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 Q_UBV 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 Q_UBV=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 -M_V) 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 Sp_UBV=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 Sp_UBVreceived 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).

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 Q_UBV 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 Sp_UBVobtained 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).