2022ApJ...940L..25M

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: \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.