The reliability of the present technique was checked on stars in the area of two young open clusters NGC 2244 and NGC 2264. In the case of NGC 2244 only those stars which have spectral estimates in either MK system (Ogura & Ishida 1981), Abastumany classification system (Voroshylov et al. 1985), or were classified via processing of unwidened low-dispersion spectrograms (Kuznetsov 1986) have been involved to the study. Photometric values and colour parameters of stars were taken from the paper by Ogura & Ishida (1981), and proper motions from the paper by Marschall et al. (1982).
Tables 3-6 give particular examples of the spectral classification on the basis of UBV photometry for open cluster NGC 2244 stars Nos. 127, 158, 239 and 278 (numbering is given here as by Ogura & Ishida 1981) with different spectral types. These examples show which way the use of average colour excess dependence on intrinsic distance modulus (Fig. 1) allows us to select a single correct spectral type among some possible ones.
Measured values: V=8.77, B-V=0.09, U-B=0.19 | ||||||||||
N | Sp | (B-V0) | E(B-V) | AV | V0 | MV | MV | V0-MV | V0-MV |
![]() |
(MS) | (ZAMS) | (MS) | (ZAMS) | |||||||
1 | A2 V | 0.05 | 0.04 | 0.13 | 8.64 | 1.3 | 1.7 | 7.34 | 6.94 | A2 V |
2 | K5 V | 1.15 | -1.06 | 0.00 | 8.77 | 7.35 | 7.3 | 1.42 | 1.47 | - |
3 | A2 III | 0.05 | 0.04 | 0.13 | 8.64 | 0.3 | - | 8.34 | - | A2 III |
4 | K3 III | 1.27 | -1.18 | 0.00 | 8.77 | 0.3 | - | 8.44 | - | - |
5 | M3 III | 1.61 | -1.52 | 0.00 | 8.77 | -0.6 | - | 9.37 | - | - |
6 | F2 I | 0.23 | -0.14 | 0.00 | 8.77 | -6.6 | - | 15.37 | - | - |
7 | K5 I | 1.60 | -1.51 | 0.00 | 8.77 | -5.8 | - | 14.57 | - | - |
QUBV=0.11,
![]() ![]() |
Measured values: V=10.25, B-V=0.66, U-B=0.04 | ||||||||||
N | Sp | (B-V0) | E(B-V) | AV | V0 | MV | MV | V0-MV | V0-MV |
![]() |
(MS) | (ZAMS) | (MS) | (ZAMS) | |||||||
1 | B3 V | -0.20 | 0.86 | 2.75 | 7.50 | -1.6 | -1.1 | 9.10 | 8.60 | - |
2 | G2 V | 0.63 | 0.03 | 0.10 | 10.15 | 4.7 | 4.7 | 5.45 | 5.45 | G2 V |
3 | M5 V | 1.64 | -0.98 | 0.00 | 10.25 | 12.3 | 12.3 | -3.05 | -3.05 | - |
4 | B5 III | -0.20 | 0.86 | 2.37 | 7.88 | -2.2 | - | 10.08 | - | - |
5 | G0 III | 0.65 | 0.01 | 0.03 | 10.22 | 1.0 | - | 9.22 | - | G0 III |
QUBV=-0.52,
![]() ![]() |
Measured values: V=11.06, B-V=0.29, U-B=-0.21 | ||||||||||
N | Sp | (B-V0) | E(B-V) | AV | V0 | MV | MV | V0-MV | V0-MV |
![]() |
(MS) | (ZAMS) | (MS) | (ZAMS) | |||||||
1 | B5 V | -0.17 | 0.46 | 1.47 | 9.59 | -1.2 | -0.5 | 10.79 | 10.09 | B5 V |
2 | G2 V | 0.63 | -0.34 | 0.00 | 11.06 | 4.7 | 4.86 | 6.36 | 6.20 | - |
3 | G6 V | 0.70 | -0.41 | 0.00 | 11.06 | 5.2 | 5.2 | 5.86 | 5.86 | - |
4 | B5 III | -0.17 | 0.46 | 1.47 | 9.59 | -2.2 | - | 11.79 | - | B5 III |
5 | G0 III | 0.65 | -0.36 | 0.00 | 11.06 | 1.0 | - | 10.06 | - | - |
6 | B9 I | -0.02 | 0.31 | 0.93 | 10.13 | -6.9 | - | 17.03 | - | - |
QUBV=-0.45,
![]() ![]() |
Measured values: V=10.21, B-V=1.04, U-B=0.69 | ||||||||||
N | Sp | (B-V0) | E(B-V) | AV | V0 | MV | MV | V0-MV | V0-MV |
![]() |
(MS) | (ZAMS) | (MS) | (ZAMS) | |||||||
1 | B9 V | -0.07 | 1.11 | 3.55 | 6.66 | 0.2 | 0.9 | 6.46 | 5.76 | - |
2 | F0 V | 0.30 | 0.74 | 2.37 | 7.84 | 2.7 | 2.8 | 5.14 | 5.04 | - |
3 | K2 V | 0.91 | 0.13 | 0.42 | 9.79 | 6.4 | 6.3 | 3.39 | 3.49 | K2 V |
4 | M0 V | 1.40 | -0.36 | 0.00 | 10.21 | 8.8 | 8.8 | 1.61 | 1.61 | - |
5 | B9 III | -0.07 | 1.11 | 3.55 | 6.66 | 0.2 | - | 6.46 | - | - |
6 | F2 III | 0.35 | 0.69 | 2.21 | 8.00 | 1.7 | - | 6.3 | - | - |
7 | G8 III | 0.94 | 0.10 | 0.32 | 9.89 | 0.8 | - | 9.09 | - | G8 III |
8 | M6 III | 1.52 | -0.48 | 0.00 | 10.21 | -0.3 | - | 10.51 | - | - |
9 | A3 I | 0.02 | 1.02 | 3.26 | 6.95 | -7.2 | - | 14.15 | - | - |
10 | G0 I | 0.76 | -0.28 | 0.00 | 10.21 | -6.4 | - | 16.61 | - | - |
11 | G8 I | 1.14 | -0.08 | 0.00 | 10.21 | -6.2 | - | 16.41 | - | - |
12 | M2 I | 1.71 | -0.68 | 0.00 | 10.21 | -5.6 | - | 15.81 | - | - |
QUBV=-0.19,
![]() ![]() |
For example, the star No. 127 at fixed value
QUBV=0.11 have a set
of seven possible spectra: A2 V, K5 V, A2 III, K3 III, M3 III, F2 I
and K5 I. It is not difficult to see that only two spectral estimates
(A2 V and A2 III) have such E(B-V) and
(V0 -MV) values which deviate
not more then
and
from the mean curve shown
in Fig. 1. All other alternative estimates of spectra are surely
discarded as wrong and a preliminary spectrum of a star is thus
determined as
A2. Then using
a new value of
E(U-B)/E(B-V) is
recomputed giving a final spectral class SpUBV=A2. Similar reasoning
allows us to determine spectra of stars Nos. 158, 239 and 278 (Tables 4-6).
The offered technique however does not permit us to classify stars on their luminosities. At the same time there is a good agreement of received results with other available estimates of spectra in the MK system on the temperature parameter.
In the process of spectral classification, about 60% of analyzed stars have been assigned to a single or two rather close estimates of spectra. In the last case an average value was adopted. Another 40% of stars have obtained two substantially different estimates of spectra. Double results of the classifying process based on UBV data originates as a rule in equal probability of some stars to be classified as young B or late F-G stars. In order to determine correct spectral type we applied the next criterion. In general, it may be modified, depending on the peculiarities of the character of the absorbing medium and star distribution in the space, and on interval of star magnitudes where spectral classification is carried out.
According to Kuznetsov (1988), on the distance of the open
cluster NGC 2244, one may observe only young O-A5 stars as
far as a limiting magnitude in the catalogue used is
mag.
It is clear therefore that if the star with an uncertain spectral
estimate is a cluster member or belongs to a far background
population it can not be of a late spectral type F-G to be
observable. In this case from two possibilities (B or F-G) the
spectral type of such a star is determined unequivocally as B. And on
the contrary, if the star is not a member of an open cluster, and
belongs to the foreground, then with a high degree of probability it
should be of a late spectral type. Thus a solution of this
particular problem is reduced to the problem of membership study, or
to separation of cluster members from field stars. This was carried
out with the use of an earlier developed method (Kuznetsov et al. 1993) of
membership study which takes advantage of star distribution in
multidimensional data space formed by all available observational
data. Normally, coordinate axes of the space correspond to proper
motions, angular astrometric positions, E(B-V), (B-V)0, and the intrinsic
distance module. In some particular cases, when a few faint stars
have no derived proper motions, computing procedure for these stars
was carried out in a data subspace of lower dimension. A star was
identified as a cluster member when calculated membership probability
P exceeded 50%.
Observational data and results of the spectral classification of
125 stars brighter than V = 14.0 mag around NGC 2244 are presented in
Table 7. In the first three columns of the table, serial numbers,
magnitudes V, and spectral classes
of stars in MK system, according
to the catalogue by Ogura & Ishida (1981) are given. The next columns
contain: estimates
of spectra in Abastumany system of spectral
classification (Voroshylov et al. 1985); spectral types
derived by
Kuznetsov (1986) from unwidened low-dispersion spectrograms;
spectra SpUBVreceived on the basis of UBV photometry with the present technique of
spectral classification; probabilities of membership P in percent
(see comments above; P are given only for the cluster members, field
stars are marked with "f'').
N | V |
![]() |
![]() |
![]() |
SpUBV | P,% |
11 | 12.37 | - | - | G6 | G6 | f |
14 | 12.66 | - | - | K2: | K3 | f |
16 | 10.99 | - | - | F8 | G3 | f |
20 | 12.50 | - | - | F6 | F3 | f |
30 | 12.72 | - | - | G0 | F2 | f |
35 | 12.90 | - | A3: | A2 | A2 | f |
36 | 13.21 | - | - | B5 | B9 | f |
39 | 12.09 | - | - | F3 | F0 | f |
45 | 10.40 | - | A1 V | A2 | A3 | f |
53 | 10.63 | - | - | G3 | G2 | f |
56 | 12.91 | - | A0-A2: | F2 | A3 | f |
59 | 12.16 | - | A2 V | B9 | B5 | f |
61 | 13.86 | - | - | B5 | B9 | f |
62 | 12.76 | - | - | B2 | B3 | f |
65 | 12.66 | - | F5-F8: | G0 | F3 | f |
67 | 12.99: | - | - | K0 | K0 | f |
69 | 12.87 | - | - | A0 | A2 | f |
72 | 12.42 | - | - | F6 | F4 | f |
74 | 12.39 | - | - | B7 | B7 | 93 |
79 | 10.62 | B2 V | B0 | - | B3 | 99 |
80 | 9.29 | B0,5 V | B0 V | - | B0 | 99 |
84 | 8.19 | O8 V | O8 V | - | B0,5 | 89 |
85 | 13.16 | - | F5-F6: | F6 | A9 | f |
95 | 13.80 | - | - | B7 | A2 | f |
100 | 12.45 | - | - | F5 | F2 | f |
106 | 11.82 | - | - | G8 | G2 | f |
108 | 11.40 | B8 III | B8 V | B8 | B7 | 83 |
109 | 13.71 | A0 III | A0: | A0 | A1 | 62 |
110 | 10.72 | - | - | K5 | K6 | f |
111 | 12.07 | - | - | K3 | K2 | f |
115 | 7.90 | B0 V | B1 III | - | B0 | 89 |
116 | 12.73 | - | B | B8 | B8 | 100 |
118 | 12.35 | - | - | K7 | K5 | f |
127 | 8.77 | A2 V | A2 V | - | A2 | f |
128 | 9.36 | B1,5 V | B2 V | - | B0 | 100 |
130 | 11.60 | B2,5 V | B5 | B3 | B3 | 100 |
133 | 11.69 | - | A2 V | A1 | B9 | 98 |
135 | 11.65 | - | - | G2 | G0 | f |
136 | 13.45 | - | - | G8 | G2 | f |
137 | 13.43: | - | A0 V | A0 | B9 | f |
140 | 13.84 | - | - | A3 | A2 | 61 |
141 | 13.91 | - | - | F3 | F3 | f |
142 | 10.09: | F6 IV | - | F8 | F5 | f |
145 | 12.75 | - | - | F5 | F4 | f |
153 | 12.55 | - | A2 V | A5 | A2 | f |
156 | 10.69 | F6 V | - | F5 | G2 | f |
158 | 10.25 | G0 IV | - | F8 | G2 | f |
160 | 12.68 | - | A0 V | B8 | B8 | 91 |
163 | 13.02 | - | - | K3 | K5 | f |
164 | 13.21 | - | - | B8 | B5 | f |
167 | 10.73 | B2 V | B2 V | B2 | B0 | 100 |
169 | 12.46 | - | - | F8 | G2 | f |
171 | 12.81 | - | - | G8 | G2 | f |
172 | 11.18 | B2,5 V | B2 V | B2 | B0 | 100 |
173 | 10.28 | - | - | K3 | K1 | f |
180 | 8.17 | O9 V | O9 III | - | B0 | 100 |
183 | 11.33 | - | A2 V | A3 | A7 | f |
188 | 11.71 | - | - | K2 | K4 | f |
189 | 11.15: | - | - | A0 | A0 | f |
190 | 11.24: | B2,5 V | - | - | B5 | 100 |
192 | 12.40 | - | B3 V | - | B7 | 78 |
193 | 10.31 | B2 V | B0 V | - | B1 | 100 |
194 | 11.95 | B6 V | A5 V | A3 | B3 | 100 |
197 | 12.57 | - | - | A0: | B7 | 100 |
198 | 12.64 | - | A | B8 | B8 | 99 |
200 | 8.54 | B0 V | B1 V | - | B0 | 100 |
201 | 9.71 | B1 V | B1 V | - | B1 | 100 |
207 | 11.43 | - | - | G8 | G2 | f |
210 | 11.50: | - | - | K7 | K5 | f |
213 | 11.51: | F2 | A2p | A3 | F0 | f |
215 | 13.40 | - | - | F5: | F2 | f |
217 | 12.86 | - | - | F6 | F0 | f |
219 | 11.33 | - | - | F5 | F0 | f |
223 | 13.16 | - | - | F3 | F0 | f |
225 | 12.09 | - | - | K5 | K5 | f |
N | V |
![]() |
![]() |
![]() |
SpUBV | P,% |
227 | 13.66 | - | - | K0: | K1 | f |
229 | 13.42: | - | - | G3 | G2 | f |
231 | 12.56 | B5 | - | B5 | B5 | 97 |
233 | 12.85 | - | - | G0: | G2 | f |
236 | 13.60 | - | - | G0 | G2 | f |
239 | 11.06 | B0 V | B9 V | B7 | B6 | 98 |
241 | 11.07 | B8 V | A2 V | B5 | B3 | 99 |
245 | 12.53: | - | B8 V | A0 | A2 | 96 |
253 | 10.76 | B3 V | B5 V | B5 | B2 | 100 |
263 | 12.35 | - | - | K7 | K7 | f |
264 | 13.50 | - | - | G3: | G2 | f |
267 | 12.84: | - | - | B7 | B6 | 100 |
268 | 12.75 | - | - | A5 | A2 | 97 |
274 | 11.31 | - | B5-B7 | B5 | B2 | 99 |
276 | 12.41 | - | A0 V | A0 | A1 | f |
278 | 10.21 | K3 III | - | K3 | K2 | f |
282 | 11.60: | - | - | K2 | K0 | f |
283 | 10.00: | - | - | K7 | K5 | f |
289 | 13.39: | - | - | G0 | G2 | f |
290 | 11.73 | - | A0 V | A0 | A2 | f |
292 | 11.89 | - | A2 V | A3 | A2 | f |
299 | 13.70 | - | - | G8 | G1 | f |
301 | 12.55: | - | - | A2 | B9 | 95 |
305 | 12.10 | - | B8 V | B9 | B8 | 95 |
308 | 13.67 | - | - | A0 | A2 | 85 |
312 | 12.51 | - | A1 V | A1 | B8 | 73 |
315 | 11.17 | - | - | G0 | G2 | f |
319 | 12.78 | - | - | A3 | A0 | 98 |
323 | 12.56 | - | A0 V | A0 | B9 | 97 |
327 | 13.56 | - | A0 V | A1 | B9 | 68 |
328 | 14.01 | - | - | B5: | B2 | f |
330 | 10.64 | - | - | K7 | K5 | f |
331 | 12.68 | - | A0 V | B9 | B7 | 98 |
332 | 13.56 | - | A0: | A0 | A0 | 90 |
334 | 12.88 | - | - | B7 | B3 | 99 |
336 | 12.60 | - | A2:III | A5 | A0 | f |
337 | 12.61 | - | A0: | B8 | B8 | 98 |
340 | 13.03 | - | - | G8 | G2 | f |
342 | 12.78 | - | F2-F6: | G5 | G2 | f |
343 | 13.11: | - | A2 V | A2 | A2 | f |
345 | 12.85: | - | B5: | B5 | B2 | 93 |
348 | 9.08: | - | B8 III | - | B4 | 86 |
349 | 12.02 | - | B5: | B5 | A9 | 98 |
356 | 13.70 | - | - | B9 | A1 | 62 |
358 | 10.12 | - | B9 V | - | A0 | f |
359 | 13.62 | - | - | G2: | G2 | f |
361 | 13.82: | - | - | F5 | G0 | f |
362 | 12.47 | - | - | G3 | G2 | f |
363 | 13.02 | - | - | G5 | K2 | f |
365 | 11.86 | - | - | G5 | G2 | f |
Comparison of the present spectral classification results (Col. 6 in the Table 7) with Ogura & Ishida (1981) spectral estimates in MK system (Col. 3) is shown in Fig. 2 by triangles. One can notice a fair agreement of compared results. Also good results give a comparison with spectra derived by Kuznetsov (1986) from unwidened low-dispersion spectrograms (cf. Cols. 5 and 6).
![]() |
Figure 2: Comparison of the spectral classification results obtained with different methods (vertical axis) with those in MK system (horizontal axis). Triangles - present results versus MK estimates by Ogura & Ishida (1981); direction ofNGC 2244. Crosses - present results versus MK spectra by Walker (1956) and Young (1978); direction of NGC 2264. Asterisks - spectra in Abastumany system (Voroshylov et al. 1985) versus estimates by Walker (1956) and Young (1978); direction of NGC 2264 |
In Fig. 2 the number of NGC 2244 stars later than O8-A3 was found to be insufficient for comparison purposes. Therefore, in order to check reliability of the present spectral classification for these spectral classes, a similar study of stars in the area around NGC 2264 has been carried out. Due to the cluster being a relatively short distance from the Sun (Arshutkin et al. 1990; Walker 1956), the O-K3 spectral range where spectral classification in MK system had already been fulfilled by Voroshylov et al. (1985), Walker (1956), and Young (1978) is much wider.
To bypass multiplicity of NGC 2264 stellar spectral estimates
at a certain fixed QUBV value, data on interstellar light absorption
dependence versus distance (Arshutkin et al. 1990; Cohen & Kuhl
1979; Walker 1956; Williams & Cremin 1969) were used as the main
criterion to be fitted. In the case with double largely diverging
spectral estimates (B or F-G) we also took into account probabilities
of membership derived by Arshutkin et al. (1990). Table 8 presents
results of spectral classification for 75 NGC 2264 stars. In the
first three columns of the table serial numbers, magnitudes V, and
spectral classes
of stars in MK system according to Walker (1956)
catalogue are given. The next three columns contain estimates of spectra
found by Young (1978), Voroshylov et al. (1985)
,
and spectra SpUBVobtained on the basis of the present technique of spectral
classification.
N | V |
![]() |
![]() |
![]() |
SpUBV |
2 | 9.68 | A7 III-IV | A7 V | A7 V | A7 |
6 | 8.17 | - | - | F3 V | F0 |
7 | 7.74 | - | B3 V | B3 V | B1 |
17 | 12.87 | - | - | A0: | A0 |
20 | 10.27 | F2 III | F0 V | F3 V | F2 |
25 | 7.80 | - | - | F3 V | F4 |
26 | 11.78 | - | - | F8 V | G2 |
28 | 12.29 | - | - | F8 V | G2 |
30 | 10.75 | - | A0 V | A0 V | A4 |
33 | 11.67 | - | K1 V | M | K0 |
35 | 10.35 | - | - | A1 V | A7 |
36 | 10.88 | - | B9 V | A0 V | A1 |
39 | 11.32 | - | - | A2 V | A3 |
43 | 10.50 | A7 III | A7 V | A5 V | A5 |
46 | 9.19 | A5 III | A3 V | A2 III | A5 |
50 | 8.11 | B3 V | B3 V | B2 III | B0 |
65 | 11.71 | - | - | F5 V | G2 |
67 | 10.80 | B2 V | B2 V | B0 | B0 |
68 | 11.72 | G0 IV-V | F4 V | G0: | G2 |
69 | 8.26 | K3 II-III | - | K1 III | K9 |
70 | 11.08 | - | - | G3 V | G3 |
73 | 9.32 | G5 III | - | G5 V | G1 |
83 | 7.93 | - | B2 V | B1 III | O9 |
87 | 10.74 | - | A0 V | - | A4 |
88 | 9.02 | B5 V | B5 V | B3 III | B1 |
92 | 11.69 | K0 IV | - | G8 | K0 |
99 | 10.80 | - | - | F5 V | F2 |
100 | 9.98 | A2 IV | B8 V | A0 V | A3 |
104 | 11.36 | A5 IV | - | A5 | A4 |
107 | 8.81 | - | B6 V | B9 III | B5 |
108 | 11.87 | G0 III-IV | F7 V | F8: | F8 |
109 | 9.08 | B6 V | B3 V | B5 V | B2 |
112 | 10.77 | A0 V | A0 V | B9 V | B8 |
114 | 11.54 | - | - | F8 V | G1 |
116 | 11.58 | F5 III-IV | F7 V | F6 V | G2 |
125 | 12.29 | F6-G0III-V | F9 V | F5 | G1 |
132 | 10.23 | - | B8 V | B8 V | B7 |
134 | 12.38 | - | G5 V | - | G2 |
137 | 9.88 | - | B5 V | - | B5 |
145 | 10.64 | A0 V | A0 V | A0 V | A4 |
151 | 12.53 | - | - | G3 | G2 |
152 | 9.10 | - | - | B8 IV | B5 |
157 | 10.06 | - | B8 V | B5 III | B7 |
158 | 10.36 | A7-F0IV-V | F0 V | A7 5 | A9 |
159 | 10.97 | A0 V | A0 V | A0 V | A4 |
172 | 10.04 | - | B8 V | B8 V | B6 |
177 | 9.20 | - | - | G5 III | G1 |
179 | 9.95 | - | B9 V | - | B8 |
180 | 12.86 | - | - | G5: | G5 |
181 | 10.03 | B9-A0IV-V | B9 V | B8 V | B7 |
182 | 10.31 | A2 V | - | A2 V | A2 |
187 | 9.21 | - | B8 V | B8 V | B7 |
189 | 11.20 | - | - | G0: | G2 |
190 | 12.26 | - | G0 V | - | G1 |
193 | 9.77 | A7 IIIp | - | A5 V | A8 |
196 | 11.46 | F6-F8 IV | - | G0 V | G1 |
202 | 8.98 | B2 V | - | B3 V | B2 |
203 | 12.90 | - | - | F8 | G1 |
205 | 10.60: | - | A8 V | F2 V | F3 |
206 | 8.70: | - | B8 V | B8 V | B7 |
209 | 11.29 | - | F2 V | F2 V | F5 |
212 | 7.47 | B2,5 V | B3 V | B1 III | B2 |
215 | 9.29 | A0 IV-V | A0 V | B9 III | B9 |
216 | 11.69 | - | - | G5: | G2 |
220 | 9.69 | - | - | F6 V | G2 |
221 | 12.12 | - | - | F3 | F4 |
222 | 9.88 | A3 IV | A2 V | A2 III | A2 |
223 | 10.86 | - | - | F0 V | F3 |
224 | 11.49 | - | F5 V | G5 V | F1 |
226 | 9.59 | A3 III | - | A2 III | A2 |
227 | 11.77 | - | - | G2: | G1 |
228 | 11.07 | - | F0 V | - | F2 |
231 | 8.96 | - | B5 V | - | B1 |
233 | 9.54 | G0 V | - | G3 V | G1 |
237 | 9.44 | K2 III | K2II-III | G5 III | K5 |
A comparison of obtained spectra with estimates in the standard MK system (Walker 1956; Young 1978) for NGC 2264 is shown in Fig. 2 by crosses. In the same figure we give (by asterisks) a comparison of the spectral classification results in the Abastumany system (Voroshylov et al. 1985) with those in MK system (Walker 1956; Young 1978). Approximately equal scattering of points in both cases testify that on its accuracy the present technique matches that of the Abastumany system of criteria (Kharadze & Bartaya 1960).
Copyright The European Southern Observatory (ESO)