Results on V356 Sgr from IUE and Voyager ultraviolet (500 to 3200 A) observations obtained in 1986 and 1987, primarily during 2 total eclipses are presented. The eclipse of Aug. 15, 1986 was fully covered with IUE low dispersion images and 9 hr of Voyager UVS data. The eclipse of Mar. 25, 1987 was covered with IUE low dispersion images and 1 high dispersion SWP image. During both eclipses the total strength of the emission lines is found to be invariant. An uneclipsed UV continuum is detected at wavelengths shorter than 1500 A. The high dispersion SWP spectrum reveals that the emission lines are extremely broad, almost symmetrical emissions with weak, slightly blue shifted absorption components. No evidence of carbon, C I, C II, C III, or C IV, is seen in the emission or absorption spectrum of V356 Sgr in eclipse. Models for this binary system are presented

Polidan, R. S.

English

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N89- 107 1 9
PREC._rNG PAC, E BLAN'K NDT 171LMED
A NEW STUDY OF THE INTERACTING BINARY STAR V356 SGR
R. S. Polidan
Lunar and Planetary Laboratory-West
University of Arizona
Tucson, AZ 85721
ABSTRACT
In this paper we present new results on V356 Sgr from IUE
and Voyager ultraviolet (500-3200 A) observations obtained in
1986 and 1987, primarily during two total eclipses. The eclipse
of 15 Aug 86 was fully covered with IUE low dispersion
images and 9 hours of Voyager UVS data. The eclipse of 25
Mar 87 was covered with IUE low dispersion images and 1
high dispersion SWP image. During both eclipses the total
strength of the emission lines were found to be mvariant.
Also, an uneclipsed UV continuum was detected at
wavelengths shorter than 1500 A. The high d2spersion SWP
spectrum revealed that the emission lines are extremely broad,
almost symmetrical emissions with weak, slightly blue shifted
absorption components. No evidence of carbon, C I, C II, C
IIl, or C IV, is seen in the emission or absorption spectrum of
V356 Sgr in eclipse. Models for this binary system are
presented.
Keywords: Interacting Binary Stars, Ultaviolet Emission Lines,
CNO Processing, Ultraviolet Spectroscopy
I INTRODUCTION
V356 Sgr _s a massive echpsmg binary with a period of 8.896
days and a total eclipse of II hours duration. Both
components of the binary are spectroscopically visible m the
optical. Eclipse depths in V are 0.87 and 0.39 magnitudes,
respectively, for primary and secondary minimum. Period
changes suggest a steady, very low rate of mass transfer (-10 -v
M@ yr -1) is occurring within the system. V356 Sgr also has
some of the best determined physical dimensions of any
interacting binary (Ref 3). Table 1 lists the physical
characteristics of the binary. The binary _s moderately
reddened, with an E(B-V) as determined from the indwidual
colors of the components and from the 2200 A feature of
0.23-+0.03 magnitudes. The most quoted model for V356 Sgr is
that proposed by Wilson and Caldwell (Ref. 6) In this model
they propose the existence a thick, opaque non-luminous ring
or disk around the B3 star. Ultraviolet emission lines were
subsequently discovered in this star by Plavec et al. (Ref. 4).
The UV emission lines are amongst the strongest detected in
any algol system. Despite this they are detectable only during
the total eclipse of the system, the ultraviolet flux from the B3
star overwhelming them at all other tmles. No optical emission
lines, including Ha, have ever been reported in the binary
Recent very high S/N t-1_ eclipse observations,
contemporaneous with the UV observations, also show no
evidence of optical emissions
2. OBSERVATIONS
The new IUE and Voyager UVS observations were obtained in
1986 and 1987, with most of the data taken during two total
primary (when the B3 star is occulted) eclipses. The eclipse of
15 Aug 86 was covered with 12 SWP and 8 LWP IUE low
dispersion images and 9 hours of Voyager UVS data. The
echpse of 25 Mar 87 was covered with 7 SWP and 6 LWP IUE
low dispersion images and 1 high dispersion SWP image. No
Voyager UVS data was obtained during this March eclipse.
Additional IUE SWP and LWP low dispersion images were
obtained after the two observed eclipses Supplemental
Voyager UVS data were obtained prior to the eclipse on 15
Aug 86 and in May 87.
3. OPTICAL LIGHT CURVE
The IUE FES was used to obtain an optical light curve for the
echpses m V356 Sgr. ]'he FES light curve suggests two
conclus:ons. 1) The quadratic ephemeris of Hall, Henry, and
Murray (Ref. 1), 2433900766 + 8896106E + 35x10-SE z, best
fits the data. The significance of the second order term is
confirmed. 2) There is suggestive evidence of changes in the
shape of the echpse light curve from the two FES light curves.
4. ULTRAVIOLET LIGHT CURVE
UV hght curves were generated from the low dispersmn
images in three narrow (25 A, 65 A, and 250 A) continuum
bands in the UV. These three light curves and the FES light
curve were ratioed to the observed flux (approprmtely scaled
and reddened) of the A3 star 38 Lyn. The 1908 and 2625 A
light curves appear relatively normal. The 1262 A curve,
however, showed a substantial excess over that expected.
During the total phase of the eclipse the B3 star can contribute
no flux. So this UV excess cannot arise in the B3 star.
Voyager 500-1200 A observations produced only a marginal
detection of V356 Sgr during eclipse. This suggests that the
characteristic temperature of the UV excess is comparable to or
slightly cooler than that of the B3 star (~16500 K from outside
of eclipse UVS observatmns) Since the source of the flux is
(predominantly) uneclipsed by the A211 star it must either arise
outside the domain of the B3 star or be of significantly larger
radius than the A2 giant Analysis of the shape of the flux
excess argues in favor of the extended region Figure I shows
the Voyager and IUE spectra V356 Sgr outside and during the
total eclipse In this figure the eclipse spectrum is compared to
the scaled and reddened spectrum of 38 Lyn, an A3 star, and
to a synthetic spectrum obtained by combining the scaled and
reddened 38 Lyn spectrum with a simulated electron scattering
of the system light. This "scattered hght" spectrum was
obtained by taking the observed maxmmm spectrum and
A Decade of UV Astronomy with IUE, Proc. Celebrator), Symposmm, GSFC. Greenbelt, USA, 12-15 Aprd 1988, ESA SP-281, 14_1 1 (June 1988)
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I 1 I I I I I I I I r I I I I I i i i I |
V356 Sgr Maximum and R_nlmum Spectra
Sca}ed 38 Lyn + !% Ha> imum Spectrum
I I I I I I I I I I I I I I I I I I I
1500 2000 2500 3000
Wavelength (A)
Figure 1. Observed Voyager and IUE spectra of V356 Sgr at
maximum and during the total eclipse of the B3V star. An
IUE spectrum of a A3 reference star, 38 Lyn, scaled to the
visual magnitude of the A2II component in V356 Sgr and
reddened by 0.23 magmtudes in B-V is also shown. Also
shown is a synthetic spectrum obtained from summing the
scaled and reddened 38 Lyn spectrum with one percent (1%) of
the observed maximum light V356 Sgr spectrum (Voyager plus
IUE) smoothed to simulate electron scattering. Note the
surprisingly good agreement between the synthetic spectrum
and the observed V356 Sgr primary eclipse spectrum.
Figure 2, Si IV 1400 line strength as a function of phase
during the eclipse of 15 Aug., 1986.
2.50
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H
0.50
V356 Sgr
$1 IV 1400 A
15 Aug 86
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0.95 0.96 0.97 0.98 0 99 1 O0 10l I 02 I 03 I 04
Phase
Figure 2
A NEW STUDY OF V356 SGR 207
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Figure 3_
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V356 $gr m Echpse C IV region
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1540 1545 1550 1555 1580 1565
Figure 3b Wc3velerqcj_h (A)
V356 Sgr A211 star
>_ O80
E
2 080
c
_" 0 20
CI --
f +
AI II
-0 20 .... ' .... ' .... ' ......... ' ..............
1645 1650 1655 1660 1668 1670 1675 1680 1685
gloure 4a Wavelength (A)
_+os I
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_" i V555 Sgr
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¢'°°I
20O i
400 600 Boo 10o3 1200
Figure 4b WX ("'_) _ II 126a
Figure 3. a) Emission lines of N V ),1238 and 1242 (at +964
km s-t in this velocity plot), Si IV )`1393, and Si II1 )`1206
from the high dispersion SWP image obtained during the
eclipse of 25 March, 1987. Dotted lines are drawn at -+Ve,e_,r_
for the B3 star b) The C IV ),1550 region from the same
high d_spersion image. Dashed curve is the spectrum of the
A2III reference star f Sgr. Note the absence of any detectable
C IV hne Dotted line is drawn at the expected level of the
scattered light shown in Figure I
Figure 4. a) Region of the expected strong photospheric C I
line in the A211 component in V356 Sgr. The dashed curve is
the spectrum of the A2[II reference star _ Sgr. Note the
complete absence of a carbon line in the A21I star's
atmosphere. The sharp C I hne is the interstellar component.
b) Photospheric C Ill ),1247 and S_ 11 ),1264 equivalent widths
for the B3V component in V356 Sgr and five non-binary stars
of similar spectral type.
Figure 5. The A1 111 ),1860 region in the echpse spectrum.
The dashed curve _s the spectrum of the A2111 reference star _"
Sgr. Note the AI Ill emission and the total absence significant
absorption of the A star photospheric light at the central
absorption core of the AI IIl lines.
V356 Sgr: A211 Star
60
1 +0
1 20
1 00
E
-- 0 80
0 60
0
_ 040
0 20
0 O0
-0 20
1840 _845 1880 1855 1860 1865 1870 1875 1880
Figure 5 Wavelength (A)
208 R S POLIDAN
smoothing it with a 20 /k boxcar smoothing function As can
be seen in Figure 1 an excellent fit is obtained to the observed
eclipse spectrum by assuming one percent (1%) of the total
system light-is scattered in the line of sight during eclipse.
Thus, the origin of the flux excess appears to be electron
scattering from an extended cloud surrounding the system.
5. ULTRAVIOLET EMISSION AND ABSORPTION LINES
Using the data obtained from the low dispersion images it was
discovered that the total strength of the UV emission lines was
invariant during both eclipses (Figure 2). This suggests that
the line formation region is outside the immediate domain of
the two stars in the binary and suggests a possible association
with the region responsible for the scattered light. The high
dispersion SWP spectrum obtained in March, 1987 revealed that
the emission lines are extremely broad, FWHM -1100 km s -1,
almost symmetrical emissions with weak, slightly blue shifted
absorption components (Figure 3). Also, apparent in the high
dispersion IUE image is that the "C IV" emission line reported
in previous investigations is not the carbon doublet but rather
dissolves into numerous weak emissions of, probably, Fe III.
No evidence of carbon, C II, C III, or C IV, is seen in the
emtsslon spectrum of V356 Sgr during eclipse. Line profile
fits to the emission line data suggest that all lines can be fit
with the same line shape parameters. No evidence supporting a
stratified line formation region was found. The strength of the
carbon emission lines C II, C III, and C IV) in V356 Sgr are at
least a factor of -twenty weaker than the silicon (S_ II, Si I11,
and Si IV) lines. Similarly, no evidence is found for carbon in
the photosphere of the A21I star in V356 Sgr (Figure 4).
Inspection of the high dispersion non-eclipse IUE images of
V356 Sgr reveal a relatively normal B3/B4 stellar spectrum,
including carbon lines. Figure 4 also presents data on the
relative strengths of the photospheric C II1 ),1247 and Si 11
),1264 lines for V356 Sgr and a set of B2 5 to B5 stars with
smlilar rotational velocities. It is quite apparent that the C Ill
line is of at least normal strength, indeed for its spectral type
the C 11I hne in V356 Sgr appears somewhat stronger than
expected. Thus, we must conclude that the B3V component of
V356 Sgr has an essentmlly normal carbon abundance while the
A211 star has no observable carbon. How are these two carbon
line observahons compatible? Simple arguments (cf. those
presented for other binaries in Ref. 2) can show that the
secondary (A211) star has likely lost enough mass to have
reached its CNO processed layers These layers will be
virtually devoid of carbon and slightly mtrogen rich (cf Ref
2) Thus, a compositional difference _s expected to exist
between the two components of the binary system.
One final question remains: what is the geometrical
distribution of the high ionization gas. F_gure 5 shows the AI
II1 emission hnes seen superposed on the photosphere of the
A2II star. Despite the obviously saturated central absorption of
the AI I11 lines no significant absorption of the A star's
photospheric light is detected. Thus, the line formation regaon
must be outside our line of sight, i.e. outside the orbital plane
S_milar results are obtained for S_ I11 and the scattered hght
continuum (Figure 3).
6. MODEL
Shu (Ref 5) has proposed a model for a "magnetically assisted
accretion driven" wind for critically rotating pre-main sequence
stars. His model can, with little modification, be directly
applied to V356 Sgr. Carbon poor material will be transferred
to a small accretion disk surrounding the B3 star. Using Shu's
arguments, a portion (~75%) of this disk material will be
accreted by the B3 star with the remainder being driven away
in a wind with a characteristic velocity near the escape velocity
of the B3 star and a preferential direction away from the
orbital plane Thus, this model predicts a gas outside the
orbital plane with the chemical composition of the secondary
but the velocity characteristics of the primary.
This work is supported by NASA Grants NAGW-587 and
NAG5-441.
1.
6.
7. REFERENCES
Hall, D S, Henry, G W, and Murray, W H 1981, Acta
Astr., 31, 383.
Peters, G J and Polidan, R S 1984, Astrophys. J., 283,
745.
Popper, D M 1980, Ann. Rev. Astr. Astrophys., 18, 115.
Plavec, M J, Dobias, J J, Etzel, P B, and Weiland, J L
1984, in "Future of UV Astronomy Based on Six Years of
IUE Research", NASA CP-2349, eds. J. M. Mead, R. D
Chapman, and Y. Kondo, p 420.
Shu, F H 1987, paper presented at the Fourth Trieste
Symposium, Sept. 1987
Wilson, R E and Caldwell, C N 1978, Astrophys. J., 221,
917.
Table 1. V356 Sgr
Period (days)
Spectrum B3V A211
Mass (M®) 12.1+1.1 4.7-+0 6
Radius (R®) 6.0+0.7 14.0-+ 1.5
Log g 3 96-+0.10 2.82_+0.10
Vescape (km s-1) 877+65 358-+30
V sin i (kin s-a) 350 90
VBync (kin s -i) 33 77
Tei-f (K) 16500*_+750 8600_+300
1b 4 x 10 -9
M (M® yr -1) 4 x 10 -7
8.896106


A new study of the interacting binary star V356 Sgr

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