Discovery of ionized circumstellar gas emission around the long-period Cepheid $\ell$ Carinae with ALMA

Abstract

Cepheid circumstellar emissions have previously been detected via both infrared excess and infrared interferometric observations at a few stellar radii. Those 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, $\ell$~Car ($P=35.56$day) at millimeter-wavelengths in order to detect possible effects of ionized gas emission. We presented ALMA observations of $\ell$ Car in two spectral setups in Band~6 (near 212 and 253\,GHz, respectively) and we compared the measured flux density to one expected for the stellar continuum. We also derived the spectral index and probed the presence of Radio Recombination Lines (RRL). We report statistically significant emission of about 3.5mJy in the two spectral ranges, which is about 2.5 times the stellar continuum emission. For the first time, we are also able to derive the spectral index of the flux density ($S_{\nu }\propto {\nu }^{\alpha }$), $\alpha =+1.26\pm$0.44 ($\sim$3$\sigma$ error), which is characteristic of partially optically thick ionized gas emission. Additionally, we discovered an emission line from a RRL of hydrogen H29$\alpha$ centered on the stellar rest velocity, smaller in spatial extent than about 0\farcs2 ($\lesssim 100$AU), with a symmetric profile with a width at half power of 55.3$\pm$7.5\,\kms (1$\sigma$ error). It confirms the presence of ionized gas emission near $\ell$~Car. The millimeter emission detected from $\ell$ 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.

Accepted for publication in Astronomy and Astrophysics. 7 pages, 5 figures

References in corpus (17)

• Gaia Data Release 3: Summary of the content and survey properties
• CASA, the Common Astronomy Software Applications for Radio Astronomy
• A new calibration of Galactic Cepheid Period-Luminosity relations from B to K bands, and a comparison to LMC PL relations
• Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2 - I. Binarity from proper motion anomaly
• Extended envelopes around Galactic Cepheids III. Y Oph and alpha Per from near-infrared interferometry with CHARA/FLUOR
• On the accuracy of the ALMA flux calibration in the time domain and across spectral windows
• The Secret Lives of Cepheids: $\delta$ Cep -- the Prototype of a New Class of Pulsating X-ray Variable Stars
• The Secret Lives of Cepheids: Evolutionary Changes and Pulsation-Induced Shock Heating in the Prototype Classical Cepheid δ Cep
• Understanding hydrogen recombination line observations with ALMA and EVLA
• Study of changes in the pulsation period of 148 Galactic Cepheid variables
• A thin shell of ionized gas explaining the IR excess of classical Cepheids
• VELOcities of CEpheids (VELOCE) I. High-precision radial velocities of Cepheids
• ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse
• The new Herschel/PACS Point Source Catalogue
• Mid-infrared circumstellar emission of the long-period Cepheid l Carinae resolved with VLTI/MATISSE
• A Search for Mass Loss on the Cepheid Instability Strip using HI 21-cm Line Observations
• First Detection of Radio Emission Associated with a Classical Cepheid