SMHASH: a new mid-infrared RR Lyrae distance determination for the Local Group dwarf spheroidal galaxy Sculptor

We present a new distance estimation for the Milky Way dwarf spheroidal satellite Sculptor
obtained from multi-epoch mid-infrared observations of RR Lyrae stars (RRLs). The 3.6 μm
observations have been acquired with the Infrared Array Camera onboard the Spitzer Space
Telescope as part of the SMHASH program. Mid-infrared light curves for 42 RRLs were
obtained, from which we measured Sculptor’s distance modulus to be μ = 19.60 ± 0.02
(statistical) ± 0.04 (photometric) mag (with σsys = 0.09 mag), using the 3.6 μm empirical
period–luminosity relations derived from the Galactic globular clusterM4, orμ=19.57±0.02
(statistical) ± 0.04 (photometric) mag (with σsys = 0.11 mag) using empirical relations in the
same passband recently derived from the Large Magellanic Cloud globular cluster Reticulum.
Both these measurements are in good agreement with values presented in previous works with
Sculptor RRLs in optical bands, and are also consistent with recent near-infrared RR Lyrae
results. Best agreement with the literature is found for the lattermodulus which is equivalent to
a distance of d = 82 ± 1 (statistical) ± 2 (photometric) kpc (with σsys = 4 kpc). Finally, using
a subsample of RRLs with spectroscopic metallicities, we demonstrate that these distance
estimates are not affected by metallicity effects.