HST observations of the LMC field around SN87A: distance determinations with Red Clump and Tip of the Red Giant Branch stars

Abstract

We have used HST-WFPC2 multiband observations of a field around SN 1987A in the Large Magellanic Cloud to measure its distance from the Sun. The observations allowed us to carefully determine the interstellar extinction along the line of sight to a large number of stars and to measure the LMC distance by using two stellar distance indicators: the Red Clump and the Tip of the Red Giant Branch. From an application of the Red Clump method we obtain a distance modulus (m-M)o,rc(LMC)=18.59+-0.04+-0.08 mag (statistical plus systematic error), in good agreement with the distance derived by using the Tip of the Red Giant Branch stars, namely (m-M)o,trgb(LMC)=18.69+-0.25+-0.06 mag (statistical plus systematic error). Both values agree well with the distance to the SN 1987A as determined from a study of its inner ring fluorescent echo ((m-M)(SN87A)=18.55+-0.05 mag, Panagia 1998), thus excluding distance moduli lower than 18.43 to a 99.7% significance level. Differences with respect to previous results obtained using the same distance indicators are discussed.