3 The isophotal atlas

For isophotal analysis purposes we extracted digitized images of pairs and multiplets from the DSS. The plates are from the SERC Southern Sky Survey and include IIIa-J long (3600 s), V band medium (1200 s) and V band short (300 s) exposures. The PDS pixel values correspond to photographic density measures from the original plates, and are not calibrated. The images were processed with the IRAF package at the Instituto de Física - UFRGS, applying a 2-d Gaussian filter to smooth out individual stars, producing isodensity maps.

We show the isophotal atlas of SMC star cluster pairs and multiplets in Figs. 1 throughout 7, where morphological evidence of interactions can be searched for. In each panel we point members and provide designations.

 

 

Table 2: Age, reddening and morphological classification

Name	Age	E(B-V)	Age $_{\rm OGLE}$	$E(B-V)_{\rm OGLE}$	Morphologies	Comments
	Myr		Myr
(1)	(2)	(3)	(4)	(5)	(6)	(7)
BS3					b	mT-1 outside OGLEII
H86-2					b	mT-1 outside OGLEII
BS4						mT-1 outside OGLEII
H86-38	251 $\pm$ 25	0.10				mP-5
BS10	>560	0.10				mP-5
B15	>1000	0.07				mP-8
H86-57	>1000	0.07				mP-8
NGC 220,SMC_OGLE8	65 $\pm$ 13	0.07	100 $\pm$ 23	0.07	d	mT-2
NGC 222,SMC_OGLE9	70 $\pm$ 7	0.07	100 $\pm$ 23	0.07	d	mT-2
B23,SMC_OGLE170	<100	0.07			d	mT-2
NGC 231,SMC_OGLE11	65 $\pm$ 8	0.12	79 $\pm$ 18	0.08		mP-10
BS15	70 $\pm$ 7	0.10				mP-10
B29	150 $\pm$ 70	0.10				mP-12
HW16,SMC_OGLE13	<20	0.07	20 $\pm$ 15	0.05		mP-12
NGC 241,SMC_OGLE17	55 $\pm$ 5	0.12	79 $\pm$ 18	0.10	b	mP-13
NGC 242,SMC_OGLE18	65 $\pm$ 10	0.10	79 $\pm$ 18	0.10	b	mP-13
B31,SMC_OGLE19,SMC_OGLE175	280 $\pm$ 30	0.10	400 $\pm$ 92	0.08		mT-4
BS20,SMC_OGLE20	450 $\pm$ 50	0.08	400 $\pm$ 92	0.08		mT-4
H86-70,SMC_OGLE21	450 $\pm$ 50	0.07				mT-4
BS27,SMC_OGLE177	<25	0.18	79 $\pm$ 37	0.08		mP-14
SMC-N10	HII Region					mP-14
NGC 248n,SMC_OGLE26n	HII Region					mP-15
NGC 248s,SMC_OGLE26s	HII Region					mP-15
B39,SMC_OGLE27	450 $\pm$ 50	0.07			b	mP-16
BS30	450 $\pm$ 50	0.07			b	mP-16
B36	315 $\pm$ 50	0.03				mP-17
SMC_OGLE31	450 $\pm$ 50	0.03				mP-17
H86-76,SMC_OGLE182	200 $\pm$ 20	0.14				mT-5
H86-78n,SMC_OGLE33n	<25	0.14	16 $\pm$ 9	0.15		mT-5
H86-78s,SMC_OGLE33s	<30	0.14	16 $\pm$ 9	0.15		mT-5
L31,SMC_OGLE36	250 $\pm$ 50	0.10				mT-6
H86-83,SMC_OGLE35	180 $\pm$ 50	0.10				mT-6
H86-84,SMC_OGLE185	250 $\pm$ 50	0.10				mT-6
H86-86,SMC_OGLE40	350 $\pm$ 50	0.03				mP-19
H86-87,SMC_OGLE187	140 $\pm$ 20	0.07	158 $\pm$ 36	0.04		mP-19
BS35,SMC_OGLE42	400 $\pm$ 100	0.03				mP-21
K25,SMC_OGLE45	250 $\pm$ 50	0.07	250 $\pm$ 58	0.07		mP-21
MA205	HII Region				b	mT-7
SMC-N25,SMC_OGLE189	HII Region				b	mT-7
SMC-N26	HII Region					mT-7
H86-99,SMC_OGLE190	225 $\pm$ 50	0.14			d	mP-22
H86-100,SMC_OGLE191	200 $\pm$ 50	0.14			d	mP-22
B50	<30	0.10				mT-8
BS41,SMC_OGLE194	70 $\pm$ 30	0.10	79 $\pm$ 18	0.07		mT-8
L39,SMC_OGLE54	80 $\pm$ 20	0.10	100 $\pm$ 23	0.10		mT-8
SMC-N33	HII Region				e	mP-23
MA301	HII Region				e	mP-23
SMC_OGLE56	115 $\pm$ 15	0.18				mT-9
H86-110	<20	0.18				mT-9
H86-109,SMC_OGLE58	180 $\pm$ 20	0.18	200 $\pm$ 45	0.17		mT-9
MA317	HII Region				b	mP-24
SMC-N34	HII Region				b	mP-24
B53,SMC_OGLE197	200 $\pm$ 50	0.07	250 $\pm$ 55	0.08		mP-25
B55,SMC_OGLE60	160 $\pm$ 30	0.07	250 $\pm$ 55	0.08		mP-25
SMC_OGLE199	-	-				mP-26
BS45,SMC_OGLE59	65 $\pm$ 30	0.10	63 $\pm$ 14	0.10		mP-26
H86-106w	200 $\pm$ 50	0.10			e	mP-27
H86-106e	<30/200 $\pm$ 501	0.12/0.10			e	mP-27
H86-115,SMC_OGLE63	-	-				mP-28
SMC_OGLE65	200 $\pm$ 20	0.10				mP-28
BS46,SMC_OGLE200	80 $\pm$ 10	0.07	100 $\pm$ 23	0.06	b	mP-29
H86-116,SMC_OGLE64	125 $\pm$ 15	0.10	126 $\pm$ 29	0.10	b	mP-29
BS48,SMC_OGLE201	160 $\pm$ 20	0.10				mP-30
H86-108,MA401	<30	0.12				mP-30
B59,SMC_OGLE73	100 $\pm$ 15	0.14	158 $\pm$ 75	0.11	e	mP-31
SMC-N46	HII Region				e	mP-31
BS56,SMC_OGLE77	140 $\pm$ 20	0.07	79 $\pm$ 38	0.08	d	mP-32
H86-130,SMC_OGLE78	65 $\pm$  8	0.12	79 $\pm$ 18	0.08	d	mP-32
B64,SMC_OGLE210	<10	0.12	158 $\pm$ 75	0.07		mP-33
BS57,SMC_OGLE211	<10	0.12				mP-33
H86-134w,SMC_OGLE212	-	-				mT-11
B65,SMC_OGLE83	65 $\pm$ 10	0.10	63 $\pm$ 30	0.09		mT-11
H86-134e,SMC_OGLE213	-	-				mT-11

 

Table 2: continued

Name	Age	E(B-V)	Age $_{\rm OGLE}$	$E(B-V)_{\rm OGLE}$	Morphologies	Comments
	Myr		Myr
(1)	(2)	(3)	(4)	(5)	(6)	(7)
BS61	250 $\pm$ 50	0.03				mP-34
BS255	-	-				mP-34
BS63,SMC_OGLE84	450 $\pm$ 50	0.10			d	mT-12
B67,SMC_OGLE87	450 $\pm$ 50	0.10	500 $\pm$ 115	0.10	d	mT-12
NGC 294,SMC_OGLE90	300 $\pm$ 50	0.10	316 $\pm$ 73	0.11	d	mT-12
H86-140,SMC_OGLE214	55 $\pm$ 25	0.10			b	mP-35
H86-139	<30	0.10			b	mP-35
BS67	>300	0.07				mP-36
BS68,SMC_OGLE95	500 $\pm$ 100	0.07				mP-36
B72	80 $\pm$ 10	0.10				m6
H86-143,SMC_OGLE93	200 $\pm$ 25	0.10				m6
BS257	50 $\pm$ 301	0.10				m6
SMC-N52A,SMC_OGLE94	HII Region					m6
SMC-N52B,SMC_OGLE96	HII Region					m6
H86-148	400 $\pm$ 100	0.10				m6
H86-159,SMC_OGLE102	500 $\pm$ 50	0.10				mP-39
H86-160	-	-				mP-39
BS81,SMC_OGLE223	80 $\pm$ 20/250 $\pm$ 501	0.10				mP-40
H86-172,SMC_OGLE108	280 $\pm$ 30	0.07				mP-40
H86-175,SMC_OGLE227	30 $\pm$ 10	0.10				mP-41
H86-179,SMC_OGLE112	<30	0.10	32 $\pm$ 23	0.11		mP-41
H86-177,SMC_OGLE226	<30	0.10			d	mP-42
H86-176	<30	0.10			d	mP-42
SMC-N62	HII Region					mT-13
SMC-N63,SMC_OGLE113	HII Region					mT-13
SMC-N64A,SMC_OGLE114	HII Region					mT-13
BS269	-	-				mP-43
BS270	<30	0.10				mP-43
BS271	<30	0.12				mP-44
BS272,SMC_OGLE229	<30	0.08	79 $\pm$ 18	0.05		mP-44
IC1611,SMC_OGLE118	100 $\pm$ 20	0.07	160 $\pm$ 37	0.08		m4-1
H86-186,SMC_OGLE119	180 $\pm$ 20	0.07			e	m4-1
IC1612,SMC_OGLE120	100 $\pm$ 50	0.10	50 $\pm$ 24	0.07	e	m4-1
K42,SMC_OGLE124	20 $\pm$ 10	0.10	40 $\pm$ 9	0.06		m4-1
H86-189,SMC_OGLE123	570 $\pm$ 70	0.07				mP-46
H86-190,SMC_OGLE230	125 $\pm$ 13	0.07	32 $\pm$ 23	0.08		mP-46
H86-191,SMC_OGLE231	200 $\pm$ 25	0.07				mP-47
H86-194,SMC_OGLE232	200 $\pm$ 25	0.07				mP-47
NGC 376,SMC_OGLE139	20 $\pm$ 2	0.10	32 $\pm$ 7	0.07	e	mP-51
BS114,SMC_OGLE235	250 $\pm$ 25	0.04			e	mP-51
SMC_OGLE138	<30	0.04	25 $\pm$ 20	0.04		mP-52
SMC_OGLE144,SMC_OGLE236	<30	0.07	40 $\pm$ 19	0.05		mP-52
SMC-N78A	HII Region					m5
SMC-N78B,SMC_OGLE145	HII Region		79 $\pm$ 37	0.07		m5
MA1520,SMC_OGLE147	HII Region		12 $\pm$ 9	0.06		m5
SMC-N78D	HII Region					m5
SMC_OGLE146	HII Region		20 $\pm$ 15	0.06		m5
BS130	80 $\pm$ 10	0.10				mP-54
BS132	<30/200 $\pm$ 251,2	0.10				mP-54
HW71nw					e	mP-56 outside OGLEII
HW71se					e	mP-56 outside OGLEII

Notes to Table 2: 1- Age depends on membership of bright stars; 2- Upper main sequence is possibly underpopulated.

3.1 Isophotal distortions

Cluster pairs and multiplets with evidence of physical interaction are marked in Col. 6 of Table 2. Their isophotal maps show features such as isophotal distortions, common envelope, isophotal twistings, etc. We classified these isophotal maps according to the following criteria: (i) isophotes of the members are detached (d), but showing isophotal distortions; (ii) isophotes of the members are connected by a "bridge'' (b); (iii) the members are embedded in the same isophotal envelope (e). These classification criteria are an important tool for a selection of interacting cluster pair candidates, since such isophotal morphologies are predominant in N-body model encounters (Rodrigues et al. 1994, ODB98). The classifications above were made for objects not disturbed by gas emission. It is worth noting that both bridges and envelopes imply a common outer isophotal, however the former includes a dimension which is considerably smaller than the smaller cluster diameter. The pair B59/SMC-N46 would be an envelope limiting case (Fig. 4).

Figure 1: Isophotal atlas of SMC cluster pairs and multiplets

Figure 2: Isophotal atlas of SMC cluster pairs and multiplets

Figure 3: Isophotal atlas of SMC cluster pairs and multiplets

Figure 4: Isophotal atlas of SMC cluster pairs and multiplets

Figure 5: Isophotal atlas of SMC cluster pairs and multiplets

Figure 6: Isophotal atlas of SMC cluster pairs and multiplets

Figure 7: Isophotal atlas of SMC cluster pairs and multiplets

As examples, the pair H86-186/IC 1612 (Fig. 5) is embedded in the same isophotal envelope and it is classified as e. The pair H86-140/H86-139 (Fig. 4) shows a bridge linking its members and is classificated as b. In Fig. 6 the pair NGC 376/BS114 shows isophotal distortions, however the outer isophotes of the components are not connected and it is thus classified as d.

We detected relevant isophotal features for about 25% of the sample distributed as follows: 6 candidate cluster systems with common envelope, 7 with bridge and 5 detached cases.

High isophotal densities such as those observed for B78/L51 (Fig. 5) are related to the high surface brightness which often occurs in blue clusters.

It is important to note that non relevant isophotal features can appear in the maps caused by bright stars or background fluctuations. By means of previous inspections in the original images we verified the non relevant objects. For example, the compact object in the lower right corner of the B39/BS30 isophotal map (Fig. 2) is a bright star, however the compact one to the right of the main cluster can be a star or a compact cluster, as seen in the original form. Higher resolution images would be necessary to check the presence of a third cluster.

Figure 8: Age histograms: dotted lines represent clusters with age via CMDs; solid lines are for the same sample as above plus embedded clusters in HII regions, assuming an age 3 Myr. In the latter histogram lower or upper limits in Table 2 were also included by adopting them as cluster ages

Figure 9: Angular distribution of the 176 objects in SMC cluster pairs and multiplets together with HI contours 100, 200, 400 and 600 in units 1019atoms cm-2 from Mathewson & Ford (1984)

Figure 10: The observed diameter ratio distribution for SMCcluster pairs

Figure 11: Centre-to-centre angular separation distribution for SMC cluster pairs