You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
a common property of star forming galaxies is the presence of emission lines in the spectrum .
emission lines are produced by gas that is ionized by uv radiation emitted from young massive stars .
star - forming regions are the sites where the nebular emission is produced , but these regions are also dusty environments .
it seems a natural goal for a model of star - forming galaxies to treat contextually the nebular emission and dust processing .
we present the implementation of nebular emission calculation in the spectrophotometric code grasil @xcite .
grasil is a code built to simulate the sed of galaxies including a careful treatment of starlight dust reprocessing .
the study of emission lines with grasil will give us many further constraints about :    * the dust obscuration : the optical thickness of star - forming regions can be derived from line ratio while it is very difficult to constrain optical thickness of ism only from continuum . * the star formation rate : emission lines are believed to be powerful sfr estimators .
we can now compare sfr derived from nebular lines with the results from fir , uv etc .
in our model , the galaxy is composed by two main components : the bulge and the disk .
dust within the disk is divided in : i ) molecular clouds ( mcs ) , where the star formation is active , ii ) diffuse medium ( or cirrus ) .
young stars are supposed to be born into the mcs , and leave them progressively as their age increases .
this is modelled by considering that a suitable fraction of the light of ssp is radiated inside the mcs .
this fraction is a function of age , parametrized by an escaping time .
so we have two populations : a ) young stars inside mcs , b ) older stars outside mcs .
a molecular cloud is modelled as a thick spherical shell of dense gas ( and dust ) around a central point source representing all the star content of the cloud .
the mcs , as well as old stars , are embedded in the diffuse medium . for more details on energy transport , refer to silva et al .
@xcite or silva et al .
, this conference .    to calculate emission lines we used the photoionization code cloudy 94 @xcite .
we built a library of hii regions in order to avoid the computation of entire nebular emission via cloudy at every run of the code grasil .
the hii regions of the library are calculated for different density , metallicity and radii .
the seds of ionizing sources of hii regions are obtained by integrating the light of each of the two populations for several assumption of escaping time and imf .
the seds of ionizing sources are parametrized by the number of ionizing photons for hi , hei and oii ( q(h ) , q(he ) , and q(o ) ) .
q(h ) gives the mass of ionized gas , while the ratios q(he)/q(h ) and q(o)/q(h ) are related to the hardness of ionizing flux and to the degree of ionization . in table
[ elenco ] it is reported the list of computed lines .
grasil calculates the qs of each population ( inside and outside the mc ) , interpolates on the library of hii regions , and computes the nebular emission for each population .
then , the nebular emission is extincted in the same way as the population that produced it .
.computed lines : hydrogen recombination lines ( upper pannel ) , elium and metal lines ( lower pannel ) [ cols= " < , < , < , < , < , < " , ]
vlirgs ( @xmath0 ) are the most powerful star forming galaxies at low redshift .
recent results show that : vlirgs do not follow the meurer s relation @xmath1 versus uv spectral index @xmath2 @xcite ; sfr computed from h@xmath3 , even corrected for extinction , is always lower than sfr derived from fir @xcite .
can the above problems be solved with a correct picture of the obscuration ?
we have simulated with our code a starburst galaxy ( barionic mass of @xmath4 ) with a quiescent star formation lasting all its history , plus a final ( analitycal ) burst .
we supposed that , as the burst begins , half of total ( @xmath5 ) gas is under molecular form , and a fraction ( from 70% to 99% ) of it is converted in stars during the burst .
as the molecular gas is consumed during the burst , the mcs become more and more transparent .
this is to mimic the consumption of the gas and the feedback of sne .
the initial optical thickness of mc is chosen to reproduce observed eq .
widht of h@xmath3+[nii ] and h@xmath2 ( -62.5   and 0.7  ) and the ratio @xmath6 ( 88 , poggianti et al . , 2001 ) .
the time evolution of the model is shown in figure [ risultati ] .
the temporal evolution in the @xmath2-fir / fuv plane can be described in three phases : +   1 ) vlirg phase : the burst is very obscured ; ir emission comes from the burst while uv comes from the disk .
there is no correlation between uv and ir because they come from different component of the galaxy .
+   2 ) uv - bright phase : the burst has almost consumed the gas and the mcs become transparent ; the model moves towards meurer s relation .
+   3 ) final phase : the burst is aging and the model departs from the relation .
the galaxy can not be observationally selected as a starburst any longer .    in inital vlirg phase ,
h@xmath3 corrected for extinction from balmer decrement h@xmath3/h@xmath2 can be a wrong estimate of the `` true '' h@xmath3 ( and then of the sfr , see fig .
[ risultati ] right ) .
in fact in this phase h@xmath2 can be dominated by the population outside mcs affected by a low extinction ; then , the balmer decrement is altered because of the selective extinction .
this effect can explain the low @xmath7 observed in vlirgs @xcite . concluding , we are able to interpret different uv , optical , ir properties of obscured galaxies within a unique star formation selective ( in age ) extinction scenario .
we found that the relation @xmath2  fir / fuv is not unique in the obscured phase .
for the same reason , the correction of h@xmath3 from balmer decrement can be wrong in this phase .
star forming galaxies exhibit a variety of physical conditions , from quiescent normal spirals to the most powerful dusty starbursts . in order to study these complex systems , 
 we need a suitable tool to analyze the information coming from observations at all wavelengths . 
 we present a new spectro - photometric model which considers in a consistent way starlight as reprocessed by gas and dust . 
 we discuss preliminary results to interpret some observed properties of vlirgs .