4 Comparison of the ALMA and Herschel - ESO

In DRSP contribution 2.1.2, Kinetics, Density and Temperature Profi le in Pre-Stellar Cores, it is
planned to measure the J = 2-1 transitions in carbon monoxide, CO, and isotopes, and the J = 1-0
transition of H 2
D + , as well as broadband continuum. Measurements of the H 2
D + molecule are
interesting since this appears not to deplete onto grains at high H 2
densities. The sources are 160
pc from the Sun, so the 1” angular resolution corresponds to 2.3 x 10 15 cm, or 160 Astronomical
Units. In addition to interferometer data, results from the ACA and single telescopes are required.
The peak line temperatures will be ~3 K for C 18 O, 5 K for 13 CO and 0.1 to 1 K for H 2
D + . For
the optically thin line of C 18 O, over a wide range of densities and kinetic temperatures, the rms
noise in a spectral channel gives a limit to the H 2
column density of 4 x 10 21 cm -2 . The rms noise
in continuum measurements is 5 mJy beam -1 . This RMS noise gives an H 2
column density of
5 x 10 20 cm -2 if the dust temperature is 10 K. Since the J = 1-0 transition of H 2
D + , is absorbed in
the atmosphere of the Earth, 100 hours are needed for these measurements, while 25 hours are
needed for measurements of the CO transitions.
For both DRSP programmes, the absolute calibration accuracy should be better than 10% for
continuum and spectral line measurements. The repeatability should be 3% and the relative
calibration between receiver bands 3%.
Figure 14: The lower lying energy levels of the water molecule. On the left are the levels for ortho-
H 2
O, on the right for para-H 2
O. The energy levels connected by vertical lines are the transitions to be
measured with Herschel HIFI. A few transitions have been measured from the ground. The strong maser
line at 22 GHz from ortho-H 2
O is the most widely observed. The 183 GHz line of para-H 2
O will be in
Band 5 of ALMA (provided by J. Cernicharo).
35