[Show abstract][Hide abstract] ABSTRACT: 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.
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
We use the IRAM 30m 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.
Complex molecules are detected toward all three sources at comparable
abundances with respect to CH$_3$OH to classical hot core sources. The relative
importance of CH$_3$CHO, CH$_3$CCH, CH$_3$OCH$_3$, CH$_3$CN, 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/CH$_3$OH ratios are tentatively correlated with
the ratios of NH$_3$ ice over CH$_3$OH ice in the same lines of sight, which is
consistent with new gas-grain model predictions.
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.
Astronomy and Astrophysics 01/2015; 576. DOI:10.1051/0004-6361/201323114 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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. However, individually these observations provide limited constraints 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. Towards the three MYSOs, we find multiple complex organic emission peaks characterized by different molecular compositions and temperatures. In particular, CH3CCH and CH3CN seem to always trace a lukewarm (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 complex molecule hosts mined from the 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, and may provide information about its formation history.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant
species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1–2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.
[Show abstract][Hide abstract] ABSTRACT: 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. N2 and O2 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. 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 N2 and O2 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 N2 and O2 isotopolog mixtures are used to investigate the
importance of dissociation upon irradiation. N2 photodesorption mainly occurs
through excitation of the b^1Pi_u state and subsequent desorption of surface
molecules. The observed vibronic structure in the N2 photodesorption spectrum,
together with the absence of N3 formation, supports that the photodesorption
mechanism of N2 is similar to CO, i.e., an indirect DIET (Desorption Induced by
Electronic Transition) process without dissociation of the desorbing molecule.
In contrast, O2 photodesorption in the 7 - 13.6 eV range occurs through
dissociation and presents no vibrational structure. Photodesorption rates of N2
and O2 integrated over the far-UV field from various star-forming environments
are lower than for CO. Rates vary between 10E-3 and 10E-2 photodesorbed
molecules per incoming photon.
Astronomy and Astrophysics 09/2013; 556. DOI:10.1051/0004-6361/201321533 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Molecular cloud cores, protostellar envelopes and protoplanetary disk
midplanes are all characterized by freeze-out of atoms and molecules
(other than H and H2) onto interstellar dust grains. On the grain
surface, atom addition reactions, especially hydrogenation, are
efficient and H2O forms readily from O, CH3OH from CO etc. The result is
an icy mantle typically dominated by H2O, but also rich in CO2, CO, NH3,
CH3OH and CH4. These ices are further processed through interactions
with radiation, electrons and energetic particles. Because of the
efficiency of the freeze-out process, and the complex chemistry that
succeeds it, these icy grain mantles constitute a major reservoir of
volatiles during star formation and are also the source of much of the
chemical evolution observed in star forming regions. Laboratory
experiments allow us to explore how molecules and radicals desorb,
dissociate, diffuse and react in ices when exposed to different sources
of energy. Changes in ice composition and structure is constrained using
infrared spectroscopy and mass spectrometry. By comparing ice
desorption, segregation, and chemistry efficiencies under different
experimental conditions, we can characterize the basic ice processes,
e.g. diffusion of different species, that underpin the observable
changes in ice composition and structure. This information can then be
used to predict the interstellar ice chemical evolution. I will review
some of the key laboratory discoveries on ice chemistry during the past
few years and how they have been used to predict and interpret
astronomical observations of ice bands and gas-phase molecules
associated with ice evaporation. These include measurements of thermal
diffusion in and evaporation from ice mixtures, non-thermal diffusion
efficiencies (including the recent results on frequency resolved UV
photodesorption), and the expected temperature dependencies of the
complex ice chemistry regulated by radical formation and diffusion.
Based on these examples I will argue that the combination of laboratory
experiments and observations is crucial to formulate and to test
hypotheses on key processes that regulate the interstellar ice
[Show abstract][Hide abstract] ABSTRACT: Based on astrochemical theory, the complex molecular composition around
high-mass YSOs should evolve from the outer envelope in toward the central hot
region as a sequence of temperature dependent chemical pathways are activated
in ices and in the gas-phase. The resulting complex molecules have been broadly
classified into three generations dependent on the temperature (<25, >25, and
>100 K) required for formation. We combine IRAM 30m 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, quantifying the emission and abundance
profiles of key organic molecules as a function of distance from the central
protostar. We find that emission from N-bearing organics and saturated
O-bearing organics present large increases in emission around 8000 AU and
R<3000 AU, while O-bearing molecules and hydrocarbons do not. The increase in
flux from 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 some molecules at R<3000 AU suggests the
presence of a weak hot core, where thermal ice evaporation and hot gas-phase
chemistry drives 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 to constrain the chemical
generation(s) to which different classes of molecule belong.
The Astrophysical Journal 05/2013; 771(2). DOI:10.1088/0004-637X/771/2/95 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Carbon monoxide is after H(2) the most abundant molecule identified in the interstellar medium (ISM), and is used as a major tracer for the gas phase physical conditions. Accreted at the surface of water-rich icy grains, CO is considered to be the starting point of a complex organic--presumably prebiotic--chemistry. Non-thermal desorption processes, and especially photodesorption by UV photons, are seen as the main cause that drives the gas-to-ice CO balance in the colder parts of the ISM. The process is known to be efficient and wavelength-dependent, but, the underlying mechanism and the physical-chemical parameters governing the photodesorption are still largely unknown. Using monochromatized photons from a synchrotron beamline, we reveal that the molecular mechanism responsible for CO photoejection is an indirect, (sub)surface-located process. The local environment of the molecules plays a key role in the photodesorption efficiency, and is quenched by at least an order of magnitude for CO interacting with a water ice surface.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In the cold parts of star-forming regions, molecules such as H_2O and CO are expected to be completely frozen out onto dust grains. Yet these molecules are found in cold gas in dense clouds and protoplanetary disks; their presence may be explained by UV-induced photodesorption. Recent astrophysically relevant study on ice photodesorption (Westley et al. 1995 ; Öberg et al. 2007,2009) showed that photodesorption is an efficient process when inducing desorption with an H_2 discharge lamp, which UV distribution is peaked at Lyman alpha. The UV fields around solar-type pre-main sequence stars are however dominated by discrete atomic and molecular emission lines (Bergin et al. 2003). Thus it is crucial to investigate ice photodesorption as a function of irradiation wavelength in order to provide accurate photodesorption rates and constrain this molecular mechanism. For the first time, the wavelength-dependent photodesorption of pure CO and H_2O (D_2O) ice is explored in various spectral windows between 80 and 160 nm. The experiments are performed with the ultra-high vacuum setup SPICES (LPMAA-UPMC, France) using tunable synchrotron radiation (SOLEIL, France). Ice photodesorption is simultaneously probed by infrared absorption in reflection mode (RAIRS) of the ice and by quadrupole mass spectrometry of the gas phase composition. The experimental results reveal a strong wavelength dependency. CO ice photodesorption could be monitored continuously between 80 and 160 nm, and the yield is directly linked to the vibronic transition strengths of CO ice in this wavelength region. This implies a direct photodesorption mechanism initiated by electronic transition in CO ice. For both H_2O and CO pure ices, low photodesorption yields were obtained around 121.6 nm (Lyman alpha) by comparison with the high yields peaking at longer wavelengths and corresponding to the first electronic absorption band. This information is important to implement into astrochemical networks in order to accurately predict the gas and ice phase composition in photon-rich regions.
Proceedings of the International Astronomical Union 06/2011; 280.
[Show abstract][Hide abstract] ABSTRACT: It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment such as the interstellar medium. In the last decennium more and more evidence has been found that the observed mix of small and complex, stable and highly transient species in space is the cumulative result of gas phase and solid state reactions as well as gas-grain interactions. Solid state reactions on icy dust grains are specifically found to play an important role in the formation of the more complex “organic” compounds. In order to investigate the underlying physical and chemical processes detailed laboratory based experiments are needed that simulate surface reactions triggered by processes as different as thermal heating, photon (UV) irradiation and particle (atom, cosmic ray, electron) bombardment of interstellar ice analogues. Here, some of the latest research performed in the Sackler Laboratory for Astrophysics in Leiden, the Netherlands is reviewed. The focus is on hydrogenation, i.e., H-atom addition reactions and vacuum ultraviolet irradiation of interstellar ice analogues at astronomically relevant temperatures. It is shown that solid state processes are crucial in the chemical evolution of the interstellar medium, providing pathways towards molecular complexity in space.
Proceedings of the International Astronomical Union 05/2011; 7:390 - 404. DOI:10.1017/S1743921311025142
[Show abstract][Hide abstract] ABSTRACT: Context. Ice desorption affects the evolution of the gas-phase chemistry during the protostellar stage, and also determines the chemical composition of comets forming in circumstellar disks. From observations, most volatile species are found in H 2 O-dominated ices. Aims. The aim of this study is first to experimentally determine how entrapment of volatiles in H 2 O ice depends on ice thickness, mixture ratio and heating rate, and second, to introduce an extended three-phase model (gas, ice surface and ice mantle) to describe ice mixture desorption with a minimum number of free parameters. Methods. Thermal H 2 O:CO 2 ice desorption is investigated in temperature programmed desorption experiments of thin (10–40 ML) ice mixtures under ultra-high vacuum conditions. Desorption is simultaneously monitored by mass spectrometry and reflection-absorption infrared spectroscopy. The H 2 O:CO 2 experiments are complemented with selected H 2 O:CO, and H 2 O:CO 2 :CO experiments. The results are modeled with rate equations that connect the gas, ice surface and ice mantle phases through surface desorption and mantle-surface diffusion. Results. The fraction of trapped CO 2 increases with ice thickness (10–32 ML) and H 2 O:CO 2 mixing ratio (5:1–10:1), but not with one order of magnitude different heating rates. The fraction of trapped CO 2 is 44–84% with respect to the initial CO 2 content for the investigated experimental conditions. This is reproduced quantitatively by the extended three-phase model that is introduced here. The H 2 O:CO and H 2 O:CO 2 :CO experiments are consistent with the H 2 O:CO 2 desorption trends, suggesting that the model can be used for other ice species found in the interstellar medium to significantly improve the parameterization of ice desorption.
Astronomy and Astrophysics 05/2011; 529(74). DOI:10.1051/0004-6361/201016121 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: (Abridged) The observed presence of pure CO2 ice in protostellar envelopes is attributed to thermally induced ice segregation, but a lack of quantitative experimental data has prevented its use as a temperature probe. Quantitative segregation studies are also needed to characterize diffusion in ices, which underpins all ice dynamics and ice chemistry. This study aims to quantify the segregation mechanism and barriers in different H2O:CO2 and H2O:CO ice mixtures covering a range of astrophysically relevant ice thicknesses and mixture ratios. The ices are deposited at 16-50 K under (ultra-)high vacuum conditions. Segregation is then monitored at 23-70 K as a function of time, through infrared spectroscopy. Thin (8-37 ML) H2O:CO2/CO ice mixtures segregate sequentially through surface processes, followed by an order of magnitude slower bulk diffusion. Thicker ices (>100 ML) segregate through a fast bulk process. The thick ices must therefore be either more porous or segregate through a different mechanism, e.g. a phase transition. The segregation dynamics of thin ices are reproduced qualitatively in Monte Carlo simulations of surface hopping and pair swapping. The experimentally determined surface-segregation rates for all mixture ratios follow the Ahrrenius law with a barrier of 1080 K for H2O:CO2 and 300 K for H2O:CO mixtures. During low-mass star formation H2O:CO2 segregation will be important already at 30 K. Both surface and bulk segregation is proposed to be a general feature of ice mixtures when the average bond strengths of the mixture constituents in pure ice exceeds the average bond strength in the ice mixture. Comment: Accepted for publication in A&A. 25 pages, including 13 figures
Astronomy and Astrophysics 07/2009; 505(1). DOI:10.1051/0004-6361/200912464 · 4.38 Impact Factor