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Probing the Mass and Structure of the Ring Nebula in Lyra with SOFIA/GREAT Observations of the [CII] 158 micron line | | R. Sahai
; M. R. Morris
; M. W. Werner
; R. Güsten
; H. Wiesemeyer
; G. Sandell
; | | Date: |
28 Mar 2012 | | Abstract: | We have obtained new velocity-resolved spectra of the [CII] 158 micron line
towards the Ring Nebula in Lyra (NGC 6720), one of the best-studied planetary
nebulae, in order to probe its controversial 3-dimensional structure and to
estimate the mass of circumstellar material in this object. We used the
Terahertz receiver GREAT aboard the SOFIA airborne telescope to obtain the
[CII] spectra at eight locations within and outside the bright optical ring of
NGC 6720. Emission was detected at all positions except for the most distant
position along the nebula’s minor axis, and generally covers a broad velocity
range, ~50 km/s (FWZI), except at a position along the major axis located just
outside the optical ring, where it is significantly narrower (~25 km/s). The
one narrow spectrum appears to be probing circumstellar material lying outside
the main nebular shell that has not been accelerated by past fast wind episodes
from the central star, and therefore most likely comes from equatorial and/or
low-latitude regions of this bipolar nebula. Along lines-of-sight passing
within about 10 arcsec of the nebular center, the CII column density is a
factor 46 higher than the CO column density. The total mass of gas associated
with the [CII] emission inside a circular region of diameter 87.5 arcsec is at
least 0.11 solar masses. A significant amount of [CII] flux arises from a
photodissociation region immediately outside the bright optical ring, where we
find a CII to CO ratio of 6.5, lower than that seen towards the central region.
Comparing our data with lower-quality CI spectra, which indicate similarly
large CI/CO ratios in NGC 6720, we conclude that the bulk of elemental carbon
in NGC 6720 is divided roughly equally between CII and CI, and that the
emissions from these species are far more robust tracers of circumstellar
material than CO in this object and other evolved planetary nebulae. | | Source: | arXiv, 1203.6373 | | Services: | Forum | Review | PDF | Favorites | |
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