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AAS meeting 207 -- January 2006
FUV DETECTION OF ETA CARINAE A
K. Davidson (University of Minnesota) & N. Smith (University of Colorado)
Summary: A recently reported FUV detection of h Car’s hypothetical companion star almost
certainly represents, instead, the primary star’s wind. There’s no clear proof that any observed
radiation comes from the elusive secondary object.
The same data have at least two positive implications: (1) They imply that the 2003 spectroscopic
event was a mass-ejection episode rather than an eclipse. (2) They’re consistent with the polar
wind scenario, wherein the pseudo-photosphere is hottest near the equator.
1. The current problem
Recent studies of h Car have concentrated on
its yet-unexplained 5.5-year spectroscopic
cycle. Two high-priority questions are:

Is this object really a 5.5-year binary system?
That seems likely, but proof has not yet materialized.
Contrary to many published assertions, so far the
observations appear consistent with a single-star
thermal/ rotational recovery cycle (see refs. below).
“Spectroscopic events” occur at 5.5-year
intervals, most recently in mid-2003. If h Car
is indeed a binary, do these phenomena signify
eclipses, or are they more fundamental?

This question is important for astrophysics,
because a non-eclipse interpretation implies
an undiagnosed instability in the most massive
well-studied star. For more information see
http://etacar.umn.edu and refs. there.
* * *
Iping et al. [1] recently described FUSE
observations of h Car near l ~ 1100 Å. They
regard these as a direct detection of a hot
companion star. Here we note, however, that –
(1) much of the detected emission probably
originates in the primary star’s wind;
(2) there is no clear proof that any of it
comes from another star; and
(3) the FUSE data are more useful for the
second question above.
In this poster we mention several concepts that
appear quite likely, based on available evidence.
In order to show that FUSE detected a second
star, one must prove that all the models sketched
here are wrong.
2. The primary star is expected to account
for much of the FUV flux
As shown by Smith et al. (2003), the polar wind
of “h Car A” probably dominates the observed
spectrum, and the effective photosphere is located
in the wind. ( continued in next column )
…The photosphere is therefore prolate as sketched
in Fig. 1, with Teff ~ 14000 K near the poles but
Teff > 20000 K near the equator. Some details
may eventually prove inaccurate, but this is
currently the “best bet” picture in view of existing
evidence. (Caveat: The axial ratio shown here is
plausible but has not been measured.) As Smith
et al. emphasized, the hot equatorial region can
produce a substantial far-ultraviolet luminosity.
Fig. 2 shows, roughly, the expected UV continua
of both stars. Since no realistic wind models are
available (see below*), here the primary continuum
is a composite of near-zero-gravity Kurucz models
with Teff = 14000 K and 20000 K. The resulting
energy distribution is good enough for our purposes,
since worse uncertainties arise from other causes
[2, 3]. The hypothetical secondary continuum in
Fig. 2 is a relatively unevolved 35000 K O-type star,
normalized so that 5% of the total luminosity occurs
at ionizing wavelengths l < 912 Å -- consistent
with the relatively weak high-excitation emission
lines in the ejecta.
* { Spherical wind models ( e.g., [4] ) are unsuitable for
the FUV flux, because the radiative transfer problem
is very different. Almost any spherical model adjusted
to the longer-wavelength spectrum will underestimate
the FUV. Even a composite of two or more spherical
models shouldn’t be trusted. One can make a rough
analogy with gas dynamics, 1-d vs. 2-d. }
Fig. 2. Likely intrinsic UV continua of both stars, not
corrected for extinction and circumstellar / interstellar
absorption lines. “Primary star” means the primary
wind, see Fig. 1. In this model more than 60% of the
flux near 1100 Å comes from the primary star.
... The main point of Fig. 2 : Near l ~ 1100 Å,
we should expect substantial radiation from the
primary star, most likely exceeding the hypothetical
companion. At least some of the flux detected with
FUSE represents “h Car A,” not a companion star.
3. Spectral characteristics in the FUSE data
don’t match the expected secondary
The FUSE data show a strong and variable N II
l1085 emission feature, which Iping et al. attribute
to “h Car B”, the hypothetical companion star.
That interpretation requires an unusually dense,
probably nitrogen-rich wind with lower speeds than
expected for the companion. As Ebbets et al. [5]
noted, the primary star’s wind is a more likely place
to generate the observed N II l1085. The FUSE
data indicate wind speeds < 1100 km/s, but the
secondary star needs a wind speed close to 3000
km/s to explain the X-rays [6].
Therefore: Even without any other information, the
character of the spectrum seen with FUSE makes
a secondary-star interpretation very doubtful.
4. A successful prediction
The FUV flux practically disappeared during the
2003 spectroscopic event. Based on an eclipse
scenario, Iping et al. [1] regard this as proof that
the FUV did not come from the primary star.
However: The FUV disappearance matched a
prediction made long ago by Zanella et al., with
no reference to eclipses or a second star [7].
Those authors proposed that h Car’s spectroscopic
events are mass-ejection episodes which temporarily
quench the far UV. This idea works even better with
a polar wind [2], and a companion star might trigger
the instability. HST/STIS spectroscopy strongly
favors this type of model rather than an eclipse [8].
At least part of the detected emission came from
the primary (Fig. 2). In a normal eclipse model,
that fraction of the FUV should have remained
visible during the event. But it disappeared; the
simplest interpretation is a mass-ejection model.
This, not a detection of the secondary star, is the
most significant result of the FUSE observations.
5. Unorthodox possibilities
Detection of the secondary star is highly desirable
because that would eliminate single-star models.
Unfortunately, as we explained in sections 2—4
above, there is no proof that any emission seen with
FUSE came from a second star.
It’s not hard to imagine a single-star model.
The equatorial photosphere (Fig. 1) may be hot
enough to produce all the ionizing photons, and
the primary is known to have produced ejecta fast
enough to account for the observed X-rays [9].
The 5.5-year period may be a thermal / rotational
recovery cycle [10]. We do not strongly advocate
such a model, but it remains quite possible.
Obviously the topic needs more work! A realistic
theoretical analysis may reveal that the HST/STIS and
other data are incompatible with a single-star model,
but so far this has not been proven.
Acknowledgments: This work is part of the Hubble Treasury
Program for Eta Carinae, supported by funding from STScI.
N.S. was supported by a NASA Hubble Fellowship. We are
grateful to J.C. Martin and R.M. Humphreys for valuable
discussions. The relevance of the symbol in the upper right
corner of this poster is hinted at http://www.sixofone.org.uk/.
Refs. [1] Iping et al. (2005) Ap.J. 633, L37.
[2]
[3]
[4]
[5]
Smith et al. (2003) Ap.J. 586, 432.
Davidson et al. (1995) Astron.J. 109, 1784.
Hillier et al. (2001) Ap.J. 553, 837; (2005) preprint.
Ebbets et al. (1997) Ap.J. 489, L161.
In general, see also http://etacar.umn.edu/.
[6] Pittard & Corcoran (2002) A&A 383, 636.
[7] Zanella et al. (1984) A&A 137, 79.
[8] Martin et al. (2006) Ap.J. in press.
[9] Smith & Morse (2004) Ap.J. 605, 854.
[10] Davidson (2005) ASP Conf. 332, p. 101; and
(1999) ASP Conf. 179, pp. 304 & 374.