The Secret Lives of Cepheids: Evolution, Mass Loss, and Ultraviolet Emission of the Long-Period Classical Cepheid $l$ Carinae

TL;DR

This study reexamines the long-period Cepheid $l$ Carinae's properties, combining historical and new data to refine its pulsation period and mass, revealing insights into Cepheid evolution, mass loss, and ultraviolet emission.

Contribution

It provides new measurements of $l$ Carinae's period change and mass, compares these with stellar models, and presents UV observations that suggest links between shocks, mass loss, and UV emission in Cepheids.

Findings

$l$ Car's mass is about 8-10 solar masses.

Models and observations are only marginally consistent.

UV observations hint at shock-related mass loss processes.

Abstract

The classical Cepheid $l$ Carinae is an essential calibrator of the Cepheid Leavitt Law as a rare long-period Galactic Cepheid. Understanding the properties of this star will also constrain the physics and evolution of massive ($M \ge 8$ $M_\odot$) Cepheids. The challenge, however, is precisely measuring the star's pulsation period and its rate of period change. The former is important for calibrating the Leavitt Law and the latter for stellar evolution modeling. In this work, we combine previous time-series observations spanning more than a century with new observations to remeasure the pulsation period and compute the rate of period change. We compare our new rate of period change with stellar evolution models to measure the properties of $l$ Car, but find models and observations are, at best, marginally consistent. The results imply that $l$ Car does not have significantly enhanced mass-loss rates like that measured for $\delta$ Cephei. We find that the mass of $l$ Car is about 8 - 10 $M_\odot$. We present Hubble Space Telescope COS observations that also differ from measurements for $\delta$ Cep, and $\beta$ Dor. These measurements further add to the challenge of understanding the physics of Cepheids, but do hint at the possible relation between enhanced mass loss and ultraviolet emission, perhaps both due to the strength of shocks propagating in the atmospheres of Cepheids.