[Paper Review] Ross 19B: An Extremely Cold Companion Discovered via the Backyard Worlds: Planet 9 Citizen Science Project

Adam C. Schneider, Aaron Meisner+28 more|arXiv (Cornell University)|Aug 11, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy183 references18 citations

TL;DR

This paper presents the discovery of Ross 19B, an extremely cold T/Y-type brown dwarf companion to the nearby M dwarf Ross 19A, identified via the Backyard Worlds: Planet 9 citizen science project. Using refined astrometric and kinematic analysis with a modified BANYAN Σ formalism, the authors confirm a 100% probability of physical association, establishing Ross 19B as one of the coldest known wide-separation substellar companions with an effective temperature of 500+115−100 K and a mass of 15–40 MJup, making it a prime target for future spectroscopic characterization with Hubble or James Webb Space Telescope.

ABSTRACT

Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation ($\sim$10', $\sim$9900 au projected) substellar companion to the nearby ($\sim$17.5 pc), mid-M dwarf Ross 19. We have developed a new formalism for determining chance alignment probabilities based on the BANYAN $\Sigma$ tool, and find a 100% probability that this is a physically associated pair. Through a detailed examination of Ross 19A, we find that the system is metal-poor ([Fe/H]=$-$0.40$\pm$0.12) with an age of 7.2$^{+3.8}_{-3.6}$ Gyr. Combining new and existing photometry and astrometry, we find that Ross 19B is one of the coldest known wide-separation companions, with a spectral type on the T/Y boundary, an effective temperature of 500$^{+115}_{-100}$ K, and a mass in the range 15-40 $M_{ m Jup}$. This new, extremely cold benchmark companion is a compelling target for detailed characterization with future spectroscopic observations using facilities such as the Hubble Space Telescope or James Webb Space Telescope.

Motivation & Objective

• To identify and confirm a physically associated, extremely cold substellar companion to a nearby M dwarf using citizen science data.
• To determine the physical parameters of the Ross 19AB system, including age, metallicity, and mass, to establish a benchmark for substellar evolution models.
• To assess the formation origin of Ross 19B by investigating atmospheric composition, particularly the C/O ratio, to distinguish between in-situ formation and dynamical scattering.
• To evaluate the potential for future high-resolution spectroscopic characterization of Ross 19B using Hubble or James Webb Space Telescope.

Proposed method

• Utilized the Backyard Worlds: Planet 9 citizen science project to identify moving sources in WISE flipbooks, leading to the detection of CWISE J021948.68+351845.3 as a candidate companion to Ross 19A.
• Applied a modified BANYAN Σ formalism to compute chance alignment probabilities, achieving a 100% likelihood of physical association between Ross 19A and Ross 19B.
• Combined new Keck/MOSFIRE J-band imaging with existing astrometry and photometry from Gaia EDR3, WISE, and other surveys to refine orbital and physical parameters.
• Conducted spectral energy distribution (SED) modeling using SEDkit to estimate Ross 19B's effective temperature and mass, placing it on the T/Y spectral boundary.
• Measured Ross 19A's metallicity ([Fe/H] = −0.40 ± 0.12) and age (7.2+3.8−3.6 Gyr) using photometric and astrometric data to constrain system evolution.
• Proposed future spectroscopic observations with HST or JWST to retrieve atmospheric C/O ratios, which could distinguish formation pathways such as disk instability or turbulent fragmentation.

Experimental results

Research questions

• RQ1 Is the wide-separation, cold companion CWISE J021948.68+351845.3 physically associated with Ross 19A, or is it a chance alignment?
• RQ2 What are the precise physical parameters of Ross 19B, including its effective temperature, mass, and spectral type, and how do they compare to known substellar benchmarks?
• RQ3 What does the atmospheric composition of Ross 19B, particularly its C/O ratio, reveal about its formation mechanism—direct collapse in a disk or dynamical scattering?
• RQ4 Can Ross 19B serve as a robust benchmark for testing and calibrating substellar evolutionary models due to its extreme coldness and well-constrained age and metallicity?

Key findings

• The probability of chance alignment between Ross 19A and Ross 19B is 100%, confirming a physical association through a modified BANYAN Σ formalism.
• Ross 19A has a metallicity of [Fe/H] = −0.40 ± 0.12 and an age of 7.2+3.8−3.6 Gyr, making it a metal-poor, old M dwarf.
• Ross 19B is one of the coldest known wide-separation companions, with an effective temperature of 500+115−100 K and a spectral type near the T/Y boundary.
• The mass of Ross 19B is constrained to 15–40 MJup, placing it near the planetary mass boundary.
• Ross 19B is faint in the near-infrared (J ≈ 21.14 mag), making it a prime target for future spectroscopic characterization with Hubble or James Webb Space Telescope.
• The system's low metallicity and extreme coldness make Ross 19B a rare and valuable benchmark for testing substellar atmospheric and evolutionary models.