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

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 (MV) 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:

\begin{eqnarray*}{M}_{V}^{\mathrm{sp}}=-7.08(\pm 0.25)\mathrm{log}P-5.74(\pm 0.29);\quad \mathrm{log}P\lt -1.03\end{eqnarray*}

\begin{eqnarray*}{M}_{V}^{\mathrm{lp}}={M}_{V}^{\mathrm{sp}}+4.38(\pm 0.32)\cdot (\mathrm{log}P+1.03(\pm 0.01));\quad \mathrm{log}P\geqslant -1.03.\end{eqnarray*}

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.