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
Δ = 4.69 AU
Mazzotta Epifani et al., 2006

3. P/2004 A1 (LONEOS) What we know from observations
(direct measurements)
Before encounter π α a e i
9.78
14.56
12.17
0.20
11.39
A close encounter with Saturn in 1992 changed the orbital parameters
It has a well developed coma and tail at rh= AU post perihelion
Afρ values: cm at rh= AU post perihelion
Relative R magnitude at rh= AU post perihelion:
After encounter π α a e i
4.79
10.22
7.51
0.20
17.73

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. THE NUCLEUS THERMAL MODEL
1 - It is possible to explain this activity with a standard 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 MODEL3
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 Švoren, 2004)
T (K) AU
Ice Ih subl.
180
Ice Ia phase trans.
90-160 11
CO2 subl. 80 13
CO subl. 25
120
Production rates relative to water (Bockelée-Morvan et al., 2004)
H2O
100
CO2
3-6 CO
0.9-30

9. THE MODEL: INITIAL PARAMETERS
dust/ice 1
CO2/H2O
0.03
CO/H2O
0.3
Trapped CO
0.1
radius (km) 10
bulk K (W/K/m)
0.05
density (kg/m3)
401
initial T (K) 25
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. Gas fluxes along the orbit
THE MODEL: RESULTS
Gas fluxes along the orbit CO
Close encounter
CO2

11. Dust flux along the orbit
THE MODEL: RESULTS
Dust flux along the orbit
Close encounter
Reasonable approximation of the observed activity…

12. The interior: temperature, CO and CO2 abundances versus radius
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
dust/ice 1
CO2/H2O
0.1
CO/H2O
0.0
Trapped CO
0.2
radius (km) 10
bulk K (W/K/m)
0.05
initial T (K) 25
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 ( m size classes only) follow a different distribution, larger grains (10-4 m) are emitted only during peaks in the activity
1984
1993
2003
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