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UBC Theses and Dissertations

Deep CCD photometry in the globular cluster M12 Sato, Takashi


New UBV CCD photometry is presented for a single field measuring 2'.2 x 3'.5 in the galactic globular cluster M12 (NGC 6218). This field is located 3'.5 or 3 core radii from the cluster centre. The photometry reaches well down the main sequence to fainter than V = 23.5. In the colour-magnitude diagram, a possible sequence of binary stars appears separated from the upper main sequence. Excluding the possible binaries, the upper main sequence shows small intrinsic width consistent with zero spread in chemical composition and with an upper limit of Δ[Fe/H] ≤ 0.2. Stars from the M12 horizontal branch and from the field found on our CCD frames have been used to derive a reddening estimate for M12 of E(B - V) = 0.23 ± 0.04. Our fiducial main sequence is compared with that defined by the subdwarfs to determine the apparent distance modulus with the result (m - M)v = 14.25 ± 0.20. The metallicity of M12 is estimated to be [Fe/H] = -1.4 using values found in the literature in conjunction with our own estimate based on δ(U - B)[sub 0.6] = -0.15 which is measured from the colour-colour diagram. Using these parameters, comparison with theoretical isochrones of VandenBerg and Bell results in a best-estimate age of 17 ±1 Gyr, although uncertainties associated with these fundamental parameters and the oxygen abundance increase the 'true error.' A luminosity function and the corresponding mass function are also constructed using the available data. A fit to a power law mass spectrum of the form Ndm ∝ m[sup -(1+z)]dm yields x = -0.70 ± 0.16 whereas x ≈ +1 is expected from the x - metallicity relation of McClure et al. Results obtained using multimass King models indicate that the global value for the power law index is also near -0.7 for 12.

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