V I Photometry of Globular Clusters Ngc 6293 and Ngc 6541: the Formation of the Metal-poor Inner Halo Globular Clusters

We present V I photometry of the metal-poor inner halo globular clusters NGC 6293 and NGC 6541 using the planetary camera of the WFPC2 on board Hubble Space Telescope (HST). Our color-magnitude diagrams of the clusters show well-defined blue horizontal branch (BHB) populations, consistent with their low metallicities and old ages. NGC 6293 appears to have blue straggler stars in the cluster’s central region. We discuss the interstellar reddening and the distance modulus of NGC 6293 and NGC 6541 and obtain E(B− V ) = 0.40 and (m − M)0 = 14.61 for NGC 6293 and E(B − V ) = 0.14 and (m −M)0 = 14.19 for NGC 6541. Our results confirm that NGC 6293 and NGC 6541 are clearly located in the Galaxy’s central regions (RGC ≤ 3 kpc). We also discuss the differential reddening across NGC 6293. The interstellar reddening value of NGC 6293 appears to vary by ∆E(B − V ) ≈ 0.02 – 0.04 mag within our small field of view. The most notable result of our study is that the inner halo clusters NGC 6293 and NGC 6541 essentially have the same ages as M92, confirming the previous result from the HST NIC3 observations of NGC 6287. Subject headings: Galaxy: formation — Galaxy: halo — globular clusters: individual (NGC 6293; NGC 6541) — color-magnitude diagrams — stars: horizontalbranch — dust: extinction Based on Observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Department of Astronomy and Space Science, Astrophysical Research Center for the Structure and Evolution of the Cosmos, Sejong University, 98 Gunja-Dong, Gwangjin-Gu, Seoul, 143-747, Korea; [email protected] Department of Physics & Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255; [email protected] INNER HALO CLUSTERS 2