4 Comparison of the ALMA and Herschel - ESO

degrees. The GLIMPSE survey is being carried out with the Spitzer satellite (Uzpen et
al., 2005). There is also a plan to map a 100 square degree region with SPIRE and PACS.
Clearly, ALMA will be used to follow up such surveys, but it is unlikely that ALMA would
be used in a survey mode for mapping large areas in the broadband continuum. There are
no comparable programmes in the ALMA DRSP.
6.9 Evolved Stars
6.9.1 Spectral Lines
In the late stages of stellar evolution, lower mass stars go through the asymptotic giant
branch, AGB, on their way to becoming a planetary nebula. The AGB stars have extensive
envelopes which contain molecules (eg, Olofsson, 2005). The nearest AGB star with such
an envelope is R Leonis, also known as IRC+10216 (see Fig. 17). Such envelopes are close
to being spherically symmetric, so are more easily modelled. There have been extensive
line surveys of this source, but further examples should be studied to determine molecular,
atomic and isotopic abundances. We show an example of a few images of different molecular
lines in IRC+10216 in Fig. 17. Clearly the spatial distribution of CS or SiS is very different
from CN or SiC 2
. To determine the chemical development of the envelope, one needs to
have the abundance and spatial distribution of many such species.
After the AGB phase, the star passes though a short-lived proto-planetary nebula phase while
evolving to a planetary nebula (PN). The proto-PNs have intense UV fields and outflows;
two of the most studied are CRL 618 and 2688 (eg, Millar & Woods, 2005). With Herschel,
one can obtain accurate beam-averaged abundances of gas phase species and dust, but high
resolution images made with ALMA are needed to determine the actual abundances since
the source sizes are of order 10” or less.
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