32
C. Cecchi-Pestellini
The number of complex species observed in a FU Orionis object (V883 Ori, [23]),
with abundances comparable to those in comet 67P/Churyumov-Gerasimenko [20]
demonstrates that ice chemistry in discs does proceeds similarly as in hot-cores. FU
Orionis events can be described as an abrupt mass transfer from the accretion disc
onto a young, low mass T-Tauri star. Thermal instabilities initiate an outburst. The
duration of the outburst is determined by viscosity within this hot, ionized area. The
rise time of these eruptions is typically of the order of 1 year, but can be much longer.
This high-accretion, high-luminosity phase has typical lifetimes of decades. Such a
sudden increases in the luminosity of the central star quickly expands the snow lines
into the disc, creating a “sublimation front”. The natural conclusion is that normal
condition, desorption mechanisms fail in supplying complex species to the gas.
All these considerations suggest that icy mantles onto dust grains are the repository of chemical complexity in the early stages of a disc lifetime. While their ice
mantles are chemically evolving, dust grains grow through collisional agglomeration. This process ends up in the formation of loosely packed structures with much
of dust aggregate internal volume being vacuum and trapped ices. Interstitial voids
must occur even in highly organized, densely packed structures. For example, dense
packing of spheres in a face-centred cubic lattice leaves 26% of space unoccupied. In
2000, Walt Duley pointed out that in the interiors of dust aggregates, those internal
voids produced by the accretion process offers the possibility of a radical change in
the chemistry [24], i.e. the re-accretion of reaction products onto other components
of the aggregate. As desorbed products can be in an energetic state, these secondary
reactions might mimic some aspects of high-temperature chemistry. Dust aggregates
can also be impulsively heated by cosmic ray impacts, during which cosmic rays suffer negligible energy losses. During the interaction, sputtering may also occur, with
atoms of the grain materials (such as Si, Ca, Mg, Fe, and P) dislodged and ejected
from an inner or outer surface. The heat released during the collision may lead to
the partial or total vaporization of the ice filling the cavities. As a consequence, part
of the chemical species forming the ice enter a transient, warm, high-pressure gas
phase, together with sputtered atoms from the grain substrate, in a hydrogen-rich
atmosphere. Thus, the cavities within the grain aggregate bring together all the components of gas and dust, a unique situation outside planetary systems. The resulting
scenario is a reasonable analogue of the conditions that Stanley Miller envisaged as
plausible for the primitive Earth atmosphere in its famous experiment at the University of Chicago [25]. Therefore, grain aggregates may represent the equivalent in
space of terrestrial micro-laboratories containing raw materials of reducing chemical
composition suitable for conversion into complex organic species. The final products
are likely to be very similar to those obtained from laboratory chemistry under terrestrial conditions. Because of the reducing atmosphere in the cavities large organic
molecules are allowed to form, with the inclusion of sputtered atoms from the grain
substrate.
C. Cecchi-Pestellini
The number of complex species observed in a FU Orionis object (V883 Ori, [23]),
with abundances comparable to those in comet 67P/Churyumov-Gerasimenko [20]
demonstrates that ice chemistry in discs does proceeds similarly as in hot-cores. FU
Orionis events can be described as an abrupt mass transfer from the accretion disc
onto a young, low mass T-Tauri star. Thermal instabilities initiate an outburst. The
duration of the outburst is determined by viscosity within this hot, ionized area. The
rise time of these eruptions is typically of the order of 1 year, but can be much longer.
This high-accretion, high-luminosity phase has typical lifetimes of decades. Such a
sudden increases in the luminosity of the central star quickly expands the snow lines
into the disc, creating a “sublimation front”. The natural conclusion is that normal
condition, desorption mechanisms fail in supplying complex species to the gas.
All these considerations suggest that icy mantles onto dust grains are the repository of chemical complexity in the early stages of a disc lifetime. While their ice
mantles are chemically evolving, dust grains grow through collisional agglomeration. This process ends up in the formation of loosely packed structures with much
of dust aggregate internal volume being vacuum and trapped ices. Interstitial voids
must occur even in highly organized, densely packed structures. For example, dense
packing of spheres in a face-centred cubic lattice leaves 26% of space unoccupied. In
2000, Walt Duley pointed out that in the interiors of dust aggregates, those internal
voids produced by the accretion process offers the possibility of a radical change in
the chemistry [24], i.e. the re-accretion of reaction products onto other components
of the aggregate. As desorbed products can be in an energetic state, these secondary
reactions might mimic some aspects of high-temperature chemistry. Dust aggregates
can also be impulsively heated by cosmic ray impacts, during which cosmic rays suffer negligible energy losses. During the interaction, sputtering may also occur, with
atoms of the grain materials (such as Si, Ca, Mg, Fe, and P) dislodged and ejected
from an inner or outer surface. The heat released during the collision may lead to
the partial or total vaporization of the ice filling the cavities. As a consequence, part
of the chemical species forming the ice enter a transient, warm, high-pressure gas
phase, together with sputtered atoms from the grain substrate, in a hydrogen-rich
atmosphere. Thus, the cavities within the grain aggregate bring together all the components of gas and dust, a unique situation outside planetary systems. The resulting
scenario is a reasonable analogue of the conditions that Stanley Miller envisaged as
plausible for the primitive Earth atmosphere in its famous experiment at the University of Chicago [25]. Therefore, grain aggregates may represent the equivalent in
space of terrestrial micro-laboratories containing raw materials of reducing chemical
composition suitable for conversion into complex organic species. The final products
are likely to be very similar to those obtained from laboratory chemistry under terrestrial conditions. Because of the reducing atmosphere in the cavities large organic
molecules are allowed to form, with the inclusion of sputtered atoms from the grain
substrate.