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in the last decade type ia supernovae ( sne  ia ) have been extensively used for cosmology yielding evidence that we live in an accelerating universe ( riess et al .
1998 ; perlmutter et al . 1999 ) .
ongoing surveys such as essence @xcite and snls @xcite use the relation between the shape of the light curve and its peak luminosity @xcite to constrain the equation - of - state parameter for dark energy .
an important caveat is that those relations assume that sne  ia are a one parameter family .
however , in recent years there has been growing evidence for the observational diversity among sn  ia and this is of prime interest in their application as distance indicators .
subtle but unequivocal differences between events with large wavelength coverage and dense temporal sampling are evident in light curve shapes , colour evolutions , luminosities , evolution of spectral lines and expansion velocities derived from the line shifts .
the search for accurate correlations between photometric and spectroscopic properties could improve the luminosity calibration and help to shed light on the explosion mechanisms and progenitor system .
recently , @xcite have identified three classes of sne  ia based on their spectroscopic features .
a similar classification is also reported in @xcite .
interestingly , two of these classes are nearly indistinguishable in some of their photometric parameters ( e.g. @xmath0 , phillips et al .
1993 ) ; yet they show clear spectroscopic differences .
high velocity gradient ( hvg ) sne are characterized by a fast decrease in their expansion velocity over time as measured from the minimum of the si  ii ( 6355   ) absorption line . on the other hand in the low velocity gradient ( lvg ) sne group , which represents the majority of sn  ia ,
the evolution of the si  ii ( 6355   ) velocity is smooth , the ejecta expansion is slower than in hvg sne .
these spectroscopic differences may or may not affect the relations used to calibrate the luminosities of sne  ia .
subtle differences in colour may exist among the different classes and lead to significant bias in reddening estimates .
the existence of spectroscopic families suggests possible differences in the progenitor channels and/or explosion mechanisms .
@xcite for example reported that sne characterized by high expansion velocities , tend to explode in late type galaxies , pointing to progenitors arising from a young population .    in this paper
we present the observations of the hvg sn  2002dj carried out by the european supernova collaboration ( esc ) .
sn  2002dj ( @xmath1 , @xmath2 , j2000 ) was discovered in ngc  5018 on june 12.2 ut ( iauc 7918 , * ? ? ?
* ) and classified by esc members as a type ia event on june 14.15 ut ( iauc 7919 , * ? ? ?
the layout of the manuscript is as follows .
observations and data reduction are presented in sect .  2 .
we describe the reddening estimate in sect .  3 .
in sect .
4 we analyze the sn  2002dj optical and ir photometry comparing its properties with those of its kinematical twin , sn  2002bo .
we also investigate whether they are representative of the hvg sne group .
5 contains the determination of the sn  2002dj absolute luminosity and a characterization of the properties of its host galaxy .
the optical and ir spectroscopy is analyzed in sect .
6 and sect .  7 , respectively .
the expansion velocities are discussed in sect .  8 and models of the early and late spectra in sect  9 .
we conclude in sect .
10 by examining possible physical conditions in the hgv sne ejecta that could explain part of their spectroscopic and photometric behavior .
* 0.9 m telescope located at cerro tololo inter - american observatory ( ctio ) equipped with a ccd camera ( 2048@xmath32048 , pixel size = 0.40 arcsec pixel@xmath4 ) and ubvri johnsons , kron - cousin standard filters . * 1.0 m telescope located at ctio equipped with a novel double - imaging camera ( andicam ; hawaii hgcdte 1024@xmath31024 , pixel size = 0.137 arcsec pixel@xmath4 ) and jhks standard filters .
* european southern observatory ( eso ) new technology telescope ( ntt ) located at la silla observatory and equipped with the eso multi mode instrument ( emmi ) in rild mode ( 2@xmath3mit / ll ccd 2048@xmath34096 , pixel size = 0.167 arcsec pixel@xmath4 ) and bvri standard filters with eso identification numbers 605 , 606 , 608 and 610 , respectively . *
eso - ntt equipped with the son of isaac camera ( sofi ; hawaii hgcdte 1024@xmath31024 , pixel size = 0.29 arcsec pixel@xmath4 ) and jhks standard filters . *
danish 1.54 m telescope located at la silla observatory equipped with the danish faint object spectrograph and camera ( dfosc ; mat / eev ccd 44 - 82 2048@xmath32048 , pixel size = 0.39 arcsec pixel@xmath4 ) and ubvri standard filters with eso identification numbers 632 , 450 , 451 , 452 and 425 , respectively .
* eso - kueyen very large telescope ( vlt ) located at paranal observatory equipped with the focal reducer and low dispersion spectrograph ( fors1 ; 2@xmath3e2v 2048@xmath34096 pixel size = 0.2 arcsec pixel@xmath4 ) and bvri standard filters with eso identification numbers + 34 , + 35 , + 36 and + 37 , respectively . *
nordic optical telescope ( not ) located at roque de los muchachos observatory equipped with the andaluca faint object spectrograph and camera ( alfosc ; e2v 2048@xmath32048 , pixel size = 0.19 arcsec pixel@xmath4 ) and ubvr standard filters with not identification numbers 7 , 74 , 75 and 76 , respectively , and an interference i filter with number 12 . *
isaac newton group ( ing ) jacobus kapteyn telescope ( jkt ) located at roque de los muchachos observatory equipped with a ccd camera ( site2 2048@xmath32048 , pixel size = 0.33 arcsec pixel@xmath4 ) and ubvri standard filters with ing identification numbers 3 , 27 , 30 , 37 and 44 , respectively . *
ing - isaac newton telescope ( int ) located at roque de los muchachos observatory equipped with the wide field camera ( wfc ; 4 thinned eev 2048@xmath34096 , pixel size = 0.33 arcsec pixel@xmath4 ) and ubvri(sloan ) standard filters with ing identification numbers 204 , 191 , 192 , 193 and 215 , respectively .
we define the transmission function of a give passband @xmath8 as : @xmath9 + where @xmath10 is the filter transmission function , @xmath11 is the detector quantum efficiency , @xmath12 is the continuum atmospheric transmission profile , @xmath13 is the line atmospheric transmission profile , @xmath14 is the mirror reflectivity function , @xmath15 the number of reflections and @xmath16 is the lens throughput .
we downloaded @xmath10 and @xmath11 from the instrument web sites .
for @xmath14 we used a standard aluminium reflectivity curve , while we did not find information about the lens transmission for any of the instruments . in the construction of the atmosphere model the weight of the terms @xmath12 and @xmath17 change considerably going from @xmath18 to @xmath19 bands . in the optical the atmosphere transmission is mainly defined by a continuum function , while in the ir in most of the instruments the passband cut - offs are defined by deep atmospheric absorption bands .
the latter make the use of a fixed instrumental passband not a good approximation of the real transmission function , because it depends on the atmospheric conditions . to a much lesser degree
this is also a problem for the optical @xmath20 and @xmath21 filters , where some telluric features fall in the middle of the passbands .
these telluric lines do not change linearly with airmass , therefore their variation is not properly accounted by the absorption coefficients . for la palma we obtained @xmath12 from @xcite , while for la silla , paranal and ctio we have used the ctio transmission curve in the iraf reduction package ( stone & baldwin 1983 , baldwin & stone 1984 ) .
for the ir sky spectrum of la silla and ctio we downloaded @xmath22 from the 2mass web side[multiblock footnote omitted ] .
the instrumental filter bands are compared with the standard ones in fig .
[ figa.1 ] and fig .
[ figa.2 ] . + as usual ( see e.g. pignata et al . 2004 ) , we checked how well the constructed instrumental passbands match the real ones and compared the colour terms derived from standard stars ( @xcite for the optical and @xcite for the ir ) with those we computed by integrating the flux of spectrophotometric standards of @xcite for the optical and sirius , vega and the sun for the ir .
we decided to modify the instrumental passband only in cases where the difference between the two colour terms reported in table  [ taba.1 ] and table  [ taba.2 ] were larger than three times the error associated to both `` synthetic '' and `` photometric '' colour terms .
the lack of a suitable number of spectrophotometric ir standards with an establish magnitude make the colour term comparison less effective than at optical wavelengths .
as part of the european supernova collaboration we obtained extensive photometry and spectroscopy of the type ia sn  2002dj covering epochs from 11 days before to nearly two years after maximum . 
 detailed optical and near - infrared observations show that this object belongs to the class of the high - velocity gradient events as indicated by si , s and ca lines . 
 the light curve shape and velocity evolution of sn  2002dj appear to be nearly identical to sn  2002bo . 
 the only significant difference is observed in the optical to near - ir colours and a reduced spectral emission beyond 6500   . for high - velocity gradient type ia supernovae , we tentatively identify a faster rise to maximum , a more pronounced inflection in the v and r light curves after maximum and a brighter , slower declining late - time b light curve as common photometric properties of this class of objects . 
 they also seem to be characterized by a different colour and colour evolution with respect to `` normal '' sne  ia . 
 the usual light curve shape parameters do not distinguish these events . 
 stronger , more blueshifted absorption features of intermediate - mass elements and lower temperatures are the most prominent spectroscopic features of type ia supernovae displaying high velocity gradients . 
 it appears that these events burn more intermediate - mass elements in the outer layers . 
 possible connections to the metallicity of the progenitor star are explored .    
 [ firstpage ]    supernovae : general - supernovae : individual : sn  2002dj