The HOSTS survey: Suspected variable dust emission and constraints on companions around θ Boo

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D bus 2401, 3001 Leuven, Belgium
² Department of Astronomy and Steward Observatory, The University of Arizona, 933 N Cherry Ave, Tucson, AZ 85719, USA
³ Large Binocular Telescope Observatory, The University of Arizona, 933 N Cherry Ave, Tucson, AZ 85719, USA
⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA
⁸ Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK
⁹ Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹⁰ Department of Physics and Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹¹ Lowell Observatory, 1400 W. Mars Hill Rd, Flagstaff, AZ 86001, USA
¹² Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
¹³ Sydney Institute for Astronomy, School of Physics, University of Sydney, Sydney, NSW 2006, Australia

^★ Corresponding author.

Received: 18 October 2024
Accepted: 7 May 2025

Abstract

Context. During the Hunt for Observable Signatures of Terrestrial Systems (HOSTS) survey by the Large Binocular Telescope Interferometer (LBTI), an excess emission from the main-sequence star, θ Boo (F7V spectral type, 14.5 pc distance) was observed. This excess indicates the presence of exozodiacal dust near the star’s habitable zone (HZ). Previous observations with Spitzer and Herschel show no evidence of outer cold dust within their respective detection limits. Because exozodiacal dust is generally believed to originate from material located farther out in the system, its source around θ Boo remains unclear.

Aims. We conducted additional nulling and high-contrast adaptive optics (AO) observations to spatially constrain the dust distribution, search for variability, and directly image potential companions in the system. This study presents the results of these observations and provides an interpretation of the inner system’s architecture.

Methods. We observed the star using the LBTI’s N′-band nulling mode during three epochs in 2017, 2018, and 2023. For each epoch, we modeled and constrained the dust distribution using the standard LBTI nulling pipeline, assuming a vertically thin disk with a face-on inclination. We also performed high-contrast AO observations in the L′-band and H-band to constrain the presence of substellar companions around the star.

Results. We find several solutions for the dust distribution for each epoch. However, the LBTI nulling observations are unable to discriminate between them. Using upper limits from previous observations, we constrain the representative size of the dust grains to approximately 3-5 μm. We also measured a tentative increase in dust brightness at the Earth-equivalent insolation distance between 2017 and 2023. This increase corresponds to the injection of 4 × 10⁻⁸−4 × 10⁻⁷ M_⊕ of new material into the disk. We consider several options to explain the origin of the observed dust and its variability, but no clear sources are identified from the current observations, partly because our high-contrast AO observations could only constrain the presence of companions only down to 11 M_Jup at 1.3″ separation.