CCD Photometry of the Galactic Globular Cluster NGC 6144

Abstract

We present CCD photometry in the BVI-filter passbands for the Galactic globular cluster NGC 6144. This cluster is located in the direction of the ρ Ophiuchi dust cloud, approximately 30' northwest of the bright star α Scorpii (Antares). Our color-magnitude diagrams (CMDs) of NGC 6144 extend from above the tip of the first-ascent red giant branch (RGB) to ~3 mag below the horizontal branch (HB). The morphology of the HB is predominantly blueward of the RR Lyrae instability strip, while the RGB is relatively steep, signifying a low-to-intermediate metal abundance. Our CMD also reveals the presence of three candidate post-asymptotic giant branch stars. We find VHB = 16.30 ± 0.07; coupling the measured RGB color at the level of the HB with a polynomial describing the shape of the RGB, we have utilized the simultaneous reddening and metallicity method of Sarajedini to estimate a metallicity of [Fe/H] = -1.81 ± 0.12 (on the Zinn & West scale) and a mean reddening of E(B-V) = 0.41 ± 0.02. In addition to this mean level, our observations reveal that the reddening has a spatially variable component generally increasing from north to south consistent with the expected density variations in the ρ Ophiuchi dust cloud.