# Eta Carinae left a curious ladder to climb

Theodore R. Gull Exoplanets & Stellar Astrophysics Laboratory, NASA/Goddard
Space Flight Center, Greenbelt, MD 20771, USA  Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218, US  Henrik Hartman
Materials Science & Applied Mathematics, Malmö University, SE-20506 Malmö,
Sweden  Michael F. Corcoran CRESST & X-ray Astrophysics Laboratory,
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA  The Catholic
University of America, 620 Michigan Ave., N.E. Washington, DC 20064, USA
Augusto Damineli Universidade de S$\tilde{a}$o Paulo, IAG, Rua do
Mat$\tilde{a}$o 1226, Cidade Universit$\acute{a}$ria S$\tilde{a}$o Paulo-SP,
05508-090, Brasil  Thomas Madura Department of Physics & Astronomy, San Jose
State University, One Washington Square, San Jose, CA 95192, USA  Anthony F.
J. Moffat D$\acute{e}$pt. de physique, Univ. de Montr$\acute{e}$al, C.P. 6128,
Succ. C-V, Montr$\acute{e}$al, QC H3C 3J7, Canada  Centre de Recherche en
Astrophysique du Qu$\acute{e}$bec, Canada  Noel D. Richardson Department of
Physics & Astronomy, Embry-Riddle Aeronautical University, 3700 Willow Creek
Rd, Prescott, AZ 86301, USA  Gerd Weigelt Max Planck Institute for Radio
Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany

(Received tbd; Revised tbd; Accepted tbd)

###### Abstract

Eta Carinae underwent the Great Eruption in the 1840s and a Lesser Eruption in
the 1890s. Its apparent spectrum, modified by intervening ejecta, the
Homunculus and Little Homunculus, continues to evolve but contains information
pertaining to events in the 19th century. The LOS spectrum contains narrow
absorption velocities, from $-$122 to $-$1665 km s-1: rungs of a broken ladder
caused by shells formed by the interacting winds. Estimated shell origin dates
correlate with origin dates of expanding emission structures preceding the
Great Eruption. The LOS absorption velocities extend the record post Great
Eruption to the Lesser Eruption. We suggest that these shells originated from
a binary merger within a triple system. Shells formed not only from periastron
passages of the current secondary, but also from ear-like extensions preceding
and following the periastron event. Additional models need to be considered.

stars: massive, Eta Carinae

††journal: ApJ

##  1 Introduction

Eta Carinae ($\eta$ Car) and its ejecta have intrigued astronomers for two
centuries. Observers from several southern stations recorded the apparently
erratic behavior of $\eta$ Argus, as it was known in the early 1800s, but paid
closer attention as it brightened in the 1840s to rival Sirius, then faded to
visual obscurity (Frew, 2004; Smith & Frew, 2011). This event is known as the
Great Eruption. A much fainter but significant brightening, known as the
Lesser Eruption, was recorded in the 1890s.

Ejecta from these two eruptions formed the Homunculus, a dusty, bipolar shell
expanding at $\approx$650 km s-1 in the polar regions (Smith et al., 2003) and
the Little Homunculus, an ionized, bipolar shell nested within the Homunculus,
expanding at $\approx$170 km s-1 (Ishibashi et al., 2003). The total ejected
mass, most of which appears to reside in the skirt region between the two
lobes, is estimated to exceed 45 $M_{\odot}$ (Morris et al., 2017) although
other estimates are less (Smith et al., 2018a; Davidson & Humphreys, 1997).

Today $\eta$ Car is a highly eccentric binary whose period, 5.54 years, is
based upon a nebular high-ionization state ([Ne III], [Fe III], [Ar III], He
I, and He II) modulated by a short, low-ionization state ([Fe II]) (Damineli,
1996; Damineli et al., 1997; Teodoro et al., 2016). The repeating X-ray light
curve confirmed the binary orbit (Corcoran et al., 2001). Pittard & Corcoran
(2002) first estimated the wind properties of both the primary and secondary.
Refinement of the wind parameters with further observations has led to the
following: M${}_{A}\approx$ 100 $M_{\odot}$ (Strawn et al., 2023),
$\dot{M}$${}_{A}=$ 8.5$\times$10-4, $M_{\odot}\,$yr-1(Madura, 2010),
V${}_{tA}=$ 420 km s-1 (Groh et al., 2012) , M${}_{B}>$ 60 $M_{\odot}$ (Strawn
et al., 2023), $\dot{M}$${}_{B}=$ 10-5$M_{\odot}\,$yr-1, V${}_{tB}=$ 3000 km
s-1 (Pittard & Corcoran, 2002).

The visible brightness of $\eta$ Car has increased since the 1940s (Damineli
et al., 2021), but the Homunculus, used as an infrared calorimeter to measure
dust re-emission of absorbed starlight, suggests no change in the binary
luminosity over the past half century (Mehner et al., 2019).

Hillier et al. (2001, 2006) noted that the measured H$\alpha$ equivalent width
in the LOS was twice that measured of the scattered starlight off the
Homunculus. The directly measured H$\alpha$ equivalent width was much weaker
than that expected from CMFGEN modeling. They suggested that the continuum-
emitting core and a portion of the H$\alpha$-emitting, extended wind of $\eta$
Car-A must have been occulted in our LOS.

A logical concept would be that a foreground dusty structure with a well-
defined edge partially obscured $\eta$ Car. The studies of Damineli et al.
(2021) and citations therein show that the H$\alpha$ equivalent width in the
LOS has decreased over the past two decades, approaching the stable equivalent
width measured in foreground Homunculus-scattered starlight. The occulter has
been dissipating over the past few decades.

The half-century infrared stability and the X-ray light curve stability over
the past five cycles (Espinoza-Galeas et al., 2022) led to our understanding
that the binary properties have remained stable in recent decades but the
ejecta in our LOS have been evolving (Gull et al., 2023).

The causes of the two eruptions have been a mystery. The extreme abundance of
nitrogen relative to carbon and oxygen and the classification of $\eta$ Car-A
as a luminous blue variable (LBV) suggested to some that the companion star is
an evolved WN star that might have undergone a near-supernova event, but this
is unlikely as $\eta$ Car-B survived. Certainly the lower limit, $>$ 60
$M_{\odot}$, provides the case for overproduction of nitrogen at the expense
of carbon and oxygen (Ekström et al., 2012)and implies a less-evolved H-rich
WNh star (like the three nearby luminous WNh stars WR22, WR24 and WR25, the
first and third of which are massive binaries - Lenoir-Craig et al. (2022);
Gamen et al. (2006)) instead of an evolved H-poor classical WN star.

The terminal velocity derived from the X-ray spectrum is 3000 km s-1 (Pittard
& Corcoran, 2002). The energetics of the Great Eruption are comparable to a
supernova event but the energetics of the Lesser Eruption is considerably less
(Davidson & Humphreys, 1997). However no definitive observation has
demonstrated that the secondary is a WN star, except possibly for the presence
of a diluted He II $\lambda$4686 emission line outside periastron that moves
in antiphase with the primary LBV (Strawn et al., 2023). He II $\lambda$4686
is the brightest emission-line in WN stars and some extreme, hot Of stars.
Currently no observation has resolved the two stars and determined their
properties independently.

An alternative suggestion is that the two events were the result of a merger
within a triple system that evolved to a double system. Two models of such a
system applied to $\eta$ Car were provided by Portegies Zwart & van den Heuvel
(2016) and Hirai et al. (2021). We focus on the model by Hirai et al. (2021)
and discuss how a series of narrow velocity components, noticed by Gull et al.
(2006), might provide crucial information about the possible merger. In an
analogy of a ladder, these velocity components appear to be rungs that record
changing information about the merger beginning before the Great Eruption,
extending across a chaotic period, and settling across the Lesser Eruption as
the system evolved from a triplet to the present day binary.

Recently, Morse & Smith (2024) measured HST imagery and spectroscopy of the
expanding Homunculus in the light of Mg II $\lambda\lambda$2796, 2803 and
found systematic expansion velocities corresponding to ejection dates of 1847
extending as early as 1760. We will demonstrate that their observations are
compatible with our LOS absorption velocities measured by Gull et al. (2006).

Our effort is to gain insight on the events extending across the nineteenth
century. We have three sets of measurements that we will attempt to associate:
the intermittent visual magnitude estimates by observers during the event, the
expansion of the Homunculus measured from two epochs of direct images with the
Hubble Space Telescope (HST)111Based on archived observations made with the
NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science
Institute, which is operated by the Association of Universities for Research
in Astronomy, Inc., under NASA contract NAS5-26555., and a series of high-
dispersion, NUV spectra of $\eta$ Car containing a plethora of narrow
absorption lines in our LOS.

The visual magnitudes provide a brightness measure of the stellar and nebular
system across the nineteenth century. The direct images enabled an averaged
expansion in the sky plane projecting back to the Great Eruption. The NUV
HST/STIS spectra reveal absorption systems in the LOS pencil beam which extend
out of the sky plane sampling the three-dimensional (3D) Homunculus tilted at
$\approx$ 45o.

We suggest that this ladder of absorption velocities was created in the 19th
century and provides important clues to the physics that led to the two great
eruptions. Unfortunately astronomical spectroscopy did not exist at the time
of the Great Eruption. Only visual photometry was possible. However we have
this record of shell absorption velocities that may provide insight on what
happened with $\eta$ Car and possibly an explanation of the observed
pseudosupernovae seen in distant galaxies.

The observational data recording the three dozen velocity components (Gull et
al., 2006) are summarized in section 2. Linking the velocity components to the
historical events of the nineteenth century and the model of Hirai et al.
(2021) is in section 3. Conclusions and some suggested future observations are
in section 4.

##  2 The observations

Figure 1: Examples of intermediate velocity components that varied with
spectroscopic phase. Top: Mn II $\lambda$2934 weakened during the low-
ionization phase because fewer NUV photons led to excitation of upper energy
levels. Bottom: Ti II $\lambda$3080 strengthened during the minimum as the
Lyman continuum, which ionizes Ti+ to Ti++, originating from $\eta$ Car-B,
dropped across the periastron passage (reproduced from Gull et al. (2006)).

Numerous UV spectra had been previously recorded of $\eta$ Car with the
International Ultraviolet Explorer (IUE) in campaigns aimed at identifying the
complex spectral features and potentially the source of variability. Viotti et
al. (1989) provided an impressive line identification list, given that the IUE
spectral resolving power was R $=$ 10,000 with an oval aperture, 10″ $\times$
20″. The IUE aperture, dependent upon aperture position angle, included
varying contributions of nebular-scattered light from the bipolar 10″ $\times$
18″ Homunculus with expansion velocities in the range of $+/-$650 km s-1
(Smith, 2006). Hence the recorded spectra of $\eta$ Car included velocity-
smearing components leading to considerable confusion in identifying complex
absorptions. Until the mid-2010 era, the nebular-scattered component dominated
the 3″ diameter core, including $\eta$ Car, found by Damineli et al. (2021).

The ten-fold increase in spectral resolving power provided by the HST/STIS
high-resolution NUV222We define the near ultraviolet (NUV) to extend from 2400
to 3100Å, the MUV as 1700 to 2400Å and the far ultraviolet (FUV) from 1200 to
1700Å, consistent with the spectral intervals covered by the STIS. The EUV
extends shortward of 1200Å. echelle mode, the much greater dynamic range of
the NUV Multi-Anode Multi-Array detector (MAMA), and the greatly improved NUV
angular resolution, 0$\farcs$07, of HST, compared to the 10$\arcsec\times\
20\arcsec$ spatial sampling of IUE, led to detection of thousands of low-
ionization, metal absorption lines with multiple velocity components in our
LOS (Gull et al., 2006). Indeed the bulk of the UV scattered light originates
from the 1″ diameter region centered on $\eta$ Car. The velocity dispersion
across that region easily smears out the nebular absorption components. Hence
the diffraction-limited spatial resolution of HST in the NUV is critical to
the study of the LOS absorptions.

NUV HST/STIS echelle spectra, recorded between October 2001 and March 2004, of
$\eta$ Car provided an unexpected super abundance of very narrow absorption
lines on top of an already complex spectrum of broad P Cygni wind line
profiles (Gull et al., 2006). Hundreds of absorption line complexes were
resolved by the HST/STIS capabilities of high resolving power, R
$=\lambda/\delta\lambda=$ 110,000, combined with the high angular resolution,
$\delta\theta=$ 0$\farcs$07 ($=$ 170 au at the adopted distance of 2300
parsecs; the projected binary orbit is thought to be about 30 au, $\approx$ 12
milli-arcseconds).

Far ultraviolet (FUV) spectra recorded in the same time interval with the
HST/STIS using a lower resolving power, R $=$ 46,000, revealed over 800
absorption lines of H2 (Nielsen et al., 2005) all at $-$513 km s-1. Additional
metal absorption lines were identified in the FUV most notably at $-$513 and
$-$146 km s-1. The decreased resolving power, the H2 absorptions, and the
broad wind lines complicated clear separation of the narrow absorption lines,
but still the FUV spectra provided crosschecks on the information in many
cases as especially in the more recent FUV observations recorded with
R=100,000 (Gull et al., 2021, 2022, 2023).

While all of the H2 absorptions originated from the Homunculus at $-$513 km
s-1, the metal absorptions originated from multiple velocity components (Gull
et al., 2005, 2006). The strongest narrow absorption lines at $-$513 km s-1,
were identified with the Homunculus, but many strong, but broader absorption
lines came from a velocity system at $-$146 km s-1, associated with the Little
Homunculus discovered and mapped by Ishibashi (2001).

Table 1: Circumstellar Absorptions ( from Gull et al. (2006))

Velocity | $\delta v^{a}$ | Neutrals and Ions Observed within Velocity Componentb | Inferred Date
---|---|---|---
km s-1 | km s-1 |  | of origin
$-$122c | – | Mg I, Mg II, Cr II, Mn II, Fe II, Ni II | 1897.9
$-$146d | 24 | He I, Mg I, Mg II, Ti II, V II, Cr II, Mn II, Fe II, Co II, Ni II | 1894.8
$-$168c | 22 | Mg I, Mg II, Cr II, Mn II, Fe II, Ni II | 1891.9
$-$185c | 17 | Mg I, Mg II, Cr II, Mn II, Fe II | 1889.7e
$-$323f | 138 | Mg II, Mn II, Fe II | 1871.6
$-$335f | 12 | Mg II, Mn II, Fe II | 1870.0
$-$345f | 10 | Mg II, Mn II, Fe II | 1868.7
$-$359f | 14 | Mg II, Mn II, Fe II | 1866.9
$-$369 | 10 | Mg II, Mn II, Fe II | 1865.5
$-$389 | 20 | He I, Mg II Mg II, Ti II, Mn II, Fe II | 1862.9
$-$398 | 9 | Mg I, Mg II, Mn II, Fe II | 1861.8
$-$412 | 14 | Mg I Mg II, Ti II, Cr II, Mn II, Fe II | 1859.9
$-$436 | 24 | Mg I, Mg II, Ti II, V II, Cr II, Mn II, Fe I, Fe II | 1856.8
$-$441 | 5 | Mg I, Mg II, Ti II, V II, Cr II, Mn II, Fe I, Fe II | 1856.1
$-$457 | 16 | Mg I, Mg II, Ti II, Cr II, Mn II, Fe II | 1854.0
$-$468 | 11 | Mg I, Mg II, Ti II, V II, Cr II, Mn II, Fe I, Fe II | 1852.6
$-$478 | 10 | Mg I, Mg II, Ti II, V II, Cr II, Mn II, Fe I, Fe II | 1851.3
$-$489 | 11 | Mg I, Mg II, Mn II, Fe II | 1849.82
$-$503 | 14 | Mg I, Mg II, Ti II, V II, Cr II, Mn II, Fe I, Fe II | 1848.0
$-$513g | 10 | Na Ih, Mg I, Mg II, Si I, Sc II, Ti II, V II, Cr II, Mn II, Fe I, Fe II, Ni I, H2 | 1846.7i
$-$527 | 14 | Mg I, Mg II, Mn II, Fe II | 1844.8
$-$537 | 10 | Mg II, Mn II, Fe II | 1843.5
$-$553 | 16 | Mg I, Mg II, Mn II, Fe II | 1841.4
$-$561 | 8 | Mg I, Mg II, Mn II, Fe II | 1840.4
$-$572 | 11 | Mg I, Mg II, Mn II, Fe II | 1839.0
$-$587 | 15 | Mg I, Mg II, Mn II, Fe II | 1837.0
$-$604 | 17 | Mg II | 1834.8
$-$696 | 92 | Mg II | 1822.7
$-$705 | 9 | Mg II | 1821.5
$-$712 | 7 | Mg II | 1819.4
$-$732 | 20 | Mg II | 1818.0
$-$833 | 101 | Mg II | 1804.7
$-$1038j | 205 | Mg II | 1777.9
$-$1050j | 12 | Mg II | 1776.3
$-$1064j | 14 | Mg II | 1774.5
$-$1074j | 10 | Mg II | 1773.2
$-$1640k | 566 | C IV, Si IV, Al III | 1699.0
$-$1665k | 15 | C IV, Si IV, Al III | 1695.7

a Increment in velocity relative to the previous entry. b Two or more lines
led to velocity component identification (Gull et al., 2006). Typical
absorption widths were 2.2 km s-1 for $-$513 km s-1 and 5.5 km s-1 for $-$146
km s-1. Absorption widths for all velocity components more negative than
$-$300 km s-1 were 2.2 km s-1. cIdentified during the 2003.5 minimum spectra
only. d Unique Little Homunculus velocity component that is well isolated in
velocity. e Based upon photometric increase and plateau (Smith & Frew, 2011).
f Identified in 2004 March 4 postrecovery spectrum only. g Unique Homunculus
velocity component that is well isolated and has many absorption lines,
including H2 in FUV spectra (Nielsen et al., 2005). h Single line
identification, confirmed by transitions at VLT UVES optical wavelengths. i
Based upon Homunculus expansion (Smith, 2017). j Confirmed in Si II, Si IV, Al
II, and Al III in the MUV spectrum Nielsen et al. (2005). k Two additional
absorption velocities identified by Nielsen et al. (2005)

The bulk of the absorption lines came from additional, weaker velocity
systems. A total of 38 velocity components were identified ranging from $-$122
to $-$1665 km s-1 (Table 1). The measured velocities are spaced at intervals
varying from 5 to 24 km s-1 but with several significant, larger gaps.

Numerous examples of multi-velocity, absorption profiles are presented in
Nielsen et al. (2005); Gull et al. (2006). Strengths of the velocity
components were found to vary within the 5.54-year binary orbital phase
primarily affected by changes in the FUV and EUV fluxes as the interacting
winds shift from the high- to low-ionization states.

Strong absorptions of Mn II $\lambda$2934 weakened during the low-ionization
phase as NUV photons leading to excitation of upper energy levels decreased
(Figure 1, Top).

Strong absorptions of the Ti II $\lambda$3080 at or near $-$146 km s-1 briefly
appeared during the months-long low ionization state across periastron, but
were absent across the high ionization state which extends across five years
of the binary period (Figure 1, Bottom). Ti+, with its ionization potential,
13.58 eV, just below the ionization potential of hydrogen, indicates the
presence or absence of UV photons shortward of the Lyman continuum. Strong
variations in the four velocity systems between $-$122 and $-$184 km s-1
occurred between the high- and low-ionization states. Similar variations were
recorded in more recent, ground-based studies of Na D absorption over the last
three orbital cycles (Pickett et al., 2022).

Absorption velocities identified between $-$603 and $-$527 km s-1 required
considerable effort as the very narrow features are superimposed upon the
rising edge of a broad absorption. Examples are absorptions arising from Fe II
transitions with lower energy levels about 1 eV above the ground state.
Patterns of very sharp (FWHM $\approx$ 2 to 5 km s-1) absorptions with
velocities more negative than $-$513 km s-1 were repeatedly identified by Gull
et al. (2006) in the multiple echelle spectra displayed in their Figure 8.
Indeed the strengths of these transitions prove to be so strong that broad,
totally saturated absorptions exist between $-$513 and $-$146 km s-1.

No single absorption line provides measurable profiles for all velocities.
Less abundant metals such as titanium provide transitions with moderate
f-values leading to unsaturated velocity profiles for systems with velocities
less negative than $-$513km s-1. The much more abundant iron in singly-ionized
state has transitions with strong f-values around 2500Å that lead to totally
saturated profiles ranging from $-$513 to $-$380 km s-1 and $-$210 to $-$100
km s-1(see the many examples in Figure set 9 in Gull et al. (2006))

Two additional high-velocity absorption components, $-$1640 and $-$1665 km s-1
were noted in the MUV by Nielsen et al. (2005). The rest of the absorption
components are at $-$1074 or closer to the velocity of $\eta$ Car and
occurring at much more frequent intervals. Likely these two absorption
velocities are associated with the high velocity emission structures noted by
Kiminki et al. (2016) and the ghost nebula originally identified by Currie et
al. (2002).

##  3 Discussion

Gull et al. (2005) analyzed absorption profiles of singly-ionized metals in
the two strongest velocity components, $-$513 and $-$146 km s-1. They found
the lower levels from which the absorptions originated, were thermally
populated. Across the high-ionization state, the $-$146 km s-1 system metals
had a kinetic temperature of 6400 K while the kinetic temperature of the
$-$513 km s-1 system was 760 K. These temperatures infer that, as expected,
the higher-velocity structures are thermally cooler at greater distances.

Many atomic and ionic species, identified in the NUV spectra as noted in Table
1, were identified in the low-velocity components. The number of species
decreased with more negative velocities with the noticeable exception of
$-$513 km s-1 and nearby velocities. At most velocities more negative than
$-$604 km s-1, only absorptions in Mg II were identified. Six velocity
components at or more negative than $-$1038 km s-1are exceptions as they are
seen in multiply-ionized states despite being more distant.

Considerable changes occurred between the high-ionization and the low-
ionization states (Gull et al., 2006). The kinetic temperature of the $-$146
km s-1 metals dropped from 6400 K to 5000 K from the high- to low-ionization
state in late 2003, then increased by March 2004. While change in the kinetic
temperature of the $-$513 km s-1 shell was not noticeable, virtually all of
the H2 absorptions disappeared across the low-ionization state due to the
significant drop in the STIS FUV flux (Gull et al., 2022). Little or no
radiation shortward of Ly$\alpha$ was present. Hence the H2 relaxed to ground
state and H2 destruction temporarily decreased.

These velocity systems do not originate from individual clumps in our LOS, but
from spatially-extended, clumpy shells as demonstrated by Gull et al. (2023).
Long slit spectra with the HST/STIS in the NUV, with the G230MB grating and
CCD detector (R=10,000), were recorded during the high-ionization states in
cycles 10 and 13. While not at the resolving power of 100,000, the spatially
resolved (0$\farcs$1 limited by the CCD format) spectra demonstrated that 1)
the merged velocity systems extended across the nebula, centered on the
position of $\eta$ Car, and 2) many velocity components disappeared due to the
ten-fold increase of FUV flux in the high states between cycles 10 and 13.

The velocity systems, while clumpy in character as noted by the changes in
equivalent widths across the high-ionization state of cycle 10, form shells
extending across significant portions of the foreground Homunculus. Hereon we
will address these absorption systems as shells formed across multiple
periastron passages.

This series of multiple absorption velocities, effectively form a ladder with
rungs defined by individual absorption velocities. This velocity ladder
records wind velocities that decreased across a very extended time interval,
namely across the nineteenth century and offers clues to understanding the
phenomena that caused the two major outbursts.

In the following subsections, we will now build the case that associates the
absorption velocities with shells. We initially sketch how the shells formed
(subsection 3.1) and suggest a mechanism that led to the narrowing of
individual velocity profiles (subsection 3.2). In subsection 3.3 we describe
how the shells have evolved over the past two decades, which provides insight
on how the shells formed and survived throughout the nineteenth and twentieth
centuries. We provide connection to the Great and Lesser Eruptions, which then
leads to possible dates when the individual shells formed (subsection 3.4). In
subsection 3.5 we try to establish an orbital period between the two
eruptions. Subsection 3.6 mentions the possibility that the different
absorption velocities might have been formed by a few discrete ejections.
Subsection 3.7 adjusts the velocities to form a linear change in velocity with
time. Subsection 3.8 considers individual wind flows seen in other OB stars.
We finally discuss in subsection 3.9 how the velocity ladder may follow a
merger scenario which appears to be the more appealing explanation but still
has shortcomings .

###  3.1  How did the shells form and how did they survive until very
recently?

Colliding winds of massive binaries can lead to continuously-formed structures
in the form of equidistant, expanding shells. These winds are modulated by the
physical conditions and changes thereof. The very noticeable spiral structures
surrounding WR140 are the result of colliding winds leading to dust formation
under optimal physical conditions (Lau et al., 2022). Nearly eighteen nested
spiral shells of dust were imaged expanding at uniform velocity indicating the
colliding winds have been stable across eighteen orbits of the binary WR140.
At specific phases of the highly-ellipitical (like $\eta$ Car with e $=$ 0.9)
binary orbit, dust formed in abundance.

Similar evidence of stable colliding winds from the current binary of $\eta$
Car was found by Teodoro et al. (2013) in the form of forbidden emission from
three nested hemispherical shells located within 0$\farcs$5 (1150 au) of
$\eta$ Car. From imagery in [Fe II] and [Ni II] mapped at intervals from 2009
to 2013, they found the shells were expanding at 470 km s-1, slightly more
than the terminal velocity of $\eta$ Car-A, 420 km s-1, previously determined
by Groh et al. (2012). The much faster, lower density, secondary wind collides
with the primary wind which is compressed and accelerated from 420 to 470 km
s-1.

3D hydrodynamical models suggest that these shells were formed within the
primary wind as the secondary with its wind passed through each periastron
passage, thus compressing a portion of the primary wind into thin, outwardly-
moving shells modulated by the low-density, fast secondary wind (Madura &
Groh, 2012; Madura et al., 2013).

Additional shells, expanding at 470 km s-1, were not detected at greater
distances. Slow-moving, much more-massive ejecta, exemplified by the Weigelt
clumps B, C and D (Weigelt & Ebersberger, 1986; Weigelt & Kraus, 2012)
provided a barrier expanding at $-$40 km s-1.

Apparently a dusty occulter, now nearly dissipated (Damineli et al., 2023;
Gull et al., 2023), is part of an extensive slow-moving structure that has
protected the multiple shells originating in the nineteenth century from
before the Great Eruption to shortly after the Lesser Eruption. Multiple
structures, seen in forbidden emission, including the Weigelt clumps, with
expansion velocities less than 100 km s-1, were mapped by Gull et al. (2016)
within the central arcsecond surrounding $\eta$ Car.

Much of the ejecta was compressed. The gases cooled, then formed molecules and
dust. Due to the paucity of carbon and oxygen, unusual dust formed, possibly
influenced by nitrogen-dominated chemistry, leaving considerable metals in
atomic or ionic state. Detection of Sr II, Sc II and V II, both in absorption
and forbidden emission in ejecta close to $\eta$ Car is unique (Hartman et
al., 2004, 2005). No previous records of absorption lines from these ions had
been identified in the ISM. Thus the carbon- and oxygen-deprived chemistry of
the dust formed by $\eta$ Car appears to be unique.

Over the past century, multiple shells, separated by ever-decreasing velocity
intervals, have been expanding outward from $\eta$ Car. The molecules and
dust, within the slow-moving ejecta close to $\eta$ Car, gradually are being
destroyed leading to increasing amounts of ionizing radiation. The neutral and
singly-ionized gas shells have now become doubly-ionized shells with
absorptions well into the UV beyond Ly$\alpha$. An extreme example is the
nearly complete destruction of the $-$513 km s-1 H2 between 2004 and 2016 in
the Homunculus (Gull et al., 2022).

In the long term, the Homunculus will continue to expand, the dust and
molecules in the shells will be destroyed and the multiply-ionized shells will
increasingly become invisible to the observer because most resonant
transitions of multiply-ionized metals are located below Ly$\alpha$. The high
dispersion UV observations of the ejecta over the past two decades record
major changes of the ionization state of the Homunculus in our LOS.

The observational records show that $\eta$ Car faded to seventh visual
magnitude in the late 1890s through the late 1930s, then began to gradually
brighten (Smith & Frew, 2011). Today, $\eta$ Car is again a naked eye object
recently approaching fourth magnitude (Damineli et al., 2019). While some
suggest that the 1940s brightening was yet another event (Abraham et al.,
2014), most are convinced that this was the result of gradual dissipation of
the Homunculus originating from the nineteenth century and continues through
the present (Smith et al., 2018b; Damineli et al., 2023; Gull et al., 2023).

Morris et al. (2017) found that the peak infrared emission originates from a
region about two arcseconds north of $\eta$ Car. They estimated the total mass
inferred from this dust emission to exceed 40 $M_{\odot}$. Such is consistent
with most of the slowly-moving ejecta flowing outward on the far side of the
binary orbit in the general direction of the current periastron as seen by the
observer.

###  3.2 A mechanism leading to narrowing of the absorptions

The result of colliding winds is a velocity distribution about a momentum-
balanced velocity broadened by thermal and turbulent contributions. Yet the
absorption components in the ladder are extremely narrow compared to the wind
absorption profiles (absorptions extending to $-$500 km s-1): shells with the
LOS velocities more negative than $-$383 km s-1 are narrower, fwhm at 2.2 km
s-1, than the $-$183 to $-$136 km s-1 components associated with the Lesser
Eruption, fwhm at 5 km s-1(Gull et al., 2006).

How did the individual absorption profiles narrow in width? Some insight comes
from comparing the multiple shells around WR140 and the ladder shells. As the
binary orbital plane approaches face-on (inclination $=$ 119o (Thomas et al.,
2021)), the dust spiral around WR140 appears to be uniformly spaced (Lau et
al., 2022), indicating that the balance of the colliding winds and the
ionizing radiation has been constant for the last 18 orbits. This in turn
suggests that the physical conditions leading to the dust formation in WR140
have not varied significantly over those many orbits, which exceeds a century
in time.

In contrast, the absorption ladder detected in the LOS of $\eta$ Car is a
record of a long-term decrease in wind velocity over many cycles. Over that
long term, likely $\eta$ Car-B, given that WNh stars have a long lifetime
($\approx$ 106 years), had a constant wind over the past few centuries.

Today, the wind of $\eta$ Car-A is very extended. Teodoro et al. (2016) mapped
three hemispherical shells in [Fe II] and [Ni II] that by position and
velocity were shown to be formed over the three low-ionization states when
$\eta$ Car-B plunged into the extended primary wind across the periastron
passage. For a few months the primary wind flowed into the surrounding volume
while $\eta$ Car-B formed a highly-ionized volume separating a portion of the
primary wind forming into a partial shell. When $\eta$ Car-B emerged from the
primary wind, its faster, less -massive wind cleared out a complimentary
hemisphere. The compressed, thin shell, highly-ionized by $\eta$ Car-B flowed
outward, analogous to the highly-ionized wind shells of WR140. However, as
noted above, only three shells survive before being absorbed by slower moving
clumps beyond (Teodoro et al., 2016; Gull et al., 2023).

Narrowing of wind absorption lines, generated by gas flowing from a bright
spot, has been demonstrated repeatedly for OB stars as done by Kaper et al.
(1997) in monitoring programs with the IUE. A bright spot, originating as a
perturbation, causes a faster flow than the rest of the wind initially, that
turns into a slower flow as the star rotates and the cooler stellar surface
under it can no longer support such a fast flow, resulting in a broad
absorption in the P Cygni profile. Extreme ultraviolet radiation (EUV)
accelerates the highly-ionized, absorbing gas. A few strong FUV resonant or
near-resonant absorptions in Si IV or C IV appear at low, broad velocities,
then accelerate to higher velocities, narrowing in dispersion and terminating
near the maximum (terminal) velocity possibly limited by velocity crowding.

Sufficient EUV originates from most OB stars to multiply ionize the escaping
wind and by momentum transfer accelerates slower moving ions up to a terminal
velocity where the radiation is totally absorbed.

The current colliding winds of $\eta$ Car create two very different structures
dependent upon the binary separation. For most of the orbit, the EUV radiation
of $\eta$ Car-B multiply-ionizes and compresses the wind of $\eta$ Car-A which
results in a highly-ionized, low-density set of shells not readily detected.
Across periastron, $\eta$ Car-B dives into the extended atmosphere of $\eta$
Car-A which results in a highly-compressed, high density shell.

The greatly extended envelope of $\eta$ Car-A absorbs most of the EUV from
$\eta$ Car-B with the result of a compressed shell with metals mostly in
singly-ionized state. Three shells, seen in [Fe II] and [Ni II] were tracked
by Teodoro et al. (2016) and shown to originate from the three previous
periastron passages.

Today, the EUV, FUV originating from the secondary companion, assumed not to
have changed very much since the 19th century, ionizes most of the gas from
each cycle outward to the dissipating, occulting structure, thought to be
located less that 2000 AU from $\eta$ Car (Gull et al., 2023). Beyond the
occulter, resonant and near-resonant transitions of neutral and singly-ionized
metals are much more numerous throughout the MUV and FUV compared to the EUV.
The observed MUV spectrum of $\eta$ Car is almost a desert due to saturated
metal absorptions by the multiple shells. The NUV, where fewer, weaker
resonant transitions reside, repeatedly show patterns of narrow-line
absorptions for individual resonant lines, defined by velocities of the
individual shells.

Each absorbed photon leads to incremental momentum transfer. The accumulation
of hundreds of resonant and near-resonant transitions in the MUV and NUV
narrows the absorption profile at a terminal velocity, usually somewhat larger
than the initial terminal velocity of the material leaving the extended
envelope. The net result is a narrowed absorption profile for each individual
shell. Note that absorptions from the four shells with least negative
velocities (closest to $\eta$ Car) have significantly broader absorption
profiles. Their large column densities and higher kinetic temperatures have
led to absorption profiles that have narrowed less that the older, more
distant shells with more negative velocities.

The EUV originating from $\eta$ Car in the nineteenth century would have been
even more strongly absorbed by the greatly extended envelope due to the Great
Eruption. The envelope surrounding the merging binary would have puffed up
considerable, possibly with decreasing terminal velocity over the merger
event. Observers commented that $\eta$ Car had a very red color (Smith, 2006;
Smith & Frew, 2011). The light echo spectrum, that has been tracked moving
across relatively distant reflection nebulae, appears to be close to that of
an F star (Rest et al., 2012). The primary absorbers within the shells would
be neutral and singly-ionized metals, not ionized hydrogen and helium nor
multiply-ionized metals.

With the EUV depleted, the metals in the shells would be in neutral and
singly-ionized states. Hundreds of resonant and near-resonant lines provide
considerable momentum transfer greatly narrowing the velocity dispersions of
each shell. Radiation transfer by photons, long ward of Lyman continuum, to
the metals would be strongest in the most recent shell, accelerating the gas
to increased velocity, but scaled by d-2 and most likely limited by a
decreasing terminal velocity.

Hence the shell would be accelerated to a maximum velocity very quickly. The
width of the velocity absorptions is systematically broader for the lowest
four velocity components associated with the Little Homunculus which suggests
either that the gas column density was quite large or more likely the FUV/NUV
flux was blocked by material moving more slowly, i.e. structures including the
Weigelt clumps.

###  3.3 Changes across the past two decades

The inventory of these absorption velocities was accomplished with HST/STIS
echelle spectra recorded in the NUV between 2000 and 2004 (Gull et al., 2006).
Variations in the strength of the absorptions increased notably as the binary
moved from the high-ionization state to the low-ionization state as the FUV
(and implied EUV) flux, originating from $\eta$ Car-B, dropped across the
periastron passage while the companion was immersed within the extended
envelope of $\eta$ Car-A for a few months (Gull et al., 2023).

Greater changes occurred between 2000 and 2018 as the apparent FUV flux seen
by the shells increased nearly ten-fold due to the gradual dissipation of a
dusty occulter located close to $\eta$ Car (Damineli et al., 2023; Gull et
al., 2021, 2023). The increasing FUV radiation (as a proxy to the EUV below
Ly$\alpha$) led to increasing ionization starting with the shells closest to
$\eta$ Car, extending increasingly to the more distant shells. Virtually all
neutral and singly-ionized absorption lines disappeared in the FUV (Gull et
al., 2021). Only moderate, long slit spectra (R=10,000) were obtained in the
NUV since 2004. These spatially-resolved spectro-images showed that the line
absorptions decreased in strength in our LOS and extended across a portion of
the foreground Homunculus lobe (Gull et al., 2023).

Hillier et al. (2001, 2003) and Damineli et al. (2023) hypothesized, then
demonstrated that the increase in flux extending from the visible to the FUV
is caused by a dissipating occulter in our LOS. Damineli et al. (2023)
predicted that by the 2030s, the effects of this occulter will have completely
disappeared, allowing the full EUV flux to ionize most of the material within
the Homunculus close to our LOS. Fortunately high-dispersion, STIS
observations were obtained in the FUV. In the 1200 to 1700Å spectra interval,
most signatures of the shells, seen in absorptions of singly-ionized metals,
disappeared in the FUV and nearly all of the H2 at $-$513 km s-1was destroyed
(Gull et al., 2021, 2023).

###  3.4 Associating velocity components with the Great and Lesser Eruptions

The measured velocities listed in Table 1 are linear sequences separated by
significant gaps. We now estimate dates of origin by associating the two most
significant absorption systems, $-$513 and $-$146 km s-1, with the two
historical events: the Great Eruption and the Lesser Eruption.

Smith (2017) measured a date of origin, 1847.1 +/- 0.8 years, for the
expanding Homunculus using a 13-year baseline with HST/WFPC2 images through
the F631 filter and a 4-year baseline with HST/ACS/HRC images through the 550M
filter. This correlates well with visual photometry recorded across the Great
Eruption (Smith & Frew, 2011). The $-$513 km s-1 absorption system is by far
the strongest, best-defined shell. Nielsen et al. (2005) identified over 800
strong absorption lines of H2 in HST/STIS FUV spectra. Gull et al. (2006)
identified hundreds of absorption lines of singly-ionized metals at this
radial velocity. We therefore associate the $-$513 km s-1 absorption system
with the expansion date of ejection, 1847.1 +/- 0.8 years.

The importance of H2 as a tracer of the Great Eruption is demonstrated by the
3D mapping of the H${}_{2}~{}\lambda 2.12\mu$ emission line (see Figure 4 in
Steffen et al. (2014)). The H2 emission forms a well-defined bipolar
structure. The massive ejection associated with the Great Eruption led to
considerable gas that compressed to conditions ideal to the formation of H2 in
all directions forming the bi-lobed structure.

Assigning an ejection date to absorption velocities of the Lesser Eruption is
more challenging. Ishibashi et al. (2003) discovered the Little Homunculus, a
bi-lobed, ionized shell internal to the Homunculus, expanding at 170 km s-1.
They measured the positions of forbidden line emission knots and arrived at
two estimates of origin: 1897 +/- 10 and 1904 +/- 10 years. In our LOS, this
structure is associated with $-$146 km s-1 absorption.

The ejection of the slow-moving, emission-line Weigelt-clumps – B, C, and D –
(Weigelt & Ebersberger, 1986; Hofmann & Weigelt, 1988) is associated with the
Lesser Eruption. Several independent measures of the Weigelt clumps are
summarized by Weigelt & Kraus (2012). They found the more reliable measure to
be 1880 +/- 20 years. Artigau et al. (2011) examined the many emission line
clumps that trace an outline of a butterfly surrounding $\eta$ Car. Dates of
origin for these clumps, based upon position and velocity, clustered around
the dates of the Great Eruption and the Lesser Eruption. Proper motion
measures simply do not sufficiently tie down the Lesser Eruption.

The visual photometry, recorded in the nineteenth century as summarized by
Smith & Frew (2011), indicates that the Lesser Eruption started with a rise in
flux starting around 1887.2 with a plateau that extended to 1896, then a
decline that extended well into the twentieth century. Assuming that the
current binary orbit has not changed since that the 1890s, Smith & Frew (2011)
projected that the current binary period, 5.54 years, phased with the most
current periastron, back to 1887.2, coincides with the photometric trigger
point.

Implicit in this discussion is that the wind of the companion star modulates
the wind(s) of the primary (merging) star across the nineteenth century. An
individual shell, emerging in our LOS, formed across each periastron event
leading to an absorption velocity. The measured velocities – rungs of the
ladder – are listed in Table 1.

The multiple absorption velocities record changes in the wind velocity that
decreases over the long term. The train of decreasing velocities is evidence
that the terminal velocity is dropping with time. We likely would not see a
train of increasing velocity absorptions as subsequent shells with increasing
terminal velocities would overtake previous shells, given a sufficiently-long
orbit. We posit that the multiple velocity absorptions are caused by passages
of the current companion star through a complex, evolving envelope at various
stages of the merger.

Figure 2: Observed absorption velocities plotted against estimated year of
ejection based upon best information on historical observations of the Great
and Lesser Eruptions. The reference dates for the x-axis are 1847.1 +/- 0.8
based upon the expansion rate of the Homunculus using HST imagery (Smith,
2017). In our LOS, the very strong H2 and metal absorptions at $-$513 km s-1
corespond to this structure. The photometric rise of the Lesser Eruption was
pegged at 1887.2 with a plateau extending through 1896 (Smith & Frew, 2011).
With the assumption of a linear period between the 1847.1 and 1887.2 events,
the absorption velocities are plotted against implied date. Note the apparent
similarities of slope across the nineteenth century with major
discontinuities. What is this telling us about the possible merger?

The four lowest absorption velocities listed in Table 1, $-$122, $-$146,
$-$168 and $-$185 km s-1, are separated by a large gap in velocity from the
previous train of absorption velocities that end abruptly at $-$323 km s-1.
The LOS $-$146 km s-1 absorption velocity corresponds to the $-$170 km s-1
expansion velocity of the 3D Little Homunculus. These four velocities sample
changes in the combined wind velocity across the Lesser Eruption. The
photometric rise and plateau began in 1887.2 and extended to 1896 (Smith &
Frew, 2011). We assign the year 1887.2 to the date of origin of the earliest
absorption velocity, $-$185 km s-1. Velocity changes across the plateau are
sampled by $-$168, $-$146 and $-$122 km s-1 absorptions. These four shell
velocities track changes across the Lesser Eruption beginning after the major
discontuity from $-$323 km s-1 to $-$186 km s-1 at the beginning of the event.
A final discontinuity drops from $-$126 to $-$40 km s-1, the velocity of very
slow-moving ejecta, notably the Weigelt clumps (Gull et al., 2016). The
overall system velocity, $-$8.1 km s-1, was measured of the nebular emissions
of the H2 and [Fe II] in the infrared (Smith, 2004).

The orbital parameters of the binary (Teodoro et al., 2020) projects back to a
periastron that occurred across 1887.2 $+/-$ 0.1 years which correlates well
with the date of the photometric rise, 1887.2 (Smith & Frew, 2011).

###  3.5 An attempt to establish a binary period between the two eruptions

Fifteen absorption velocities were identified between the $-$513 and $-$146 km
s-1 absorption velocities. An attempt to estimate a period would suggest a
shell absorption event that may have occurred on average every 2.51 $+/-$ 0.10
years, which is less than half of the current binary period.

A first thought was to assume that the absorptions are periodic in time. Hence
we apply an average interval, 2.51 years, to estimate the dates when each
shell came into our LOS.

The absorption velocities in our LOS (Table 1) are plotted against implied
year of absorption in Figure 2. The velocities bunch into four linear segments
separated by large discontinuities. The first three groups, $-$1074 to $-$1038
km s-1, $-$732 to $-$696 km s-1, and $-$604 to $-$323 km s-1, have very
similar slopes. The similar slopes do not tie down the 2.51-year interval but
suggest that decrease in velocity appears to be similar for for lengthy
periods. Some absorptions may have been missed, or simply are not present.
More likely there was an event in the system that caused a sudden drop in wind
velocity. The remaining group, $-$182 to $-$122 km s-1, has a noticeably
shallower slope which may indicate an even slower decrease in velocity during
or after the Lesser Eruption.

The behavior of the changing velocities displayed in Figure 2 is very puzzling
and appears non-physical. Why would the velocity increments change in a linear
manner from well before the Great Eruption to just before the Lesser Eruption,
then shift abruptly just as there is a photometric change at the beginning of
the Lesser Eruption?

The assumption that each absorption velocity originates only from the
periastron passage does not make sense. The bulk of the ejecta originated in
the Great Eruption, not the Lesser Eruption. Any change in orbital period
would likely occur during the merger, thought to occur during the Great
Eruption. The jump from $-$323 to $-$186 km s-1 is not due to a change in
orbital period.

Why are there linear segments of decreasing velocities with large jumps
between them? An alternate explanation with some physical understanding is
needed.

We conclude this discussion that an orbital period close to 5.5 years is much
more likely than 2.5 years.

###  3.6 Absorptions created by multiple clumps or shells in a few ejections

The multiple absorption velocities might have been caused by multiple clumps
of varying opacity and ejection velocity originating from a single event. As
presented in Gull et al. (2023), the spatially extended absorptions recorded
as HST/STIS CCD spectro-images with resolving power, R $=$ 10,000 and angular
resolution, 0$\farcs$1, extended spatially across the position of $\eta$ Car,
well across a few arcseconds of the Homunculus. We interpret these absorptions
to be from extended shells, not individual clumps.

Additionally, it is difficult to understand the nearly linear spacing in the
velocity shifts. A more acceptable interpretation is based upon what we see
today, namely that the current hot secondary and its massive wind dives deeply
into the extended primary wind across periastron. Analogously, in a triple
star system merging into a double stellar system, the more distant star
encounters portions of the distorted, merging envelope each binary period.

We suggest a more physical solution is necessary.

###  3.7 Aligning the linear segments

Figure 3: Absorption velocities now plotted by shifting the segments to form a
continuous line. Each of the four groups of velocities are shifted
independently. The dates of origin now extend to the 1770s which are similar
to dates of origin measured of Mg II emission clumps external to the
Homunculus based upon two epochs of HST/ACS images (Morse & Smith, 2024).

Our LOS passes through the wall of the Homunculus well away from its axis of
symmetry. Madura (2010); Madura et al. (2012) modeled the colliding wind
structures to interpret high-spatial-resolution mappings recorded by HST/STIS
of forbidden emissions projected within an arcsecond of $\eta$ Car. They
determined that the axis of the binary orbital plane was closely aligned to
the Homunculus axis of symmetry and that the periastron passage of $\eta$
Car-B was on the far side of $\eta$ Car-A.

We consider an alternate approach to understand the velocity ladder. Perhaps
the large changes in velocities between the linear stretches are due to no
shells in our LOS for a period of time. The Homunculus has a large,
continuous, well-defined bipolar structure as mapped by the H2 3D structure
(Steffen et al., 2014). We know that the bulk of the dust appears to be in the
orbital plane on the far side of $\eta$ Car (Morris et al., 2017, 2020), where
also a large structure seen in NH is present as mapped by the Herschel
Observatory (Gull et al., 2020). The equivalent widths of absorption profiles
decrease from that of $-$513 to $-$323 km s-1 then disappear until an
absorption appears at $-$186 km s-1 as shown in Figure 1 and multiple examples
shown in (Gull et al., 2006).

The velocity increment appears to be continuous from well before the Great
Eruption to well after the Great Eruption. Hence we assume that the wind
velocities generated by the system across the Great Eruption was a continuous
mass loss process. Despite the huge increase in mass loss at one specific
time, the wind velocities changed continuously.

We use the segmented, linear behavior of the terminal winds and shift the
segments to align with a constant slope as displayed in Figure 3. The aligned
segments suggest that shells formed in our LOS as early as the 1760s, that
there was a wind velocity that systematically decreased across nearly 130
years, but that there were intervals where shells did not extend across our
LOS. This may agree with the highly variable equivalent widths of the
absorption profiles seen across the high- and low-ionization states. Such
might be the result of the mass in the winds decreasing or the orientation of
the colliding winds changing with time.

The ladder velocities are from only one narrow sample through the ejecta
(Figure 3) and show similar dates of origin of shells, 1760 to 1847, of shells
with velocities more negative than $-$ 513 km s-1. The less negative
velocities originate from shells within the Homunculus, thus extending the
record of the wind structures associated with the Great Eruption to dates
closer to the Lesser Eruption.

The several gaps in absorption velocities provide clues to the shapes of the
individual shells. Likely the shells are densest in the orbital plane, but may
drop off well before the polar region. Two effects may contribute to the lack
of shells at large angles out of the plane: less material flows towards the
orbital pole and more EUV radiation escapes to the orbital pole. The latter
would lead to far less momentum transfer as very few resonant transitions
occur in the EUV compared to the NUV. Hence wind flows toward the polar
regions would not evolve narrow absorption profiles.

If the shells formed from a binary wind interaction, as shells appear to form
in recent times (Teodoro et al., 2016), then we would expect to see eight
absorption velocities evenly distributed between the $-$513 and the $-$185 km
s-1 absorptions. Fifteen were identified plus a large gap of 138 km s-1 just
before the $-$185 km s-1 absorption identified with the beginning of the
Lesser Eruption.

Likewise, two gaps in velocity occur between $-$ 684 and $-$ 604 km s-1 and
between $-$1038 and $-$832 km s-1 (ignoring the $-$833 km s-1 absorption).
Apparently something led to shells not being in our LOS before the advent of
the Great Eruption.

We also note that the equivalent widths of the absorption velocity components
decrease from $-$513 to $-$323 km s-1 which would be expected if the column
densities of the shells decrease with time.

Figure 4: Frequency of velocities extending from before the Great Eruption to
afterwards. The two most populated velocities, 10 and 14 km s-1 may be
modulated substructure of the merger. Estimated errors of measure are one to
two km s-1.

In an attempt to look for periodicity, we examined the frequency of velocity
shifts from $-$1074 to $-$323 km s-1 as displayed in Figure 4. The two most
frequent changes in velocity were 10 and 14 km s-1, but there is a large
dispersion of very low and very high velocity changes, ignoring the large
discontinuities mentioned above.

Recently Morse & Smith (2024) presented expansion measures of outer ejecta
using HST/WFC3 images of Mg II $\lambda$2800 recorded in 2018 and 2020. They
found about half of the clumps moved at transverse velocities consistent with
the 1847.1 origin of the bipolar Homunculus as previously measured by Smith
(2017). The structures measured by Morse & Smith (2024) are part of or
external to the Homunculus. The transverse velocities plotted in their Figure
2 demonstrate a certain symmetry of the SE and NW lobes. The other half,
located outward of the Homunculus, had positions and transverse velocities
consistent of origins distributed before 1847 as early as 1760. Such suggests
that the merger extended over many decades, not as a singular event.

The dates of origin, inferred from the linear change in velocity with time as
plotted in Figure 3, are listed in Table 1, right hand column. Notably the
dates of origin extend from 1847.1 to the 1770’s, which trends similar to the
dates of origin of the emission structures measure by Morse & Smith (2024).

###  3.8 Could the shells be due to DACs?

Discrete absorption components, or DACs, are seemingly ubiquitous in massive
hot stars (Howarth & Prinja, 1989). These are the observed effects of co-
rotating interaction regions, or CIRs, in the winds as modeled by Cranmer &
Owocki (1996). These are often seen in ultraviolet resonance lines for main-
sequence O stars, as well as sometimes seen in evolved massive stars. The DACs
are seen to start at low velocities and propagate to the terminal wind speed
fairly broad at first, becoming ever narrower as Vinf is approached. Time-
series of ultraviolet wind lines of luminous blue variables are not as
frequently seen as those of other hot stars, owing in large part to their
higher amount of interstellar extinction than many OB stars thus making them
harder to observe with IUE in the past. However, Richardson et al. (2011)
presented an intense time-series of optical H$\alpha$ spectroscopy of the LBV
P Cygni, the only other Galactic LBV with an observed eruption. With more than
a decade of uniform instrumentation, they found a DAC-like feature with a long
recurrence time of 1700 days, much longer than the expected rotation period of
the star.

With eta Car, we can expect that the velocity shells we observe were from
material ejected during the Great Eruption. One possible way to explain these
shells would be to assume that these were similar DACs as those seen in the
H-alpha line of P Cyg reported by Richardson et al. (2011). In this case, we
could envision that the terminal velocity of the very hot, recently merged
object would be at -1000 km/s or similar, but the newly merged object could
start to expand and cool. Thus, each DAC would have a different terminal
velocity, resulting in the half dozen or so shells we see at very high
velocity. Then, as the merger progresses, the star will be puffed up to show a
lower effective temperature, which in turn provides a lower terminal velocity.
Now our DACs will have much lower terminal speeds and hence velocities in
today’s observed shells than those in the early parts of the eruption. This
much lower effective temperature is supported with the spectral properties of
the Great Eruption’s light echoes (Prieto et al., 2014; Rest et al., 2012). As
the star continues to cool and reach an equilibrium, the DACs build up shells
of various column density based on the mass-loss rate that was likely not
constant during the eruption. A similar scenario could have existed during the
Lesser Eruption to account for the relatively low velocities in the family of
absorptions.

While the concept of DACs or CIRs cannot be completely dismissed, the scale of
the material in the shells appears to be much larger than the typical DAC. We
remind the reader that the current binary system produces shell-like
structures during each periastron passage (Teodoro et al., 2016) which is a
very different mechanism that those which produce DACs and CIRs.

###  3.9 Associating the velocity ladder with the merger scenario.

Figure 5: Transitional, bi-lobed common envelope structure from the Great
Eruption to the Lesser Eruption. The cores of two massive stars have merged
during the Great Eruption, while the extended atmospheres formed a distorted,
bi-lobed envelope that gradually conforms to the newly formed core. The Lesser
Eruption appears to be a final disgorgement of the common envelope leading to
the current, very extended primary wind of the current binary. (Concept
adapted from Hirai et al. (2021), Figure 1)

We now consider how this absorption ladder may support a binary merger in a
triple system. Alternative scenarios may explain the ladder of absorptions.
Indeed we encourage others to suggest other viable models that explain the
observations!

Concepts of a triple system explaining the behavior of $\eta$ Car go back as
far as Livio & Pringle (1998) who suggested that the current $\eta$ Car is a
triple system. However Teodoro et al. (2016) found that the present-day
behavior of $\eta$ Car is well explained by the binarity discovered by
Damineli (1996). Portegies Zwart & van den Heuvel (2016) provided a
computationally intensive model of a binary merger within a triple system that
addressed the Great Eruption and the current binary system. However more
detail was necessary to compare theory to observtations.

Recently Hirai et al. (2021) investigated triple systems that evolved by
mergers to binaries with ejecta as in the $\eta$ Car and Homunculus system.
Their Figure 1 summarizes the various stages leading to the merger: a
relatively stable triple system that lasts for several million years where the
member stars basically evolve independently; an unstable triple with a
thousand to ten thousand year interval as one or more stars swell up and begin
to interact, leading to a chaotic series of interactions; the merger period
where two evolved stars spiral into each other initially creating a common
envelope with much material flowing outward leading to the actual merger of
two stars. Finally the wind-ejecta interaction stage begins with the merged
star and the remaining companion interacting with each other and influencing
the ejecta.

We focus on the last stage described by the Hirai et al. (2021) model: the
core merger of two stars that led to settling surrounded by a bi-lobed
envelope while the third star orbits around the merging structure. We present
a sketch of this stage in Figure 5. Hirai et al. (2021) suggested that the
settling time would be decades to a century. This timescale is comparable to
the period covered by the velocity ladder which begins decades before the
merger (the Great Eruption) and lasts through the final settling (the Lesser
Eruption) of the distorted, bi-lobed, common envelope of the merged stars. As
the two stars approach the core-merger event, a common envelope develops that
persists well beyond the actual core merger.

Hirai et al. (2021) suggested that the common envelope gradually dissipates
but persists for up to a hundred years. The rungs of velocity ladder, plotted
in Figure 3, would be generated by the third star passing through the complex,
shared envelope of the merging binary with a period close to 5.5 years (Figure
5). Throughout most of the period of the third star orbiting around the
merging system, its wind clears out the shared envelope. Each time that the
third star encounters the merging, rotating structure, a shell of gas moves
toward our LOS and is compressed by colliding winds and radiation.

However the merging structure was a highly distorted structure with extended
ears beyond the newly-core star. The third star, now the binary companion,
would always pass through the greatly extended envelope of the merged star.
Additionally the companion star might pass through the extended lobes
resulting from the merger as the extended envelope slowly settles to a new
normal. If the merging stars had been co-planar with the third star, then the
companion might pass through both lobes left over from the merger.

Possible evidence of the three stars being nearly co-planar in orbit may be
the nearly symmetrical bipolar shape of the H2 structure traced out by Steffen
et al. (2014) as displayed in their Figure 4. Of note are two ’ears’ that
protrude from the bipolar structure: one extends to the NE and the other to
the SW. Such may be due to small assymetries of the merger, but notable is the
fact that the axis of symmetry of the Homunculus is closely aligned to the
orbital pole of the current binary. Such strongly suggests that the merging
binary had a orbit that at the time of merger was close to the orbital plane
of the binary plus third star.

The velocity ladder appears to be a record across the nineteenth century of
the merged star and common envelope being modulated by the fast wind of the
current secondary star. The wind of this secondary star interacted with the
merged envelope and merged star, interrupting the slow-moving gas across the
bulk of the orbit. In analogy to the modern-day binary system (Madura et al.,
2013), the third star during its orbit first dove through the bipolar envelope
then the core atmosphere and through the opposite portion of the bipolar
envelope potentially interrupting the wind flows three times forming up to
three shells each 5.5-year orbit (see Figure 10 in Madura et al. (2013).

The merger apparently took place across several cycles in the 1840s with the
bulk of the ejecta occurring in 1847. The common envelope existed well before
that time, possibly as early as the 1760s. The gaps in velocity may be a
record of times when abrupt changes occurred as the two merging stars spiraled
inward. The gaps, between $-$1038 and $-$732 km s-1 and $-$696 and $-$604 km
s-1, may be caused by the chaotic behavior of the triple system prior to the
merger suggested by Hirai et al. (2021). It does seem peculiar that the gaps
are seen as drops, not increases in expansion velocities. However shells with
increasing velocities with time would overtake earlier shells and wipe them
out.

The slope, defined by the four lowest velocities, is less than the slope
defined by the higher velocity segments. One interpretation is that the slope,
defined by these four lowest velocities, may be the final settling of the
common envelope during the Lesser Eruption.

The jump from $-$323 to $-$185 km s-1 appears to be associated with the event
corresponding to the Lesser Eruption. Only four shell velocities were recorded
below $-$323 km s-1 and may indicate the final dissipation of the merged
envelope by 1895. The origin of the Weigelt clumps is consistent with the
1890s; they likely are the final ejection of the merged envelope lifting off
at very slow velocities estimated to be about $-$40 km s-1.

Our LOS views the current orbital plane tilted at 45o. Given the chaotic
nature of a ninteenth-century triple system, intermittent gaps in the shells
that we view would be expected.

In the meantime, the bipolar envelope from the merged binary would be
gradually relaxing, but in a chaotic way. The observed velocity ladder
suggests a relaxation time of 40 to 50 years from the actual merger until the
last major wind flow comes out in the form of very slowly-moving clumps.

One puzzlement is the apparent linear decrease in wind velocity with time. The
Great Eruption led to about 40 $M_{\odot}$ being ejected. A change in velocity
increment might be expected at the time of merger. However, we note that mass
loss, while most at the time of the Great Eruption, appears to span well
before and well after the merger.

A second puzzlement is that we apparently do not detect each shell caused by
passage of the secondary through the extended primary wind. We should expect
to see nine absorption velocities at equal intervals between $-$513 and $-$146
km s-1. We do not. Could this be evidence that the primary envelope shrunk to
below 2 au diameter for several cycles preceding the Lesser Eruption? The
final drop in velocity from $-$126 to $-$40 km s-1 may support a temporarily
shrinking of the newly-formed primary structure. Perhaps accelerated shells
did form briefly late in the velocity gap and wiped out some slower,
previously-formed shells.

Is there an alternate explanation?

##  4 Conclusions

A series of 38 narrow-absorption velocities, measured in our LOS from $\eta$
Car provides a record of wind velocities that decreased with time. Referencing
two key absorption velocities to the Great and Lesser Eruptions, we find this
velocity ladder spans from the 1890s possibly back to the 1770s.

However, this velocity ladder is only a partial record of events that occurred
during the 18th and 19th centuries. While we find many possible
interpretations, the simplest interpretation appears to be that the
absorptions rise from shells that record interactions of a companion star to a
merging binary. Analogous to Occam’s razor, this may sever the Gordian knot of
complexity that is the ejecta formed from major events.

The velocities, more negative than $-$513 km s-1 which correlates with the
1847.2 measure of the Great Eruption (Smith, 2017), suggest dates of origin
that are similar to dates of origin of emission structures determined by Morse
& Smith (2024) to be as early as the 1760s.

The velocity ladder includes velocities less negative than $-$513 km s-1 which
leads to shells originating later than 1847.2, into the 1860s but well before
the start of the Lesser Eruption in 1887.2

Large discontinuities exist between sequences of velocities that show a near
linear decrease in velocity with time. We suggest these discontinuities in our
LOS are due to weakening of the wind velocities and mass loss of $\eta$ Car
with time. Offsetting the sequences of velocities to form a straight line in
velocity increments leads to inferred dates of origin that extend back to the
1760s and forward to the 1890s.

The frequency of the absorption velocities exceeds that which would be
expected from the secondary star passing through the extended primary envelope
each orbit. Instead, placing all of the velocities on a linear curve, suggests
that the secondary passed through the preceding and following ears of the
structure caused by the merger.

This velocity ladder, considered in context of the well-defined bipolar H2
shell (Steffen et al., 2014) with minimal asymmetries, suggests that the three
stars across this interval of time were nearly co-planar in orbit.

The current binary is stable on the scale of decades as witness the half-
century calorimetry measures and the repeatability of the X-ray light curve.
The limited FUV observations with the HST/STIS are also consistent with a
stable binary system with no evidence for changing stellar parameters over
recent decades.

Two lingering puzzles are 1) the linear decrease with time of the shell
velocity and 2) the absence of an absorption velocity caused by periastron
passages in several orbits preceding the Lesser Eruption.

Further studies and models are necessary to test this suggestion that the
ladder of velocities are a record of a merging binary in a triple system.

Additional monitoring with the UV HST/STIS, especially during both the high-
and the low-ionization periastron passages, is needed to follow the ladder
velocities to determine how excitation, ionization, and the measured
velocities evolve with time. As the apparent UV fluxes of $\eta$ Car have
increased ten-fold in our direction over the past two decades, even higher
resolving power spectra with high S/N are possible and must be pursued. Many
singly-ionized metal absorptions will disappear during the high-ionization
state only to reappear during the low-ionization state. Additional absorption
velocities may appear in our LOS.

Observations in the high-ionization state are likewise needed to follow the
evolution of the shells as the extreme UV continues to increase. However the
singly-ionized metal resonant absorptions, accessible in the MUV and NUV, are
being replaced by doubly-ionized metal absorptions located shortward of Ly
$\alpha$, a spectral interval that was found to be very complex as shown by
the FUSE observatory with its limited spectral and angular resolutions (Iping
et al., 2005).

New insight will also come from JWST and ALMA observations capable of
penetrating the dust and parsing out the molecular and neutral gas structures.

This discussion was not funded by any facility. Many thanks by the primary
author to the Eta Carinae ZOOM team and the numerous STScI personnel that
assisted in the original observations. The material is based upon work
supported by NASA under award number 80GSFC21M0002. HST(STIS)

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