Photoelectric spectrum scans of five southern Wolf-Rayet stars in the spectral range lambda lambda 4600-4720 were analyzed to study the variability of brightness and of emission line strengths. No variations of any kind in short time scale were found. However, in WC stars night-to-night variations of three to four percent were detected in the emission line strengths

Bahng, J. D. R.

English

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PHOTOELECTRIC SPECTROPHOTOMETRY OF WOLF-RAYET STARS
by
J.D.R. Bahng*
Astronomy Department, Northwestern University
Evanston, Illinois 60201, U.S.A.
(Received
(NASA-CR-140743) PHOTOELECTRIC N75-11810
SPECTROPHOTOMETRY OF WOLF-RAYET STARS
(Northwestern Univ.) 15 p HC $3.25
CSCL 03A Unclas
G3/89 53835
ABSTRACT
Photoelectric spectrum scans of 5 Southern Wolf-Rayet stars in the
spectral range AXX 4600-4720 were analyzed to study the variability of bright-
ness and of emission line strengths. No variations of any kind in short time
scale were found. However, in WC stars night-to-night variations of 3 to 4
percent were detected in the emission line strengths.
*Visiting Astronomer, Cerro Tololo Inter-American Observatory, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
under contract with the National Science Foundation.
https://ntrs.nasa.gov/search.jsp?R=19750003738 2020-03-23T02:57:26+00:00Z
2I. INTRODUCTION
There is enough evidence to indicate that variations in line intensities
and profiles occur in the spectra of Wolf-Rayet stars (Kuhi 1968). The exact
time scale of these variations is not yet known precisely, but among the bi-
nary W stars the changes appear to be related to the orbital period. The
changes are also observed in non-binary W stars; in these cases the changes
appear to be similar to those found by Oke (1954) among Of stars. Brucato
(1971) found variations in the strengths of emission lines of Of stars with
a time scale on the order of 10 minutes. Thus, one might expect that the
emission lines of W stars may exhibit similar rapid variations. Sanyal, Weller,
and Jeffers (1974) have in fact found a 154-second periodic change in the re-
gion around He II X4686 for y2 Velorum. Other recent studies have reported
variations in the spectra of W stars (Irvine and Irvine 1973; Moffat and
Haupt 1974; Wood, Schneider, and Austin 1974).
The present investigation is an attempt to detect rapid variations in
emission line strengths of the brightest southern Wolf-Rayet stars. Observa-
tions were made on y2 Vel as well. However, the results found for this star
were quite unique, and because of the interest generated by the observations
of Sanyal et al. (1974) and Wood et al (1974), they will be discussed in a
separate paper at a later date.
2. OBSERVATIONS
Observational material was obtained with a two-channel low-resolution
photoelectric spectrum scanner at the Cassegrain focus of the 92-cm re-
3flector of the Cerro Tololo Inter-American Observatory in March 1973. Two
circular entrance apertures of 18 arc-sec in diameter and separated by 2.6
arc-min were used to record simultaneously the spectral intensities of the
star plus sky and sky alone. Each scan consists of integrated signal counts
at every 4 A with an exit slit width of 10 A in the region XX4600 - 4720 A.
This spectral region was chosen so that the strongest emission lines in 
the
visual region, He II X4686 and C III - IV X4650 in WC stars or N III X4640
in WN stars, could be measured.
Each program star was observed continuously for about two hours, so 
that
a series of spectrum scans of the same star was obtained in which each 
scan
was separated by an equal time interval. During this time, an off-set 
guider
was used to insure a proper positioning of the star at the center of the aper-
ture. In every case, an attempt was made to accumulate signal counts of ap-
proximately 104 in the continuum level.
In order to evaluate reproducibility and the stability of data, four
standard stars were observed during the course of each night. These 
stars
were observed in the same manner as the W stars, except for the 
fact that for
each program star, a standard star was observed once before and 
once afterward.
Inspection of the data on standard stars shows that shifts 
in the mean levels
of the spectra between two sets, before and after, are for the most 
part less
than one percent: only on two occasions the shifts were greater 
at 1.2 and 1.6
percent. After removing these small shifts, two scans of 
each standard star
on each night were compared at each corresponding spectral element. 
The r.m.s.
deviations were found to be always less than one percent, in the 
range of 0.5
to 0.8 percent. It is concluded that the instrumental system was extremely
stable, and that the sky conditions were of excellent photometric quality.
The accuracy obtainable is, therefore, essentially limited by the photon-
4counting statistics. Pertinent parameters of observations on 5 W stars and
4 standard stars are given in Table 1.
3. RESULTS AND DISCUSSION
(a). Mean scans
For each night, individual scans were combined to form a single mean
scan for each star observed. After the sky contribution (from the second
channel) was removed, the observed signal counts at each spectral element
were combined without further correction. The resulting mean scans were then
rectified and normalized. The continuum level near 4720 A was generally 2
to 3 percent higher than that near 4600 A. The rectification was carried out
by assuming a linear increase of the intensity in the continuum with wave-
length over this interval. The scans were than normalized to this continuum
level. The resulting mean scans are shown in Figures 1-5. Dependence of the
relative strengths of emission lines of C and N class is clearly seen. Also
the difference between WC6 and WC7 is clearly indicated. There appear to be
some changes in the spectra between successive nights, particulary among WC
stars, which will be discussed in a later section.
(b). Variations in brightness
From each set of observations "integrated magnitude", m, for each scan
was computed as follows:
m = 2.5 log [EF (X)/ F(X)] (1)
where F(X)..is the actual observed signal count at each wavelength point, with
5the subscript o referring to the mean scan. Then the r.m.s. variations of m
were computed for each set. In order to remove effects of atmospheric extinc-
tion, linear and quadratic trends were removed and the r.m.s. recomputed. In
all cases, the results remained essentially unchanged. Because of the short
base line in wavelength and the fact that in all cases the stars were observed
near the meridian, the extinction effect is negligible. The r.m.s. variations
in brightness are shown in Table 2. They are all very small. This result re-
inforces the conclusion reached with regard to observations of standard stars
that the observing conditions were extremely stable and that the uncertainty
is essentially due to the photon-counting statistics. It is concluded that
these stars do not exhibit any brightness variations in a time scale of a few
minutes to about two hours. Since these analyses are confined to sets of con-
tiguous observations only, and since no attempt was made to establish actual
magnitude scale, no conclusions can be drawn with respect to the possible
night to night variations in brightness.
(c). Variations in emission line strenghts
Equivalent widths of emission lines were computed from each scan to study
possible variations of emission strength. In the case of WC stars, 
C III-IV
A4650 and He II 4686 overlap so much that the separation of the two can not 
be
made. For these stars, the total equivalent width, W(X4650 + X4686), was ob-
tained. For WN stars, there is still some overlap between N III X4640 and
He II X4686, but a well defined minimum is present at about 4665 A. 
W(X4640)
and W(X4686) were computed separately by using the data points only up 
to this
minimum point from each side.
The result is a series of equivalent widths as a function of time spaced
6at equal interval of time. Since a number of contiguous scans in each set
is rather small, no attempt was made to utilize a power spectrum analysis to
detect possible periodic variations. Instead the r.m.s. variations were
computed for each set of equivalent widths. in order to assess whether these
r.m.s. values represent real variations, or due to the observational errors,
theoretical r.m.s. values were computed by assuming the photon counting sta-
tistics alone. The procedure for calculating the theoretical r.m.s. values,
aphot' is essentially the same as that used by Williams, Frantz, and 
Breger
(1974).
The results are summarized in Table 3. In all cases, aobs is greater
than a phot, but never more than a factor 2. Williams et al (1974) also find'
that such is the case in their study of peculiar A stars. Since no other
source of errors other than the photon statistics is allowed in these calcu-
lations, this is not surprising. If aobs were substantially larger than
ophot , by say a factor or 4 or more given the quality 
of the data on hand, it
may be inferred that there are real variations. It is concluded, therefore,
that there is no short-term variability of emission strengths in these stars.
The inspection of the mean equivalent widths, however, shows that night
to night variations may exist in some cases. For WC stars there appears to
be a very strong indication that this is the case. The r.m.s. variations of
mean equivalent widths from night to night are at least twice as large as aobs
for a single night, 4 percent for HD 113904, and 3 percent for HD 152270.
Among WN stars, they are always equal to or less than Gobs for a single night.
That there may be some variations from night to.night are present in the WC
stars, can be seen from the Figures 1 and 2.
7It so happens that among the brightest W stars chosen for this study,
all the WC stars are known binaries, while all the WN stars, as far as is
known at present, are non-binaries. With this small sample, it is of course
dangerous to generalize and draw a conclusion that the binaries show varia-
tions of this sort while singles do not. Moffat and Haupt (1974) have found
some long-term variability over a three-hour interval in He II X4686 emission
strength for HD 93131 which is a single WN star. The problem thus requires
further observations of more stars and of longer durations.
I wish to express my deep gratitude to the staff of Cerro Tololo Inter-
American Observatory for their hospitality and valuable assistance during
my visit to Chile. This research was supported in part by a NASA grant
(NGR 14-007-122).
REFERENCES
Brucato, R.J. 1971, M.N.R.A.S., 153, 435.
Irvine, C.E., and Irvine, N.J. 1973, P.A.S.P., 85, 403
Kuhi, L.V. 1968, Wolf-Rayet Stars, ed. Gebbie and Thomas (Washington, D.C.:
National Bureau of Standards), p.12 8 .
Moffat, A.F.J., and Haupt, W. 1974, Astron. & Astrophys., 32, 435.
Oke, J.B. 1954, Ap. J., 120, 22.
Sanyal, A., Weller, W., and Jeffers, S. 1974, Ap._., 187, L31.
Smith, L.F. 1968, M.N.R.A.S., 138, 109.
Williams, W.E., Frantz, R.L., and Breger,,M. 1974, Astron. & Astrophys.,
(in press).
Wood, F.B., Schneider, W.H., and Austin, R.R. 1974, Bull. Am. Astro. Soc.,
6,222.
Table 1. Parameters of Observations
(1) (2) (3) Number of Scans each night, 
1973 (UT)
ED NO. Sp. Ty. mV  t(sec) AT(iain) Mar. 22 Mar. 23 Mar. 24 Mar. 25 Mar. 26
68324 B3 V 5.40 4
92749 WN 7 6.41 8 5.02 22 - 22 - 20
92938 B3 V 4.80 4
93131 WN 7 6.7 8 5.02 - 21 - 21 -
113902 B8 V 5.70 4
113904 WC6 + 09.51 5.50 4 2.85 28 33 38 42 35
128345 B5 V 4.04 2
151932 WN 7 6.45 8 5.02 22 - 22 - 22
152270 WC7 + 05-8 6.61 8 5.02 - 15 - 22 -
(1) Spectral Classisfication of W-R stars from Smith (1968)
(2) Net integration time per spectral element.
(3) Time interval between successive scans.
Table 2. Brightness Variations in W stars
R.M.S. magnitued Variations
HD NO. Mar. 22 Mar. 23 Mar. 24 Mar. 25 Mar. 26
92740 0.003 0.003 0.003
93131 0.003 0.002
113904 0.004 0.007 0.002 0.006 0.007
151932 0.004 0.005 0.003
152270 0.004 0.003
Table 3. Emission Strength Variations in W stars
W(X4650+X4686) W(X4640) I W(X4686)
WC stars N(A) c obs(%) 0phot (%)  WN stars l (A) aobs (%) Ophot(%) W(A) 
0obs % ) phot
HD 113904 HD 92740
Mar. 22 -42.9 2 1 Mar. 22 -10.9 4 2 -22.4 2 1
Mar. 23 -43.1 2 1 Mar. 24 -10.9 4 2 -22.6 2 
1
Mar. 24 -40.5 2 1 Mar. 26 -11.7 4 2 -22.7 
2 1
Mar. 25 -40.6 2 1
Mar. 26 -38.9 2 1 HD 93131
Mar. 23 - 9.4 4 3 -26.7 2 1
iD 152270 Mar. 25 - 9.7 4 
3 -26.7 2 1
Mar. 23 -222. 1 0.5
Mar. 25 -233. 1 0.5 HD 151932
Mar. 22 -20.3 3 2 -38.6 2 1
Mar. 24 -19.3 3 2 -37.7 2 1
Mar. 26 -19.1 3 2 -38.4 2 1
o
I I I
HD 113904
6.- WC 6 + O 9.5 I
Mar. 22
5
(1)4 Mar. 23
0
U Mar. 24
4 3-Ul
Mar. 25
2
Mar. 26
I I I
4600 4650 4700
WAVELENGTH (A)
Figure 1. Mean Scans for HD 113904 with a 10 A exit slit.
12
I I
7-
6 HD 152270
WC 7+ O05-8
5
u/) 4-Zz
0O
U
+ 3
L-:' 2 Mar.23
1 Mar. 25
I I I
4600 4650 4700
WAVELENGTH (A)
Figuew 2. Mean Scans for ID 152270 with a 10 A exit slit.
13
5- HD 92740
WN 7
4-
Mar. 22
z 3
0
Zm
O
U
+ 2 Mar 2
U-
S 1 Mar. 2
I I I
4600 4650 4700
WAVELENGTH (.)
Figure 3. Mean Scans for HD 92740 with a 10 A exit slit.
14
HD 93131
4-
WN 7
3
O 
Mar. 23
2
+
uL Mar 25.
1
LL.
I I I
4600 4650 4700
WAVELENGTH (A)
Figure 4. Mean Scans for HD 93131 with a 10 A exit slit.
15
HD 151932
5
WN 74
A--
Mar. 22
Z 3
0
U
+ Mar 24
2 -
UMar 26_
1-
I I I
4600 4650 4700
WAVELENGTH (A)
Figure 5. Mean Scans for HD 151932 with a 10 A exit slit.


Photoelectric spectrophotometry of Wolf-Rayet stars

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