THE ASTRONOMICAL JOURNAL, 118 : 1684È1699, 1999 October ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.
HST /FOS SPATIALLY RESOLVED SPECTRAL CLASSIFICATION OF COMPACT OB GROUPS IN THE LARGE MAGELLANIC CLOUD1 NOLAN R. WALBORN Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 ; walborn=stsci.edu
LAURENT DRISSEN Departement de Physique, Universite Laval, Ste-Foy, PQ G1K 7P4, Canada ; ldrissen=phy.ulaval.ca
JOEL WM. PARKER Southwest Research Institute, Suite 426, 1050 Walnut Street, Boulder, CO 80302 ; joel=boulder.swri.edu
ABHIJIT SAHA Kitt Peak National Observatory, National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85726 ; saha=noao.edu
AND JOHN W. MACKENTY AND RICHARD L. WHITE Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 ; mackenty=stsci.edu, white=stsci.edu Received 1999 March 4 ; accepted 1999 July 9
ABSTRACT Blue-violet spectrograms of individual components in four compact OB groups of the Large Magellanic Cloud, obtained with the Hubble Space T elescope (HST ) Faint Object Spectrograph (FOS), are presented and discussed. Two of the massive multiple systems are in the 30 Doradus periphery, while the other two represent the core and the peripheral, triggered associations in the giant shell H II region Henize N11. Uncontaminated spectrograms of three Wolf-Rayet and two very early Of components have been obtained for the Ðrst time ; they can be observed only as composites with their close companions from the ground. Many of the companions have also been observed separately with the HST FOS, and several are of special interest in their own right. These observations provide information on the initial masses and ages of the peculiar objects, and on the evolutionary relationships among di†erent spectral categories within the presumably coeval systems. Key words : binaries : close È Magellanic Clouds È open clusters and associations : general È stars : early-type È stars : fundamental parameters È stars : Wolf-Rayet 1.
INTRODUCTION
Koter, Heap, & Hubeny 1998). There are many additional interesting compact OB groups in the LMC accessible to sharpened investigation with HST , which o†er progress toward understanding the evolutionary status of their peculiar members, as well as the upper stellar mass limit and initial mass function. In this paper, we present spatially resolved HST spectroscopy of four such systems in special locations, for which we had previously obtained HST direct images (Walborn et al. 1995a, 1995b). These systems are the WN]OB groups Breysacher 73 in 30 Doradus B and NGC 2044 West (HDE 269828) in 30 Doradus C ; and the central WC ] OB object HD 32228 in the giant shell H II region Henize N11, along with the O3 III(f *) ] OB group LuckeHodge 10/Parker 3209 in the largest nebula of the N11 shell. The observational histories of these objects will be given in conjunction with their respective present results below.
The propensity of massive stars to form in compact multiple systems presents a fundamental observational challenge, which can lead to misinterpretations of particular objects and to systematic errors in determinations of the upper initial mass function, if this multiplicity remains unresolved or unrecognized. The problem, of course, is aggravated with increasing distance. Even in the solar neighborhood, un- or barely resolved hierarchical OB multiple systems occur, e.g., q CMa (van Leeuwen & van Genderen 1997), HD 93206 (Leung, Mo†at, & Seggewiss 1979 ; Morrison & Conti 1980), HD 167971 (Leitherer et al. 1987), and HD 193322 (McKibben et al. 1998). The Magellanic Clouds provide the most favorable extragalactic laboratory for massive stellar evolution, but in the LMC 0A. 1 already subtends 5000 AU, while readily detected massive spectroscopic binaries have typical separations of 0.1 AU, so that a signiÐcant gap remains in the coverage of spatial scales by current instrumentation. Nevertheless, the Hubble Space T elescope (HST ) o†ers a considerable advance over previous capabilities to address this problem, which is eminently worth pursuing, as shown by the most famous (or notorious) case of R136, the concentrated core of the 30 Doradus ionizing cluster (Massey & Hunter 1998 ; de
2.
OBSERVATIONS AND DATA REDUCTIONS
All of the target objects were Ðrst imaged with the HST cameras, to obtain positions and magnitudes within the compact groups for the subsequent spectroscopic programs, as well as photometry for scientiÐc analysis. The imaging and spectroscopic data acquisition and reductions will be described in turn.
ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Based on observations with the NASA/ESA Hubble Space T elescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
2.1. Imagery and Photometry The direct imaging has been presented brieÑy by Walborn et al. (1995a, 1995b) and will be more extensively 1684
COMPACT OB GROUPS IN THE LMC in subsequent papers by Parker et al. Hence the presentation here will also be relatively brief. Short F336W (approximately U), F439W (DB), and F569W (DV ) exposures of 30 Dor B and C were obtained with the spherically aberrated Wide Field/Planetary Camera I (WF/PC I) in WFC mode during 1993 August under HST program ID No. 4322 ; and of Lucke-Hodge 10 in N11 similarly during 1993 October. Fortunately, the observations of HD 32228 in N11 were carried over to the optically corrected WFPC2,
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and they were obtained with the PC chip during 1994 September under program ID No. 5678. The WF/PC I measurements in the compact groups have been problematic and the principal source of delay in the presentation of these results. Some preliminary photometry in 30 Dor B and C was given by Walborn et al. (1995b), but various anomalies it exhibited motivated complete remeasurements, in which object lists from the V deconvolutions described in that paper were used as inputs to the
FIG. 1.ÈHST WFC I V image of 30 Doradus B (NGC 2060) and Breysacher 73, taken from Walborn et al. (1995b). The smaller frame at upper left shows the raw, full
[email protected] square Ðeld, while the larger frame shows the deconvolved image of Brey 73 enlarged to the scale given below. North is up and east to the left in each panel, as in all subsequent direct images shown. The stellar identiÐcation numbers are from Testor et al. (1988), augmented with letters for newly resolved components.
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two runs. Manual remedies were applied to these cases, which will be noted in the presentation of results below. Typical formal errors of the WF/PC I magnitudes and colors for the spectroscopically classiÐed stars are 0.05 mag, although close pairs of comparable magnitudes (including most of those in LH 10-3209) have errors of 0.1È0.2 mag, and larger random errors can occur in a few cases. As expected and shown by the results, the WFPC2 errors are much smaller, with consistent values of 0.02 mag in V and 0.03 mag in the colors for the spectroscopically classiÐed stars. Zero points to the UBV system have been derived from ground-based photometry of isolated stars in 30 Dor B and C by Schild & Testor (1992), and in the N11 Ðelds by Parker et al. (1992).
6
4
2
3250
3500
3750
4000
4250
4500
4750
FIG. 2.ÈSpectrum of the WN component No. 1A in Breysacher 73 as observed by the HST FOS. The axes are wavelengths in angstroms and intensities in relative, rectiÐed continuum units, as in all subsequent spectral plots shown. The alternating relative intensities of consecutive Pickering series members reveal a substantial hydrogen content, as indicated in the spectral type. A weak He I j4471 P Cygni proÐle is present. The spectrum is very similar to those of the WNL stars in the Carina Nebula (Walborn 1974).
DoPHOT (Saha et al. 1994) program for all Ðlters. Interestingly, the results of the two procedures were comparable, with very small di†erences between them in most of the derived magnitudes, but large discrepancies in a small number of individual cases that were di†erent between the
2.2. Spectroscopy and Spectral ClassiÐcation Spatially resolved spectroscopy of the brighter components of four interesting, compact multiple systems within the imaged Ðelds was subsequently performed with the HST Faint Object Spectrograph (FOS). These observations would not have been possible without the optical correction of the spherically aberrated input beam to FOS by the COSTAR system. The WN ] OB system Breysacher 73 in 30 Doradus B was observed in 1996 January under program ID No. 6032, while HD 32228 and LH 10-3209 in N11 were observed in 1996 September and NGC 2044 West in 30 Doradus C in 1997 January under ID No. 6508. The FOS 0A. 3 aperture and the red Digicon with grating G400H provided a nominal resolution of 3 Ó per diode (oversampled by a factor of 4) and wavelength coverage from 3240 to 4780 Ó. Individual exposure times ranged from 6 to 60 minutes. In each group, the components were acquired by consecutive o†sets from reference stars that were reacquired every few exposures, based on relative positions measured from the WFPC images. This procedure
TABLE 1 OBSERVED AND DERIVED PARAMETERS FOR THE COMPONENTS OF BREYSACHER 73 No.a
Xa
Ya
V
B[V
U[B
E
14 . . . . . . 25 . . . . . . 22 . . . . . . 3 ....... 21 . . . . . . 12 . . . . . . 6 ....... 13 . . . . . . 19 . . . . . . 8 ....... 28 . . . . . . 16 . . . . . . 18 . . . . . . 15 . . . . . . 2 ....... 9 ....... 10 . . . . . . 17 . . . . . . 20 . . . . . . 5 ....... 26 . . . . . . 7 ....... 1 .......
78.36 99.94 93.43 50.89 90.08 77.15 63.29 77.23 87.24 73.06 108.98 81.55 84.82 78.66 42.37 73.89 74.87 83.79 89.37 60.43 100.39 66.40 42.35
588.73 585.21 570.10 607.14 572.85 594.73 568.73 556.56 561.72 583.53 537.67 591.26 567.15 601.29 570.58 574.73 544.34 589.71 531.56 566.40 575.49 567.36 562.85
14.13 14.30 14.75 14.83 15.10 15.19 15.30 15.30 15.38 15.40 15.49 15.71 16.04 16.15 16.59 16.89 16.96 17.04 17.19 17.34 17.60 17.67 18.53
0.13 0.19 0.25 0.15 0.20 0.18 0.06 0.12 0.21 0.03 [0.02 0.65 0.41 0.22 0.19 0.19 [0.01 0.10 0.13 1.40 0.25 0.65 0.84
[0.84 [0.91 [0.87 [0.94 [1.00 [1.23 [1.06 [1.11 [1.08 [0.58 [0.96 [0.56 [0.84 [0.82 [0.95 [0.76 [0.03 [0.81 [1.07 [0.88 [0.70 [1.34 [0.82
0.45 0.51 ... 0.47 0.52 0.49 0.38 0.42 0.49 0.28 0.27 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43
B~V
log T
M bol
BC
Spectral Type
4.683 4.585 ... 4.599 4.613 4.569 4.613 4.539 4.464 4.407 4.503 3.885 3.883 4.301 4.368 4.522 3.999 4.509 4.500 3.689 4.507 3.885 3.820
[10.20 [9.52 ... [8.96 [8.94 [8.45 [8.32 [7.92 [7.64 [6.66 [7.02 [4.13 [3.80 [5.74 [5.69 [6.25 [3.15 [6.00 [5.88 [2.74 [5.43 [2.17 [1.32
[4.39 [3.69 ... [3.79 [3.89 [3.58 [3.89 [3.37 [2.96 [2.62 [3.11 0.05 0.05 [2.00 [2.39 [3.25 [0.22 [3.15 [3.17 [0.18 [3.14 0.05 0.04
O4 III(f )p O7ÈO8 II WN6.5h O7.5 V((f )) O7 V O8 III O7 V (N strong) O9.5 V O9.5ÈB1pe B1ÈB2 :p(e) B0.2 V
Alternate IDb Tes Tes Tes Tes Tes Tes Tes Tes Tes Tes Tes
a Running number and detector coordinates from the present photometry, to be presented in full by Parker et al. in a subsequent paper. b Testor et al. 1988, with letters added here to the numbers to denote subsequently resolved components.
2A 3 1A 7 1B 2B 5 4 1C 2C 6
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COMPACT OB GROUPS IN THE LMC
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2A O4 III(f)p
3
O7-8 II
1B O7 V
5
O7 V (N str)
7
O7.5 V((f))
2B O8 III
4000
4100
4200
4300
4400
4500
4600
4700
FIG. 3a FIG. 3.ÈHST FOS blue-violet, rectiÐed spectrograms of OB components in Breysacher 73. The spectrograms are separated by 0.5 continuum intensity units, as in all subsequent montages shown. The wavelengths of the identiÐed spectral features are given in Walborn & Fitzpatrick (1990). The O4 star No. 2A is a possible blue straggler ; it and the other peculiar objects are discussed in the text.
was successful in every case except NGC 2044, for which the imagery guide-star acquisition had failed and the telescope pointing was maintained by the gyroscopes only ; despite e†orts to compensate for them, the resulting positional uncertainties evidently caused the spectroscopic targets to fall at the edge of the aperture, since the counts obtained are small percentages of those expected. Typical signal-to-noise ratios in the spectrograms are 50 to 70, except in NGC 2044, for which they are 20 to 25. Altogether, satisfactory data were obtained for 39 individual components in the four systems. The reduction of the FOS data was quite difficult because of a number of artifacts resembling spectral lines, which di†ered among the runs and were not removed by the standard Ñat Ðelds. Hence, tailored Ñat-Ðelding procedures had to be used. For NGC 2044 and LH 10-3209, composite Ñats from calibrations bracketing the observing runs were provided by T. Keyes. We obtained even better results for Brey 73 and HD 32228, with their larger numbers of goodquality data, by constructing empirical Ñats from the summed data themselves with the real spectral lines masked. The spectral classiÐcation of the OB stars was done fol-
lowing the procedures discussed for the similar data of Drissen et al. (1995), with reference to the digital atlas of Walborn & Fitzpatrick (1990), and without knowledge of the photometric magnitudes of the stars. No particular difficulties were encountered, and the spectral types are believed to be close to the absolute system, despite the lack of standards obtained during the observing runs. The good correspondence between the derived and calibration absolute visual magnitudes for most of the stars, especially in HD 32228, which has the most reliable photometry, supports that conclusion. The WN stars were classiÐed in the system of Smith, Shara, & Mo†at (1996). 3.
RESULTS AND DISCUSSION
The background, present observational results, and analysis will be discussed for each of the four multiple systems in turn in this section. 3.1. Breysacher 73 in 30 Doradus B 30 Doradus B (NGC 2060), on the southwest periphery of the main 30 Doradus Nebula, contains one of the youngest supernova remnants known (Chu & Kennicutt 1988 ; Chu
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4
O9.5 V
6
B0.2 V
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1C O9.5-B1pe
2C B1-2:p(e)
4000
4100
4200
4300
4400
4500
4600
4700
FIG. 3b
et al. 1992 ; Chu 1997) and a recently discovered 16 ms X-ray pulsar (Marshall et al. 1998 ; Cusumano et al. 1998). Its rich population of early O stars was Ðrst revealed by Schild & Testor (1992). Previously, the brightest object in the association was known as a WN ] OB system, discovered by Azzopardi & Breysacher (1979, their No. 10) and cataloged as No. 73 by Breysacher (1981). From the
FIG. 4.ÈH-R diagram of Breysacher 73, from the stellar parameters listed in Table 1 and with procedures as discussed in the text. The Ðlled circles correspond to spectroscopically classiÐed stars and the open circles to those with photometry only, as in all subsequent HRDs shown. The evolutionary tracks are from Schaerer et al. (1993) with Z \ 0.008 ; isochrones for ages of 1 and 3 Myr are plotted as dashed lines.
start, there was evidence for the composite nature of Brey 73, and a high-resolution ground-based study by Testor, Llebaria, & Debray (1988) isolated 12 components. Figure 1 shows the deconvolved WFC I V image of Brey 73, taken directly from Walborn et al. (1995b). FOS spectroscopy was obtained for the 11 brightest components in the system, including the WN (No. 1A), which has been identiÐed and observed without contamination from its OB companions for the Ðrst time, as shown in Figure 2. The blue-violet classiÐcation regions in the spectrograms of the companions are presented in Figures 3a and 3b, smoothed by 3 pixels to a formal resolution of 2.25 Ó. The spectroscopic and photometric results for Brey 73 are listed in Table 1, which is ordered by increasing V magnitudes and also includes all spectroscopically unclassiÐed stars with acceptable photometry within a 10A ] 10A Ðeld. (Note that the component star numbers used in the Ðgures and the present spectroscopic discussions are the previous designations given in the last column of the tables, rather than the new numbers from the present photometry given in the Ðrst column.) Remarkably, over half of the OB companions observed also have emission lines or other spectral peculiarities ! No. 2A is a very early O star with broad N III j4640 and He II j4686 emission, the latter with a weak absorption reversal ; although the photospheric absorption lines are not highly broadened in this case, the spectrum is likely related to the Onfp category of Walborn (1973). No. 3 has He II j4686 Ðlled in by emission, leading to the high luminosity class (Walborn 1973). No. 5 is an example of a nitrogen-enhanced dwarf O star, very similar to HD 48279 (Walborn & Fitzpatrick 1990). No. 7 is a normal O dwarf with weak N III j4640 emission. Finally, Nos. 1C and 2C are Be stars, the former with very strong Balmer and weak Fe II
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FIG. 5.ÈHST WFC I V raw image of NGC 2044 in 30 Doradus C (upper left), with successive deconvolved enlargements of the West subclustering (HDE 269828) and its multiple component No. 5, taken from Walborn et al. (1995b). The stellar identiÐcation numbers are from Heydari-Malayeri et al. (1993), augmented by letters for newly resolved components.
j4583 emission (as well as the Balmer jump strongly in emission, not shown), and the latter with Balmer lines Ðlled in by emission. Figure 4 is an H-R diagram (HRD) for Brey 73 ; the individual stellar parameters are also listed in Table 1. The reddenings and e†ective temperatures of the spectroscopically classiÐed stars ( Ðlled circles) have been determined from the spectral types, according to the intrinsic colors of Johnson (1966) and the T calibrations of Vacca, eff Garmany, & Shull (1996) and Humphreys & McElroy (1984), the latter being used for the early B stars and scaled to the Vacca et al. values at B0 V and O9.5 I to avoid
discontinuities. (Note that Wolf-Rayet [W-R] stars are not plotted in the present HRDs.) The absolute visual magnitudes are computed with V [ M \ 18.6 and R \ 3.0, 0 V functions of T as while the bolometric corrections are eff & speciÐed by Massey et al. (1989a), Massey, Parker, Garmany (1989b), and Parker & Garmany (1993), based on the calibrations of Flower (1977) and Chlebowski & Garmany (1991). The average E \ 0.43 of the spectroB~V adopted for all of the scopically classiÐed stars has been stars with photometry only (open circles) ; the e†ective temperatures of the latter are determined from the reddeningindependent parameter Q when its value is in the applicable
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6
4
2
3250
3500
3750
4000
4250
4500
4750
FIG. 6.ÈSpectrum of the WN component No. 5C in NGC 2044 West as observed by the HST FOS. The nonmonotonic Pickering series decrement also indicates some hydrogen in this spectrum, as denoted in the spectral type. The spectrum is similar to that of HD 187282, shown by Smith, Shara, & Mo†at (1996).
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range of [0.96 to 0.00 ; outside of that range T is adopted eff from a (B[V ) relation. Of course, the results for the stars 0 with photometry only are substantially more uncertain than those for the stars with spectroscopy. The evolutionary tracks are those of Schaerer et al. (1993) for Z \ 0.008, and corresponding isochrones for ages of 1 and 3 Myr are also plotted as dashed lines. An original objective of this program was to obtain estimates of the initial masses and ages of the W-R components in the multiple systems from the characteristics of their ““ normal ÏÏ OB companions. The companions of Brey 73 are rather too interesting in their own right, and the resulting HRD presents a puzzle. Star 2A lies on the 1 Myr isochrone and has nearly twice the mass of the next most massive stars, which Ðt a 3 Myr isochrone. Five possible interpretations can be suggested. (1) Star 2A is an interloper from the larger 30 Dor B Ðeld. However, the structures of Brey 73 (Fig. 1) and the larger Ðeld (Schild & Testor 1992) make that appear improbable. (2) Star 2A is an unresolved multiple system. However, the problem is caused by the very early spectral type, which dominates the luminosity, so that invocation of companions does not solve it unless they are of similar early type. But the derived mass of No. 2A is as expected for a single star of its spectral type. (3) The compact system is simply not coeval. Not too much can be said for or against this possibility, although it is unsatisfying
9A O4 If+
7
O4 III
11AB O7.5 V
8
O8 III
3
O9 V
5A B0 I
4000
4100
4200
4300
4400
4500
4600
4700
FIG. 7.ÈHST FOS blue-violet, rectiÐed spectrograms of OB components in NGC 2044 West. The O4 If] star No. 9A is observed without contamination from later-type companions for the Ðrst time, and a second O4 component (No. 7) has been discovered. The apparent emission at He II j4686 in No. 5A is contamination from its close WN companion No. 5C (see Figs. 5 and 6).
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TABLE 2 OBSERVED AND DERIVED PARAMETERS FOR THE COMPONENTS OF NGC 2044 WEST No.a
Xa
Ya
V
B[V
U[B
36 . . . . . . 40 . . . . . . 24 . . . . . . 35 . . . . . . 13 . . . . . . 42 . . . . . . 26 . . . . . . 27 . . . . . . 23 . . . . . . 41 . . . . . . 1 ....... 29 . . . . . . 8 ....... 33 . . . . . . 28 . . . . . . 16 . . . . . . 9 ....... 39 . . . . . . 30 . . . . . . 45 . . . . . . 20 . . . . . . 34 . . . . . . 43 . . . . . . 7 ....... 25 . . . . . . 44 . . . . . . 37 . . . . . . 3 ....... 14 . . . . . . 31 . . . . . . 22 . . . . . . 38 . . . . . . 21 . . . . . . 19 . . . . . . 12 . . . . . . 17 . . . . . . 10 . . . . . . 18 . . . . . .
154.69 157.79 131.03 154.70 112.79 162.21 132.13 133.74 130.83 160.60 58.88 135.41 100.32 145.55 134.62 120.21 101.94 157.18 136.70 176.86 126.57 153.51 165.74 92.10 131.59 166.39 156.51 80.34 113.16 139.30 129.73 156.60 129.73 126.38 110.68 120.58 104.43 125.87
174.32 241.41 201.21 227.01 180.69 217.85 199.62 197.54 200.29 239.39 207.25 237.25 221.51 193.95 200.39 246.67 204.22 238.34 236.23 264.93 199.87 198.74 265.81 201.15 216.87 235.85 286.85 224.85 209.92 215.80 263.86 286.17 252.95 307.92 294.95 207.32 216.00 218.97
13.09 13.42 13.96 14.01 14.14 14.30 14.45 14.59 14.84 14.88 15.00 15.15 15.21 15.32 15.39 15.40 15.42 15.68 15.77 15.88 15.91 16.38 16.41 16.77 16.79 16.88 16.90 16.97 17.00 17.05 17.34 17.42 17.54 17.60 17.68 17.74 18.16 18.23
0.02 ... ... 0.01 0.04 0.01 0.06 0.01 [0.02 [0.01 0.09 0.09 0.08 0.04 0.51 0.13 0.03 0.66 0.14 [0.02 0.19 [0.08 0.25 [0.06 [0.14 0.08 0.67 0.04 0.01 0.16 [0.13 0.26 0.00 0.43 0.34 0.21 0.05 [0.35
[0.93 ... ... [1.03 [0.92 [0.88 [0.75 [0.53 [1.14 [0.13 [0.84 [0.74 [0.87 [0.90 [1.05 [0.83 [0.86 0.17 0.21 [0.80 [1.16 [0.71 [0.78 [0.43 [0.74 [0.88 [1.24 [0.77 [0.70 [0.78 [0.08 [0.44 [0.76 [0.47 [0.73 [1.05 [0.21 [0.91
E
B~V ... 0.36c 0.36c 0.33 0.35 0.32 0.36 ... 0.36 0.36 0.36 0.41 0.40 0.35 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36
log T ... 4.678 4.476 4.683 4.585 4.569 4.438 ... 4.667 4.034 4.537 4.599 4.652 4.555 3.913 4.555 4.516 3.856 4.315 4.432 4.204 4.326 4.072 4.154 4.316 4.568 3.853 4.443 4.368 4.528 4.910 4.307 4.410 3.954 3.874 4.160 4.083 4.330
M bol ... [10.61 [8.76 [9.97 [9.21 [8.84 [8.04 ... [9.12 [5.21 [8.03 [8.48 [8.76 [7.81 [4.27 [7.76 [7.46 [3.94 [5.99 [6.57 [5.19 [5.44 [3.91 [4.03 [4.98 [6.37 [2.72 [5.54 [5.07 [5.92 [8.33 [4.29 [4.78 [2.15 [1.94 [3.10 [2.23 [3.62
BC
Spectral Type
Alternate IDb
... [4.35 [3.03 [4.39 [3.69 [3.58 [2.81 ... [4.28 [0.41 [3.35 [3.79 [4.17 [3.48 0.02 [3.48 [3.20 0.06 [2.08 [2.77 [1.42 [2.14 [0.64 [1.12 [2.08 [3.57 0.06 [2.83 [2.39 [3.29 [5.99 [2.03 [2.64 [0.07 0.06 [1.16 [0.70 [2.17
WN7d O4 If] B0 I O4 III O8 Vd O8 III
Brey 65 H-M 9A H-M 5Aa H-M 7 H-M 6 H-M 8 H-M 5B H-M 5C H-M 5Ab H-M 9B
WN4(h)
O7.5 V O5.5 Vd O9 V
H-M H-M H-M H-M
11A 20 3 5E
H-M 11B H-M 5D
a Running number and detector coordinates from the present photometry, to be presented in full by Parker et al. in a subsequent paper. b Heydari-Malayeri et al. 1993, with letters added here to the numbers to denote subsequently resolved components. c The colors for these components were discrepant, so the average color excess of the other spectroscopically classiÐed components was adopted instead. d Spectral type from Schild & Testor 1992 or Heydari-Malayeri et al. 1993.
FIG. 8.ÈH-R diagram of NGC 2044 West, from the stellar parameters listed in Table 2. See the Fig. 4 legend for other speciÐcations. The dashed lines are isochrones for ages of 1 and 1.5 Myr.
morphologically and philosophically. (4) The age of the system is 1 Myr and the gap between No. 2A and the other stars is merely a result of small-number statistics. However, the spectra of Stars 3 and 5 show evidence of evolution, the former directly and the latter from the interpretation of its class as due to close binaries with nitrogen enrichment from more massive, evolved companions (Walborn 1976 and references therein). The Be stars, on the other hand, could be preÈmain-sequence objects similar to HD 100546 (Malfait et al. 1998) ; infrared observations should be able to conÐrm or deny that possibility. (5) Star 2A is a blue straggler, perhaps a massive stellar merger. Theoretical reasons to expect such events in compact massive clusters have recently been discussed by Portegies Zwart et al. (1999). In that case, the age of the system is 3 Myr, and Star 2A may be providing a clue to the origin of at least some members of the Onfp class, for which there is spectroscopic evidence of rapid rotation and disk structures, possibly related to a recent merger. However, the evidence for the 3 Myr age depends essentially on the properties of one star, No. 3, and there is considerable scatter in the HRD, so an exotic interpretation of No. 2A remains a suggestion at the present time. Finally, there is a question whether the low-mass, cool
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FIG. 9.ÈHST WFC I V raw image of Lucke-Hodge 10 in N11 (upper left), with deconvolved enlargements of three O3 members identiÐed by their Parker et al. (1992) numbers, taken from Walborn et al. (1995a) with the addition of letter designations for the six components of No. 3209.
stars might also be preÈmain-sequence objects. The possibility cannot be ruled out from the present data, but they could also be Ðeld stars, or their HRD locations may be due to photometric errors ; such clumps at the faint end of the observational sample tend to move about or disappear when improved data are obtained (e.g., Parker et al. 1992 ; Parker & Garmany 1993). 3.2. NGC 2044 W est in 30 Doradus C The interstellar structure of the complex region 30 Doradus C (NGC 2044), further west of 30 Dor A and B and near SN 1987A, is also interpreted as an (older) supernova remnant (Chu & Kennicutt 1988 ; Chu 1997). The remarkable collection of stellar clusterings in the region has been investigated with successively higher spatial resolutions by Lortet & Testor (1984), Schild & Testor (1992), Heydari-Malayeri et al. (1993), and Walborn et al. (1995b). As for all of the systems discussed in this paper, the brightest subclustering, called b by Lortet & Testor and
NGC 2044 West by Walborn et al., also has a stellar designation, HDE 269828, since that was its appearance in the early, low-resolution surveys. Walborn (1977) noted the triple structure and composite spectrum of this object. It is listed as No. 65 in the LMC W-R catalog of Breysacher (1981), and that designation is now retained for the bright component at the northwest corner of the system, but Schild & Testor and Heydari-Malayeri et al. showed that the system also contains a second WN and an Of component. Figure 5 shows deconvolved WFC I V images of NGC 2044 W, taken directly from Walborn et al. (1995b). Useful FOS data were obtained for seven components of this system (albeit with lower signal-to-noise ratio than for the others, as discussed above), including the second WN object, which is here identiÐed as component 5C and observed without contamination from its close companions for the Ðrst time (Fig. 6). It is noteworthy that No. 5C is a WNE type, whereas Brey 65 is a WNL. The elongated
No. 4, 1999
COMPACT OB GROUPS IN THE LMC
1693
A O3 III(f*)
C O7 V
B O9 V
E O9 V
D O9.5 V
F O9.5 V
4000
4100
4200
4300
4400
4500
4600
4700
FIG. 10.ÈHST FOS blue-violet, rectiÐed spectrograms of the six O-type components in LH 10-3209. The pure O3 spectrum of the brightest component, Star A, is observed without contamination from its later O companions for the Ðrst time.
FIG. 11.ÈH-R diagram of the six spectroscopically classiÐed components of LH 10-3209, from the parameters listed in Table 3. The isochrone corresponds to an age of 1 Myr.
object 5A has now been resolved into two east-west components, hereafter 5Aa and 5Ab, with 5Aa the eastern and brighter of the pair, to which the observed spectral type has been ascribed. No spectroscopic information is available for component 5Ab or 5B. The spectrograms of the companions are presented in Figure 7, smoothed by 5 pixels to a formal resolution of 3.75 Ó. The spectroscopic and photometric results for NGC 2044 W are listed in Table 2, which also includes previous ground-based spectral types for three additional components, and all spectroscopically unclassiÐed stars with acceptable photometry within a 16A. 5 ] 18A. 5 Ðeld. The most important results in Figure 7 are the discovery of two O4 stars and, in particular, that No. 9A is a pure O4 If] type. Heydari-Malayeri et al. (1993) observed a spectrum with both early- and late-O (or early-B) features from the location of this object and assigned an intermediate O7 If spectral type ; however, Walborn et al. (1995b) reclassiÐed that spectrogram as O4 If] OB, and the spatially resolved FOS result conÐrms the latter interpretation, showing that the strong He I lines in the ground-based result came from other nearby stars. (The ““ ] ÏÏ in the present spectral type indicates Si IV j4089 in emission, in
FIG. 12.ÈHST PC2 V image of HD 32228 shown as an outset to a CTIO 4 m Ha plate of N11, taken from Walborn et al. (1995a) with the addition of stellar component identiÐcations from Heydari-Malayeri & Testor (1983, letters) and Schertl et al. (1995, numbers). HD 32228 dominates the association LH 9 within the cavity of the giant nebular shell, while LH 10 ionizes the largest nebula to the north.
COMPACT OB GROUPS IN THE LMC
16
14
12
10
8
6
4
2
3250
3500
3750
4000
4250
4500
4750
FIG. 13.ÈSpectrum of the WC component No. 2 in HD 32228 as observed by the HST FOS.
addition to the other Of features.) The spectrogram of No. 5A shows some emission contamination from the WN component No. 5C at He II j4686. The stellar content of NGC 2044 W is reminiscent of the Galactic cluster NGC 6871, which has O4 If] (HD 190429A), O6.5 III(f ) (HD 190864), WN4]O9.7 Iab (HD 190918), and B0.7 Ib (HD 190919) members (Walborn 1972, 1974). Figure 8 presents the HRD for NGC 2044 W, from the stellar parameters listed in Table 2. All of the speciÐcations are as in the Brey 73 HRD above, except that the average E \ 0.36 from the classiÐed stars has been applied to B~V with photometry only, and the isochrones shown are those for 1 and 1.5 Myr. It is concluded that the age of the system lies in that range, from which it follows that the initial masses of the WN objects were D90 M , although the HRD location of the B0 I star No. 5Aa is_discrepant. The HRD of NGC 2044 W is rather similar to that of Brey 73, but without a gap between the most massive and lower mass stars (and the spectra of the former are not peculiar). Again, the question whether the stars to the right of the main sequence are still approaching it must be left open, in view of the uncertainties.
1695
3.3. L H 10-3209 in Henize N11 Henize N11 is the second-ranked H II region in the LMC. It is a giant shell, in which an evolved, central association has evacuated a cavity and triggered a secondary, peripheral starburst. That picture has been established by the detailed study of the stellar content of the central (Lucke & Hodge 1970 No. 9) and largest peripheral (LH 10) associations by Parker et al. (1992). The interpretation of N11 as a two-stage starburst was further discussed by Walborn & Parker (1992). Three new O3 stars were discovered in LH 10 by that study. One of them, Parker No. 3209, displayed strong He I lines along with the O3 features in its spectrum ; accordingly, it was classiÐed O3 III(f *) ] OB. (The ““ f * ÏÏ in O3 spectral types signiÐes N IV j4058 emission stronger than N III j4640 ; that line ratio always correlates with the He absorption-line ionization criteria.) The other two stars (Parker Nos. 3058, 3061) showed pure O3 spectra. The WF/PC I imaging of the present program provided strong support for the interpretation of the ground-based spectral morphology, by revealing six components in LH 10-3209 and no close companions to the other two stars (Walborn et al. 1995a). As will be seen, the spatially resolved FOS spectroscopy in LH 10-3209 completes the story. Figure 9 shows the deconvolved WFC I V images of the three O3 stars in LH 10 ; it is taken from Walborn et al. (1995a), with the addition of component identiÐcations in No. 3209. (Actually, components C ] D ] E were just resolved by Parker et al. 1992 as their No. 3205, and F as No. 3211.) All six of the components resolved by WFC I were observed with FOS, and their spectrograms are shown in Figure 10, smoothed by 3 pixels to a formal resolution of 2.25 Ó. The observational data and derived parameters for the six stars are listed in Table 3. The principal result is immediately apparent : the brightest component, A, has indeed a pure O3 III(f*) spectrum, with no He I detected, while the fainter companions are all mid- or late-O types with strong He I lines, explaining the composite groundbased result. An H-R diagram for LH 10-3209 is presented in Figure 11, with all procedures as before and a 1 Myr isochrone included. The fact that a 120 M star lies on that isochrone is consistent with the absence _ of WN stars in LH 10 and establishes that value as a reasonable age estimate for the association. 3.4. HD 32228 in Henize N11 HD 32228, also cataloged as Radcli†e (R) 64 and Breysacher 9, dominates the central association LH 9 in N11. It
TABLE 3 OBSERVED AND DERIVED PARAMETERS FOR THE COMPONENTS OF LH 10-3209 No.a
Xa
Ya
V
B[V
U[B
17 . . . . . . 18 . . . . . . 14 . . . . . . 16 . . . . . . 15 . . . . . . 19 . . . . . .
584.15 584.53 573.82 575.74 574.46 595.06
102.76 105.73 100.73 100.70 96.60 94.66
12.73 14.26 14.49 15.57c 15.76 15.88
[0.07 0.07 0.04 ... 0.00 0.00
[1.13 [0.20 [1.28 ... [0.82 [0.90
E
B~V 0.25 0.38 0.36 0.32c 0.30 0.30
log T 4.707 4.555 4.613 4.555 4.539 4.539
M bol [11.17 [8.95 [9.07 [7.47 [7.10 [6.98
BC
Spectral Type
[4.56 [3.48 [3.89 [3.48 [3.37 [3.37
O3 III(f*) O9 V O7 V O9 V O9.5 V O9.5 V
Alternate IDb Star Star Star Star Star Star
A B C E D F
a Running number and detector coordinates from the present photometry, to be presented in full by Parker et al. in a subsequent paper. b Fig. 9. c The photometry for this component was discrepant, so the magnitude di†erence from component C in the V deconvolution and the average color excess of the other components were adopted for it instead.
1696
WALBORN ET AL.
4000
4100
1
O9 Ib
i
O8.5 II(f)
3
O7.5 III
4
O8 V
k
O9 Vn (sb2?)
5
O9.5 V
9
B0.5-1 V
4200
4300
4400
Vol. 118
4500
4600
4700
FIG. 14a FIG. 14.ÈHST FOS blue-violet, rectiÐed spectrograms of OB components in HD 32228 ; (b) is designed as an atlas of late-O and early-B main-sequence spectra from the present data.
had been known from the earliest surveys as one of the brightest ““ stars ÏÏ in the LMC, with a WC ] OB spectral type, but it is now recognized as a massive compact cluster that, although currently at least an order of magnitude less massive than R136 in 30 Doradus, occupies an analogous central position in the giant H II region. Indeed, many if not most giant H II regions harbor such luminous central objects, e.g. HD 97950 in the most massive optically visible Galactic H II region NGC 3603 (Drissen et al. 1995 and references therein). The structure of HD 32228 has been investigated with successively higher spatial resolutions by Walborn (1977), Heydari-Malayeri & Testor (1983), Bauer et al. (1996), Schertl et al. (1995), and Walborn et al. (1995a). Figure 12 is reproduced from Walborn et al. (1995a), with the addition of stellar identiÐcations in HD 32228 ; it shows the present PC2 V image as an outset to a CTIO 4 m Ha plate of N11 in its entirety. The brightest 15 components of HD 32228 were observed with FOS, including the WC (No. 2) without contamination from its companions for the Ðrst
time (Fig. 13). The blue-violet spectrograms of the companions are shown in Figures 14a and 14b, smoothed by 3 pixels to a formal resolution of 2.25 Ó. The observational and derived parameters for the spectroscopically classiÐed stars are given in Table 4, along with those for all stars with photometry only within a 13A. 7 ] 13A. 7 Ðeld. Other than the WC component, there are no peculiar spectra among the brightest stars in HD 32228, although Stars 1 and i are quite luminous, and Star k may be a double-lined spectroscopic binary. Figure 14b is designed as a small atlas of late-O and early-B main-sequence spectra from the present data. The HRD of HD 32228 is presented in Figure 15, with all speciÐcations as above except that the average E \ 0.07 from the classiÐed stars has been applied to thoseB~V with photometry only, and the isochrone shown is for an age of 3.5 Myr. The correspondence between the spectral types and the HRD locations of the classiÐed stars is remarkable and strongly suggests that much of the scatter in the previous
No. 4, 1999
COMPACT OB GROUPS IN THE LMC
d
O8 V((f))
8
O8.5 V
7
O9 V
6
O9.5 V
1697
12 O9.5 V
f
B0 V
13 B1 V
4000
4100
4200
4300
4400
4500
4600
4700
FIG. 14b
HRDs is due to the poorer quality of the WF/PC I photometry. (As already noted, the spectral classiÐcations were done without knowledge of the photometric magnitudes of the stars.) Hence, one can conclude with some conÐdence that the model age of HD 32228 is 3.5 Myr, and the initial mass of the WC star was about 45 M . This age is rather _ less than adopted in previous discussions of LH 9 (e.g., Walborn & Parker 1992), but in view of the age derived for LH 10-3209 above, the age di†erence between the two associations and the two-stage starburst interpretation of N11 remain as before. In the case of HD 32228, the morphology of the HRD for the lower mass stars to right of the main sequence may be more consistent with a preÈmainsequence interpretation. 4.
SUMMARY AND FUTURE WORK
We have obtained spatially resolved spectroscopy and photometry with HST of four interesting, massive multiple systems in the LMC, adding substantially to knowledge of their stellar content and ages. The spectra of three W-R and
two early Of stars have been observed without contamination from their OB companions for the Ðrst time, and improved estimates of their ages and initial masses have been derived. The evolutionary status of Breysacher 73 in 30 Doradus B is somewhat obscured by the possible presence of a massive blue straggler, but that drawback is o†set by the surprising diversity of peculiar objects discovered to coexist within that system. NGC 2044 West is a rather similar system containing an early Of along with both WNE and WNL members. Improved age estimates have been obtained for massive multiple systems in the initial, central and triggered, peripheral associations of the giant shell H II region N11, contributing to its interpretation as a two-stage starburst. While there was prior ground-based evidence for the multiplicity of the LMC objects discussed here, similar systems in more distant galaxies will not be as readily recognized, and they may introduce systematic errors in determinations of the upper stellar mass limit and initial mass function, unless they are identiÐed and removed from the samples.
TABLE 4 OBSERVED AND DERIVED PARAMETERS FOR THE COMPONENTS OF HD 32228 No.a
Xa
Ya
V
B[V
U[B
30 . . . . . . 89 . . . . . . 44 . . . . . . 62 . . . . . . 21 . . . . . . 84 . . . . . . 50 . . . . . . 41 . . . . . . 51 . . . . . . 68 . . . . . . 61 . . . . . . 72 . . . . . . 36 . . . . . . 87 . . . . . . 75 . . . . . . 69 . . . . . . 94 . . . . . . 59 . . . . . . 29 . . . . . . 65 . . . . . . 80 . . . . . . 76 . . . . . . 46 . . . . . . 42 . . . . . . 43 . . . . . . 70 . . . . . . 26 . . . . . . 25 . . . . . . 73 . . . . . . 22 . . . . . . 23 . . . . . . 53 . . . . . . 52 . . . . . . 31 . . . . . . 47 . . . . . . 49 . . . . . . 83 . . . . . . 40 . . . . . . 16 . . . . . . 48 . . . . . . 37 . . . . . . 90 . . . . . . 86 . . . . . . 54 . . . . . . 24 . . . . . . 81 . . . . . . 19 . . . . . . 38 . . . . . . 66 . . . . . . 78 . . . . . . 85 . . . . . . 88 . . . . . . 18 . . . . . . 97 . . . . . . 91 . . . . . . 45 . . . . . . 77 . . . . . . 98 . . . . . . 60 . . . . . . 28 . . . . . . 56 . . . . . . 63 . . . . . . 20 . . . . . . 93 . . . . . . 27 . . . . . . 14 . . . . . . 92 . . . . . . 67 . . . . . . 58 . . . . . . 95 . . . . . . 71 . . . . . .
400.57 510.44 422.08 452.36 362.33 491.31 430.01 413.36 430.13 465.26 452.05 470.67 408.46 498.10 472.59 466.53 548.65 449.10 394.27 460.41 483.57 474.26 423.55 415.18 418.60 467.50 383.30 379.46 471.15 368.73 368.74 437.04 437.02 402.26 424.60 429.88 487.47 412.76 335.54 428.29 409.41 517.08 496.58 439.20 369.03 484.42 354.16 410.59 463.51 477.08 493.89 506.38 352.30 573.66 517.18 423.50 476.08 597.99 450.75 393.50 445.71 452.56 359.37 536.33 386.75 319.54 536.01 464.00 447.76 552.48 469.32
397.98 482.69 376.50 394.18 434.83 508.04 340.37 381.60 412.28 394.78 409.25 447.43 412.93 348.03 441.56 508.70 557.00 403.81 350.96 374.90 414.54 396.79 387.09 426.87 544.24 457.60 420.34 458.86 379.78 473.45 457.68 422.46 389.52 433.61 339.32 316.20 437.05 407.60 522.49 480.35 424.00 428.18 433.43 540.13 413.74 470.25 390.36 317.73 512.59 334.93 562.73 366.32 426.97 552.23 357.07 321.48 563.50 340.39 540.76 467.42 326.11 474.47 343.37 578.35 479.90 326.40 454.17 311.45 305.55 413.79 329.68
12.56 12.99 13.39 13.57 13.82 14.19 14.66 14.74 14.77 14.87 14.88 14.90 14.91 15.04 15.08 15.18 15.41 15.48 15.56 15.61 16.13 16.15 16.17 16.35 16.41 16.41 16.53 16.55 16.67 16.87 16.92 16.95 16.96 17.01 17.18 17.34 17.41 17.55 17.58 17.59 17.66 17.66 17.72 17.72 17.73 17.74 17.76 17.76 17.93 17.99 18.15 18.15 18.20 18.23 18.23 18.25 18.36 18.42 18.44 18.45 18.46 18.54 18.54 18.65 18.65 18.67 18.72 18.90 18.92 19.02 19.10
[0.18 [0.17 [0.09 [0.21 [0.26 [0.24 [0.26 [0.23 [0.24 [0.28 [0.27 [0.19 [0.27 [0.24 [0.28 [0.19 [0.24 [0.26 [0.29 [0.25 [0.12 [0.21 [0.24 [0.28 [0.14 [0.14 [0.17 [0.20 [0.24 [0.09 [0.17 [0.18 [0.21 [0.14 [0.05 [0.17 [0.27 [0.04 [0.07 [0.15 [0.15 [0.10 [0.05 [0.18 [0.17 [0.11 [0.24 0.18 [0.10 [0.08 [0.18 0.02 [0.21 0.12 [0.18 [0.07 [0.23 0.08 0.02 0.06 [0.12 0.06 0.08 [0.14 [0.16 0.17 [0.20 [0.25 [0.01 [0.25 [0.09
[1.01 [1.10 [0.58 [0.99 [0.99 [0.99 [0.95 [1.00 [0.96 [0.99 [0.92 [1.02 [1.03 [0.91 [0.83 [1.06 [0.96 [0.88 [0.92 [0.95 [0.87 [0.84 [0.71 [0.84 [0.93 [0.94 [0.76 [0.80 [0.76 [0.67 [0.81 [0.47 [0.73 [0.53 [0.69 [0.74 [0.67 [0.72 [0.69 [0.71 [0.51 [0.54 [0.55 [0.41 [0.46 [0.49 [0.60 [0.02 [0.65 [0.44 [0.50 [0.22 [0.37 [0.61 [0.43 [0.48 [0.56 [0.15 [0.54 [0.65 [0.10 [0.21 [0.63 [0.44 [0.32 [0.31 [0.28 [0.26 [0.56 [0.30 [0.55
E
B~V 0.13 0.14 ... 0.11 0.05 0.07 0.05 0.07 0.06 0.03 0.00 0.11 0.04 0.06 [0.01 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07
log T 4.515 4.553 ... 4.585 4.585 4.555 4.585 4.539 4.539 4.570 4.445 4.539 4.555 4.523 4.445 4.551 4.430 4.354 4.368 4.415 4.427 4.351 4.242 4.313 4.466 4.474 4.312 4.326 4.276 4.289 4.350 4.127 4.270 4.176 4.324 4.298 4.203 4.352 4.314 4.287 4.160 4.199 4.227 4.097 4.126 4.165 4.174 3.934 4.270 4.149 4.143 4.077 4.070 3.964 4.107 4.176 4.156 3.985 4.254 4.354 3.998 4.085 3.985 4.126 4.064 3.939 4.037 4.018 4.253 4.031 4.209
M
bol [9.63 [9.50 ... [9.05 [8.62 [8.10 [7.78 [7.44 [7.38 [7.41 [6.57 [7.40 [7.29 [6.99 [6.34 [7.08 [6.16 [5.64 [5.64 [5.87 [5.42 [4.95 [4.29 [4.52 [5.37 [5.42 [4.34 [4.40 [3.99 [3.86 [4.17 [2.82 [3.66 [3.05 [3.76 [3.44 [2.82 [3.56 [3.30 [3.13 [2.31 [2.54 [2.65 [1.88 [2.04 [2.26 [2.30 [1.07 [2.69 [1.92 [1.72 [1.32 [1.24 [0.68 [1.42 [1.81 [1.58 [0.55 [2.09 [2.67 [0.56 [0.98 [0.43 [1.12 [0.75 [0.17 [0.52 [0.23 [1.60 [0.19 [1.16
BC
Spectral Type
Alternate IDb
[3.20 [3.46 ... [3.69 [3.69 [3.48 [3.69 [3.37 [3.37 [3.59 [2.85 [3.37 [3.48 [3.25 [2.85 [3.45 [2.76 [2.31 [2.39 [2.67 [2.74 [2.29 [1.65 [2.06 [2.97 [3.02 [2.06 [2.14 [1.85 [1.92 [2.28 [0.96 [1.81 [1.25 [2.13 [1.98 [1.42 [2.30 [2.07 [1.91 [1.16 [1.39 [1.56 [0.79 [0.96 [1.19 [1.24 [0.02 [1.81 [1.10 [1.06 [0.66 [0.62 [0.10 [0.84 [1.25 [1.14 [0.16 [1.72 [2.31 [0.21 [0.71 [0.16 [0.96 [0.59 [0.03 [0.43 [0.32 [1.71 [0.40 [1.45
O9 Ib O8.5 II(f ) WC4 O7.5 III O8 V((f )) O9 Vn (sb2 ?) O8 V O9.5 V O9.5 V O8.5 V B0.5ÈB1 V O9.5 V O9 V B0 V B1 V
Sch 1, H-M a H-M i Sch 2, H-M b1 Sch 3, H-M g1 H-M d H-M k Sch 4, H-M c Sch 5, H-M b2 Sch 6 Sch 8, H-M g2 Sch 9, H-M g3 Sch 12, H-M h1 Sch 7 H-M f Sch 13, H-M h2
a Running number and detector coordinates from the present photometry, to be presented in full by Parker et al. in a subsequent paper. b (Sch) Schertl et al. 1995 ; (H-M) Heydari-Malayeri & Testor 1983, with arabic numerals added here to the letters to denote subsequently resolved components.
COMPACT OB GROUPS IN THE LMC
1699
The stars observed spectroscopically and discussed in this paper are a tiny fraction of those found in the HST direct images of their Ðelds. Complete photometric analyses of these images are being undertaken by Parker et al. and will be presented in future papers. Our objective is to derive color-magnitude diagrams and initial mass functions for all of them. Initial mass functions for the multiple systems discussed here, especially HD 32228, which has the best photometric data, will also be derived and compared with those of the surrounding Ðelds, to further investigate the starformation process and stellar evolution in these massive young regions.
FIG. 15.ÈH-R diagram of HD 32228, from the parameters listed in Table 4. The 3.5 Myr isochrone provides an excellent Ðt to the cluster turno† as deÐned by the most luminous, spectroscopically classiÐed stars.
Support for this work was provided by NASA through grants GO-4322.01-92A, GO-6032.01-94A, GO-6508.0195A, and AR-7545.01-96A from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
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