In the region of Sgr B2 there are several condensations heated externally by nearby hot stars .
Therefore H _ { 2 } O far–IR lines are expected to probe only an external low–density and high temperature section of these condensations , whereas millimeter-wave lines can penetrate deeper into them where the density is higher and T _ { k } lower .
We have conducted a study combining H _ { 2 } O lines in both spectral regions .
First , Infrared Space Observatory observations of several H _ { 2 } O thermal lines seen in absorption toward Sgr B2 ( M ) at a spectral resolution of \sim 35 km s ^ { -1 } have been analyzed .
Second , an IRAM –30m telescope map of the para–H _ { 2 } O 3 _ { 13 } -2 _ { 20 } line at 183.31 GHz , seen in emission , has also been obtained and analyzed .
The H _ { 2 } O lines seen in absorption are optically thick and are formed in the outermost gas of the condensations in front of the far–IR continuum sources .
They probe a maximum visual extinction of \sim 5 to 10 mag .
Radiative transfer models indicate that these lines are quite insensitive to temperature and gas density , and that IR photons from the dust play a dominant role in the excitation of the involved H _ { 2 } O rotational levels .
In order to get the physical conditions of the absorbing gas we have also analyzed the CO emission toward Sgr B2 ( M ) .
We conclude , based on the observed CO J =7–6 line at 806.65 GHz with the Caltech Submillimeter Observatory , and the lack of emission from the far–IR CO lines , that the gas density has to be lower than \sim 10 ^ { 4 } cm ^ { -3 } .
Using the values obtained for the kinetic temperature and gas density from OH , CO , and other molecular species , we derive a water column density of ( 9 \pm 3 ) \times 10 ^ { 16 } cm ^ { -2 } in the absorbing gas .
Hence , the water vapor abundance in this region , \chi ( H _ { 2 } O ) , is \simeq ( 1-2 ) \times 10 ^ { -5 } .
The relatively low H _ { 2 } O/OH abundance ratio in the region , \simeq 2-4 , is a signature of UV photon dominated surface layers traced by far–IR observations .
As a consequence the temperature of the absorbing gas is high , T _ { K } \simeq 300-500 K , which allows very efficient neutral–neutral reactions producing H _ { 2 } O and OH .
On the other hand , the 183.31 GHz data provide a much better spatial and spectral resolution than the far-IR ISO data .
This maser line allows to trace water deeper into the cloud , i.e. , the inner , denser ( n ( H _ { 2 } ) \geq 10 ^ { 5 - 6 } cm ^ { -3 } ) and colder ( T _ { k } \sim 40 K ) gas .
The emission is very strong toward the cores .
The derived water vapor abundance for this component is a few \times 10 ^ { -7 } .
There is also moderate extended emission around Sgr B2 main condensations , a fact that supports the water vapor abundance derived from far–IR H _ { 2 } O lines for the outer gas .