Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud

TL;DR

This paper reports the discovery and analysis of a peculiar Cepheid-like star near the Small Magellanic Cloud exhibiting unusual photometric behavior and potential proper motion, challenging existing models of Cepheid variables.

Contribution

It presents detailed photometric and spectroscopic observations of a unique Cepheid-like star with unprecedented variability and proper motion data, suggesting it may be a foreground object or an unusual SMC Cepheid.

Findings

Unusual change in mean magnitude and amplitude of the star's light curve

Radial velocity of 104 km/s and metallicity of -0.4 dex

Possible detection of non-zero proper motion indicating a foreground object

Abstract

For seven years, the EROS-2 project obtained a mass of photometric data on variable stars. We present a peculiar Cepheid-like star, in the direction of the Small Magellanic Cloud, which demonstrates unusual photometric behaviour over a short time interval. We report on data of the photometry acquired by the MARLY telescope and spectroscopy from the EFOSC instrument for this star, called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid HR 7308. The light curve of our target is analysed using the Analysis of Variance method to determine a pulsational period of 5.5675 days. A fit of time-dependent Fourier coefficients is performed and a search for proper motion is conducted. The light curve exhibits a previously unobserved and spectacular change in both mean magnitude and amplitude, which has no clear theoretical explanation. Our analysis of the spectrum implies a radial velocity of 104 km s$^{-1}$ and a metallicity of -0.4$\pm$0.2 dex. In the direction of right ascension, we measure a proper motion of 17.4$\pm$6.0 mas yr$^{-1}$ using EROS astrometry, which is compatible with data from the NOMAD catalogue. The nature of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may have detected a non-zero proper motion for this star, which would imply that it is a foreground object. Its radial velocity, pulsational characteristics, and photometric data, however, suggest that it is instead a Cepheid-like object located in the SMC. In such a case, it would present a challenge to conventional Cepheid models.