4 Comparison of the ALMA and Herschel - ESO

DRSP 4.1.3, Chemistry in the atmospheres of Venus and Mars, is a proposal to search for and then
image mm/sub-mm spectral lines of molecules such as SO 2
, H 2
S, HCl, 16 O 18 O, H 2
CO, H 2
O 2
and
NO. This will require 92 hours, and should be repeated two years later for the purpose of monitoring.
There is an additional DRSP, 4.1.2, to study HDO from Mars. In DRSP 4.1.4, it is proposed
to study the stratospheres of the giant planets in lines of CO, HCN, CS, HC 3
N and HDO. There
would also be blind searches; in all, this would require 154 hours. In DRSP contribution 4.1.8,
it is proposed to measure the atmospheres of Triton and Pluto in spectral lines, and DRSP 4.2.1
proposes to study the albedos sizes and surface properties of transneptunian objects.
6.9.4 Modelling and Analysis of the Data
The data sets that will be produced by ALMA and Herschel will be so large that the analysis
and comparisons with models will have to be streamlined and automated as much as possible.
Various groups around the world have started to address this issue and substantial
progress is being made.
For molecular clouds, chemical abundances and physical parameters have been derived
traditionally using simple tools, such as rotation diagrams assuming a constant excitation
temperature T ex
or statistical equilibrium calculations using an escape probability approximation
model for a single temperature, T, and density, n. In both cases, comparison with a
“standard” molecule such as CO is needed to derive the abundance with respect to H 2
. It is
well-known that this method can lead to abundances that are in error by more than an order
of magnitude. For example, the use of n = 10 5 versus 10 6 cm -3 for the interpretation of the
HCN J = 4-3 line with a critical density, n*, of ~10 7 cm -3 gives a factor of 10 difference in
derived HCN abundance. Densities are known to range from 10 4 to >10 7 cm -3 across starforming
regions. Also the neglect of beam dilution and optical depth effects can result in
large errors. For example, the emission from the complex organic molecules may originate
in a region of only 1”, much smaller than the observational single-dish beam of >15” which
applies to the more extended CO.
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