The Cepheid Impostor HD 18391 and its Anonymous Parent Cluster

TL;DR

This study confirms that HD 18391, a supergiant off the Cepheid instability strip, exhibits small-amplitude pulsations and is associated with a young, heavily-reddened cluster, following the Cepheid period-luminosity relation despite not being a classical Cepheid.

Contribution

It identifies HD 18391 as a Cepheid impostor and characterizes its cluster environment, providing insights into its pulsation and evolutionary status.

Findings

HD 18391 shows small-amplitude variability with a 123-day period.

The star is part of a young, heavily-reddened cluster at ~1661 pc.

HD 18391 follows the Cepheid period-luminosity relation despite not being a classical Cepheid.

Abstract

New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in order to confirm the nature of the variablity previously detected in the star, which lies off the hot edge of the Cepheid instability strip. Small-amplitude variability at a level of \Delta V = 0.016+-0.002 is indicated, with a period of P=123.04+-0.06 d. A weaker second signal may be present at P=177.84+-0.18 with \Delta V = 0.007+-0.002, likely corresponding to fundamental mode pulsation if the primary signal represents overtone pulsation (123.04/177.84 = 0.69). The star, with a spectroscopic reddening of E(B-V) = 1.02, is associated with heavily-reddened B-type stars in its immediate vicinity that appear to be outlying members of an anonymous young cluster centered ~10 arcmin to the west and 1661+-73 pc distant. The cluster has nuclear and coronal radii of r_n=3.5 arcmin and R_c=14 arcmin, respectively, while the parameters for HD 18391 derived from membership in the cluster with its outlying B stars are consistent with those implied by its Cepheid-like pulsation, provided that it follows the semi-period-luminosity relation expected of such objects. Its inferred luminosity as a cluster member is M_V=-7.76+-0.10, its age (9+-1)x10^6 years, and its evolutionary mass ~19 M_{\sun}. HD 18391 is not a classical Cepheid, yet it follows the Cepheid period-luminosity relation closely, much like another Cepheid impostor, V810 Cen.