A Potential Signature of HD 7977's Passage Among Observed Long-Period Comet Orbits

Abstract

It is generally presumed that the tidal field of the Milky Way's disk is the main perturbation that has driven observed long-period comets (LPCs) from the Oort cloud into the inner solar system. The tide's influence on the Oort cloud should produce a distinct anisotropy in the arguments of perihelion ($ω$) of dynamically new LPCs with semimajor axes ($a$) over 10$^4$ au. Simulating LPC production dominated by the Galactic tide, we find that observed dynamically new LPCs are more isotropic than expected. Meanwhile, our simulation exhibits much better agreement between simulated and observed ``returning'' LPCs that have made a handful of passages through the inner solar system prior to discovery. The isotropy of new LPCs can be explained if the Oort cloud is much less centrally concentrated than the conventional Oort cloud formation model predicts. However, a second possibility also exists. Additional simulations we perform show that the observed $ω$ distributions of new and returning LPCs can both be well-replicated if the star HD 7977 passed within $\sim$6000--10000 au of the Sun $\sim$2.5 Myrs ago. In such a scenario, our solar system is still undergoing the latter stages of a comet shower. These simulations imply the modern observed LPC flux is $\sim$twice as high as the longer-term (tide-dominated) rate. This also implies that estimates of the Oort cloud's population should be revised downward by a factor of $\sim$2. Our LPC analysis predicts the upcoming Gaia data release will favor an HD 7977 impact parameter of $\sim$6000--10000 au.