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

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France,
e-mail: vincent.hocde@oca.eu
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁶ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
⁷ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁸ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁹ Univ. Grenoble Alpes, CNRS, IPAG, 38000, Grenoble, France
¹⁰ NASA Goddard Space Flight Center, Astrophysics Division, Greenbelt, MD 20771, USA
¹¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1090 GE Amsterdam, The Netherlands
¹² Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, Vienna, Austria
¹³ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
¹⁴ Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, PO Box 9010, MC 62, 6500 GL Nijmegen, the Netherlands
¹⁵ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
¹⁶ Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstraße 15, 24118, Kiel, Germany
¹⁷ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
¹⁸ CSFK Lendület Near-Field Cosmology Research Group, Budapest, Hungary
¹⁹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²⁰ Institute for Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
²¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937, Köln, Germany
²² Zselic Park of Stars, 064/2 hrsz., 7477 Zselickisfalud, Hungary
²³ Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, NSW 2006, Australia
²⁴ NOVA Optical IR Instrumentation Group at ASTRON, Oude Hoogeveensedijk 4 7991 PD Dwingeloo, The Netherlands
²⁵ ASTRON (Netherlands), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

Received: 22 February 2021
Accepted: 24 March 2021

Abstract

Context. The nature of circumstellar envelopes (CSEs) around Cepheids is a matter of ongoing debate. The physical origin of their infrared (IR) excess could be shown to either be made up of a shell of ionized gas, a dust envelope, or a combination of both.

Aims. This study is aimed at constraining the geometry and the IR excess of the environment of the bright long-period Cepheid ℓ Car (P = 35.5 days) at mid-IR wavelengths in order to understand its physical nature.

Methods. We first used photometric observations in various bands (from the visible domain to the infrared) and Spitzer Space Telescope spectroscopy to constrain the IR excess of ℓ Car. Then we analyzed the VLTI/MATISSE measurements at a specific phase of observation in order to determine the flux contribution as well as the size and shape of the environment of the star in the L band. Finally, we tested the hypothesis of a shell of ionized gas in order to model the IR excess.

Results. We report the first detection in the L band of a centro-symmetric extended emission around ℓ Car, of about 1.7 R_⋆ in full width at half maximum, producing an excess of about 7.0% in this band.This latter value is used to calibrate the IR excess found when comparing the photometric observations in various bands and quasi-static atmosphere models. 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 ± 0.02 R_⋆) that is also fainter (IR excess of 1% in the L band).

Conclusions. We provide new evidence supporting a compact CSE for ℓ Car and we demonstrate the capabilities of VLTI/MATISSE for determining common properties of CSEs. While the compact CSE of ℓ Car is likely to be of a 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 Cepheid 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.