As a PhD candidate at the University of Michigan, I have led several programs investigating the role of environment and primary mass on multiple star formation, including: a) searching for milli-arcsecond (mas) companions to intermediate mass stars using long-baseline interferometry, b) developing a method to detect companions around a wide range of host masses below the diffraction limit with the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST), and c) creating a technique to use wavefront sensing information on JWST to perform high-contrast imaging with theoretical point-spread function (PSF) models.
Multiplicity of Stars and Brown Dwarfs in the Orion Nebula Cluster with HST
A large portion of my dissertation was devoted to detecting close companions to stars and brown dwarfs within the Orion Nebula Cluster (ONC) using HST to determine the role of the birth environment on the formation of multiple systems. In three papers, De Furio et al. (2019, 2022a, 2022c), I developed a binary PSF fitting algorithm using empirical PSF models from HST (Anderson & King 2006) on wide-field images from the Advanced Camera for Surveys (ACS). With rigorous injection and recovery analyses, my technique is proven to reliably identify companions at separations ≥ 0.5λ/D (~ 25 mas), see Fig. 1. The limit of my technique is competitive with other advanced post-processing techniques, such as aperture masking interferometry (AMI) with adaptive optics and kernel phase interferometry (KPI) with HST. With well defined empirical PSFs across the field of view, my technique is applicable to the many sources simultaneously observed in wide-field images (e.g. in star clusters) while other techniques are typically applied on a source by source basis with the need for a calibrator star.
With archival HST data in the ONC, I used my PSF fitting algorithm to find companions to low-mass stars and brown dwarfs, and characterize their populations with a Bayesian analysis. A part of this program involved mentoring an undergraduate student who is second author on De Furio et al. (2022c). I found: 1) the companion frequency (CF) of low-mass stars in the ONC is consistent with the field over all mass ratios and orbital separations of 10-200 au, in contrast to findings from low-density regions like Taurus, 2) the power-law fit to the mass ratio distribution of low-mass primaries in the ONC is consistent with that in the field and Taurus, and 3) the CF to brown dwarfs with separations > 20 au in the ONC is in excess of the field, requiring further dynamical evolution destroying binaries with low binding energy to resemble the field. I also served as the science PI on a successful HST Cycle 30 program, collaborating across four institutions, which was awarded 7 orbits (ID: 17141) to explore the stellar multiplicity of NGC 1333, a moderate density star-forming region.
Companion Search to a Cool Y-dwarf below the Diffraction Limit on JWST
Building on my experience with empirical PSF fitting on HST, I analyzed JWST NIRCam and NIRISS Guaranteed Time Observation (GTO) data as part of the NIRCam Science team. Prior work on WISE1828+2650, a Teff~300K field Y-dwarf, could not explain the observed spectrum with state-of-the-art spectral and evolutionary models, suggesting the object could be a binary with a composite SED. This object was observed with NIRCam and NIRISS (ID: 1189) to obtain imaging data and search for a close companion. I constructed 4x super-sampled empirical PSFs based on contemporaneous apparent single stars within the NIRCam and NIRISS fields of view. I then used those empirical PSFs as input to a Bayesian binary fitting tool using a nested sampling approach which derives the best-fit binary solution to any input image. In Fig. 2, I show our ability to detect companions to a moderate signal-to-noise target on NIRCam in the F360M filter with a 400s exposure. In De Furio, et al. (2023), I demonstrate the ability to recover companions down to at least 0.5λ/D (~ 50 mas) with NIRCam. This technique will also be applied to a sample of 20 field Y-dwarfs observed with NIRCam through an approved Cycle 1 program (ID: 2473) on which I am co-I.
High-contrast Imaging with Theoretical PSFs on JWST
In addition to my application of empirical PSF models, I collaborated with a group of researchers at the NASA Jet Propulsion Laboratory through the Strategic University Research Partnership program to test a novel high-contrast imaging analysis for full aperture and AMI observing modes. Most high-contrast imaging routines require observations of a reference star to remove the stellar contribution or calibrate phase errors which can typically take half of the total observing time. This new technique takes advantage of the stability of JWST and uses the measured wavefront error (WFE) from a phase calibration observation (performed roughly every 48 hours) as prior information to estimate the WFE of a given observation and simultaneously search for faint companions. The model of the wavefront includes three low order Zernike coefficients per primary mirror segment, using the Hexike basis, and knowledge of the high frequency errors that are derived during phase calibration. The most likely WFE and astrophysical scene are determined by generating synthetic reference PSFs from the Hexike coefficients and comparing to the data. In De Furio et al. (submitted), I demonstrate the effectiveness of this technique on simulations of planned NIRISS AMI imaging programs (GO-1843), recovering a companion with a contrast of 8 magnitudes and a separation of 0.205″ (1.5λ/D, 3.16 NIRISS pixels) relative to the primary star, see Fig. 3. Without the need to observe a reference star, significant telescope time can be saved. I am applying this technique to real data, and once validated, will publicly release the code for use in JWST Cycle 2.
A-Star Small Separation Companion Population with Long-baseline Interferometry
On top of my space-based programs, I also lead a ground-based observing program to detect close companions to field A-type stars using long-baseline interferometry at the Center for High Angular Resolution Astronomy (CHARA) Array. Previous observations by DeRosa et al. (2014) are incomplete to companions < 30 au, while others (Murphy et al. 2018) are sensitive over a small range of parameter space, 0.6-3.6 au. In De Furio et al. (2022b), we describe results from two nights of observation awarded through NSF’s NOIRLab to probe separations from 0.5-300 mas (0.025-15 au at 50 pc) for a small sample of intermediate mass stars. We identified five companions with separations between 0.29-2.8 au which motivated a follow-up campaign of four nights obtained through CHARA’s internal consortium. With a larger sample, we will characterize the companion population in terms of companion frequency, mass ratios, and separations to inform our understanding of star formation based on primary mass.