Photographic spectra of Cen X-3 show that the primary star has a spectral type near 06.5 with weak, variable emission at wavelength 4640 and 4686. No orbital motion of the emission or absorption lines is detected; for the latter the upper limit is approximately + or - 50 km/s. Analysis of the available data indicates that the primary is a factor of 2-3 less massive than expected from normal evolutionary models while the X-ray source has a solar mass near 1.5

Hiltner, W. A.

Osmer, P. S.

Whelan, J. A. J.

English

NASA Technical Reports Server

PHYS CAL PARAMETERS OF THE CEN X-3 SYSTEM
Patrick S. Osmer
Cer o Tololo Inter-American Observatory*
W. A. Hiltnert
University of Michigan
J. A. J. Whelan
Institute of Astronomy, Cambridge
Abstract. Photographic spectra of Cen X-3 show that the primary star has a
spectral type near 06.5 with weak, variable emission at AX4640 and 4686. No
orbital motion o the emission or absorption lines is detected; for the latter
the upper limit a ~50 Ikm s . Analysis of the available data indicates that
the primary is afactor of 2-3 less massive than expected from normal evolu-
tionary models, vile the X-ray source has a mass near 1.5 Me.
/(NASA-CR-139569) : PHYSICAL PARAWETERS OF N714-31322
THE CEN X-3 SYSTEM (Cerro Tololo
Inter-American Observatory) 10 :p HC
$4.00 - .CSCL 03A Unclas
\~__G3/30 46651
I + - ,+
*Operated by the Association of Universities for Rebearch in Astronomy, Inc.,
under contract with the National Science Foundation.
tVisiting Astronomer, Cerro Tololo Inter-American Observatory.
https://ntrs.nasa.gov/search.jsp?R=19740023209 2020-03-23T03:57:24+00:00Z
-2-
The Cen X-3 yatem is the only binary X-ray source with a massive cornm-
panion for which both the orbital parameters of the X-ray source (Schreier /
et al. 1972) and the optical ellipsoidal light curve (Krzeminski 1973, 1974,
Petro (1974) have been determined. Consequently it is the only massive system
for which a fundamental determination of parameters such as the radius of
the primary and masses of the components can be made. In this paper we present
spectroscopic observations of. the ,primary star which yield a spectral type
near. 06.5 andan intermediate,iuminosityclass. Then consideration of Krzeminski's
light curve shows that, the primary is a third to half as maive as expected
from nqrmal evolutionary model.while, he X-ray source is more massive than
originally, thought. In the accompanying paper, Petro (1974) reaches similar
conclusions from a detailed, independent analysis of a new light curve obtained
by him..
SWe used the Cassegrain.spectrograph on the Cerro Tololo, 1.5-m telescope
to obtain 102 X/mm, 03ramm wide spectrograms on nitrogen-bakedIIa-O emulsion.
We madesix exposures of Cen X-3: two of 4 hr on,1974 Mar 18 and the rest of
7 hr on 1974 Mar,19, 23, 24, and 29 (Table 1). The coverage in phase was
satisfactory, except for.a.ap ,between 0.1 and 0.4 (phase 0.0 corresponds to
X-ray eclipse). Since the 7 hr exposures covered 1/7 of the orbital period,
there was a loss of time resol.ution. However,, for a first reconnaissance of
the system, especially in view of the rather weak-line4 nature of 0 star
spectra, we Judged that the long exposures were preferable to using an image
tube.
Examination of the spectra (Figure 1) leaves no doubt that lines of H,
I I 
.
,' /
-3-
He I, and He II are present in absorption. Both A14200 and 4541 of He II
are seen, and their approximate equality, on the average, with A)4026 and
4471 of He I, respectively, suggests a spectral type near 06.5. The rapid
decrease of the He II/He I ratio with advancing spectral type means that
the star cannot be much later than 07. Although the visibility of the
absorption lines is not the same on all plates, it is difficult to estimate
how significant the differences are relative to the plate naise. On plate
C-3988 (Figure ib) He II .A45 4i is not seen and Hy is weak. H6 appears to have
aP Cyg profile. However, the He II AX4200/He I - He II 4024 ratio is approx-
imitely the same as on plate ,C-3979 (Figure la), for example, which rules out
a large change tin temperature. Spectrograms with a higher signal-to-noise
ratio are needed to make a more,accurate classification, and settle the question
of variability. i,' .. ,
Weak, ill-defined emission can be sen on the spectrograms at X46140 and
4686. There-is no obvious variation of the emission-line velocities with time,
which ,are not distinguishably different from the absorption-line velocities.
At our dispersion Cen X-3 4s.,very similar to HD 153919 = 2U1700-37 (Figure le)
except fqr having weaker emission, which implies that the primary star has a
lower luminosity, probably-near class II or III.
The main difference between our results and those of Rickqrd (1974) and
Vidal et al. (1974), who derived. a spectral type ,of 09-B0. from image tube
spectrogramst yis that our plates show the presence of He II X14200 and 4541
in absorption INote that the absence of He II AX4686 in absorption is not in
itself an argument, aginat. the ,earUer spectral type found by us since the line
changes from absoprtion to,emissionwijh reasing l1minosity. Our experience
indicates that pbptograph1iq ,spectra are to be preferred for classifying 0 stars
Ii.,
because wak absorptio lines are more difficult to see on image tube
spectra of similar dis ersion. There is, of course, the possibility of a
long-term variation i spectral type of Cen X-3, but the lack of such var-
iation in other massie X-ray binaries and the low ratio of X-ray luminosity
to total optical 1 nosity in Cen X-3 do not provide support for the idea
at present.
Radial,velocit measures of the absorption, lines show a wide scatter
on any one plate an the mean of the A3835, Hy, and X4471 velocities (the
lines most readily, easurable), shos no obvious variation with phase (Table 1).
Fitting a sine., c ,to the resultagives residuals indistinpishable from
fitting a straight line, 38 km s in the 4een. The interstellar K-line
velocity,,is -12 km s-I with an rms deviation of 22 km s-I for a single plate.
The velooity, ata for the absorption lines give an upper limit for the
semi-amplitude of .he orbital motion of th& primary of about 50 km s
Since the correspq ding velocity.ox the X-ray source is '400 km -a (Schreier
et al. 1972) an uppertlimit for the mass ratio q = M /Mprimary is 1/8. That
x prima.ry
this limit .is,,ealready of some significance, and very important to improve upon,
is apparent from, consideration of the ellipsoidal light curve.
We have taken advantage of the visit of one of us (P.S.O.) to Cambridge
to bring the results of a different light-curve synthesis program (that of
J.A.J.W) together with those given by Petro (197T4). Several groups have dev-
eloped suqh programs, but there has been! little direct comparison of the results
published as yet. In the fpllowing discussion we consider the implication of
the,,X-r;7y, orbital data, the amplitudes of, Krzeminsk' a light curve (Petro's
V.I
II
data were not available then), and the spectroscopic results Just described.
Petro (1974) presents in the accompanyng paper a comprehensive analysis of
his new light curve.
The observations of Krzeminski (1974) indicate that the depths of the
minimatin,V)are Am1 = 0.09 ± 0.02 mag at phase 0.5 and Am2 =.0.07 ± 0.02
mag at phase 0.0.. These values are larger than predicted by Wilson (1972)
from calculations of.the light, curve that satisfy the duration of the X-ray
eclipse,(Schreier et al. 1972). ,Wilson derived an upPer limit of q ~ 0.02
on the assumption the X-ray eclipses are produced by the stellar photosphere
at its Roche limit. However, a larger mass ratio is necessary to produce
the observed light curve; :to be consistent with the observed eclipse duration
this implies either that the primary overfills its Roche lobe, perhaps as
described by Weedman and Hall. (1972), or that X-ray occulting material extends
beyond the photosphere. I . ,
Using a light-curve syntiesis program based on. that described by
Strittmatter et,9 a_. (1973), whichfassumes co-rotation, we constructed a
series of light curves for conditions appropriate to Cen X-3: temperatures
of 30,0000 and 40,000K at the. "back" of the primary, no X-ray heating,
inclination of the orbit, i, = 900, and mass ratios 0.018 . q _. 0.20. The
effects .Qf altering the input parameters to include (1) overfilling or under-
filling the Roche lobe, (2) a small amount of X-ra5 heating, and (3) a decrease
of inclination i are illustrated in Figure 2 of Whelan (1973), where further
deoails of the technique minay also be, found (see,also,Hutchings 1974). The
first two effects are small here. Solutions of the light-curve amplitudes
alone are possible as;i.decreases, .but the resulting increase in q is limited
both by the long eclipse,,,which can hardly be satisfied even at 900, and the
upper bound on the orbital velocity of the primary. All presently available
I
6-
data require that 90.
We adopt i - 00 and q 0.08 as the solution which gives the best fit
to the amplitudes 'of the light curve and note that it implies an orbital
velocity of -33 kA s " for the primary, well within the' observed limit, and
an eclipse angle E of 370 . Although 37o is below the formal lower bound of
420 of Schreier e al. (1972), it is not unreasonable in view of the possibility
of the primary ov rfilling the Roche lobe or that a region which is larger than
the photosphere, _9paque to X-rays, and caused by a stellar wind may exist (Pringle
1973).
When combined with the X-ray mass function and a primary Te of -36,000.
the solution ippliqeathat II Ii
M 1 8 ± 1 . . o . 4
primary aM MX ray 1-4 04M
R -12 R and L1 x 1039 erg s1
primary 0 , primary
iI
where R is the equivalent spherical radius. The derived luminosity is con-
sistent with that of an 06.5 III star and indicates a distance of -10 kpc. The
37 -1 "'
corresponding X-ray luminosity is "3 x 10 erg a '. The mass of the X-ray
object is plausible and shows that it need not be of very low mass (Sofia 1972,
Wilson 1972). We especially note: (1) the primary cannot be a low-mass
highly evolved star, regardless of the value of i; (2) M ri < < 50 Mo,
which' is the mass value derived from the luminosity and normal stellar models
(Papaloizou and Whelan 1973) (3) Lprimary (Cen.X-3)- Lprimary (SMC X-1),primary pri.mar y
for which the luminosity is known independently; and (14) the primary mass of
Cen X-3 is similar to that:.ptimated by Osmer and Hiltner (1974) for SMC X-l.
. I I I, , . ' i ,
'A'
Evidently, the masses of the optical counterparts of the X-ray binaries
should not be derived solely by fitting spectral types and luminosities to
C
evlutionary models; rather the masses must be 4etermined directly from the
radial velocity and light curve data. For this reason it appears to be
premature to conclude that 2U0900-40 (Wicklamasinghe et al. 1974) and 2U1700-
37 (Bessell et al. 1974) are black holes.
We are 'indebted to Mark Hanna for the photographic vork involved in
producing Figure 1 anqYo Klaus Czuia for assistance at the telescope. We
also thank Dr. Nolan Walbo for useful discussions. This wor4, was supported
kn part by NASA under grant NGR 23-005-464.
44.
i i"I
I.
TABLE 1
Spectroscopic Observations
J.D. Heliocentric radial
Plate 2442 Phase velocity (km s-2)*
C-3977 124.601 0.54 -47
C-3978 124.783 6 63 - 7
C-3979 125.697 o 0o 57
C-3988 129.704 0.99 20
C-3991 130.683. 0.46 -41
C-3993 135.660 0.84 -15
I -
*Mean of A3835, Hy, and A447Tlvelocities. Mean error of a single.
value -30 km' .
J
REFERENCES
Bessell, M. S., Peterson;, B. A., Wickramasinghe, D. T., and Vidal, N. V.
1974, p . 87, 355.
Hutchings, J. B. _ ., 188, 4 .
Krzeminski, W. 1973,I.A.U. Circular No. 2612.
1974, Ap. J. Letters, in press.
Osmer, P. S., and Hiltner, W. A. 1974, Ap. J. Letters, 188, L5.
Papaloizou, J. C. B. and Vhelan, J. A. J. 1973, M.N.R.A.S., 164, 1.
Petro, L. D. 1974, Ap. J. Letters, preprint.
Pringle, J. E. 1973, Nature Phys. Sci., 243, 90.
Rickard, J. J. 1974, Ap. 4. Letters, 189, L113.
Schreier, E., Levinson, R., Gursky, H., iKellogg, E., Tananbaum, H., and
, Giacconi, R. 1972, Ap. J. Letters, 17, L79.
Sofia, S. 1972, Ap. J. Letter,,174, L31.
Strittmatter, P. ,A, Scott, J., Whelan, J.,, Wickramasinghe, D. T., and Woolf,
N. J. 1973, Astr. andAp., 25, 275.
Vidal,,N. V., Wickramasinghe, D. T., Peterson, B. A., and Bessell, M. S.
1974,,Ap. J. Letters, inpress.
Weedman, D. V., and Hall,,D. S. 1972, Ap. J. Letters, 176, LI9.
Whelan, J. 1973, ,Ap. J. Letters, 185, L127..
Wickramasinghe, D. T., Vidal, N. V.,:Bessell, M. S., Peterson, B. A., and
Perry, M. E. 1974, A .J., 188, 167.
Wilson, R. E. 1972, Ap. J.iLetters, 174, L27.
-10
FIGURE CAPTION
Figure 1. Spectr of the Cen X-3 primary and associated comparison
stars. (a) Cen X-3, plate C-3979, phase 0.07; (b) Cen X-3,
plate C-3988, phase 0.99; () Cen X-3, plate C-3991, phase
0.46; (d) Cen X-3, plate 0-3993, phase 0.84; (e) HD 153919 =
2U170 -37;'(f) 15 Mon, MK standard; (g) X Ori, MK standard;
(h) %1Ori, MK standard. The strength of the He II absorption
lines in Cen X-3 indicate that its spectral type is similar
to that of HD 153919 but the weaker emission at 1 4640 and
4686 plies a lower luminoity.
'I j
'U , | . ' . I
I .. . I


Physical parameters of the Cen X-3 system

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server