between 5 K and 50 K (Bonev et al. 2007). Conversion between the two forms in the
gas phase by radiative transitions or by collisions is strictly forbidden (BockeléeMorvan et al. 2004a) with a radiative transition rate calculated to be 6 10
À14 yr.
À1
(Pachucki and Komasa 2008). The OPR is thus believed to remain constant throughout the coma. The slow rate of conversion of OPR is consequently thought to form a
probe of the temperature of the particular species at the time of its formation. On the
other hand, Fillion et al. (2012) have noted that the interpretation of the spin
temperatures of molecules formed or condensed on grains is perhaps premature,
because very little is known concerning the relaxation of spin isomers in cold solids,
on surfaces of astrophysical interest, and after thermal/non-thermal desorption.
Recent theoretical work by Chapovsky (2019) now suggests that lifetimes are shorter
than the lifetime of the Solar System. Nuclear spin conversion also appears to be
more efficient when there are molecular impurities, for example.
If the interpretation is correct, however, the results may be extremely useful. The
observations by Bonev et al. (2007) at 2.9 μm in the infrared at a resolution of around
25,000 (λ/Δλ) suggest differences in OPR between comets from 1.8 to 3 and
formation temperatures between 20 and 40 K. Repeated measurements of 103P/
Hartley 2 at two different apparitions gave the same value of OPR within error
(Bonev et al. 2013) suggesting no variation in OPR with depth in the nucleus.
Other species of interest can exhibit ortho-para states including NH 3 and CH 4 .
Shinnaka et al. (2011), for example, determined OPR for NH 3 based on studies of the
dissociation product, NH 2 , for 15 comets and concluded that formation temperatures
around 30 K were consistent with the observations. CH 4 has been studied by
Kawakita et al. (2005) from which the spin temperature was derived to be
33 K. These results are significant as they may be an indicator of the internal
temperature of the nucleus—a value which is needed to compute the thermal balance
(Sect. 2.9.3) and describe possible amorphous to crystalline transitions (Sect. 2.9.
3.4).
280
3 Gas Emissions Near the Nucleus
gas phase by radiative transitions or by collisions is strictly forbidden (BockeléeMorvan et al. 2004a) with a radiative transition rate calculated to be 6 10
À14 yr.
À1
(Pachucki and Komasa 2008). The OPR is thus believed to remain constant throughout the coma. The slow rate of conversion of OPR is consequently thought to form a
probe of the temperature of the particular species at the time of its formation. On the
other hand, Fillion et al. (2012) have noted that the interpretation of the spin
temperatures of molecules formed or condensed on grains is perhaps premature,
because very little is known concerning the relaxation of spin isomers in cold solids,
on surfaces of astrophysical interest, and after thermal/non-thermal desorption.
Recent theoretical work by Chapovsky (2019) now suggests that lifetimes are shorter
than the lifetime of the Solar System. Nuclear spin conversion also appears to be
more efficient when there are molecular impurities, for example.
If the interpretation is correct, however, the results may be extremely useful. The
observations by Bonev et al. (2007) at 2.9 μm in the infrared at a resolution of around
25,000 (λ/Δλ) suggest differences in OPR between comets from 1.8 to 3 and
formation temperatures between 20 and 40 K. Repeated measurements of 103P/
Hartley 2 at two different apparitions gave the same value of OPR within error
(Bonev et al. 2013) suggesting no variation in OPR with depth in the nucleus.
Other species of interest can exhibit ortho-para states including NH 3 and CH 4 .
Shinnaka et al. (2011), for example, determined OPR for NH 3 based on studies of the
dissociation product, NH 2 , for 15 comets and concluded that formation temperatures
around 30 K were consistent with the observations. CH 4 has been studied by
Kawakita et al. (2005) from which the spin temperature was derived to be
33 K. These results are significant as they may be an indicator of the internal
temperature of the nucleus—a value which is needed to compute the thermal balance
(Sect. 2.9.3) and describe possible amorphous to crystalline transitions (Sect. 2.9.
3.4).
280
3 Gas Emissions Near the Nucleus