A segmented period-luminosity relation for nearby extragalactic $\hat{I}´$ Scuti stars

Abstract

The period-luminosity relations (PLR) of Milky Way $\hat{I}´$ Scuti ($\hat{I}´$ 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 non-linear behaviour in the PLR of $\hat{I}´$ Sct stars. Using the largest sample of $\sim 3700$ extragalactic $\hat{I}´$ Sct stars from data available in the literature $-$mainly based on OGLE and SuperMACHO survey 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 $\log P=-1.03\pm 0.01$ (or $0.093\pm 0.002$ d) for shorter periods (sp) and longer periods (lp) than the break-point: $$M_V^{sp}=-7.08(\pm 0.25)\log P-5.74(\pm 0.29);\log P<-1.03$$ $$M_V^{lp}=M_V^{sp}+4.38(\pm 0.32)\cdot (\log P+1.03(\pm 0.01));\log P\ge -1.03$$ 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 $\hat{I}´$ Sct stars. The origin of the segmented relation at $\sim 0.09$ days remains unexplained based on the current data.

9 pages, 3 figures, 1 table. Accepted for publication into The Astrophysical Journal Letters