W aterem ission in N GC 1333-IRAS 4 ? The physicalstructure ofthe envelope

W e reportISO -LW S far infrared observationsofCO ,waterand oxygen linestowardsthe protobinary system IRAS4 in the NG C1333 cloud.W e detected severalwater,O H,CO rotationallines,and two [O I]and [CII] nestructurelines.G iven therelatively poorspectraland spatialresolution oftheseobservations,assessing the origin ofthe observed em ission is not straightforward.In this paper,we focus on the water line em ission and explore the hypothesis that it originates in the envelopes that surround the two protostars,IRAS4 A and B,thanks to an accurate m odel.The m odelreproduces quite wellthe observed water line uxes,predicting a density pro le,m assaccretion rate,centralm ass,and waterabundancepro lein agreem entwith previousworks. W e hence conclude thatthe em ission from the envelopesisa viable explanation forthe observed waterem ission, although we cannot totally rule out the alternative that the observed water em ission originates in the out ow. The envelopes are form ed by a static envelope where the density follows the r 2 law,at r 1500 AU,and a collapsing envelope where the density follows the r 3=2 law.The density ofthe envelopes at 1500 AU from the centeris 4 106 cm 3 and thedusttem perature is 30 K ,i.e.abouttheevaporation tem perature ofCO -rich ices.This m ay explain previous observations that claim ed a factor of10 depletion ofCO in IRAS4,as those observationsprobetheouter 30 K region oftheenvelope.Thewateris 5 10 7 lessabundantthan H 2 in the outerand cold envelope,whereasitsabundancejum psto 5 10 6 in theinnerm ostwarm region,atr 80 AU wherethedusttem peratureexceeds100 K ,theevaporation tem peratureofH 2O -rich ices.W ederivea m assof0.5 M foreach protostar,and an accretion rateof5 10 5 M yr 1 ,im plying an age ofabout10000 years,ifthe accretion rate rem ainsconstant.W e nally discussthe di erence between IRAS4 and IRAS16293-2422,where a sim ilaranalysishasbeen carried out.W efound thatIRAS4 isprobably a youngersystem than IRAS16293-2422. Thisfact,coupled with the larger distance ofIRAS4 from the Sun,fully explainsthe apparentdi erence in the m olecularem ission ofthese two sources,which ism uch richerin IRAS16293-2422.