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

Deka, Mami and Kanbur, Shashi M. and Deb, Sukanta and Das, Susmita and Kurbah, Kerdaris (2022) Period-colour and amplitude-colour relations for OGLE-δ Scuti stars in the Galactic bulge and LMC. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 517 (2). pp. 2251-2263. ISSN 0035-8711

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Abstract

We present an analysis on the behaviour of the Galactic bulge and the Large Magellanic Cloud (LMC) δ 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 δ Scuti stars are exploited for the analysis. It has been found that the Galactic bulge δ Scuti stars obey flat PC relations at maximum/mean/minimum light, while the LMC δ Scutis have sloped, sloped, and flat PC relations at maximum, mean, and minimum light, respectively. Both the Galactic bulge and the LMC δ Scutis have sloped, flat, and sloped AC relations at maximum, mean, and minimum light, respectively. These relations also show that Galactic δ Scutis are hotter as compared to their LMC counterparts. The period-amplitude (PA) relations for δ 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 δ 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 δ 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 δ 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.

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