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the most successful p - process models available to - date call for the synthesis of the stable neutron - deficient nuclides heavier than fe in the o / ne layers of type ii supernovae ( snii ) ( rayet et al .
1995 , hereafter rahpn ) . in spite of their many virtues in reproducing the solar - system p - nuclide abundance distribution ,
they however suffer from some shortcomings .
one of them concerns their persistent underproduction of the light mo ( @xmath2 , @xmath3 ) and ru ( @xmath4 , @xmath5 ) p - isotopes .
some have tried to remedy this situation with exotic solutions , calling in particular for accreting neutron stars or black holes ( e.g. schatz et al . 1998 ) .
the level of the contribution of such sites to the solar system content of the nuclides of concern here is quite impossible to assess in any reliable way .
in contrast , it has been emphasized many times over the last decade that the problem might just be due to some misrepresentation of the production in the he - burning core of massive stars of the s - nuclide seeds for the p - process ( e.g. arnould et al .
1998 ) .
the aim of this letter is to scrutinize the latter , ` non - exotic ' , solution in a quantitative way by duly taking into account the uncertainties that still affect the rate of the @xmath1 reaction , as they appear in the nacre compilation of reaction rates ( angulo et al .
clearly , these uncertainties in the key neutron producer in conditions obtained during central he burning in massive stars have a direct impact on the predicted abundances of the s - nuclide seeds for the p - process , as already analyzed quantitatively by meynet & arnould ( 1993 ) . another potential embarassment of the p - process predictions identified by rahpn is a snii overproduction of oxygen relative to the p - nuclides . we show that this problem might be cured along with the one of the underproduction of the light mo and ru isotopes if the @xmath1 rate is modified adequately within a range permitted by the nacre compilation . for the sake of illustration
, we just discuss here the case of a 25 m@xmath6 solar metallicity ( @xmath7 ) star .
a more complete study dealing in particular with a set of stars with different masses and metallicities , and analyzing the impact of the uncertainties in the rates of a variety of reactions , is currently under way .
the adopted input physics is briefly described in sect . 2 , and some results are presented in sect .
conclusions are drawn in sect .
the calculations reported here are based on a model for a @xmath8 8 m@xmath6 helium star already considered by rahpn .
it corresponds to a main sequence mass of about 25 m@xmath6 and is evolved from the beginning of core helium burning to the supernova explosion .
details about this model can be found in hashimoto ( 1995 ) , and are summarized in rahpn . as in rahpn ,
20 o / ne - rich layers with explosion temperatures peaking in the ( 1.8 - 3.3)@xmath9 k range are selected as the p - process layers ( ppls ) .
their total mass is approximately 0.58  m@xmath6 .
the deepest ppl is located at a mass of about 1.94  m@xmath6 , which is far enough from the mass cut for all the nuclides produced in this region to be ejected during the explosion .
the p - process reaction network and its numerical solver are described by rahpn .
a series of their selected nuclear reaction rates are updated , however . in particular , the nacre ` adopted ' rates are used for charged particle captures by nuclei up to @xmath10 . for heavier targets ,
the rates predicted by the hauser - feshbach code most ( goriely 1997 ) are used , except for the experimentally - based neutron capture rates provided by beer et al .
( 1992 ) .    as already pointed out in sect . 1
, we turn our special attention to the impact of the uncertainties remaining in the rate of @xmath1 . for temperatures of about @xmath11 k at which the s - process typically develops during core he burning in massive stars ( e.g. rayet & hashimoto 2000 ) ,
the nacre upper limit on this rate is 50 - 500 times larger than the ` adopted ' value ( see angulo et al .
1999 for details ) . in order to quantify the consequences of this situation for the predicted abundance distribution of the s - nuclide seeds for the p - process , and ultimately for the p - nuclide yields themselves
, we perform nucleosynthesis calculations for five different rates ranging from the nacre adopted value to its upper limit .
these rates , labelled r@xmath12 ( @xmath13=1 to 5 ) in the following , are defined and displayed in fig .  1 .
cross section ( j.w .
hammer & m. jaeger , private communication ) .
rate estimates based on these new data are not available yet .
it seems , however , that the revised upper limit might range somewhere between r@xmath14 and r@xmath15 they are used in the @xmath8 25 m@xmath16 star referred to above to calculate the abundances of the s - process nuclides at the end of core he burning .
the results are shown in fig .  2 for the s - only nuclides .
use of r@xmath17 leads to the classical ` weak ' s - process component pattern ( e.g. rayet & hashimoto 2000 ) , exhibiting a decrease of the overproduction ( with respect to solar ) of the s - nuclides by a factor ranging from @xmath18100 to about unity when the mass number @xmath19 increases from about 70 to 100 . in the heavier mass range ,
the s - process ` main component ' supposed to originate from low- or intermediate - mass stars takes over .
this ` canonical ' picture changes gradually with an increase of the @xmath1 rate , more @xmath20 having time to burn , releasing more neutrons , before he exhaustion in the core .
the direct result of this is a steady increase of the overproduction of heavier and heavier s - nuclides .
for example , with the extreme r@xmath21 rate , the overproduction factor increases from @xmath22 to @xmath23 for @xmath19 varying from about 70 to 90 , before decreasing to a value around unity for @xmath24 only .    at first sight
, it might be felt that the s - process abundance distributions obtained with large enough r@xmath12 values exhibit some unwanted or embarrassing features .
one of these concerns the underproduction of the @xmath25 s - nuclides relative to the @xmath26 ones .
another one relates to the fact that a more or less substantial production of heavy s - nuclides ( like in the ba region ) would screw up the pattern of the s - process main component ascribed to lower - mass stars . in our opinion
, none of these predictions can really act as a deterrent to @xmath1 rates substantially in excess of r@xmath17 . on the one hand ,
the absence of ab initio self - consistent calculations of the s - process in low- and intermediate mass stars does not allow at this time to predict the exact shape of the main s - process component which is classically assigned to these stars . as a consequence , a contribution to the main component by massive stars can not be excluded , even if it may disturb some traditional views on the subject . on the other hand ,
the reduction of the light s - process nuclide production by massive stars could well be compensated by their increased synthesis by some low- or intermediate - mass stars when rates larger than r@xmath17 are considered ( goriely & mowlavi 2000 ) .
the classical @xmath27 overproduction problem found in the massive star s - process ( e.g. rayet & hashimoto 2000 ) could also be eased with increased @xmath1 rates , as demonstrated by fig .  2 .
for these same rates , note that @xmath27 is not overproduced either in some of the calculations of goriely & mowlavi ( 2000 ) which predict high yields of the other light s - nuclides .
figure  2 also suggests that a discrepancy , if any , between the observed ba overabundance in the sn1987a ejecta and the model predictions could be cured in a natural way by increasing the adopted @xmath1 rate .
the [ ba / fe]@xmath28/[ba / fe]@xmath29 ratio is observationally still quite uncertain , values between about 5 and 20 having been reported ( e.g. mazzali & chugai 1995 ) .
prantzos et al . ( 1988 ) have calculated lower values of 2.6 to 4.7 with the @xmath1 rate of fowler et al .
this rate is on average comparable to r@xmath17 in the temperature range of relevance to the s - process .
we have not conducted any new s - process calculation in a specific sn1987a progenitor model . instead , some rough estimates based on the procedure of prantzos et al .
( 1988 ) in which their adopted s - process ba mass fraction is replaced by the one calculated for the model star adopted here have been made .
assuming that the lmc metallicity is one third of the solar one , we predict [ ba / fe]@xmath28/[ba / fe]@xmath29 ratios from 3 to 14 for rates increasing from r@xmath17 to r@xmath30 .
theory could thus account for quite substantial sn1987a ba productions with high enough @xmath1 rates ( compatible with the nacre data ) .
as discussed by rahpn , it is a fair approximation to adopt the s - process abundance distributions of fig .  2 as seeds for the p - process .
for the @xmath31 species , the initial abundances in the ppls are taken from the detailed stellar models .
although these models have been obtained with rates that may differ from the nacre ones adopted here , this inconsistency is certainly not responsible for any intolerable distorsion in the predicted s - process seeds or p - process yields .
the various seed abundances of fig .  2 are used to compute the production of the p - nuclides in the ppls of the @xmath8 25 m@xmath16 star considered here . as in rahpn
, the abundance of a p - nuclide @xmath13 is characterized by its mean overproduction factor @xmath32 , where @xmath33 is its solar mass fraction ( anders & grevesse 1989 ) , and @xmath34 where @xmath35 is the mass fraction of isotope @xmath13 at the mass coordinate @xmath36 , @xmath37 is the total mass of the ppls , the sum running over all the ppls ( @xmath38 corresponds to the bottom layer ) .
an overproduction factor averaged over all 35 p - nuclei is calculated as @xmath39 , and is a measure of the global p - nuclide enrichment in the ppls .
so , if the computed p - nuclei abundance distribution were exactly solar , the normalized mean overproduction factor @xmath40 would be equal to unity for all @xmath13 .
figure 3 shows the normalised p - nuclide overproduction factors derived from the seed abundance distributions calculated with the @xmath1 rates r@xmath17 , r@xmath41 and r@xmath21 .
changes in the shape of the p - nuclide abundance distribution are clearly noticeable , at least for @xmath42 .
the use of r@xmath17 leads to a more or less substantial underproduction of not only @xmath2 , @xmath3 , @xmath4 and @xmath5 , a ` classical ' result in p - process studies ( see rahpn ) , but also of @xmath43 and @xmath44 , which was not predicted in previous calculations .
this new feature directly relates to the larger abundances around @xmath45 used by rahpn ( dashed curve in fig .
2 ) , in contrast to the much flatter seed distribution obtained with r@xmath17 .
this kr - sr - mo - ru trough is gradually reduced , and in fact essentially disappears , for @xmath1 rates of the order or in excess of r@xmath41 .
this situation is most clearly illustrated by fig .
4 . in these very same conditions ,
@xmath40 for @xmath46 and @xmath47 comes much closer to unity as well .
it has to be noticed that this situation does not result from a stronger production of these two nuclides by the p - process , but instead from their increased initial abundances associated with a more efficient s - process when going from r@xmath17 to r@xmath21 .
in contrast , the @xmath40 pattern does not depend on the adopted @xmath1 rate for @xmath48 .
this is expected from a mere inspection of the s - nuclide seed distributions displayed in fig .  2 .
in particular , @xmath49 and @xmath50 remain underproduced .
this can not be considered as an embarassment as these two nuclides can emerge from the s - process in low- or intermediate - mass stars .
in addition , the overall efficiency of the p - nuclide production substantially increases with increasing @xmath20 burning rates .
more specifically , @xmath51 is multiplied by a factor of about 15 when going from r@xmath17 to r@xmath21 .
this could largely ease , and even solve , the problem of the relative underproduction of the p - nuclides with respect to oxygen identified by rahpn .
for their considered 25 m@xmath6 model star calculated with the @xmath52 rate from caughlan et al .
( 1985 ) , they obtain @xmath53 and report a value of 4.4 for the ratio of the oxygen to p - process yields .
this value would come close to unity for @xmath1 rates in the vicinity of r@xmath41-r@xmath14 , as the p - nuclides would be about 3 to 6 times more produced than in rahpn .
this letter makes plausible that the long - standing puzzle of the underproduction with respect to solar of the p - isotopes of mo and ru in snii explosions could be quite naturally solved by just assuming an increase of the @xmath1 rate over its ` nominal ' value .
more specifically , this could be achieved by multiplying the nacre ` adopted ' rate by factors of about 10 to 50 in the temperature range at which the s - process typically develops during core he burning in massive stars .
these factors are well within the uncertainties reported by nacre . as an important bonus ,
this increased rate would also largely avoid ( i ) the underproduction of @xmath43 and of @xmath44 which we predict here for the first time to be concomitant to the light mo and ru one , ( ii ) the too low production of @xmath46 and @xmath47 , and ( iii ) the overall underproduction of the p - nuclides with respect to oxygen noted by rahpn . in direct relation with an increased @xmath1 rate , more s - process ba
could also be ejected by snii events .
our predictions confortably overlap the range of ba overabundances reported for sn1987 .
this array of pleasing features has of course not to be viewed as a proof of the validity of the assumption that the true @xmath1 rate is higher than usually thought .
it may just be a hint that there might be ways around exotic solutions .
this conclusion applies at least if one relies on the simplistic ( and the only ones to be available for our purpose ) supernova models used here and in previous p - process calculations ( see rahpn et references therein ) , as well as in a myriad of other explosive nucleosynthesis calculations .
the most detailed calculations of the p - process call for its development in the o / ne layers of type ii supernovae . in spite of their overall success in reproducing the solar system content of p - nuclides , they suggest a significant underproduction of the light mo and ru isotopes . on grounds of a model for the explosion of a 25 @xmath0 star with solar metallicity , we demonstrate that this failure might just be related to the uncertainties left in the rate of the @xmath1 neutron producing reaction . 
 the latter indeed have a direct impact on the distribution of the s - process seeds for the p - process .