Searching for Wide Companions and Identifying Circum(sub)stellar Disks through PSF-Fitting of Spitzer/IRAC Archival Images

Abstract: Direct imaging surveys have discovered wide-orbit planetary-mass companions that challenge existing models of both star and planet formation, but their demographics remain poorly sampled. We have developed an automated binary companion point spread function (PSF) fitting pipeline to take advantage of Spitzer's infrared sensitivity to planetary-mass objects and circum(sub)stellar disks, measuring photometry across the four IRAC channels of 3.6 $\mu$m, 4.5 $\mu$m, 5.8 $\mu$m, and 8.0 $\mu$m. We present PSF-fitting photometry of archival Spitzer/IRAC images for 11 young, low-mass ($M\sim0.044$-0.88 $M_{\odot}$; M7.5-K3.5) members of three nearby star-forming regions (Chameleon, Taurus, and Upper Scorpius; $d\sim$ 150 pc; $\tau\sim$ 1-10 Myr) that host confirmed or candidate faint companions at $\rho = 1.68^{\prime\prime}-7.31^{\prime\prime}$. We recover all system primaries, six confirmed, and two candidate low-mass companions in our sample. We also measure non-photospheric $[3.6]-[8.0]$ colors for three of the system primaries, four of the confirmed companions, and one candidate companion, signifying the presence of circumstellar or circum(sub)stellar disks. We furthermore report the confirmation of a $\rho=4.66^{\prime\prime}$ (540 au) companion to [SCH06] J0359+2009 which was previously identified as a candidate via imaging over five years ago, but was not studied further. Based on its brightness ($M_{[3.6]}=8.53$ mag), we infer the companion mass to be $M=20\pm5$ $M_\mathrm{Jup}$ given the primary's model-derived age of 10 Myr. Our framework is sensitive to companions with masses less than 10 $M_\mathrm{Jup}$ at separations of $\rho = 300$ au in nearby star-forming regions, opening up a new regime of parameter space that has yet to be studied in detail, discovering planetary-mass companions in their birth environments and revealing their circum(sub)stellar disks.