A study of the stellar wind properties of O-type close binary systems is presented. The main objective of this program was to search for colliding winds in four systems, AO Cas, iota Ori, Plaskett's star, and 29 UW CMa, through an examination of high dispersion UV spectra from IUE and optical spectra of the H alpha and He I lambda 6678 emission lines

Gies, Douglas R.

English

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COLLIDING STELLAR WINDS IN O-TYPE CLOSE BINARY SYS S
NASA Astrophysics Data Program - Grant NAG 5-1218-1
FINAL TECHNICAL REPORT
June 1989 through June 1991
Principal Investigator: Douglas R. Gies
Department of Physics and Astronomy
Georgia State University
Atlanta, GA 30303
Phone: (404) 651-2932
(NASA-CR-18_610) COLLIDING STELLAR WINDS IN
O-TYP? CLOSE BI_ARY SYSTEMS Finml Technical
R_poFt t Jun. 1989 - Jun. 1991 (Georgia
State Univ.) 8 p CSCL 03B
N91-25981
Unclds
0020341
https://ntrs.nasa.gov/search.jsp?R=19910016667 2020-03-19T16:52:07+00:00Z
1. INTRODUCTION
This report summarizes a study of the stellar wind properties of O-type close binary
systems conducted in the Department of Physics and Astronomy of Georgia State Univer-
sity. The main objective of the program was to search for colliding winds in four systems,
AO Cas, t Ori, Plaskett's Star, and 29 UW CMa, through an examination of high disper-
sion UV spectra from IrfE and optical spectra of the Ha and He I _6_78 emission lines
obtained at the University of Texas McDonald Observatory. The luminous O-type stars
continuously shed their outer gaseous layers in a supersonic outflow or stellar wind. In
systems of close binary stars, the winds from each star will collide between the stars and
be diverted along a bow shock (Luo, McCray, & Mac Low 1991, Ap. J., 362, 267). The
high velocity part of the flow is best observed in the UV resonance lines formed in the
wind (Howarth & Prinja 1989, Ap. J. 3_ppl., 69, 527), while the low velocity, high density
component can be traced through optical emission lines (Ebbets 1082, Ap. J. Suppl., 48,
399). Our program was designed to combine these two types of data to develop a consistent
picture of how stellar winds interact in close binary systems.
We obtained the IUE spectra during a visit in August 1989 to the NASA IUE Regional
Data Analysis Facility at the University of Colorado in Boulder. The data consist of high
resolution spectra of the N V _1240, Si IV )_1400, C IV )d550, He II _1640, and N IV
_1718 line profiles. There are typically ten to thirty spectra of each star which are well
distributed in orbital phase. These spectra have been reduced to a common rectified format
for analysis. Similarly, all the McDonald Observatory optical spectra were reduced to a
common format for straight forward comparison with the UV data.
The analysis of wind features formed the basis of a Master of Science Degree thesis by
Michael S. Wiggs (under the direction of D. R. Gies), and the results are summarized in
§2. In brief, we have found evidence for high density regions between the stars in three
of the four systems, and each of these regions probably form the apex of a bow shock.
The UV line profiles also show orbital phase related changes that are consistent with the
existence of bow shocks in these systems.
During the course of this work, we also began a study of photospheric lines in the
UV spectra using a tomography algorithm familiar in medical applications to derive the
separated spectra of each component star. The algorithm for this application was developed
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by W. G. Bagnuolo, Jr., and our results to date are summarized in §3. A full bibliography
of published work associated with this project appears in §4.
The NASA ADP support has made possible (through matching funds) the purchase
of a DECstation 3100 computer (plus tape drive and laser printer) which has been the
primary tool for this analysis. We are grateful for this financial support that has been so
important for the completion of this project.
2. EVIDENCE FOR COLLIDING WINDS
A0
We began our search for spectroscopic evidence of colliding stellar winds in binary
systems of O-type stars with a study of AO Cas, a short period, double-lined spectroscopic
binary. We examined the orbital phase related variations of the Ha and He I _6678 line
profiles to derive the location and motions of high density circumstellar gas in the system.
We compared these profile variations with those observed in the UV stellar wind lines in
IUE archival spectra. The IUE spectra were also used to derive a system mass ratio by
constructing cross correlation functions of a single-lined phase spectrum with each of the
other spectra. The resulting mass ratio, q = Mo/Mp = 1.47 ± 0.08 is consistent with the
rotational line broadening of the primary (more luminous) star, if the primary is rotating
synchronously with the binary system. The other system dimensions were estimated by
comparing model light curves and line profiles with the observations. The best fit models
have an inclination of i = 61.1 ° -4- 3.0 ° and have a primary which is close to filling its
critical Roche lobe. The primary is undermassive for its spectral classification (O9 III),
and the secondary has a mass and radius typical of a late O-type main sequence star. We
formed Ha difference profiles by subtracting model photospheric lines from the observed
profiles, and the velocity curve of these profiles indicates that the emission originates near
the hemisphere of the secondary that faces the primary star. The large peak-to-peak width
and the lack of significant equivalent width changes in the Ha difference profiles suggest
that the emission forms in a bow shock near the secondary where the winds collide.
The UV stellar wind lines are dominated by the wind of the primary, and the absence
of any large changes in the absorption edge velocities of these lines indicates that the bow
shock never substantially occults the primary. However the red emission peak of the Si IV
P Cygni type line reaches a maximum intensity near superior conjunction of the primary
which is consistent with a lack of wind material in the region where the primary's wind
sweeps past the secondary.
Plaskett's Star
The strong Ha emission found in this system apparently contains two components, a
sharp emission peak that displays a radial velocity curve similar to that of the primary
star, and a very broad emission plateau that shows no discernible orbital motion but does
change significantly from night to night. The former component probably forms in a bow
shock very close to the primary while the second component may form in instabilities in
the shock region between the stars. Placement of the bow shock close to the primary star
suggests that the secondary star in fact has the stronger wind which is surprising given
the relative weakness of the spectral lines of the secondary. On the other hand, there is
evidence from the tomographic analysis (§3) that the secondary is a rapid rotator so that
its lines appear weak due to large rotational Doppler broadening.
The UV wind profiles show little evidence of orbital motion and consequently these
profiles cannot be used directly to infer which star has the dominant wind. However,
the C IV and N V P Cygni absorption troughs show a significantly weakening following
inferior conjunction of the primary star. We suggest that this weakening is caused by the
trailing bow shock surrounding the primary star. The density of scattering ions within the
shock cone is significantly lower than in the undisturbed wind of the secondary, and thus
at orbital phases when the bow shock cone is projected against the wind of the secondary
there is a reduction in number of scattering ions in our direction and the absorption trough
weakens.
29UW CMa
The strong Ha emission in thisbinary system shows littlevidence of orbital motion,
which suggests an origin between the stars close to the center of mass. The emission
intensity peaks at both conjunction phases. If the line emission is optically thick then
the emission strength depends on the projected area of the emitting cloud, and thus we
suspect that the cloud forms a disc shaped region with a surface normal to the axis joining
the stars.
The Ha emission suggests the existence of a bow shock between the stars with a wide
opening angle. The UV wind lines also support this picture. The blue edge velocities of
the P Cygni absorption troughs decrease near primary superior conjunction when the bow
shock occults the wind of the primary, and the bow shock effectively removes the highest
velocity part of the flow oriented in our direction during these phases. We also find that
the P Cygni emission peaks reach greatest intensity at the conjunction phases when the
bow shock volume enclosing the secondary presents the smallest projected area on the sky.
This system has the highest eccentricity in our sample (e = 0.76) and we are partic-
ularly interested in how the wind conditions change near periastron. The Hc_ profile in
this system shows no trace of emission except very close to periastron when broad, weak
emission slightly alters the appearance of the profile. The UV P Cygni features appear rel-
atively constant with orbital phase (particularly the blue edge velocity) but there is some
evidence of the orbital motion of the primary in the red emission peak of C IV, which
suggests that the primary has the dominant wind as expected. The results suggest that
the wind of the primary is relatively undisturbed through most of the orbit, but the wind
may become focused in the direction of the secondary close to periastron passage when
the primary comes close to filling its effective Boche volume (Stevens 1988, M.N.R.A.S.,
235,523).
3. TOMOGRAPHIC SEPARATION OF COMPOSITE SPECTRA
The UV spectra of hot stars contain many weak photospheric lines in addition to the
resonance lines formed in the wind. However in close binary systems, the lines of the
individual components are often too blended to permit an analysis of the line strengths
or allow a spectral classification. We have begun a program of separating the individual
component spectra by using a tomography algorithm to re-construct the individual spec-
tra from a set of combined spectra of differing orbital Doppler shifts. The tomography
algorithm is an iterative method that makes successive corrections to the spectra of each
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componentusing an assumed intensity ratio and the velocity curves for each component.
The resulting spectra tend to have a better S/N ratio than any individual spectrum since
data from many spectra are combined in the re-construction. Furthermore, because the
UV photospheric lines form in higher excitation transitions than optical lines, the resulting
spectra are less prone to contamination from circumstellar emission components. Tomog-
raphy appears to be an extremely useful technique for studying the spectra and physical
properties of both the primary and especially the less luminous secondary components of
binary systems. Here we summarize the results from tomography for three of the program
stars; we plan to study the fourth star, t Ori, later this year.
AO Cas
The IUE data were analyzed with the tomography algorithm to to produce the separate
spectra of the two stars in six spectral regions. The spectral classifications of the primary
and secondary, 09.5 III and 08 V, respectively were estimated through a comparison of
UV line ratios with those in spectral standard stars. We also estimated the intensity ratio
to be in the range 0.5 - 0.7 (primary brighter) at 1600 A through a comparison of line
strength in several features.
Plaskett's Sta_
A cross-correlation analysis showed that the secondary produces significant lines in the
UV, and we fitted the cross-corrdation functions for both components to derive a velocity
curve for the secondary star. The resulting mass ratio is q = 1.18 +0.12 (secondary slightly
more massive). Tomography was again used to produce the separate spectra of the two
stars in six spectral regions. From a comparison with spectral standard stars, we derive
interpolated spectral classifications of 07.3 I and 06.2 I for the primary and secondary,
respectively. The intensity ratio in the UV is 0.53 :k 0.05 (primary brighter). However, the
secondary apparently has a stronger stellar wind since we find that the N IV M718 line
is a well developed P Cygni profile in the secondary star spectrum while the feature is a
pure absorption line in the spectrum of the primary. The secondary lines are rotationally
broadened, and we estimate the projected rotational velocity to be 310 4- 20 km s -1.
29 UW CMa
A preliminary cross-correlation study of the UV photospheric lines indicates that the
secondary is visible in the UV but it is faint (intensity ratio _ 0.25). A velocity curve for the
secondary derived from the cross-correlation functions yields a mass ratio of q = 0.95+0.18.
The spectral classifications are approximately 07 I and 09.5 III for the primary and
secondary, respectively, from a preliminary tomographic analysis. The N IV )d718 feature
is a P Cygni profile for the primary only, and so the primary dearly possesses the stronger
wind.
4. BIBLIOGRAPHY
Bagnuolo, W. G., Jr., and Gies, D. R. 1991, "Tomographic Separation of Composite Spec-
tra: The Components of the O-Star Spectroscopic Binary AO Cassiopeiae", Astrophysical
Journal, in press (July 20 issue).t
Bagnuolo, W. G., Jr., Gies, D. R., and Wiggs, M. S. 1991, "Tomographic Separation of
Composite Spectra. I. The Components of Plaskett's Star", submitted to Astrophysical
Journal (attached here as an appendix).
Bagnuolo, W. G., Jr., Gies, D. R., and Wiggs, M. S. 1991, "Tomographic Separation of
Composite Spectra. II. The Components of 29 UW Canls Majoris", in preparation.
Bagnuolo, W. G., Jr., Gies, D. R., and Wiggs, M. S. 1991, "Tomographic Separation of
Composite Spectra. III. The Components of Iota Orionis", in preparation.
Gies, D. R., and Wiggs, M. S. 1991, "A Spectroscopic Search for Colliding Stellar Winds in
O-Type Close Binary Systems. I. AO Cassiopeiae", Astrophysical Journal, 375,321-334.t
Gies, D. R., and Wiggs, M. S. 1991, "A Spectroscopic Search for Colliding Stellar Winds
in O-Type Close Binary Systems. IV. Iota OrionJs", in preparation.
Wiggs, M. S. 1991, "A Spectroscopic Search for Colliding Stellar Winds in O-Type Close
Binary Systems", M.S. degree thesis, Georgia State University.
7
Wiggs, M. S., and Gies, D. 1t. 1991, "Rapid Variability of Ha and He I A6678 in Plaskett's
Star (HD 47129)", in Proceedings of the ESO Workshop on Rapid Variability of OB-StarJ:
Nature and Diagnostic Value (ESO CWP 36), ed. D. Baade, 25-26.t
Wiggs, M. S., and Gies, D. R. 1991, "A Spectroscopic Search for Colliding Stellar Winds
in O-Type Close Binary Systems. II. Plaskett's Star", in preparation.
Wiggs, M. S., and Gies, D. R. 1991, "A Spectroscopic Search for Colliding Stellar Winds
in O-Type Close Binary Systems. HI. 29 UW Canis Majoris", in preparation.
t Included in the Semiannual Status Report, May 1990 - December 1990.
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Colliding stellar winds in O-type close binary systems

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