THERMAL MODEL OF THE ACTIVE CENTAUR P2004 A1 LONEOS

1
THERMAL MODEL OF THE ACTIVE CENTAUR P/2004 A1
(LONEOS)

• M.T. Capria 1, A. Coradini 2, M.C. De Sanctis1,
  E. Mazzotta Epifani 3, P. Palumbo 4, M. Fulle 5 ,
  G. Cremonese 6
• (1) INAF-IASF, Roma (2) INAF-IFSI, Roma (3)
  INAF-OAC, Napoli (4) Univ. Parthenope, Napoli
  (5) INAF-OAT, Trieste (6) INAF-OAPD, Padova

2
P/2004 A1 (LONEOS)

• P/2004 A1 (LONEOS), discovered during the LONEOS
  program in 2004, is an active object with
• q 5.46 AU
• e 0.308
• i 8.2
• P 22.2 years
• It is a Centaur
• It was observed at TNG telescope on April 3rd,
  2005, when at r 5.54 AU and
• ? 4.69 AU

Mazzotta Epifani et al., 2006
3
P/2004 A1 (LONEOS)
What we know from observations (direct
measurements)
Before encounter

• A close encounter with Saturn in 1992 changed the
  orbital parameters
• It has a well developed coma and tail at
  rh5.5-5.6 AU post perihelion
• Af? values 309-257 cm at rh5.5-5.6 AU post
  perihelion
• Relative R magnitude at rh5.5-5.6 AU post
  perihelion 11.-12.

After encounter
4
P/2004 A1 (LONEOS)
What we know from the interpretation of the
observations

• In the tail there are grains released even close
  to the past aphelion
• The tail is steady (not deriving from impulsive
  events)
• There is evidence of particles 1 cm big at least
• Dust mass loss between 100 and 200 kg s-1
• An abundant dust production was probably present
  since before the close encounter with Saturn
• Upper limit on nucleus radius 9.2 km

Mazzotta Epifani et al., 2006
5
P/2004 A1 (LONEOS)

• We will try to answer to the following questions
• It is possible to explain this activity with a
  standard thermal model (sublimation from ices
  and/or trapped gases, entraining dust)?
• Which was the effect of the orbit perturbation on
  the observed activity?

6
1 - It is possible to explain this activity with
a standard thermal model?
THE NUCLEUS THERMAL MODEL
The water ice can be initially in the amorphous
phase and then it can undergo an irreversible,
exothermic phase transition to crystalline form.
A part of the volatiles can be initially trapped
in the amorphous ice. The temperature on the
surface is obtained by a balance between the
solar input and the energy re-emitted in the
infrared, conducted in the interior and used to
sublimate surface ices. The numerical code
computes how the heat diffuses in the nucleus,
inducing the water ice phase transition and the
sublimation-recondensation of water and
volatiles.

• The nucleus is a porous sphere composed by a
  mixture of ices (water, CO, CO2) and a
  refractory component.
• Energy and mass conservation equations are solved
  for the whole nucleus

Coradini et al., 1997 Capria et al., 2002
7
THE NUCLEUS THERMAL MODEL
3
Free particles can move toward the surface and be
blown off or accumulate to form a devolatilized
layer.

2
When the ice sublimates the embedded particles
become free and can undergo the drag exerted by
the gas flux. Pores are widening...
1
The refractory material is described as a
distribution of spherical grains distributed in
different size classes. The grains are initially
embedded in the ice.
8
THE MODEL INITIAL PARAMETERS
Onset of activity (Meech and voren, 2004)

Production rates relative to water
(Bockelée-Morvan et al., 2004)
9
THE MODEL INITIAL PARAMETERS

Surface temperature
Some CO (10 percent wrt water) is trapped in the
amorphous ice and released during the transition
to crystalline phase
we are assuming that the body was active even
before the change in the orbital parameters
10
THE MODEL RESULTS
Gas fluxes along the orbit

CO
Close encounter
CO2
11
THE MODEL RESULTS
Dust flux along the orbit

Close encounter
Reasonable approximation of the observed activity
12
THE MODEL RESULTS
The interior temperature, CO and CO2 abundances
versus radius

CO
CO2
surface
At the moment of the observation
13
OTHER POSSIBILITIES?
All of CO is trapped in the amorphous ice a lot
of CO2

• Can we explain the observed activity assuming
  that
• the CO flux is only the result of the
  amorphous-crystalline ice transitionis and the
  consequent release of trapped gases?
• a lot of CO2 was present?

CO (20 percent wrt water) is trapped in the
amorphous ice and released during the transition
to crystalline phase
14
OTHER POSSIBILITIES?
All of CO is trapped in the amorphous ice a lot
of CO2

CO2
CO
H2O
CO
CO2
Please note that the scales are different!
CO initially present both as an ice and as
trapped gas
15
OTHER POSSIBILITIES?
CO present in the interior both as an ice and as
a trapped gas

• We obtain a good approximation of the
  observations, and we explain even more distant
  activity but
• We need a lot of CO, close to the surface we
  need CO (also) as an ice.

All of CO is trapped in the amorphous ice a lot
of CO2

• We explain the activity on the new orbit, but
• it is more difficult to explain the distant
  activity, if any
• It is unclear how much (and how many) of a
  volatile can be trapped in the amorphous ice

We cannot rule out the determinant contribution
of volatiles different than CO, but they should
have been much more abundant than ever observed
16
2- Which was the effect of the orbit perturbation
on the observed activity?
The dust particles are initially (in the nucleus)
distributed in 5 size classes, following a
gaussian distribution
The emitted dust particles (10-6 -10-4 m size
classes only) follow a different distribution,
larger grains (10-4 m) are emitted only during
peaks in the activity
1984
2003
1993
A clearly observable spike in the activity a
well known phenomenon
17
CONCLUSIONS
It is possible to explain the activity of P/2004
A1 (LONEOS, even, if any, on the old orbit, with
a standard thermal model ?
Yes it is, if a supervolatile-rich body is
assumed
Which was the effect of the orbit perturbation on
the observed activity?
An increase in the activity, probable emission of
larger grains
Can we explain the emission of the larger dust
grains?
Yes, if we assume that they are surface fragments