Person:

Fayolle, Edith C.

Complex molecules have been broadly classified into three generations dependent on the mode of formation and the required formation temperature (<25, 25-100 K, and >100 K). Around massive young stellar objects (MYSOs), icy grain mantles and gas are exposed to increasingly higher temperatures as material accretes from the outer envelope in toward the central hot region. The combination of this temperature profile and the generational chemistry should result in a changing complex molecular composition with radius around MYSOs. We combine IRAM 30 m and Submillimeter Array observations to explore the spatial distribution of organic molecules around the high-mass young stellar object NGC 7538 IRS9, whose weak complex molecule emission previously escaped detection. We find that emission from N-bearing organics and CH3OH present substantial increases in emission around 8000 AU and R < 3000 AU, while unsaturated O-bearing molecules and hydrocarbons do not. The increase in line flux for some complex molecules in the envelope, around 8000 AU or 25 K, is consistent with recent model predictions of an onset of complex ice chemistry at 20-30 K. The emission increase for many of the same molecules at R < 3000 AU suggests the presence of a weak hot core, where thermal ice evaporation and hot gas-phase reactions drive the chemistry. Complex organics thus form at all radii and temperatures around this protostar, but the composition changes dramatically as the temperature increases, which is used together with an adapted gas-grain astrochemical model to constrain the chemical generation(s) to which different classes of molecules belong.

Interstellar complex organic molecules were first identified in the hot inner regions of massive young stellar objects (MYSOs), but have more recently been found in many colder sources, indicating that complex molecules can form at a range of temperatures. Individually these observations provide limited constraints, however, on how complex molecules form, and whether the same formation pathways dominate in cold, warm and hot environments. To address these questions, we use spatially resolved observations from the Submillimeter Array of three MYSOs together with mostly unresolved literature data to explore how molecular ratios depend on environmental parameters, especially temperature. Toward the threeMYSOs, we find multiple complex organic emission peaks characterized by different molecular compositions and temperatures. In particular, CH3CCH and CH3CN seem to always trace a luke-warm (T∼60 K) and a hot (T>100 K) complex chemistry, respectively. These spatial trends are consistent with abundance-temperature correlations of four representative complex organics – CH3CCH, CH3CN, CH3OCH3 and CH3CHO – in a large sample of complexmolecule hostsmined fromthe literature. Together these results indicate a general chemical evolution with temperature, i.e. that new complex molecule formation pathways are activated as a MYSO heats up. This is qualitatively consistent with model predictions. Furthermore, these results suggest that ratios of complex molecules may be developed into a powerful probe of the evolutionary stage of a MYSO, as well as provide information about its formation history.

Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: (N_2) and (O_2) are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure (N_2) and (O_2) thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic (N_2) and (O_2) isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: (N_2) photodesorption mainly occurs through excitation of the (b^1\sqcap_u) state and subsequent desorption of surface molecules. The observed vibronic structure in the (N_2) photodesorption spectrum, together with the absence of (N_3) formation, supports that the photodesorption mechanism of (N_2) is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, (O_2) photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of (N_2) and (O_2) integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between (10^{-3}) and (10^{-2}) photodesorbed molecules per incoming photon.

Over the last four years we have illustrated the potential of a novel wavelength-dependent approach in determining molecular processes at work in the photodesorption of interstellar ice analogs. This method, utilizing the unique beam characteristics of the vacuum UV beamline DESIRS at the French synchrotron facility SOLEIL, has revealed an efficient indirect desorption mechanism that scales with the electronic excitations in molecular solids. This process, known as DIET – desorption induced by electronic transition – occurs efficiently in ices composed of very volatile species (CO, N2), for which photochemical processes can be neglected. In the present study, we investigate the photodesorption energy dependence of pure and pre-irradiated CO2 ices at 10–40 K and between 7 and 14 eV. The photodesorption from pure CO2 is limited to photon energies above 10.5 eV and is clearly initiated by CO2 excitation and by the contribution of dissociative and recombination channels. The photodesorption from “pre-irradiated” ices is shown to present an efficient additional desorption pathway below 10 eV, dominating the desorption depending on the UV-processing history of the ice film. This effect is identified as an indirect DIET process mediated by photoproduced CO, observed for the first time in the case of less volatile species. The results presented here pinpoint the importance of the interconnection between photodesorption and photochemical processes in interstellar ices driven by UV photons having different energies.

Context. Massive young stellar objects (MYSOs) with hot cores are classic sources of complex organic molecules. The origins of these molecules in such sources, as well as the small- and large-scale differentiation between nitrogen- and oxygen-bearing complex species, are poorly understood. Aims. We aim to use complex molecule abundances toward a chemically less explored class of MYSOs with weak hot organic emission lines to constrain the impact of hot molecular cores and initial ice conditions on the chemical composition toward MYSOs. Methods. We use the IRAM 30 m and the Submillimeter Array to search for complex organic molecules over 8−16 GHz in the 1 mm atmospheric window toward three MYSOs with known ice abundances, but without luminous molecular hot cores. Results. Complex molecules are detected toward all three sources at comparable abundances with respect to CH3OH to classical hot core sources. The relative importance of CH3CHO, CH3CCH, CH3OCH3, CH3CN, and HNCO differ between the organic-poor MYSOs and hot cores, however. Furthermore, the N-bearing molecules are generally concentrated toward the source centers, while most O- and C-bearing molecules are present both in the center and in the colder envelope. Gas-phase HNCO/CH3OH ratios are tentatively correlated with the ratios of NH3 ice over CH3OH ice in the same lines of sight, which is consistent with new gas-grain model predictions. Conclusions. Hot cores are not required to form complex organic molecules, and source temperature and initial ice composition both seem to affect complex organic distributions toward MYSOs. To quantify the relative impact of temperature and initial conditions requires, however, a larger spatially resolved survey of MYSOs with ice detections.