arXiv subjects

A. L. Wallace

Publications and source records attributed to A. L. Wallace.

6 recordsLinked to original sources

arXiv2026

Perhaps there is no brown dwarf desert? A study of sub-stellar companions with Gaia DR3

The brown dwarf desert describes a range of orbital periods (<5 years) in which fewer brown dwarf-mass companions have been observed around Sun-like stars, when compared to planets and low mass stellar companions. It is therefore theorised that brown dwarf companions are unlikely to form or remain in this period range. The Gaia space telescope is uniquely sensitive to companions in this period range, making it an ideal tool to conduct a survey of the brown dwarf desert. In this study, we use Bayesian inference to analyse data from nearby (<200pc) Sun-like stars in Gaia's DR3 catalogue, assuming single companions. From this, we identify 2673 systems (2.41% of the sample) with possible brown dwarf companions in this period range. Accounting for observational biases, we find that 10.4+0.8-0.6% of nearby Sun-like stars have astrometric errors consistent with a brown dwarf-mass companion with a period less than 5 years, significantly higher than previous studies which reported occurrence rates of <1%. However, we acknowledge the limitations of DR3 and are unable to make a definitive statement without epoch data. By simulating epoch data with multiple companions, we find that, while some of the data can be explained by multiple low-mass brown dwarf companions and high-mass planets (>10MJ), high-mass brown dwarfs (>50MJ) in this period range are comparatively rare. Finally, we used our studies of the brown dwarf distribution to predict the number of companions in the brown dwarf desert we can expect to discover in DR4.

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arXiv2025

Detection and Characterisation of Giant Planets with Gaia Astrometry

Astrometric observations with Gaia are expected to play a valuable role in future exoplanet surveys. With current data from Gaia's third data release (DR3), we are sensitive to periods from less than 1 year to more than 4 years but, unlike radial velocity are not as restricted by the orbital inclination of a potential planet. The presence and potential properties of a companion affect the primary's renormalised unit weight error (RUWE) making this a valuable quantity in the search for exoplanets. Using this value and the fitted astrometric tracks from Gaia, we use Bayesian inference to constrain the mass and orbital parameters of companions in known systems. Combining this with radial velocity measurements, we show it is possible to independently measure mass and inclination and suggest HD 66141 b is a possible brown dwarf with maximum mass 23.9$^{+7.2}_{-6.4}$ M$_{\mathrm{J}}$. We show how this method may be applied to directly imaged planets in the future, using $β$-Pictoris c as an example but note that the host star is bright and active, making it difficult to draw clear conclusions. We show how the next Gaia data release, which will include epoch astrometry, will allow us to accurately constrain orbital parameters from astrometric data alone, revolutionising future searches for exoplanets. Combining predicted observational limits on planet mass with theoretical distributions, we estimate the probability that a star with a given RUWE will host a detectable planet, which will be highly valuable in planning future surveys.

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arXiv2023

Photometric Determination of Main-Sequence Binaries with Gaia

Since its launch in 2013, the Gaia space telescope has provided precise measurements of the positions and magnitudes of over 1 billion stars. This has enabled extensive searches for stellar and sub-stellar companions through astrometric and radial velocity measurements. However, these surveys require a prior knowledge of any unresolved companion affecting the results which can be identified using photometry. In this work, Gaia's magnitude measurements are combined with near-infrared observations from 2MASS and WISE and simulation-based inference is applied to constrain astrophysical parameters and search for hidden companions. This method is first tested on simulated sets of binary stars before expanding to Gaia's non-single star catalogue. Using this test, a region is identified on the H-R diagram in which the method is the most accurate and all Gaia sources within that region are analysed. This analysis reproduces a known anti-correlation between metallicity and binary fraction. Finally, the method is applied to the nearby star cluster M67 and, using previous studies of the metallicity distribution, it is possible to improve constraints on binary fraction. From this the binary fraction in the cluster is calculated to vary from 30% in the outer cluster to 45% near the core. This is found to be significantly higher the 23% binary fraction calculated for the wider stellar neighbourhood.

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arXiv2021

Constraints on Planets in Nearby Young Moving Groups Detectable by High-Contrast Imaging and Gaia Astrometry

The formation of giant planets is best studied through direct imaging by observing planets both during and after formation. Giant planets are expected to form either by core accretion, which is typically associated with low initial entropy (cold-start models) or by gravitational instability, which corresponds to a high initial entropy of the gas (hot-start models). Thus, constraining the initial entropy provides insight into the planet formation mechanism and determines the resultant brightness evolution. We find that, by observing planets in nearby moving groups of known age both through direct imaging and astrometry with Gaia, it will be possible to constrain the initial entropy of giant planets. We simulate a set of planetary systems in stars in nearby moving groups identified by BANYAN $Σ$ and assume a model for planet distribution consistent with radial velocity detections. We find that Gaia should be able to detect approximately 50% of planets in nearby moving groups greater than ~0.3 M$_\text{J}$. Using 5$σ$ contrast limits of current and future instruments, we calculate the flux uncertainty, and using models for the evolution of the planet brightness, we convert this to an initial entropy uncertainty. We find that, for future instruments such as MICADO and METIS on E-ELT and VIKiNG with VLTI, the entropy uncertainty is less than 0.5 $k_{B}$/baryon, showing that these instruments should be able to distinguish between hot and cold-start models.

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arXiv2020

High-resolution survey for planetary companions to young stars in the Taurus Molecular Cloud

Direct imaging in the infrared at the diffraction limit of large telescopes is a unique probe of the properties of young planetary systems. We survey 55 single class I and class II stars in Taurus in the L' filter using natural and laser guide star adaptive optics and the near-infrared camera (NIRC2) of the Keck II telescope, in order to search for planetary mass companions. We use both reference star differential imaging and kernel phase techniques, achieving typical 5-sigma contrasts of ~6 magnitudes at separations of 0.2" and ~8 magnitudes beyond 0.5". Although we do not detect any new faint companions, we constrain the frequency of wide separation massive planets, such as HR 8799 analogues. We find that, assuming hot-start models and a planet distribution with power-law mass and semi-major axis indices of -0.5 and -1, respectively, less than 20% of our target stars host planets with masses >2 MJ at separations >10 AU.

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arXiv2019

The Likelihood of Detecting Young Giant Planets with High Contrast Imaging and Interferometry

Giant planets are expected to form at orbital radii that are relatively large compared to transit and radial velocity detections (>1 AU). As a result, giant planet formation is best observed through direct imaging. By simulating the formation of giant (0.3-5$M_{J}$) planets by core accretion, we predict planet magnitude in the near infrared (2-4 $μ$m) and demonstrate that, once a planet reaches the runaway accretion phase, it is self-luminous and is bright enough to be detected in near infrared wavelengths. Using planet distribution models consistent with existing radial velocity and imaging constraints, we simulate a large sample of systems with the same stellar and disc properties to determine how many planets can be detected. We find that current large (8-10m) telescopes have, at most a 0.2% chance of detecting a core accretion giant planet in the L' band and 2% in the K band for a typical solar type star. Future instruments such as METIS and VIKiNG have higher sensitivity and are expected to detect exoplanets at a maximum rate of 2% and 8% respectively.

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