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

TL;DR

This study empirically maps the intrinsic instability strip (IS) for classical Cepheids in the Small Magellanic Cloud (SMC) using OGLE-IV data, separating fundamental-mode (F) and first-overtone (1O) pulsators. By tracing IS edges from color distributions and applying evolutionary-crossing analyses with $MESA$/$RSP$, the authors identify breaks in IS slopes between $P \approx 1.4$ and $3$ days and show that the SMC IS is broader than the LMC, with a red edge that is redder despite lower metallicity. The work finds good agreement between the blue IS edge and some theoretical models but notable discrepancies for the red edge and short-period Cepheids, highlighting sensitivities to metallicity, convection, and blue-loop behavior. The results constrain pulsation/evolution models, emphasize the need to account for metallicity gradients when deriving P-L relations, and advocate using long-period ($P>3$ days) F Cepheids for distance measurements in this low-metallicity regime.

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.

Figures (11)

• Figure 1: CMD of the final sample of F (blue) and 1O (red) SMC Cepheids. The objects discarded in our cleaning procedure are shown as crosses. Distributions of the intrinsic color (V-I)$_0$ and absolute magnitude $M_I$ are shown in the upper and right subpanels, respectively.
• Figure 2: Same as Fig. \ref{['fig:ISfull']} but independently for 1O- (left panel) and F (right panel) Cepheids.
• Figure 3: CMD showing empirical IS edges (red empty circles) separately for SMC (left panels) and LMC (right panels), 1O (upper panels) and F (lower panels) Cepheids. Black empty circles represent the median value of the intrinsic color of each bin of the corresponding sample. Fits for the blue and red edges, considering changes in slope at different pulsation periods, are shown as solid blue and red lines, respectively. Gray-shaded areas mark the blue loop extent (delimited by its bluest extreme and the tip of the RGB) for evolutionary tracks with representative $Z$ for the SMC and LMC galaxies. The gray areas are created using tracks from $2$ to $7$ M$_\odot$ for the SMC, and $3$ to $7$ M$_\odot$ for the LMC. The upper limits for the areas are defined by the evolutionary tracks for $7$ M$_{\odot}$ and would extend further up if tracks for higher masses are considered.
• Figure 4: Histogram of the number of LMC (upper panels), and SMC (lower panels) Cepheids, including F and 1O, as a function of fundamentalized pulsating period. Colored points show the expected number of Cepheids on each bin for different IS crossings, calculated using Eq. \ref{['eq:1']}. The sum of the expected numbers of stars for all IS crossings is shown as a black solid line. Vertical blue, red and green lines mark pulsation periods of $0.6$ (the shortest period of our LMC models), $3$, and $2.5$ days, respectively.
• Figure 5: CMD of F and 1O SMC Cepheids. Angular separation of Cepheids from the SMC center $(12^\circ.54,-73^\circ.11)$ is shown with a color gradient. The IS edges for the full sample and for only those within a $0.6$ degree radius from the SMC center are shown as solid and dashed lines, respectively.