Multiple Collimated Molecular Flows in the Young Planetary Nebula NGC 7027

1
Multiple Collimated Molecular Flows in the Young
Planetary Nebula NGC 7027
Zhen-Yuan Huanga, Dinh-Van-Trunga, Tatsuhiko
Hasegawaa, Sun Kwoka,b, Aran Lyoa,  Sebastien
Mullera, Naomi Hiranoa, Jeremy Lima, Muthu
Mariappanc a. ASIAA b. University of Hong Kong
c. Aryabhatta Research Institute of Observational
Sciences
Abstract The young planetary nebula NGC 7027 was
observed in HCO (3-2) and HCN (3-2) with SMA.
HCO envelope is consistent with the
photodissociated walls which had been discussed
widely in previous H2 results. Owing to several
extruding components which are independent to the
nebula, we know that active ejecting events have
been engraving the walls for a period. Two close
bipolar outflows identified in HCO and HCN are
the high-velocity wings in the early observed
spectra. SMA observations of CO(2-1) and CO(3-2)
show another bipolar outflow which has an
opposite tilted direction. It could be an
evidence for precessing and collimated flows in
this transient stage.
Introduction Observations NGC 7027 is a
planetary nebula stepping out the stage of
asymptotic giant branch (AGB) with plentiful line
and continuum emission at all wavelength. The
central star has temperature T? 1980,000 K
(Latter et al. 2000). In its carbon-rich
envelope, the C/O ratio of 3 engenders a series
of carbon-based molecules, including CO, HCO and
HCN, which are less studied spatially before. It
has been known that CO emission extended about
60 in diameter (Jaminet 1991) and there is an
equatorial torus with possible outflows at the
north-to-south axis (Bieging 1991 Graham 1993).
New H2 images (Latter 2000 Cox 2002) showed a
biconical symmetric structure and several
point-symmetry bipolar flows. The thin shell
where H2 exist is generally known as
characteristics of photodissociation regions
(PDRs Hollenbach Tielens 1999 Latter 2000
Cox 2002). To study the molecular kinematics
around the PDR, we carried out the observations
of HCO(3-2) and HCN(3-2) with SMA compact
configuration on July 12th 15th, 2005. Besides,
early SMA observations of CO(2-1) and CO(3-2)
with higher spatial resolution are compared here.
Figure 1 - HST image of NGC 7027. Red is H2
emission at 2.12µm, white is HII region, and blue
represents dust scattered visible light.
Results
Figure 2 - The color image is the integrated
intensity of HCO(3-2) line emission in the
central velocity range of 16-34 km/s. The white
contours show H2 emission excited by UV radiation
(Cox et al. 2002). The HCO emission is at the
same layer between the central ionized regions
and outer extended molecular clouds. It confirms
the expectation of chemistry reactions in the PDR
(Hasegawa et al. 2000). However, this thin shell
did not appear in HCN (3-2) maps since the
extended and weaker HCN emission is resolved out.
Figure 3 - The position-velocity diagrams of
HCO(3-2) along major axis and minor axis at the
upper and bottom panel, respectively. The zero
velocity is related to systemic velocity of 25
km/s. The upper plot indicates that north-west
tilted towards the observer and the south-east
tilted away. This result is consistent with what
have been drawn in the Br ? and H2 observations
(Cox et al. 2002). The bottom plot marks the
equatorial ring which maintains an original
expanding velocity of 15 km/s. The opening bulges
correspond to the bipolar flows at the
high-velocity wings. Presumably that the nebula
is expanding radically, the ratio of the extreme
velocities in the PV diagrams along the major and
minor axis gives an inclination angle of 64
degree.
Figure 4 - Upper two figures show HCO(3-2) and
HCN(3-2) line emission of fast components
(VLSRlt10km/s and VLSRgt43km/s). Two bipolar flows
are resolved with spatial resolution about 3
arcsec. The blue and red contours represent the
flow with an outflow velocity of 21 km/s. The
green and orange contours represent the other
flow with an outflow velocity of 18 km/s. These
two outflows are symmetric with respect to
systemic velocity and are inside of PDR on the
sky screen. The bottom-left figure shows another
bipolar outflow found in CO(2-1) and CO(3-2) with
an outflow velocity of 19 km/s and 17.5 km/s,
respectively. The red and blue contours show the
CO(2-1) flow the orange and green contours show
the CO(3-2) flow. Note that this outflow at the
outer envelope tilted away from us. Because of
the projection effect, the extruding flows could
be inside or outside the central cavity. The
cartoon illustrates what we have known of NGC
7027 outflows from SMA observations. It seems
that NGC 7027 has experienced several highly
collimated mass-loss events since AGB period in
addition to the main expansion of the whole
nebula. It is likely that bipolar outflows have
procession and result in current diverse
morphology.
References Bieging J. H. 1991, ApJ, 379, 271
Cox, P. J. et al. 2002, AA, 384, 603 Graham, J.
R. 1993, AJ, 105, 250 Hasegawa, T. I. et al.
2000, ApJ, 532, 944 Hollenbach Tielens
1999 Jaminet, P. A., 1991, ApJ, 380, 461 Latter,
W. B. et al. 2000, ApJ, 539, 783

• Summary
• The thin layer of HCO (3-2) emission is
  consistent with H2 walls.
• Two close bipolar outflows are identified in
  HCO(3-2) and HCN(3-2). Another outflow tilted
  away from the observer is confirmed in CO(2-1)
  and CO(3-2).
• NGC 7027 appears to be in a special epoch for
  experiencing multiple collimated mass loss.