X TrA through the eyes of MATISSE: More evidence of clumpy molecular layers around C-type asymptotic giant branch stars^★

¹ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
² European Southern Observatory, Alonso de Córdova, 3107 Vitacura, Santiago, Chile
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁴ Institute of Fundamental Physics (CSIC), Department of Molecular Astrophysics, Serrano 123, 28006 Madrid, Spain
⁵ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
⁶ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden
⁷ Instituto de Astrofísica, Facultad de Ciencias Exactas, Universidad Andrés Bello, Chile
⁸ Centro de Astrobiología (CAB), CSIC-INTA. ESAC-campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ Institute of Fundamental Physics (CSIC), Department of Molecular Astrophysics. Serrano 123, 28006 Madrid, Spain
¹⁰ NASA Goddard Space Flight Center, Greenbelt, MD, USA
¹¹ Schmidt Sciences, New York, NY, USA
¹² National Science Foundation, Alexandria, VA, USA
¹³ HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Observatory, Budapest, Hungary
¹⁴ Universidad Nacional Autónoma de México. Instituto de Astronomía. A.P. 70-264, 04510, Ciudad de México, 04510, Mexico
¹⁵ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
¹⁶ Department of Physics and Astronomy, Division of Astronomy and Space Physics, Theoretical Astrophysics, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹⁷ Max-Planck-Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 September 2025
Accepted: 26 November 2025

Abstract

Aims. The goal of this study is to further the understanding of the wind formation mechanism in asymptotic giant branch (AGB) stars through the analysis of the close environment (within a few stellar radii) of the carbon star X TrA.

Methods. X TrA was observed for the first time with the Mid-Infrared SpectroScopic Experiment instrument (MATISSE) in the L and N bands in low spectral resolution mode (R=30), and its close surroundings were mapped in specific wavelength ranges corresponding to specific molecules (C₂H₂ and HCN, at 3.1 and 3.8 μm) and dust (amorphous carbon and, for example, SiC at 11.3 μm), via image reconstruction techniques.

Results. The angular diameter of the star ranges from 10 mas in the L band pseudo-continuum (3.5 μm) to 20 mas at 3.1 and 11.3 μm. The reconstructed images show some mild elongated features (along the east-west direction) and asymmetric protrusions, which are most evident around 3.1 μm. Imaging results highlight the clumpy nature of the circumstellar environment, starting from the photospheric region up to more distant layers.

Conclusions. The angular diameters found for X TrA in the image data are in agreement with previous photospheric diameter estimates (following VLTI/MIDI 8–13 μm observations), and their wavelength dependence is similar to values found for other carbon stars observed with MATISSE (R Scl and V Hya). The 3.1 μm images presented here show highly asymmetric features, another case of a C-rich star with irregular morphologies close to the stellar disk; this supports the notion that the C₂H₂ + HCN abundance distribution usually originates from a clumpy layer around carbon stars.