Chemical Simulations For The Complex Organic Molecules In Star Forming Regions

As an important method to research the origin,evolution and distribution of differ-ent molecules in the universe,numerical simulations of astrochemistry are widely used.The chemical evolution of star forming regions plays a key role in the material cycling of the universe,so it is significant to perform chemical simulations of star forming re-gions.Abundant carbon-chain species have been observed towards lukewarm corinos L1527,B228,and L483.These carbon-chain species are synthesized in the gas phase after CH4 desorbs from the dust grain surface at the temperature around 25 K.We in-vestigate carbon-chain species formation in lukewarm corinos using a more advanced surface chemical model.We also pay attention to the significance of the finite size ef-fect.We used the macroscopic Monte Carlo method in our simulations.In addition to the two-phase model,the basic multi-phase model and the new multi-phase models were used for modelling surface chemistry on dust grains.All volatile species can sublime at their sublimation temperatures in the two-phase model while most volatile species are frozen in the ice mantle before water ice sublimes in the basic and the new multi-phase models.The new multi-phase models allow more volatile species to sublime at their sublimation temperatures than the basic multi-phase model does.The significance of the finite size effect is dependent on the duration of the cold phase.The discrepancies between the rate equation approach and the Monte Carlo method decrease as the dura-tion of the cold phase increases.When T～25 K,the abundances of gaseous CH4 and CO in the two-phase model are the highest while the basic multi-phase model predicts the lowest abundances among all models.The abundances of carbon-chain species in the basic and the new multi-phase models are lower than that in the two-phase model when T～30 K because CH4 is crucial for the synthesis of carbon-chain species.How-ever,because the abundance of electrons increases as the abundance of H3O+ decreases,some carbon-chain species abundances predicted by the basic multi-phase model may not be lower than that in the new multi-phase models.The two-phase model performs best in predicting carbon-chain species abundances to fit observations while the basic multi-phase model works the worst.The abundances of carbon-chain species predicted by the new multi-phase models agree reasonably well with observations.The amount of CH4 that can diffuse inside the ice mantle,thus sublime upon warm-up plays a crucial role in the synthesis of carbon-chain species in the gas phase.The carbon-chain species observed in lukewarm corinos may be able to gauge surface chemical models.The chemical differentiation of seven complex organic molecules(COMs)has been observed in the Sgr B2 region:CH2OHCHO,CH3OCHO,t-HCOOH,C2H5OH,and CH3NH2 were detected both in the extended region and near the hot cores Sgr B2(N)and Sgr B2(M),while CH3OCH3 and C2H5CN were only detected near the hot cores.The density and temperature in the extended region are relatively low in comparison with Sgr B2(N)and Sgr B2(M).Different desorption mechanisms have been proposed to explain the observed COMs in the cold regions.However,they fail to explain the deficiency of CH3OCH3 and C2H5CN in the extended region around Sgr B2.Based on known physical properties of the extended region around Sgr B2,we explored under what physical conditions the chemical simulations can fit the observations and explain the different spatial distribution of these seven species in the extended region around Sgr B2.We used the macroscopic Monte Carlo method to perform a detailed parameter space study.A static physical model and an evolving physical model including a cold phase and a warm-up phase were used,respectively.In addition to photodesorption that is included in all models,we investigated how chain reaction mechanism,shocks,and an X-ray burst could affect the results of chemical modeling.All gas-grain chemical models based on static physics cannot fit the observations.The simulations based on evolving physical conditions can fit six COMs when T～30-60 K in the warm-up phase,but the best-fit temperature is still higher than the observed dust temperature of 20 K.The best agreement between the simulations and all seven observed COMs at a lower temperature T～27 K is achieved by considering a short-duration ≈102 yr X-ray burst with ζCR=1.3×10-13 s-1 at the early stage of the warm-up phase when it still has a temperature of 20 K.The reactive desorption is the key mechanism for producing these COMs and inducing the low abundances of CH3OCH3 and C2HSCN.We conclude that the evolution of the extended region around Sgr B2 may have begun with a cold,T≤10 K phase followed by a warm-up phase.When its temperature reached about T～20 K,an X-ray flare from the Galactic black hole Sgr A*with a short duration of no more than 100 years was acquired,affecting strongly the Sgr B2 chemistry.The observed COMs in Sgr B2 are able to retain their observed abundances only several hundred years after such a flare,which could imply that such short-term X-rays flares occur relatively often,likely associated with the accretion activity of the Sgr A*source.