4 Comparison of the ALMA and Herschel - ESO

Figure 12: Data for IC10. The integrated CII emission is shown as contours. The shaded region is the
sampled area (Madden et al., 1997). The rectangles show the field of view of the SPIRE array and the
PACS array of Herschel. The smallest rectangle shows the size of the spectrometer.
6.6 Star Formation and Molecular Clouds
6.6.1 Protostars
The study of the star formation process is one of the primary science goals of ALMA. In Fig.
13 we show a sketch of the four stages of star formation, from the collapse of a molecular
cloud to the formation of a star surrounded by a disk. For column densities, N >10 18 cm -2
and densities n > 10 2 cm -3 , interstellar gas consists of molecular hydrogen, H 2
, and helium.
The H 2
molecule will not produce emission lines if kinetic temperatures are below ~100K
and there are no shock waves. Then the abundances of the H 2
molecules must be traced
indirectly. Cloud collapse requires high interstellar gas densities and low kinetic temperatures.
Under such conditions, grain properties change and the constituents of the gas will
condense onto grains. With the broadband data from SPIRE and PACS one can accurately
determine the total luminosity and with the spectrometers, measure fine structure lines of
atomic species. With Herschel HIFI it will be possible to take spectra without absorption
in the Earth’s atmosphere. This is especially useful for H 2
O. The higher angular resolution
ALMA images will help to refine the analysis of models based on Herschel data. The final
result will be the distribution of H 2
(which has no spectral lines at low temperatures), of
selected atoms, a number of molecules and dust, as well as the dynamics.
Herschel HIFI can measure spectra with a velocity resolution of better than 0.1 kms -1 , while
ALMA can measure spectra with a velocity resolution exceeding 0.01 kms -1 . Measurements
with such a fine velocity resolution are important since models show that the collapse velocities
of low mass protostars are of order 0.1 kms -1 .
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