Publications

2023

Long-term Stability of Planetary Systems Formed from a Transitional Disk

Transitional disks are protoplanetary disks with large and deep central holes in the gas, possibly carved by young planets. Dong & Dawson simulated systems with multiple giant planets that were capable of carving and maintaining such gaps during the disk stage. Here we continue their simulations by evolving the systems for 10 Gyr after disk dissipation and compare the resulting system architecture to observed giant planet properties, such as their orbital eccentricities and resonances. We find that the simulated systems contain a disproportionately large number of circular orbits compared to observed giant exoplanets. Large eccentricities are generated in simulated systems that go unstable, but too few of our systems go unstable, likely due to our demand that they remain stable during the gas-disk stage to maintain cavities. We also explore whether transitional-disk inspired initial conditions can account for the observed younger ages of 2:1 resonant systems orbiting mature host stars. Many simulated planet pairs lock into a 2:1 resonance during the gas-disk stage, but those that are disrupted tend to be disrupted early, within the first 10 Myr. Our results suggest that systems of giant planets capable of carving and maintaining transitional disks are not the direct predecessors of observed giant planets, either because the transitional disk cavities have a different origin or another process is involved, such as convergent migration that packs planets close together at the end of the transitional disk stage.

Figure 10 from the paper, showing an eccentricity CDF for the simulated planets compared to observed planets. When comparing with all simulations, the systems are too circular compared to observed planets. In contrast, when restricting only to those systems that have experienced instabilities, the simulations are too eccentric compared to observations.

 

2022

MIRAC-5: a ground-based mid-IR instrument with the potential to detect ammonia in gas giants

We present the fifth incarnation of the Mid-Infrared Array Camera (MIRAC-5) instrument which will use a new GeoSnap (3 – 13 microns) detector. Advances in adaptive optics (AO) systems and detectors are enabling ground based mid-infrared systems capable of high spatial resolution and deep contrast. As one of the only 3 – 13 micron cameras used in tandem with AO, MIRAC-5 will be complementary to the James Webb Space Telescope (JWST) and capable of characterizing gas giant exoplanets and imaging forming protoplanets (helping to characterize their circumplanetary disks). We describe key features of the MIRAC-5 GeoSnap detector, a long-wave Mercury-Cadmium-Telluride (MCT) array produced by Teledyne Imaging Sensors (TIS), including its high quantum efficiency (> 65%), large well-depth, and low noise. We summarize MIRAC-5’s important capabilities, including prospects for obtaining the first continuum mid-infrared measurements for several gas giants and the first 10.2-10.8 micron NH3 detection in the atmosphere of the warm companion GJ 504b (Teff ~ 550 K) within 8 hours of observing time. Finally, we describe plans for future upgrades to MIRAC-5 such as adding a coronagraph. MIRAC5 will be commissioned on the MMT utilizing the new MAPS AO system in late 2022 with plans to move to Magellan with the MagAO system in the future.

Figure 4 from the paper, showing a predicted spectra for the warm companion GJ504b alongside optimized filter bandpasses needed to achieve a significant detection of ammonia

 

2021

Exoplanets with ELT-METIS I: Estimating the direct imaging exoplanet yield around stars within 6.5 parsecs

Direct imaging is a powerful exoplanet discovery technique that is complementary to other techniques and offers great promise in the era of 30 meter class telescopes. Space-based transit surveys have revolutionized our understanding of the frequency of planets at small orbital radii around Sun-like stars. The next generation of extremely large ground-based telescopes will have the angular resolution and sensitivity to directly image planets with R < 4 Earth radii around the very nearest stars. Here, we predict yields from a direct imaging survey of a volume-limited sample of Sun-like stars with the Mid-Infrared ELT Imager and Spectrograph (METIS) instrument, planned for the 39 m European Southern Observatory (ESO) Extremely Large Telescope (ELT) that is expected to be operational towards the end of the decade. Using Kepler occurrence rates, a sample of stars with spectral types A-K within 6.5 pc, and simulated contrast curves based on an advanced model of what is achievable from coronagraphic imaging with adaptive optics, we estimated the expected yield from METIS using Monte Carlo simulations. We find the METIS expected yield of planets in the N2 band (10.10 – 12.40 microns) is 1.14 planets, which is greater than comparable observations in the L (3.70 – 3.95 microns) and M (4.70 – 4.90 microns) bands. We also determined a 24.6% chance of detecting at least one Jovian planet in the background limited regime assuming a 1 hour integration. We calculated the yield per star and estimate optimal observing revisit times to increase the yield. We also analyzed a northern hemisphere version of this survey and found there are additional targets worth considering. In conclusion, we present an observing strategy aimed to maximize the possible yield for limited telescope time, resulting in 1.48 expected planets in the N2 band.

Figure 2 from the paper, showing the discovery space around the five target stars.

2020

The Michigan infrared test thermal ELT N-band (MITTEN) cryostat

We introduce the Michigan Infrared Test Thermal ELT N-band (MITTEN) Cryostat, a new facility for testing infrared detectors with a focus on mid-infrared (MIR) wavelengths (8-13 microns). New generations of large format, deep well, fast readout MIR detectors are now becoming available to the astronomical community. As one example, Teledyne Imaging Sensors (TIS) has introduced a long-wave Mercury-Cadmium-Telluride (MCT) array, GeoSnap, with high quantum efficiency (< 65 %) and improved noise properties compared to previous generation Si:As blocked impurity band (BIB) detectors. GeoSnap promises improved sensitivities, and efficiencies, for future background-limited MIR instruments, in particular with future extremely large telescopes (ELTs). We describe our new test facility suitable for measuring characteristics of these detectors, such as read noise, dark current, linearity, gain, pixel operability, quantum efficiency, and point source imaging performance relative to a background scene, as well as multiple point sources of differing contrast. MITTEN has an internal light source, and soon an accompanying filter wheel and aperture plate, reimaged onto the detector using an Offner relay. The baseline temperature of the cryostat interior is maintained is < 40 K and the optical bench maintains a temperature of 16 K using a two-stage pulse-tube cryocooler package from Cryomech. No measurable background radiation from the cryostat interior has yet been detected.

The MITTEN cryostat in IRIS lab.