Discovery of a Hypervelocity L Subdwarf at the Star/Brown Dwarf Mass Limit (2407.08578v1)

Abstract: We report the discovery of a high velocity, very low-mass star or brown dwarf whose kinematics suggest it is unbound to the Milky Way. CWISE J124909.08+362116.0 was identified by citizen scientists in the Backyard Worlds: Planet 9 program as a high proper motion ($\mu$ $=$ 0''9/yr) faint red source. Moderate resolution spectroscopy with Keck/NIRES reveals it to be a metal-poor early L subdwarf with a large radial velocity ($-$103$\pm$10 km/s), and its estimated distance of 125$\pm$8 pc yields a speed of 456$\pm$27 km/s in the Galactic rest frame, near the local escape velocity for the Milky Way. We explore several potential scenarios for the origin of this source, including ejection from the Galactic center $\gtrsim$3 Gyr in the past, survival as the mass donor companion to an exploded white dwarf. acceleration through a three-body interaction with a black hole binary in a globular cluster, and accretion from a Milky Way satellite system. CWISE J1249+3621 is the first hypervelocity very low mass star or brown dwarf to be found, and the nearest of all such systems. It may represent a broader population of very high velocity, low-mass objects that have undergone extreme accelerations.

• The paper identifies J1249+3621 as a hypervelocity early-type L subdwarf with a speed of approximately 456 km/s, suggesting it may be unbound from the Milky Way.
• It utilizes a combination of proper motion analysis, moderate-resolution spectroscopy, and advanced atmospheric modeling to determine its low mass and metal-poor composition.
• The study advances our understanding of stellar dynamics and proposes new insights into the mechanisms responsible for ejecting extremely low-mass objects from galaxies.

Discovery of a Hypervelocity L Subdwarf at the Star/Brown Dwarf Mass Limit

The paper by Burgasser et al. (2024) presents the identification and characterization of a uniquely high velocity, very low-mass star or brown dwarf, designated CWISE J124909.08+362116.0, hereafter referred to as J1249+3621. This research adds a significant data point to the small cohort of known hypervelocity stars, particularly remarkable for probing the star/brown dwarf mass threshold.

Summary of Findings

The essence of this work is the identification of J1249+3621 as a hypervelocity star. Churching multiple observational techniques, the researchers ascertain that this source exhibits a proper motion of 0.9 arcseconds per year and a radial velocity of -103 ± 10 km/s. Spectroscopic analysis using Keck/NIRES affirms its classification as a metal-poor early-type L subdwarf. The inferred speed of 456 ± 27 km/s suggests that J1249+3621 may not be bound to the Milky Way, with its estimated distance from the Earth being approximately 125 parsecs.

Notable Outcomes and Claims

• Hypervelocity Status: J1249+3621 is potentially unbound from the Milky Way's gravitational pull, a claim substantiated by its high tangential velocity near Milky Way's local escape velocity.
• Mass Estimates and Composition: Using atmosphere modeling, J1249+3621's mass is deduced to be around the star/brown dwarf boundary, with a probability of containing a mass marginally above the hydrogen-burning limit. The source is largely metal-poor, yet showcases alpha enrichment.
• Potential Origin Scenarios: Several formation hypotheses are explored, involving ejection from the Galactic center, survival post-compact binary supernova explosion, or even potentially representing high-velocity remnants from a globular cluster.

Methodology and Analysis

The paper employs moderate-resolution spectroscopy for classification and radial velocity measurements besides utilizing atmosphere models (Sonora Elf Owl and SAND) for parameter estimation. The forward modeling technique in spectral analysis coupled with Metropolis-Hastings Markov Chain Monte Carlo fitting is notable in pinning down atmospheric features and motion parameters from the infrared spectra. The paper's robust approach in blending observational data with sophisticated modeling underlines the credibility of its estimates.

Implications and Future Prospects

The discovery of J1249+3621 represents a novel contribution to the paper of hypervelocity stars, particularly in the regime of very low-mass, metal-poor objects. Theoretical implications include enhancing understanding of stellar dynamics and interactions within the Milky Way, particularly in terms of ejection mechanisms and population statistics of such hypervelocity objects.

This research recasts the investigative focus on substellar objects as potential hypervelocity candidates, challenging traditional models of galactic ejection and evolution. Practically, it underscores the capability of survey techniques, like those harnessed by the Backyard Worlds: Planet 9 collaboration, in detecting faint, fast-moving sources that might signify extreme astrophysical processes.

Future studies could amplify focus on direct parallax measurements for more precise distance and kinematic modeling, along with detailed atmospheric composition analysis—which could unravel formation narratives through chemical signatures. Additionally, the extension of this observational framework to potential analogs in other galactic systems could broaden the understanding of such phenomena beyond the Milky Way.

In sum, Burgasser et al. (2024) present a rigorous examination that fortifies our comprehension of the extremes young stars may traverse, leveraging advanced techniques to reveal the story of a potential vagrant at the very edges of the Milky Way's gravitational comfort.