Seeds of Life in Space (SOLIS)

Context. The interstellar complex organic molecules (iCOMs) are C-bearing molecules containing at least six atoms; two main proposals for their formation are suggested: a direct formation in the icy mantle of the dust grains and formation through the reaction in gas phase of released grain mantle species. The shocked gas along outflows driven by low-mass protostars is a unique environment to study how the iCOMs can be formed as the composition of the dust mantles is sputtered into the gas phase. Aims. The chemical richness in shocked material associated with low-mass protostellar outflows has been so far studied in the prototypical L1157 blue-shifted outflow to investigate the iCOM formation routes. To understand whether the case of L1157-B1 is unique, we imaged and studied the IRAS 4A outflows in the NGC 1333 star forming region. Methods. We used the NOrthern Extended Millimeter Array interferometer as part of the IRAM Seeds Of Life in Space (SOLIS) Large Program to image the large-scale bipolar outflows driven by the IRAS 4A system in the 3 mm band, and we compared the observation with the GRAINOBLE+ astrochemical model. Results. We report the first detection, in the IRAS 4A outflows, of several iCOMs: six lines of methanol (CH3OH), eight of acetaldehyde (CH3CHO), one of formamide (NH2CHO), and four of dimethyl ether (CH3OCH3), all sampling upper excitation energy up to ~30 K. We found a significant chemical differentiation between the southeast outflow driven by the IRAS 4A1 protostar, showing a richer molecular content, and the north–southwest one driven by the IRAS 4A2 hot corino. The CH3OH/CH3CHO abundance ratio is lower by a factor of ~4 in the former; furthermore, the ratio in the IRAS 4A outflows is lower by a factor of ~10 with respect to the values found in different hot corinos. Conclusions. After L1157-B1, the IRAS 4A outflow is now the second outflow to show an evident chemical complexity. Given that CH3OH is a grain surface species, the astrochemical gas-phase model run with GRAINOBLE+ reproduced our observation assuming that acetaldehyde is formed mainly through the gas-phase reaction of the ethyl radical (CH3CH2) and atomic oxygen. Furthermore, the chemical differentiation between the two outflows suggests that the IRAS 4A1 outflow is likely younger than that of the IRAS 4A2. Further investigation is needed to constrain the age of the outflow. In addition, observation of even younger shocks are necessary. In order to provide strong constraints on the CH3CHO formation mechanisms it would be interesting to observe CH3CH2, but given that its frequencies are not known, future spectroscopic studies on this species are needed.

Compaction versus reactive flow: How does melt fraction change in crustal mush reservoirs?

<p>Chemical differentiation requires the relative motion of melt and crystals during multicomponent phase change.  Compaction is often invoked as the mechanism that allows this in crystal rich ‘mush’ reservoirs.  Compaction is a term used broadly to describe the coupled processes of buoyancy-driven melt flow through permeable crystalline matrix and matrix deformation in response to the extraction or accumulation of melt.  One key challenge to melt segregation models that invoke compaction is that textural evidence for crystal deformation in the residual material left after melt extraction is largely absent (Holness, 2018).</p><p>Here, we test the relative contribution of compaction and reactive flow to melt fraction change in crustal mush reservoirs using a modified version of the reactive flow model of (Solano et al., 2014).  Reactive flow changes melt, solid and bulk composition and is essential to chemical differentiation in crustal mush reservoirs but has been largely neglected in models of melt segregation.  We find that melt fraction changes in response to reactive flow can be as important as those caused by compaction, irrespective of the phase behaviour tested.  That compaction may account for only half the melt fraction change observed in mush reservoirs could help to explain why textural evidence for mush deformation remains enigmatic.</p><p> </p><p>References<br>Holness, M. B. (2018). Melt segregation from silicic crystal mushes: a critical appraisal of possible mechanisms and their microstructural record. Contributions to Mineralogy and Petrology, 173(6):48.<br>Solano, J. M. S., Jackson, M. D., Sparks, R. S. J., and Blundy, J. (2014). Evolution of major and trace element composition during melt migration through crystalline mush: Implications for chemical differentiation in the crust. American Journal of Science, 314(5):895–939.</p><p> </p>

Questioning the spatial origin of complex organic molecules in young protostars with the CALYPSO survey

Context. Complex organic molecules (COMs) have been detected in a few Class 0 protostars but their origin is not well understood. While the usual picture of a hot corino explains their presence as resulting from the heating of the inner envelope by the nascent protostar, shocks in the outflow, disk wind, the presence of a flared disk, or the interaction region between envelope and disk at the centrifugal barrier have also been claimed to enhance the abundance of COMs. Aims. Going beyond studies of individual objects, we want to investigate the origin of COMs in young protostars on a statistical basis. Methods. We use the CALYPSO survey performed with the Plateau de Bure Interferometer of the Institut de Radioastronomie Millimétrique to search for COMs at high angular resolution in a sample of 26 solar-type protostars, including 22 Class 0 and four Class I objects. We derive the column densities of the detected molecules under the local thermodynamic equilibrium approximation and search for correlations between their abundances and with various source properties. Results. Methanol is detected in 12 sources and tentatively in one source, which represents half of the sample. Eight sources (30%) have detections of at least three COMs. We find a strong chemical differentiation in multiple systems with five systems having one component with at least three COMs detected but the other component devoid of COM emission. All sources with a luminosity higher than 4 L⊙ have at least one detected COM whereas no COM emission is detected in sources with internal luminosity lower than 2 L⊙, likely because of a lack of sensitivity. Internal luminosity is found to be the source parameter impacting the COM chemical composition of the sources the most, while there is no obvious correlation between the detection of COM emission and that of a disk-like structure. A canonical hot-corino origin may explain the COM emission in four sources, an accretion-shock origin in two or possibly three sources, and an outflow origin in three sources. The CALYPSO sources with COM detections can be classified into three groups on the basis of the abundances of oxygen-bearing molecules, cyanides, and CHO-bearing molecules. These chemical groups correlate neither with the COM origin scenarios, nor with the evolutionary status of the sources if we take the ratio of envelope mass to internal luminosity as an evolutionary tracer. We find strong correlations between molecules that are a priori not related chemically (for instance methanol and methyl cyanide), implying that the existence of a correlation does not imply a chemical link. Conclusions. The CALYPSO survey has revealed a chemical differentiation in multiple systems that is markedly different from the case of the prototypical binary IRAS 16293-2422. This raises the question of whether all low-mass protostars go through a phase showing COM emission. A larger sample of young protostars and a more accurate determination of their internal luminosity will be necessary to make further progress. Searching for correlations between the COM emission and the jet/outflow properties of the sources may also be promising.