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evolutionary population synthesis predictions depends strongly on the stellar inputs .
one of them is the library of stellar spectra used to compute the integrated population spectrum . to interpret the integrated light of post - starburst galaxies in the near - ir , where the bulk of the energy of luminous evolved stars is emitted
, one needs to introduce in some way the spectral signatures of such stars ( essentially molecular absorption bands ) .
asymptotic giant branch stars ( agb stars ) are characterized by low temperatures as well as high luminosity .
the third dredge - up phenomenon on the agb , due to the recurrent penetration of the convective envelope into the carbon - rich layers , brings the products of the helium burning nucleosynthesis to the surface ( see mowlavi , 1998 , for a review ) , and is responsible for the eventual conversion of some m stars to carbon stars .
this process is highly efficient in metal - poor systems , where the c stars can dominate the agb stars population luminosity at low metallicity ( groenewegen 1998 ) .
another particularity of the most evolved agb stars is their high rate of mass loss ( @xmath2 , zijlstra 1998 ) .
these stars , surrounded by a dusty envelope , emit most of their energy at wavelengths from a few to a few hundred microns .
they can be missed in optical surveys , and accounting for circumstellar extinction reduces the predicted emission of the agb population even in the near - ir spetral region ( see habing , 1996 , for detailed review ) .    for intermediate age populations one must take into account all the agb properties pointed out above .
the different librairies used in the literature , empirical or synthetic ones , ( terndrup et al .
1990 , lanon & rocca - volmerange 1992 , fluks et al .
1994 , lejeune et al .
1998 ) have introduced cool static giant stars , but they are not appropriate to distinguish between red giant stars and agb stars .
bressan et al .
( 1998 ) have taken the presence of the agb stars into account in their modelling of the intermediate age population by assigning , to each point of an isochrone , parameters of a dusty envelope model and performing radiative transfer calculations to correct the spectra of static giant star .
this method is promising but it takes into account only the effects of the circumstellar shells around agb stars . our principal motivation is to recognise the presence of agb stars in the integrated near - ir spectrum of a stellar population and to introduce all the effects pointed out above to be able to interpret accurately the spectrophotometric propreties of intermediate age stellar populations .
the scope of this paper is to construct a new stellar library taking into account the relevant processes affecting the near - ir spectra of agb stars .
the spectra of our sample ( lanon & wood , 1997 ) cover a broad spectral range ( 0.52.5@xmath3 m ) quasi - instantaneously , i.e. with no phase mixing .
this sample consists of a wide range of cool objects : @xmath4 o - rich miras and @xmath5 c - rich miras , with various periods ( between @xmath6 and @xmath7 days , corresponding to different luminosities ) , observed @xmath8 times or more on different phases ( up to @xmath9 times ) . for reference and comparison ,
the sample also includes spectra of non - variable m - type giants , supergiants , lmc and galactic bulge variables , and oh / ir and c / ir . the near - ir spectra , taken at 2.3 m anu telescope at siding spring observatory , have a resolution of @xmath10 and are connected with overlapping low resolution optical spectra , taken at mt stromlo observatory .
the quality of the spectra is good ; the s / n per resolved element usually reaches @xmath11 in atmospheric windows and the typical uncertainty on ( i - k ) is @xmath12mag .
+ the data confirm that the long period variables display much deeper near - ir molecular absorption bands than the cool and luminous static stars ( bessell et al .
to estimate the luminosity of our spectra , we have used the l@xmath13-p relation of kanbur et al .
( 1998 ) , and an instantanous bolometric correction , obtained from the individual spectra themselves ( mouhcine et al . in preparation ) .
+ the effective temperature ( t@xmath14 ) determination is more problematic ( see haniff et al .
1995 for the essential reasons , which we can not discuss in enough detail here ) , and will strongly affect in the synthetic integrated spectra .
the shape of the spectral energy distribution of the o - rich long - period variables ( lpvs ) is similar at most phases to that of static m giants ( fig.1 ) .
alvarez et al .
( in preparation ) find that the optical spectra of lpvs can be fitted well with static giant model atmospheres .
this might be interpreted as an indication that the t@xmath14 scale of the lpvs and static giants are equivalent .
a large grid of static giant spectra was used to determine the effective temperatures of the spectra of our sample . feast ( 1996 ) has derived an alternative temperature scale for lpvs using a data set grouping m mira and non - mira stars .
the colour - t@xmath14 relation derived is steeper than the one derived using the static giants models : cool mira spectra are assigned lower t@xmath14 values .
this relation could be biased since the t@xmath14 used in feast s fit are derived from angular diameter measurements , who are interpreted using uncertain models both to correct for limb - darkening and to convert a monochromatic radius to the effective radius ( rosseland optical depth 2/3 or 1.0 ) .
the situation is complicated by the uncertainties about the pulsation mode of some lpvs .
in addition , the t@xmath14 derived can be biased if there is some scattering source ( dust ? ) in the upper atmospheres of lpvs that makes them appear bigger than they are ( wood , private communication ) .
+ to be conservative we have constructed two different librairies considering the two different t@xmath14-(j - k ) relations .
schematically , the thermal pulses move stars up and down in luminosity along the evolutionary tracks , while the lpv pulsations ( periods of @xmath15 days ) occur perpendicular to the tracks ( large t@xmath14 shifts ) , deeply modifying the spectral type , the shape of the global energy distribution and the spectral signatures of the stars .
the evolutionary tracks represent the evolution of static parent stars of the pulsating long period variables . how do we deal with this problem ? how to determine the stellar spectrum to associate with each point of the evolutionary tracks
? there are two approaches to adopt .
one is to obtain properly weighted averages of the phase dependent spectra of individual stars , for various masses and evolutionary stages .
this approach is , in principle , correct , but needs a huge amount of data or large pulsating model grids , which makes this approach very hard practically .
the second approach , which was adopted , is to average the instantaneous spectra by temperature bin , disregarding phase and the pulsation properties .
this approach is supported by the fact that , in our sample , no systematic correlation between the molecular indices and the luminosity or amplitude was found .
in addition , this approach needs much less data .
+ our library also contains carbon stars . using interferometric angular diameters , in combinaison with bolometric flux , dyck et al .
( 1997 ) have found that there is small range of effective temperatures for c - stars over a large range in spectral type , and derived that t@xmath16 k. finally , the library includes 5 oh / ir and several observations of 1 c / ir star .
this library of averaged spectra is constructed for population synthesis modelling .
it should become public before the end of the year .
we acknowledge the fruitful collaboration with r. alvarez , b. plez , m. scholz , p. wood , for different aspects of this work .
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a self - consistent spectrophotometric modelling of intermediate age post - starburst requires accurate stellar ingredients taking into account a principal feature of the stars dominating the near - ir emission during this phase : the variability of the agb stars . 
 a new library of stellar spectra based on averages of the empirical spectra of variable agb stars is presented . 
 this library is designed for convenient use in the population synthesis models . 
 we discuss meaningful ways to compute these averages , and the non - trivial connection with the theoretical stellar parameters . our library covers the near infrared wavelength range between @xmath0 m and @xmath1 m and exhibits fundamental differences when compared to the standard libraries using only static giants .