ORCID Profile
0000-0002-6591-5290
Current Organisations
Australian National University
,
Monash University
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Publisher: Oxford University Press (OUP)
Date: 17-09-2019
Abstract: Giant planets are expected to form at orbital radii that are relatively large compared to transit and radial velocity detections ( au). As a result, giant planet formation is best observed through direct imaging. By simulating the formation of giant (0.3–5MJ) planets by core accretion, we predict planet magnitude in the near-infrared (2–4 μm) 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 s le of systems with the same stellar and disc properties to determine how many planets can be detected. We find that current large (8–10 m) telescopes have at most a 0.2 per cent chance of detecting a core-accretion giant planet in the L’ band and 2 per cent 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 per cent, respectively.
Publisher: Oxford University Press (OUP)
Date: 26-02-2021
Abstract: In the Gaia era, the majority of stars in the Solar neighbourhood have parallaxes and proper motions precisely determined while spectroscopic age indicators are still missing for a large fraction of low-mass young stars. In this work, we select 756 overluminous late K and early M young star candidates in the southern sky and observe them over 64 nights with the ANU 2.3-m Telescope at Siding Spring Observatory using the Echelle (R = 24 000) and Wide Field spectrographs (WiFeS, R = 3000–7000). Our selection is kinematically unbiased to minimize the preference against low-mass members of stellar associations that dissipate first and to include potential members of diffuse components. We provide measurements of Hα and calcium H& K emission, as well as of Li i 6708 Å in absorption. This enables identification of stars as young as 10–30 Myr – a typical age range for stellar associations. We report on 346 stars showing detectable lithium absorption, 318 of which are not included in existing catalogues of young stars. We also report 125 additional stars in our s le presenting signs of stellar activity indicating youth but with no detectable lithium. Radial velocities are determined for WiFeS spectra with a precision of 3.2 km s−1 and 1.5 km s−1 for the Echelle s le.
Publisher: Oxford University Press (OUP)
Date: 27-09-2021
Abstract: The formation of giant planets can be 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, associated with high initial entropy of the gas (hot-start models). Thus, constraining the initial entropy can provide insight into a planet’s formation process and determines the resultant brightness evolution. In this study, 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 25 per cent of planets in nearby moving groups greater than $\\sim 0.3\\, M_\\text{J}$. Using 5σ 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 future instruments such as METIS on E-ELT as well as GRAVITY and VIKiNG with VLTI should be able to constrain the entropy to within 0.5 kB/baryon, which implies that these instruments should be able to distinguish between hot- and cold-start models.
Publisher: Oxford University Press (OUP)
Date: 17-08-2020
Abstract: 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 5σ contrasts of ∼6 mag at separations of 0.2 arcsec and ∼8 mag beyond 0.5 arcsec. 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 semimajor axis indices of −0.5 and −1, respectively, less than 20 per cent of our target stars host planets with masses & MJ at separations & au.
No related grants have been discovered for Alexander Wallace.