Photo-processing and thermal desorption of astrophysical ice mixtures on olivine grains: TPD and mass spectra analyses

An ever-increasing number of interstellar organic complex molecules, iCOMs, are continuously detected along the formation process of a Sun-like star thanks to millimeter and centimeter observations. Large interferometers in the (sub)mm range, such as IRAM-NOEMA and ALMA, showed the presence of iCOMs from the early stages of star formation such as pre-stellar dense cores [1], to protoplanetary disks [2], the place where planets form. 
 The discovery of new complex molecules in different space environments leads us to ask how organic chemistry works in space. To interpret the distribution and abundance of iCOMs observed along the formation process of a Sun-like star, it is fundamental to understand their formation mechanisms, the chemical transformations they undergo, the processes responsible for their release in the gas phase such as the thermal desorption process, and how the solid phase interactions between iCOMs and grain surfaces influence the desorption and the subsequent gas phase presence of molecular species. 

 In laboratory, it is possible to simulate interstellar ices analogs formed by iCOMs mixed with grains, process them through UV irradiation, simulate their thermal desorption through Temperature Programmed Desorption (TPD) experiments, study their evolution, the photolysis processes they undergo, and the formation of new species through mass spectra analyses. 
 We will show our published laboratory results on TPD experiments of astrophysical relevant ice mixtures of water, acetonitrile, and acetaldehyde from olivine grains used as interstellar dust analogs on which the icy mixtures were condensed at 17 K, showing how the interactions between the molecules and the surface of grains can modify the thermal desorption process and their release in gas phase [3]. Moreover, the ice mixtures were subjected to in situ UV irradiation to study both photolysis processes and the formation of new molecules [4].

Experimental method: We assembled an ultra-high vacuum (UHV) chamber (P∼ 6.68 · 10^-10 mbar) with feedthroughs for gas-phase deposition from a prechamber (P ∼ 10^-7 mbar), where the ice mixtures were prepared controlling the partial pressures. The UHV chamber interfaces with a Quadrupole Mass Spectrometer for mass spectrometry, with an ARS closed-cycle helium cryocooler able to get a temperature of 11 K, and with a 300 W UV-enhanced Newport Xenon lamp to UV irradiation.

Results:  We found that in the presence of grains, only a fraction of acetaldehyde and acetonitrile desorbs at about 100 K and 120 K respectively, while 40% of the molecules are retained by grains of the order of 100 μm up to 80 K higher temperatures. In protoplanetary disks, submicrometric interstellar grains begin to agglomerate into fluffy aggregates of hundreds of microns. Our results show that in protoplanetary regions with temperatures higher than 100 K, where we expect to no longer have iCOMs in the solid phase, a fraction of these molecules can instead survive on the grains. The presence of the grains can allow the delivery of molecules in the innermost part of the disks, in the Earth-like planets forming region broadening the snow lines of O- and N-bearing molecules. The snow lines should therefore be thought of as “snow regions”.

 Moreover, we found that UV irradiation and olivine are efficient in producing new species possibly deriving from photodissociation, recombination, isomerization, and hydrogen addition reactions. Among these, formamide, urea, glycolaldehyde, and ethanolamine are worthy of mention for their role as prebiotic building blocks.

 Through laboratory studies, it is possible to improve our understanding of the chemical-physical interactions between molecules and the surface of grains, a process that can significantly affect the presence of molecular species both in the gas phase and in the small planets on short orbits providing an estimate of the fraction of molecules released at various temperatures. Through mass analysis, it is possible to study both photolysis processes and the formation of new molecules. These studies offer the necessary support to the observational data and may help our understanding of the formation and origin of iCOMs.

References:

[1] Bacmann A. et al (2012) A&A, 541, id.L12.

[2] Walsh C. et al. (2016) ApJL, 823.   

[3] Corazzi M. A. et al (2021) ApJ, 913.    

[4] Corazzi M. A. (2002) submitted on ApJ.