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.
Highlights
Circumstellar envelopes (CSEs) around Cepheids have been spatially resolved by long-baseline interferometry in the K band with the Very Large Telescope Interferometer (VLTI) and the Center for High Angular Resolution Astronomy (CHARA; Kervella et al 2006; Mérand et al 2006)
Our aim is to determine its IR excess from photometric measurements in various bands and Spitzer spectroscopy, while inferring the size of the circumstellar envelopes (CSEs) and its flux contribution thanks to the unique capabilities provided by the Multi AperTure mid-Infrared SpectroScopic Experiment (VLTI/MATISSE; Lopez et al 2014; Allouche et al 2016; Robbe-Dubois et al 2018) in the L (2.8–4.0 μm), M (4.5–5 μm), and N bands (8–13 μm)
Thanks to the VLTI/MATISSE measurements of flux contribution of the CSE in the L band (7%), we can fix this offset parameter consistently to a 0.03 magnitude in order to force the IR excess in the L band from SPIPS to reach 7%, which is within the uncertainties
Summary
Circumstellar envelopes (CSEs) around Cepheids have been spatially resolved by long-baseline interferometry in the K band with the Very Large Telescope Interferometer (VLTI) and the Center for High Angular Resolution Astronomy (CHARA; Kervella et al 2006; Mérand et al 2006). We study the long-period Cepheid Car. Our aim is to determine its IR excess from photometric measurements in various bands and Spitzer spectroscopy, while inferring the size of the CSE and its flux contribution thanks to the unique capabilities provided by the Multi AperTure mid-Infrared SpectroScopic Experiment (VLTI/MATISSE; Lopez et al 2014; Allouche et al 2016; Robbe-Dubois et al 2018) in the L (2.8–4.0 μm), M (4.5–5 μm), and N bands (8–13 μm). Near-IR excess modeling with SPIPS Due to their intrinsic variability, the photospheres of the Cepheids are difficult to model along the pulsation cycle It is an essential prerequisite for deriving both the IR excess in a given photometric band and the expected angular stellar diameter at a specific phase of interferometric observations We combine this average IR excess with Spitzer mid-infrared spectroscopy
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