us to accurately isolate the nucleus signals from the surrounding comae. By combining with scrutinized available data obtained with ground-based telescopes, we accumulated a sample of 51 cometary nuclei, 44 ecliptic comets (ECs) and 7 nearly-isotropic comets (NICs) using the nomenclature of Levison (1996). We analyze color distributions and color-color correlations as well as correlations with other physical parameters. We present our compilation of colors of 232 outer solar system objects - separately considering the different dynamical populations, classical KBOs in low and high-inclination orbits (respectively CKBO-LI and CKBO-HI), resonant KBOs (practically Plutinos), scattered-disk objects (SDOs) and Centaurs - of 12 candidate dead comets, and of 85 Trojans. We perform a systematic analysis of all color distributions, and conclude by synthesizing the implications of the dynamical evolution and of the colors for the origin of the minor bodies of the solar system. We find that the color distributions are remarkably consistent with the scenarios of the formation of TNOs by Gomes (2003) generalized by the "Nice" model (Levison et al., 2008), and of the Trojans by Morbidelli et al. (2005). The color distributions of the Centaurs are globally similar to those of the CKBO-HI, the Plutinos and the SDOs. However the potential bimodality of their distributions allows to possibly distinguish two groups based on their (B-R) index: Centaurs I with (B-R) > 1.7 and Centaurs II with (B-R) < 1.4. Centaurs I could be composed of TNOs (prominently CKBO-LI) and ultra red objects from a yet unstudied family. Centaurs II could consist in a population of evolved objects which have already visited the inner solar system, and which has been scattered back beyond Jupiter. The diversity of colors of the ECs, in particular the existence of very red objects, is consistent with an origin in the Kuiper belt. Candidate dead comets represent an ultimate state of evolution as they conspicuously appear more evolved than the Trojans and Centaurs II.
To appear in: Icarus
For preprints, contact Philippe Lamy: philippe.lamy@oamp.fr
We present the results of a wide-field survey for distant Sedna-like
bodies in the outer solar system using the 1.2-m Samuel Oschin Telescope
at Palomar Observatory. We searched 12,000 deg2 down to a mean
limiting magnitude of 21.3 in R. A total number of 53 Kuiper belt
objects and Centaurs have been detected, 25 of which were discovered in
this survey. No additional Sedna-like bodies with perihelia beyond 70 AU
were found despite a sensitivity to motions out to 1000 AU. We
place constraints on the size and distribution of objects on Sedna
orbits.
Published in: The Astrophysical Journal Letters, 694, L45 (2009 March 20)
For preprints, contact mschwamb@gps.caltech.edu
or on the web at
http://www.iop.org/EJ/abstract/1538-4357/694/1/L45/
A novel method of generating artificial scintillation noise is
developed and used to evaluate occultation rates and false positive
rates for surveys probing the Kuiper Belt with the method of
serendipitous stellar occultations. A thorough examination of
survey design shows that: (1) diffraction-dominated occultations are
critically (Nyquist) sampled at a rate of 2 Fsu-1, corresponding
to 40 s-1 for objects at 40 AU, (2) occultation detection rates
are maximized when targets are observed at solar opposition, (3)
Main Belt Asteroids will produce occultations lightcurves identical
to those of Kuiper Belt Objects if target stars are observed at
solar elongations of:
, or
,
and (4) genuine KBO occultations are likely to be so rare that a
detection threshold of 
should be adopted to
ensure that viable candidate events can be disentangled from false
positives.
To appear in: The Astronomical Journal
For preprints, contact bick@astro.princeton.edu
or on the web at http://arXiv.org/abs/0902.3457
The Taiwanese-American Occultation Survey (TAOS) operates four fully automatic telescopes to search for occultations of stars by Kuiper Belt Objects. It is a versatile facility that is also useful for the study of initial optical GRB afterglows. This paper provides a detailed description of the TAOS multi-telescope system, control software, and high-speed imaging.
Published in: Publications of the Astronomical Society of the Pacific, 121, 138
Preprints available on the web at
http://arxiv.org/abs/0802.0303
We present here HST NICMOS F110W and F160W observations of Haumea, and
its two satellites Hi'iaka and Namaka. From the measured (F110W-F160W)
colours of
, 
, and 
mag for
each object, respectively, we infer that the 1.6 m water-ice
absorption feature depths on Hi'iaka and Namaka are at least as deep as
that of Haumea. The light-curve of Haumea is detected in both filters,
and we find that the infrared colour is bluer by 
at the
phase of the red spot. These observations suggest that the satellites of
Haumea were formed from the collision that produced the Haumea
collisional family.
Published in: The Astrophysical Journal Letters, 695, L1 (2009 April)
For preprints, contact fraserw@gps.caltech.edu
or online at http://arxiv.org/abs/0903.0860
Context. Previously known as 2003 EL61, (136108) Haumea, is the largest member of a group of trans-Neptunian objects (TNOs) with similar orbits and `unique' spectral characteristics in the form of a neutral slope in the visible and the deepest water ice absorption bands observed in the trans-neptunian belt (TNb). Studying the surface of 2003 EL61 provides useful constraints of the origin of this particular group of TNOs and about the outer Solar System's history.
Aims. In this work we attempt to study the composition of the surface of 2003 EL61.
Methods. We present visible and near-infrared spectra of 2003 EL61 obtained with the 4.2m WHT and the 3.6m TNG telescopes at the ``Roque de los Muchachos'' Observatory (Canary Islands, Spain). Near-infrared spectra were obtained at different rotational phases covering almost one complete rotational period. Spectra are fitted using scattering models based on Hapke theory and constraints on the surface composition are derived.
Results. The observations confirm previous results that the 2003 EL61 spectrum is neutral in color and exhibits deep water-ice absorption bands. They also provide new facts about the surface of this object: the lack of significant variations in the spectral slope (in the near-infrared) and the depth of the water-ice absorption bands at different rotational phases clearly evident in the data, suggest that the surface of 2003 EL61 is homogeneous. The scattering models indicate that a 1:1 intimate mixture of crystalline and amorphous water ice is the most probable surface composition of this big TNO, and constrain the presence of other minor constituents to a maximum traction of 8%.
Conclusions. The derived composition suggests that: a) cryovolcanism is unlikely to be the resurfacing process that retains the surface of this TNO, and the other members of this population, covered mainly by water ice; b) the surface is older than 108 yr, which constrains the timescale of any catastrophic event, such as the collision suggested to be the origin of this population, to at least 108 yr; c) the surface of 2003 EL61 is depleted of carbon-bearing species. According to the orbital parameters of the population, this implies that is a possible source of carbon-depleted, Jupiter-Family comets.
Published in: Astronomy & Astrophysics 496, 547 (2009 March)
For preprints, contact npinilla@tng.iac.es
or on the web at http://arxiv.org/abs/0803.1080
Using precise relative astrometry from the Hubble Space Telescope and the W. M. Keck Telescope, we have determined the orbits and masses of the two dynamically interacting satellites of the dwarf planet (136108) Haumea, formerly 2003 EL61. The orbital parameters of Hi'iaka, the outer, brighter satellite, match well the previously derived orbit. On timescales longer than a few weeks, no Keplerian orbit is sufficient to describe the motion of the inner, fainter satellite Namaka. Using a fully-interacting three point-mass model, we have recovered the orbital parameters of both orbits and the mass of Haumea and Hi'iaka; Namaka's mass is marginally detected. The data are not sufficient to uniquely determine the gravitational quadrupole of the non-spherical primary (described by J2). The nearly co-planar nature of the satellites, as well as an inferred density similar to water ice, strengthen the hypothesis that Haumea experienced a giant collision billions of years ago. The excited eccentricities and mutual inclination point to an intriguing tidal history of significant semi-major axis evolution through satellite mean-motion resonances. The orbital solution indicates that Namaka and Haumea are currently undergoing mutual events and that the mutual event season will last for the next several years.
To appear in: The Astronomical Journal
For preprints, contact darin@gps.caltech.edu
or on the web at http://arxiv.org/abs/0903.4213
We calculate the 
-ray albedo due to cosmic-ray interactions with
debris (small rocks, dust, and grains) in the Oort Cloud. We show that
under reasonable assumptions a significant proportion of what is called
the ``extragalactic -ray background'' could be produced at the
outer frontier of the solar system, and may be detectable by the Large
Area Telescope, the primary instrument on the Fermi Gamma-ray Space
Telescope. If detected, it could provide unique direct information about
the total column density of material in the Oort Cloud that is difficult
to access by any other method. The same -ray production process
takes place in other populations of small solar system bodies, such as
Main Belt asteroids, Jovian and Neptunian Trojans, and Kuiper Belt
objects. Their detection can be used to constrain the total mass of
debris in these systems.
Published in: The Astrophysical Journal Letters, 692, L54 (2009 February 10)
Preprints, on the web at http://arxiv.org/abs/0901.0304
We accept submissions for the following sections:
Distant EKOs is not a refereed publication, but is a tool for furthering communication among people interested in Kuiper belt research. Publication or listing of an article in the Newsletter or the web page does not constitute an endorsement of the article's results or imply validity of its contents. When referencing an article, please reference the original source; Distant EKOs is not a substitute for peer-reviewed journals.
