A Segmented Period-Luminosity Relation for Nearby Extragalactic Delta Scuti Stars

Abstract

The period-luminosity relations (PLRs) of Milky Way δ Scuti (δ Sct) stars have been described to the present day by a linear relation. However, when studying extragalactic systems such as the Magellanic Clouds and several dwarf galaxies, we notice for the first time a nonlinear behavior in the PLR of δ Sct stars. Using the largest sample of ~3700 extragalactic δ Sct stars from data available in the literature-mainly based on the Optical Gravitational Lensing Experiment and the Super MAssive Compact Halo Object project in the Large Magellanic Cloud (LMC) - we obtain that the best fit to the period-luminosity (M_V) plane is given by the following piecewise linear relation with a break at logP = -1.03 ± 0.01 (or 0.093 ± 0.002 days) for shorter periods (sp) and longer periods (lp) than the break-point:

M_V = -7.08(±0.25) logP-5.74(± 0.29); logP<-1.03

M_V^lp = M_V^sp +4.38(± 0.32)\cdot (logP+1.03(± 0.01)); logP≥ -1.03.

Geometric or depth effects in the LMC, metallicity dependence, or different pulsation modes are discarded as possible causes of this segmented PLR seen in extragalactic δ Sct stars. The origin of the segmented relation at ∼0.09 days remains unexplained based on the current data.