Period-colour and amplitude-colour relations for OGLE-delta Scuti stars in the Galactic bulge and LMC

We present an analysis on the behaviour of the Galactic bulge and the Large Magellanic Cloud (LMC) delta Scuti stars in terms of period-colour and amplitude-colour (PCAC) relations at maximum, mean, and minimum light. The publicly available Optical Gravitational Lensing Experiment-IV (OGLE-IV) light curves for Galactic bulge and OGLE-III light curves for LMC delta Scuti stars are exploited for the analysis. It has been found that the Galactic bulge delta Scuti stars obey flat PC relations at maximum/mean/minimum light, while the LMC delta Scutis have sloped, sloped, and flat PC relations at maximum, mean, and minimum light, respectively. Both the Galactic bulge and the LMC delta Scutis have sloped, flat, and sloped AC relations at maximum, mean, and minimum light, respectively. These relations also show that Galactic delta Scutis are hotter as compared to their LMC counterparts. The period-amplitude (PA) relations for delta Scutis exhibit different behaviour in the Galactic bulge and the LMC. The LMC variables are found to have higher amplitudes at a given period. The amplitude of the Galactic bulge delta Scuti shows a bimodal distribution which can be modelled using a two-component Gaussian Mixture Model: one component with a lower amplitude and another with a higher amplitude. The observed behaviour of the delta Scuti PCAC relations can be explained using the theory of the interaction of hydrogen ionization front (HIF) and stellar photosphere as well as the PA diagram. We use mesa-rsp to calculate theoretical non-linear hydrodynamical pulsation models for delta Scuti stars with input metallicities of Z = 0.02 and Z = 0.008 appropriate for the Galactic bulge and LMC, respectively. The observed PCAC relations and theoretical calculations support the HIF-photosphere interaction theory.