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statistical model and hydro - inspired model calculations indicate the picture of early chemical freeze - out(cfo ) for strange hadrons in relativistic heavy ion collisions@xcite .
the cfo temperature can be extracted by fitting the model calculation of particle ratios for hadrons to the experimental data . on the other hand ,
one obtains the thermal freeze - out temperature from the slope of transverse momentum distributions by assuming some radial flow profile .
naturally , it is therefore motivating to see how this scenario can be adopted by more realistic models , where a full three - dimensional hydrodynamical evolution is taken into account . in this work ,
we present results of hydrodynamical model calculations , inspired by this picture .
the hydrodynamical model we employed is based on smoothed particle hydrodynamic(sph ) algorithm@xcite , in this model , the matter flow is parametrized in terms of discrete lagrangian coordinates , of the so - called sph particles . as a result
, the hydrodynamic equations are reduced to a system of coupled ordinary differential equations .
the code which implements the entropy representation of the sph model for relativistic high energy collisions , and which has been developed within the so paulo - rio de janeiro collaboration , is called spherio .
as it has been shown , the model is an efficient and robust method to tackle the problems concerning relativistic high - energy nucleus - nucleus collisions , which are characterized by highly asymmetrical configurations .
it has been successfully used to investigate the effects of the initial - condition fluctuations and adopting the continuous emission scenario for the description of decoupling process @xcite .    in the present calculation ,
the model has been further improved in order to consider strangeness conservation and to adopt the scenario of cfo . the strangeness conservation is implemented by explicitly incorporating strangeness chemical potential into the code , and correspondingly a different set of equation of state(eos ) has been built and utilized . as a good approximation , we assume local strangeness neutrality throughout the hydro evolution .    to adopt the picture of cfo , as a first step , we take a simple postulate that for strange hadrons such as @xmath0 , @xmath1 , @xmath2 and @xmath3 , the cfo takes place immediately after they are completely hadronized .
these particles cease to have inelastic collisions and therefore their abundances are fixed and determined only by partonic eos . instead of introducing a specific temperature ,
cfo is incorporated in a simple and parameter - free approach .
this is inspired by the earlier estimation from statistical model@xcite that cfo takes place close to the phase transition region .
it is also because at this point , we would like first to check whether the picture of cfo would qualitatively improve the result of hydrodynamic model without going into the details of parameters .
with the system being cooled and rarefied further , the thermal freeze - out occurs at a lower temperature @xmath4 which serves as an adjustable parameter in our calculation .
as in the previous calculations , we use nexus event generator to produce event - by - event fluctuating initial conditions . we make use of a rescaling factor@xcite to fix the pseudo - rapidity distribution for all charged particles . in the present calculation , we introduce another parameter @xmath5 which provides initial transverse velocity in addition to the one provided by nexus .
the reason for the introduction of @xmath5 is because nexus almost does not provide any initial transverse velocity .
thus the initial transverse expansion reads @xmath6 where @xmath7 is the radial distance from orgin .      as a preliminary application
, we have calculated the pseudo - rapidity distribution for all charged particles . in fig.1 , we present the pseudo - rapidity distribution for the most central au+au collisions at 200a gev .
the experimental data are from phobos collab , taken in the most central au+au at 200a gev .
we show in fig.2 , the experimental transverse momentum distribution data for all charged particles , which can be well reproduced with a choice of @xmath8@xmath9 and freeze - out temperature @xmath10 mev .
the experimental data are from star collab@xcite , taken in the most central au+au at 200a gev , with @xmath11 .        with the parameters chosen , in the following , we calculate the spectra for various hadrons . in fig.3
we show the transverse mass spectra of pions , protons and kaons for most central collisions at mid rapidity , as well as experimental data from brahms collab@xcite .
the same spectra for @xmath0 , @xmath1 and @xmath2 are depicted in fig.4 , together with data from star collab@xcite .
we use dotted lines to represent the results obtained without incorporating cfo , and solid lines for those with cfo switched on .
it is observed that the present hydrodynamic model gives good description of the experimental transverse momentum spectra for pions , kaons and protons even without turning on cfo for these particles .
although it gives the correct slopes for the spectra of strange hyperons such as @xmath0 , @xmath1 and @xmath2 , the disagreement comes from the multiplicities of the spectra . by introducing cfo ,
those results are improved significantly . while the spectra for pions , kaons and protons almost remain the same , ( this is not shown in the figures . )
it provides a good fit of strange hyperons @xmath0 , @xmath1 and @xmath2 . the cfo amplifies the multiplicities of strange hadrons with respect to the ones in the original model owing to higher cfo temperature , as compared with the thermal freeze out one .
meanwhile the slopes of the spectra remain unchanged .
it was indicated experimentally in ref.@xcite , as one goes to large rapidity region where the baryon density differs sizably from zero , the outcome of cfo would be much more significant .
further work on this topic is under progress .
it is worth noting that the effect of cfo has been discussed by several authors . in ref@xcite , the early cfo
is studied in term of hybrid model where the hydodynamic evolution is complemented with a hadronic cascade model .
the pure hydrodynamic evolution calculation has been carried out either only in the transverse plane assuming bjorken s scaling@xcite or with the baryon chemical potential taken to be zero@xcite .
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we investigate the effect of chemical freeze - out on identified particle spectra at 200agev au - au collisions at rhic , by utilizing a full three - dimensional hydrodynamical calculation . 
 the hydrodynamical code spherio we employed is based on the smoothed particle hydrodynamic algorithm . in order to describe the spectra of strange hadrons , 
 the code has been further improved by explicitly incorporating the strangeness conservation and a chemical freeze - out mechanism . in our model , 
 strange hadrons such as @xmath0 , @xmath1 , @xmath2 and @xmath3 undergo the chemical freeze - out immediately after the hadronization , and their multiplicities are fixed thereafter . at a lower temperature 
 the thermal freeze - out takes place for all the particles . 
 it is shown that the present model provides a reasonably good description for the spectra of identified particles , in particular , considerable improvement is observed for those of strange hadrons .