Letter to the Editor

Discovery of ionized circumstellar gas emission around the long-period Cepheid ℓ Carinae with ALMA

¹ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Rabiańska 8, 87-100 Toruń, Poland
³ Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA, Leiden, The Netherlands
⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris-Cité, 5 Place Jules Janssen, 92195 Meudon, France

^⋆ Corresponding author; vhocde@camk.edu.pl

Received: 29 November 2024
Accepted: 22 January 2025

Abstract

Cepheid circumstellar emissions have previously been detected using both infrared (IR) excess and IR interferometric observations at a few stellar radii. These studies have shown that these circumstellar emission can be produced by ionized gas, however, there is no direct observational evidence to confirm this hypothesis. In this letter, we explore the continuum emission and a spectrum of the bright and long-period Cepheid ℓ Car (P = 35.56 day) at millimeter-wavelengths to detect possible effects of ionized gas emission. We used ALMA observations of ℓ Car in two spectral setups in Band 6 (near 212 and 253 GHz, respectively) and compared the measured flux density to what would be expected for the stellar continuum. We also derived the spectral index and probed the presence of radio recombination lines (RRLs). We report statistically significant emission of about 3.5 mJy in the two spectral ranges, which is about 2.5 times the stellar continuum emission. For the first time, we have also been able to derive the spectral index of the flux density, (S_ν ∝ ν^α), α = +1.26 ± 0.44 (∼3σ error), which is characteristic of partially optically thick, ionized gas emission. Additionally, we discovered an emission line from a RRL of hydrogen H29α centered on the stellar rest velocity, smaller in spatial extent than about 0″​.2 (≲100 AU), with a symmetric profile with a width at half power of 55.3 ± 7.5 km s⁻¹ (1σ error). These findings confirm the presence of ionized gas emission near ℓ Car. The millimeter emission detected from ℓ Car can be attributed to ionized gas emission from the Cepheid’s chromosphere. Further radio interferometric observations are necessary to confirm the occurrence of these ionized gas envelopes around Cepheids of different pulsation periods.