THE INFLUENCE OF PARTICLE SHAPES AND SIZESIN THE CO ICE BAND PROFILE THE INFLUENCE
R. Vilaplana1, M.A. Satorre1 and F. Moreno2
Universidad Politécnica de Valencia, Dpto. Applied Physics, Alcoy, Spain
2 Instituto de Astrofísica de Andalucía (CSIC), Dpto. Solar System, Granada, Spain
1Dust particles play a key role in the balance of thermal control in different astrophysical environments such as interstellar and circumstellar media and cometary dust tails. Thegrain morphology can change from one astrophysical environment to another. However, ices such as H2O, CO and CO2 are typical constituents. They form a mantle covering thesmall core grains. The coagulation could lead to the formation of heterogeneous mixtures (ice/silicates/carbonaceous material) in a volume or an aggregate. Light is scatteredand absorbed by these dust particles and emitted in the infrared (IR) range. Absorption and emission spectra and extinction curves give information about the constituents. The
peak profiles and their position are influenced by different factors, such as molecular environment [1, 2], temperature [3, 4, 5], irradiation history [6, 7] and particle shape andsize [8, 9].

Our knowledge of chemical and physical properties of ices in astrophysical environments ismainly based on the comparison between observations and laboratory measurements.

The laboratory measurements of IR ice spectra of astrophysical interest are obtained fromfilms formed by deposition at low temperature on a flat substrate. However, in the astrophysicalenvironments grains grow and ices are mixed in a volume or aggregate.

Scheme of the experimental setup

The spectrometer Bruker IFS 66v/S, the movable cold finger
and the bolometer

Regarding the observations, there are two independent solid CO components which are observed along most lines of sight (one narrow band generally centered at about 2140
cm-1 and a broader one at 2136 cm-1). Laboratory studies with astrophysical relevant mixtures of CO and CO2 ices have shown that the narrow CO band at 2140 cm-1 occurs inmixtures dominated by non-polar molecules (CO2, O2, N2), and the broad feature is due to mixtures with polar molecules such as H2O ice as have been shown by some authors[2, 10, 11, 12].

These authors also carried out calculations with scattering codes trying to understand the possible influence of the physical characteristics of the grains on these features
profiles. Therefore, it is of great interest to study how the grain morphology (shape and size) could affect the CO and CO2 ice band profiles.

CALCULATIONS:CALCULATIONS:
We have calculated the extinction factors of some CO and CO2 ice bands
using Mie and Discrete Dipole Approximation codes. Shapes such asspheres, spheroids of axis ratio 5, the so-called compact irregular particle(DW1996) and aggregates of spheres randomly generated with a size
monomer of 0.17 µm have been considered. The aggregates have been
generated by For the generation of the aggregates we performed a Fortran
implementation of both a particle-cluster aggregation (PCA) and a clustercluster aggregation (CCA) method. PCA process starts with a single
sphere in a fix position. mechanism. Initially, the calculations have been
carried out by considering only ice as constituent. The results are thefollowings:

We have tested the influence of including silicate as constituent. In order to do this we have randomlychanged the refractive index of 25% of the dipoles. A comparison between the extinction factors
obtained for a mixture of ice (75%) and silicate (25%) and the extinction factors previously calculatedwith only CO ice as a constituent for the aggregates showed in Fig. 3 and for size parameters of 0.5,
1, 1.5 and 2 is shown in the next figure.

Fig.4: The extinction factors
corresponding to aggregates of
CO ice with silicate (dotted line)with the shapes shown in Fig. 3.
The radius of the monomers is
set to 0.1 microns and the size
parameter to 0.5 (up-left), 1 (upright), 1.5 (bottom-left) and 2(bottom-right) compared withthe results previously obtained
to these aggregates without
silicate (solid line).

For the generation of the aggregates we performed a Fortran implementation of both a particlecluster aggregation (PCA) and a cluster-cluster aggregation (CCA) method. Fig. 5 shows an image of
two new aggregates. We set the radius of the monomers of these aggregates to 0.1 µm and the size
parameter of the whole particle to X=1. Both aggregates were generated with the CCA technique,which gives fluffier particles than PCA method. In Fig. 6 the extinction factors of the aggregates (withradius of the monomers 0.1µµm) are compared to the extinction factors obtained for a sphere of X=1.
As it is shown, the profiles of both of the aggregates are almost the same, and quite close to thosecorresponding to the sphere.
Fig.1: The extinction factors of the 2140 cm-1 band of CO ice sphere (up) and657.8 and 2341 cm-1 bands of CO2 ice sphere (center and bottom) versus thewavenumber for values of size parameter of 0.5, 1, 2 and 3.

Fig. 5: An image of two aggregates
1 and 2 generated with PCA method.
The radius of the monomer was set
to 0.1 µm and the size parameter ofthe particles to X = 1.

Fig. 6: Comparison of the calculated extinction factors of the aggregate 1 (solid line) and aggregate 2(dotted line) in Fig. 5 and the extinction factors obtained with a sphere (dashed line) of about X=1.
RESULTS:RESULTS:
1.- A broad scattering peak, slightly displaced towards red with respect to the absorption peak forsize parameters larger than X=1, for all studied cases.

Fig.2: Comparison of the extinction factors of the 2140 cm-1 band of CO ice forshapes: sphere (top left), ellipsoid (top right), DW1996 (bottom left) andaggregate randomly generated with a size monomer of the 0.17 µm (bottomright) from values of the size parameter from 0.5 to 4. These aggregates areshown in the next figure.

2.- Some particle shapes with size parameter smaller or equal 1 (the ellipsoid of axis ratio 5 and theaggregate with radius of the monomer 0.7 µm), the profile of the CO ice band appears as double peakstructure due to the surface modes.
3.-The calculations including silicate have shown variations in the band strength but not significant
changes in the feature of the profile. the random method used to add the silicate inclusions would be
improved by changing the refractive index of not only of isolated dipoles if not of several dipoles
surrounding every one.
REFERENCES:Fig. 3: An image of five aggregates randomly generated with a radius monomerof 0.17 µm. The size parameters of each aggregate are 0.5, 1, 1.5, 2 and 2.5,
respectively (from left to right).

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