Hubble Space Telescope Wide Field Planetary Camera 2 Imaging of ...

THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 109 : 473È480, 1997 April ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.

HUBBL E SPACE T EL ESCOPE WIDE FIELD PLANETARY CAMERA 2 IMAGING OF THE CRAB NEBULA. I. OBSERVATIONAL OVERVIEW1 WILLIAM P. BLAIR,2 KRIS DAVIDSON,3 ROBERT A. FESEN,4 ALAN UOMOTO,2 GORDON M. MACALPINE,5 AND RICHARD B. C. HENRY6 Received 1996 August 26 ; accepted 1996 November 8

ABSTRACT We present Hubble Space Telescope WFPC2 images of a portion of the Crab Nebula supernova remnant obtained using narrow passband Ðlters. These images show a wide range of Ðlament morphologies including small individual isolated knots, long smooth arcs, and chaotic Ðlament complexes, with evidence that some Ðlament cores are far denser and cooler than had been expected. Although several Ðlaments show resolved ionization structures, in general such structure is not seen on a Ðlament-byÐlament basis. In areas where the Ðlaments are mainly on the remnantÏs near side, ““ shadowing ÏÏ features are seen against the bright synchrotron emission background because of absorption by dust in the Ðlaments. While most of these dust features appear to be the cores of knotty-type Ðlaments, we identify a remarkable sinuous dust shadow that maintains a coherent structure over D25A (0.24 pc, assuming d \ 2 kpc). The obscuration caused by this narrow structure requires surprisingly dense dust concentrations. The HST imaging data reveal relatively few emission-line structures that are unresolved at WFPC2Ïs 0A. 10 resolution, with characteristic angular knot sizes of 0A. 4 to 0A. 8, corresponding to physical scales of 1.2È2.4 ] 1016 cm at the assumed distance of 2 kpc. Such sizes are roughly in accord with expectations from theoretical models of the Ðlaments. Future papers in this series will concentrate on speciÐc aspects of the data set in more detail. Subject headings : ISM : individual (Crab Nebula) È ISM : structure È supernova remnants 1.

INTRODUCTION

1995, 1996). Because its expansion velocity is unusually low for a young SNR (\1500 km s~1 ; Clark et al. 1983 ; Lawrence et al. 1995), imaging with narrow passband Ðlters permits analyses of much of the remnantÏs Ðlamentary emission. The best imaging of the Crab Nebula from the ground has indicated considerable structure at less than 1A (van den Bergh & Pritchet 1986). These data, together with highresolution images of the synchrotron continuum near the pulsar (van den Bergh & Pritchet 1989), led us and others (Hester et al. 1995, 1996) to image the remnant using the Hubble Space Telescope (HST ). Using [O I] and [O III] HST images, Hester et al. (1996) have investigated largescale Ðlament structures in terms of magnetic RayleighTaylor instabilities at the interface between the pulsar-driven synchrotron nebula and the ejecta (see ° 5 of Davidson & Fesen 1985, and references therein ; we shall refer to such structures as ““ Ðnger ÏÏ instabilities below). We have used the HST and WFPC2 camera to perform a more global analysis of optical Ðlament morphologies in the Crab Nebula in several important optical wavelength bands. In particular, we are interested in both (1) ascertaining the sizes of the smallest Ðlaments and (2) searching for changes in morphology as Ðlaments are observed in the light of di†erent emission lines. Discovering a limiting Ðlament size would provide valuable constraints on the dynamics of the explosion and subsequent expansion of the supernova and remnant. At the same time, studying the appearance of representative Ðlaments at various wavelengths allows one to understand the ionization structure of Ðlaments and to search for spatial abundance variations. Because the angular extent is larger than the WFPC2 Ðeld of view, only a portion of the object has been imaged in our survey. Also, because of the ^1500 km s~1 velocity spread of the expanding nebulosity, interpretation of the relative brightnesses of Ðlaments is restricted to portions of

The Crab Nebula ranks among the brightest Galactic supernova remnants (SNRs) across a broad range of wavelengths. This, together with its relative youth (SN 1054) and its energetic pulsar, has made it the object of intense study for more than half a century (see Davidson & Fesen 1985 ; Kafatos & Henry 1985, and references therein). This research has included phenomena such as supernova explosion dynamics and nucleosynthesis, and Ðlament abundances and kinematics, as well as neutron star physics and pulsar emission mechanisms. The Crab Nebula is also the prototype for an important class of centrally bright or ““ plerionic ÏÏ remnants, the evolution of which may play a role in understanding the so-called ““ composite SNRs ÏÏ that exhibit both shell and central emission structures (see Weiler & Sramek 1988). Lying 200 pc below the Galactic plane at a distance of about 2 kpc (Trimble 1973), the Crab Nebula o†ers a close look at the Ðne-scale structure of undiluted SN ejecta from a Type II supernova event. Many Ðlaments have undergone strong dynamical modiÐcation by the relativistic wind from the Crab NebulaÏs powerful central pulsar (Hester et al. 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. 2 Department of Physics and Astronomy, Johns Hopkins University, Charles and 34th Streets, Baltimore, MD 21218. 3 Astronomy Department, 116 Church Street SE, University of Minnesota, Minneapolis, MN 55455. 4 Department of Physics and Astronomy, 6127 Wilder Laboratory, Dartmouth College, Hanover, NH 03755. 5 Department of Astronomy, University of Michigan, Ann Arbor, MI 48109. 6 Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019.

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TABLE 1 HST WFPC2 IMAGING LOG AND FILTER BANDPASS ANALYSIS Exposure Information (s)

Position Anglea (deg)

*j FWHM (A )

1 ] 2000 3 ] 2300 4 ] 2000

43.25

6576È6604

38.11

6709È6756

1995 Jan 5

1 ] 1923 3 ] 2000

38.54

6552È6574

F547M . . . . . . . F502N . . . . . . .

1995 Jan 7 1995 Jan 5

4 ] 800 4 ] 2000

38.97 38.70

5210È5699 4999È5026

F300Wb . . . . . . F631Nc . . . . . .

1995 Jan 7 1994 Mar 10

2 ] 1000 2 ] 2000

38.96 64.00

2560È3288 6291È6321

Filter

Date (UT)

F658N . . . . . . .

1994 Apr 15

F673N . . . . . . .

1995 Jan 2

F656N . . . . . . .

Line ID [N II] j6583 Ha j6563 [S II] j6717 [S II] j6731 Ha j6563 [N II] j6548 [N II] j6583 ... [O III] j5007 [O III] j4959 ... [O I] j6300

Velocity Range (km s~1) [320 ]590 [360 [980 [500 ]180 [1410

to ]960 to ]1870 to ]1740 to ]1110 to ]500 to ]1190 to [410 ... [480 to ]1140 º2400 ... [429 to ]1000

a Astronomical position angle of each exposure using the WF4 CCD readout direction (basically the vertical axis in Fig. 1) as the reference direction, converted from the ORIENTATION keyword in the original Ðle headers. b Near-UV continuum band ; used 2 ] 2 on-chip binning for these data. c Archival image from Hester et al. 1996.

the nebula where line-of-sight velocities fall within the FWHM of the Ðlter bandpasses. Nonetheless, the resolving power of the HST permits us to measure structures that are physically interesting and that are well below the level attainable with ground-based optical observations. At the assumed 2 kpc distance, the 0A. 10 pixels of the wide-Ðeld CCDs correspond to a linear size of 0.00097 pc (\3.0 ] 1015 cm), while those on the PC chip are roughly a factor of 2 smaller. In this paper, we provide an overview of the observations and discuss both the limitations of and some initial Ðndings from them. In ° 2, our observational technique and the data processing are described, followed in ° 3 by a comparison with Fabry-Perot velocity information, a description of some areas of particular interest within the region we have surveyed, and an analysis of knot and Ðlament sizes. Future papers will discuss speciÐc aspects of the data set in more detail. 2.

OBSERVATIONS AND DATA PROCESSING

As part of a Guest Investigator program, we obtained WFPC2 exposures of the Crab Nebula in bandpasses centered on strong optical emission lines and two continuum bands largely uncontaminated by line emission. The observations were originally scheduled for 1994 April. However, only a set of four exposures with the F658N Ðlter was obtained at that time, the remaining exposures being lost because of a spacecraft saÐng event. The observations were rescheduled and executed successfully during the Ðrst week of 1995 January. We attempted to match the earlier observationÏs position as closely as possible within the constraints imposed by the di†erent time of year. The primary di†erence was that the orientation angle in 1995 January was di†erent by D5¡. Table 1 provides a log of the observational parameters for our images, including the Ðlters used, the observation times, and the position angles. The observation with each Ðlter consisted of two pairs of exposures, with each exposure nominally the same length, and each exposure pair spatially o†set by a set amount (12 WFC pixels in x and y). We have processed these data using the Ðles supplied by the STScI and standard procedures available in the

IRAF/STSDAS7 environment. Each positionally coincident pair of exposures was combined (averaged), and cosmic rays were removed. The resulting two independent intermediate frames were then aligned and combined, again with cosmicray rejection. This procedure yielded a single, very clean image representing the average of four exposures with each Ðlter. In Figures 1aÈ1e (Plates 3È7), we show the complete WFPC2 mosaics for each of our primary bandpasses. These images include corrections for small distortions across the CCDs, have been trimmed and aligned, and show the PC1 CCD with the WF CCDs combined into a single image. The F300M image (not shown) looks similar to the F547M exposure, but at lower signal-to-noise ratio. The pulsar is visible in Figure 1d as the lower of the two stars in the upper left corner of the WF3 CCD. The PC1 CCD samples Ðlaments due east of the pulsar, WF2 contains a bright arc of southern Ðlaments, WF3 contains the region studied most extensively with IUE (Davidson et al. 1982), and WF4 contains the region known as the northern ““ hook ÏÏ (the backward question mark of Ðlaments trailing o† to the lower left). This nomenclature will be used below to refer to these regions of Ðlaments. These Ðgures show the extent of the region surveyed and the general character of the emission seen in each bandpass. We highlight the smoother, more ““ di†use ÏÏ general appearance (or perhaps the absence of as much knotty, Ðne-scale structure) in F502N compared with the other emission-line images (see Davidson 1973, who predicted this structure). However, these Ðgures only hint at the wealth of detail, which is undersampled in this presentation. Below we look at subsets of these data in more detail. 3.

DISCUSSION AND ANALYSIS

3.1. Global Comparison with Fabry-Perot Velocity Data The WFPC2 emission-line Ðlters are not ideal for an object like the Crab Nebula. The large Doppler shifts of the 7 The Image Reduction and Analysis Facility (IRAF) is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract to the National Science Foundation. The Space Telescope Science Data Analysis System (STSDAS) is distributed by the Space Telescope Science Institute.

HST WFPC2 IMAGING OF THE CRAB NEBULA. I.

No. 2, 1997

Ðlaments (^1500 km s~1), in conjunction with the narrowÐlter bandpasses of di†erent widths, make accurate comparison of the features in the di†erent images difficult. Table 1 summarizes the Ðlter characteristics, the primary emission line or lines expected within each Ðlter bandpass, and the velocity range(s) for each emission line passed by each Ðlter. (The F631N Ðlter, for which an archival exposure is used in the discussion below, is shown for completeness.) In very general terms, the Ðlters sample emission below ^ a few hundred km s~1 fairly well, and for higher velocity material tend to provide better coverage on the redshifted side than on the blueshifted side of the expanding nebula. Hence, the speciÐc Ðlter bandpasses are di†erent enough that changes from Ðlter to Ðlter could be because of varying relative line intensities (see Davidson 1978, 1979 ; Fesen & Kirshner 1982 ; MacAlpine et al. 1989, 1996), material moving into or out of the various bandpasses, or some combination of both. In terms of velocity response, the F673N Ðlter does somewhat better with blueshifted emission by virtue of the fact that the j6731 line has to shift further before it moves out of the bandpass. However, j6717 (which accounts for roughly half the [S II] emission) is well out of the Ðlter range by the time j6731 has been blueshifted this much, thus a†ecting relative intensities. The F656N (nominally Ha) and F658N (nominally [N II] j6583) exposures have an additional complication ; these lines are close enough in wavelength that they can shift from one bandpass to the next. For instance, as shown in Table 1, any highly blueshifted material that emits in [N II] j6583 will show up in the F656N exposure (along with Ha emission within a few hundred km s~1 of rest), providing coverage to even more negative velocities than the F673N Ðlter does for [S II]. Likewise, highly redshifted Ha enters into the F658N exposure along with lower velocity [N II] j6583. The F502N Ðlter is cleanest in this regard, even e†ectively rejecting the [O III] j4959 line, but at the expense of restricted velocity coverage (again missing highly blueshifted Ðlaments). We have used the Fabry-Perot (FP) velocity-intensity maps of the Crab Nebula in [O III] j5007 reported by Lawrence et al. (1995) to assess which areas in our images are well sampled by the various Ðlters and which are not.

475

The FP data were obtained with a 320 km s~1 bandpass sampled in roughly 130 km s~1 steps over the entire velocity range of interest. By performing a frame-to-frame di†erential analysis, one can estimate (to roughly ^65 km s~1 accuracy) the velocity ranges over which many of the larger scale features in the Crab Nebula emit. Of course, the FP data do not provide this information at the smallest spatial scales available in the HST data. Also, FP data at [O III] do not always indicate what is happening with Ha, [S II], or [N II], since the relative line intensities are quite variable. Nonetheless, this analysis provides a useful framework in which to view the HST data. In Table 2, we summarize the velocity ranges for various features visible in our images, many of which shall be discussed below. The FP data demonstrate the remarkable fact that many of the larger scale coherent features can be followed over hundreds of km s~1 in velocity space, although the brightest emission is usually restricted to a range of a few hundred km s~1 or less. Comparison of the information in Table 2 with the mosaic images in Figure 1 permits a few general comments. Many of the brighter Ðlaments stretching across the PC1 and WF3 CCDs, running roughly east-west across the middle of the nebula, are part of the so-called ““ high-helium torus ÏÏ identiÐed by Uomoto & MacAlpine (1987) and MacAlpine et al. (1989). (Note : ““ East of pulsar ÏÏ and ““ IUEP1 ÏÏ regions in Table 2.) These Ðlaments are mostly on the near side, approaching our line of sight with relatively high velocity. These large blueshifts tend to move some of the emission out of the FWHM range of the HST Ðlter bandpasses. Thus, in many instances we are sampling only a portion of the velocity distribution of these Ðlaments. (Knowing this makes the dominant appearance of the IUE-P1 region on Fig. 1e (F502N) even more impressive.) We are more fortunate with the WF2 and WF4 CCDs, which cover regions south and north of the torus, respectively. The FP data indicate that much of the material in these regions is at low to moderate redshifts that are mostly within the Ðlter bandpasses. However, even in these Ðlaments the relative intensities can be confused by blending of Ha and [N II] emission in the F656N and F658N frames, or by the di†ering velocity response to the two separate [S II]

TABLE 2 VELOCITY RANGES FOR CRAB NEBULA REGIONS OF INTEREST Region

CCD Figure

Description

Bright southern Ðlament . . . . . .

WF2 Fig. 2

Northern hook . . . . . . . . . . . . . . . .

WF4 Fig. 3

West of hook . . . . . . . . . . . . . . . . . . N6 Argoknot . . . . . . . . . . . . . . . . . . East of pulsar . . . . . . . . . . . . . . . . . .

WF4 Fig. 4 WF4 Fig. 4 PC1 Fig. 5

IUE-P1 region . . . . . . . . . . . . . . . . .

WF3 Fig. 6

Southern RT Ðlament . . . . . . . . .

WF2 Fig. 7

Total Bright only Top East wall Bottom Total Total Total Total (blue) Total (red) Total Bright only (Red Ðlament) Total

FP Framesa 2a-10 2b-2 2b-1 2b-3 2b-4 2b-1 2a-4 2b-1 2a-4 2b-3 2a-6 2a-5 2b-2 2b-3

to to to to to to to to to to to to to to

2b-6 2b-5 2b-4 2b-7 2b-9 2b-9 2a-8 2b-4 2a-8 2b-6 2b-1 2a-10 2b-5 2b-7

Velocity Rangeb (km s~1) [150 ]350 ]220 ]450 ]600 ]220 [900 ]220 [900 ]450 [1050 [820 ]350 ]500

to to to to to to to to to to to to to to

]650 ]550 ]420 ]750 ]1000 ]1000 [600 ]420 [600 ]650 ]50 [350 ]700 ]700

*V b (km s~1) 800 200 200 300 400 780 300 200 300 200 1100 470 350 200

a Range of Fabry-Perot data frames for which feature of interest is seen ; frames are referenced as Ðgure number-frame number from Lawrence et al. 1995. b Approximate absolute velocity range and velocity spread over which feature of interest is seen, from a ““ di†erential ÏÏ analysis of the FP data. Uncertainties are of order ^65 km s~1.

476

BLAIR ET AL.

lines in F673N, as discussed above. For instance, the fading of the tail of the northern hook feature on the F656N image is primarily a velocity e†ect ; the main thing getting through the F656N Ðlter from this region is some portion of the [N II] j6548 line, which is much fainter. In reality, these general comments are an oversimpliÐcation ; many regions have spotty emission or knots over a much larger velocity range, or even have red and blueshifted systems of Ðlaments seen in projection. For instance, emission on the PC1 and WF3 CCDs are dominated by the brighter near-side (blueshifted) Ðlaments, but the FP data analysis shows that faint redshifted Ðlaments should be seen in the same spatial regions. Hence, a detailed comparison (i.e., Ðlament by Ðlament) needs to be done to separate velocity from line intensity e†ects. The entries in Table 2 are meant to refer to selected features or a main body of Ðlaments in each region that appear to be a cohesive unit in the FP data. We shall revisit these issues on a case by case basis in the discussion below. 3.2. Discussion of SpeciÐc Regions The images provide so much information about the detailed morphology of the Crab Nebula Ðlaments that it can be overwhelming. In this section, we choose several speciÐc regions of the images for more detailed inspection in order to provide an overview of the range of features present in the data. The set of exposures for each Ðlter was obtained at a di†erent rotation on the sky (see Table 1), so we have rotated them all to a common reference frame using the orientation information in the Ðle headers. All of the following Ðgures will be presented in this reference frame, where north is 45¡ clockwise from vertical (upper right) and east is 45¡ counterclockwise (upper left). This minimizes the areas lost at the frame edges, and allows all of the frames to be resampled in a similar fashion. All images were then aligned and trimmed for comparison. In addition to presenting the observed intensity maps, we present two other types of image comparisons below. One involves the subtraction of a Gaussian-smoothed version of an image from itself (using p \ 2.0 pixels), which results in a ““ contrast-enhanced ÏÏ image useful for highlighting Ðnescale structures. The other comparison is the creation of ratio maps between pairs of images, which highlight di†erences in the spatial distributions of various emission lines. Resulting line ratio di†erences across an image can be because of changing relative line intensities (\abundance variations or ionization e†ects), di†erences in velocity coverage of the respective Ðlters, or a combination of both. Hence, careful comparison with the FP data are used in interpreting these ratio maps. We Ðrst examine the bright region of Ðlaments in the southern part of the area surveyed (WF2). This region was not studied in previous HST work on the Crab Nebula (Hester et al. 1995, 1996). MillerÏs (1978) spectroscopic position 1 cuts through this grouping of Ðlaments. This region is well sampled by all of the Ðlters, with a mean velocity of about ]400 km s~1. Figures 2aÈ2d (Plate 8) show enlargements of an aligned 40A region from the F502N, F656N, F673N, and F658N Ðlter images corresponding to [O III] j5007, Ha j6563, [S II] jj6716, 6731, and [N II] j6583, respectively. These Ðgures are scaled in intensity to show the brightest Ðlaments to best advantage. (The F547M image in this region is relatively faint and shows little struc-

Vol. 109

ture, and is omitted from this comparison.) There are striking di†erences from Ðlter to Ðlter in the detailed Ðlament structures that extend to the fainter emission features as well. Figures 2eÈ2f (Plate 9) show the contrast-enhanced versions of the [O III] and [S II] images. These reveal details of the Ðne-scale structure in the Ðlaments not obvious from the intensity maps directly. (Note : white circular patches with black centers are stars.) Interestingly, the brightest region of Ðlaments nearly disappears into the background in Figure 2e (F502N) because of the relatively smooth distribution of the [O III] ; the other emission-line images, represented by the F673N map in Figure 2f, show more knotty-type Ðlaments. Figures 2gÈ2h show ratio maps of the same region, which highlight the spatial di†erences between pairs of images. In each case, regions appearing dark indicate where the numerator image is relatively strong, while white areas show regions with relatively strong emission by the denominator image. The ratio maps contain examples of both real relative line intensity variations and velocity e†ects. The dark region just left of center in Figure 2h, for instance, highlights a real change in relative line intensities. This region is on the main body of Ðlaments that is well sampled by the Ðlters. It stands out well on the [S II] : [N II] ratio because it is not only enhanced in [S II] but also faint in [N II], as can be seen by comparison to Figures 2c and 2d. In contrast, there are Ðlaments that appear in white in Figure 2g, indicating relative strength in the denominator image (F656N). These are mostly faint Ðlaments, apparently unrelated to the main body of bright Ðlaments. From Table 1, we see that the only thing we should be picking up in F656N that we are not in the other Ðlters is highly blueshifted emission (\1000 km s~1) from [N II] j6583. Indeed, comparison with the FP data shows some candidate Ðlaments in [O III] at high negative velocities that are probably responsible for this emission. Figure 3 (Plates 10È11) highlights another 40A region, this one from WF4, centered on the northern hook region and where the FP data again indicate relatively good sampling by the HST Ðlters. Figures 3aÈ3f show the aligned images, including F547M and also an archival F631N image that registers [O I] j6300 (see Hester et al. 1996). (The region at lower left of the F631N image is a residual of the rotation and alignment process, since this image was obtained at a substantially di†erent position and orientation from ours.) Faint emission from the hook is visible in the ““ continuum ÏÏ image (Fig. 3f ), probably due to faint emission lines of [Fe II], [Fe III], and He II j5412 within the bandpass (see Fesen, Kirshner, & Chevalier 1978). The white patch in the lower right portion of Figure 3f is due to foreground dust that partially blocks the synchrotron continuum. Fesen & Blair (1990, hereafter FB90) catalogued many of the most prominent dust features in the Crab Nebula, and this one corresponds with their feature 4A. The Ðlament containing this dust can be seen prominently in Figures 3b, 3c, and 3e at the same position. Figures 3gÈ3h show the contrastenhanced versions of the F502N and F656N images in Figures 3aÈ3b. In these, the hook appears less prominent against the general distribution of small-scale structure, although linear features and rims of some Ðlaments are enhanced. Finally, Figures 3iÈ3l show line ratio maps between the various Ðlter pairs, as indicated on each panel. Several things are worth noting about this Ðeld. We

No. 2, 1997


expected the comparison of the F631N exposure ([O I]) to F502N ([O III]) to be particularly interesting, since these two ions are from the same element and the velocity ranges sampled by the two Ðlters are similar (cf. Table 1). Comparison of Figures 3a and 3e shows the dramatic di†erences in morphology of the Ðlaments as viewed in these two ionization stages, with the relatively smooth distribution in [O III] and the very clumpy appearance in [O I]. The ratio of these two images (Fig. 3l) highlights this di†erence, while demonstrating that there is relatively little obvious ionization structure on a Ðlament by Ðlament basis. (One obvious exception is a knot near the lower right edge of the frame that shows a dark center and white rim.) For example, the eastern wall of the hook (i.e., the region that is most prominent in the F502N image) is mainly seen in the 450È750 km s~1 range in the FP data, and should be well sampled by the Ðlters ; Figure 3l shows [O III] emission enveloping this entire region, and yet very little correlation is seen between speciÐc [O I] and [O III] features. This same region shows strong [N II] emission in comparison with [S II] (white in Fig. 3k), which should be a real e†ect for these Ðlaments as well. In contrast, the relatively weak emission in the lower left corner of the F656N exposure in Fig. 3b (in particular, the Ðlaments that appear black in the [S II] : Ha map shown in Fig. 3j) is mainly a velocity e†ect, as mentioned in ° 3.1. We also highlight the vertical ““ Ðnger ÏÏ Ðlament to the left of the eastern wall of the hook, which is most prominent in F656N (Fig. 3b), and is clearly present in all of the other emission-line exposures except F502N. Since this Ðnger is present in [O I] and absent in [O III] (which cover the same velocity range), this must be a real e†ect. This is conÐrmed by the FP data, which do not show an indication of this Ðlament at other velocities. The region shows up bright white in Figure 3i and dark in Figure 3k, indicating strong Ha and [S II] emissions and weak [N II]. A smaller region (20A in extent) extracted from the lower right portion of the WF4 CCD (see Fig. 1) is shown in Figure 4 (Plates 12È13). Figures 4aÈ4e show the aligned intensity maps through each of our primary Ðlters. Figures 4fÈ4h show selected contrast-enhanced versions of the emission-line images, and Figures 4iÈ4l show selected line ratio images. Many of the brighter Ðlaments in this region are part of a much larger arc of Ðlaments that are seen over moderately blueshifted velocities ([450 to [800 km s~1. Hence, many of these Ðlaments may be sampled incompletely by some of the Ðlters. The FP data also indicate some fainter background emission at high redshifted velocities (near ]800 to ]1000 km s~1) within the Ðeld shown. Even with these limitations, this small region contains a variety of di†erent small-scale Ðlamentary structures. The circular feature at upper left is knot ““ N6 ÏÏ from MacAlpine et al. (1994), one of the so-called ““ argoknots ÏÏ that exhibit unusually strong [Ar III] j7135 emission in optical spectra. This feature is remarkable in that it appears isolated in the 220È420 km s~1 range (well sampled by the Ðlters), and does not connect into any other velocity structures. It shows a well-deÐned ionization structure where [O III] emission appears to form a shell around the lower ionization ions, especially [S II] (see Fig. 4l). A similar ionization structure is apparent for the elongated Ðlament at middle left in the Ðgures, which appears to be a small ““ Ðnger instability ÏÏ Ðlament as discussed by Hester et al. (1996). This Ðlament is seen at moderate blueshifts in the FP data ( D [400 km s~1), placing it on the near side of the expand-

477

ing shell. It is possible that these two features represent similar structures seen in di†erent geometries, but the uniformity in appearance of N6 and its isolated nature in velocity space make it more likely that N6 is a distinct morphological structure. Such clear examples of ionization structure on a Ðlament by Ðlament basis are seen relatively infrequently in the images. We note the presence of a distinct ““ dust shadow ÏÏ in Figure 4e at the position of N6, with a less distinct counterpart for the Ðlament below. (These correspond to FB90Ïs dust features 4B and 4C.) This is opposite to what one might expect since N6 is more redshifted and, hence, deeper into the nebula along the line of sight. Indeed, with a small positive velocity, one might expect that N6 is actually within the synchrotron emission region, with only a portion of the synchrotron being background in this case. The dust feature from N6 would probably be very dark if it were located on the near side of the nebula, but its presence even at the observed level indicates that N6 must contain considerable dust. Further discussion of the HST data on this Ðlament and the other knots showing similar spectroscopic signatures is deferred to a separate paper. Although plenty of structure is evident in [O III], this emission is again seen to be fairly uniform and di†use in comparison with the other emission-line images. The Ha (F656N) and [S II] (F673N) images look quite similar, but the [N II] (F658N) image shows numerous di†erences in detail. The linear Ðlaments extending toward the lower central region, seen especially in F656N and F673N, are Ðnger instabilities. Interestingly, none of these Ðlaments show the local ionization structure (e.g., are rimmed by [O III] emission) or dust shadows as seen for knot N6 and the Ðlament at left. Figure 5 (Plate 14) shows a 512 ] 512 pixel subset of the PC1 image that corresponds to a 23A. 6 Ðeld of view just east of the pulsar. Figures 5aÈ5d show the emission-line images, Figure 5e shows the continuum band, and Figure 5f shows the contrast-enhanced version of F673N. Because of the smaller pixel size of PC1, the signal-to-noise ratio per pixel is lower on these images, but for the brighter Ðlaments a higher spatial resolution is achieved, and some structures are seen down to or very near the limit of the resolution. In general, however, the Ðlaments appear to be resolved at 0A. 1 resolution (see ° 3.3, below). In addition, numerous linear and arclike Ðlaments are seen in this Ðeld (cf. Fig. 5f ). These appear to be coherent structures that are just barely resolved in one dimension and are a factor of 10 or more longer than they are wide. With the resolution of HST it is now clear that this is a distinct Ðlament morphology ; these Ðlaments are not composed of chains of small, knotty Ðlaments or other unresolved structure. Unfortunately, Ðlament velocities in this region are not well sampled by the Ðlters, making direct comparisons of relative intensities unreliable. Hence, no ratio maps are shown for this Ðeld. On the other hand, because the Ðlaments are mostly blueshifted and the background brightness of the synchrotron nebula is relatively high, a relatively large number of Ðlament dust shadows are visible in this region. These dust features correspond with the region 3 complex identiÐed by FB90 and most closely align with structures seen in the F673N image, although there is not a one-to-one correlation. These dust shadows show considerable structure at HST resolution and are not always correlated with the brightest appearing Ðlaments.

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Figure 6 (Plates 15È16) shows a 51A. 2 region from WF3, centered in the west-central portion of the nebula, including the region observed as MillerÏs (1978) spectroscopic position 2. This Ðeld includes the region studied most extensively in the ultraviolet with IUE (Davidson et al. 1982) and the Hopkins Ultraviolet Telescope (Blair et al. 1992) and is part of the high-helium torus. The individual emission-line Ðlter images are shown in Figures 6aÈ6d. Although the region was known to contain much substructure within (for instance) the IUE 10A ] 20A aperture positions, the exquisite detail in the HST images is still impressive. As with Figure 5, these Ðlaments are largely blueshifted, and only a portion of the Ðlament emission is sampled by the Ðlter bandpasses (cf. Table 2). Hence, no ratio images are shown for this Ðeld, although at least some variations in this region are the results of real changing relative line intensities (see MacAlpine et al. 1996). The dust shadows in this region deserve special mention. Figures 6e and 6f show two representations of the F547M continuum exposure at low and high contrast and with linear and log scaling. This is necessary because of the relatively large spatial change in the intensity of the synchrotron region sampled and to better highlight the dust feature shadowing. In particular, we highlight the remarkable sinuous or ropelike shadow in the lower right quadrant of the images that stretches across D25A in length. At least parts of this shadow correspond to Ðlamentary structures in the other images, a point that is most obvious from comparison with Figures 6gÈ6h of the same plate, which show the contrast-enhanced versions of the F502N and F673N images. A portion of this feature was identiÐed as feature 1D by FB90, but its overall structure was not realized prior to the HST imaging. Cross cuts through this feature on the F547M image show that is has a width of 0A. 40 and causes roughly a 20% attenuation of the background synchrotron emission. (This is a lower limit, assuming the Ðlament causing the shadow is on the near side and that the synchrotron nebula is entirely background emission, which may not be strictly true.) The implications of this feature from even a cursory analysis are astonishing. Assuming standard dust, abundances, and extinction characteristics, a 20% attenuation at (roughly) V corresponds to A \ 0.24 mag, or E(B[V ) \ 0.08 mag. In the interstellarVmedium, one magnitude of color excess requires an atomic column density of about 5.8 ] 1021 cm~2 (Savage & Mathis 1979). Thus E(B[V ) \ 0.08 mag normally implies a column density of order 4.6 ] 1020 cm~2. Since the rope is only 0A. 40 or 1016 cm across, such a column density would require an average internal density of order 40,000 atoms cm~3, which is larger by more than a factor of 10 than the highest ionized-gas densities indicated by [O II] and [S II] line ratios in the Crab Nebula (see Davidson 1979 ; Fesen & Kirshner 1982). Furthermore, if the rope is cool neutral gas roughly in pressure equilibrium with the normal ionized emission-line gas surrounding it, then its temperature must be less than 1000 K. Although the Ðlament cores may not be strictly in thermal equilibrium with the low-density surrounding gas, temperatures this low would be surprising because synchrotron X-rays should keep the cores of the Ðlaments much warmer than this (Davidson & Tucker 1970). Moreover, the width of the rope divided by 900 yr gives a maximum internal velocity dispersion of ¹4 km s~1. Although this is only a Ðrst-order analysis, the inferred high densities and low-

Vol. 109

velocity dispersion point toward a probable magnetic conÐnement mechanism for this feature. Similar comments pertain to the other localized extinction knots, for example, knot N6 mentioned above, or the other primary dust shadow in Figure 6, which corresponds to feature 1A in FB90. The latter feature matches most closely with a small cluster of Ðlaments visible in the F673N exposure. Considerable substructure is visible within this dust shadow complex, with several knots considerably darker (lighter in Figs. 6e and 6f ) than the rope. Many of these features do not look much darker in the near-UV F300W image (not presented here). One possible implication is that the extinction curve for dust in the Crab Nebula does not rise very sharply toward the UV. If so, this may indicate large grain sizes in comparison with typical interstellar grains. We will address the nature of the dust extinction features in the HST data in more detail in a future paper. A few structures in the images are conspicuous because they look morphologically peculiar compared with the more typical structures observed in the Crab Nebula (cf. knot N6 discussed above). In Figure 7 (Plates 17È18), we show a peculiar thumb-shaped feature from a small region of the WF2 just to the north of the region shown in Figure 2. Figures 7aÈ7e show the individual intensity maps for a 10A region centered on the Ðlament. We have subsampled these images by a factor of 2 in order to reduce the appearance of pixelation. Figures 7fÈ7k show ratio maps of the same region. (No contrast-enhanced versions are shown for this small Ðeld, since they do not show signiÐcant structure beyond that already visible in the intensity maps.) This feature stands out in part because it is unusually noticeable in the F547M ““ continuum ÏÏ image and because of its well-deÐned geometry. The brightest Ðlaments are sometimes weakly visible in the F547M image, as we mentioned earlier, because of faint emission lines in the F547M Ðlter bandpass. However, the structure in Figure 7 is unusually prominent in the continuum relative to the emission lines. Ground-based spectroscopy of this region could discern whether it has unusual emission lines around 5500 A , or whether it somehow manages to be visible in the nonthermal continuum. If this Ðlament is a Ðnger instability as seen elsewhere in the Crab Nebula, its position in the overall remnant structure makes it likely that we are viewing it at a highly oblique angle as it protrudes into the synchrotron nebula from the far side. This picture is supported by the FP data, which show the feature at moderately redshifted velocities near ]700 km s~1. It does not have a corresponding dust shadow as some of the other Ðlaments do, consistent with a picture where it is located on the backside of the nebula and is protruding toward us. Although the same basic structure is seen for this feature in the emission-line images, each is slightly di†erent in detail, demonstrating the complexity of interpreting relative line intensities even for a single coherent structure that is well sampled by the Ðlters. The ratio maps seem to show a leading edge to the protrusion that is relatively bright in Ha and [N II], causing an apparent arc in Figures 7g and 7i. (Compare them also with some of the Ðnger Ðlaments seen more from the side in Figures 3 and 4.) 3.3. Characteristic Knot Sizes Our discussion of characteristic knot sizes in the Crab Nebula must be tempered by a concern about the complex-

No. 2, 1997


ity of the nebulosity (including projection e†ects) and the incomplete sampling in velocity space. Nevertheless, with increased resolution, Ðner scale structure has always been seen in the Crab Nebula (see van den Bergh & Pritchet 1986). Since the spatial resolution of HST WFPC2 is signiÐcantly higher than any previous optical imaging, it is worth asking whether typical emission knots are fully resolved in the images or whether signiÐcant structure is likely to remain at still smaller spatial scales. We have examined the data for the PC1 Ðeld (Fig. 4) using cuts across individual features and radial plots for isolated emission knots. On each image we measured a stellar proÐle and the proÐles of several of the smallest scale knots and/or Ðlament widths that were reasonably bright and well isolated. The stellar proÐles all had full widths at half-maximum (FWHM) values of 0A. 12 ^ 0A. 02, assuming Gaussian proÐle Ðts. The point spread function, however, is undersampled by the PC1, so Gaussian Ðts are only a rough approximation. Fits of the radial plots indicate even somewhat smaller FWHM values are appropriate for stars. For comparison, while a few of the smallest knotty Ðlaments that we measured approached the stellar proÐle, we found none that had FWHMs as small as stars, indicating that all of the physically interesting emission-line structure is being resolved, at least on the PC1. (The smallest structures may be undersampled on the WF2, WF3, and WF4 CCDs where the pixel size is 0A. 10.) We conclude from this that there is no evidence for signiÐcant unresolved structure on the PC1 image. Although the smallest knots may approach the resolution limit, most of the knots and Ðlaments appear well resolved even in the WFC images. To quantify this, we have selected several subsections of the WF2 Ðeld containing groupings of reasonably bright and isolated knotty Ðlaments for analysis with an autocorrelation technique. The WF2 Ðeld was chosen because of the reasonably good velocity sampling for most of the emission in this region. The technique is described in detail by Morse et al. (1996) ; since an autocorrelation compares an image against itself assuming different o†sets, any characteristic size shows up as a central peak in the autocorrelation image, where the width of the peak is related to the size of the characteristic structure. To Ðrst order, this technique quantiÐes morphology, providing a global picture of the selected region without the tedium of measuring many knots individually. The exact knots that show up in a given subimage from Ðlter to Ðlter are not always the same. Hence, it is not feasible to use this analysis to investigate how individual knots may change in size from one exposure to the next. However, from a more global perspective, all of the subregions studied give characteristic knot sizes in the range 0A. 4 to 0A. 8, with data from the F502N exposure tending toward the upper end of the range and data from the other emission lines tending toward the lower and middle portions of the range. This may be reÑective of the somewhat smoother appearance or of a slightly more extended structures in [O III]. However, it appears that the bulk of the detailed Ðlamentary structures in the Crab Nebula are in the subarcsecond range accessible to HST . At the assumed distance of 2 kpc, these characteristic angular knot sizes correspond to physical scales of 1.2È2.4 ] 1016 cm. How does this compare with theoretical expectations of Ðlament sizes ? To Ðrst order, most of the Ðlamentary emission is thermal gas photoionized by the synchrotron emis-

479

sion, with a temperature near 104 K. At temperatures such as this, Ðlaments need to have densities near 103 cm~3 in order to be in approximate pressure equilibrium with the nonthermal particles. This is roughly in accord with observations. Hence, observed Ñuxes can be used to estimate the ionization parameter required, which in turn determines the expected geometrical thickness of the ionization structures (see Williams 1967 ; Davidson & Tucker 1970 ; Davidson 1973 ; Henry & MacAlpine 1982 ; Pequignot & Dennefeld 1983). Davidson & Tucker (1970) used this kind of information to calculate crude models of the Ðlamentary structures, and estimated characteristic Ðlament sizes of roughly 2È 5 ] 1016 cm, only slightly larger than we observe. In addition, the Ðlaments are being accelerated outward at a rate of g D 3 ] 10~4 cm s~2 (derivable from the di†erence between the expansion age and the time since the explosion ; see Trimble 1968, 1970), which corresponds to roughly 100 km s~1 in 1000 yr. With this acceleration, a pseudohydrostatic scale length for a Ðlament would be h D 2kT / mg D 5 ] 1015 cm, again close to the size scale observed. Since we have now apparently resolved the Ðlamentary structures with HST , the actual measured sizes of Ðlaments can be used to constrain future models of this type. 4.

CONCLUSIONS

We have presented HST WFPC2 data for a portion of the Crab Nebula supernova remnant. While the images have some limitations caused by the expansion velocity of the nebula and the narrow Ðlter widths, a tremendous wealth of detailed structure is revealed with the excellent resolution of the HST WFPC2 combination. We have shown a representative sample of the morphologies visible in the images, concentrating on the nebular Ðlamentary structures, by presenting not only the intensity maps but also contrast-enhanced images and ratio maps where appropriate. Comparison with published FP data has been used to provide insight into the variations seen between the di†erent images. On a global perspective, the emission as seen in [O III] j5007 appears more smoothly distributed, while emission in the lower ionization lines appears more structured and clumpy. On a smaller scale, we Ðnd that the Ðlamentary structures are largely resolved at the WFPC2 resolution, with a relatively small amount of structure seen at or near the limiting resolution. However, typical knot and Ðlament sizes are in the subarcsecond regime accessible mainly to HST , and appear to be roughly in accord with theoretical expectations. Some individual Ðlaments show a characteristic ionization structure, with rims of [O III] emission surrounding cores of lower ionization emission, but these Ðlaments are more the exception than the rule. We Ðnd numerous examples of ““ dust shadowing,ÏÏ where dust in dense Ðlament cores on the near side of the nebula block the background synchrotron emission. Most of these features show knotty-type structures like most of the Ðlaments, but one remarkable ropelike feature maintains a coherent structure over D25A. The amount of absorption caused by this thin rope Ðlament indicates possible core densities of order a factor of 10 higher than measured previously for the ionized gas. Future papers in this series will delve into speciÐc aspects of these data in more detail. Support for this work was provided by NASA/STScI grants GO-5354.04-93A to the Johns Hopkins University,

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GO-5354.01-93A to the University of Minnesota, GO5354.02-93A to the University of Michigan, GO-5354.03-

93A to Dartmouth College, and GO-5354.05-93A to the University of Oklahoma.

REFERENCES Blair, W. P., et al. 1992, ApJ, 399, 611 MacAlpine, G. M., et al. 1994, ApJ, 432, L131 Clark, D. H., Murdin, P., Wood, R., Gilmozzi, R., Danziger, J., & Furr, MacAlpine, G. M., Lawrence, S. S., Sears, R. L., Sosin, M. S., & Henry, A. W. 1983, MNRAS, 204, 415 R. B. C. 1996, ApJ, 463, 650 Davidson, K. 1973, ApJ, 186, 223 MacAlpine, G. M., McGaugh, S. S., Mazzarella, J. M., & Uomoto, A. 1989, ÈÈÈ. 1978, ApJ, 220, 177 ApJ, 342, 364 ÈÈÈ. 1979, ApJ, 228, 179 Miller, J. S. 1978, ApJ, 220, 490 Davidson, K., et al. 1982, ApJ, 253, 696 Morse, J. A., et al. 1996, AJ, 112, 509 Davidson, K., & Fesen, R. A. 1985, ARA&A, 23, 119 Pequignot, D., & Dennefeld, M. 1983, A&A, 120, 249 Davidson, K., & Tucker, W. 1970, ApJ, 161, 437 Savage, B. D., & Mathis, J. S. 1979, ARA&A, 17, 73 Fesen, R. A., & Blair, W. P. 1990, ApJ, 351, L45 (FB90) Trimble, V. 1968, AJ, 73, 535 Fesen, R. A., & Kirshner, R. P. 1982, ApJ, 258, 1 ÈÈÈ. 1970, AJ, 75, 926 Fesen, R. A., Kirshner, R. P., & Chevalier, R. A. 1978, PASP, 90, 32 ÈÈÈ. 1973, PASP, 85, 579 Henry, R. B. C., & MacAlpine, G. M. 1982, ApJ, 258, 11 Uomoto, A., & MacAlpine, G. M. 1987, AJ, 93, 1511 Hester, J. J., et al. 1995, ApJ, 448, 240 van den Bergh, S., & Pritchet, C. J. 1986, Nature, 321, 46 ÈÈÈ. 1996, ApJ, 456, 233 ÈÈÈ. 1989, ApJ, 338, L69 Kafatos, M. C., & Henry, R. B. C. 1985, The Crab Nebula and Related Weiler, K. W., & Sramek, R. A. 1988, ARA&A, 26, 295 Supernova Remnants (Cambridge : Cambridge Univ. Press) Williams, R. E. 1967, ApJ, 147, 556 Lawrence, S. S., MacAlpine, G. M., Uomoto, A., Woodgate, B. E., Brown, L. W., Oliversen, R. J., Lowenthal, J. D., & Liu, C. 1995, AJ, 109, 2635

FIG. 1.ÈReduced WFPC2 mosaics of the region of the Crab Nebula observed in our program. These pictures are undersampled, but show the overall structures and the extent of variations seen from image to image. The rotation varies slightly (\5¡) between the images. North is roughly toward the upper right, and east is toward the upper left. (a) F673N ([S II]). (b) F658N (Mainly [N II]). (c) F656N (Ha and/or [N II], depending on velocity). (d) F547M (mainly continuum). (e) F502N ([O III]). BLAIR et al. (see 109, 474)

PLATE 3

FIG. 1ÈContinued BLAIR et al. (see 109, 474)

PLATE 4


PLATE 5


PLATE 6


PLATE 7

FIG. 2.ÈMultipanel Ðgure showing the observed characteristics of a 40A region surrounding the brightest Ðlaments in the southern part of the Crab Nebula. In this and all following Ðgures, the images have been rotated so that north is directly to the upper right and east is to the upper left. (aÈd) Aligned images from the four Ðlters centered on emission lines, as indicated. (eÈf ) Contrast-enhanced versions of the F502N and F673N exposures, respectively (see text). (gÈh) Image ratio maps as indicated. In these and following Ðgures of ratios, regions appearing dark indicate where the numerator image is relatively strong, while white areas show strong emission by the denominator image. BLAIR et al. (see 109, 476)

PLATE 8


PLATE 9

FIG. 3.ÈA 40A region of WF4 centered on the ““ northern hook ÏÏ region of the Crab Nebula. (aÈf ) Aligned images, including an archival F631N image. The region at lower left is a residual of the rotation and alignment process. (gÈh) Contrast-enhanced versions of the F502N and F656N images, respectively. (iÈl) Ratio maps between the various Ðlter pairs, as indicated on each panel. BLAIR et al. (see 109, 476)

PLATE 10


PLATE 11

FIG. 4.ÈA 20A region of WF4, which is a portion of a large arch of Ðlaments extending from the northern hook down toward the high-helium torus (cf. Fig. 1). Numerous Ðlament morphologies are seen within this small region. (a-e) Aligned intensity maps. ( fÈh) Selected contrast-enhanced versions. (iÈl) Ratio maps, as indicated. BLAIR et al. (see 109, 477)

PLATE 12


PLATE 13

FIG. 5.ÈEnlargements of a 23A. 6 Ðeld from PC1 showing Ðlaments just east of the pulsar. (aÈd) Emission-line images. (e) Continuum band. ( f ) Contrast-enhanced map of F673N. No ratio maps are shown since many of the Ðlaments in this region have high negative velocities. BLAIR et al. (see 109, 477)

PLATE 14

FIG. 6.ÈA 51A. 2 region from WF3 that is approximately centered on the region observed extensively with IUE. (aÈd) Emission-line Ðlter images. (eÈf ) Two di†erent representations of the F547M continuum exposure, at low and high contrast and with linear and log scaling. (gÈh) Contrast-enhanced versions of the F502N and F673N images. BLAIR et al. (see 109, 478)

PLATE 15


PLATE 16

FIG. 7.ÈEnlargements of a 10A Ðeld from WF2 showing a possible Ðnger instability feature. (aÈe) Intensity maps of the region. ( fÈk) Ratio maps, as indicated on the Ðgures. The data for this Ðgure were subsampled by a factor of 2 to reduce the visual e†ects of pixelation. BLAIR et al. (see 109, 478)

PLATE 17


PLATE 18