Exploring the Role of Primary Mass and Star-forming Environment on the Formation of Stellar and Sub-stellar Multiple Systems

Abstract

Multiplicity is a common outcome of the star formation process that occurs across orders of magnitude in primary mass and orbital separation, from brown dwarfs to O-stars and out to thousands of au. Initial companion properties depend on the mass and specific angular momentum of molecular cloud cores, and are further modulated by continued accretion and orbital migration. Dynamical processes likely sculpt the companion population based on cluster environment. In this thesis, we explore crucial regimes of star-forming environment and primary mass to understand their impact on the formation and evolution of companion populations which are valuable inputs to a predictive star and planet formation theory.

We investigated the companion population to known low-mass stellar and sub-stellar members of the Orion Nebula Cluster (ONC), a high-density star-forming region, for comparison to results in low-density star-forming regions. With data from the Hubble Space Telescope, we created a double point-spread function fitting routine to detect companions beyond 0.5$lambda$/D, 10au at the distance to the ONC. We detected fifty-one companions (twenty-four are new) and characterized companion demographics using Bayesian analyses. We demonstrate that cluster dynamics are likely important at early times, shaping the companion population of the ONC to resemble the Galactic field, unlike low density associations. We argue that low binding-energy brown-dwarf binaries in the ONC can still be disrupted. We also find evidence for initial companion formation processes which do not depend on stellar density, but are dynamically processed depending on local stellar density.

Utilizing similar detection techniques, we explored the multiplicity of Y-dwarfs, the lowest temperature ($<$ 500 K) known free floating objects, with JWST. We probe the low-mass limit of companion formation around the lowest primary masses explored to date. We demonstrated the ability to recover companions beyond 0.05'' in the NIRCam long wavelength channel, 0.5$lambda$/D. We show that WISE1828+2650, a peculiar low-mass brown dwarf, has no companion beyond 0.5 au. Either one exists at closer separations or this object has a more complex atmosphere than can be currently modeled. We report the first detection of a Y-Y dwarf binary system, WISE0336-0143, with a separation=1 au, primary mass=8.5-18 M$_{J}$, and companion mass=5-11.5 M$_{J}$ assuming ages from 1-5 Gyr. This result confirms that multiplicity occurs through the star formation process even approaching the opacity limit of fragmentation. These data and future JWST observations will allow us to fully characterize the companion population near the fragmentation limit and constrain their formation processes.

Lastly, we probed the close companion population to A-type stars (1.5-2.5 M$_{odot}$) using long-baseline interferometry with MIRC-X/MYSTIC at the Center for High Angular Resolution Astronomy Array. We detected seven companions across 0.29-8.86 au with mass ratios=0.21-0.96. We find no evidence for a difference between the companion frequency of A-type and solar-type (0.7-1.3 M$_{odot}$) stars over 0.01-27.54 au and mass ratios of 0.1-1.0. However, we find tentative evidence for a difference in the companion frequency between A-type and more massive B-type stars. This result is potentially indicative of an increase in circumstellar disk fragmentation for higher mass primaries that produces more close companions.

All together, these results identify key features of the companion population based on star-forming environment and primary mass that will further our understanding of star formation processes and how companions impact the early conditions of disk evolution and planet formation.