Hubble Space Telescope WFPC2 Observations of the Binary Fraction ...

THE ASTROPHYSICAL JOURNAL, 477 : 705È710, 1997 March 10 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.

HUBBL E SPACE T EL ESCOPE WFPC2 OBSERVATIONS OF THE BINARY FRACTION AMONG PREÈMAIN-SEQUENCE CLUSTER STARS IN ORION DEBORAH L. PADGETT Infrared Processing and Analysis Center, Jet Propulsion Laboratory, MC 100-22, 4800 Oak Grove Drive, Pasadena, CA 91108 ; dlp=ipac.caltech.edu

STEPHEN E. STROM Five College Astronomy Department, University of Massachusetts, Amherst, MA 01003

AND ANDREA GHEZ Department of Astronomy, University of California, Los Angeles, Los Angeles, CA 90024 Received 1995 December 21 ; accepted 1996 October 3

ABSTRACT We present estimates of the binary frequency for optically visible (I \ 19 mag) members of the HSTwith NGC 2024, NGC 2068, dense (mean star separation D0.05 pc) embedded stellar clusters associated and NGC 2071 (distance \ 460 pc) based on Hubble Space T elescope (HST ) Wide Field Planetary Camera 2 (WFPC2) observations. Of 99 targets, 15 are found to be double stars with projected linear separations of 138È1050 AU (0A. 3 \ h \ 2A. 3). The resulting multiplicity fraction is 15/99 or 0.15 ^ 0.04 in this separation range, while the comparable values observed for solar neighborhood G and K stars are 0.11 and 0.09, respectively. We also examined the archival F547M WFPC2 images of the Trapezium cluster obtained by OÏDell & Wen and found a binary frequency of 7/50 or 0.14 ^ 0.05 in the separation range 138È828 AU for stars with V \ 17. Our results compare well with the value of 0.16 ^ 0.03 HST determined recently by Reipurth & Zinnecker from observations of 238 preÈmain-sequence (PMS) stars in nearby (d D 150 pc) low-density (mean star separation D0.3 pc) star-forming regions over comparable separation ranges. Our results suggest an excess of PMS binaries over main-sequence binaries with the same separations. In addition, we Ðnd no evidence that the fraction of binaries formed in relatively dense clusters di†ers from that characterizing low-density star-forming regions. Subject headings : binaries : visual È ISM : individual (Orion Nebula) È open clusters and associations : general È stars : preÈmain-sequence È stars : statistics 1.

INTRODUCTION

forming primary star as preferred mechanisms for binary or multiple star formation (Burkert & Bodenheimer 1993 ; Boss 1988 ; Clarke & Pringle 1991 ; Adams, Ruden, & Shu 1989). Unseen companions also play havoc with the placement of young stars on theoretical preÈmain-sequence tracks, causing underestimates of stellar age and overestimates of stellar masses because of the excess luminosity (Simon, Ghez, & Leinert 1993). Furthermore, a number of studies Ðnd that both submillimeter and millimeter emission from circumstellar disks are suppressed in close preÈ main-sequence binaries (Jensen, Mathieu, & Fuller 1994 ; Simon et al. 1995), possibly by the truncation of the outer disk region, indicating that the presence of close stellar companions is crucial to the evolution of protoplanetary disks. However, the conclusion that most solar-type stars may be formed in binaries is based upon studies of the binary frequency of YSOs in relatively sparsely populated star formation regions unrepresentative of the giant molecular cloud (GMC) complexes that are believed to be the primary sites of star formation in the Milky Way. These GMCs give birth to stellar aggregates and clusters, where typical star separations range from 0.05 (NGC 2024) to 0.003 pc (Trapezium) (McCaughrean & Stau†er 1994), rather than loose T Tauri star associations such as Taurus-Auriga with mean stellar separations of 0.3 pc (Gomez et al. 1993). Based on prerepair Hubble Space T elescope (HST ) Planetary Camera images of the Trapezium, Prosser et al. (1994) have claimed a binary frequency more consistent with the solar

Recent high angular resolution observations aimed at assessing multiplicity among preÈmain-sequence (PMS) stars have led to a result that could profoundly alter our understanding of the star formation process : many, and possibly all, of the youngest (t \ 3 Myr) optically visible solar-type stars appear to be members of binary or multiple systems (Ghez 1992 ; Ghez, Neugebauer, & Matthews 1993 ; Leinert et al. 1993 ; Simon et al. 1992 ; Richichi et al. 1994 ; Reipurth & Zinnecker 1993 ; Simon et al. 1995). This result is based primarily on ground-based speckle-imaging and lunar occultation studies of T Tauri stars located in the nearby (d \ 150 pc) Taurus-Auriga and Ophiuchus complexes that have found a high frequency of binaries with separations of 15 AU \ r \ 225 AU. Ghez et al. (1993) Ðnd a multiplicity fraction among Taurus-Auriga preÈmainsequence stars that is larger by a factor of 3 than the multiplicity of solar neighborhood stars with similar mass (Duquennoy & Mayor 1991 ; Fischer & Marcy 1992). At somewhat larger separations of 150 AU \ r \ 1200 AU, the young stellar object (YSO) systems studied by Reipurth & Zinnecker (1993) have a multiplicity fraction 1.6 times larger than the solar neighborhood value. If most young, solar-type stars are indeed members of binary or multiple systems, then there are a number of fundamental implications for our understanding of star formation. Such binary systems must take form early, thus pointing toward fragmentation of protostellar cores or growth of instabilities in massive disks surrounding a 705

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PADGETT, STROM, & GHEZ

neighborhood value. Their results suggests that the formation mechanism or survival time of binaries in dense regions must di†er signiÐcantly from that characterizing lower density star-forming regions such as Taurus, in which the binary formation frequency or survival probability might be higher. In order to determine whether the binary formation/ survival fraction in GMC clusters di†ers from lower density star-forming regions or the solar neighborhood, we have carried out HST imaging of three clusters (NGC 2024, NGC 2068, and NGC 2071) located in the nearest GMCÈ that associated with the Orion star-forming complex at a distance of 460 pc. The Wide Field Planetary Camera 2 (WFPC2) installed during the 1993 December repair mission enables us to resolve binaries spanning the range 100È1500 AU in these dense clusters. In addition, we have examined 1994 archival WFPC2 images of the Trapezium cluster located at the same distance as our target clusters. 2.

OBSERVATIONS

In 1994 February and 1995 April, we obtained F814W (j \ 8140 AŽ and *j \ 2500 AŽ , hereafter I ) WFPC2 0 HST in NGC images of three Ðelds in NGC 2024, three Ðelds 2068, and one Ðeld in NGC 2071. Figure 1 (Plate 20) is a WFPC2 mosaic of a Ðeld in NGC 2024. Note that a WFPC2 mosaic consists of three Wide Field (WF) camera images with a scale of 0A. 097 pixel~1 and a single Planetary Camera (PC) frame with 0A. 046 pixel~1. However, the shadow of the pyramid obscures one edge of each chip, e†ectively reducing the size of the Ðeld. Thus, a single WFPC2 image covers an area of 18,000 arcsec2 on the sky. The results presented are based on two 100 s exposures for each Ðeld that are displaced by 0.5 pixels in order to compensate in part for the undersampling of the di†raction core of images taken with the Wide Field Camera (WFC). This ““ dithering ÏÏ also enabled us to subtract ““ hot ÏÏ pixels that evolve over time on the WFPC2 CCD chips. These images were corrected for bias and dark current using the procedures of the WFPC2 science team and were Ñattened

Vol. 477

using on-orbit Ñat-Ðeld data. Cosmic rays were removed by subtracting a di†erenced image of the two 100 s exposures after the frames were registered to the same subpixel position. This procedure removed virtually all of the many hundred cosmic rays present in the raw data. Note that there are two ““ ghost images ÏÏ associated with each extremely saturated star, but they are easily identiÐed since they are always on a line between the star and the chip center. The 1994 WFPC2 images of the Trapezium cluster obtained by OÏDell & Wen (1994) consist of a pair of 100 s F547M (j \ 5470 AŽ and *j \ 700 AŽ , hereafter V ) expo0 HST sures centered on the cluster core. These images were reduced in the same way as the NGC 2024 and NGC 2068 images, except that no dithering was introduced between the image pairs. 3.

BINARY IDENTIFICATION AND COMPARISON WITH SIMULATIONS

We used the DAOPHOT package inside IRAF to identify possible stars with I \ 19, which translated to a signal-to-noise ratio B60HSTon the WFPC2 frames. This enables us to detect companions that are approximately 2 mag fainter than the primary at a 5 p level. Each candidate was carefully screened and examined by eye for multiplicity. Due to the undersampling of the WF point-spread function, we have chosen to present results only for binary candidates with r [ 3 pixels (B138 AU) because of the possibility of incompleteness for smaller separations, as suggested by our simulations discussed below. In addition, we have excluded pairs in which both components are saturated stars (I \ 14). Binary candidates with small HSTexamined on each of the raw frames to separations were conÐrm their extended nature. In our seven WFPC2 mosaics of NGC 2024, NGC 2068, and NGC 2071, we have identiÐed a total of 99 stars with I \ 19. Table 1 lists 15 HSTbetween 138 and 1050 binary candidates with separations AU (0A. 29È2A. 3 ; 3È24 WFC pixels) found in our sample. Three additional candidates with separations of about 2.5 pixels are also listed, but they are excluded from our sta-

TABLE 1 BINARY CANDIDATES NGC 2024, NGC 2068, AND NGC 2071 IdentiÐcation Numbera

X

Y

Field

Chip

R.A. (2000)

Decl. (2000)

2024-1 . . . . . . . 2024-2 . . . . . . . 2024-3 . . . . . . . 2024-4 . . . . . . . 2024-5 . . . . . . . 2024-6 . . . . . . . 2024-7 . . . . . . . 2024-8 . . . . . . . 2068-1 . . . . . . . 2068-2 . . . . . . . 2068-3 . . . . . . . 2068-4 . . . . . . . 2068-5 . . . . . . . 2071-1 . . . . . . . 2071-2 . . . . . . . 2024-9* . . . . . . 2068-6* . . . . . . 2068-7* . . . . . .

585 260 214 246 122 224 599 183 527 567 331 135 735 722 214 163 689 277

676 119 99 668 113 769 262 526 708 720 430 362 581 519 185 113 726 117

1 1 1 1 1 1 2 3 1 1 2 3 3 1 1 1 1 2

PC1 WF3 WF4 WF4 WF4 WF4 WF3 WF2 WF2 WF3 WF3 PC1 WF3 WF4 WF4 WF3 WF2 WF4

05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05

[01 [01 [01 [01 [01 [01 [01 [01 ]00 ]00 ]00 ]00 ]00 ]00 ]00 [1 ]00 ]00

41 41 41 41 41 41 41 41 46 46 46 46 46 47 47 41 46 46

38.8 36.3 35.9 36.3 36.5 36.5 35.0 44.0 47.35 40.30 34.63 47.45 40.96 05.31 05.74 36.4 48.03 37.63

53 54 53 52 53 52 56 55 04 04 06 03 02 23 22 53 04 06

23.5 04.6 34.8 39.4 35.1 27.7 12.2 20.0 09.1 57.0 43.4 18.7 55.0 11.7 11.3 55.1 04.2 54.7

Iprimary HST ... 17.2 18.0 16.0 18.9 16.7 15.2 18.4 17.1 14.3 15.5 ... 15.1 19.0 18.7 16.0 18.7 19.0

*I HST 0.6 0.0 4.5 : 1.9 2.7 0.6 1.8 1.9 0.7 2.0 2.5 3.9 3.5 1: 0.8 0: 0: 0:

*r (pixels)

P b ps (%)

10 (5 WF) 3.5 8 21 9 7 4.5 21 12 5 10 42 (21 WF) 11 4: 18 2.5 2.5 2.5 :

99 99 95 75 94 95 99 99 96 99 98 98 98 99 99 99 99 99

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a The Ðrst part of the identiÐcation number identiÐes the NGC designation of the cluster. Objects marked with an asterisk have less than 0A. 3 separation and are excluded from our statistics. b Probability that binary is a physical system considering the stellar density within the chip Ðeld.

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BINARY FRACTION AMONG PMS CLUSTER STARS IN ORION

707

TABLE 2 TRAPEZIUM BINARY CANDIDATES IdentiÐcation Number

X

Y

Chip

R.A. (2000)

1T . . . . . . . . . . . . 2T . . . . . . . . . . . . 3T . . . . . . . . . . . . 4T . . . . . . . . . . . . 5T . . . . . . . . . . . . 6T . . . . . . . . . . . . 7T . . . . . . . . . . . . 8Tb . . . . . . . . . . .

583 168 261 52 714 491 284 693

75 65 414 20 217 200 375 253

WF2 WF3 WF3 WF3 WF3 WF4 WF4 WF3

05 05 05 05 05 05 05 05

35 35 35 35 35 35 35 35

22.09 18.04 15.95 18.13 17.97 14.65 15.71 17.70

Decl. (2000) [05 [05 [05 [05 [05 [05 [05 [05

24 24 24 24 25 24 23 25

12.2 37.0 54.7 24.7 33.42 09.7 47.8 32.3

V

(primary)

HST 14.5 : 17.7 15.9 17.1 14.9 16.6 14 : 16.1

*V

HST 3: 2: 0: 0: 1: 0.3 1: 0:

*r (pixels)

P a ps (%)

18 7 5.4 4.3 3: 12 5 2.5

83 97 97 98 98 85 98 99

a Probability that binary is a physical system considering the stellar density within the chip Ðeld. b This object has less than 0A. 3 separation and is excluded from our statistics.

tistics. T hus, for separations of 138 AU \ r \ 1050 AU, we have found 15 binaries out of a sample of 99 stars for a multiplicity fraction 0.15 ^ 0.04. The uncertainty is estimated using Poisson statistics. If we include the two candidates with separations of 2.5 pixels, the multiplicity fraction increases to 0.167. Subimages of the binary candidates with separations between 0A. 3 and 2A. 3 are presented in Figure 2 (Plates 21È24). In quoting multiplicity statistics for any survey, it is crucial to understand the completeness limits as a function of magnitude di†erence and separation. In order to assess our survey completeness, we have simulated binaries with varying separations and Ñux ratios. Using the TINYTIM software courtesy of J. Krist at STScI, we produced a simulated WFPC2 point-spread function (PSF) appropriate for a K star observed with the F814W Ðlter on the WF4 chip. Since this PSF was oversampled by a factor of 6 compared with the actual WFC, the e†ects of subpixel shifts could be explored without interpolation by performing integral pixel shifts on the oversampled image and then rebinning to the WFC resolution. The simulated binaries were co-added to an actual WF subimage without stars in order to add an appropriate amount of background emission. We Ðnd that our survey should be complete at the faint limit for r [ 3 pixels and *I [ 2 for I (primary) \ 19. Because of the undersampledHST WFC PSF,HST the fraction of detected binaries drops below 100% for smaller physical separations and larger magnitude di†erences. At larger separations, in which the secondary is beyond the wings of the primaryÏs PSF, the detectable *I is º4. HST stellar densities in N2024, and especially The projected the Trapezium, are sufficiently high to warrant examination of the possibility that some of our binary candidates might be chance superpositions. To check this possibility, we created simulated star Ðelds in which single ““ stars ÏÏ were laid down at random for a projected stellar surface density appropriate for either individual frames or the entire cluster. These simulations enabled us to calculate the number of chance superpositions as a function of projected separation. By running 100 such simulations, we were able to establish the probability that a candidate visual binary represents a true physical system. Running simulations for each frame enables us to quantify the reliability of individual binary candidates, given the considerable variations in local stellar density from frame to frame. The probability that a given candidate binary system is a physical system rather than a chance superposition is listed in Tables 1 and 2 (P ). Using the mean cluster stellar surface density, we ps

have also evaluated the probability that at least one pair of stars with a separation less than or equal to a given value is a chance superposition. These probabilities (P ) are superposition presented in Table 3. The analysis of the archival Trapezium images proceeded in much the same way. However, because of the higher and more variable background (º200 DN in Trapezium vs. 50 DN in NGC 2024) in these F547M (V ) images, the detection threshold was set to V \ 17.HST We found a binary HST frequency of 7/50 in the periphery of the Trapezium cluster (r [ 35A), where the nebular background is low enough to enable a comparable, though not identical, survey. The typical projected separation of cluster members in the peripheral region of the Trapezium is comparable to the densest regions of NGC 2024 and NGC 2068. Again, simulations indicate that we should detect all binary pairs provided r º 3 pixels, *V ¹ 2 for V (primary) ¹17. We HST identify seven of theseHST as strong binary candidates with separations ranging from 3 to 18 pixels. Thus, we Ðnd a binary frequency in the Trapezium periphery of 0.140 ^ 0.05 in the separation range between 138 and 828 AU. Table 2 lists the binary candidates that we have identiÐed in the periphery of the Trapezium. We did, however, note an apparent deÐciency of close binaries among the low-mass population near the very core of the TrapeziumÈthe region within 35A of h1 C Orionis in which the mean separation between cluster stars is B0.003 pc. Although one of the O stars is a close binary, we found no close (¹0A. 5) solar-type binaries in the inner part of the Trapezium. There are 40 primary candidates in this region, and for separations º3 pixels and *V ¹ 2, we expect to HST Ðnd at least two companions with separations between 0A. 3 and 0A. 5 if the binary frequency is similar to that in the solar neighborhood or on the Trapezium periphery. Even presuming that companions of lesser brightness are masked by bright circumstellar and interstellar nebulosity, there may TABLE 3 CLUSTER SUPERPOSITION PROBABILITIES Cluster

P (2A. 3)a superposition (%)

P (1A)a superposition (%)

Trapezium . . . . . . NGC 2024 . . . . . . NGC 2068 . . . . . .

100 40 20

20 10 2

P

superposition (%)

(0A. 5)a

4 1 ¹1

a Probability, at a given separation, that binary is a chance superposition of stars based on the mean stellar density of cluster Ðelds surveyed.

708

PADGETT, STROM, & GHEZ

be a deÐcit of equal brightness binaries in this region. The question of whether this is a signiÐcant result or a statistical Ñuke will require a deeper, higher angular resolution survey aimed at careful study of a larger sample. 4.

DISCUSSION

4.1. Comparison with Other Binary Surveys In Figure 3, we present the observed binary frequency (number of binary systems/number of single systems]number of binary systems) as a function of projected separation (bins of 0.3 dex) for two samples of lowmass preÈmain-sequence stars. The Ðrst sample is taken from the compilation of Reipurth & Zinnecker (1993) and includes binary pairs (separations 1 \ h \ 12A) located in four nearby (d D 150 pc) star-forming regions. The second sample derives from our HST WFPC2 observations and includes binary pairs (separations 0.3 \ h \ 3A. 6) in NGC 2024, NGC 2068, and NGC 2071 (d D 450 pc) ; note that the projected separation range, r/AU, spanned by the two samples is similar (138È1050 AU for the current study ; 150È 1200 AU for Reipurth & Zinnecker). The Ðrst sample should include all binary pairs with I-band Ñux ratios *I \ 2.5 mag. The second sample should be complete forHST *I \ 2 mag for h [ 0A. 3. The dashed distribution (dashedHST line, Fig. 3) shows the binary frequency found for G and K stars by Duquonnoy & Mayor (1991). For separations greater than 100 AU, the uncertainty in the overall main-sequence binary frequency is D25% (Mathieu 1995). These data suggest that there is no signiÐcant di†erence between the observed YSO binary frequency for stars located in the periphery of extremely dense PMS clusters, such as the Trapezium or relatively dense star-forming

Vol. 477

associations such as N2024/N2068 (D8000 stars pc3 with stellar separations D0.05 pc), and the observed YSO binary frequency for stars located in the relatively low density regions (D40 stars pc3 with stellar separations D0.3 pc) surveyed by Reipurth & Zinnecker (1993). Using a twosample s2 test, our sample is indistinguishable from the Reipurth & Zinnecker T association sample with a 96% conÐdence level. We Ðnd no evidence to suggest that the Taurus-Auriga and GMC modes of star formation produce di†erent initial multiplicity fractions. If there are di†erences between the binary frequencies of individual clouds, as suggested by Durisen & Sterzik (1994), it is not evident from our data. A comparison of our distribution of binary separations with those found for solar neighborhood G and K stars by Duquennoy & Mayor (1991) suggests an excess of preÈ main-sequence binaries, especially in the bin from 300 to 600 AU. In the separation range convered by our survey, we Ðnd that the multiplicity fraction of preÈmain-sequence stars in Orion is 1.3È1.4 times the solar neighborhood G and K star fraction. However, a two-sample s2 test comparing our sample with the G star distribution and the K star distribution found that the samples were di†erent at only the 44% and 78% conÐdence level, respectively (Mathieu 1995). Thus, the statistical signiÐcance of our excess taken by itself is not high. However, it is consistent with the trend apparent in other star-forming regions, independent of stellar density. Larger numbers of both preÈmain-sequence and main-sequence stars must be surveyed for binarity before preÈmain-sequence binary excesses at these separations can be conÐrmed. We have found some evidence that suggests that binary

FIG. 3.ÈComparison of the binary frequency for PMS stars in T associations observed by Reipurth & Zinnecker (1993) (top panel) and Orion clusters observed by HST (bottom panel). The dashed lines in each panel represent the binary frequency observed for solar neighborhood G stars by Duquennoy & Mayor (1991). The total binary frequency of these two samples is consistent within the errors.

No. 2, 1996

BINARY FRACTION AMONG PMS CLUSTER STARS IN ORION

709

formation might be a†ected by the intracloud star-forming environment. In the inner 40A of the Trapezium, we have found no equal brightness binaries among the low-mass stellar population, which suggests that the binary formation process may be inhibited in the densest regions of GMCs. Proximity to massive stars in the Trapezium core may also a†ect the formation and evolution of binary systems. A similar lack of weak-lined T Tauri star binaries noted by Brandner et al. (1995) in the Scorpius OB association is attributed to the environmental e†ects of massive stars on the low-mass star-forming medium. However, the small number statistics (two expected close binaries of nearly equal brightness ; none observed) require a deeper survey with much larger sample before this e†ect can be conÐrmed. 4.2. Possible Explanations for the Y SO Binary Excess If indeed there is an excess of binaries among low-mass preÈmain-sequence stars as compared with solar neighborhood G, K, and M stars, the overabundance may arise from a combination of the following factors : 1. Gravitational interactions with other stars or with the clumpy molecular cloud medium that result in altering the semimajor-axis distribution. 2. Gravitational interactions that result in disruption of wide binaries. 3. Luminosity evolution of the very low mass secondaries (those with masses below the hydrogen-burning limit). Such evolution could drive the secondary star luminosities for solar neighborhood Ðeld stars below current survey limits if the ages of those stars exceed D1 Gyr. Available published data provide the basis for preliminary discussion of the third possibility. In Figure 4, we plot (top panel) the frequency distribution of luminosity ratios Q \ [L (secondary)/L (primary)] derived from I-band Ñux ratios1 for a sample of 17 primary stars selected from the Reipurth & Zinnecker (1993) sample. These 17 stars were selected from among all of the stars in their sample with published spectral types to have K7 \ spectral type \ M3 (masses in the range 0.55È0.3 M according to the tracks _ al. [1994] ; these tracks computed recently by Swenson et yield masses D20% larger than those of DÏAntona & Mazzitelli [1994] and were chosen to provide a conservative estimate of secondary masses). The derived Q-values were used along with the mass-luminosity ratio published by Burrows et al. (1993) (which is virtually identical to that derived by Swenson et al. and DÏAntona & Mazzitelli) to derive the mass ratio M(secondary)/M(primary) (second panel, Fig. 4). In turn, these ratios, combined with the primary masses derived from the observed spectral types and an assumed age of 1 Myr (third panel), were used to derive the frequency distribution of secondary masses shown in the fourth panel. Note that 5/17 stars have masses that nominally place them below the hydrogen-burning limit. A similar analysis for the Trapezium binary sample (Prosser et al. 1994) yields a fraction 7/18 of candidate nominally substellar mass objects. Work is in progress to derive the spectral types of the secondaries reported in the current paper by using an IR spectral classiÐcation (M. Meyer 1995, private communication). 1 For M-type binary pairs, the ratio of I-band Ñuxes provides an estimate of bolometric luminosity to within 30% over most of the range from M0 to M7 (see, for example, Hartigan, Strom, & Strom 1994).

FIG. 4.ÈThe presence of young brown dwarf companions to PMS stars is suggested by estimated secondary masses. In the top panel, we plot the frequency distribution of luminosity ratios for 17 K7ÈM3 stars in the Reipurth & Zinnecker (1993) sample of PMS stars in T associations. Using the mass-luminosity ratio of Burrows et al. (1993) and the Q-values, we have estimated the mass ratios for these stars in the second panel. Primary masses have been derived using the observed spectral type and an assumed age of 1 Myr (third panel). Combining the primary masses with the mass ratios, we have estimated the frequency distribution of secondary masses in the fourth panel. Note that 5/17 stars appear to have masses below the hydrogen-burning limit.

It is thus conceivable that as many as one-third of the companions to low-mass PMS stars may be substellar. If so, their descendents may well have escaped detection in surveys of solar neighborhood K and M dwarfs. Should this be the case, then the ““ observed ÏÏ solar neighborhood binary frequency must be increased by a factor 1.5, close to the factor required to bring the solar neighborhood and young star-forming region binary frequencies into coincidence. We hasten to add that these arguments do not require the faint companions to be substellar. For example, some of the secondaries that appear faint at I could be bolometriHST partially embedded cally far more luminous stars still within their protostellar cores (e.g., Moneti & Zinnecker 1991) As noted above, the masses derived for the primaries from PMS tracks are also uncertain. Nevertheless, the possibility that some of the apparent excess of PMS binaries reÑects the presence of luminous substellar companions should be pursued via additional photometric and spectroscopic studies. 5.

SUMMARY AND CONCLUSIONS

We have observed an area of 35 arcmin2 in the preÈmainsequence clusters NGC 2024, NGC 2068, and NGC 2071 in Orion using the HST WFPC2. Our analysis reveals 15 binaries with separations of 0A. 3 \ r \ 2A. 3 (138È1050 AU at

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PADGETT, STROM, & GHEZ

Orion) out of a sample of 99 stars. The resulting multiplicity fraction of 0.15 ^ 0.03 is consistent with the binary frequency of 0.16 ^ 0.03 found for solar-type preÈmainsequence stars by Reipurth & Zinnecker (1993) for the separation range from 150 to 1200 AU. A similar analysis of the archival Trapezium F547M WFPC2 image from OÏDell & Wen (1994) yields seven binaries out of 50 stars in the cluster periphery. These results suggest that local stellar density does not a†ect the initial multiplicity fraction of stars, at least in regions where the stellar density varies from 40 to 5000 stars pc~3. However, there is a hint that in regions of very high stellar density (mean separation of D700 AU), binary frequency may be lower. This latter result requires conÐrmation from observation of a much

larger sample. As in Reipurth & Zinnecker (1993), we Ðnd an apparent excess of preÈmain-sequence binaries over the separation range of 138È1050 AU compared with solar neighborhood G and K stars. However, the statistical signiÐcance of this excess is not robust because of our relatively small sample of objects. The authors acknowledge helpful comments from C. Dougados, M. Meyer, R. Mathieu, L. Allen, K. Strom, S. Edwards, and P. Hartigan. We also thank K. Stapelfeldt for assistance in data reduction. This paper is based on data obtained with the Hubble Space T elescope under General Observer proposal 5355. Deborah Padgett acknowledges support from the NASA Origins of Solar Systems program.

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FIG. 1.ÈHST WFPC2 mosaic of one Ðeld in the Orion young stellar cluster NGC 2024. The F814W Ðlter was used for this co-addition of two 100 s exposures. The WFC images are each 80A by 80A, while the PC image (small corner) is 40A by 40A. PADGETT, STROM, & GHEZ (see 477, 706)

PLATE 20

NGC 2024 PC1 POS1 (585,676)

NGC 2024 WF3 POS1 (260,119)

NGC 2024 WF4 POS1 (246,668)

NGC 2024 WF4 POS1 (224,769)

NGC 2024 WF4 POS1 (214,99)

NGC 2024 WF4 POS1 (122,113)

NGC 2024 WF3 POS2 (599,262)

NGC 2024 WF2 POS3 (183,526)

FIG. 2a FIG. 2.ÈWFPC2 images of candidate binary systems in (a) NGC 2024, (b) NGC 2068, (c) NGC 2071, and (d) the Trapezium. The pixel sizes are 0A. 1, except in the frames marked PC1 in which the pixels are 0A. 046. Information about candidate objects is listed in Tables 1 and 2. PADGETT, STROM, & GHEZ (see 477, 707)

PLATE 21

NGC 2068 WF2 POS 1 (527,708)

NGC 2068 WF3 POS 1 (567,720)

NGC 2068 WF3 POS 2 (331,430)

NGC 2068 WF3 POS3 (735,581)

NGC 2068 PC1 POS 3 (135,362) FIG. 2b PADGETT, STROM, & GHEZ (see 477, 707)

PLATE 22

NGC 2071 WF4 (722,519)

NGC 2071 WF4 (214,185) FIG. 2c PADGETT, STROM, & GHEZ (see 477, 707)

PLATE 23

FIG. 2d PADGETT, STROM, & GHEZ (see 477, 707)

PLATE 24