Cs. Kiss, P. Ábrahám, R. J. Laureijs, and S. M. Birkmann
CONSTRAINTS ON THE NATURE OF DUST PARTICLES BY INFRARED OBSERVATIONS (Poster)

CONSTRAINTS ON THE NATURE OF DUST PARTICLES BY INFRARED OBSERVATIONS

Cs. Kiss(1), P. Ábrahám(1), R. J. Laureijs(2), and S. M. Birkmann(3)
(1)Konkoly Observatory, PO Box 67, H-1525 Budapest, Hungary
(2)European Space Agency, Astrophysics Division, Kepleraan 1, 2201AZ Noordwijk, The Netherlands
(3)Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

The far-infrared emissivity ($\epsilon$) of the large interstellar grains can be obtained for many molecular and moderate density regions thanks to the availability of more far-infrared and submillimetre observations and on-line star count (optical extinction) data. Several papers reported on the detection of an enhancement of the far-infrared dust emissivity in various sky regions, which was interpreted as an increase in grain size. However, the observed emissivities show a large scatter and due to the relatively low number of observations the trend was not well determined.

We compiled a list of interstellar coulds observed with ISOPHOT (the photometer onboard ISO) at least at two infrared wavelengths ($\sim$100$\mu$m and $\sim$200$\mu$m), constructed FIR emission maps, calculated dust temperatures, created extinction maps using 2MASS and USNO data, and calculated far-infrared emissivity for each cloud. This is the largest homogeneously reduced database constructed so far. During the data analysis special care on was taken on possible systematic errors and we conclude that these do not affect the final $\epsilon$ values significantly. Our far-infrared emissivity values have a clear temperature dependence, and show a general $\epsilon$ increase of a factor of 2 prior to the diffuse ISM in the low temperature (12K$\le$T$\le$14K) regime. This suggests the presence of grain growth, however, the limited emissivity excess resticts the possible grain growth processes to ice-mantle formation and coagulation of silicate grains, and excludes the coagulation of carbonaceous particles. The latter procedures might be important in some specific clouds (e.g. LDN1251), but are not efficient enough in most regions.