Molecular distributions in the inner regions of protostellar disks

Abstract

Abstract. The distributions of molecules in the inner regions of a protostellar disk are presented. These were calculated using an uncoupled chemical/dynamical model, with a numerical integration of the vertical disk structure. A comparison between models with and without the effects of X-ray ionisation is made, and molecules are identified which are good tracers of the ionisation level in this part of the disk, notably CN and C2H. In the region considered in this paper (r ≤ 10 AU), the chemistry is dominated by the thermal desorption of species from grains. This shows that a critically important detail in this region of the disk, as far as molecular distributions are concerned, is the temperature profile. We find that not all of the gaseous material is frozen onto grain surfaces at 10 AU, and we identify species, including some organic molecules, which should exist in observable quantities in the inner regions of protostellar disks.

Formation of simple organic molecules in inner T Tauri disks

“…Our model predicts that CH 4 has a large abundance in the very inner disk (<0.5 AU). However, it is less abundant than C 2 H 2 and HCN throughout the rest of the disk, unlike in the model by Markwick et al (2002) where it is the most abundant organic species. The reasons for this difference are unclear.…”

“…For simplicity, we neglect the effect of stellar X-rays, although in the case of strong X-ray emitter T Tauri stars the abundances of certain molecules, which are sensitive to the ionization rate such as HCO + or HNC, may be substantially affected. The influence of an enhanced ionization rate on the molecular abundances has been discussed by Aikawa & Herbst (1999) and Markwick et al (2002). Concerning the formation of simple organics such as C 2 H 2 , HCN, and CH 4 , an enhanced ionization rate does not greatly change the results from those obtained when only a FUV radiation field is considered.…”

“…The reasons for this difference are unclear. In the model by Markwick et al (2002), the large gas phase abundance of methane is likely due to its desorption from grain surfaces. However, since they do not include formation of molecules on grain surfaces, CH 4 must have been previously formed through a mechanism of gas phase reactions, which is not explicitly detailed.…”

“…However, since they do not include formation of molecules on grain surfaces, CH 4 must have been previously formed through a mechanism of gas phase reactions, which is not explicitly detailed. We, nevertheless, do not model the same disk region studied by Markwick et al (2002). We only consider the upper disk layers exposed to FUV radiation, while they consider the full disk in the z direction down to the midplane, where most of material is contained.…”

“…Willacy et al (1998) studied the chemistry in the disk midplane, shielded from stellar and interstellar UV photons, and found that some organic species could be easily formed on grain surfaces. Markwick et al (2002) studied the chemistry in the inner 10 AU, including stellar and interstellar UV radiation, stellar X-rays, and radionuclides decay as sources of ionization and adsorption/desorption on grains, but not mantle chemistry, i.e., formation and destruction of molecules occurs only in the gas phase. They calculated large abundances of organic species such as CH 4 but they did not discuss the main chemical routes to form them.…”