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Paper text:
evolutionary stellar population synthesis , i.e. modelling spectral energy distributions emitted by evolving stellar populations , is a natural approach to studying the stellar content of different galaxies
. one of the main ingredients of these models are the stellar libraries , which can be empirical or theoretical .
stellar population models usually consider only the total metal content of stars and , therefore , ignore the different chemical abundance patterns that are present in individual stars .
however , different chemical abundance patterns have a strong influence on the shape of the spectra .
in particular , it is well known that stellar population models based on empirical libraries ( which are mostly composed of solar neighbourhood stars ) can not reproduce the high values of mg abundances found in giant elliptical galaxies .
this is commonly interpreted as a consequence of high [ mg / fe ] in these systems , most likely due to a rapid star formation history compared to the more quiet one of the solar neighbourhood ( tinsley 1980 ) .
an obvious solution , explored recently by several authors ( e.g. coelho _ et al .
_ 2007 ; walcher _ et al .
_ 2009 ; lee , worthey & dotter 2009 ) , would be to use theoretical libraries with the desired coverage in chemical abundances .
however , while theoretical libraries have improved dramatically in the last few years ( chavez , malagnini & morossi 1997 ; murphy & meiksin 2004 ; rodriguez - merino _ et al .
_ 2005 ; munari _ et al .
_ 2005 ; martins _ et al .
_ 2005 ; coelho _ et al .
_ 2005 ; frmaux _ et al .
_ 2006 ; bertone _ et al .
_ 2008 ) , they still do not reproduce real stars of all spectral types , with especial problems depending on the wavelength range ( i.e. martins & coelho 2007 ; bertone _ et al . _ 2008 ) .
some of the remaining problems are the incompleteness of the atomic and molecular line opacity lists in the blue region of the spectrum and for cool stars ( t@xmath1 @xmath2 4500 k ) as well .
another approach is to compute , with the help of model atmospheres , response functions to characterise the variation of specific spectral characteristic ( usually lick indices ) to variation of different elements ( see trippico & bell 1995 ; korn , maraston & thomas 2005 ) .
models using these response functions are those of tantalo , chiosi & bressan ( 1998 ) , trager _
( 2000a ) , and thomas , maraston & bender ( 2003 ) .
however , the accuracy of these theoretical predictions have not been tested empirically yet .
it is the main objective of this work to provide [ mg / fe ] abundance ratios for one of the most complete empirical stellar libraries currently available ( miles ) ( * m*id - resolution * i*saac newton telescope * l*ibrary of * e*mpirical * s*pectra , snchez - blzquez _ et al .
the miles database , which was especially designed for stellar population modelling , contains flux calibrated optical spectra of high signal - to - noise ratio ( s / n ) for 985 stars covering @xmath33525 - 7500  with a homogeneous resolution @xmath4 = fwhm = 2.3  . the parametric coverage of sample stars in the three - dimensional h - r diagram is quite wide : 2800 @xmath5 t@xmath1 @xmath5 50400 k , 0.0 @xmath5 @xmath6 g @xmath5 @xmath75.0 , and @xmath82.7 @xmath5 [ fe / h ] @xmath5 @xmath71.0 dex , where [ fe / h ] = log(fe / h)@xmath9 @xmath8 log(fe / h)@xmath10 such that formally log(fe ) = log(@xmath11(fe)/@xmath11(h ) ) + 12 , log(h ) = 12 and @xmath11(fe ) and @xmath11(h ) are the numerical densities ( @xmath12 ) of iron and hydrogen atoms respectively .
the scales for these photospheric parameters were carefully defined by cenarro _
their precisions , respectively @xmath13100 k , @xmath130.2 and @xmath130.1 dex , makes miles good for ssp modelling . for the present work , we excluded those stars with uncertain or wrong atmospheric parameters ( see vazdekis _
_ 2010 for details about how these stars were identified ) .
the miles [ mg / fe ] catalogue presented here consists of two measurement sets . the first one is composed of measurements obtained from the literature from high spectral resolution analyses properly calibrated to a common system .
the second set assembles abundances measured by us directly from the miles mid - resolution ( hereafter mr ) spectra and calibrated using the high - resolution ( hereafter hr ) sample .
the catalogue is represented in two separate tables for field and cluster stars .
both tables are only available in electronic form .
the paper layout is as follows : section 2 describes the compilation of hr abundance measurements from the literature and their calibration ; section 3 shows the mg abundance measurements from the miles spectra ; section 4 compiles the miles [ mg / fe ] catalogue and analyses its coverage over the library parameter space ; section 5 compares our stellar data with predictions of theoretical models focusing on the behaviour of some lick indices with [ mg / fe ] ; and , finally , section 6 plans for applications to building new simple stellar population models with variable @xmath0-enhancement .
section 7 summarizes the whole paper and final conclusions .
there are also three appendices : appendix a confronts the compiled hr data with a well - known stellar spectrum library , appendix b presents comparisons of the miles photospheric parameter scales with those from the compiled hr studies , and appendix c compares the results for cluster stars with hr studies .
the first step of this work consisted of performing a bibliographic compilation of magnesium abundances from high - resolution spectroscopic analyses for the miles library stars . to guarantee homogeneity between the measurements provided by several studies we performed a calibration and correction of systematic differences among sources and a chosen standard reference system , following a similar procedure as in cenarro _
et al . _ ( 2001 , 2007 )
for instance , feltzing & gustafsson ( 1998 ) give a detailed error analysis of elemental abundances for g and k metal - rich dwarfs also including comparisons with other studies . in this section , we describe the chosen reference sample and the procedures we followed to homogenize the measurements to a single uniform scale of [ mg / fe ] .      our reference sample to define a scale for the mg / fe abundance ratio is from borkova & marsakov ( 2005 , hereafter bm05 ) .
the catalogue of borkova & marsakov is a robust compilation of the atmospheric parameters t@xmath1 , log @xmath14 and [ fe / h ] plus [ mg / fe ] from high - s / n high - resolution analyses of field stars published between 1989 and 2003 ( covering 36 studies with mg abundance determinations for around 900 stars ) .
bm05 computed weighted average values and their errors through an iterative procedure in order to correct for the systematic deviations of each data set relative to reduced mean homogeneous scales .
the uncertainty of [ mg / fe ] in bm05 is 0.05 and 0.07 dex , respectively , for metal - rich ( [ fe / h ] @xmath15 @xmath81.0 dex ) and metal - poor stars ( [ fe / h ] @xmath5 @xmath81.0 dex ) .
the bm05 catalogue contains 218 stars in common with the miles library ( all with log @xmath14 @xmath16 3.0 ) .
bm05 was also the reference work adopted in cenarro _
( 2009 ) , where mg and ca abundances were compiled for 192 stars of their calcium triplet stellar library ( hereafter cat ) of 706 objects .
as there are many miles stars in common with the cat sample ( 132 stars ) , this work provides values that we can compare with ( see appendix a ) .
we first checked the possible presence of systematic differences in the scales of miles and bm05 atmospheric parameters ( [ fe / h ] , log @xmath14 and t@xmath1 ) , however we did not find any ( see appendix b for details ) .
apart from the bm05 compilation , we obtained [ mg / fe ] for 97 more stars from 15 other hr studies , as listed in table 1 .
their abundance ratios were then carefully transformed onto the adopted scale as described next .
we have obtained [ mg / fe ] measurements for 76.3% of the stars in the miles spectral library used for understanding stellar atmospheres and stellar populations in galaxies and star clusters . 
 these abundance ratios were obtained through ( 1 ) a compilation of values from the literature using abundances from high - resolution spectroscopic studies and ( 2 ) a robust spectroscopic analysis using the miles mid - resolution optical spectra . 
 all the [ mg / fe ] values were carefully calibrated to a single uniform scale , by using an extensive control sample with results from high - resolution spectra . the small average uncertainties in the calibrated [ mg / fe ] values ( respectively 0.09 and 0.12 dex with methods ( 1 ) and ( 2 ) ) and the good coverage of the stars with [ mg / fe ] over stellar atmospheric parameter space of the library will permit the building of new simple stellar populations ( ssps ) with empirical @xmath0-enhancements . 
 these will be available for a range of [ mg / fe ] , including both sub - solar and super - solar values , and for several metallicities and ages . 
 these models will open up new prospects for testing and applications of evolutionary stellar population synthesis .    
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