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3 Spectral  classification  of  stars  up  to   V=13 - 14 mag in the directions of open clusters NGC 2244 and NGC 2264

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.


 

 
Table 3: A set of possible photometric and spectral characteristics of the star No. 127 at a fixed QUBV value
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 $\overline{Sp}$
            (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, $\overline{Sp}=$ A2, Q'UBV=0.12, SpUBV= A2, $Sp_{{\rm MK}}=$ A2 V



 

 
Table 4: Same as Table 3 for the star No. 158
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 $\overline{Sp}$
            (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, $\overline{Sp}=$ G1, Q'UBV=-0.55, SpUBV= G1, $Sp_{{\rm MK}}=$ G0 IV



 

 
Table 5: Same as Table 3 for the star No. 239
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 $\overline{Sp}$
            (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, $\overline{Sp}=$ B5, Q'UBV=-0.41, SpUBV= B6, $Sp_{{\rm MK}}=$ B7 V



 

 
Table 6: Same as Table 3 for the star No. 278
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 $\overline{Sp}$
            (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, $\overline{Sp}=$ K0, Q'UBV=-0.03, SpUBV= K5, $Sp_{{\rm MK}}=$ K3 III


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 $3\sigma \{ E(B-V)\}$ and $3\sigma \{ V_0 - M_V \}$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 $\overline{Sp}=$A2. Then using $\overline{Sp}$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 $V_{{\rm lim}} =14.0$ 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 $Sp_{{\rm MK}}$of stars in MK system, according to the catalogue by Ogura & Ishida (1981) are given. The next columns contain: estimates $Sp_{{\rm Ab}}$ of spectra in Abastumany system of spectral classification (Voroshylov et al. 1985); spectral types $Sp_{\rm K}$ 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'').


 

 
Table 7: Photometric and spectral characteristics of stars brighter than V=14.0 mag in the direction of open cluster NGC 2244
N V $Sp_{{\rm MK}}$ $Sp_{{\rm Ab}}$ $Sp_{\rm K}$ 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



 
Table 7: continued
N V $Sp_{{\rm MK}}$ $Sp_{{\rm Ab}}$ $Sp_{\rm K}$ 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).


  \begin{figure}\includegraphics[0,0][90mm,90mm]{DS1736F2.EPS}\end{figure} 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 ŒŠ 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 $Sp_{{\rm MK}}$ of stars in MK system according to Walker (1956) catalogue are given. The next three columns contain estimates of spectra $Sp_{\rm Y}$found by Young (1978), Voroshylov et al. (1985) $Sp_{\rm V}$, and spectra SpUBVobtained on the basis of the present technique of spectral classification.


 

 
Table 8: Photometric and spectral characteristics of stars brighter than V=13.0 mag in direction of open cluster NGC 2264
N V $Sp_{{\rm MK}}$ $Sp_{\rm Y}$ $Sp_{\rm V}$ 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).


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