HIGH-RESOLUTION IMAGING OF PROTOÈPLANETARY NEBULAE: THE EFFECTS OF ORIENTATION1 KATE Y. L. SU,2,3 BRUCE J. HRIVNAK,3,4,5 AND SUN KWOK2,4,5,6

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1 THE ASTRONOMICAL JOURNAL, 122:1525È1537, 2001 September ( The American Astronomical Society. All rights reserved. Printed in U.S.A. HIGH-RESOLUTION IMAGING OF PROTOÈPLANETARY NEBULAE: THE EFFECTS OF ORIENTATION1 KATE Y. L. SU,2,3 BRUCE J. HRIVNAK,3,4,5 AND SUN KWOK2,4,5,6 Received 2001 March 15; accepted 2001 May 21 ABSTRACT Deep Hubble Space T elescope (HST ) F814W images were obtained of six protoèplanetary nebulae (PPNs), each of which shows a bipolar morphology but with a di erence in orientation. Two of these (IRAS 17245[3951 and IRAS 22574]6609) show a dark lane separating bipolar lobes and appear to be seen approximately edge-on. The other four (IRAS 16594[4656, 17106[3046, 19477]2401, and 20028]3910) appear to be at intermediate orientations, and in all but one case, the central star is seen. In addition, one bright PPN (IRAS 20136]1309) was observed that is slightly extended, and we suggest that it may be a bipolar nebula seen edge-on. Visible-band HST images also exist for six of these, and color images were formed to analyze the dust distribution. New ground-based photometry was combined with satellite data to delineate the spectral energy distribution (SED) from 0.5 to 100 km for each of these. The orientation e ects on the optical morphologies and the SEDs are discussed. In general, the ratio of dust to photospheric Ñux is higher as the orientation increases toward edge-on, although there are some exceptions. Some numerical models were constructed and used to show quantitatively the e ects that di erences in the asymmetry of the circumstellar envelope or in the size of the cavity opening angle can have on this ratio. The general di erences in appearance and SED of these PPNs are attributed primarily to the di erent viewing orientations. These results, when combined with those of previous imaging studies of PPNs, strengthen the idea that PPNs possess a basic bipolar structure due to an asymmetric circumstellar dust shell. Key words: circumstellar matter È planetary nebulae: general È stars: AGB and post-agb È stars: mass loss 1. INTRODUCTION Bipolar morphologies are common in both the early (young stars) and late (planetary nebulae) stages of stellar evolution. The bipolar morphologies of planetary nebulae (PNs) have been noted for nearly a century (Curtis 1918), and modern optical and radio images have shown that most PNs are not spherically symmetric (Aaquist & Kwok 1996; Stanghellini, Corradi, & Schwarz 1993). The origins of the overall morphologies of PNs and of the detailed structures pose a major problem for our understanding of the dynamics of PNs. Since the nebulae have their origins in the circumstellar envelopes (CSEs) ejected by their progenitor asymptotic giant branch (AGB) stars, the fact that the CSEs are remarkably symmetric suggests that these aspherical morphologies are created or enhanced during the evolution from the AGB to the PN stages. In the past decade, there have been great advances in our understanding of the dynamical structure of PNs. It is now believed that the expansion and morphology of PNs are the result of the interaction of a fast central-star wind with the ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 This work was based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS Department of Physics and Astronomy, University of Calgary, Calgary AB T2N 1N4, Canada; ysu=iras.ucalgary.ca, kwok= iras.ucalgary.ca. 3 Department of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383; kate.su=valpo.edu, bruce.hrivnak=valpo.edu. 4 Visiting Astronomer, United Kingdom Infrared Telescope, which is operated by the Royal Observatories on behalf of the UK Particle Physics and Astronomy Research Council. 5 Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, which is operated by AURA, Inc., under contract with the National Science Foundation. 6 Canada Council Killam Fellow remnant CSE of the AGB progenitor (Kwok 1982; Balick 1987). Detailed hydrodynamical models conðrm that many of the common morphologies of PNs can be produced by the interacting stellar winds model (Frank & Mellema 1994; Mellema 1995). While the dynamical models have been successful in explaining the end results, they do not address the cause ÏÏ of the axial symmetry. Popular theories to create the density contrast in the AGB CSE necessary for the formation of an axisymmetric nebula include a binary companion, stellar rotation, and stellar magnetic Ðelds (Livio 1995; Garc a- Segura et al. 1999). However, the question of nurture ÏÏ or nature ÏÏ remains: do the bipolar morphologies develop gradually as the interacting stellar winds process progresses as Balick (1987) suggests, or do they form early in the post- AGB evolution? A speciðc test of this question is to observe the morphologies of proto-pns (PPNs), objects in transition between the AGB and PN phases. In recent years, many new PPNs have been discovered as the result of ground-based surveys of cool IRAS sources (Kwok 1993). They are characterized by strong far-infrared excesses due to the remnant of the AGB wind and by heavily reddened photospheres. Ground-based observations have revealed a bipolar structure for several of the larger PPNs, such as AFGL 2688 (the Egg Nebula; Ney et al. 1975), IRAS 17150[3224, and IRAS 17441[2411 (Kwok et al. 1996), and subarcsecond (D0A.7) observations of smaller ones have shown that many are not round (Hrivnak et al. 1999b). However, it is the Hubble Space T elescope (HST ), with its tenfold improvement in resolution, that has Ðnally permitted detailed studies of the morphology of the small nebulae around these transitional objects. In this paper are presented the results of an HST imaging study of seven PPNs. They were chosen on the basis of

2 1526 SU, HRIVNAK, & KWOK Vol. 122 ground-based imaging that indicated that they were extended (Hrivnak et al. 1999b). The only exception is IRAS 20136]1309, which was chosen on the basis of the unusual appearance of its spectral energy distribution (SED). Four of these PPNs have been observed previously with the HST, either in the V -band study of two PPNs by Hrivnak, Kwok, & Su (1999a) or the snapshot survey of PPNs by Ueta, Meixner, & Bobrowsky (2000). However, in this study we have observed deeper to examine the faint structure and have emphasized the F814W bandpass to determine complementary color information. We discuss the morphology and SED of each of these seven. Based upon these and other published images, we arrive at some general conclusions about the basic structure of PPNs and the e ect of the viewing orientation on the observed morphology. 2. GROUND-BASED OBSERVATIONS As part of a systematic program to identify and study IRAS candidates for PPNs, we identiðed optical counterparts of each of these objects. This was carried out using a bolometer at 10 km to search around the rather uncertain IRAS positions to obtain more precise coordinates and to see whether there were coincident optical counterparts. Finding charts are presented for four of these in Figure 1, and the coordinates of all seven are listed in Table 1, based upon the HST images discussed later. Finding charts and ground-based measurements of the other three objects have been given previously (IRAS 16594[4656 and 17245[3951, Hrivnak, Kwok, & Su 1999a; IRAS 22574]6609, Hrivnak & Kwok 1991). Follow-up visible, near-infrared, and mid-infrared observations were made for each of these PPNs. Data for the three listed above have been published, as have mid-infrared measurements of IRAS 19477]2401 (Kwok, Hrivnak, & Borieko 1987). The previously unpublished measurements are listed in Table 2. Several of the objects are found to be quite red: (B[V ) \ 3.0 and (V [I ) \ 3.1 for IRAS 17106[3046, (B[V ) \ 1.7 and (V [I ) \ C 2.6 for IRAS 20028]3910, and (V [I ) \ 5.3 for C IRAS 19477]2401. IRAS 19477]2401 is C very faint (V \ 22.2 FIG. 1.ÈFinding charts of IRAS 17106[3046 (top left), IRAS 19477]2401 (top right), IRAS 20028]3910 (bottom left), and IRAS 20136]1309 (bottom right), extracted from the Digitized Sky Survey. The vertical axis of each chart is 4@.0.

3 No. 3, 2001 HIGH-RESOLUTION IMAGING OF PROTOÈPLANETARY NEBULAE 1527 TABLE 1 HST POSITIONS OF THE PPNS l b IRAS ID R.A. (J2000) Decl. (J2000) (deg) (deg) 16594[ [ [ [ [ [ [ [ ] [ ] ] [ ] NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. mag), and it is uncertain if the two di erent sets of visible measurements indicate photometric variability. The di erence in the two V measurements of this source are at the level of the uncertainty in the measurements, and the di erence in the two I measurements may be due to the large color term involved in the standardization. Some additional ground-based data have been published by others for a few of these objects, and they agree well with the data in Table 2. An exception is IRAS 19477]2401, for which the nearinfrared data of Garc a-lario et al. (1997) are approximately 1.5 mag brighter than ours. Variations this large are unlikely for a PPN, and thus it appears that the same object was not observed by both groups. Since we have identiðed the IRAS source at 10 km and obtained both accurate coordinates and an accurate Ðnding chart, we believe that we have measured the correct source in the near-infrared. Visible spectra are available for only two of these PPNs. A spectral type of B7 has been assigned to IRAS 16594[4656 (Van de Steene, Wood, & van Hoof 2000). A published spectrum appears rather featureless, except for emission at Ha,Hb, and [O I] at 6300 Ó (Garc a-lario et al. 1999). We have obtained a visible spectrum of IRAS 20136]1309, and it appears to have the spectral type of a G0-F8 supergiant. 3. HST OBSERVATIONS AND REDUCTIONS The seven PPNs were imaged using the Wide Field Planetary Camera 2 (WFPC2) on the HST, under a program to study bipolar PPNs and to search for circumstellar arcs (HST program 8210; principal investigator, B. Hrivnak). The planetary camera was used, with the wide-band Ðlters F606W (SjT \ nm, *j \ nm) and F814W (SjT \ nm, *j \ nm). For all but one object, the exposures were relatively long, so that faint nebulosity could be detected. In order to remove the cosmic-ray hits in the long-exposure images and to permit an improvement in the combined image resolution, a predeðned dithered pattern (two, three, or four steps) was used with each Ðlter. The HST observing log is listed in Table 3. The data were reduced using standard bias and dark subtraction and Ñat-Ðeld corrections. Because the WFPC2 undersamples the image, the individual dithered images in each Ðlter were combined to obtain a drizzled ÏÏ image that slightly improved the resolution because of the improved spatial sampling. The point-spread functions after drizzling were found to have a FWHM in the range 0A.083È0A.102 in the F814W images. Standard aperture photometry was carried out on the objects, using elliptical apertures that approximately matched the sizes of the nebulae. For consistency, we deðned the outer extent of the nebulae in the sky-subtracted images at the conservative value of 10 p (i.e., S/N \ 10). sky The magnitude measurements determined based on the photometric header, PHOTFLAM, are on the HST WFPC2 magnitude system (STMAG). These were transformed to the standard Johnson V and Cousins I systems C using the Space Telescope Science Data Analysis System TABLE 2 GROUND-BASED VISIBLE AND INFRARED PHOTOMETRY A. VISIBLE PHOTOMETRY IRAS ID Date Observatory B V R C a I C a 17106[ May 19 CTIO ^ ^ ^ ^ ] Jun 5 CFHT ^ ^ Oct 1 USNOb... [22.3 ^ ^ ] Oct 1 USNOb ^ ^ ^ ] Oct 1 USNOb ^ ^ ^ 0.01 B. NEAR-INFRARED PHOTOMETRY IRAS ID Date Observatory J H K L L@ M@ 17106[ May 7 CTIO ^ ^ ^ ^ Aug 22 UKIRT ^ ^ ] Aug 17 UKIRT ^ ^ ^ ^ ] Oct 6È9 KPNOc ^ ^ ^ ^ ^ ] Aug 22 UKIRT ^ ^ 0.12 C. MID-INFRARED PHOTOMETRY IRAS ID Date Observatory 8.75 km 9.7 km N 11.5 km 12.5 km Q 17106[ Aug 22 UKIRT 4.68 ^ ^ ^ ^ ^ 0.09 [1.14 ^ ] Aug 22 UKIRT 2.93 ^ ^ ^ ^ ^ ^ 0.04 a The R and I magnitudes are in the Cousins system. b CCD photometry was kindly carried out by H. C. Harris at the US Naval Observatory (USNO) in Flagsta, AZ. c Near-infrared photometry was kindly carried out by R. Joyce at Kitt Peak National Observatory (KPNO).

4 1528 SU, HRIVNAK, & KWOK Vol. 122 TABLE 3 HST OBSERVING LOG Exposure Times IRAS ID Filter (s) Observation Date 16594[ F814W 2 ] 10, 2 ] 100, 2 ] Jun [ F606W 2 ] 40, 3 ] May 30 F814W 2 ] 40, 3 ] May [ F814W 2 ] 40, 3 ] Jun ] F814W 3 ] May ] F814W 3 ] May ] F606W 2 ] 5, 4 ] 10, 1 ] May 25 F814W 2 ] 5, 4 ] 10, 1 ] May ] F814W 1 ] 200, 3 ] Jun 21 synthetic photometry software SYNPHOT. These transformations require knowledge of the spectral type or temperature of the objects. For this, we used the e ective blackbody temperature of the reddened photosphere of each object determined from the SEDs, which we discuss later in 4. The results of these measurements are listed in Table 4. We have also used the available HST F606W and F555W images to determine the V magnitudes of the four PPNs for which these images have previously been published and have included them in this table. There is good agreement between the transformed HST magnitudes and the ground-based observations. Thus it seems that the transformations worked well. The lone exception is the published V magnitude of IRAS 22574]6609 (Hrivnak & Kwok 1991); it appears that the marginal ground-based detection (V \ 24) yielded an incorrect value. After the application of transformations, a complementary color image (V [I ) was also constructed using the available C images. To enhance the details of the images, an unsharp mask Ðlter was applied; this locates pixels that di er by a certain factor from surrounding pixels within a speciðed radius and further increases the pixelsï contrast. While it will be these unsharp images that we will present and upon which we will base our discussion of the details of the morphology, we have checked the original drizzled images to conðrm that all the features discussed are real and not artifacts of this process. 4. DISCUSSION OF HST IMAGES TABLE 4 HST MEASUREMENT RESULTS 4.1. Individual Objects IRAS 17245[3951.ÈThis object clearly shows two bipolar lobes separated by a dark lane running approximately east-west with a position angle (P.A.) of 117 (see Fig. 2). Thus it is a bipolar nebula viewed nearly edge-on. The line connecting the brightness peaks in the two lobes is oriented nearly perpendicular to this dark lane at a P.A. of 11. We assume the central star to be located at the crossing point of these two lines at R.A. \ 17h28m04s.61, decl. \ [39 53@44A.4 (J2000). Thus the dark lane appears to be the result of a disk that is obscuring the central star while permitting light to escape approximately perpendicular to it. The two lobes are of approximately equal size, with the northern lobe slightly brighter with a Ñux ratio of 1.2. Some additional structure is seen in the lobes; in particular, two parallel jetlike ÏÏ features are seen pointing away from the embedded central star in the southern lobe. These same basic features were seen in the shorter exposure (480 s) HST WFPC2 F606W image by Hrivnak et al. (1999a), who named the object the Walnut Nebula.ÏÏ There is no indication of circumstellar arcs around this nebula, as have been seen in all the other nearly edge-on PPNs. The nebula is surrounded by a halo that is elliptical in appearance of size 4A.8] 3A.9, where the outer edge of the halo is deðned as the place where the Ñux in the nebula has decreased to 3 p. The elliptical halo is oriented at a P.A. sky of 12, and thus it has the same orientation as the lobes, with the major axis of the halo approximately perpendicular to the dark lane. The elliptical appearance of the halo may represent a change in the mass loss at the end of the AGB from spherical symmetry or simply the extent to which photons can penetrate because of a latitudinal variation in the density of the material around the star. The color image clearly shows that the bright lobes are bluer than the dark lane. The brighter northern lobe is bluer than the dark lane by 0.9 mag, while the slightly dimmer Aperture Sizea Halo Sizeb IRAS ID (arcsec) STMAG F606W STMAG F814W V I C (arcsec) 16594[ ] c c ] [ ] ] [ ] c c ] ] ] ] ] ] d d ] ] ] ] ] ] d d ] 1.4 a Aperture size (major axis ] minor axis), based upon 10 p on the F814W images. b Halo size, based upon 3 p on the F814W longer exposure sky image. c Based upon the F606W image sky of Hrivnak et al. 1999a. d Based upon the archival F555W image; see Ueta et al

5 No. 3, 2001 HIGH-RESOLUTION IMAGING OF PROTOÈPLANETARY NEBULAE 1529 FIG. 2.ÈHST WFPC2 broadband F814W and color images of IRAS 17245[3951 (top) and IRAS 22574]6609 (bottom). In the V [I gray-scale images, a color scale in magnitudes is included on the right edge; the darker shades represent the redder colors. These both appear to be edge-on bipolar C nebulae. The bright and dark X-shaped patterns in the color image of IRAS 17245[3951 are due to di raction spikes in the V and I images, which were observed with the spacecraft in di erent orientations. southern lobe is redder than the northern lobe by D0.4 mag. This sense in the color is consistent with the idea of a dusty disk and reñection from the lobes. Since the southern lobe is fainter and redder than the northern lobe, it appears to su er more extinction, suggesting that the northern lobe is tilted toward us and the southern lobe away from us. The color of the halo is the same as that of the bluer lobe. IRAS 22574]6609.ÈThis object shows a basic bipolar morphology, with a dark lane running approximately northeast to southwest at a P.A. of 23, dividing the nebula into two lobes (see Fig. 2). The axis through the brightness peaks in each of the two lobes is oriented approximately perpendicular to the dark lane at a P.A. of 125. We assume that the location of the central star occurs where the axis connecting the two brightness peaks crosses the dark lane, R.A. \ 22h59m18s.30, decl. \ 66 25@48A.3 (J2000). The southeastern lobe is the brighter of the two, with a Ñux ratio of 1.1. In addition to this basic bipolar structure, several other features are seen. The southeastern lobe shows an extension of the nebula to the southwest, while the northwestern lobe shows an extension toward the north-northwest and an extension toward the northeast. The nebula is surrounded by an elliptical halo of the size 1A.80] 1A.38 with a P.A. of 158, which is not perpendicular to the dark lane. There also seem to be some additional discrete structures or blobs ÏÏ in the northwestern lobe and in the dark lane between the lobes. This same basic morphology can be seen in the shorter exposure F814W image of Ueta et al. (2000), although not with as much detail. A color map was formed from the combination of our F814W image and the F555W image of Ueta et al. The object is very red, as was known from the previous groundbased photometry and from the HST images. The resulting color image (V [I ) is rather noisy. Nevertheless, it does show that the two lobes C are bluer than the dark lane, with a di erence of *(V [I ) \ 1.1 mag. This is consistent with the general expectation C of a dusty disk and reñection from the lobes. Because the halo is too faint in the F555W image, its color could not be determined. One of the blobs lies along the dark lane, and it appears to be the image of a star located 0A.23 northeast of the assumed location of the central star. It appears to be very

6 1530 SU, HRIVNAK, & KWOK Vol. 122 red, redder than even the dark lane. It is possible that this is a binary companion of the central embedded source. This possibility will be discussed later. IRAS 16594[4656.ÈThe F814W image (Fig. 3) of this object shows the central star surrounded by a relatively large bipolar nebula. In the longer exposure images, the central star is saturated. The appearance suggests that the southwestern lobe is tilted toward us at an intermediate orientation. Extending outward from the nebula are three approximately point-symmetric radial arcs or petals.ïï These features were seen in the shorter (480 s) F606W image of this nebula published by Hrivnak, Kwok, & Su (1999a), who called this the Water Lily Nebula.ÏÏ In this new image, we observe deeper in a bandpass where the object is brighter. This results in the detection of four circumstellar arcs, of which two are seen to the west-northwest of the main bright central nebula, one is seen to the east-northeast, and one is seen to the south. The arcs are concentric, centered on the central star. These are discussed in detail by Hrivnak, Kwok, & Su (2001). The nebula is surrounded by an elliptical halo of the size 12A.3] 8A.8. A color image was produced by the combination of the new F814W image and the previous F606W image. Since the longest exposure images were used, the central star is saturated. The bright lobes, especially the southwest one, appear to be bluer than the part of the nebula perpendicular to the lobes by D0.6 mag, and the fainter petals of the lobes are bluer by an additional D0.2 mag. Using the shorter exposure images to measure the central star, its color is found to be (V [I C ) \ 2.3. IRAS 17106[3046.ÈThe HST broadband images and the color image of IRAS 17106[3046 were presented in detail by Kwok, Hrivnak, & Su (2000). The images reveal a collimated bipolar outñow emerging from the center of a visible disk, viewed at an intermediate angle. These images, in fact, present the most direct evidence that the bipolar outñows seen in PPNs are collimated by equatorial disks. The central star is clearly resolved, although it is saturated in the longer exposure images in both Ðlters. The lobes are bluer than the disk, and the star is reddest of all. The lobes and disk are surrounded by an elliptical halo. Although the images have been presented elsewhere, we included IRAS FIG. 3.ÈHST WFPC2 broadband F814W and color images of IRAS 16594[4656 (top) and IRAS 20028]3910 (bottom). These both appear to be bipolar nebulae viewed at intermediate orientations. The central star in IRAS 16594[4656 is saturated in this long exposure, and consequently the starïs color is not represented correctly. Di raction spikes are visible in the images of IRAS 16594[4656. The color scale is the same as in Fig. 2.

7 No. 3, 2001 HIGH-RESOLUTION IMAGING OF PROTOÈPLANETARY NEBULAE [3046 in this study to compare its component colors and its SED with those of the other six PPNs. IRAS 20028]3910.ÈThis object appears to be a bipolar nebula viewed at an intermediate orientation (see Fig. 3). The southeastern lobe is very bright. In what initially appears to be the northwestern lobe, one Ðnds, under careful examination, several small circumstellar arcs, which are also seen approximately symmetrically around the object on the southeastern side of the nebula. In addition, there appears to be a pair of faint, intersecting searchlight beams separated by an angle of D14. These arcs and searchlight beams are discussed in detail by Hrivnak, Kwok, & Su (2001). The northwestern lobe, in fact, is not seen and must be obscured by a circumstellar torus. This implies that the torus, which must be oriented at a relatively large angle to our line of sight, is quite opaque and relatively large in radial extent. Surprisingly, the bright pointlike structure of the central star is not seen in the image. This is in contrast to the other bipolar PPNs viewed at intermediate orientations, IRAS 16594[4656, 17106[3046, and possibly 19477]2401 (discussed below), in which the central star is clearly seen. This indicates a relatively high extinction along the line of sight through the southeastern lobe. The color image shows the interior of southeastern lobe to be bluer than the edge of the torus by 1.0 mag. IRAS 19477]2401.ÈThe deep F814W image (Fig. 4) shows a faint pointlike source imbedded in a three-lobed, cloverleaf-like nebula. For this reason, we call it the Cloverleaf Nebula.ÏÏ However, in spite of this detailed appearance, the basic structure seems to be axisymmetric, with a polar axis running approximately north-south (P.A. \ 8 ) and the central star visible in the northern lobe. Thus it appears that the bipolar nebula is viewed at an intermediate orientation. The faint additional extension is seen to the northeast, similar to the additional extensions seen in IRAS 22574]6609. Since these are the Ðrst HST images of this object and it was observed only with the F814W Ðlter, no color image of the nebula can be made. The nebula is surrounded by an elliptical halo of the size 2A.6] 2A.1 with a P.A. of 31. IRAS 20136]1309.ÈThis object appears to show a pointlike structure in both F606W and F814W (Fig. 4) images. However, the FWHM of the object is signiðcantly larger than that of the Ðeld stars, 0A.124 as compared with 0A.073, respectively, in the F606W Ðlter. This suggests the presence of nebulosity, and the radial proðles imply that this nebulosity has a circular or spherical symmetry. Using the shorter exposure images to measure the central star, its color is found to be (V [I ) \ C 4.2. Comparison with Images from a Bipolar Model Five of the PPNs discussed above appear to have a clear bipolar structure, and IRAS 19477]2401 appears to us to have a basic structure that is also bipolar. In classifying an object as bipolar, we mean that it has axial symmetry in the optical appearance. This is attributed to the asymmetric CSE with a higher density region about a plane through the star and lower density along the axis perpendicular to this plane. The higher density region will produce an obscuring lane (or disk) when viewed edge-on in the case that the material is optically thick. The visible light will be scattered into view preferentially from the direction of the axis and produce a bipolar appearance. An object like IRAS 20136]1309 could, in fact, possess a bipolar structure but be viewed along the symmetry axis. Su et al. (1998) computed a two-dimensional radiation transfer model to Ðt the visible images and SED of the edge-on bipolar PPN IRAS 17441[2411. The model was similar to the one described here ( 5.2), but it also included an explicit optically thick disk. Their paper also shows simulated V -band images produced with the model for four di erent viewing orientations: 0 (pole-on), 30, 60, and 90 (edge-on). These model images can usefully be compared with the observed HST images to obtain approximate FIG. 4.ÈHST WFPC2 broadband F814W images of IRAS 19477]2401 (left) and IRAS 20136]1309 (right). In the full image of IRAS 19477]2401, the contrast has been adjusted to reveal the faint halo; in the inset, a contour plot shows the basic bipolar structure. The di raction spikes and point-spread function pattern are visible in the image of the bright central star in IRAS 20136]1309.

8 1532 SU, HRIVNAK, & KWOK Vol. 122 orientations, with the following results: IRAS 17245[3951 and 22574]6609 at i B 90 ; IRAS 16594[4656, 17106[3046, and 19477]2401 at i B 60 ; and IRAS 20028]3910 at i B 30. These have an uncertainty of *i D 10 in comparison with this speciðc model. The image of IRAS 20136]1309 is consistent with that of a bipolar model viewed nearly pole-on (i B 0 ). 5. DISCUSSION 5.1. PPN Component Colors The availability of HST V (F606W or F555W) and I (F814W) images has permitted color information to be obtained for the various components of the PPNsÈlobes, halo, disk, and, where visible, the central star. These are summarized in Table 5. Also included for each object is an estimate of the interstellar extinction, derived from the studies of Neckel & Klare (1980) and Burstein & Heiles (1982) and assuming typical distances of 1 to 3 kpc, as well as an estimate of the corresponding reddening, based upon the extinction relations of Cardelli, Clayton, & Mathis (1989). In the four cases for which such data are available, the color of the equatorial obscuring region is redder than the color of the lobes, consistent with the proposed asymmetry in which the disk is along the direction of higher optical depth and the lobes are along the direction of lower optical depth. The optical Ñux is from the reddened stellar photosphere plus the stellar light scattered o the dust grains, which blues the color. Thus, we also expect the color of the lobes to be bluer than the color of the central star. There are data to test this in only two cases: in IRAS 17106[3046, the star is much redder than the lobes, but in IRAS 16594[4656, this is not so. The colors of IRAS 16594[4656 are perhaps distorted in some way by the strong Ha emission present (Van de Steene et al. 2000), which would be included in the F606W Ðlter. The halos are bluer than the stars or disks and are similar to the lobes in color. Only for IRAS 17106[3046 is complete information available on the colors of the various components. The central star is reddest among all the morphological components, with an observed (V [I ) \ 3.9. If the central star C has, for example, a spectral type of G2 I (a typical type for a PPN central star), then it would have an intrinsic color of (V [I ) \ 0.7. Since the contribution of interstellar C reddening is not large [E(V [I )IS \ 0.4], the central star of IRAS 17106[3046 must be located C inside a very dense environment. The colors for the other morphological components (lobes, halo, and disk) are bluer (less red), indicating that they are all viewed in scattered light SEDs and Flux Ratios In the previous section, images were presented of the PPNs from which it was argued that six of the seven display a basic bipolar structure viewed at di ering orientations. They were then classiðed based upon their orientation to the line of sight as edge-on or intermediate angle, and approximate orientation angles were assigned based upon comparison with published model results. An unambiguous case for a bipolar structure for IRAS 20136]1309 cannot be made based upon its image alone; however, we will discuss it with the other six as an example of what a bipolar PPN viewed pole-on would look like. One can also expect in a bipolar structure that the orientation of the asymmetric CSE will a ect the observed SED of the object, most especially in the optically thick case. In optically bright PPNs, a double-peaked SED is observed, with about equal amounts of Ñux observed from the reddened photosphere (in the visible and near-infrared) and from the dust (in the midinfrared; see, e.g., Hrivnak, Kwok, & Volk 1989). In a bipolar nebula viewed nearly edge-on, the light of the photosphere is dimmed or totally obscured by the equatorial density enhancement (or disk), but the escaping stellar light in the polar direction can be seen through scattering into the light of sight by dust grains. Meanwhile, the light from the dust in the mid-infrared, where the nebula and disk are optically thin, will be independent of viewing angle. In the edge-on, optically thick case, the amount of Ñux observed from the photosphere will be much less than that from the dust (see examples of such SEDs by Hrivnak & Kwok 1991). To examine this e ect in these PPNs, the SEDs of all seven have been constructed. In addition to the new ground-based and HST photometry included here, other ground-based data from the literature have been included. In the mid-infrared, we have included the IRAS photometry, the IRAS and Infrared Space Observatory (ISO) spectra where available, and recent Ñux measurements from the Midcourse Space Experiment (MSX) that are available TABLE 5 COLOR MEASUREMENTS OF THE PPNS COLOR (V [I C ) IRAS ID ORIENTATION Lobes Halo Disk Star A V IS E(V [I C )IS SPECTRAL TYPE 16594[ Intermediate 2.6È a B [ Intermediate 2.5È È [ Edge-on 1.9È È3.0 b ] Intermediate c c c c ] Intermediate 2.2 d 3.2 b ] Pole-on(?) e e a G0-F ] Edge-on 2.5 d 3.6 b a No disk visible. b No star visible. c No V image. d Too noisy to determine accurately. e Although the object appears to be extended, no photometric observations can be made of the lobes or halo.

9 No. 3, 2001 HIGH-RESOLUTION IMAGING OF PROTOÈPLANETARY NEBULAE 1533 for Ðve of these PPNs. The MSX observations were made through Ðlters with isophotal wavelengths of 8.28, 12.13, 14.65, and km, and these were extracted from the MSX Point Source Catalog (Egan et al. 1999). The MSX Ñux measurements for these PPNs are listed in Table 6, along with the IRAS measurements from the IRAS Point Source Catalog. The SEDs of these seven PPNs are displayed in Figure 5. Since the observed Ñuxes contain not only the e ects of local extinction, but also interstellar extinction and reddening along the line of sight to these objects, we have attempted to correct for this interstellar component using the approximate values listed in Table 5. The interstellar extinction law of Cardelli et al. (1989) was used to correct the data. For comparison, photometry points both before and after the correction for interstellar extinction are plotted. In some cases, such as IRAS 16594[4656, the corrections are quite large. The Ñuxes for Ðve of the objects are reasonably well represented by two blackbody curves, one representing the reddened photosphere and the other the dust shell. This is what would be expected from a star surrounded by a detached CSE containing cool dust. In the other two objects, IRAS 20136]1309 and 22574]6609, there is additional emission in the 2È8 km range that Ðlls in the gap between the two blackbody curves and that may be due to the presence of warm dust in the systems. The observed color temperatures of the reddened photospheres (T ) range p from 1600 to 3000 K and of the dust (T ) from 150 to 250 K. d When corrected for interstellar extinction, the reddened photospheric temperatures ) increase to a range of 1700 to 3500 K. The ratio between p the peak Ñuxes of the two blackbody curves, dust to reddened photosphere, was also computed. For comparison, these ratios were computed both without (R) and with correction (R@) for interstellar extinction. These various values are listed in Table 7. In some cases, the corrections can be large, reducing the ratio by half; however, for these seven, they do not a ect the order of the ratios from highest (IRAS 20028]3910) to lowest (IRAS 20136]1309). To enlarge the sample, we have included these same values for the two other PPNs that we have studied with HST imaging, IRAS 17150[3224 and 17441[2411 (Kwok, Su, & Hrivnak 1998; Su et al. 1998; Kwok et al. 1996) and also the well-known PPN AFGL 2688 (Sahai et al. 1998; Hrivnak & Kwok 1991). These three are all bipolar and viewed nearly edge-on, with an obscuring lane separating the two lobes. In general, the higher the value of R (R@), the more edge-on is the orientation. However, there are two exceptions, IRAS 17245[3951 and 20028]3910. IRAS 20028]3910 appears to be viewed at an intermediate angle but has a very high value of R (R@). This object was noted as unique among those at an intermediate angle because its central star was not seen, implying a larger optical depth toward the star. It may be that its CSE is more massive, contains more dust, and has a higher optical depth than the other objects, which has both reduced the observed photospheric Ñux and increased the dust emission. IRAS 17245[3951 is an edge-on bipolar TABLE 6 MID-INFRARED FLUX MEASUREMENTS MSX (Jy) IRAS (Jy) IRAS ID F 8.3 F 12.1 F 14.6 F 21.4 F 12 F 25 F 60 F [ ^ 5% 52 ^ 3% 87 ^ 4% 253 ^ 6% 45 ^ 5% 298 ^ 5% 131 ^ 17% 34 ^ 15% 17106[ ^ 5% 3.39 ^ 5% 10.7 ^ 4% 48.6 ^ 6% 4.0 ^ 5% 62 ^ 4% 51 ^ 12% 17 ^ 14% 17245[ ^ 5% 2.94 ^ 5% 8.5 ^ 4% 37 ^ 6% 3.4 ^ 9% 45 ^ 5% 38 ^ 14% \ ] ^ 5% 13.7 ^ 3% 19.7 ^ 4% 40.1 ^ 6% 11.2 ^ 5% 55 ^ 4% 27 ^ 11% \ ] ^ 5% 50 ^ 3% 86 ^ 4% 176 ^ 6% 42 ^ 4% 211 ^ 4% 143 ^ 17% 47 ^ 10% 20136] ^ 4% 9.7 ^ 6% 2.1 ^ 9% \ ] ^ 3% 29 ^ 4% 21 ^ 7% 7 ^ 18% TABLE 7 PARAMETERS OF THE SEDS Orientation A V T p T T d IRAS ID (deg) (mag) (K) (K) (K) R R@ Ordera 16594[ D [ D [ b [ D [ b ] D ] D ] D ] D AFGL D75c :d 3500:d :d 440:d 1 a Order of R (and R@) from greatest to least. b From Su et al c Estimated value based upon optical appearance in Sahai et al d The values for AFGL 2688 are uncertain because of variations in the aperture sizes used in the measurements of this extended object and the uncertainty in Ðtting the photospheric blackbody curve to the SED.

10 1534 SU, HRIVNAK, & KWOK Vol. 122 λ(µm) λ(µm) K 150K 1700K 3000K 2500K 1600K K 2100K 150K K 2000K 180K K 1600K 180K K 2200K 170K K 3000K 250K λ(µm) λ(µm) FIG. 5.ÈSED plots for the seven PPNs. Symbols used in the SEDs are as follows: Ðlled squares for ground-based photometry, open squares for ground-based photometry after the correction for interstellar extinction, Ðlled triangles for HST measurements, open triangles for HST measurements after extinction correction, Ðlled diamonds for IRAS Ñuxes, and crosses for MSX measurements. Dashed lines are IRAS low-resolution spectrometer (LRS) or ISO short-wavelength spectrometer (SWS) spectra, solid lines are Ðtted blackbody curves (photosphere and dust emission), and the dotted lines are the Ðtted photospheric blackbody curves after the interstellar extinction corrections. nebula with a low value of R (R@). Perhaps this implies a relatively small amount of circumstellar dust and a lower optical depth and, thus, a reduced dust emission. Thus the ratio of the dust to the photospheric emission generally corresponds to the orientation of the bipolar nebula, but it is not uniquely correlated with the orientation. It is seen that in a general way the ratio of peak Ñuxes (dust to photosphere) is correlated with the orientation of a bipolar nebulae. This would be expected to be strictly true only if all the nebulae were exactly the same except for our viewing orientation. However, one would also expect them to di er in the amount of circumstellar matter and perhaps

11 No. 3, 2001 HIGH-RESOLUTION IMAGING OF PROTOÈPLANETARY NEBULAE 1535 in the degree of asymmetry in the envelope. The possible qualitative e ects that a di erence in the amount of dust in the envelope can have on this ratio were mentioned above. Here we will discuss the results of an investigation into the e ects of di ering asymmetry on this ratio. We began by constructing a series of models assuming an axially symmetric, latitudinally varying density distribution (o P r~2hb, h is the colatitude), with the degree of asymmetry controlled by the exponent b. The optical depth was Ðxed at the equator and pole. Isotropic single scattering was assumed. For more details of the model, see Su, Volk, & Kwok (2000) and Su et al. (2001). The Ñux ratio was calculated from the modeled SEDs at di erent orientations (from pole-on, i \ 0, to edge-on, i \ 90 ). When the degree of the asymmetry is larger (large b), more stellar light can escape, and the Ñux ratio will be smaller, since the total visible Ñux of the object is brighter. This is seen in Figure 6a. Next we added to the asymmetry of this model by introducing a polar cavity (lobe) that has a open-cone shape and uniform but very low density inside; this cavity is characterized by an opening angle (h ). To demonstrate this combined e ect, a series of models o was constructed assuming a varying density distribution (o P r~2h1.5) with di erent opening angles of a cavity. SEDs were formed from the models, ranging from the case without a cavity (h \ 0 ) to o the case with a large cavity (h \ 80 ). The resulting Ñux o ratios are a combination of the orientation angle (i) e ect and the opening angle (h ) e ect. As illustrated in Figure 6b, o the Ñux ratio is not sensitive to the orientation angle when the opening angle is large. This could help explain why the Ñux ratio is low but the orientation is nearly edge-on in IRAS 17245[3951. A high Ñux ratio in a PPN at an intermediate orientation, as seen in IRAS 20028]3910, would suggest a small asymmetry and a small opening angle, and this appears to be the case based upon the optical image. An optically thick circumstellar dust shell is assumed in the models constructed above. For an optically thick CSE, the ratio of peak Ñuxes increases, in general, as the orientation increases toward edge-on. In addition, the ratio is expected to be smaller and not sensitive to the orientation as the degree of asymmetry increases. For a completely optically thin CSE, there will be no di erence in the ratio of peak Ñuxes at di erent orientations, since the central star is directly seen with no dimming. For the cases in between, one expects to see a similar trend in the ratio of peak Ñuxes as in the optically thick case, but it will not be as pronounced Comparison with the PPN Imaging Study of Ueta et al. In light of the observations of these 10 PPNs (the seven presented here and the three additional ones included in Table 7), one can ask whether all PPNs possess a basic bipolar structure, and if so, whether the orientation is the dominant factor in the morphological appearance of a PPN. The only study of a large sample of PPNs is that of Ueta et al. (2000; hereafter UMB), who observed 27 PPN candidates. Their sample includes both candidates with bright central stars and candidates with obscured central stars. They found that most (21 of 27) possess optical reñection nebulosities and that all those with nebulosity display an axisymmetry. UMB classiðed them in two groups of about equal numbers. The Ðrst group (which they called SOLE) was characterized by a bright central star and a faint, extended nebulosity. The second group (DUPLEX) was characterized by a clear bipolar structure in which the central star is completely or partially obscured. UMB suggest a basic bipolar model for each of these two groups but then go on to argue that these do not represent the same bipolar structure simply viewed at di erent orientations. Instead, they propose that the two groups are distinguished by circumstellar dust shells of low and high optical depths, respectively. We have examined the images of the PPNs in which UMB found nebulosity and also reach the conclusion that these objects can be represented by a basic bipolar structure. This is obvious for the DUPLEX objects. However, even in some of their SOLE objects, one can see the e ects of orientation in a bipolar structure. When we refer to the e ects of orientation, we do not have in mind only the optically thick case in which the central star is obscured FIG. 6.È(a) Flux ratios (dust to photosphere) calculated from simulated SEDs at di erent orientations with di erent degrees of asymmetry in the CSE. This diagram shows that when the asymmetry is larger (larger b), the Ñux ratio between the dust and photospheric components of the SED is smaller. (b) Flux ratios calculated from simulated SEDs at di erent orientations with di erent opening angles of a cavity (h ) for b \ 1.5. This diagram shows that when the opening angle is large, it is difficult to distinguish among the di erent orientations based upon the Ñux ratio o between the dust and photospheric components of the SED.

12 1536 SU, HRIVNAK, & KWOK Vol. 122 when viewed edge-on. We also consider the optically thin case in which the central star can always be seen, but in which there is nevertheless a visible asymmetry in the brightness or size of the lobes and/or the position of the central star when viewed at an intermediate orientation. Of the 11 objects that UMB classify as SOLE, the central star appears to be o -center in four (IRAS 05341]0852, 07430]1005, 17436]5003, and 18095]2704), and in these and one additional case (IRAS 06530[0213), one lobe is brighter than the other; both of these e ects are expected to be due to the orientation of the nebulae. As long as the nebulosity has an axisymmetric bipolarity, one can expect that the lobe that is closer to us will appear to be brighter than the other one that is further away, and the central star, if seen, will appear to be o -center. Thus when one examines this combined sample of D25 di erent PPNs imaged with the HST, one Ðnds that almost all display an axisymmetric and, in particular, a bipolar structure and that, in most of these, the e ects of the orientation of the nebula can be seen in the morphology Parameters Derived from the SEDs The total Ñux of each object, corrected for interstellar extinction, is listed in Table 8. An approximate distancedependent luminosity (L kpc~2) has been computed for _ each object assuming an isotropic Ñux distribution. We know that this is only approximate and depends somewhat upon viewing orientation; for example, in the edge-on case, some of the visible light of the central star that is escaping along the polar axis will be missed. However, since the nebulae are assumed to be optically thin in the midinfrared, the Ñux density in the mid-infrared will be independent of viewing orientation. Since more than half of the Ñux is emitted in the mid-infrared, the calculated values of the luminosity will be accurate to much better than a factor of 2. Assuming that these PPNs have a total luminosity of 6000 L, which is the theoretical luminosity for a post-agb _ star with a core mass of 0.60 M (Scho nberner 1987), the _ estimated distances are also listed in Table 8. The distances of these PPNs are in the range of 1È5 kpc except for IRAS 20136]1309, which appears to be further away. Expanding CSEs have been detected by molecular emission in each of the seven PPNs. The measured expansion velocities can be used, together with the observed halo sizes, to obtain minimum (distance-dependent) dynamical ages (q /D) for these nebulae; these have also been included in Table dyn 8. These are minimum values, limited by our ability to see any fainter extent to the halo, and these ages refer to the minimum time since extensive mass loss was initiated during the AGB phase. Because IRAS 22574]6609 is very faint, the halo size and, thus, the corresponding dynamical age may be signiðcantly underestimated. The dynamical ages of the inner radii of the dust shells for three of these PPNs, IRAS 16594[4656, 19477]2401, and 22574]6609, have recently been calculated based upon new mid-infrared spectra and detailed model Ðtting of the SEDs (Hrivnak, Volk, & Kwok 2000). These ages would represent the time since the start of the PPN phase and would be expected to be smaller than the dynamical ages based upon the halos; indeed, the distance-dependent dynamical ages of the inner radii are each a factor of 3È5 smaller than the distancedependent dynamical ages of the measured halos Possible Binary System in IRAS 22574]6609 It was noted in the discussion of the HST images of IRAS 22574]6609 that there is a very red blob of light that appears to be the image of a star located along the dark lane separating the two lobes. Could this represent a binary companion, and could this companion have inñuenced the mass outñow in the PPN? The object appears to have the intensity proðle of a star, so we assume that it is indeed a star and not, for example, a hole in the disk or a dense region of shock-heated gas. It is very red, with (V [I ) \ C 4.4, so it appears to represent light that has passed through the disk. This places the star either within the disk or behind it. The chance of this being an optical binary with the red star located behind the PPN is rather small, given the location o the galactic plane (l \ 112, b \ 6 ) and the large distance calculated to the PPN (5.8 kpc). Thus it seems likely to be a binary companion. The angular separation between the red star and the calculated position of the central star is 0A.23, which translates to a projected linear separation of D230D/(1 kpc) AU. This is much too large a separation for the companion to have inñuenced the mass loss of the PPN. Only if the eccentricity is greater than 0.99 will the companion pass within 1D/ (1 kpc) AU of the central star of the PPN, and such a large eccentricity also seems highly unlikely. Nevertheless, it is TABLE 8 PARAMETERS DERIVED FROM THE SEDS f total L /D2 Da V exp q dyn /Db IRAS ID (ergs s~1 cm~2) (L _ kpc~2) (kpc) (km s~1) (yr kpc~1) 16594[ E[ º16c ¹ [ E[ d [ E[ e ] E[ f ] E[ g ] E[ h ] E[ f 140 a Assuming L \ 6000 L ; for a di erent value of L, D@ \ D(L /6000 L )1@2. b q /D \ (angular size _ of halo)/v. _ c CO dyn emission, from Loup et al exp d OH maser emission, from Silva et al e OH maser emission, from Sevenster et al f CO emission, from Hrivnak et al g CO emission, from Likkel et al h Assumed value; OH emission detected by Lewis 1992, but no V listed. exp