APEX SV projects

The properties of swept-up molecular shells in planetary nebulae

Coordinator: L. Ake-Nyman and P. Bergman

Data:
Program is available and data products can be downloaded

Background

One of the most exciting challenges facing theories of post-main sequence 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 appear largely spherically symmetric whereas many Protoplanetary Nebulae (PPNe) and PNe show bipolar outflows with high-velocity winds (having velocities of hundreds of km/s). It was long believed that this transition is caused by the interaction of a spherically symmetric high velocity wind (that starts to operate in the last stages of the AGB phase) with an equatorially dense AGB circumstellar envelope, causing the swept-up shell to expand faster in the polar directions than in the equatorial region (Kwok 1982). The problem with this model is that observations of AGB envelopes normally show no density enhancements. Observations with the HST show many bipolar PNe surrounded by concentric shells, hence Sahai and Trauger (1998) proposed that the bipolarity can be caused by a high speed collimated jet that starts to operate in the last stages of the AGB, or in the early PPNe stage, which interacts with a spherically symmetric AGB envelope. The collimated outflow can be caused by e.g. a binary companion.
CO emission has been detected toward many PPNe and PNe. In the PNe the CO molecule seems to survive for some time in dense clumps that protects it from being photodissociated by the ionizing radiation field from the central star. In some PNe, the CO emission sometimes outlines expanding, dense torii or shells that are being swept-up by a high-velocity ionized wind, and it traces the geometry and kinematics of the shell. However, very little modelling has been done to study the geometry and kinematics of these shells.

Proposal

We have developed a program that models the CO emission from expanding shells with different geometry and kinematics. With SEST we have mapped a set of PNe in CO(1-0) and CO(2-1) as well as their 13CO isotopes. We now propose to continue the observations with APEX in the CO(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 accompanying figures. NGC6563 has an elliptical, hollow shell, whereas NGC6072 is more toroidal (there are also some lines from molecular clouds in the line 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 (from elliptical in NGC6563 to toroidal in NGC6072), and in that case how do the properties of the shell change with time.
The APEX observations will help us to determine the temperature and 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 data will be valuable to determine the column densities and thus the total mass of the shells more accurately.
Preliminary observations show that the CO(3-2) line strength observed with APEX is about factor of 5 stronger than the CO(2-1) line strength observed with SEST, which means that we also will obtain better S/N ratio compared to the SEST data, even for integration times of 60s/position. The APEX beam is also somewhat narrower than the SEST beam at these frequencies (18" compared to 23"). This will help us to make 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 an integration time of 30s ON source for NGC6072 and 60s for NGC 6563 and a separation of 11". The observations will be done in position switching with an OFF position 2 arcmin in +RA from the center position.
The integrations have to be subdivided in integrations of 15s because of baseline instabilities so an integration of 30s ON source will take 15 (ON) + 15 (OFF) times 2 times a factor 1.6 in overhead which is 96 seconds. We need to calibrate every 10 minutes (or 5 positions) 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

References

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