Revisiting candidates for non-pulsating stars located in the Cepheid instability strip in the Large Magellanic Cloud

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Abstract
Context. The instability strip (IS) is a region on the Hertzsprung-Russell diagram occupied by various types of pulsating stars. However, among them there are also stars that appear photometrically stable at a certain level of detectability. Although several scenarios have been proposed to explain this phenomenon, their nature remains unknown. Aims. We analyzed photometric and spectroscopic data for 11 candidates for non-pulsating stars located in the Cepheid IS of the Large Magellanic Cloud (LMC) in order to investigate the reasons for the lack of pulsations. Methods. We used available temperature calibrations based on photometric colors to estimate the effective temperatures of the candidates, which served as initial parameters for the spectroscopic analysis. We also applied surface brightness–color relations calibrated for Cepheid variables, giants, and supergiants to estimate stellar radii. The spectral analysis was performed using the spectral synthesis method to determine the atmospheric parameters; namely, the effective temperature, metallicity, surface gravity, microturbulent velocity, projected rotational velocity, and chemical abundances for up to ∼30 elements. Results. For most objects, only a single spectrum was available. However, no significant variations in radial velocities were detected among the stars with repeat observations; therefore, all stars were treated as single object in the analysis. Two stars exhibit broad spectral lines, which may indicate high rotational velocities or possible binarity; however, additional spectra are required to confirm this interpretation. One of these objects also shows asymmetric line profiles, which might be related to the presence of non-radial pulsation modes causing line-shape variations. A common feature among all analyzed candidates is an enhancement of barium-peak s -process elements compared to solar values. This may indicate past mass transfer from a companion during its post-asymptotic giant branch phase. Conclusions. This study provides an insight into the physical parameters of candidates for non-pulsating stars residing within the Cepheid IS in the LMC. Most of the stars have parameters very similar to Cepheids and the lack of pulsations remains a mystery and challenges pulsation theory. Additional spectroscopic observations are required to confirm and further investigate the possible origins of the observed properties of these objects.
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