Slitless Spectroscopy of LMC Planetary Nebulae. A Study of the Emission Lines and Morphology

Abstract

HST STIS slitless spectroscopy of LMC PNs is the ideal tool to study their morphology and their ionization structures at once. We present the results from a group of 29 PNs that have been spatially resolved, for the first time, in all the major optical lines. Images in the light of \ha, \nii, and \oiii are presented, together with line intensities, measured from the extracted 1D and 2D spectra. A study on the surface brightness in the different optical lines, the electron densities, the ionized masses, the excitation classes, and the extinction follows, illustrating an ideal consistence with the previous results found by us on LMC PNs. In particular, we find the surface brightness decline with the photometric radius to be the same in most emission lines. We find that asymmetric PNs form a well defined cooling sequence in the excitation -- surface brightness plane, confirming their different origin, and larger progenitor mass. Comment: 12 Figures Figure 3 in 7 parts (3a through 3g). Astrophysical Journal, in press

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arXiv:astro-ph/0204158v1 9 Apr 2002
Slitless Spectroscopy of LMC Planetary Nebulae. A Study of the
Emission Lines and Morphology.1
Letizia Stanghellini2
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218,
USA; lstanghe@stsci.edu
Richard A. Shaw
National Optical Astronomy Observatory, 950 N. Cherry Av., Tucson, AZ 85719, USA;
shaw@noao.edu
Max Mutchler
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218,
USA; mutchler@stsci.edu
Stacy Palen, Bruce Balick
Astronomy Department, Box 351580, University of Washington, Seattle WA 98195-1580;
palen@astro.washington.edu, balick@astro.washington.edu
and
J. Chris Blades
Space Telescope Science Institute; blades@stsci.edu
Received
;accepted
2Affiliated with the Astrophysics Division, Space Science Department of ESA; on leave
from INAF-Osservatorio Astronomico di Bologna

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ABSTRACT
HST STIS slitless spectroscopy of LMC PNs is the ideal tool to study their
morphology and their ionization structures at once. We present the results from
a group of 29 PNs that have been spatially resolved, for the first time, in all the
major optical lines. Images in the light of Hα , [Nii] , and [Oiii] are presented,
together with line intensities, measured from the extracted 1D and 2D spectra. A
study on the surface brightness in the different optical lines, the electron densities,
the ionized masses, the excitation classes, and the extinction follows, illustrating
an ideal consistence with the previous results found by us on LMC PNs. In
particular, we find the surface brightness decline with the photometric radius to
be the same in most emission lines. We find that asymmetric PNs form a well
defined cooling sequence in the excitation – surface brightness plane, confirming
their different origin, and larger progenitor mass.
Subject headings: Stars: AGB and post-AGB — stars: evolution — planetary
nebulae: general — Magellanic Clouds
1. Introduction
Planetary Nebulae (PNs) have been used extensively to gain insight into the late
stages of evolution of low and intermediate mass stars. In particular, the observed nebular
morphology reflects the geometry of the initial mass ejection during and shortly after the
1Based on observations made 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 NAS 5–26555

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AGB phase of stellar evolution followed by the cumulative dynamical effects of winds,
heating by ultraviolet photons, plus the sudden increase in pressure from the passage
of an ionization front. It is now widely accepted that PNs are the ejecta of ∼ 1 to 8
M⊙ progenitor stars, expelled at the end of the thermally pulsating, Mira–like phase on
the AGB. These ejecta travel at a low velocity, and remove most of the stellar envelope
eventually exposing a white dwarf core.
Most PNs show some degree of asymmetry. Classical round PNs, which might represent
isotropic mass loss, are the minority. Balick (1987) defined several morphological classes
based on the outline of the nebular core, such as elliptical and butterfly. Balick argued
that a large fraction of PNs have a relatively dense equatorial waistband. Prolate elliptical,
bipolar, and closely related nebular geometries develop as a consequence. Manchado et
al. (1996a) use the interior symmetry to define classes. The major morphological classes
defined by Manchado et al. (2000) are round (R), elliptical (E), bipolar or quadrupolar (B),
and pointsymmetric (P) planetary nebulae. Bipolar core (BC) PNs, defined by Stanghellini
et al. (1999), are those planetary nebulae characterized by a bi-nebulosity in the core, wich
may have lobes below detectability.
Bipolar PNs appear to be associated with particularly dense tori or other types of
collimation “nozzles” that form as a normal part of their evolution (Balick 1987; Icke,
Preston & Balick 1989). Interestingly, Galactic bipolar PNs are located preferentially in the
plane—i.e., their scale height (z) distribution is similar to that of Pop I stars initially more
massive than about 2 M⊙. Other less extreme morphological types of PNs have a larger
scale height and evolve from less massive progenitors (Stanghellini, Corradi, & Schwarz
1993; Manchado et al. 2000).
Bipolar and other extremely axisymmetric PNs also tend to be enriched in N and
O, but relatively depleted in C/O as found, e.g., by Peimbert (1978); Torres-Peimbert

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& Peimbert (1997). These results provide important insight into the shortcomings of
present models for AGB envelope ejection (Frank 2000). Peimbert (1978); Torres-Peimbert
& Peimbert (1997) suggested that convective dredging operates differently in stars of
different initial masses, as expected from the models of stellar evolution (van den Hoek &
Groenewegen 1997).
Therefore Galactic PNs suggest that initial stellar mass, and, perhaps, local chemical
composition determine the outcome of PN abundances and morphologies. The trends found
in the Galactic PNs are supported by a sizable collection of data; nonetheless, they suffer
two key impediments for this type of study: namely, the long-standing uncertainties in the
distances of Galactic PNs and the selection effects due to absorption by interstellar dust.
One important aspect of morphological studies is the dependence of PN morphology on
the emission line in which they are imaged. It has been demonstrated that, while generally
the Hα and [Oiii] morphologies nicely represent the overall nebular volume, low-excitation
lines such as [Nii] emphasize some particularly interesting small-scale morphological
features, such as organized ensembles of low-ionization knots and point symmetry and
quadrupolarity (Stanghellini, Corradi, & Schwarz 1993; Manchado, Stanghellini, & Guerrero
1996b). A series of narrow-band images of PNs is the ideal database to start answering
some of the current questions on the morphology formation mechanisms, and to study
morphological types in detail.
Statistical studies of the properties of PN morphologies, masses, luminosities, ionization,
and chemical abundances in which selection bias is minimized and uncertainties in distances
are eliminated can be pursued in the Magellanic Clouds. However, any comparison of the
morphological and stellar properties of Galactic and MC PNs require the spatial resolution
of the HST in order to resolve the nebulae and to obtain accurate photometry of the
central stars in very crowded fields. Using HST, MC PNs can be observed with a physical

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resolution that is comparable to ground-based observations of typical Galactic PNs.
Since 1999 we have embarked in a large study of MC PN morphologies, evolution,
central stars, and progenitors. At this time, several HST programs that aim at collecting
a very large dataset of STIS slitless spectroscopy and broad band photometry of LMC
and SMC PNs have been completed or are active, and other programs are scheduled to be
executed in Cycle 10.
This paper presents data from the Cycle 8 HST snapshot survey of LMC PNs (program
8271), consisting of a survey of 29 LMC PNs, acquired in broad band imaging and slitless
spectroscopy with STIS. The broad band images of these PNs have been already published
by Shaw et al. (2001) (hereafter Paper I), with an extensive discussion on PN morphology
and evolution for the sample of program 8271 and of other HST archived images of MC PNs.
Here we present a study of the monochromatic images of the LMC PN sample, obtained by
using STIS slitless spectroscopy. After calibration, the slitless images are functionally the
same as calibrated narrow-band filter images since Doppler smearing is insignificant. We
discuss the data acquisition strategy and analysis procedures in §2. In §3 we present the
images, the integrated spectral line intensities, and surface brightness estimates. Some of
the surface brightness results are unexpectedly correlated, so we discuss the data trends
and their significance. Finally, in §4, we give a summary of our work so far, and a glance to
the future of our extensive Magellanic Cloud Planetary Nebulae project.
2. Data calibration and analysis
2.1. Slitless strategy
The observations presented in this paper are from HST GO program 8271, using the
Space Telescope Imaging Spectrograph. See Paper I for the observation log, observing