A moderately precise dynamical age for the Homunculus of Eta Carinae based on 13 years of HST imaging

Abstract

The Hubble Space Telescope archive contains a large collection of images of η Carinae, and this paper analyses those most suitable for measuring its expanding Homunculus Nebula. Multiple intensity tracings through the Homunculus reveal the fractional increase in the overall size of the nebula; this avoids registration uncertainty, mitigates brightness fluctuations, and is independent of previous methods. Combining a 13 yr baseline of Wide Field Planetary Camera 2 images in the F631N filter, with a 4 yr baseline of Advanced Camera for Surveys/High Resolution Channel images in the F550M filter, yields an ejection date (assuming linear motion) of 1847.1 (±0.8 yr). This result improves the precision, but is in excellent agreement with the previous study by Morse et al., that used a shorter time baseline and a different analysis method. This more precise date is inconsistent with ejection during a periastron passage of the eccentric binary. Ejection occurred well into the main plateau of the Great Eruption, and not during the brief peaks in 1843 and 1838. The age uncertainty is dominated by a real spread in ages of various knots, and by some irregular brightness fluctuations. Several knots appear to have been ejected decades before or after the mean date, implying a complicated history of mass-loss episodes outside the main bright phase of the eruption. The extended history of mass ejection may have been largely erased by the passage of a shock through clumpy ejecta, as most material was swept into a thin shell with nearly uniform apparent age.

Exceptionally fast ejecta seen in light echoes of Eta Carinae’s Great Eruption

“…From the adopted time delay for the echo of ∼160 years, this suggests that the broad emission wings became prominent in the mid-1850s. It is interesting that the emergence of fast material in spectra occurs shortly after the ejection date of 1847.1 (±0.8 yr) deduced from proper motions of the Homunculus (Smith 2017). The broad emission wings are also absent in the extracted spectrum of a field star that was included in the same IMACS slit aperture about an arcminute away from EC2 (Figure 4), confirming that they are not instrumental.…”

Light echoes from the plateau in Eta Carinae’s Great Eruption reveal a two-stage shock-powered event

“…Another interesting outcome, noted in Figure 15, is that the ejection date of the Homunculus from its measured kinematic age is solidly in between Stage 1 and Stage 2. From a recent study of available archival HST imaging over more than a decade, the proper motion expansion of the Homunculus gives a fairly precise date of origin for the Homunculus (extrapolating from linear motion observed today) of 1847.1 ±0.8 yr (Smith 2017). Of course, if there was strong CSM interaction that accelerated slow pre-shock material and decelerated the fast ejecta, as in a Type IIn supernova, the true ejection date might be slightly different (most likely a short time after this).…”

“…In the two-stage merger scenario proposed above, the ejected mass that constitutes the Homunculus Nebula actually leaves the star over a time period of several decades or more. The single, well-constrained apparent ejection date for the Homunculus of 1847.1 ±0.8 yr from proper motions (Smith 2017;Morse et al 2001) arises because a strong shock from an explosive event sweeps up this previously ejected material into a thin cooled shell with a single dynamical age. The shock cooling, which provides the luminosity of the plateau, also allows this swept up material to collapse to a very thin layer as seen today in the walls of the Homunculus (Smith 2006a(Smith , 2013.…”

Imaging the expanding knotty structure in the close environment of the LBV star $η$ Carinae

“…The star is surrounded by a bipolar nebula, known as the "Homunculus nebula", ejected during the 19 th -century Great Eruption (around 1841 Gaviola 1950;Morse et al 2001;Smith 2017). The lobes of this nebula extend about 20 on the sky (4.5 × 10 4 au), and show a complex and highly clumped density structure (Morse et al 1998).…”