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Multiple Collimated Molecular Flows in the Young Planetary Nebula NGC 7027

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Multiple Collimated Molecular Flows in the Young Planetary Nebula NGC 7027. Zhen-Yuan Huang a , Dinh-Van-Trung a , Tatsuhiko Hasegawa a , Sun Kwok a,b , Aran Lyo a ,  Sebastien Muller a , Naomi Hirano a , Jeremy Lim a , Muthu Mariappan c

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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 γ 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 (VLSR<10km/s and VLSR>43km/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, A&A, 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.
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