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

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.

• The paper analyzes proper motions of Eta Carinae's ejecta using Hubble data, identifying three distinct eruption events around 1250 AD, 1550 AD, and the 1840s.
• The study reveals that older eruptions (1250 and 1550 AD) were less symmetric than the Homunculus Nebula, suggesting complex, potentially one-sided mass loss.
• The findings challenge single-star models, suggesting complex interactions likely drove Eta Carinae's recurrent, episodic eruptions.

Analysis of η Carinae's Historical Eruptions: Proper Motions and Implications

The paper entitled "Ancient eruptions of η Carinae: A tale written in proper motions" published in MNRAS, presents a detailed study of the historical mass-loss events of the star η Carinae. The authors, Megan M. Kiminki, Megan Reiter, and Nathan Smith, utilize eight different epochs of Hubble Space Telescope (HST) imaging to analyze the proper motions of nearly 800 knots in η Carinae's outer ejecta. This study provides a refined understanding of the star's episodic mass-loss history with implications for theoretical models of its behavior.

Key Observations and Findings

The study reveals that η Carinae has undergone several significant eruptions prior to the well-documented Great Eruption in the 1840s. Proper motion analysis shows that the ejecta can be distinctly categorized into three groups, with ejection events dated around 1250 A.D., 1550 A.D., and during or shortly before the 1840s Great Eruption. This chronology suggests that η Carinae experienced major eruptions much earlier than previously thought.

The data highlights a lack of axisymmetry in the older eruptions compared to the Homunculus Nebula, which is known to be highly bipolar. The oldest material from the 1250 event appears particularly asymmetric, or even one-sided, which provides essential constraints for understanding the historical physical mechanisms driving these eruptions.

Implications for η Carinae's Eruptive Mechanisms

The study's results necessitate a re-evaluation of the mechanisms proposed for η Carinae's eruptive behavior. Traditional theories involving single-star models struggle to explain the recurrent, asymmetric, and episodic mass-loss events observed. Thus, binary or hierarchical triple system models have been considered, suggesting dynamic interactions or possible mergers during these historical burst periods.

One significant observation is the absence of large-scale deceleration in outer ejecta. This undermines previous assertions that η Carinae’s soft X-ray emitting shell is driven by slower-moving knots decelerating as they impinge upon circumstellar material. Instead, the soft X-rays are likely produced by fast, rarefied ejecta from the 1840s overtaking older and denser material. This finding reinforces the notion of significant mass ejections at various epochs, each contributing uniquely to the circumstellar environment.

Theoretical and Practical Considerations

The research provides substantive evidence that η Carinae's behavior cannot be simply attributed to isolated outbursts but involves recurrent mechanisms influenced by complex stellar interactions. Such results have profound implications for understanding the life cycles of massive stars and the evolutionary processes of luminous blue variables (LBVs).

Furthermore, these findings imply that models considering η Carinae's future evolution should incorporate multi-epoch ejection events, possibly influenced by stellar dynamics within a binary or triple system.

Prospects for Future Research

Future studies may explore the structures and dynamics of η Carinae’s past eruptions, incorporating more sophisticated models that account for the asymmetric and episodic characteristics observed. Additional high-resolution spectral data may provide a three-dimensional perspective of the ejecta, enriching the understanding of the velocity and density profiles associated with these historical eruptions.

In conclusion, this meticulous examination of η Carinae’s past mass-loss events expands the prevailing knowledge of its eruptive history and presents new challenges for existing stellar evolution theories. It underscores the necessity of considering complex, multi-object interactions in the modelling of massive star behavior and life cycles, contributing valuable insights to the domain of astrophysical research.