Empirical instability strip for classical Cepheids II. The Small Magellanic Cloud galaxy

TL;DR

This study empirically determines the edges of the classical Cepheids instability strip in the SMC, compares them with models, and discusses implications for stellar evolution and distance measurements.

Contribution

It provides the first empirical determination of the SMC Cepheid instability strip edges and compares them with theoretical models, highlighting discrepancies and metallicity effects.

Findings

The blue edge of the IS agrees with models, red edge shows significant differences.

The SMC IS is wider and redder than the LMC, despite lower metallicity.

Breaks at periods 1.4-3 days influence the IS shape and Cepheid distribution.

Abstract

Aims. This study aims to determine empirical intrinsic edges of the classical Cepheids instability strip (IS) in the Small Magellanic Cloud (SMC) galaxy, considering various effects that alter its shape, and compare them with theoretical models and other galaxies. Methods. We used the data of classical fundamental-mode (F) and first-overtone mode (1O) SMC Cepheids from the OGLE-IV variable star catalog, with the final cleaned sample including 2388 F and 1560 1O Cepheids. The IS borders are determined by tracing the edges of the color distribution along the strip. Based on that, and using evolutionary tracks, the IS crossing times are computed. Results. We obtained the blue and red edges of the IS in V- and I-photometric bands and in the HR diagram, and detected breaks at periods between 1.4 and 3 days. A comparison with existing theoretical models showed good agreement for the blue edge and significant differences for the red edge. We also found that the IS of the SMC is wider than that of the Large Magellanic Cloud (LMC), with its red edge being redder despite its lower metallicity. The analysis of crossing times showed that the expected number of Cepheids as a function of period agrees with the observed distribution for P > 1 days but differs for P < 1 days. Conclusions. Slope changes along the SMC IS borders are most likely explained by the distribution of metallicity. The behavior of the blue loops at the SMC metallicity is not consistent with observations, and at the LMC metallicity, the blue loops are too short for lower-mass stars. A comparison of theoretical edges with our empirical ISs imposes constraints on the models and enables the identification of valid ones. Based on the positions of the breaks, our study also suggests that fundamental-mode Cepheids with periods longer than 3 days should be used for distance determination.