Hot exozodiacal dust around Fomalhaut: The MATISSE perspective

Context . Excess over the stellar photospheric emission of main-sequence stars has been found in interferometric near-infrared observations, and is attributed to the presence of hot exozodiacal dust (HEZD). As part of our effort to detect and characterize HEZD around the nearby A3 V star Fomalhaut, we carried out the first interferometric observations with the MATISSE instrument at the VLTI in the photometric bands L and M for the Fomalhaut system. Aims . We investigate whether the new MATISSE data indicate the presence of HEZD around Fomalhaut. If detected, we aim to constrain the dust grain size, location, dust species, and total dust mass based on these data. We also investigate if the possibly detected circumstellar radiation could have an alternative explanation. Methods . Assuming a dust distribution either as a narrow ring or spherical shell for modeling the HEZD, we aim to constrain the HEZD parameters by generating visibilities and fitting them to the MATISSE data using different approaches. Results . The MATISSE L band data provide a marginal detection of circumstellar radiation, potentially caused by the presence of HEZD, which is only the second detection of HEZD emission in the L band. An analysis of the data with different fitting approaches showed that the best-fit values for the HEZD parameters are consistent with those of previous Fomalhaut observations, which again underlines the functionality of MATISSE. We derived the following best-fit HEZD parameter values: Assuming a dust ring, it would have an inner ring radius of 0.11 au, an outer ring radius of 0.12 au, a narrow dust grain size distribution around a dust grain radius constrained by 0.53 μm, and a total dust mass of 3.25 × 10 −10 M ⊕ . However, even with an additional consideration of previous VINCI ( K band) and KIN ( N band) measurements, we cannot further tighten the constraints for the HEZD properties than in previous Fomalhaut studies. Because our data cannot directly constrain the morphology of the excess radiation source, even the presence of a stellar companion can reproduce the detected marginal visibility deficit. Moreover, the MATISSE data neither imply nor exclude the existence of a double ring structure close to Fomalhaut. Finally, the results indicate that the choice of the geometric model has a more significant impact on the derived dust-to-star flux ratio than the specific fitting approach applied. Conclusions . Since different dust-to-star flux ratios can result from the applied fitting approaches, this also has an impact on the parameter values of the HEZD around Fomalhaut. This circumstance should also be investigated for HEZD systems analyzed so far only with the fitting approach usually applied. Moreover, further NIR and MIR data are required for a more comprehensive description of the emission originating in the close vicinity of Fomalhaut.