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to accurately characterize dust emissivity properties represents a major challenge of nowadays astronomy . in the submillimeter domain , large grains at thermal equilibrium ( e.g. @xcite )
dominate the dust emission .
this thermal dust is characterized by a temperature and a spectral dependence of the emissivity which is usually simply modelled by a spectral index .
the temperature , density and opacity of a molecular cloud are key parameters which control the structure and evolution of the clumps , and therefore , star formation .
the spectral index ( @xmath0 ) of a given dust grain population is directly linked to the internal physical mechanisms and the chemical nature of the grains .
it is generally admitted from kramers - knig relations that 1 is a lower limit for the spectral index .
@xmath0 = 2 is particularly invoked for isotropic crystalline grains , amorphous silicates or graphitic grains .
however , it is not the case for amorphous carbon , which is thought to have a spectral index equal to 1 .
spectral indices above 2 may exist , according to several laboratory measurements on grain analogs .
observations of the diffuse interstellar medium at large scales favour @xmath0 around 2 ( e.g. @xcite , @xcite ) . in the case of molecular clouds ,
spectral indices are usually found to be between 1.5 and 2 .
however , low values ( 0.2 - 1.4 ) of the spectral index have been observed in circumstellar environments , as well as in molecular cloud cores .
pronaos ( programme national dobservations submillimtriques ) is a french balloon - borne submillimeter experiment @xcite .
its effective wavelengths are 200 , 260 , 360 and 580 , and the angular resolutions are 2@xmath1 in bands 1 and 2 , 2.5@xmath1 in band 3 and 3.5@xmath1 in band 4 .
the data analyzed here were obtained during the second flight of pronaos in september 1996 , at fort sumner , new mexico . this experiment has observed various phases of the interstellar medium , from diffuse clouds in polaris @xcite and taurus @xcite to massive star - forming regions in orion ( @xcite , @xcite ) , messier 17 @xcite , cygnus b , and the dusty envelope surrounding the young massive star gh2o 092.67 + 03.07 in ncs .
the @xmath2 ophiuchi low - mass star - forming region has also been observed , as well as the edge - on spiral galaxy ngc 891 @xcite .
we fit a modified black body law to the spectra : @xmath3 , where @xmath4 is the spectral intensity ( mjy / sr ) , @xmath5 is the emissivity at @xmath6 of the observed dust column density , @xmath7 is the planck function , @xmath8 is the temperature and @xmath0 is the spectral index . in most of the areas ,
we use either only pronaos data or pronaos + iras 100  data .
we restrain the analysis to all fully independent ( 3.5@xmath1 side ) pixels for which both relative errors on the temperature and the spectral index are less than 20% .
this procedure allows to reduce the degeneracy effect between the temperature and the spectral index .
dupac et al .
( 2001 , 2002 ) have shown by fitting simulated data that this artificial anticorrelation effect was small compared to the effect observed in the data .
ophiuchi ( grey asterisks ) , polaris ( black squares ) , taurus ( grey square ) , ncs ( grey cross ) and ngc 891 ( black crosses ) . the full line is the result of the best hyperbolic fit : @xmath9 ]    we present in fig .
[ compil ] the spectral index - temperature relation observed .
the temperature in this data set ranges from 11 to 80 k , and the spectral index also exhibits large variations from 0.8 to 2.4 .
one can observe an anticorrelation on these plots between the temperature and the spectral index , in the sense that the cold regions have high spectral indices around 2 , and warmer regions have spectral indices below 1.5 .
in particular , no data points with @xmath10 35 k and @xmath11 1.6 can be found , nor points with @xmath12 20 k and @xmath13 1.5 .
this anticorrelation effect is present for all objects in which we observe a large range of temperatures , namely orion , m17 , cygnus and @xmath2 ophiuchi .
it is also remarkable that the few points from other regions are well compatible with this general anticorrelation trend .
the temperature dependence of the emissivity spectral index is well fitted by a hyperbolic approximating function .
several interpretations are possible for this effect : one is that the grain sizes change in dense environments , another is that the chemical composition of the grains is not the same in different environments and that this correlates to the temperature , a third one is that there is an intrinsic dependence of the spectral index on the temperature , due to quantum processes such as two - level tunneling effects .
additional modeling , as well as additional laboratory measurements and astrophysical observations , are required in order to discriminate between these different interpretations .
more details about this analysis and the possible interpretations can be found in @xcite .
we combine data sets from cobe ( dirbe data points and firas spectrum ) and wmap in order to investigate the galactic dust spectral energy distribution on the whole sky .
we convolve all data to the firas 7@xmath14 angular resolution , in order to obtain a consistent data set .
this is done by co - adding higher resolution data from dirbe and wmap into 7 degree - sided pixels . the resulting spectrum is shown in fig . [ sp ] for the galactic center area .
it is well fitted by a 21 k , @xmath0=1.86 one - dust component model .
these values are well compatible with the pronaos temperature - spectral index relation ( see section [ pro ] ) .
more results and conclusions from this cobe - wmap joint analysis will soon be available .
we present a compilation of pronaos - based results concerning the temperature dependence of the dust submillimeter spectral index , including data from galactic cirrus , star - forming regions , dust associated to a young stellar object , and a spiral galaxy . 
 we observe large variations of the spectral index ( from 0.8 to 2.4 ) in a wide range of temperatures ( 11 to 80 k ) . 
 these spectral index variations follow a hyperbolic - shaped function of the temperature , high spectral indices ( 1.6 - 2.4 ) being observed in cold regions ( 11 - 20 k ) while low indices ( 0.8 - 1.6 ) are observed in warm regions ( 35 - 80 k ) . 
 three distinct effects may play a role in this temperature dependence : one is that the grain sizes change in dense environments , another is that the chemical composition of the grains is not the same in different environments , 
 a third one is that there is an intrinsic dependence of the dust spectral index on the temperature due to quantum processes . 
 this last effect is backed up by laboratory measurements and could be the dominant one .    
 we also briefly present a joint analysis of wmap dust data together with cobe / dirbe and cobe / firas data .