Circumstellar Disk Composition and Evolution
Explaining the remarkable variety of planets that encircle other stars requires that we study the early history of planets and how they interact with their natal environment, the circumstellar disk.
Do most stars possess disks massive enough to form planets?
How is material distributed radially and vertically in the disk? The density of material controls the timescales for planet formation, and the temperature and viscosity of the disk control the transport of solids.
How does disk material dissipate?
How do disks and planetesimals interact?
How do planets obtain their compositions and volatiles?
How do giant impacts shape planetary architectures and habitability?
Using the high spatial resolution and sensitivity of the Hubble Space Telescope and ground-based telescopes such as Magellan, I am studying dusty circumstellar disks as the birthplaces of planetary systems. The observations elucidate disk geometries and dust composition and in an ensemble fashion teach us about the evolution of disks and the timescales for planet formation within them. I try to connect the disks around other stars to our understanding of planet formation in our own Solar System and in other systems.
Visual and near-infrared imaging provides detailed morphologies and colors of resolved disks. I have been PI or co-I on many programs HST programs over the years to image disks, particulary nearby dusty debris disks. Debris dust arises from the collisions and evaporation of planetesimals, and it is these same planetesimals which are the building blocks for planets (e.g. Debes et al. 2017). We are making porous aggregate dust models to improve our ability to retrieve dust composition from measurements of disk colors (and, in the best cases, spectroscopy). The goal is to be able to estimate the organic-to-silicate-to-ice ratios of the dust to constrain the place of formation and subsequent processing. By studying the grain composition directly with spectroscopy over a range of distances from the star, I try to learn about the processes of planet building and collisions that occur in disks. (e.g. Lomax et al. 2018, Arnold et al. 2018).
Infrared Spectroscopy - MagNIFIES: A Near-Infrared Spectrograph for the (Giant) Magellan Telescope
From the youngest stars still enshrouded in clouds of gas and dust to the bulge of stars that record the history of the assembly of the Galaxy, Carnegie’s Magellan Telescopes have an amazing view of the Milky Way. We propose to build an incredibly capable near-infrared spectrograph – it will have the largest simultaneous wavelength grasp, be the most efficient, and therefore have the best sensitivity, of any such instrument ever.
MagNIFIES will take the collecting power of the 6.5 m Magellan telescope and disperse and collect all the light at wavelengths from 1.07 to 5.3 μm simultaneously in six sub-spectrographs aligned on a common platform. With spectral resolutions in excess of 40,000, MagNIFIES will have the largest spectral grasp of any high dispersion instrument in the world. It will be made possible by a collaboration between Carnegie, Giant Magellan Telescope Organization, University of Texas at Austin, and the Korean Astronomy and Space Science Institute with major users across the Magellan Consortium.
Disk Evolution Studies
With precisely measured distances, we can compare the luminosities of young stars in stellar associations. The astrometric camera, CAPSCam, at the Carnegie Institution's du Pont Telescope at Las Campanas Observatory, was used to obtain trigonometric parallaxes to stars in the TW Hydrae Association, a nearby loose association of young stars that is perhaps related to Scorpius-Centaurus, and the Upper Scorpius association, which is the youngest part of the much larger Scorpius-Centaurus star forming region. In our paper on TWA, we showed that the association had an average age of 7.9 Myr, identified two new members, and showed that it is difficult to measure a model-independent traceback age given the expected velocity dispersions of young stellar groups. We published parallaxes to 52 low-mass stars in Upper Scorpius. We measured ages of the individual stars by combining our measured parallaxes with pre-main-sequence evolutionary tracks. Surprisingly, we find a significant difference in the ages of stars with and without circumstellar disks. The stars without disks have a mean age of 4.9 +/- 0.8 Myr and those with disks have an older mean age of 8.2 +/- 0.9 Myr. This counterintuitive result is intriguing evidence for the effects of accretion on resetting young stellar ages or perhaps for the influence of magnetic fields in controlling both stellar evolution and disk dissipation.
Brown Dwarfs and Planets
With Alan Boss, we have an astrometric search for gas giant planets and brown dwarfs orbiting nearby low-mass dwarf stars. We use the 2.5 m duPont Telescope at Carnegie's Las Campanas Observatory in Chile. We have followed about 100 nearby (primarily within 10 pc) low-mass stars, principally late M, L, and T dwarfs, for 10-15 years (e.g. Boss et al. 2009). We are searching for giant planets in architectures similar to that of the solar system, which could permit the existence of habitable, Earth-like planets on shorter-period orbits. These stars are generally too faint and red to be included in ground-based Doppler planet surveys. The smaller masses of late M dwarfs also yield correspondingly larger astrometric signals for a given mass planet. Our search will help to determine whether gas giant planets form primarily by core accretion or by disk instability around late M dwarf stars. This figure shows the astrometric orbit of a previously known brown dwarf binary known as Epsilon Indi BC; from measuring the orbit we were able to determine the masses of the two "failed stars" circling about their common center of mass (Dieterich et al. 2018).