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Near Infrared Photometry of Galactic Globular Clusters M56 and M15. Extending the Red Giant Branch vs. Metallicity Calibration Towards Metal Poor Systems

Valentin D. Ivanov·Jordanka Borissova·Almudena Alonso-Herrero·Tatiana Russeva
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Abstract

Infrared JKs-band photometry of the Galactic globular clusters M15 and, for the first time, M56 is presented. We estimated the reddening (E(B-V)=0.18+/-0.08 mag) and distance modulus ((m-M)_V=15.43+/-0.30 mag) towards the poorly studied globular cluster M56. We combined our data with observations of other clusters from the literature (12 in total) to extend the [Fe/H] vs. Red Giant Branch (RGB) slope relation towards metal-poor clusters. Our best fit yields to [Fe/H]=-3.40(+/-0.22)-27.74(+/-2.35)x(RGB Slope), with an r.m.s.=0.20. The broader metallicity baseline greatly reduced the uncertainties compared to other existing calibrations. We confirmed a previously obtained calibration of the relation between the RGB color (J-Ks)_0(RGB) at M_Ks=-5.5 vs. [Fe/H]: [Fe/H]=-6.90(+/-0.99)+6.63(+/-1.05)x(J-Ks)_0(RGB) with an r.m.s.=0.33. Finally, using the new RGB slope calibration we estimated the abundance of the super metal-rich cluster Liller 1 [Fe/H]=+0.34+/-0.22.

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Valentin D. Ivanov, Jordanka Borissova, Almudena Alonso-Herrero, Tatiana Russeva. 2000-02-04. Near Infrared Photometry of Galactic Globular Clusters M56 and M15. Extending the Red Giant Branch vs. Metallicity Calibration Towards Metal Poor Systems. https://arxiv.org/abs/astro-ph/0002118

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