We present an analysis of high latitude δ Scuti stars (|b| > 1○) in the Galactic bulge region (−8○.3 < l < 9○.4) using a clean sample of the photometric data of 7,440 stars recently released by the OGLE-IV project. The geometrical parameters of the bulge are determined based on Maximum Likelihood (ML) analysis in five-dimensional parameter space. More refined values of these parameters as well as their uncertainties are obtained from a fully Bayesian Markov Chain Monte Carlo (MCMC) analysis. Approximating the bulge as an ellipsoid, the distribution of the number density of stars as a function of Galacto-centric distance has been modelled using three distribution functions: two Exponential ($\rm E_{1},\rm E_{2}$) types and one Gaussian ($\rm G$) type. Based on the AIC and BIC values, the exponential model $\rm E_{1}$ is chosen as the best statistical model for the parameter values obtained from the MCMC analysis. The MCMC analysis yields the following results: the mean distance to the Galactic center (GC) is found to be R0 = 8.034 ± 0.012stat ± 0.586sys kpc; the bulge δ Scuti distribution has a triaxial shape with normalized (a ≡ 1) axes ratios (a: b: c) as 1.000 ± 0.005 : 0.348 ± 0.002 : 0.421 ± 0.002. Here a is the semi-major axis lying in the Galactic plane and pointing towards us; b and c are the two semi-minor axes, the former lying in the Galactic plane and the later perpendicular to it. Smaller values of b as compared to a obtained for Galacto-centric distances R ≥ 2.0 kpc indicate the presence of a bar-like structure of the bulge with a bar angle of 22○.006 ± 2○.078.

This is the second of a series related to the study of geometry of the Magellanic Clouds based on multi-wavelength photometry of classical Cepheids. In this paper we determine the geometrical and viewing angle parameters of the Small Magellanic Cloud (SMC) using the Leavitt law for classical Cepheids with/without a break in the law at a certain period as reported in the literature. The study utilises photometric data for more than 3400 common classical Cepheids (Fundamental (FU) and First overtone (FO)) in optical (V, I), near-infrared (Y, J, Ks) and mid-infrared ([3.6] μm and [4.5] μm) photometric bands. We obtain statistical reddening and distance modulus free from the effect of reddening to each of the individual Cepheids with respect to the mean distance modulus and reddening of the SMC. The reddening maps of the SMC obtained from the analyses with/without breaks in the Leavitt law show good agreement with each other as well as with other maps available in the literature. The Cartesian coordinates of individual stars with respect to the galaxy plane are obtained using the information of equatorial coordinates (α, δ) as well as extinction-free distance measurements. Modelling the observed three dimensional distribution of the Cepheids as a triaxial ellipsoid, we obtain the geometrical and viewing angle parameters of the SMC. The weighted average yields the following values of parameters for the SMC: the geometrical axes ratios of 1.000 ± 0.001: 1.544 ± 0.002: 9.742 ± 0.030 and the viewing angle parameters having inclination angle i = 3○.465 ± 0○.030 with respect to the longest axis from the line of sight and position angle of line of nodes (major axis) θlon = 63○.086 ± 0○.117.

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/flat PC relations at maximum/mean/minimum light. Both the Galactic bulge and the LMC δ Scutis have sloped/flat/sloped AC relations at maximum/mean/minimum. 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.

We present an analysis of the theoretical and observed light curve parameters of the fundamental mode (FU) classical Cepheids in the Magellanic Clouds in -and -photometric bands. The state-of-the-art 1D non-linear radial stellar pulsation (RSP) code in MESA ( -) has been utilized to generate the theoretical light curves using four sets of convection parameters. Theoretical light curves with two chemical compositions: = 0.008 and = 0.004 appropriate for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively, covered a wide range of periods (3 < (d) < 32). The observed light curves are taken from the OGLE-IV database. We compare theoretical and observed Fourier parameters (FPs), and investigate the period-luminosity (PL), period-colour (PC), and amplitude-colour (AC) relations as a function of pulsation phase for short (log < 1), long (log > 1) and all periods. The multiphase relations obtained from theoretical and observed light curves in the PL/PC/AC plane are found to be dynamic in nature, with the effect more pronounced at Φ ∼ 0.75 − 0.85. Furthermore, a contrasting behaviour of the theoretical/observed multiphase PL and PC relations between the short and long periods has been found for both LMC and SMC. The analysis shows that multiphase PL relations are more stringent to test the models with observations over the FPs. Distances to the LMC/SMC determined using long period Cepheids are found to be in good agreement with the literature values when the term 21 is added to the PL relation.

We present an analysis of the theoretical and observed light curve parameters of the fundamental mode (FU) classical Cepheids in the Magellanic Clouds in V- and I- photometric bands. The state-of-the-art 1D non-linear radial stellar pulsation (RSP) code in MESA (mesa-rsp) has been utilized to generate the theoretical light curves using four sets of convection parameters. Theoretical light curves with two chemical compositions: Z = 0.008 and Z = 0.004 appropriate for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively, covered a wide range of periods ($3&lt;P (\rm {d})&lt;32$). The observed light curves are taken from the OGLE-IV database. We compare theoretical and observed Fourier parameters (FPs), and investigate the period-luminosity (PL), period-colour (PC), and amplitude-colour (AC) relations as a function of pulsation phase for short (log P &lt; 1), long (log P &gt; 1) and all periods. The multiphase relations obtained from theoretical and observed light curves in the PL/PC/AC plane are found to be dynamic in nature, with the effect more pronounced at Φ ∼ 0.75 − 0.85. Furthermore, a contrasting behaviour of the theoretical/observed multiphase PL and PC relations between the short and long periods has been found for both LMC and SMC. The analysis shows that multiphase PL relations are more stringent to test the models with observations over the FPs. Distances to the LMC/SMC determined using long period Cepheids are found to be in good agreement with the literature values when the term R21 is added to the PL relation.