Mid-infrared circumstellar emission of the long-period Cepheid l Carinae resolved with VLTI/MATISSE

TL;DR

This study uses VLTI/MATISSE mid-infrared interferometry to resolve and analyze the circumstellar environment of the long-period Cepheid l Carinae, revealing a compact, likely gaseous, circumstellar envelope and testing models of ionized gas shells.

Contribution

First direct mid-IR interferometric detection of a circumstellar envelope around l Car, constraining its size, shape, and nature, and evaluating models of ionized gas shells.

Findings

Detected a 1.7 R* extended emission in L band around l Car

No clear dust emission detected in N band from VLTI/MATISSE and Spitzer data

Modeled shell of ionized gas suggests a more compact, fainter CSE, inconsistent with observed IR excess

Abstract

The nature of circumstellar envelopes (CSE) around Cepheids is still a matter of debate. The physical origin of their infrared (IR) excess could be either a shell of ionized gas, or a dust envelope, or both. This study aims at constraining the geometry and the IR excess of the environment of the long-period Cepheid $\ell$ Car (P=35.5 days) at mid-IR wavelengths to understand its physical nature. We first use photometric observations in various bands and Spitzer Space Telescope spectroscopy to constrain the IR excess of $\ell$ Car. Then, we analyze the VLTI/MATISSE measurements at a specific phase of observation, in order to determine the flux contribution, the size and shape of the environment of the star in the L band. We finally test the hypothesis of a shell of ionized gas in order to model the IR excess. We report the first detection in the L band of a centro-symmetric extended emission around l Car, of about 1.7$R_\star$ in FWHM, producing an excess of about 7.0\% in this band. In the N band, there is no clear evidence for dust emission from VLTI/MATISSE correlated flux and Spitzer data. On the other side, the modeled shell of ionized gas implies a more compact CSE ($1.13\pm0.02\,R_\star$) and fainter (IR excess of 1\% in the L band). We provide new evidences for a compact CSE of $\ell$ Car and we demonstrate the capabilities of VLTI/MATISSE for determining common properties of CSEs. While the compact CSE of $\ell$ Car is probably of gaseous nature, the tested model of a shell of ionized gas is not able to simultaneously reproduce the IR excess and the interferometric observations. Further Galactic Cepheids observations with VLTI/MATISSE are necessary for determining the properties of CSEs, which may also depend on both the pulsation period and the evolutionary state of the stars.