Empirical instability strip for classical Cepheids: I. The Large Magellanic Cloud galaxy

25.01.2023

The instability strip (IS) of classical Cepheids has been extensively studied theoretically. Comparison of the theoretical IS edges with those obtained empirically, using the most recent Cepheids catalogs available, can provide us with insights into the physical processes that determine the position of the IS boundaries. In this study, we investigate the empirical positions of the IS of the classical Cepheids in the Large Magellanic Cloud (LMC), considering any effect that increases its width, to obtain intrinsic edges that can be compared with theoretical models. 

CMD of F and 1O LMC Cepheids. The boundaries of our empirical IS are shown as red circles. Fits for the upper and lower part of the red and blue edges are shown as black solid lines. Periods for these stars are shown with a color gradient. For 1O Cepheids, periods were fundamentalized. Dashed lines of constant periods are overplotted. In most cases, error bars are smaller than the size of the points.
CMD of F and 1O LMC Cepheids. The boundaries of our empirical IS are shown as red circles. Fits for the upper and lower part of the red and blue edges are shown as black solid lines. Periods for these stars are shown with a color gradient. For 1O Cepheids, periods were fundamentalized. Dashed lines of constant periods are overplotted. In most cases, error bars are smaller than the size of the points.

We use data of classical fundamental-mode (F) and first-overtone (1O) LMC Cepheids from the OGLE-IV variable star catalog, together with a recent high-resolution reddening map from the literature. Our final sample includes 2058 F and 1387 1O Cepheids. We studied their position on the Hertzsprung-Russell diagram and determined the IS borders by tracing the edges of the color distribution along the strip. We obtain the blue and red edges of the IS in V- and I-photometric bands, in addition to logTeff and logL. The results obtained show a break located at the Cepheids' period of about 3 days, which was not reported before.

Same as the previous Fig., but for 1O (left panel) and F (right panel) independently
Same as the previous Fig., but for 1O (left panel) and F (right panel) independently

We compare our empirical borders with theoretical ones published in the literature obtaining a good agreement for specific parameter sets. The break in the IS borders is most likely explained by the depopulation of second and third crossing classical Cepheids in the faint part of the IS, since blue loops of evolutionary tracks in this mass range do not extend blueward enough to cross the IS at the LMC metallicity. Results from the comparison of our empirical borders with theoretical ones prove that our empirical IS is a useful tool for constraining theoretical models.

CMD showing the empirical IS limits (red points), separately for 1O (left panel) and F-mode (right panel) LMC Cepheids. Fits for the blue and red edge, considering a break at P ∼ 3 days, are shown as solid lines. Shaded gray areas mark the blue loop extent (delimited by its bluest extreme and the tip of the RGB) for evolutionary tracks with masses from 3 to 7 M⊙, and for Z = 0.006, 0.008, and 0.009. Note that the upper limits for the areas are defined by the evolutionary tracks for 7 M⊙ and would extend further up if tracks with higher masses are considered. The I-band absolute magnitude distributions of the 1O and F LMC Cepheids are shown on the right side of each panel.
CMD showing the empirical IS limits (red points), separately for 1O (left panel) and F-mode (right panel) LMC Cepheids. Fits for the blue and red edge, considering a break at P ∼ 3 days, are shown as solid lines. Shaded gray areas mark the blue loop extent (delimited by its bluest extreme and the tip of the RGB) for evolutionary tracks with masses from 3 to 7 M⊙, and for Z = 0.006, 0.008, and 0.009. Note that the upper limits for the areas are defined by the evolutionary tracks for 7 M⊙ and would extend further up if tracks with higher masses are considered. The I-band absolute magnitude distributions of the 1O and F LMC Cepheids are shown on the right side of each panel.
https://ui.adsabs.harvard.edu/abs/2023arXiv231115849E/abstract
Share