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

(Abridged) Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. This study aims to experimentally quantify the broad-band UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K under ultra-high vacuum conditions. The reaction products are mainly identified by RAIRS and TPD experiments. Complex organics are readily formed in all experiments, both during irradiation and during a slow warm-up of the ices to 200 K 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, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3, HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. 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. The experiments show that ice photochemistry in CH3OH ices is efficient enough to explain the observed abundances of complex organics around protostars and that ratios of complex molecules can be used to constrain their formation pathway.Comment: Accepted for publication in A&A. 65 pages including appendice

The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime

Context. Near- to mid-infrared observations of molecular emission from protoplanetary disks show that the inner regions are rich in small organic volatiles (e.g., C2H2 and HCN). Trends in the data suggest that disks around cooler stars (Teff ≈ 3000 K) are potentially (i) more carbon-rich; and (ii) more molecule-rich than their hotter counterparts (Teff ≳ 4000 K). Aims. We explore the chemical composition of the planet-forming region (<10 AU) of protoplanetary disks around stars over a range of spectral types (from M dwarf to Herbig Ae) and compare with the observed trends. Methods. Self-consistent models of the physical structure of a protoplanetary disk around stars of different spectral types are coupled with a comprehensive gas-grain chemical network to map the molecular abundances in the planet-forming zone. The effects of (i) N2 self shielding; (ii) X-ray-induced chemistry; and (iii) initial abundances, are investigated. The chemical composition in the “observable” atmosphere is compared with that in the disk midplane where the bulk of the planet-building reservoir resides. Results. M dwarf disk atmospheres are relatively more molecule rich than those for T Tauri or Herbig Ae disks. The weak far-UV flux helps retain this complexity which is enhanced by X-ray-induced ion-molecule chemistry. N2 self shielding has only a small effect in the disk molecular layer and does not explain the higher C2H2/HCN ratios observed towards cooler stars. The models underproduce the OH/H2O column density ratios constrained in Herbig Ae disks, despite reproducing (within an order of magnitude) the absolute value for OH: the inclusion of self shielding for H2O photodissociation only increases this discrepancy. One possible explanation is the adopted disk structure. Alternatively, the “hot” H2O (T ≳ 300 K) chemistry may be more complex than assumed. The results for the atmosphere are independent of the assumed initial abundances; however, the composition of the disk midplane is sensitive to the initial main elemental reservoirs. The models show that the gas in the inner disk is generally more carbon rich than the midplane ices. This effect is most significant for disks around cooler stars. Furthermore, the atmospheric C/O ratio appears larger than it actually is when calculated using observable tracers only. This is because gas-phase O2 is predicted to be a significant reservoir of atmospheric oxygen. Conclusions. The models suggest that the gas in the inner regions of disks around cooler stars is more carbon rich; however, calculations of the molecular emission are necessary to definitively confirm whether the chemical trends reproduce the observed trends

Results from a VLT-ISAAC survey of ices and gas around young stellar objects

General results from a 3-5 micron spectroscopic survey of nearby low-mass young stellar objects are presented. L and M-band spectra have been obtained of \~50 low mass embedded young stars using the ISAAC spectrometer mounted on UT1-Antu at Paranal Observatory. For the first time, a consistent census of the CO, H2O ices and the minor ice species CH3OH and OCN- and warm CO gas present around young stars is obtained, using large number statistics and resolving powers of up to R=10000. The molecular structure of circumstellar CO ices, the depletion of gaseous CO onto grains in protoplanetary disks, the presence of hot gas in the inner parts of circumstellar disks and in outflows and infalls are studied. Furthermore, the importance of scattering effects for the interpretation of the spectra have been addressed.Comment: To appear in the proceedings of the conference "Chemistry as a Diagnostic of Star Formation", University of Waterloo, Canada, 21-23 August 200

Infrared spectroscopy of solid CO-CO2 mixtures and layers

The spectra of pure, mixed and layered CO and CO2 ices have been studied systematically under laboratory conditions using infrared spectroscopy. This work provides improved resolution spectra (0.5 cm-1) of the CO2 bending and asymmetric stretching mode, as well as the CO stretching mode, extending the existing Leiden database of laboratory spectra to match the spectral resolution reached by modern telescopes and to support the interpretation of the most recent data from Spitzer. It is shown that mixed and layered CO and CO2 ices exhibit very different spectral characteristics, which depend critically on thermal annealing and can be used to distinguish between mixed, layered and thermally annealed CO-CO2 ices. CO only affects the CO2 bending mode spectra in mixed ices below 50K under the current experimental conditions, where it exhibits a single asymmetric band profile in intimate mixtures. In all other ice morphologies the CO2 bending mode shows a double peaked profile, similar to that observed for pure solid CO2. Conversely, CO2 induces a blue-shift in the peak-position of the CO stretching vibration, to a maximum of 2142 cm-1 in mixed ices, and 2140-2146 cm-1 in layered ices. As such, the CO2 bending mode puts clear constraints on the ice morphology below 50K, whereas beyond this temperature the CO2 stretching vibration can distinguish between initially mixed and layered ices. This is illustrated for the low-mass YSO HH46, where the laboratory spectra are used to analyse the observed CO and CO2 band profiles and try to constrain the formation scenarios of CO2.Comment: Accepted in A&

Benefits of collaborative learning

AbstractCollaborative learning is an educational approach to teaching and learning that involves groups of learners working together to solve a problem, complete a task, or create a product. This review article outlines benefits of learning in collaboration style, begins with the concept of the term and continues with the advantages created by collaborative methods. This paper sets out major benefits of collaborative learning into four categories of; social, psychological, academic, and assessment benefits. Each of them is further subdivided to more specific themes

Imaging galactic diffuse clouds: CO emission, reddening and turbulent flow in the gas around Zeta Oph

Methods: 12CO emission is imaged in position and position-velocity space analyzed statistically, and then compared with maps of total reddening and with models of the C+ - CO transition in H2-bearing diffuse clouds. Results: Around Zeta Oph, 12CO emission appears in two distinct intervals of reddening centered near EBV = 0.4 and 0.65 mag, of which < 0.2 mag is background material. Within either interval, the integrated 12CO intensity varies up to 6-12 K-km/s compared to 1.5 K-km/s toward Zeta Oph. Nearly 80% of the individual profiles have velocity dispersions < 0.6 km/s, which are subsonic at the kinetic temperature derived from H2 toward Zeta Oph, 55 K. Partly as a result, 12CO emission exposes the internal, turbulent, supersonic (1-3 km/s) gas flows with especial clarity in the cores of strong lines. The flows are manifested as resolved velocity gradients in narrow, subsonically-broadened line cores. Conclusions: The scatter between N(CO) and EBV in global, CO absorption line surveys toward bright stars is present in the gas seen around Zeta Oph, reflecting the extreme sensitivity of N(12CO) to ambient conditions. The two-component nature of the optical absorption toward Zeta Oph is coincidental and the star is occulted by a single body of gas with a complex internal structure, not by two distinct clouds. The very bright 12CO lines in diffuse gas arise at N(H2) ~ 10^21/cm^2 in regions of modest density n(H) ~ 200-500/cc and somewhat more complete C+-CO conversion. Given the variety of structure in the foreground gas, it is apparent that only large surveys of absorption sightlines can hope to capture the intrinsic behavior of diffuse gas.Comment: 2009 A&A, in pres