IUE observations of HR 1105 over its 596-day orbit show strong orbital modulation of both continuum and emission lines. These are most intense just before both conjunctions and nearly disappear near quadratures, the most intense phase being just before the hot component passes in front of the S star. High dispersion observations exhibit a blue-shifted absorption feature in Mg II, representing an outflow of material of about 55 km/s. These observations are consistent with the UV source being an optically thin gas stream between the components of the system, which is partially eclipsed when the S star is in front

Ake, Thomas B., III

Bopp, Bernard W.

Johnson, Hollis R.

English

NASA Technical Reports Server

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A, IV. Sha/trr (ed.)
The Unusual Interacting S Star Binary HR 1105
Thomas B. Ake, Ill
Astronomy Programs, Computer Sciences Corporation, Code 68I/CSC,
Goddard Space Flight Center, Greenbelt, MD 20771
HoUis R. Johnson
Astronomy Dept., SW 319, Indiana University, Bloomington, IN 47405
Bernard W. Bopp
Dept. o/Physics and Astronomy, Ritter Astrophysical Research Center,
Llnit,ersity of Toledo, Toledo, OH 43606
Abstract. IUE observations of HR 1105 over its 596-day orbit show
strong orbital modulation of both continuum and emission lines. These
are most intense just before both conjunctions and nearly disappear near
quadratures, the most intense phase being just before the hot component
passes in front of the S star. High dispersion observations exhibit a blue-
shifted absorption feature in Mg II, representing an outflow of material of
about 55 km s -I. These observations are consistent with the UV source
being an optical])' thin gas stream between the components of the system
(Shcherbakov & Tuominen 1992), which is partially eclipsed when the S
star is in front.
1. Introduction
HR 1105 is a symbiotic-like, interacting binary comprised of an $3.5/2 red-
giant primary and a hot compact companion. The orbit of the primary was
determined by Griffin (1984) to be nearly circular at 596 days. Peery (1986)
discovered the hot secondary with IUE, and Ake et al. (1988) found evidence for
orbital modulation of the UV flux, although they could not distinguish between
possible sources of the emission (an accretion disk, streaming, or atmospheric
heating of the red giant by the hot star). Shcherbakov & Tuominen (1992) found
that He I A10830 has a complex variation of the absorption-emission profile over
the orbit, which they argued is due to gas-streaming near the inner Lagrangian
point of system. Thus HR 1105 is a prime example of a Tc-deflcient S star that
is not a thermally pulsing AGB star, but rather is a system where mass transfer
has modified the composition of the red giant when the secondary component,
which is now a white dwarf, was on the AGB (Johnson et al. 1993, and references
therein).
We have undertaken further lUE observations of HR 1105 over nearly a
complete orbit from 1992-1993, including critical phases near conjunctions of
413
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Figure 1. SWP spectra of HR 1105. Phases are determined from the
Griffin (1984) orbit, such that the secondary is in front of the S star at
¢ = 0.33, and the S star is in front at ¢ = 0.88 The other two figures
are near quadratures.
the components and at velocity quadratures. Using other observations taken
over several different cycles, we now have IUE data over the full orbit.
2. Observations and Analysis
In our IUE monitoring program, we obtained low dispersion SWP and LWP/R
spectra, and LWP/R high dispersion images of the Mg II emission lines. Because
the red giant contributes significant UV light in the LW region, the $WP spectra
show the most dramatic variations (Fig. 1). The UV line and continuous emis-
sion is most intense near inferior conjunction of the hot companion (¢ = 0.3),
but reaches a secondary maximum half a cycle later (¢ = 0.8). This secondary
maximum rules out the UV source as being due to atmospheric heating of the
S star by a hot companion since it occurs when the heated face faces away from
_he observer. At the brightest phases, lines due to N V A1240, O I A1300, C II
A1335, S IV A1400, C IV A1550, Si III] A1892, and C III] A1909 are present.
However at ¢ = 0.8, N V disappears, indicating that the hottest area of the UV
source is occulted by the red giant.
It,
Figure 2. C¢
phase. Open
closed squares
indicates the t
In Figure 2, w,
a 100/_ region cent
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with increasing N_,
denser region then.
that the inclination
There is also evid(
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!
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S Star Binary HR 1105
)
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Figure 2. Continuum and line measurements as a function of orbital
phase. Open squares are data from 1982-1989 taken over 4 orbits;
closed squares are during the same orbit in 1992-1993. The arrow
indicates the phase at which the secondary object should be eclipsed.
415
10 InFigureO/_ 2, we show the orbital variations of various SWP features, i,Ve use
a region centered at A1460 as a continuum point, f14c,o, since it is relatively
free of emission lines. Other measurements were made of the net integrated line
fluxes of the various emission lines. We find that the continuum and emission
Lines (as illustrated by the behavior of C IV) vary together, reaching maxima
near to, but not at, the conjunctions. The Si III]/C III] ratio, which increases
with increasing N r, is also highest at these phases, indicating we are seeing into a
denser region then. The lowest recorded fluxes occur at ¢ = 0.89, which is a sign
that the inclination of the system is high and the secondary is partially eclipsed.
There is also evidence, based on the multiple observations near ¢ = 0.3, of a
secular variation of the UV flux.
In the LW region, we find that the flux at 3100 ]t shows no cyclical variation
because of the dominance of the flux from the red giant photosphere in this
region• The Mg II lines and flux at A2600 show similar variations as seen in
the SWP, but of smaller amplitude. The minimum Mg II flux is consistent with
t;
il
416 Ake, Johnson, and Bopp
_hat of an M giant of similar V-K color as HR 1105. Thus the Mg II llnes have
contributions from both the S-star chromosphere and the UV source.
In high dispersion, the Mg II profiles show changes in the ratio of the violet
_o red emission peaks around the centrally reversed absorption core. At _ - 0.68,
the line is blue-shifted with respect to the central absorption, indicating it arises
close to the red giant since the S star is at maximum velocity of approach at that
phase. At _ = 0.3, when the hot component is in front, the Mg II line is much
stronger than at other phases, and an absorption trough appears at -78 km s -*.
Compared to the systemic velocity,thisrepresentsan flow of about 55 km s-I
towards the observer.Itisunclearwhether thisfeatureisseen in alldata (i.e.,
itarisesfrom a wind from the S star),or ifititstrongestat thisconjunction
(i.e.,itisformed in a stream).
3. Discussion
Of the possible scenarios for the orbital modulation of the UV flux in HR 1105,
the Shcherbakov & Tuominen gas-streaming model is the most consistent with
the IUE observations. Because the UV source nearly disappears far from con-
.junction, it cannot be due to atmospheric heating or an accretion disk. The
secondary maximum also rules against atmospheric heating. Although detailed
observations near the conjunctions are not available, there is evidence that the
peak emissions occur at A_ ~ 0.1 before conjunctions, i.e., when looking down a
stream of material from the S star towards the secondary. This is also consistent
with the appearance of the blue-shifted absorption feature in Mg II. The stream
is optically thin since it is only seen at certain orientations. The absence of N V
and low fluxes near inferior conjunction of the S star indicate parts of the UV
sourceare ecUpsed.
lnlemctin@ BinarV Stars
ASP Conference Series, Vol. 5¢
A. W. ShaIter (ed.)
Separation of C
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References
Ake, T.B., Johnson, H.R., & Peery, B.F. Jr. 1988, in A Decade of Astronomy
with the IUE Satellite, ESA SP-281, p. 245
Griffin, R.F. 1984, Observatory, 104,224
Johnson, H.R., Ake, T.B., & Ameen, M.M. 1993, ApJ, 402, 667
Peery, B.F. Jr. 1986, in New Insights in Astrophysics: 8 Years of Observations
with IUE, ed. E. Rolfe, ESA SP-263, p. 117
Shcherbakov, A.G. & Tuominen, I. 1992, A&A, 255,215
1. Introduction
Because of the severe
systems, reliableinfor_
components has been
an iterativescheme usi
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1991; Bagnuolo et at.
the spectraltypes an(
(V sin i), a magnitude
differences. Using IU.
systems because for tl
UV features are high e:
circumsteUar gas emis_
*Guest Observer with the I
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Inc. under contract to the


The unusual interacting S star binary HR 1105

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