The Closest Known Flyby of a Star to the Solar System (1502.04655v1)

Abstract: Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 ("Scholz's star") to calculate its past trajectory. Integrating the Galactic orbits of this $\sim$0.15 M${\odot}$ binary system and the Sun, we find that the binary passed within only 52$^{+23}{-14}$ kAU (0.25$^{+0.11}_{-0.07}$ parsec) of the Sun 70$^{+15}_{-10}$ kya (1$\sigma$ uncertainties), i.e. within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently ($\sim$0.1 Myr$^{-1}$). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is $\sim$98%, but the probability of penetrating the dynamically active inner Oort Cloud ($<$20 kAU) is $\sim$10$^{-4}$. While the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars.

• The paper identifies Scholz’s star’s flyby as the closest known encounter, occurring ~70,000 years ago at about 52,000 AU (0.25 parsecs) from the Sun.
• It employs precise velocity measurements and integrates 10,000 orbital trajectories to determine the encounter’s timing and proximity with quantified uncertainties.
• The findings show a 98% chance that the star intruded the outer Oort Cloud, indicating a negligible effect on increasing inner comet flux.

Stellar Flyby of the Solar System: Detailed Analysis

The paper by Mamajek et al. provides an in-depth examination of the closest recorded stellar encounter with our solar system. The paper focuses on the low-mass binary system WISE J072003.20-084651.2, known as Scholz’s star, which offers new insights into stellar dynamics and their potential perturbations on the Oort Cloud.

Overview of Scholz’s Star Encounter

The primary finding from this paper is the identification of a past flyby of Scholz's star within approximately 52,000 astronomical units (AU) of the Sun. This event took place around 70,000 years ago. The encounter marks the closest known passage of a stellar object to the solar system, occurring within the outer regions of the Oort Cloud. Given the flyby's distance of 0.25 parsecs from the solar system, the analysis dedicates considerable effort to quantifying its dynamic effects on the solar neighborhood.

Methodology and Computation

The research utilizes precise velocity measurements and galactic orbit integrations to simulate the past trajectories of both Scholz's star and the Sun. The forward modeling of 10,000 possible orbits ensures a comprehensive analysis of the encounter's proximity and timing, with uncertainties well-delineated. The star's classification places it as a binary system composed of a low-mass star and a brown dwarf, refining the binary's mass estimates to approximately 0.15 solar masses.

Perturbations in the Oort Cloud

Given Scholz's star's low mass and high velocity, the paper concludes that its gravitational influence during the flyby was minimal. The authors calculate a 98% probability that the star intruded the outer Oort Cloud, but only a 0.01% probability for significant entry into the dynamically active inner regions of the cloud (<20,000 AU). Consequently, the event is unlikely to have substantially increased the flux of long-period comets into the inner solar system.

Implications of the Study

The results highlight the rarity of such close stellar encounters. The authors estimate these passages occur roughly once every 9.2 million years. Furthermore, the encounter underscores the potential of nearby stars as perturbers to the Oort Cloud, suggesting further investigation into other low-mass stars in the vicinity. The findings indicate the necessity for continuous monitoring of proper motions and radial velocities of nearby stars using ongoing and future surveys such as Gaia, which could reveal other close stellar flybys not previously detected.

Future Considerations

The paper by Mamajek et al. draws attention to the need for improved astrometric and radial velocity accuracy to refine the understanding of such encounters. Advancements in these areas will help astronomers accurately project future perturbations on the Oort Cloud and assess their implications for cometary dynamics and potential threats to Earth.

In summary, this work establishes a critical reference for understanding stellar motions in our galactic neighborhood and sets the stage for identifying future dynamics involving the solar system's periphery, particularly concerning the Oort Cloud's stability and comet flux variation.