Ancient eruptions of Eta Carinae: A tale written in proper motions

TL;DR

This study uses Hubble imaging to trace the proper motions of Eta Carinae's outer ejecta, revealing multiple eruptions over centuries with distinct ages, directions, and dynamics, challenging simple models of its eruptive history.

Contribution

It provides the first detailed proper motion analysis of Eta Carinae's outer ejecta, identifying multiple pre-19th-century eruptions with different geometries and dynamics, constraining eruption mechanisms.

Findings

Ejecta divided into three age groups: ~1250, ~1550, and 1840s.

Ejecta show ballistic expansion with no large-scale deceleration.

Older ejecta predate the Homunculus and suggest episodic mass loss.

Abstract

We analyze eight epochs of Hubble Space Telescope H$\alpha$+[N II] imaging of Eta Carinae's outer ejecta. Proper motions of nearly 800 knots reveal that the detected ejecta are divided into three apparent age groups, dating to around 1250 A.D., to around 1550 A.D., and to during or shortly before the Great Eruption of the 1840s. Ejecta from these groups reside in different locations and provide a firm constraint that Eta Car experienced multiple major eruptions prior to the 19th century. The 1250 and 1550 events did not share the same axisymmetry as the Homunculus; the 1250 event was particularly asymmetric, even one-sided. In addition, the ejecta in the S ridge, which have been associated with the Great Eruption, appear to predate the ejection of the Homunculus by several decades. We detect essentially ballistic expansion across multiple epochs. We find no evidence for large-scale deceleration of the observed knots that could power the soft X-ray shell by plowing into surrounding material, suggesting that the observed X-rays arise instead from fast, rarefied ejecta from the 1840s overtaking the older dense knots. Early deceleration and subsequent coasting cannot explain the origin of the older outer ejecta---significant episodic mass loss prior to the 19th century is required. The timescale and geometry of the past eruptions provide important constraints for any theoretical physical mechanisms driving Eta Car's behavior. Non-repeating mechanisms such as the merger of a close binary in a triple system would require additional complexities to explain the observations.