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type ii supernovae ( sne ) emerge from shock - driven explosion of massive ( @xmath12 ) stars initiated by core collapse @xcite . in particular , plateau type ii sne ( sne iip ) result from core collapse of supergiants that have massive hydrogen - rich envelope . the light curves of sne iip are characterized by a constant luminosity plateau lasting about 80 - 120 days after explosion @xcite . recently , the progenitors of a few sne iip have been directly identified on pre - explosion images @xcite , revealing that these are @xmath13 supergiants , close to the low - mass theoretical limit of the core collapse process . sn  2004dj in ngc  2403 is an outstanding sn discovered by itagaki @xcite on july 31 , 2004 ( jd 2453218 ) , because it is one of the brightest and closest supernova observed ever . based on its first optical spectrum @xcite sn  2004dj was quickly classified as a `` normal '' type iip event . unfortunately , it was discovered during the plateau phase ( @xmath14 month after explosion ) , thus , the maximum of the light curve could not be covered . due to its proximity ( @xmath15 mpc ) @xcite , sn  2004dj was also detected in radio @xcite , infrared @xcite and x - ray bands @xcite . @xmath16-band optical photometry and high - resolution @xmath17 spectroscopy obtained during the plateau phase was published by @xcite . @xcite presented @xmath18 light curve and full optical spectra in the nebular phase . they estimated the amount of @xmath19ni as @xmath20 , and pointed out an asymmetric , bipolar distribution of the ejected nickel from modeling the nebular @xmath17 profile . the optical light curve observed in the batc ( intermediate - band ) system was also studied by @xcite . they concluded that sn  2004dj was a typical sn iip that ejected a @xmath21 envelope and synthesized @xmath20 of radioactive nickel . particular interest was paid on the association of sn  2004dj with the compact cluster sandage  96 which is thought to be the host of the progenitor star @xcite . @xcite and @xcite analysed the spectral energy distribution ( sed ) of sandage  96 ( based on archival ccd photometry ) , and concluded that s96 is indeed a compact cluster with age of @xmath22 myr and total stellar mass of @xmath23 ( the uncertainty is due to the different reddening determination ) . from population synthesis , the initial mass of the progenitor star of sn  2004dj was estimated as @xmath24 , which is in the upper part of the mass range of other sne iip progenitors ( see above ) .    in this paper we present time - resolved optical photometry and spectroscopy of sn  2004dj covering its first year after discovery . our primary goal is to derive a reasonable distance to this sn and its host galaxy ngc  2403 by using all available information . the new observational data are presented in section 2 and 3 . in section 4 we apply a variant of the expanding photosphere method ( epm ) and the sne iip standard candle method ( scm ) for distance determination , and critically compare the results with other distance estimates of the host galaxy . based on the improved distance , we estimate some of the physical properties of sn  2004dj and s96 in section 5 . finally , section 6 summarizes our results . [ cols="<,^,^,^,^,^,^,^,^",options="header " , ]     the resulting parameters corresponding to the minimum of @xmath7 are listed in table  13 and plotted in fig .  19 . because the metallicity of ngc 2403 at the position of s96 is probably subsolar ( @xmath25o / h@xmath26 = -0.24 $ ] , @xcite ) , we have used the models with @xmath27 and @xmath28 metallicities from both grids . comparing these results with the published ones ( shown in the last three rows of table  13 ) , it is visible that our analysis resulted in the same kinds of solutions as the previous studies : a `` young '' population with @xmath8 myr and an `` old '' population with @xmath29 myr . the `` old '' solutions are very similar to the ones given in the previous studies . the `` young '' solution , however , has considerably less age . it is interesting that this solution comes out using both model grids , regardless of metallicity ( but with different reddenings and cluster masses , of course ) . based on the @xmath7 values , the `` young '' solution from the bc03 grid gives the best description of the observed data . however , we agree with @xcite that if the observed sed is undersampled , such as the one presented here , it may be misleading to accept or reject solutions based on @xmath7 values only . thus , we conclude that , at present , there are three probable solutions for the sed fitting of s96 : the old one corresponds to @xmath30 myr , the young one with @xmath31 myr , and a solution with age between these two , @xmath32 myr . each solutions have their own combinations of cluster reddening and total mass , between the values of @xmath33 and @xmath34 . due to the correlations between these parameters , it is not possible to assign a unique solution for all these quantities simultaneously .    regarding the potential progenitor of sn  2004dj , it is seen in table  13 that the sn mass ( determined from the padova isochrones , @xcite ) ranges from @xmath35 to @xmath36 , depending on age . the uncertainty of the age determined from sed fitting is at least 10 % @xcite , implying an error of the sn - mass of about @xmath37 for the young- and @xmath38 for the old solution . it is interesting that the young ( 8 myr ) solution results in significantly higher mass than all the sne iip progenitor masses detected so far . the highest mass of a detected sn type ii progenitor is that of sn  1999ev @xcite with @xmath39 . this is similar to the progenitor mass of sn  2004et , being @xmath40 @xcite and sn  1999gi with @xmath41 @xcite . contrary to these , the progenitors of sn  2003gd @xcite and sn  2005cs @xcite , also detected on pre - explosion images , are found within the mass range of @xmath42 . these two recent discoveries have led to a conclusion that perhaps all type iip sne emerge from relatively less massive progenitors , so that @xmath43 . this is supported by the non - detection of the progenitors of sn  1999em , sn  1999gi and sn  2001du @xcite giving @xmath44 . however , the possible masses of the progenitor of sn  2004dj determined above do not fully support this scenario . if the @xmath45 myr solution of s96 is indeed valid , then the progenitor mass of sn  2004dj must have been higher than @xmath46 . this is significantly higher than the masses found for the other progenitors , even for sn  1999ev ( see above ) . since the other two solutions for s96 give progenitor masses that are consistent with the mass range of the others , it can not be stated that sn  2004dj had definitely a more massive progenitor , but it can not be ruled out at present . in principle , the cluster masses in table  13 imply that there might be at least a few stars with @xmath11 when sn  2004dj exploded . from the integration of the salpeter imf , the number of stars with @xmath11 is @xmath47 , which gives @xmath48 for @xmath49 . the actual number of massive stars in s96 may , of course , differ from this value , but statistically it is possible that at least a few such stars are indeed there . @xcite estimated that s96 contains about 12 red and perhaps 2 - 3 blue supergiants , based on their sed parameters . the spectrophotometric observations of the remainder of s96 after the sn faded away would help to clarify the picture . comparing the observations of the co vibrational bands in the mid - ir with theoretical explosion models combined with radiative transfer calculations , @xcite presented evidence for a red supergiant with @xmath50 as the likely progenitor of sn  2004dj . again , this is at the upper end of the detected progenitor masses for type ii sne , close to the prediction of the @xmath51 myr cluster solution by @xcite . @xcite estimated the parameters of the pre - supernova star using the formulae given by @xcite . these formulae relate the length of the plateau @xmath52 , the middle - plateau absolute magnitude @xmath53 and expansion velocity at the photosphere @xmath54 to the explosion energy ( @xmath55 ) , ejected mass ( @xmath56 ) and progenitor radius ( @xmath16 ) . @xcite derived @xmath57 days , @xmath58 mag and @xmath59 kms@xmath60 , and they got @xmath61 erg , @xmath62 and @xmath63 . from the more detailed analysis presented in this paper , we estimate these parameters as @xmath64 days ( sect . 2.1.1 , allowing @xmath65 days for the peak duration before the plateau , see @xcite ) , @xmath66 mag and @xmath67 kms@xmath60 ( sect . 4.2 and 4.3 ) . these parameters result in @xmath68 erg , @xmath69 and @xmath70 . it is interesting that the ejected mass is , again , close to @xmath71 , although with high uncertainty . it is concluded that the presently available information suggest that the progenitor mass of sn  2004dj is @xmath72 . the sed fitting of s96 may suggest a cluster age as low as 8 myr , indicating @xmath73 . this massive progenitor may also be suspected from theoretical sn models . the conclusions of this paper are summarized as follows :    * new @xmath0 photometric observations of sn  2004dj are presented . the date of explosion is estimated as june 30 , 2004 ( jd @xmath74 ) , about 1 month before discovery . the plateau phase lasted about @xmath75 days after explosion . the most probable value of the reddening toward sn  2004dj is @xmath76 mag @xcite . the progenitor cluster s96 is also detected on pre - explosion frames . * the new , plateau - phase optical spectra reveal great similarity between sn  2004dj and sn  1999em , a typical sn iip . radial velocities from metallic lines and @xmath77 are presented . the spectral features are successfully modeled with the spectrum synthesis code _ synow _ with @xmath78 k as the photospheric temperature . the observed p cyg profile of @xmath77 can be better described by an increased emission of the photosphere relative to the blackbody intensity at the rest wavelength of @xmath77 . a single nebular spectrum confirms the @xmath77 asymmetry reported by @xcite . * the distance to sn  2004dj is estimated from two methods applicable for sne iip . the epm distance is @xmath79 mpc , while the scm gives @xmath80 mpc . these are in good agreement with other distance estimates to the host galaxy ngc  2403 , based on cepheids and tf - relation , although the cepheid - distance is slightly less than the sn - distances . the averaged distance is @xmath81 mpc , which is consistent with all available information . * using the updated distance , the nickel mass of sn  2004dj is calculated as @xmath82 from the shape of the tail light curve . the progenitor mass has been estimated from fitting theoretical seds to the observations of s96 . the solutions are similar to those of @xcite and @xcite , except that a `` young '' solution corresponding to @xmath8 myr is also possible . such a young cluster age would mean that the mass of the progenitor was @xmath11 . the sn - masses of the other solutions are less than @xmath83 , being in better agreement with other sne iip progenitors . this research has been supported by hungarian otka grants no.ts 049872 and t042509 . the authors are grateful to the directors and staff of konkoly observatory , szeged observatory ( hungary ) , david dunlap observatory ( canada ) and f. l. whipple observatory ( usa ) for generously allocating telescope time . they also want to express their warm thanks to prof . david branch and his group members at university of oklahoma for providing access and support to the synow code . thanks are also due to an anonymous referee for the very thorough report and numerous suggestions that helped us to improve the paper . the nasa astrophysics data system , the simbad and ned databases , the canadian astronomy data centre and the supernova spectrum archive ( suspect ) were used to access data and references . the availability of these services are gratefully acknowledged .
new @xmath0 photometry and optical spectroscopy of the type iip supernova 2004dj in ngc  2403 , obtained during the first year since discovery , are presented . 
 the progenitor cluster , sandage  96 , is also detected on pre - explosion frames . 
 the light curve indicates that the explosion occured about 30 days before discovery , and the plateau phase lasted about @xmath1 days after that . 
 the plateau - phase spectra have been modelled with the @xmath2 spectral synthesis code using h , na  i , ti  ii , sc  ii , fe  ii and ba  ii lines . the sn distance is inferred from the expanding photosphere method ( epm ) and the standard candle method ( scm ) applicable for sne iip . 
 they resulted in distances that are consistent with each other as well as earlier cepheid- and tully - fisher distances . 
 the average distance , @xmath3 mpc is proposed for sn  2004dj and ngc  2403 . 
 the nickel mass produced by the explosion is estimated as @xmath4 . 
 the sed of the progenitor cluster is reanalysed by fitting population synthesis models to our observed @xmath0 data supplemented by @xmath5 and @xmath6 magnitudes from the literature . 
 the @xmath7-minimization revealed a possible `` young '' solution with cluster age @xmath8 myr , and an `` old '' solution with @xmath9 - @xmath10 myr . 
 the `` young '' solution would imply a progenitor mass @xmath11 , which is higher than the previously detected progenitor masses for type ii sne . 
 [ firstpage ]    stars : evolution  supernovae : individual ( sn  2004dj )  galaxies : individual ( ngc  2403 )