The properties of swept-up molecular shells in planetary nebulae

Coordinator: L. Ake-Nyman and P. Bergman

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One of the most exciting challenges facing theories of post-mainsequence evolution today is to understand how Asymptotic Giant Branch(AGB) red giants transform themselves into aspherical Planetary Nebulae(PNe). The extended circumstellar envelopes of most AGB stars appearlargely spherically symmetric whereas many Protoplanetary Nebulae(PPNe) and PNe show bipolar outflows with high-velocity winds (havingvelocities of hundreds of km/s). It was long believed that thistransition is caused by the interaction of a spherically symmetric highvelocity wind (that starts to operate in the last stages of the AGBphase) with an equatorially dense AGB circumstellar envelope, causingthe swept-up shell to expand faster in the polar directions than in theequatorial region (Kwok 1982). The problem with this model is thatobservations of AGB envelopes normally show no density enhancements.Observations with the HST show many bipolar PNe surrounded byconcentric shells, hence Sahai and Trauger (1998) proposed that thebipolarity can be caused by a high speed collimated jet that starts tooperate in the last stages of the AGB, or in the early PPNe stage,which interacts with a spherically symmetric AGB envelope. Thecollimated outflow can be caused by e.g. a binary companion.
CO emission has been detected toward many PPNe and PNe. In the PNethe CO molecule seems to survive for some time in dense clumps thatprotects it from being photodissociated by the ionizing radiation fieldfrom the central star. In some PNe, the CO emission sometimes outlinesexpanding, dense torii or shells that are being swept-up by ahigh-velocity ionized wind, and it traces the geometry and kinematicsof the shell. However, very little modelling has been done to study thegeometry and kinematics of these shells.

We have developed a program that models the CO emission fromexpanding shells with different geometry and kinematics. With SEST wehave mapped a set of PNe in CO(1-0) and CO(2-1) as well as their 13COisotopes. We now propose to continue the observations with APEX in theCO(3-2) and 13CO(3-2) lines towards two of them; NGC 6072 and NGC 6563.The CO(2-1) maps of these sources are shown in the accompanyingfigures. NGC6563 has an elliptical, hollow shell, whereas NGC6072 ismore toroidal (there are also some lines from molecular clouds in theline of sight towards NGC6072). The goals of the observations are:
1.To determine the geometry and the kinematics of the shells.
2.To determine the temperature and density distribution of the shells.
3.To determine the expansion time scales.
4.To estimate the molecular mass of the shells.
The results of the modelling will address the following questions:
1.Are these shells formed by interacting winds?
2.What are the physical properties of the shells (geometry, kinematics, expansion time scales, mass, fragmentation)?
3.Do these objects fit into an evolutionary sequence (fromelliptical in NGC6563 to toroidal in NGC6072), and in that case how dothe properties of the shell change with time.
The APEX observations will help us to determine the temperatureand mass distribution in the shells since we are sampling hotter,denser gas in the CO(3-2) lines compared to the (2-1) and (1-0) lines.Since the main isotopic data may be affected by opacity, the 13CO datawill be valuable to determine the column densities and thus the totalmass of the shells more accurately.
Preliminary observations show that the CO(3-2) line strengthobserved with APEX is about factor of 5 stronger than the CO(2-1) linestrength observed with SEST, which means that we also will obtainbetter S/N ratio compared to the SEST data, even for integration timesof 60s/position. The APEX beam is also somewhat narrower than the SESTbeam at these frequencies (18" compared to 23"). This will help us tomake maps of higher accuracy in terms of line shape and resolution.

Time estimate
In CO (3-2) we intend to observe 49 positions/source with anintegration time of 30s ON source for NGC6072 and 60s for NGC 6563 anda separation of 11". The observations will be done in positionswitching with an OFF position 2 arcmin in +RA from the centerposition.
The integrations have to be subdivided in integrations of 15sbecause of baseline instabilities so an integration of 30s ON sourcewill take 15 (ON) + 15 (OFF) times 2 times a factor 1.6 in overheadwhich is 96 seconds. We need to calibrate every 10 minutes (or 5positions) and a calibration takes about 1 minute.
Total time for NGC 6072: 49 positions times 96 (seconds) + 10 calibrations times 1 minute = 88 minutes.
Total time for NGC6563: 49 positions times 96 times 2 (seconds) + 20 calibrations times 1 minute= 176 minutes
We intend to spend the same time for the 13CO(3-2) observations with longer integrations times, but fewer positions.
We therefore need in total 88+88+176+176 minutes (9h) + 1h for pointing and focus observations, thus 10h in total.

Source list
Name RA(2000) DEC(2000) V_LSR
NGC6563 18 12 02.75 -33 52 07.1 -25
NGC6072 16 12 58.08 -36 13 46.1 +15

Kwok S., 1982, ApJ 258, 280
Sahai R, Trauger J., 1998, AJ 116, 1357