THE ASTROPHYSICAL JOURNAL, 503 : 253»277, 1998 August 10. 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.. (. HUBBL E ...
THE ASTROPHYSICAL JOURNAL, 503 : 253È277, 1998 August 10 ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.
HUBBL E SPACE T EL ESCOPE OBSERVATIONS OF PLANETARY NEBULAE IN THE MAGELLANIC CLOUDS. VII. CYCLE 3 AND ARCHIVE NARROWBAND [O III] 500.7 NANOMETER IMAGING E. VASSILIADIS,1 M. A. DOPITA,2 S. J. MEATHERINGHAM,3 R. C. BOHLIN,4 H. C. FORD,5 J. P. HARRINGTON,6 P. R. WOOD,2 T. P. STECHER,7 AND S. P. MARAN8 Received 1997 October 28 ; accepted 1998 March 16
ABSTRACT We present the continuation of our analysis of narrowband [O III] images, obtained using the Hubble Space T elescope (HST ), of planetary nebulae (PNs) in the Magellanic Clouds. Six objects, Ðve in the Large Magellanic Cloud, are observed as part of the General Observer program. An additional 15 objects are retrieved from the HST archive. The majority of the images are obtained prior to the Ðrst maintanence and refurbishment mission and are dominated by nebular [O III] 500.7 nm emission : the PN central stars are generally not seen. The raw images are deconvolved using 100 iterations of the Richardson-Lucy image restoration algorithm. The nebular dimensions and morphologies are determined. The nebular dynamical ages are derived using [O III] 500.7 nm expansion velocities from the literature. Using the techniques presented in Paper IV, the measured nebular ages are compared to theoretical evolutionary ages for H-burning and He-burning PN nuclei. Approximately 50% of the PN central stars are He burners in both the Large and Small Magellanic Clouds. Subject headings : Magellanic Clouds È planetary nebulae : general 1.
INTRODUCTION
PNs (Jacoby 1980). Speckle interferometry has been used to measure PN sizes, but can only be used for bright and, therefore, young and compact sources (Wood, Bessell, & Dopita 1986). Angular sizes down to 0A. 1 are measured. High-speed direct imaging has been used to measure larger and fainter PNs with angular diameters greater than 0A. 7 (Wood et al. 1987). The Planetary and Faint Object Cameras (hereafter, PC and FOC, respectively) on board the Hubble Space T elescope (HST ) are ideally suited to imaging Magellanic Cloud PNs with diameters down to 0A. 1, even with the presence of spherical aberration of the primary mirror (e.g., Blades et al. 1992). The geometry of the nebular gas can be directly observed, and the optical thickness can be determined. This information can be combined with UV and optical spectrophotometry to yield accurate nebular photoionization models (Dopita et al. 1993, 1994 ; Liu et al. 1995). In Dopita et al. (1996, hereafter Paper IV), narrowband [O III] 500.7 nm images are obtained for 11 LMC and four SMC PNs. As a continuation of this General Observer (GO) program, a further Ðve LMC and one SMC object are imaged in the same manner during Cycle 3. In addition to these GO data, a search of the HST Archive revealed that [O III] images are available for 10 LMC and Ðve SMC objects, which are unique to the Guaranteed Time Observer (GTO) programs. Using the techniques developed in Paper IV, the LMC images presented here are used to determine the ratio of He-burning to H-burning PN nuclei. For the moment, the results from the Ðrst GO sample are not included within this analysis. The six SMC images presented here are analyzed in a similar manner, in conjunction with the four SMC objects presented but not analyzed in Paper IV.
The observational characteristics of low-mass stellar evolution from the asymptotic giant branch (AGB) through to the planetary nebula (PN) regime are poorly known because of the lack of a reliable distance scale for Galactic PNs (Terzian 1993 ; Kwok 1993). In particular, any uncertainty in the distance of a PN will have a direct consequence on the observed nebular size and central-star luminosity. Both quantities are important in aiding the comparison between theory and observation and in reconciling the discrepancy between the observed dynamical ages of PNs and the theoretical evolutionary ages of their central stars (McCarthy et al. 1990). To circumvent these problems, one can observe PNs at some independently determined distance, such as in the Galactic Bulge or in the Magellanic Clouds. The Galactic Bulge PNs are relatively close compared to those objects in the SMC and LMC, but they su†er severly from extinction. The reddening toward the clouds is only E(B[V ) D 0.05 (Bessell 1991). However, at the distance of the Magellanic Clouds, the known Cloud PNs generally subtend less than 1A on the sky. Larger Magellanic Cloud PNs seem less numerous because of their lower surface brightness, but probably constitute the majority of all Magellanic Cloud 1 Instituto de Astrof• sica de Canarias, C/- Via Lactea S/N, E-38200, La Laguna, Tenerife, Spain. 2 Mount Stromlo and Siding Spring Observatories, Institute of Advanced Studies, Australian National University, Private Bag, Weston Creek PO, Weston Creek, ACT 2611, Australia. 3 Information Technology Services Centre, University College, Australian Defence Force Academy, Canberra, ACT 2600, Australia. 4 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218. 5 Johns Hopkins University, Department of Physics and Astronomy, Charles & 34th Streets, Baltimore, MD 21218. 6 University of Maryland, Department of Astronomy, College Park, MD 20742-2421. 7 NASA Goddard Space Flight Center, Code 680, Greenbelt, MD 20771. 8 NASA Goddard Space Flight Center, Code 600, Greenbelt, MD 20771.
2.
OBSERVATIONS
2.1. GO PC Data Five PNs in the LMC and one PN in the SMC are observed with chip six of the PC during the period 1994 AugustÈOctober. The exposures are taken as part of the Cycle 3 GO4940 program, and they are taken solely 253
254
VASSILIADIS ET AL. TABLE 1 GO OBSERVATION LOG SMP ID
Rootname
Date
t a exp (s)
Vol. 503
data are obtained prior to the Ðrst maintenance and refurbishment mission undertaken in 1994 December. These three objects are also observed through the Ha ] [N II] F664N Ðlter : pivot wavelength \ 663.74 nm, rms bandwidth \ 5.52 nm.
LMC SMP 7 . . . . . . . . . . SMP SMP SMP SMP
37 . . . . . . . . . 61 . . . . . . . . . 67 . . . . . . . . . 101 . . . . . . . .
W1ID0301T W1ID0302T W1ID0401T W1ID0601T W1ID0701T W1ID0801T W1ID0802T
1993 1993 1993 1993 1993 1993 1993
Oct 23 Oct 23 Aug 11 Aug 11 Aug 11 Aug 17 Aug 17
400 400 100 180 500 350 350
1993 Aug 20
400
1993 1993 1993 1993 1993 1993
400 400 400 400 400 400
SMC SMP 22 . . . . . . . . .
W1ID0201T PSF
PSF-STAR . . . . . .
W1ID0101T W1ID0102T W1ID0501T W1ID0502T W1ID0901T W1ID0902T
Aug 11 Aug 11 Aug 22 Aug 22 Oct 6 Oct 6
a Exposure time.
through the F502N narrowband Ðlter. The observation log is shown in Table 1. This sample of objects complements the sample observed during Cycle 1, in terms of coverage in the Hb excitation-class plane (Dopita & Meatheringham 1990). Not all observations are split into two exposures because of scheduling restrictions. On three separate occasions, a nearby guide star was observed with the PC to record the point-spread function (PSF). The guide star observed in each case is the same as that observed in Paper IV. The PSF observations are scheduled to fall approximately at the beginning, middle, and end of the program, so that a reliable PSF is available to restore the raw PN images su†ering from spherical aberration (Burrows et al. 1991). The F502N Ðlter has a pivot wavelength at j501.85 nm and a rms bandwidth of 1.28 nm : the FWHM is 2.97 nm (see MacKenty et al. 1992, Appendix A). The pivot wavelength is deÐned to allow exact conversion between the HST broadband Ñux densities expressed per unit wavelength and per unit frequency (Koornneef et al. 1986). The Ðgures for the pivot wavelength and rms bandwidth are taken from the image headers. The Ðlter is sensitive to the strong [O III] 500.7 nm nebular emission commonly seen in PNs. The PC, operating at f/31, has an image scale of 0A. 04389 pixel~1 (Westphal et al. 1991). The maximum signals recorded in the uncalibrated *.D0H frames are listed in Table 1 : all values are less than the saturation limit of 4095 DN. Some exposures experienced guide star recentering events, where Ðne-track guiding was temporarily lost, and guiding was under gyrocontrol. The drift in tracking is of the order of 0A. 002 s~1 (Mo & Hanisch 1995). Such incidents usually lasted for less than 10 s and had no visible e†ect on the quality of the exposures. 2.2. GT O PC Data The GTO PC data are obtained under Program 1046, with H. C. F. as principal investigator. Of the eight observations found in the HST archive (Table 2), three are of Cloud objects taken through the F502N Ðlter. All of these
2.3. GT O FOC Data The GTO FOC pre-COSTAR9 data found in the HST archive are obtained under Programs 1266, 4075, and 4821, with J. C. Blades as principal investigator. All observations are listed in Table 2, and all are taken in the standard, unzoomed 512 ] 512 pixel mode. There are 15 observations through the F501N Ðlter, which have not been made earlier in either the GO or GTO programs, of 10 LMC and Ðve SMC sources. The FOC F501N Ðlter has a pivot wavelength of 501.96 nm and a rms bandwidth of 3.55 nm. This Ðlter is sufficiently wide to detect emission from both [O III] 495.9 and 500.7 nm. The pre-COSTAR image scale is 0A. 02217 pixel~1 (Paresce 1992). One object, LMC SMP 35, which is observed as part of Program 4821, is also observed in Paper IV. In addition to the PC and pre-COSTAR FOC observations, three objects are also observed with the postCOSTAR FOC. The three objects are observed in previous programs, but these unblurred data serve as a valuable check of our analysis of the pre-COSTAR data. The postCOSTAR image scale is 0A. 01435 pixel~1 (Hodge 1994). 3.
DATA PROCESSING
3.1. PC Data The GO and GTO PC data are essentially treated in the same manner. The uncalibrated, raw *.D0H data are processed by STScI using the CALWFPC task within STSDAS.10 All subsequent analysis is performed upon the calibrated *.C0H data products. The four original 800 ] 800 pixel PC frames are largely devoid of any objects, apart from the desired targets on chip six. The narrow bandpass of the F502N Ðlter is very efficient in excluding stars over the 80A Ðeld of view. The PN and PSF observations are adequately contained within subimages of 256 ] 256 pixels, 11A. 2 ] 11A. 2, centered on each target. For those observations split across two exposures, cosmic rays are removed using the CRREJECT option of the COMBINE task within STSDAS. Single-exposure observations are processed with the IMGCLEAN v1.41 routine written by E. Deutsch and implemented within IDL.11 Any artifacts remaining after cosmic-ray removal are replaced with the median of surrounding pixel values. The 80 ] 80 pixel central regions of the calibrated and cleaned GO and GTO PC frames are shown on the left in Figures 1 and 2, respectively. The Richarson-Lucy (RL) restoration method (Richardson 1972 ; Lucy 1974), with the modiÐcation made for CCD read noise (Snyder 1990), is used to restore the calibrated raw images. To remain internally consistent with the analysis presented in Paper IV, 100 iterations of the RL method are performed for all cases. A canonical value of 20 DN is adopted for the CCD noise characteristics. The inclusion of the CCD noise helps in limiting the growth of noise 9 Corrective Optics Space Telescope Axial Replacement. 10 Space Telescope Science Data Analysis System. 11 Interactive Data Language, Research Systems Inc.
No. 1, 1998
MAGELLANIC CLOUD PLANETARY NEBULAE
255
TABLE 2 GTO OBSERVATION LOG SMP ID
HST ID
Rootnamea
Proposal ID
Date (UT)
Filter
Mode
t exp (s)b
x (pixels)
y (pixels)
LMC SMP 21 . . . . . . SMP 23 . . . . . . SMP 29 . . . . . . SMP 32 . . . . . . SMP 35e . . . . . . SMP 45 . . . . . . SMP 62 . . . . . . SMP 66 . . . . . . SMP 78 . . . . . . SMP 83f . . . . . .
SMP 97 . . . . . .
LMC-N 97 LMC-N 97 LMC-N 97 LMC-N 24 LMC-WS 9 LMC-WS 9 LMC-N 192 LMC-N 192 LMC-N 192 LMC-WS 12 LMC-LM 1-27 LMC-N 201 LMC-N 201 LMC-N 52 LMC-WS 33 LMC-WS 33 LMC-WS 35 LMC-WS 35 LMC-N 66 LMC-N 66 LMC-N 66 LMC-N 66 LMC-N 66 LMC-LM 1-61
X14T0101T X25H0301T X25H0302T X14T0501T W0KU0101T W0KU0102T X14T0601T X25H0201T X25H0202T X1CT0601T X1CT0401T X0MZ0301T X0MZ0302T X14T0701T W0KU0201T W0KU0202T W0KU0301T W0KU0302T X0MZ0201T X0MZ0202T X1CT0501T X25H0101T X25H0102T X14T0401T
4075 5185 5185 4075 1046 1046 4075 5185 5185 4821 4821 1266 1266 4075 1046 1046 1046 1046 1266 1266 4821 5185 5185 4075
1992 1994 1994 1992 1991 1991 1993 1994 1994 1993 1993 1991 1991 1992 1991 1991 1991 1991 1991 1991 1993 1994 1994 1992
Nov 18 Feb 2 Feb 2 Dec 8 Jul 28 Jul 28 Mar 7 Feb 6 Feb 6 Apr 28 Jun 20 Jun 27 Jun 27 Nov 18 May 11 May 11 May 11 May 11 Jun 26 Jun 27 Jul 10 Feb 5 Feb 5 Nov 18
F501N F501N F501Nc F501N F664N F502N F501N F501N F501Nd F501N F501N F501N F486N F501N F664N F502N F664N F502N F501N F486N F501N F501N F501Nd F501N
F96 F96 F96 F96 PC6 PC6 F96 F96 F96 F96 F96 F96 F96 F96 PC6 PC6 PC6 PC6 F96 F96 F96 F96 F96 F96
1803 1996 996 997 300 300 997 1996 996 1997 1997 734 836 997 300 300 300 300 540 974 1996 1996 996 1778
283 188 188 326 336 335 283 246 246 316 290 290 275 221 240 240 239 238 292 272 347 357 358 239
271 294 294 178 573 574 235 167 167 254 237 167 186 277 221 221 245 242 237 259 281 173 172 247
1991 1991 1993 1991 1991 1992 1993 1993 1993 1993
Jul 9 Jul 9 Apr 28 Jul 9 Jul 9 Nov 25 Apr 26 Jan 9 Jul 10 Jul 6
F501N F486N F501N F501N F486N F501N F501N F501N F501N F501N
F96 F96 F96 F96 F96 F96 F96 F96 F96 F96
996 996 1497 996 921 997 1997 1997 1996 1996
218 205 306 343 329 312 320 286 306 335
240 257 242 220 236 262 257 267 246 312
SMC SMP 2 . . . . . . . . SMP 3e . . . . . . . SMP 5 . . . . . . . . SMP SMP SMP SMP SMP
10 . . . . . . 21 . . . . . . 22g . . . . . . 23 . . . . . . 28e . . . . . .
SMC-N 2 SMC-N 2 SMC-N 4 SMC-N 5 SMC-N 5 SMC-N 18 SMC-L 305 SMC-N 67 SMC-L 343 SMC-L 536
X0MZ0101T X0MZ0102T X14T0301T X0MZ0401T X0MZ0402T X14T0801T X1CT0101T X14T0201T X1CT0201T X1CT0301T
1266 1266 4075 1266 1266 4075 4821 4075 4821 4821
a X25H data set obtained after Ðrst HST servicing mission. b Exposure time, rounded to nearest second. c Used in conjunction with F2ND neutral density Ðlter. d Used in conjunction with F1ND neutral density Ðlter. e Previously observed by HST (Paper IV). f W0KU data set previously analyzed in Paper I. g Observed in GO sample (this paper).
near the unresolved PN targets. The readout noise and gain for chip six of the PC are 14.45 electrons and 8.18 electrons DN~1, respectively (Westphal et al. 1991, Table 10.2). Extensive modeling of the restoration properties of the RL method are discussed in Paper IV. For the GO PC data, three empirical PSFs are available. The three separate observations are scheduled at the beginning, middle, and end of the Cycle 3 program. Fortunately, the PN exposures were completed within a number of weeks, meaning that interorbit variations in the PSF are minimized. Any variation between the three PSFs is attributed to intraorbit focus variations (Hasan & Bely 1994), which do not critically a†ect the following analysis. Each PN image is restored with the Ðrst PSF observation. No empirical PSFs are available for the GTO PC data. The necessary PSFs are generated using the TinyTim PSF modeling software (Krist 1993) : each PN image requires an individual PSF, derived for a given date and chip location. The restored GO and GTO PC images are shown on the right in Figures 1 and 2, respectively.
3.2. FOC Data The uncalibrated raw *.D0H data are processed by STScI using the CALFOC task within STSDAS. All the FOC data presented here are taken in the standard unzoomed 512 ] 512 pixel mode. As is the case with the PC and the F502N Ðlter, the FOC and F501N Ðlter combination is efficient in excluding the majority of background stars from the Ðeld of view. The pre-COSTAR images are reduced to 384 ] 384 pixel, 8A. 5 ] 8A. 5 subimages. The reduction in frame size may seem trivial at Ðrst glance, but amounts to 40% less data that need to be subjected to the time-intensive restoration process. The calibrated raw FOC images are restored with the same RL algorithm employed for the PC images. As the FOC is not a CCD but a photon-counting device, the read noise is set to zero and the gain to 1 when the RL software is executed. Each restoration is limited to 100 RL iterations. The original and restored images are shown in Figure 2.
LMC-SMP7 W1ID0301T/2T
0.40’’
E N
LMC-SMP37 W1IDO401T
0.40’’
E
N
FIG. 1.ÈHST PC images of the GO sample taken through the F502N Ðlter. L eft : Raw calibrated images ; right : RL restored images. The orientation of the raw images is the same as that indicated for the restored images. Each frame is 80 ] 80 pixels, or 3A. 5 ] 3A. 5. All images are displayed on a logarithmic scale from the peak to a factor of 100 below.
256
LMC-SMP61 W1ID0601T
0.40’’
E
LMC-SMP67 W1ID0701T
N
0.40’’
E
FIG. 1.ÈContinued
257
N
LMC-SMP101 W1ID0801T/2T
0.40’’
E
SMC-SMP22 W1ID0201T
N
0.40’’
E N
FIG. 1.ÈContinued
258
0.40’’
LMC-SMP21 (N97) X14T0101T
E N
0.40’’
LMC-SMP23 (N24) X14T0501T
E N
FIG. 2.ÈAs Fig. 1, but for the GTO PC and FOC images. The FOC frames shown are 166 ] 166 pixels, or 3A. 7 ] 3A. 7.
259
0.40’’
LMC-SMP29 (WS9) W0KU0102T
E
N
0.40’’
LMC-SMP32 (N192) X14T0601T
N
FIG. 2.ÈContinued
260
E
0.40’’
LMC-SMP35 (WS12) X1CT0601T
N E
0.40’’
LMC-SMP45 (LM1-27) X1CT0401T
N E
FIG. 2.ÈContinued
261
0.40’’
LMC-SMP62 (N201) X0MZ0301T
N E
0.40’’
LMC-SMP66 (N52) X14T0701T
E N
FIG. 2.ÈContinued
262
0.40’’
LMC-SMP78 (WS33) W0KU0202T
N E
0.40’’
LMC-SMP83 (WS35) W0KU0302T
N E
FIG. 2.ÈContinued
263
0.40’’
LMC-SMP83 (N66) X0MZ0201T
N E
0.40’’
LMC-SMP83 (N66) X1CT0501T
N E
FIG. 2.ÈContinued
264
0.40’’
LMC-SMP97 (LM1-61) X14T0401T
E N
0.40’’
SMC-SMP2 (N2) X0MZ0101T
E
FIG. 2.ÈContinued
265
N
0.40’’
SMC-SMP3 (N4) X14T0301T
N E
0.40’’
SMC-SMP5 (N5) X0MZ0401T
E
FIG. 2.ÈContinued
266
N
0.40’’
SMC-SMP10 (N18) X14T0801T
E N
0.40’’
SMC-SMP21 (L305) X1CT0101T
N E
FIG. 2.ÈContinued
267
SMC-SMP22 (N67) X14T0201T
0.40’’
N
SMC-SMP23 (L343) X1CT0201T
E
0.40’’
E
FIG. 2.ÈContinued
N
MAGELLANIC CLOUD PLANETARY NEBULAE
269
0.40’’
SMC-SMP28 (L536) X1CT0301T
E
N
FIG. 2.ÈContinued
The original post-COSTAR images are shown separately in Figure 3 ; these images can be compared to the preCOSTAR observed and reconstructed frames in Figure 2. Most importantly, any structure that appears in the preCOSTAR reconstructed images for these three objects is conÐrmed by the post-COSTAR images. No empirical PSFs exist for the GTO FOC data. Again, PSFs are modeled with the TinyTim software at each date at which an exposure is made. In contrast to the PC and post-COSTAR FOC data, the pre-COSTAR FOC PSF is not Ðeld dependent. A comparison of the PC and FOC PSF surface brightness proÐles is shown in Figure 4. 4.
RESULTS
4.1. Aperture Photometry The lack of a speciÐc stopping criterion for the nonlinear RL method precludes the use of the restored images to derive the dimensions or surface brightness of each target. However, as is found from simulations in Paper IV, the nebular morphology and dimensions are reliably obtained for a limited number of RL iterations. Circular apertures are used to measure the signal within annuli that are 0.5 pixel wide. The annuli normally extend toward the edge of each subimage. Given the varying surface brightness and extended emission nature of the objects, absolute aperture photometry is neither accurate nor worthwhile to attempt with HST . Preliminary measurements of the six GO PC images yielded photometry to within 0.15 mag of the more precise groundbased measurements of Jacoby, Ciardullo, & Walker (1991 ; Vassiliadis et al. 1994). 4.2. Nebular Sizes As in Paper IV, two estimates of the outer nebular diameter are made from the aperture photometry measurements : D represents the diameter that contains 85% of the total 85 and D Ñux, represents the diameter at which the surface edge
brightness begins to fall o† like the PSF. As was observed in Paper IV, the PSF surface brightness proÐle follows a R~4 power law, where R represents the radius measured from the PSF core. The diameters are initially measured in pixels and then converted into arcseconds using the image scales quoted in ° 2. The adopted distances for the LMC and SMC are 50.6 and 58.3 kpc, respectively (Feast 1991). The corresponding image scales are 0.25 and 0.28 pc arcsec~1, respectively. Results for each object are listed in Table 3. 4.3. Nebular Dynamical Ages Nebular expansion velocities in the [O III] 500.7 nm emission line are available for the majority of the SMP objects (Dopita et al. 1985 and 1988 for the LMC and SMC objects, respectively). As discussed in Paper IV, the size and velocity information from the same ion gives a selfconsistent measure of the dynamical age. Expansion velocities V for the current sample are reproduced in Table 3. Theexpappropriate derivation of the dynamical age relies on knowing the nebular morphology. Each image is classiÐed into one of four groups : spherical (denoted by S), bipolar with a central condensation (denoted by Bc), bipolar ring (denoted by BR), and ring (denoted by R). The bipolar-ring types also require knowledge of the semiminor-to-semimajor axis ratio q and, in turn, the ring inclination angle h \ cos~1 q. The following formulae for determining the dynamical age are reproduced here from Paper IV. For S types,
A
B
3.0856 ] 1013 km pc~1 R , (1) \ neb 3.1557 ] 107 s yr~1 V exp where R is in pc, and V is in km s~1. For Bc, BR, and R neb V is believed exp to be the projection onto the types, where exp radial direction of the expansion velocity of the ring, q
dyn
q
dyn
\ (1 [ q2)1@2
A
B
3.0856 ] 1013 km pc~1 R neb . 3.1557 ] 107 s yr~1 V exp
(2)
270
VASSILIADIS ET AL.
LMC-SMP21 (N97) X25H0301T
Vol. 503
LMC-SMP32 (N192) X25H0201T
N E
E
0.40’’
N
0.40’’
LMC-SMP83 (N66) X25H0101T
N E
0.40’’
FIG. 3.ÈPost-COSTAR GTO FOC images. Only the observed frames are shown : there are no reconstructions. Each frame is 244 ] 244 pixels, or 3A. 5 ] 3A. 5.
For Bc, BR, and R types, where V is believed to be the exp expansion velocity of the bipolar outÑow,
A
B
3.0856 ] 1013 km pc~1 R , (3) q \ (1 [ q2)1@2 out dyn 3.1557 ] 107 s yr~1 qV exp where R is the radius of the bipolar lobes projected onto the sky. out Values for the inclination angle and dynamical age are listed in Table 3. Using the aperture photometry conducted in ° 4.1, the surface brightness proÐle for each object is shown in Figures 5 and 6 for the LMC and SMC objects, respectively. In both Ðgures the proÐles have been grouped according to the nebular morphological classiÐcation.
5.
DISCUSSION
5.1. Observed versus T heoretical Sizes The mean observed radius from the HST images, R is HST deÐned as D ]D edge . R \ 85 HST 4
(4)
The values of R in pc are listed in Table 4 and describe HST ionization zone. the size of the O`` Theoretical values for the size of the O`` zone are calculated in the original photoionization models of Dopita &
No. 1, 1998
MAGELLANIC CLOUD PLANETARY NEBULAE
The Ðrst method incorporates the following : Stellar evolutionary models of PN central stars describe the change of stellar temperature and luminosity with time (e.g., Vassiliadis & Wood 1994 ; Blocker 1995). In addition, the nebular expansion velocity is found to follow a tight correlation with the central star position in the HertzsprungRussell diagram (Dopita et al. 1987 ; Dopita 1993),
1 2 LOG NORMALISED SURFACE BRIGHTNESS
2
271
3 4
V \ ([128 ^ 4) exp
0
] (38 ^ 2)[log T
eff
-2
-4
-1.5
-1.0 -0.5 LOG RADIUS (ARCSEC)
0.0
0.5
FIG. 4.ÈAzimuthally averaged surface brightness proÐles for the PSFs. Solid line : A theoretical R~4 power law ; curve 1 : observed PC PSF ; curve 2 : TinyTim PC PSF ; curve 3 : TinyTim pre-COSTAR FOC PSF ; curve 4 : post-COSTAR FOC PSF.
Meatheringham (1991a, 1991b). These values have not been previously published : the radii quoted in Dopita & Meatheringham, R , refer to the outer radius for the entire out nebula. The observed and theoretical radii are shown in Figure 7. The adopted uncertainty in the HST values corresponds to the absolute range in radii, o D [ D o/2. SMC SMP 22 is the only85objectedge in the sample whose dimensions have been measured from the ground. Using speckle techniques, Wood et al. (1987) Ðnd SMC SMP 22 to be less than 0A. 6 in size, which is consistent with the HST determination of D . In contrast, D is approximately a 85 factor of 2 larger. edge The diameters of Ðve objects presented in Paper IV are redetermined in order to compare the di†erences in measuring technique. The D and D values for these objects edge from Paper IV are enclosed in 85 parentheses, and they are listed under the corresponding values determined in this study (Table 3). The use of circular apertures generally gives diameters up to D15%È20% larger than the elliptical apertures employed in Paper IV : LMC SMP 35 does not follow this trend. The derivation of D relies on the PSF surface brightness to fall o† as R~4.edge The PN surface brightness proÐles are shown in Figures 5 and 6. The steeper PN proÐles are associated with the R and BR morphological types. The shallower PN proÐles are associated with the more irregular and patchy objects, like LMC SMP 83. By taking the mean of the D and D measurements, the uncertainty 85 associated withedge each measurement method is minimized. 5.2. Dynamical versus Evolutionary Ages Two semiempirical methods are presented in Paper IV, which can compute the true, or evolutionary, age of a PN.
[ (0.25 ^ 0.05) log L /L ] . _
(5)
Therefore, V is known as a function of time and can be exp integrated to yield the nebular radius. Each central-star mass yields a speciÐc radius-age relationship, as is shown in Figure 12 of Paper IV. The central-star e†ective temperature and luminosity are calculated in the photoionization models of Dopita & Meatheringham (1991a, 1991b) : the values are reproduced in Table 4. These parameters are also used to infer the centralstar mass in each case by interpolating between the theoretical stellar evolutionary tracks of Vassiliadis & Wood (1994). Masses are estimated under both the H- or He-burning central star scenarios. The Ðrst estimate of the evolutionary age, denoted by q , R is measured from the radius-age relationships presented in Paper IV, using the nebular radii and central-star masses listed in Table 4. The second method for deriving the true nebular age is to correct the observed dynamical ages presented in Table 3 for acceleration of the nebular shell. The apparent dynamical age at any given time is computed from the radius-age relationship calculated earlier. The apparent expansion velocity is the gradient of the radius-age relationship, and divided by the radius, it yields the apparent dynamical age. The ratio of the evolutionary age to this age yields the desired correction factor to be applied to the observed dynamical ages listed in Table 3. Again, di†erent correction factorÈradius relationships exist for di†erent central-star masses. The corrected dynamical ages q are listed in Ev Table 4. Ideally, the true nebular ages q and q should agree R for either Ev with the central-star evolutionary ages H- or Heburning nuclei, q . For such a comparison to be valid, the HR AGB to log T is assumed to be instantransition from the taneous, and the initial expansioneffvelocity is taken as 5 km s~1 (see McCarthy et al. 1990). The evolutionary timescale of the central star from the AGB to the PN regime depends on the rate of depletion of the stellar envelope mass (Schonberner 1981), which is a function of the nuclear hydrogen-burning mass loss rate and the stellar wind mass loss rate. The precise moment when the AGB superwind mass loss rate ceases remains unknown, and therefore, the rate of evolution through the transition region is also unknown. There is no single observation that unambiguously quantiÐes the transition time from AGB star to PN as a function of the central-star mass. Hydrodynamic models that couple the central star and nebular evolution show the nebular acceleration to be highly variable, implying that simple expressions such as equations (1)È(3) for q are a poor nebular diagnostic dyn (Marten, GeÓsicki, & Szczerba 1993). However, these results depend on the input stellar evolution models during the AGB to PN central star transition, which remain uncertain
272
VASSILIADIS ET AL. TABLE 3 INFERRED TYPES, MORPHOLOGICAL CLASSES, DIAMETERS, INCLINATION ANGLES, AND DYNAMICAL AGES q
DIAMETER (pc) OBJECT
EXCITATION CLASS
SMP SMP SMP SMP SMP SMP
07 . . . . . . . 21 . . . . . . . 23 . . . . . . . 29 . . . . . . . 32 . . . . . . . 35b . . . . . .
8.6 7.6c 3.6 8.1 8.2 5.4
SMP SMP SMP SMP SMP SMP SMP SMP
37 . . . . . . . 45 . . . . . . . 61 . . . . . . . 62 . . . . . . . 66 . . . . . . . 67 . . . . . . . 78 . . . . . . . 83 . . . . . . .
6.8 5.4 2.9 5.9 5.6c 1.2 6.0 7.6c
SMP 97 . . . . . . . SMP 101 . . . . . .
TYPE
Type I ? Type ? Type I
Type ? Type I
8.0 8.3
MORPHOLOGICAL CLASSa
R BR S Bc R BR S BR S Bc BR Bc S BR S BR ?
D
85 LMC
0.395 0.149 0.115 0.129 0.210 0.329 (0.345) 0.105 0.389 0.114 0.127 0.172 0.164 0.094 0.547 0.530 0.549 0.228 0.410
D
edge
D
theory
dyn
(kyr)
V (kmexp s~1)
h (deg)
eq. (1)
eq. (2)
eq. (3)
0.177 0.154 0.173 0.166 0.215 0.462 (0.274) 0.078 0.462 0.114 0.137 0.240 0.159 0.138 0.507 0.641 0.607 0.254 0.324
0.212 ... 0.108 0.120 0.130 0.157
44.7 49.1d 21.6 35.9 42.3 41.3
45 50 ... 30 20 50
4.3 1.5 2.6 1.8 2.4 3.9
3.1 1.1 ... 0.9 0.8 3.0
1.9 1.8 ... 1.3 0.9 6.5
0.094 0.218 0.107 0.154 ... 0.088 0.157 0.219
39.2 36.8 29.3 34.6 23.1 27.9 33.4 82.9d
0.245 0.109
46.1d 47.3d
... 50 ... 60 45 45 ... 60 60 60 ... 30
1.3 5.2 1.9 1.8 3.6 2.9 1.4 3.2 3.1 3.2 2.4 4.2
... 4.0 ... 1.6 2.6 2.0 ... 2.8 2.7 2.8 ... 2.1
... 7.3 ... 3.4 5.1 2.8 ... 5.2 6.6 6.2 ... 1.9
0.090 (0.105) 0.189 0.144 (0.145) 0.122 0.144 (0.100) 0.200 0.050 0.151 0.200 0.104 (0.100)
0.066
15.4
...
2.3
...
...
0.202 0.162
32.2 32.9
60 50
1.7 2.0
1.5 1.5
5.0 2.6
0.142 0.062
29.2 21.9
0 ...
1.6 2.6
0.0 ...
0.0 ...
... 0.056 0.199 ... 0.165
24.1 35.2 50.9d 31.7 54.4
... ... 50 ... 65
3.6 1.0 3.2 2.2 0.8
... ... 2.4 ... 0.7
... ... 1.7 ... 2.0
SMC SMP 01b . . . . . .
1.1
SMP 02 . . . . . . . SMP 03b . . . . . .
6.3 4.9
SMP 05 . . . . . . . SMP 06b . . . . . .
6.6 3.8
SMP SMP SMP SMP SMP
3.5c 6.5 8.0 ... 7.3
10 . . . . . . . 21 . . . . . . . 22 . . . . . . . 23 . . . . . . . 28b . . . . . .
S Type ?
R BR R S
Type Type Type Type
I I ? I
S S BR S Bc
0.072 (0.068) 0.113 0.134 (0.122) 0.098 0.117 (0.086) 0.178 0.072 0.331 0.145 0.088 (0.075)
a Morphological classiÐcations : S \ spherical ; Bc \ bipolar with central condensation ; BR \ bipolar ring ; R \ ring. b Values in parentheses are from Paper IV. c From Monk, Barlow, & Clegg 1988. All other excitation class measurements from Meatheringham & Dopita 1991a, 1991b. d From multicomponent Ðt. For details, see Dopita et al. 1988 for LMC objects, and see Dopita et al. 1985 for SMC objects.
as a result of the unknown mass loss prescription (see Vassiliadis 1993 ; van Hoof, Oudmaijer, & Waters 1997). Measurements of [O III] 500.7 nm expansion velocities of Magellanic Cloud PNs indicate the nebular expansion to accelerate after departure from the AGB (Dopita et al. 1987 ; Dopita & Meatheringham 1990). The use of the outer nebular radius in equation (1) means that q represents the age since the onset of the last major massdynloss episode on the AGB and not the age since mass loss ceased. The ratio of inner to outer nebular radii is typically 0.3 (Phillips 1984), which means that the elapsed time since the cessation of the superwind mass loss phase is D0.3 times the time elapsed since the onset of the superwind on the AGB. 5.3. L MC versus SMC The LMC objects in this study indicate that D36% (4/11) of the classiÐed PN nuclei are He burners. The analysis in Paper IV indicated a higher ratio for the LMC : D65% (6/9), not counting the two objects listed as ““ H/He.ÏÏ If the LMC results are combined, and unclassiÐed objects are included, then D50% (13/27) are He burners, D25% are H burners, and D25% remain unclassiÐed.
Of the 11 SMC objects imaged, only six give any reliable indication of the central-star burning status, where the ratio of H to He burners is approximately 1 : 1. For the complete SMC sample, D30% (3/10) are He burners, D30% (3/10) are H burners, and D40% (4/10) remain unclassiÐed. Despite the small size of the SMC sample, the results indicate that He burners represent a nonnegligible proportion of PN central stars. This result is in contrast with the Galactic PN population, believed to be dominated by H burners, with the distribution of central-star masses peaking at D0.6 M (Schonberner 1981 ; Gorny, Stasinska, & Tylenda _ 1997). 5.4. Revision of Stellar Parameters The determination of burning status depends not so much on the adopted theoretical tracks for central-star evolution, but on the adopted central-star parameters. Apart from the observed dynamical age, which relies on the measured nebular radius and expansion velocity, the entire subsequent analysis relies on determining the central-star mass. These masses only follow from the adopted centralstar temperature and luminosity and from the theoretical
LMC
S
Bc
2
67 29 62
0
97
1 37 61 78 23
LOG NORMALISED SURFACE BRIGHTNESS
3
BR
R
3
2
66 21 101 35 83 45
7 32
1
0 -1.5
-1.0
-0.5
0.0
-1.5
-1.0
-0.5
0.0
LOG RADIUS (ARCSEC) FIG. 5.ÈAzimuthally averaged surface brightness proÐles for the LMC PNs, arranged according to morphological type (Table 3). Dashed line : An R~4 power law.
SMC
S
Bc
2
0
28
1 1 6 21 10 23 BR
R
3
2
0 -1.5
-1.0
-0.5
5 2
1 22 3
LOG NORMALISED SURFACE BRIGHTNESS
3
0.0
-1.5
LOG RADIUS (ARCSEC) FIG. 6.ÈAs Fig. 5, but for the SMC objects
-1.0
-0.5
0.0
MAGELLANIC CLOUD PLANETARY NEBULAE
275
TABLE 4 SEMIEMPIRICAL PN AGES COMPARED TO THEORY He-BURNING AGESa OBJECT
SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP
07 . . . . . . . 21 . . . . . . . 23 . . . . . . . 29 . . . . . . . 32 . . . . . . . 35b . . . . . . 37 . . . . . . . 45 . . . . . . . 61 . . . . . . . 62 . . . . . . . 66 . . . . . . . 67 . . . . . . . 78 . . . . . . . 83 . . . . . . . 97 . . . . . . . 101 . . . . . .
log T (K)eff
log (L /L ) _
R (pc)
MHe (MMS) _
q
5.352 ... 4.813 5.301 5.274 5.072 5.190 5.064 4.771 5.104 ... 4.663 5.114 5.230 5.243 5.204
3.415 ... 3.597 3.519 3.477 3.172 3.526 3.199 3.602 3.673 ... 3.288 3.781 3.459 3.778 3.477
0.143 0.076 0.072 0.074 0.106 0.198 0.046 0.213 0.057 0.066 0.103 0.081 0.058 0.282 0.120 0.183
3.5 ... 1.5 2.8 2.5 1.0 1.8 1.0 1.5 2.0 ... 1.0 2.3 1.9 2.5 1.8
\5.6 ... 4.2 \3.5 4.5 9.9 2.9 10.5 3.7 3.4 ... 5.2 3.0 10.2 5.0 7.6
R LMC
H-BURNING AGESa
q
q
HR
MH (MMS) _
2.4 1.8 3.6 1.8 1.2 8.4 1.8 9.4 2.6 4.7 5.1 3.7 1.9 7.6 3.1 2.5
8.0 ... 3.0 8.0 10.0 50.0 13.0 50.0 3.0 5.0 ... 13.0 5.0 12.0 7.0 13.0
2.8 ... 1.4 2.0 1.9 1.5 1.8 1.5 1.4 1.7 ... 1.0 1.8 1.8 2.0 1.8
3.8 ... 6.2 3.1 4.2 9.7 3.1 10.1 5.3 4.3 ... 10.1 3.5 9.2 4.1 6.7
2.2 1.8 4.3 2.2 1.5 10.4 2.2 12.5 3.1 5.6 5.1 4.4 2.3 8.7 3.9 3.0
1.4 ... 10.0 2.1 3.0 12.0 4.1 30.0 8.0 3.5 ... [15.0 2.0 4.1 2.1 4.0
3.2 1.8 3.6 2.2 3.6 ... 1.4 2.2 ... 2.8
2.5 7.0 9.0 20.0 2.5 ... 50.0 \8.0 ... 22.0
1.1 1.8 1.4 1.5 1.6 ... 1.4 2.2 ... 1.7
7.0 3.5 5.5 5.0 4.5 ... 4.0 3.0 ... 3.5
2.9 8.4 4.3 2.6 4.4 ... 1.5 2.5 ... 3.3
13.0 3.5 26.0 9.0 2.0 ... 30.0 5.0 ... 6.0
Ev
q
R
q
q
Ev
HR
PNN TYPE
H ... He H H H H H? He ? ? ... He ? ? He ? ?
SMC SMP SMP SMP SMP SMP SMP SMP SMP SMP SMP
01 . . . . . . . 02 . . . . . . . 03 . . . . . . . 05 . . . . . . . 06 . . . . . . . 10 . . . . . . . 21 . . . . . . . 22 . . . . . . . 23 . . . . . . . 28 . . . . . . .
4.613 5.114 4.982 5.167 4.881 ... 4.978 5.322 ... 5.182
3.467 3.679 3.384 3.444 3.775 ... 3.243 3.447 ... 3.284
0.040 0.075 0.069 0.055 0.065 0.094 0.031 0.121 0.086 0.048
1.2 2.0 1.0 1.5 2.0 ... 1.0 3.0 ... 1.5
3.0 3.5 4.5 3.5 3.5 ... 2.5 4.5 ... 3.5
He H? He H He ... ? ? ... H
a All ages in units of 103 yr. b See Paper IV also.
stellar evolution tracks. The parameters used in this study and in Paper IV rely on photoionization modeling of the optical spectrophotometry of individual nebulae, calculated, prior to the availability of UV HST spectrophotometry and resolved HST narrowband images. Arguably, photoionization modeling yields the most reliable Ðgures for the stellar parameters of PN central stars, and the inclusion of as much input as is available aids in constraining the Ðnal models. The central-star parameters for the sample in Paper IV have been rederived using MAPPINGS 2 (Dopita et al. 1997, hereafter Paper V). The mean di†erence between the rederived and previous results for the e†ective temperature is [0.033 dex. The maximum di†erence is [0.131 dex for LMC SMP 20. Similarly, the mean di†erence in log (L /L ) _ is 0.07 dex. The maximum di†erence is ]0.218 dex for LMC SMP 02. As expected, the di†erences in the luminosities vary more, conÐrming that the HST observations apply much-needed constraints to the photoionization models. Photoionization modeling for the current sample will be presented in a forthcoming paper. 5.5. Objects of Note 5.5.1. SMC SMP 22
SMC SMP 22 possesses the hottest central star, log T \ 5.34, and highest N/O abundance ratio, D1.2, of eff in the SMC (Dopita & Meatheringham 1991a). The all PNs ground-based optical spectrophotometry indicates a wide
range of ionization stages, from [O I] to Ne V, and a very high electron temperature, T \ 26,600 K from the [O III] e & Dopita 1991a). emission lines (Meatheringham SMC SMP 22 is the only X-rayÈbright source identiÐed with an extragalactic PN. The observed spectrum is extremely soft, 0.16È3.5 keV, and is believed to be consistent with the emission from the surface of a single white dwarfÈ like star (Wang 1991). The energetics implied by the X-ray emission, high V and low q , indicate that the central exp dynBased on the central temstar must be relatively massive. perature and luminosity listed in Table 4, the central-star mass is D0.7 M , implying a progenitor of D3 M . _ of SMC SMP 22 shows a ring of_ material The PC image viewed at a low inclination angle with respect to the observer. The data are not sufficiently deep to indicate any bipolar lobes perpendicular to the ring plane. As remarked in Meatheringham & Dopita (1991a, 1991b), the spectrophotometric properties of SMC SMP 22 are similar to those measured for SMC SMP 28, which was imaged with the PC in Cycle 1 (Paper IV). Both objects have high values of V and low values of q . Curiously, in contrast to SMC exp22, SMC SMP 28 appears dyn SMP spherically symmetric with no evidence of any ring structure. 5.5.2. L MC SMP 83
The central star of the PN LMC SMP 83 was observed to undergo large variations in stellar temperature and luminosity during the past decade (Pen8 a et al. 1997 and references therein). These variations were serendipitously observed with HST also, and the same images presented in
276
VASSILIADIS ET AL.
Vol. 503
temperature and luminosity derived before the central star exhibited the bulk of the changes (Paper I). Objects such as LMC SMP 83 are rare, and we do not expect that the inclusion of such objects will nullify the obvious dominence of He burners.
0.35 0.30
6.
(pc) R HST
0.25
Our conclusions are the following :
0.20
0.15
0.10
0.05 0.00 0.00
CONCLUSIONS
0.05 R
0.10 theory
0.15 (pc)
0.20
FIG. 7.ÈComparison of the observed and theoretical nebular radii of the O`` ionization zone. Solid line : 1 :1 locus. The observed nebular radii are averages of the data in Table 3. Error bars : range in observed values for a given object. Solid and open symbols : LMC and SMC samples, respectively.
this study for this object were examined by Vassiliadis (1996). No variations were recorded in the nebular [O III] emission. Despite the obvious central-star changes that took place, the observed nebular age will always be the same in each of the four images. The central-star mass is based on a stellar
1. The nebular morphology is reliably determined from observations using the planetary camera of WFPC1 and the FOC. For three objects, LMC SMP 21, LMC SMP 32, and LMC SMP 83, the pre-COSTAR reconstructions agree with the post-COSTAR observations. 2. Subarcsecond nebular dimensions are measured using the encircled energy and the PN brightness proÐle. 3. The nebular ages are consistently younger than the predicted central-star evolutionary ages by at least a factor of 2. 4. A signiÐcant proportion of the LMC PN central stars (50%) are He burners, in contrast to the Galaxy, where H burners dominate. This result supports that obtained in Paper IV. 5. Likewise, the data indicate that 30%È50% of the PN nuclei in the SMC are He burners. However, this estimate is only based on six objects. The result suggests there is little distinction between the LMC and SMC with regard to PN evolution. The statistics for the LMC will be improved with the addition of 15 objects observed during Cycle 6. Based on observations with the NASA/ESA Hubble Space T elescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Astrometry and Ðnding charts were obtained using the Guide Stars Selection System Astrometric Support Program (GASP) developed at the Space Telescope Science Institute. Support for this work was provided by NASA through grant GO-2266 from the Space Telescope Science Institute and by the Instituto de Astrof• sica de Canarias, Spain.
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