The Census of Exoplanets in Visual Binaries: population trends from a volume-limited Gaia DR2 and literature search

We present an extensive search in the literature and Gaia DR2 for visual co-moving binary companions to stars hosting exoplanets and brown dwarfs within 200 pc. We found 218 planet hosts out of 938 to be part of multiple-star systems, with 10 newly discovered binaries and 2 new tertiary stellar components. This represents an overall raw multiplicity rate of 23.2±1.6% for hosts to exoplanets across all spectral types, with multi-planet systems found to have a lower duplicity frequency at the 2.2σ level. We found that more massive hosts are more often in binary configurations, and that planet-bearing stars in multiple systems are predominantly the most massive component of stellar binaries. Investigations of multiplicity as a function of planet mass and separation revealed that giant planets with masses >0.1 MJup are more frequently seen in stellar binaries than small sub-Jovian planets with a 3.6σ difference, a trend enhanced for the most massive (>7 MJup) short-period (0.5 AU). While stellar companion mass appears to have no impact on planet properties, binary separation seems to be an important factor in the resulting structure of planetary systems. Stellar companions on separations <1000 AU can play a role in the formation or evolution of massive close-in planets, while planets in wider binaries show similar properties to planets orbiting single stars. Finally, numerous stellar companions on separations <1-3 arcsec likely remain undiscovered to this date. Continuous efforts to complete our knowledge of stellar multiplicity on separations of tens to hundreds of AU are essential to confirm the reported trends and further our understanding of the roles played by multiplicity on exoplanets

Binary companions triggering fragmentation in self-gravitating discs

Observations of systems hosting close in ($<1$ AU) giant planets and brown dwarfs ($M\gtrsim7$ M$_{\rm Jup}$) find an excess of binary star companions, indicating that stellar multiplicity may play an important role in their formation. There is now increasing evidence that some of these objects may have formed via fragmentation in gravitationally unstable discs. We present a suite of 3D smoothed particle hydrodynamics (SPH) simulations of binary star systems with circumprimary self-gravitating discs, which include a realistic approximation to radiation transport, and extensively explore the companion's orbital parameter space for configurations which may trigger fragmentation. We identify a "sweet spot" where intermediate separation binary companions ($100$ AU $\lesssim a\lesssim400$ AU) can cause a marginally stable disc to fragment. The exact range of ideal binary separations is a function of the companion's eccentricity, inclination and mass. Heating is balanced by efficient cooling, and fragmentation occurs inside a spiral mode driven by the companion. Short separation, disc penetrating binary encounters ($a\lesssim100$ AU) are prohibitive to fragmentation, as mass stripping and disc heating quench any instability. This is also true of binary companions with high orbital eccentricities ($e\gtrsim0.75$). Wide separation companions ($a\gtrsim500$ AU) have little effect on the disc properties for the setup parameters considered here. The sweet spot found is consistent with the range of binary separations which display an excess of close in giant planets and brown dwarfs. Hence we suggest that fragmentation triggered by a binary companion may contribute to the formation of these substellar objects.Comment: 15 pages, 8 figures, accepted for publication in MNRA

Brown dwarfs and giant exoplanets: bridging observations and theory with statistical methods

Stellar physics has been widely studied over the last century, with theoretical models for stars robustly tested by decades of observations. In contrast, studies of brown dwarfs and extra-solar giant planets were only possible for the last two decades due to the intrinsically faint luminosity of these objects. As a result, the fundamental properties of substellar objects are still poorly constrained. Formation mechanisms for brown dwarfs and planetary-mass objects remain heavily debated, and atmospheric models widely lack empirical validation at the lowest masses and temperatures. Theoretical models are currently the only available way to infer physical parameters (e.g. mass, temperature) for isolated objects and directly-imaged companions on wide orbits, and are thus widely used by the community in spite of the extremely high uncertainties they carry. More stringent observational constraints, or new alternative methods, are essential to allow for a further and deeper understanding of brown dwarfs and giant planets. Robust population studies provide invaluable insights into formation processes and empirical trends. Statistical methodologies may thus be used to refine theoretical models and obtain a more complete overview of the properties and statistics of the substellar populations. This dissertation addresses three problems in the framework of brown dwarfs and giant exoplanets, namely, substellar binary properties, the formation of massive planets and brown dwarfs around stars, and the detection and model-independent masses of direct imaging systems. Chapter 2 presents results from a multiplicity survey investigating the binary statistics of the lowest-mass brown dwarfs. As binarity is a direct outcome of formation, observed trends as a function of mass provide valuable insights into formation processes. In Chapter 3, I conduct a search for stellar companions to stars with close-in, massive planets, as a test of formation theory for giant planets and brown dwarfs on small orbital separations. Chapter 4 introduces a dedicated tool designed to identify new wide-orbit companions and constrain the orbits of astrometric systems. The method allows for the determination of dynamical masses for directly-imaged companions, a powerful way to circumvent the large uncertainties introduced by models. The common goal to these projects is to infer new, crucial observational constraints for formation theories or atmospheric models in the substellar regime. This will in turn provide a more comprehensive view of the characteristics and demographics of brown dwarfs and exoplanets

A novel survey for young substellar objects with the W-band filter III: Searching for very low-mass brown dwarfs in Serpens South and Serpens Core

We present CFHT photometry and IRTF spectroscopy of low-mass candidate members of Serpens South and Serpens Core (∼430 pc, ∼0.5 Myr), identified using a novel combination of photometric filters, known as the W-band method. We report SC182952+011618, SS182959-020335 and SS183032-021028 as young, low-mass Serpens candidate members, with spectral types in the range M7-M8, M5-L0 and M5-M6.5 respectively. Best-fit effective temperatures and luminosities imply masses of < 0.12M⊙ for all three candidate cluster members. We also present Hubble Space Telescope imaging data (F127M, F139M and F850LP) for six targets in Serpens South. We report the discovery of the binary system SS183044-020918AB. The binary components are separated by ≈45 AU, with spectral types of M7-M8 and M8-M9, and masses of 0.08-0.1 and 0.05-0.07M⊙. We discuss the effects of high dust attenuation on the reliability of our analysis, as well as the presence of reddened background stars in our photometric sample

Astrometric accelerations as dynamical beacons : discovery and characterization of HIP 21152 B, the First T-dwarf companion in the Hyades * * Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

Benchmark brown dwarf companions with well-determined ages and model-independent masses are powerful tools to test substellar evolutionary models and probe the formation of giant planets and brown dwarfs. Here, we report the independent discovery of HIP 21152 B, the first imaged brown dwarf companion in the Hyades, and conduct a comprehensive orbital and atmospheric characterization of the system. HIP 21152 was targeted in an ongoing high-contrast imaging campaign of stars exhibiting proper-motion changes between Hipparcos and Gaia, and was also recently identified by Bonavita et al. (2022) and Kuzuhara et al. (2022). Our Keck/NIRC2 and SCExAO/CHARIS imaging of HIP 21152 revealed a comoving companion at a separation of 0.″37 (16 au). We perform a joint orbit fit of all available relative astrometry and radial velocities together with the Hipparcos-Gaia proper motions, yielding a dynamical mass of 24−4+6MJup , which is 1–2σ lower than evolutionary model predictions. Hybrid grids that include the evolution of cloud properties best reproduce the dynamical mass. We also identify a comoving wide-separation (1837″ or 7.9 × 104 au) early-L dwarf with an inferred mass near the hydrogen-burning limit. Finally, we analyze the spectra and photometry of HIP 21152 B using the Saumon & Marley (2008) atmospheric models and a suite of retrievals. The best-fit grid-based models have f sed = 2, indicating the presence of clouds, T eff = 1400 K, and logg=4.5dex . These results are consistent with the object’s spectral type of T0 ± 1. As the first benchmark brown dwarf companion in the Hyades, HIP 21152 B joins the small but growing number of substellar companions with well-determined ages and dynamical masses

Astrometric Accelerations as Dynamical Beacons: Discovery and Characterization of HIP 21152 B, the First T-dwarf Companion in the Hyades*

Benchmark brown dwarf companions with well-determined ages and model-independent masses are powerful tools to test substellar evolutionary models and probe the formation of giant planets and brown dwarfs. Here, we report the independent discovery of HIP 21152 B, the first imaged brown dwarf companion in the Hyades, and conduct a comprehensive orbital and atmospheric characterization of the system. HIP 21152 was targeted in an ongoing high-contrast imaging campaign of stars exhibiting proper-motion changes between Hipparcos and Gaia, and was also recently identified by Bonavita et al. (2022) and Kuzuhara et al. (2022). Our Keck/NIRC2 and SCExAO/CHARIS imaging of HIP 21152 revealed a comoving companion at a separation of 0.″37 (16 au). We perform a joint orbit fit of all available relative astrometry and radial velocities together with the Hipparcos-Gaia proper motions, yielding a dynamical mass of $24^{+6}_{-4}M_{Jup}$, which is 1–2σ lower than evolutionary model predictions. Hybrid grids that include the evolution of cloud properties best reproduce the dynamical mass. We also identify a comoving wide-separation (1837″ or $7.9 {\times} 10^4 au$) early-L dwarf with an inferred mass near the hydrogen-burning limit. Finally, we analyze the spectra and photometry of HIP 21152 B using the Saumon & Marley (2008) atmospheric models and a suite of retrievals. The best-fit grid-based models have $f_{sed} = 2$, indicating the presence of clouds, $T_{eff} = 1400 K$, and $log\, g=4.5dex$ . These results are consistent with the object’s spectral type of T0 ± 1. As the first benchmark brown dwarf companion in the Hyades, HIP 21152 B joins the small but growing number of substellar companions with well-determined ages and dynamical masses. * Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan

An optimized survey strategy for the ERIS/NIX imager: searching for young giant exoplanets and very low mass brown dwarfs using the K-peak custom photometric filter

International audienceWe present optimal survey strategies for the upcoming NIX imager, part of the Enhanced Resolution Imager and Spectrograph instrument to be installed on the Very Large Telescope. We will use a custom 2.2 µm K-peak filter to optimize the efficiency of a future large-scale direct imaging survey, aiming to detect brown dwarfs and giant planets around nearby stars. We use the results of previous large-scale imaging surveys (primarily SPHERE SHINE and Gemini GPIES) to inform our choice of targets, as well as improved planet population distributions. We present four possible approaches to optimize survey target lists for the highest yield of detections: (i) targeting objects with anomalous proper motion trends, (ii) a follow-up survey of dense fields from SPHERE SHINE and Gemini GPIES, (iii) surveying nearby star-forming regions, and (iv) targeting newly discovered members of nearby young moving groups. We also compare the predicted performance of NIX to other state-of-the-art direct imaging instruments

An <i>HST</i> survey of 33 T8 to Y1 brown dwarfs: NIR photometry and multiplicity of the coldest isolated objects

We present results from a Hubble Space Telescope imaging search for low-mass binary and planetary companions to 33 nearby brown dwarfs with spectral types of T8–Y1. Our survey provides new photometric information for these faint systems, from which we obtained model-derived luminosities, masses, and temperatures. Despite achieving a deep sensitivity to faint companions beyond 0.2–0.5 arcsec, down to mass ratios of 0.4–0.7 outside ∼5 au, we find no companions to our substellar primaries. From our derived survey completeness, we place an upper limit of $f \lt 4.9~{{\ \rm per\ cent}}$ at the 1σ level (<13.0 per cent at the 2σ level) on the binary frequency of these objects over the separation range 1–1000 au and for mass ratios above q = 0.4. Our results confirm that companions are extremely rare around the lowest mass and coldest isolated brown dwarfs, continuing the marginal trend of decreasing binary fraction with primary mass observed throughout the stellar and substellar regimes. These findings support the idea that if a significant population of binaries exist around such low-mass objects, it should lie primarily below 2–3 au separations, with a true peak possibly located at even tighter orbital separations for Y dwarfs