Harold Linnartz

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

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 CH3OH-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 CH3, 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 CH3OH ice or CH3OH:CH4/CO ice mixtures are used. C2H6 ,C H 3CHO, CH3CH2OH, CH3OCH3, HCOOCH3, HOCH2CHO and (CH2OH)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 CH3OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. At 20 K, the pure CH3OH 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 CH3OH ices is efficient enough to explain the observed abundances of complex organics around protostars. Some complex molecules, such as CH3CH2OH and CH3OCH3, 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 (CH2OH)2, depend on the initial ice composition and temperature and can thus be used to investigate when and where complex ice molecules form.

Hydrogenation reactions in interstellar CO ice analogues - A combined experimental/theoretical approach

Context. Hydrogenation reactions of CO in inter- and circumstellar ices are regarded as an important starting point in the formation of more complex species. Previous laboratory measurements by two groups of the hydrogenation of CO ices provided controversial results about the formation rate of methanol. Aims. Our aim is to resolve this controversy by an independent investigation of the reaction scheme for a range of H-atom fluxes and different ice temperatures and thicknesses. To fully understand the laboratory data, the results are interpreted theoretically by means of continuous-time, random-walk Monte Carlo simulations. Methods. Reaction rates are determined by using a state-of-the-art ultra high vacuum experimental setup to bombard an interstellar CO ice analog with H atoms at room temperature. The reaction of CO + Hi nto H 2CO and subsequently CH3OH is monitored by a Fourier transform infrared spectrometer in a reflection absorption mode. In addition, after each completed measurement, a temperature programmed desorption experiment is performed to identify the produced species according to their mass spectra and to determine their abundance. Different H-atom fluxes, morphologies, and ice thicknesses are tested. The experimental results are interpreted using Monte Carlo simulations. This technique takes into account the layered structure of CO ice. Results. The formation of both formaldehyde and methanol via CO hydrogenation is confirmed at low temperature (T = 12−20 K). We confirm that the discrepancy between the two Japanese studies is caused mainly by a difference in the applied hydrogen atom flux, as proposed by Hidaka and coworkers. The production rate of formaldehyde is found to decrease and the penetration column to increase with temperature. Temperature-dependent reaction barriers and diffusion rates are inferred using a Monte Carlo physical chemical model. The model is extended to interstellar conditions to compare with observational H2CO/CH3OH data.

PHOTODESORPTION OF ICES. II. H2O AND D2O

Gaseous H2O has been detected in several cold astrophysical environments, where the observed abundances cannot be explained by thermal desorption of H2O ice or by H2O gas-phase formation. These observations hence suggest an efficient nonthermal ice desorption mechanism. Here, we present experimentally determined UV photodesorption yields of H2O and D2O ices and deduce their photodesorption mechanism. The ice photodesorption is studied under ultrahigh vacuum conditions and at astrochemically relevant temperatures (18–100 K) using a hydrogen discharge lamp (7–10.5 eV), which simulates the interstellar UV field. The ice desorption during irradiation is monitored using reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed species. The photodesorption yield per incident photon, Ypd( T, x), is identical for H2O and D2O and its dependence on ice thickness and temperature is described empirically by Ypd( T, x) = Ypd( T, x > 8)(1 − e −x/ l(T ) ), where x is the ice thickness in monolayers (MLs) and l(T )i s a temperature-dependent ice diffusion parameter that varies between ∼1.3 ML at 30 K and 3.0 ML at 100 K. For thick ices, the yield is linearly dependent on temperature due to increased diffusion of ice species such that Ypd( T, x >8) = 10 −3 (1. 3+0 .032 × T ) UV photon −1 , with a 60% uncertainty for the absolute yield. The increased diffusion also results in an increasing H2O:OH desorption product ratio with temperature from 0.7:1.0 at 20 K to 2.0:1.2 at 100 K. The yield does not depend on the substrate, the UV photon flux, or the UV fluence. The yield is also independent of the initial ice structure since UV photons efficiently amorphize H2O ice. The results are consistent with theoretical predictions of H2O photodesorption at low temperatures and partly in agreement with a previous experimental study. Applying the experimentally determined yield to a Herbig Ae/Be star+disk model provides an estimate of the amount of gas-phase H2O that may be observed by, e.g., Herschel in an example astrophysical environment. The model shows that UV photodesorption of ices increases the H2O content by orders of magnitude in the disk surface region compared to models where nonthermal desorption is ignored.

Photodesorption of ices I: CO, N₂, and CO₂

Context. A longstanding problem in astrochemistry is how molecules can be maintained in the gas phase in dense inter- and circumstellar regions at temperatures well below their thermal desorption values. Photodesorption is a non-thermal desorption mechanism, which may explain the small amounts of observed cold gas in cloud cores and disk mid-planes. Aims. This study aims to determine the UV photodesorption yields and to constrain the photodesorption mechanisms of three astrochemically relevant ices: CO, N2 and CO2. In addition, the possibility of co-desorption in mixed and layered CO:N2 ices is explored. Methods. The UV photodesorption of ices is studied experimentally under ultra high vacuum conditions and at astrochemically relevant temperatures (15–60 K) using a hydrogen discharge lamp (7–10.5 eV). The ice desorption is monitored by reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed molecules. Results. Both the UV photodesorption yield per incident photon and the photodesorption mechanism are highly molecule specific. The CO photodesorbs without dissociation from the surface layer of the ice, and N2, which lacks a dipole allowed electronic transition in the wavelength range of the lamp, has a photodesorption yield that is more than an order of magnitude lower. This yield increases significantly due to co-desorption when N2 is mixed in with, or layered on top of, CO ice. CO2 photodesorbs through dissociation and subsequent recombination from the top 10 layers of the ice. At low temperatures (15–18 K), the derived photodesorption yields are 2.7(±1.3) × 10 −3 and <2 × 10 −4 molecules photon −1 for pure CO and N2, respectively. The CO2 photodesorption yield is 1.2(±0.7)×10 −3 ×(1−e −(x/2.9(±1.1)) )+1.1(±0.7)×10 −3 ×(1−e −(x/4.6(±2.2) )) molecules photon −1 ,w herex is the ice thickness in monolayers and the two parts of the expression represent a CO2 and a CO photodesorption pathway, respectively. At higher temperatures, the CO ice photodesorption yield decreases, while that of CO2 increases.

LABORATORY EVIDENCE FOR EFFICIENT WATER FORMATION IN INTERSTELLAR ICES

Even though water is the main constituent in interstellar icy mantles, its chemical origin is not well understood. Three different formation routes have been proposed following hydrogenation of O, O2 ,o r O3 on icy grains, but experimental evidence is largely lacking. We present a solid state astrochemical laboratory study in which one of these routes is tested. For this purpose O2 ice is bombarded by H or D atoms under ultrahigh vacuum conditions at astronomically relevant temperatures ranging from 12 to 28 K. The use of reflection absorption infrared spectroscopy (RAIRS)permitsderivationof reactionratesandshowsefficientformationof H2O(D2O)witharatethatissurprisingly independent of temperature. This formation route converts O2 into H2O via H2O2 and is found to be orders of magnitude more efficient than previously assumed. It should therefore be considered as an important channel for interstellar water ice formation as illustrated by astrochemical model calculations. Subject headingg astrochemistry — infrared: ISM — ISM: atoms — ISM: molecules — methods: laboratory Online material: color figures

Reaction networks for interstellar chemical modelling: Improvements and challenges.

We survey the current situation regarding chemical modelling of the synthesis of molecules in the interstellar medium. The present state of knowledge concerning the rate coefficients and their uncertainties for the major gas-phase processes—ion-neutral reactions, neutral-neutral reactions, radiative association, and dissociative recombination—is reviewed. Emphasis is placed on those key reactions that have been identified, by sensitivity analyses, as ‘crucial’ in determining the predicted abundances of the species observed in the interstellar medium. These sensitivity analyses have been carried out for gas-phase models of three representative, molecule-rich, astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the expanding circumstellar envelope IRC +10216. Our review has led to the proposal of new values and uncertainties for the rate coefficients of many of the key reactions. The impact of these new data on the predicted abundances in TMC-1 and L134N is reported. Interstellar dust particles also influence the observed abundances of molecules in the interstellar medium. Their role is included in gas-grain, as distinct from gas-phase only, models. We review the methods for incorporating both accretion onto, and reactions on, the surfaces of grains in such models, as well as describing some recent experimental efforts to simulate and examine relevant processes in the laboratory. These efforts include experiments on the surface-catalyzed recombination of hydrogen atoms, on chemical processing on and in the ices that are known to exist on the surface of interstellar grains, and on desorption processes, which may enable species formed on grains to return to the gas-phase.

Photodesorption of CO Ice

At the high densities and low temperatures found in star forming regions, all molecules other than H2 should stick on dust grains on timescales shorter than the cloud lifetimes. Yet these clouds are detected in the millimeter lines of gaseous CO. At these temperatures, thermal desorption is negligible and hence a non-thermal desorption mechanism is necessary to maintain molecules in the gas phase. Here, the first laboratory study of the photodesorption of pure CO ice under ultra high vacuum is presented, which gives a desorption rate of 3 × 10 3 CO molecules per UV (7–10.5 eV) photon at 15 K. This rate is factors of 10 2 -10 5 larger than previously estimated and is comparable to estimates of other non-thermal desorption rates. The experiments constrains the mechanism to a single photon desorption process of ice surface molecules. The measured efficiency of this process shows that the role of CO photodesorption in preventing total removal of molecules in the gas has been underestimated. Subject headings: Molecular data — Molecular processes — ISM: abundances — Physical Data and Processes: astrochemistry — ISM: molecules

Gas-Phase Electronic Spectra of Carbon-Chain Radicals Compared with Diffuse Interstellar Band Observations

This paper compares laboratory gas-phase spectra of neutral and cationic linear carbon-chain radicals with astronomical diUuse interstellar band (DIB) spectra. The origin bands of the strong electronic tran- sitions of the studied species, (n \ 3¨6),

THE c2d SPITZER SPECTROSCOPIC SURVEY OF ICES AROUND LOW-MASS YOUNG STELLAR OBJECTS. IV. NH3 AND CH3OH

NH_3 and CH_3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and CH3OCH3. Despite a number of recent studies, little is known about their abundances in the solid state. This is particularly the case for low-mass protostars, for which only the launch of the Spitzer Space Telescope has permitted high-sensitivity observations of the ices around these objects. In this work, we investigate the ~8–10μm region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10μm silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH_(3-) and CH_3OH-containing ices to help interpret the astronomical spectra. We clearly detect a feature at ~9μm in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH_3 ν_2 umbrella mode and derive abundances with respect to water between ~2% and 15%. Simultaneously, we also revisited the case of CH_3OH ice by studying the ν_4 C–O stretch mode of this molecule at ~9.7μm in 16 objects, yielding abundances consistent with those derived by Boogert et al. based on a simultaneous 9.75 and 3.53μm data analysis. Our study indicates that NH_3 is present primarily in H_2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH_3OH being in an almost pure methanol ice, or mixed mainly with CO or CO_2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH_3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10%–20% of nitrogen is locked up in known ices.

CO Ice Photodesorption: A Wavelength-dependent Study

UV-induced photodesorption of ice is a non-thermal evaporation process that can explain the presence of cold molecular gas in a range of interstellar regions. Information on the average UV photodesorption yield of astrophysically important ices exists for broadband UV lamp experiments. UV fields around low-mass pre-main-sequence stars, around shocks and in many other astrophysical environments are however often dominated by discrete atomic and molecular emission lines. It is therefore crucial to consider the wavelength dependence of photodesorption yields and mechanisms. In this work, for the first time, the wavelength-dependent photodesorption of pure CO ice is explored between 90 and 170 nm. The experiments are performed under ultra high vacuum conditions using tunable synchrotron radiation. Ice photodesorption is simultaneously probed by infrared absorption spectroscopy in reflection mode of the ice and by quadrupole mass spectrometry of the gas phase. The experimental results for CO reveal a strong wavelength dependence directly linked to the vibronic transition strengths of CO ice, implying that photodesorption is induced by electronic transition (DIET). The observed dependence on the ice absorption spectra implies relatively low photodesorption yields at 121.6 nm (Lyα), where CO barely absorbs, compared to the high yields found at wavelengths coinciding with transitions into the first electronic state of CO (A1Π at 150 nm); the CO photodesorption rates depend strongly on the UV profiles encountered in different star formation environments.