Laboratory studies on Temperature-Programmed Desorption analyzes of prebiotic molecules in space

Through laboratory analyses it is possible to study the physical and chemical processes involving prebiotic molecules, the building blocks of life. Today thanks to the advent of new generation of (sub-) millimeter and centimeter interferometers (ALMA, NOEMA, ngVLA), an increasing number of interstellar complex organic molecules (iCOMs) is observed in star forming regions, pre-stellar dense cores, hot corinos, jets and outflow ardound protostars [1, 2, 3]. Hot corinos are inner compact (<100 au) and hot (>100 K) regions of some protostars [4].  In these high-temperature regions, thermal desorption is the physical process responsible for the sublimation of frozen mantles into the gas phase and so for the presence of a rich chemistry [5] in the gas phase. On the other hand, iCOMs are difficult to observe in protoplanetary disks, where, for a solar-type star, the region where the temperature reaches the values for the desorption and release into the gaseous phase of water and iCOMs (water snow line) is too close to the star (≤5 au). This region is difficult to solve but a new perspective is provided by objects such as the FU Ori systems in which the young central star undergoes a sudden increase in brightness which leads to heating of the disk and quick expansion of the snow lines to large radii. This phenomenon has been observed in the protoplanetary disk around the protostar V883 Ori [6]. Thanks to the increase in temperature of the disk and the consequent thermal desorption of the molecules, five iCOMs were recently detected: methanol, acetone, acetonitrile, acetaldehyde, and methyl formate [7]. Moreover, outbursting young stars are good new targets for looking for organic complex molecules that thermally desorb from icy mantles. The interpretation of observations can benefit from laboratory activities, where it is possible to simulate the thermal desorption process and UV irradiation of complex molecules under simulated space conditions. Furthermore, laboratory studies on thermal desorption are fundamental to constrain parameters such as the thermal desorption temperature of a given molecule and its fragments, and the binding energies involved.

 

Here we reported the results that we have recently published about temperature-programmed desorption (TPD) analysis of pure formamide (HCONH₂) ice and in the presence of TiO₂ dust, before and after UV irradiation. We found that pure formamide desorbed at 220 K in high vacuum regime and after UV irradiation it fragmented mainly into NH₂, HCO and CH₂NO. These fragments are more volatile and desorbed before formamide (~180 K) at the same desorption temperature of water. The presence of water was due to residual deposition in the high vacuum chamber. It is reasonable to think, therefore, that the sublimation of water ice was responsible for releasing more volatile species. The same phenomenon probably occurs in the space where water is the most abundant molecule. Going forward with our investigation, we reproduced the condensation, irradiation, and desorption experiments with a substrate of TiO₂ dust. In presence of grains, we observed evidence for a change in desorption temperature. Formamide desorption from TiO₂ dust occurred at higher temperatures, that is, around 30 K above the temperature at which desorption takes place for pure formamide. A higher desorption temperature is direct evidence of the interactions described by the Van der Waals forces that were occurring between the molecule and the grains. The molecule interacts and diffuses into the grains and this is confirmed by the values of the binding energy that we found. When formamide desorbed directly from the cold finger of the cryostat (copper chromate surface), the binding energy found was (5.9 ± 0.3)·10³ K; while when it desorbed from TiO₂ dust, the binding energy found was (1.35 ± 0.08)·10⁴ K, a value two times higher [8]. Therefore, in the chemical models of sublimation, it is essential to take into account physisorption of iCOMs on grain surfaces and their diffusion to correctly describe the desorption process, that is, to constrain desorption temperatures and binding energies [8]. Furthermore, our experiments show something more than the desorption temperature and the binding energy of formamide.  The molecular fragments observed in laboratory (NH₂, HCO and CH₂NO) can be used to indirectly measure the presence of formamide through detection of relative abundance (e.g., [NH₂]/[HCO]∼4).

We reported also preliminary results on laboratory studies about thermal desorption of ice mixtures of acetaldehyde and acetonitrile from crystalline olivine grains.

These studies offer support to observational data and improve our understanding of the role of the grain surface in enriching the chemistry in space.

 

References

[1]Beltrán, M. T. et al. 2009, The Astrophysical Journal Letters, 690


[2] Rivilla, V. M. et al. 2017, Astronomy & Astrophysics, 598


[3] Codella, C. et al. 2015, Mon. Not. R. Astron. Soc., 449


[4]Ceccarelli, C. et al. 1999, Astronomy and Astrophysics, 342

[5] López-Sepulcre, A. et al. 2015, MNRAS, 449

[6] Cieza, L. A. et al. 2016, Nature, 535, Issue 7611

[7] Lee, J. et al. 2019, Nature Astronomy,3, 314

[8] Corazzi, M. A. et al. 2020, A&A, 636, A63