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M. Bobrowsky
CTA INCORPORATED, 6116 Executive Blvd., #800, Rockville, MD
20852
and shows a partially filled circular
shell with an angular radius of 
and a thickness of
. There is also a halo extending out
from the center. At an estimated distance of 4kpc,
the radius of the shell is 0.033pc. Hu 2-1 was imaged using two
filters---H
and [O III] 5007Å. Hu 2-1 displays an equatorial
ring with bubbles of gas above and below the ring. The dimensions of
the ring are 
which, if
actually circular, implies a tilt of 
. The bubbles extend
from the central star. A halo, brighter in the
equatorial plane, extends out several arc-seconds further. The
morphologies of He 2-1 and M 1-58 are quite different. M 1-58 appears
rather amorphous, circularly symmetric, and filamentary. Hu 2-1, on
the other hand, is quite asymmetric, and bears a striking resemblance
to He 3-1357---another compact PN that was spatially resolved in the
same snapshot survey.
Keywords: Planetary Nebulae, Circumstellar Matter
A number of compact planetary nebulae (PNe) were imaged as part of a snapshot survey with the Hubble Space Telescope (HST) Planetary Camera. Two of them, which are structurally very dissimilar, are described herein. One of them, Hu 2-1, had been considered to be a young PN based on its high density and low expansion velocity (Maciel & Pottasch 1980, Martin 1981, Sabbadin et al. 1984). Also, its central star is located in the H-R diagram at the beginning of the evolutionary tracks of PNe (Martin 1981). The other PN, M 1-58, has a more spherically symmetric and more fragmentary structure. It is probably not as young as Hu 2-1 and has a larger outer halo.
Two exposures of M 1-58 were made in April 1993, using the Planetary
Camera with the H
(F487N) filter. The exposure time was 350 s
for each image and the guiding mode was coarse track. Four exposures
of Hu 2-1 were made in April and May 1993, also with the Planetary
Camera. Two exposures were made with the H
filter and two with
the [O III] 5007Å filter (F502N). The guiding mode for the Hu 2-1
exposures was fine lock and the exposure times were 180s for H
and 140s for [O III]. The CCD chip used was PC6, with an image scale
of 
pixel
. The routine scientific data
processing included bias, preflash, dark, and flat-field corrections.
After removing bad pixels caused by cosmic rays, both images from the
same filter were aligned and averaged. Most of the effects from the
spherical aberration were removed using 100 iterations of the
Richardson-Lucy deconvolution algorithm.
The images of Hu 2-1 in H
and 5007Å are morphologically
similar. Consequently, a composite image, including
emission from both the H
(F487N) and [O III] 5007Å
(F502N) filters, is presented in Figure 1.
Figure: A composite (H
and [O III] 5007Å)
image of the planetary nebula Hu 2-1
Several components are visible. There is the central star; a toroidal structure (hereafter, the ring) close to the star; a roughly spherical nebula with a radius more than twice the semi-major axis of the ring; additional emission extending much farther, also in the direction of the ring's major axis; and an even more extended halo, not visible in this image, but known from previous, lower-resolution observations (Miranda, et al. 1995).
The small ring, close to the central star, has a major axis
long and a position angle (PA) of 
.
Since the minor axis has an angular extent of 
, the
polar axis is tilted 
with respect to the line of
sight if the toroidal structure is actually circular. Using this ring
to define an equatorial plane, the polar direction would then have a PA
of 
. This differs from the definition of a polar axis by
Miranda et al. (1995), who gave a polar axis PA of 
.
This may be because they were not able to resolve the small ring and
took the more easily seen extended emission in the equatorial plane to
be bipolar lobes. Without the high-resolution images, this would have
been a reasonable interpretation in view of the collimated appearance
of the extended emission.
Beyond the inner ring, the nebula is brightest approximately 
from
the central star. This nebulosity is similar to the bubbles of gas
seen above and below an equatorial ring in another, but not quite so
young planetary nebula, He 3-1357 (Bobrowsky 1994). The equatorial ring in
He 3-1357 is four times the size of the ring in Hu 2-1. Assuming a
distance of 2.35kpc (Maciel & Pottasch 1980, Martin 1981), the linear
size of the ring in Hu 2-1 is 0.004pc (800AU). Then, if the ring is
expanding at 19kms-1 (Miranda 1995), the kinematical age of the ring
is only 200yr! This is one-tenth of the expansion age estimated for
He 3-1357 (Bobrowsky 1994).
Based on the HST images, the flux ratio F(5007Å)/F(H
) is 5.4, and
log F(H
) = 
, corrected for reddening. This is somewhat higher
than previously published H
fluxes. For example, Barker (1978)
gave log F(H
) = 
. An increase in H
flux is not
surprising for such a young object, which is expected to become more
ionized with time. Similar rapid evolution was observed with He 3-1357
(Parthasarathy et al. 1993, Bobrowsky 1994).
The axisymmetric structure of Hu 2-1 may be the result of the interaction of a binary companion with the primary's envelope during common envelope evolution (Livio & Soker 1988, Taam & Bodenheimer 1989). The moderate density contrast between the equatorial and polar regions may indicate that the progenitor had evolved to a late AGB stage before encountering the common envelope phase (Bond & Livio 1990). While there are other possible origins for the bipolar structure of Hu 2-1, the binary star origin finds some support in the photometric variability that has been observed for the last two decades (Arkhipova et al. 1994). The ubv magnitudes of Hu 2-1 showed weak irregular variations of 0.2--0.3mag. The cause of these variations is not yet understood.
An H
image of M 1-58 is shown in Figure 2. It appears as a
Figure: An H
(filter F487N) image of the planetary nebula M 1-58
filamentary, but fairly symmetric, thick shell. It is brightest at a
radius of 
and has a thickness of about
, with a fainter halo extending out several
arc-seconds from the center. At an estimated distance of 4.0kpc
(Cahn et al. 1992), the radius of the brightest part of the shell
is 0.033pc.
Based on the HST observation, the H
flux of M 1-58, corrected for
reddening, is 
erg cm
s
.
From the ratio of He II 4686 to H
, M 1-58
was determined (Kostyakova 1977) to be optically
thin according to the Seaton criterion I (Seaton 1960) of
I(4686)/I(H
) < 1.22. For M 1-58, I(4686)/I(H
) = 0.56.
A surface brightness plot (Fig. 3) shows that M 1-58 would be
Figure: A plot of the H
surface brightness of M 1-58
classified as an ``attached-halo multiple-shell PN,'' using the terminology of Stanghellini & Pasquali (1995). In this respect it is quite similar to NGC 3242.
It is frequently conjectured that many symmetric PNe are actually bipolar PNe seen pole-on. However, M 1-58 is sufficiently different from a PN like Hu 2-1 that it seems quite unlikely that M 1-58 is simply an Hu 2-1 seen pole-on. If Hu 2-1 were seen pole-on, it would look less fibrous and more structured than M 1-58. Apparently, these two PNe experienced significantly different evolutionary histories.
These observations were made with the NASA/ESA Hubble Space Telescope and obtained at the Space Telescope Science Institute which is operated AURA, Inc., under NASA contract NAS5-26555. Support for this work was provided by NASA through grant number GO-3603.01-91A from the Space Telescope Science Institute.
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