Formation rates of complex organics in UV irradiated CH₃OH-rich ices

Abstract

Context. Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes, so-called hot cores. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. There is, however, a general lack of quantitative data on UV-induced complex chemistry in ices. Aims. This study aims to experimentally quantify the UV-induced production rates of complex organics in CH 3 OH-rich ices under a variety of astrophysically relevant conditions. Methods. The ices are irradiated with a broad-band UV hydrogen microwave-discharge lamp under ultra-high vacuum conditions, at 20-70 K, and then heated to 200 K. The reaction products are identified by reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), through comparison with RAIRS and TPD curves of pure complex species, and through the observed effects of isotopic substitution and enhancement of specific functional groups, such as CH 3 , in the ice. Results. Complex organics are readily formed in all experiments, both during irradiation and during the slow warm-up of the ices after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH 3 OH ice or CH 3 OH:CH 4 /CO ice mixtures are used. C 2 H 6 , CH 3 CHO, CH 3 CH 2 OH, CH 3 OCH 3 , HCOOCH 3 , HOCH 2 CHO and (CH 2 OH) 2 are all detected in at least one experiment. Varying the ice thickness and the UV flux does not affect the chemistry. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH 3 OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. At 20 K, the pure CH 3 OH photodesorption yield is 2.1(±1.0) × 10 −3 per incident UV photon, the photo-destruction cross section 2.6(±0.9) × 10 −18 cm 2 . Conclusions. Photochemistry in CH 3 OH ices is efficient enough to explain the observed abundances of complex organics around protostars. Some complex molecules, such as CH 3 CH 2 OH and CH 3 OCH 3 , form with a constant ratio in our ices and this can can be used to test whether complex gas-phase molecules in astrophysical settings have an ice-photochemistry origin. Other molecular ratios, e.g. HCO-bearing molecules versus (CH 2 OH) 2 , depend on the initial ice composition and temperature and can thus be used to investigate when and where complex ice molecules form.

Astrochemistry of dust, ice and gas: introduction and overview

“…The emission comes from a region of only 25 AU in radius, i.e., comparable to the orbit of Uranus in our own solar system 207 . Its abundance with respect to related complex molecules like methyl formate and ethylene glycol is consistent with laboratory experiments of mild photoprocessing of methanol-rich ice mantles 61 , although other reaction routes such as discussed by Boamah et al, this volume, are not excluded. The above scenario requires that the grains spend some time at elevated temperatures so that the radicals become mobile (see § 5.2).…”

Surface formation routes of interstellar molecules: hydrogenation reactions in simple ices

“…This approach makes it possible to derive fundamental and molecule specific parameters, like reaction rates and diffusion barriers, which can then be included in astrochemical models to simulate the ice evolution under much longer timescales (10 5 years) than accessible in the laboratory (\1 day). The work presented in the next section follows a bottom-up approach and summarizes a representative sample of relevant experiments (e.g., Watanabe and Kouchi 2002;Watanabe et al 2004Watanabe et al , 2006Fuchs et al 2009;Miyauchi et al 2008;Ioppolo et al 2008Ioppolo et al , 2010Ioppolo et al , 2011aMatar et al 2008;Oba et al 2009Oba et al , 2010Cuppen et al 2010;Mokrane et al 2009;Romanzin et al 2011;Ö berg et al 2009). These experiments prove that species like H 2 CO, CH 3 OH and H 2 O can be formed at low temperatures by simple hydrogenation (i.e., without the need for thermal, UV or cosmic ray processing) and provide the basic molecular data to simulate their formation on astronomical timescales (e.g., Cuppen et al 2009), even though the ice as a whole is not representative for a realistic astronomical ice.…”

Complex molecule formation around massive young stellar objects

“…1 The ratio of (CH 3 CHO+HCOOCH 3 )/(CH 3 OCH 3 +CH 3 CH 2 OH) abundances toward cold sources (pre-stellar cores and an outflow), low-mass protostellar envelopes, low-mass hot cores, and traditional high-mass hot cores 11 . Based on laboratory experiments the relative abundance of CHO-bearing molecules should trace the relative importance of cold (CO-ice rich) and warm ice COM chemistry 18 . Literature column densities without uncertainties were assigned a 20% uncertainty.…”