The search for disks or planetary objects around directly imaged companions: a candidate around DH Tauri B

Abstract

Context. In recent decades, thousands of substellar companions have been discovered with both indirect and direct methods of detection. While the majority of the sample is populated by objects discovered using radial velocity and transit techniques, an increasing number have been directly imaged. These planets and brown dwarfs are extraordinary sources of information that help in rounding out our understanding of planetary systems. Aims. In this paper, we focus our attention on substellar companions detected with the latter technique, with the primary goal of investigating their close surroundings and looking for additional companions and satellites, as well as disks and rings. Any such discovery would shed light on many unresolved questions, particularly with regard to their possible formation mechanisms. Methods. To reveal bound features of directly imaged companions, whether for point-like or extended sources, we need to suppress the contribution from the source itself. Therefore, we developed a method based on the negative fake companion technique that first estimates the position in the field of view (FoV) and the flux of the imaged companion with high precision, then subtracts a rescaled model point spread function (PSF) from the imaged companion, using either an image of the central star or another PSF in the FoV. Next it performs techniques, such as angular differential imaging, to further remove quasi-static patterns of the star (i.e., speckle contaminants) that affect the residuals of close-in companions. Results. After testing our tools on simulated companions and disks and on systems that were chosen ad hoc, we applied the method to the sample of substellar objects observed with SPHERE during the SHINE GTO survey. Among the 27 planets and brown dwarfs we analyzed, most objects did not show remarkable features, which was as expected, with the possible exception of a point source close to DH Tau B. This candidate companion was detected in four different SPHERE observations, with an estimated mass of ~1MJup, and a mass ratio with respect to the brown dwarf of 1∕10. This binary system, if confirmed, would be the first of its kind, opening up interesting questions for the formation mechanism, evolution, and frequency of such pairs. In order to address the latter, the residuals and contrasts reached for 25 companions in the sample of substellar objects observed with SPHERE were derived. If the DH Tau Bb companion is real, the binary fraction obtained is ~7%, which is in good agreement with the results obtained for field brown dwarfs. Conclusions. While there may currently be many limitations affecting the exploration of bound features to directly imaged exoplanets and brown dwarfs, next-generation instruments from the ground and space (i.e., JWST, ELT, and LUVOIR) will be able to image fainter objects and, thus, drive the application of this technique in upcoming searches for exo-moons and circumplanetary disks.

First Doppler Limits on Binary Planets and Exomoons in the HR 8799 System

“…Indeed, only three of the hundreds of known moons in the solar system are found within 2 AU of the Sun. So far, only Lazzoni et al (2020) have published constraints on moons orbiting planets at similar orbital radii to the HR 8799 planets, and their direct imaging constraints only probe moons orbiting far from their host planets. Our constraints are first probes of moons and binary planets in a new parameter space (planet semimajor axis 10 AU and moon orbital period 10 days).…”

“…However, despite the prevalence of planets in our galaxy and the ubiquity of moons orbiting the planets in our solar system, astronomers have not yet securely detected any exomoons and have only a handful of unconfirmed candidates (e.g. Lazzoni et al 2020;Limbach et al 2021, but see also Kreidberg et al 2019). Given the importance of Earth's moon on our planet's spin dynamics (Li & Batygin 2014) and the potential habitability of icy moons in the outer solar system (e.g.…”

“…Many different exomoon detection methods have been proposed (e.g. Sartoretti & Schneider 1999;Hook 2005;Cabrera & Schneider 2007;Lewis et al 2008;Kipping 2009;Agol et al 2015;Heller 2016;Sengupta & Marley 2016;Forgan 2017;Hwang et al 2018;Vanderburg et al 2018;Lazzoni et al 2020) but so far only transit photom-etry (Kipping et al 2015;) and direct imaging (Lazzoni et al 2020) have yielded constraints on the presence of exomoons. Future space observations from the Roman Space Telescope microlensing survey (Spergel et al 2013;Penny et al 2019) are expected to either detect or constrain planets/moons orbiting lowmass brown dwarfs or free-floating planets, although unambiguous detections will be challenging (Hwang et al 2018).…”

On the Detection of Exomoons Transiting Isolated Planetary-mass Objects

“…Many methods have been suggested to search for the moons of planets outside the solar system, which are often called "exomoons." As reviewed by Heller (2018), about a dozen signals possibly attributable to exomoons have been described in the literature based on gravitational microlensing (Bennett et al 2014;Miyazaki et al 2018), signatures in transit spectra (Oza et al 2019;Gebek & Oza 2020), gaps in circumplanetary rings (Kenworthy & Mamajek 2015), transit-timing variations (TTVs) accompanied by exomoon transits of the host star (Rodenbeck et al 2018;Teachey et al , 2020Kreidberg et al 2019), TTVs (Fox & Wiegert 2020;Kipping 2020), direct imaging (Lazzoni et al 2020), and absorption by gas possibly associated with an orbiting moon (Ben-Jaffel & Ballester 2014). Follow-up, confirmation, and further characterization of these exomoon candidates have proven difficult, making it important to devise more methods for detecting exomoons.…”

“…This includes looking for exomoons via direct imaging, radial velocity monitoring, astrometric variations, and transits. Earlier authors considered applying these methods to directly imaged exoplanets orbiting stars (Cabrera & Schneider 2007;Agol et al 2015;Heller 2016;Vanderburg et al 2018;Lazzoni et al 2020), but for IPMOs, the observational requirements are more easily met because high-contrast imaging is unnecessary to detect the IPMO. Indeed, the gravitational-lensing technique has already been used to identify a signal (MOA-2011-BLG-262Lb) that could have arisen from an exomoon orbiting an IPMO, although the data do not strongly rule out the possibility that the signal is from a planet orbiting a low-mass star (Bennett et al 2014).…”