New Insights into Circumstellar Disk Evolution in the Era of ALMA

Citation

Barenfeld, Scott Alexander (2019) New Insights into Circumstellar Disk Evolution in the Era of ALMA. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5DK0-H549. https://resolver.caltech.edu/CaltechTHESIS:05152019-180537558

Abstract

In recent decades, it has become clear that solar systems such as our own form within the circumstellar disks of gas and dust that surround young stars. Thus, to understand how these solar systems come to be, it is necessary to study the conditions within these disks. Until recently, such studies have required a focus on intrinsically brighter and younger disks that are easier to observe. However, a full picture of planet formation requires the characterization of older disks to determine how these systems change over time. The unprecedented capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) now present the opportunity to study populations of these disks in detail for the first time. In this thesis, I present a study of over 100 such disks in the 5-11 Myr old Upper Scorpius OB Association (Upper Sco) using ALMA, as well as Keck Observatory, with the aim of comparing the properties of these systems to younger disks in order to shed new light on disk evolution.

Following background discussion on disks and their evolution, ALMA measurements of the continuum and CO line fluxes of these disks at 0.88 mm are reported in Chapter 3. The continuum fluxes are used to show that the majority of these systems contain less than 1 M_⊕ of dust. It is then shown that dust masses around these stars are on average a factor of ∼4.5 lower than their younger counterparts in the Taurus star-forming region, placing important constraints on the mass evolution of these systems. Finally, constraints on depletion of gas in these disks are discussed using the CO measurements.

The spatial distributions of the gas and dust within these Upper Sco disks are modeled in Chapter 4. The radial extents of gas and dust are measured and compared, with several systems showing evidence of the gas being more extended. The sizes of the dust disks are compared to younger systems, showing that these disks shrink by a factor of approximately three as they age. These results suggest that dust evolves from the outside-in within disks, perhaps through radial drift. Despite this evolution, dust disks in Upper Sco fall on the same correlation between size and millimeter luminosity as their younger counterparts. This implies a link between the radial structures of disks of different ages, perhaps indicating that these systems are composed of optically-thick dust substructure.

Of course, an understanding of planetary system formation would be incomplete without accounting for the presence of stellar companions, which are common around young stars and are expected to shorten disk lifetimes by truncating their sizes. As such, Chapter 5 presents a search for stellar companions in the Upper Sco disk sample analyzed in Chapters 3 and 4. Using adaptive optics imaging and aperture masking observations with the NIRC2 instrument on the Keck II telescope, stellar companions are identified in 27 of 112 systems. It is then shown that the companion fraction of systems with disks is lower than those without, confirming the harmful effects of stellar companions on disks seen in younger systems. However, the fraction of disk systems in Upper Sco with a close companion is shown to match that of younger disks in Taurus. This indicates that these effects occur within the first ∼Myr of disk evolution, after which stellar companions have little to no effect. Additionally, while the millimeter luminosities of disks with stellar companions are observed to be lower than those around single stars in Taurus, there exists no such difference in Upper Sco. This provides further support for outside-in dust evolution, as the shrinking of disks around single stars would cause them by the 5-11 Myr age of Upper Sco to match the sizes and brightnesses of truncated disks in binary systems.

Taken together, the results presented in this thesis show the masses and radial extents of the dust-component of circumstellar disks decrease with age. This thesis concludes by summarizing these results and discussing their link within a scenario of outside-in dust evolution involving radial drift and dust substructure. To close, potential avenues are presented to continue the study of disk evolution with ALMA.

Thesis Files