Mass function and chemical abundances of the three distinct populations in the globular cluster NGC 6752 Roman Gerasimov 1 Adam Burgasser 1 Derek Homeier 2,3 Luigi R. Bedin 4 Michele Scalco 5,4 Jon Rees 6 Maurizio Salaris 7,8 Jay Anderson 9 1University of California San Diego 2Förderkreis Planetarium Göttingen 3Aperio Software Ltd. 4INAF -- Osservatorio Astronomico di Padova 5Università di Ferrara 6Lick Observatory 7Liverpool John Moores Uni. 8INAF -- Osservatorio Astronomico di Abruzzo 9Space Telescope Science Institute Photometric abundances Halo globular clusters are valuable tracers of the early history of chemical en- richment in the Milky Way due to their old ages [10] and comparatively uniform coeval stellar populations contained within. The latter allow constraints on the age and composition of a given cluster to be derived from its colour-magnitude diagram. The colours of ultracool dwarf (UCD) members of the cluster (including low Main Sequence stars and brown dwarfs) are expected to be most sensitive to element abundances due to molecular opacity and chemistry enabled by the low effective temperatures (Teﬀ) of such objects. Furthermore, brown dwarfs cool continuously throughout the lifetime of the cluster and, therefore, their magnitudes are directly related to the cluster age [4]. While no brown dwarfs in globular clusters have been detected so far, they are expected to be uncovered in large numbers in scheduled cycle 1 observations with the James Webb Space Telescope [2, 3]. In this study, we derive photometric constraints on the composition of the glob- ular cluster NGC 6752, known to host three distinct stellar sub-populations [11]. Theoretical isochrones were calculated from new grids of stellar evolutionary models and model atmospheres and iteratively fitted to existing Hubble Space Telescope (HST) observations (GO-15096, GO-15491; PI: Bedin). Our modelling setup is described in [5]. At Teﬀ> 4600 K, model atmospheres are calculated with the ATLAS 9 code [8, 9]. UCD model atmospheres are computed with PHOENIX 15 [6, 1, 7], including the Allard & Homeier cloud model. Evolu- tionary models are computed using MESA [13] with surface boundary conditions derived from our model atmospheres. Figure 1 summarizes the effect of enhancement of individual elements on the colour of UCDs in NGC 6752. We found the photometric effect of [Ca/M], [Co/M], [Cu/M], [Mn/M], [N/M], [Ni/M], [Si/M], [Ti/M] and [V/M] to be negli- gible within 3000 K < Teﬀ< 4500 K. The variation in colour shown in the figure is computed with respect to the approximate abundances of the red-most sub- population of NGC 6752 and does not include the effect of perturbed abun- dances on stellar evolution. Note that every enhancement shown in the figure displays a unique dependence on temperature and may therefore be inferred from the colour-magnitude diagram of the population. 3000 3300 3600 3900 4200 4500 Effective temperature [K] C O Al Mg Na M 0.1 mag blue red Figure 1: Sensitivity of HST/WFC3 mF110W −mF160W colours in the red-most sub-population of NGC 6752 to en- hancements of individual elements as well as the overall metallicity ([M/H]) as a function of effective temperature. Sensitivity is represented by the width of the cyan and pink bars and is defined as the difference in colour magni- tude as the enhancement is varied by 1 dex compared to the nominal abundance. Cyan/pink bars correspond to the case where the increased abundance offsets the colour blueward/redward. Isochrones 0.4 0.5 0.6 0.7 0.8 mF110W mF160W 15 16 17 18 19 20 21 22 mF160W 3300 3600 3750 3900 4200 Pop A [M/H] = 1.65 [Na/M] = 0.03 [Mg/M] = 0.51 [Al/M] = 0.65 [O/M] = 0.67 [C/M] = 0.2 Pop B [M/H] = 1.61 [Na/M] = 0.26 [Mg/M] = 0.49 [Al/M] = 1.1 [O/M] = 0.5 [C/M] = 0.3 Pop A Pop B Pop C [prelim] Teff [K] Observation 0.2 0.4 0.6 0.8 mF110W mF160W 16 18 20 22 24 26 28 mF160W Pop B M dwarfs L dwarfs T dwarfs Prelim Observation HBL Figure 2: Observed colour-magnitude diagram of NGC 6752 and b