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supernova 2001du was visually discovered by r.  evans ( 2001 ) on august 24.7 ( ut dates are used throughout this paper ) at the western end of the bar in the spiral galaxy ngc 1365 ( about 90  w and 10  s of the nucleus ) . smartt , kilkenny , & meikle ( 2001 ) classified the sn as type ii ; wang et al .  ( 2001 ) , from a spectrum obtained on august 30 ut , confirmed the sn as type ii - plateau ( ii - p ) before optical maximum . ngc 1365 has also been host to sn 1957c ( of unknown type ) and the type ic sn 1983v . using _ hubble space telescope _ ( _ hst _ ) observations of cepheids , silbermann et al . ( 1999 ) determined the distance modulus of ngc 1365 to be @xmath1 mag , corresponding to a distance of about 18.2 mpc . van dyk , li , & filippenko ( 2003 ) recently measured the absolute position of sn 2001du and , applying an independent astrometric grid to pre - sn _ hst _ archival wfpc2 images , isolated three possible candidate progenitor stars within the uncertainty of the measured sn position : one reddish star with a detection in both the f555w and f814w bands , and two stars with only f555w detections , implying bluer colors for these latter two stars . also , multi - band wfpc2 images of the sn were obtained for _ hst _ program go-9041 ( pi : s.  smartt ) in 2001 november . these images are now in the public domain , and in this paper we analyze them and compare them to the pre - sn images to further constrain the nature of the sn progenitor . ( smartt et al . 2003 have also recently analyzed their images and , in a somewhat similar manner , come to the same basic conclusion that we reach here . ) pre - sn images , each of total exposure time 300 s in the bands f336w , f555w , and f814w , were obtained by gto-5222 on 1995 january 15 . late - time multi - band images of the sn were obtained by go-9041 on 2001 november 26 . these include deep images in f336w , f439w , f555w , f675w , and f814w , as well as shorter exposures in f336w , f555w , and f814w . the available _ hst _ data are summarized in table 1 , with total exposure times given for each band .    in van dyk et al . ( 2003 ) we adopted the position of the sn , measured from an image obtained with the yalo 1-m telescope at ctio ( and available on the internet ) , as @xmath2 = 3@xmath333@xmath429@xmath515 , @xmath6 = @xmath7360831@xmath85 ( j2000 ) , with total uncertainty @xmath9 . the sn site is located on the wf3 chip . in figure 1 we reproduce figure 21 from van dyk et al . , showing the sn environment in the f555w band . in figure 2 we show the late - time f555w image ( the sn is nearly centered on the pc chip for all the go-9041 exposures ) . as one can see , the actual sn position is within the @xmath10 error circle , which provides additional confidence in the astrometric method used by van dyk et al . when isolating the various sn sites = 3@xmath333@xmath429@xmath514 , @xmath6 = @xmath7360831@xmath83 ( j2000 ) , @xmath11 in each coordinate . we initially used the routine hstphot ( dolphin 2000a , b ) to obtain photometry of the objects imaged on the pc chip . hstphot automatically accounts for wfpc2 point - spread function ( psf ) variations and charge - transfer effects across the chips , zeropoints , aperture corrections , etc . , and can return as output magnitudes transformed from the _ hst _ flight system to standard johnson - cousins bands . van dyk et al .  ( 2003 ) discuss the similarity in the results from hstphot , compared with those from the packages dophot ( schechter , mateo , & saha 1993 ) and daophot / allstar ( stetson 1987 , 1992 ) . and @xmath12 mag . ]    the sn profile is saturated in the longer f555w ( @xmath13 ) , f675w ( @xmath14 ) , and f814w ( @xmath15 ) exposures . thus , no brightness measurement is possible . the sn is not saturated in the shorter f555w and f814w exposures , the long f336w ( @xmath16 ) and f439w ( @xmath17 ) exposures , and also the short f336w exposure . however , the results from hstphot for the f336w exposures varied , depending on how these exposures were combined with the other bands as input into the routine , and the results from the long and short exposures for this band were also discrepant with each other ( the results ranged from about @xmath18 to 18.7 mag for different runs of hstphot ) . we do not know why this occurred , but we speculate that it possibly may be due to the low signal - to - noise ratio for both exposure sets . for f336w we then applied to both the long and short exposures daophot / allstar within iraf as a check , using a tinytim psf ( krist 1995 ) , the zeropoint from holtzman et al . ( 1995 ) , and aperture corrections ( @xmath190.00 mag ) derived from artificial star tests . ( checks for the f439w and short f555w and f814w exposures with daophot show good agreement with the hstphot results . ) we obtained @xmath20 and 18.87@xmath21 mag for the short and long exposures , respectively . we then averaged these daophot magnitudes with the hstphot magnitudes at f336w , and obtained @xmath22 mag . the sn was quite red by late november 2001 , with @xmath23 mag . whether using the transformations from flight system to johnson - cousins magnitudes from holtzman et al . ( 1995 ) , dolphin ( 2000b ) , or via synphot applied to the bruzual spectral synthetic atlas ( see van dyk , filippenko , & li 2002 ) , this color is too red to obtain a reliable transformation from @xmath24 to @xmath16 . however , we do apply a transformation , with uncertainty @xmath190.2 mag . the transformations for all other bands are from dolphin ( 2000b ) , applied internally within hstphot . the final results for the photometry of sn 2001du in the _ hst _ images are presented in table 2 .    in figure 3 we show preliminary , unpublished @xmath25 light curves for sn 2001du , based on data from the yalo 1-m telescope ( n.  suntzeff & k.  krisciunas , 2003 , private communication ) , and we include the @xmath26 photometry from the _ hst _ data . for comparison we show the @xmath17 and @xmath13 light curves for the plateau - type sn ( ii - p ) 1999em in ngc 1637 ( leonard et al . it is clearly evident that sn 2001du is indeed a sn ii - p as well . if this sn were a linear - type event ( ii - l ) , it should have markedly declined in brightness by the time of the _ hst _ observations ( @xmath1994 days since discovery ) . in van dyk et al .  ( 2003 ) we had isolated three faint objects , a  c , detected and determined to be stellar by hstphot , within the error circle around the sn position . stars a and b , to the south and west , respectively , are both blue , with @xmath27 ( @xmath28 ) @xmath29 mag and @xmath30 mag , respectively , and no detections at f814w ( @xmath31 ) . ( blue , here , is relative , and includes stars with roughly early - g spectral type or earlier , based on the detection limits ; see below . ) star c , to the east , is relatively red , with @xmath32 and @xmath33 mag . we had assumed that , since sn 2001du is of type ii - p , the most plausible of the three candidates is the redder star . however , that was argued to be still unlikely , based on the assumed luminosity and color of star c.    we have used the late - time sn images , specifically the @xmath13 image , and attempted to match them up with the pre - sn images . this was done by using the task _ geotran _ in iraf to transform both the late - time sn images to the pre - sn images and vice versa . we can therefore locate the position of the sn on the pre - sn images as being 0.70@xmath340.15 wf pixel ( @xmath35 ) northeast of the position of star b. it would be tempting to assign star b , then , as the progenitor of sn 2001du . however , ( 1 ) the offset of the sn position from star b ( which corresponds spatially to 6.2 pc at the distance of ngc 1365 ) is significantly large , and the accuracy of the measurement is appreciably high ; ( 2 ) sne  ii - p are thought to arise from the explosions of red supergiants ; and ( 3 ) no evidence exists that sn 2001du is peculiar in any way and that it arose from a bluer , more compact star like the progenitor of sn 1987a ( e.  cappellaro , 2002 , private communication ) . thus , it is more likely , as we previously had suspected ( van dyk et al . 2003 ) , that the progenitor simply is not detected in the pre - sn images . we can use the detection limits of both the f555w and f814w pre - sn images to constrain the nature of the progenitor . we do this by inferring limits on the luminosity and , therefore , the mass from the limits on the star s apparent magnitude . these hstphot detection limits in the sn environment are @xmath36 and @xmath37 mag . before converting into absolute magnitude , we must first assess the amount of extinction to the sn . in van dyk et al . ( 2003 ) we attempted to use stars in the environment to estimate the reddening . the galactic extinction toward ngc 1365 is @xmath38 mag ( schlegel , finkbeiner , & davis 1998 ; and ned ) . we had found that stars in the larger sn environment implied larger amounts of extinction , @xmath39 mag . ( unfortunately , although the post - sn images are deeper than the pre - sn ones , the sn is far too bright , and its image is spread over too many surrounding pixels , to perform a more thorough study of the environment . ) using the light curve in figure 3 we can actually derive more specific information about the possible reddening toward the sn from the sn itself ( which is information that was not available to us previously ) . the _ hst _ photometry of the sn on november 26 is about 94 days after discovery , i.e. , near the end of the light - curve plateau , where we can still estimate the sn s color temperature with reasonable accuracy ( e.g. , hamuy et al . these colors are @xmath40 and @xmath41 mag . for comparison , the colors of the very well - studied type ii - p sn 1999em at the same age ( about 95 days after discovery ) are @xmath42 and @xmath43 mag ( leonard et al . leonard et al .  and hamuy et al .  ( 2001 ) conclude that sn 1999em is reddened by @xmath44 mag . since @xmath45 is affected by line blanketing in the @xmath17 band ( e.g. , hamuy et al . 2001 ) , the small difference in @xmath45 ( sn 1999em is 0.05 mag redder than sn 2001du ) could be due to differences in metallicity between the two sne , which might affect this color . the marginally redder @xmath46 color of sn 2001du ( by 0.02 mag ) may imply that this sn is slightly more reddened than sn 1999em . however , we consider these differences in color between the two sne to be small and likely within the uncertainties in the colors , so we adopt @xmath47 mag for sn 2001du , as found for sn 1999em . if we assume the cardelli , clayton , & mathis ( 1989 ) reddening law , then @xmath48 mag and @xmath49 mag for sn 2001du . for the distance modulus @xmath50 mag , @xmath51 and @xmath52 mag for the progenitor star . in a similar vein to the estimation of the progenitor mass limit for sn ii - p 1999em by smartt et al . ( 2002 ) and sn ii - p 1999gi by smartt et al . ( 2001 ) , the absolute magnitude limits can be converted to the luminosity of the likely supergiant progenitor , assuming the full range of possible stellar surface temperatures . this is accomplished using a similar relation to equation ( 1 ) in smartt et al . ( 2001 , 2002 ) , where we also have assumed the temperatures and stellar bolometric corrections for supergiants from drilling & landolt ( 2000 ) and have computed , using synphot within iraf and the bruzual spectral atlas , the conversions of @xmath27 and @xmath53 to johnson @xmath13 for the range in spectral type . the luminosity limits for both f555w ( @xmath13 ) and f814w ( @xmath15 ) are shown in figure 4 . stars with luminosities brighter than these limits ( shown by the _ heavy lines _ ) should have been detected in the pre - sn images .    for comparison we have plotted ( also in a similar fashion to smartt et al . ) the model stellar evolutionary tracks for a range of masses from lejeune & schaerer ( 2001 ) , assuming enhanced mass loss for the most massive stars and that a metallicity @xmath54 is appropriate for the sn environment . indication for a metallicity somewhat greater than solar in this sector of ngc 1365 comes from the abundance study by roy & walsh ( 1997 ) of h  ii regions in the galaxy . the closest h  ii regions to the sn site are their nos .  20 and 21 , where at this radial distance from the nucleus , the o / h abundance is @xmath199 dex or , in fact , slightly greater , based on model assumptions ( the solar o / h abundance is 8.8 dex ; grevesse & sauval 1998 ) . the luminosity limits at each band in figure 4 show that the pre - sn images are not sensitive to the bluest high - mass supergiants , so we can not be absolutely sure that such a star is not the progenitor . if we exclude this possibility , the @xmath13 band sets an upper limit to the progenitor mass of @xmath55 , although this ranges upward to as high as @xmath56 , depending on the effective temperature . the @xmath15 band is more sensitive to less - massive red supergiants , ranging downward to @xmath57 ( possibly even less ) at the lowest effective temperatures . these results from figure 4 are roughly consistent with what we showed in figure 22 of van dyk et al . we therefore estimate that the upper mass limit for the sn 2001du progenitor is @xmath0 . this limit is consistent with the limit leonard et al . ( 2002b ) find for sn ii - p 1999gi ( @xmath58 ) and the @xmath59 limit one finds for sn ii - p 1999em , if the distance to the sn 1999em host is assumed to be the cepheid distance , @xmath60 mpc ( leonard et al . low masses for the progenitors of sne  ii - p , relative to that indicated for sne ii - l ( linear ) and sne  iin ( narrow ) , are also inferred from their optical properties ( e.g. , schlegel 1996 ; filippenko 1997 ; chugai & utrobin 2000 ) and suggested by the inferred paucity of circumstellar matter from radio and x - ray observations ( e.g. , weiler et al . 1989 ; schlegel 2001 ; pooley et al . comparing _ hst _ archival wfpc2 images obtained of the type ii - p sn 2001du in ngc 1365 at late times with images obtained before explosion , we have located the position of the sn progenitor star to within 1 wf chip pixel of the candidate star b identified by van dyk et al . this star , however , is more than likely _ not _ the progenitor . we use the detection limits of the pre - sn f555w and f814w images to place constraints on the progenitor s nature ; specifically , @xmath61 . ( smartt et al . 2003 find a similar mass limit of @xmath62 . ) this limit is consistent with what has been found and inferred for other sne  ii - p . the proximity of the sn to the blue star b implies that the progenitor may have been part of a small stellar association ( the separation between the progenitor and star b is likely too large for the blue star to be a binary companion ) . the faint emission underlying stars a , b , and c identified by van dyk et al . ( 2003 ) suggests that an association complex may lie in the sn environment . it would be interesting to image the sn again in multiple bands with _ hst _ at high spatial resolution , presumably with the advanced camera for surveys ( acs ) , when the sn has significantly dimmed . the stars in the environment of the fading sn would then become more obvious , and the age and mass of the progenitor star might be further constrained , based on the characteristics of its surviving neighbors . this research made use of the nasa / ipac extragalactic database ( ned ) which is operated by the jet propulsion laboratory , california institute of technology , under contract with nasa , and the leda database ( http://leda.univ-lyon.fr ) . the work of a.v.f.s group at uc berkeley is supported by nasa grants ar-8754 , ar-9529 , and go-8602 from the space telescope science institute , which is operated by aura , inc . , under nasa contract nas5 - 26555 . we thank d.  c.  leonard and m.  hamuy for useful discussions . we also thank the referee , k.  krisciunas , for helpful comments and for providing unpublished ground - based photometry of sn 2001du . lcclc 1995 jan 15 & f336w & 300 & gto-5222 & & f555w & 300 & & & f814w & 300 & & 2001 nov 26 & f336w & 100 & go-9041 & n & & 460 & & n & f439w & 400 & & n & f555w & 40 & & n & & 600 & & y & f675w & 400 & & y & f814w & 40 & & n & & 600 & & y
_ hubble space telescope _ ( _ hst _ ) wfpc2 archival images obtained years prior to the explosion of the type ii - plateau supernova ( sn ) 2001du in ngc 1365 previously have been analyzed to isolate the progenitor star . 
 the sn site was located using precise astrometry applied to the _ hst _ images , with significant uncertainty , leaving three possible progenitor candidates . 
 images of the fading sn have recently become publicly available in the _ hst _ archive , allowing us to pinpoint the sn s exact location on the pre - explosion images . 
 we show that the sn occurred in very close proximity to one of the blue candidate stars , but we argue that this star is not the actual progenitor . instead 
 , the progenitor was not detected on the pre - sn images , and we constrain the progenitor s mass to be less than @xmath0 . 
 this is consistent with previous constraints on the progenitor masses of other type ii - plateau supernovae ( sne ) , suggesting that such sne arise from the iron core collapse of massive stars at the lower extreme of the possible mass range .