4 Comparison of the ALMA and Herschel - ESO

Figure 17: Images of emission from IRC+10216 from the species identified in the upper left corner of
each panel. The contours are the integrated intensities. IRC+10216 is a solar mass star on the Asymptotic
Giant Branch (AGB). Images from Lucas et al., 1995.
The high spatial resolution, large FOV and good sensitivity of Herschel will lead to great
progress in our understanding of the mass loss process in evolved stars, the time evolution
of the mass loss rate, how it influences the sha ping of the surrounding nebula, properties
of the dust formed and the formation site of the dust. Spectra of AGB stars and PNe are
ideal tracers of solid and molecular species that form in the cool circumstellar envelope.
Most of the astronomical solid state features are found in the near and mid-infrared range.
However, dust species like Forsterite Mg 2
SiO 4
at 69 µm (a sensitive dust thermometer),
Calicite (CaCO 3
) at 92.6 µm, crystalline water-ice at 61 µm, and other dust species and ices
will be measured by Herschel in our galaxy and other nearby galaxies. In addition in some
circumstellar envelopes, powerful H 2
O and SiO masers are present. These will be easily
detected with ALMA. ALMA can image the SiO quasi-thermal millimetre line emission
in the outflows of the stars. With distance from the star, the SiO molecule first freezes out
onto dust grains as the temperature drops; further still, SiO is photodissociated by the interstellar
UV-radiation field. Measurements of SiO, when combined with CO line emission
will be used to follow the envelope evolution (gas expansion and acceleration) within 20-30
stellar radii. These data will provide a database of dust formation and evolved-star physics
that will be used as a template to study the spectra of high-redshift galaxies that will be
targeted with ALMA.
40