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Register a new userSeeds of Life in Space (SOLIS). VI. Chemical evolution of sulfuretted species along the outflows driven by the low-mass protostellar binary NGC1333-IRAS4A
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Context. Low-mass protostars drive powerful molecular outflows that can be observed with mm and sub-mm telescopes. Various sulfuretted species are known to be bright in shocks and could be used to infer the physical and chemical conditions throughout the observed outflows. Aims. The evolution of sulfur chemistry is studied along the outflows driven by the NGC1333-IRAS4A protobinary system located in the Perseus cloud to constrain the physical and chemical processes at work in shocks. Methods. We observed various transitions from OCS, CS, SO, and SO$_2$ towards NGC1333-IRAS4A in the 1.3, 2, and 3mm bands using the IRAM NOEMA array and we interpreted the observations through the use of the Paris-Durham shock model. Results. The targeted species clearly show different spatial emission along the two outflows driven by IRAS4A. OCS is brighter on small and large scales along the south outflow driven by IRAS4A1, whereas SO$_2$ is detected rather along the outflow driven by IRAS4A2 that is extended along the north east - south west (NE-SW) direction. Column density ratio maps estimated from a rotational diagram analysis allowed us to confirm a clear gradient of the OCS/SO$_2$ column density ratio between the IRAS4A1 and IRAS4A2 outflows. SO is detected at extremely high radial velocity up to 25 km/s relative to the source velocity, clearly allowing us to distinguish the two outflows on small scales. Conclusions. The observed chemical differentiation between the two outflows of the IRAS4A system could be explained by a different chemical history. The outflow driven by IRAS4A1 is likely younger and more enriched in species initially formed in interstellar ices, such as OCS, and recently sputtered into the shock gas. In contrast, the longer and likely older outflow triggered by IRAS4A2 is more enriched in species that have a gas phase origin, such as SO$_2$.
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We present observations of SO and $rm SO_2$ lines toward the shocked regions along the L1157 chemically rich outflow, taken in the context of the Seeds Of Life In Space IRAM-NOrthern Extended Millimeter Array Large Program, and supported by data from Submillimeter Array and IRAM-30 m telescope at 1.1--3.6 mm wavelengths. We simultaneously analyze, for the first time, all of the brightest shocks in the blueshifted lobe, namely, B0, B1, and B2. We found the following. (1) SO and $rm SO_2$ may trace different gas, given that the large(-scale) velocity gradient analysis indicates for $rm SO_2$ a volume density ($rm 10^5text{--}10^6,cm^{-3}$) denser than that of the gas emitting in SO by a factor up to an order of magnitude. (2) Investigating the 0.1 pc scale field of view, we note a tentative gradient along the path of the precessing jet. More specifically, $rm chi({SO/SO_2})$ decreases from the B0-B1 shocks to the older B2. (3) At a linear resolution of 500--1400 au, a tentative spatial displacement between the two emitting molecules is detected, with the SO peak closer (with respect to $rm SO_2$) to the position where the recent jet is impinging on the B1 cavity wall. Our astrochemical modeling shows that the SO and $rm SO_2$ abundances evolve on timescales less than about 1000 years. Furthermore, the modeling requires high abundances ($2times10^{-6}$) of both $rm H_2S/H$ and S/H injected in the gas phase due to the shock occurrence, so pre-frozen OCS only is not enough to reproduce our new observations.
Aim: In the past, observations of protostellar shocks have been able to set constraints on the formation route of formamide (NH2CHO), exploiting its observed spatial distribution and comparison with astrochemical model predictions. In this work, we follow the same strategy to study the case of acetaldehyde (CH3CHO). Method: To this end, we used the data obtained with the IRAM-NOEMA interferometer in the framework of the Large Program SOLIS to image the B0 and B1 shocks along the L1157 blueshifted outflow in methanol (CH3OH) and acetaldehyde line emission. Results: We imaged six CH3OH and eight CH3CHO lines which cover upper level energies up to 30 K. Both species trace the B0 molecular cavity as well as the northern B1 portion, i.e. the regions where the youngest shocks (1000 yr) occurred. The CH$_3$OH and CH$_3$CHO emission peaks towards the B1b clump, where we measured the following column densities and relative abundances: 1.3 x 10^16 cm-2 and 6.5 x 10-6 (methanol), and 7 x 10^13 cm-2 and 3.5 x 10-8 (acetaldehyde). We carried out a non-LTE LVG analysis of the observed CH3OH line: the average kinetic temperature and density of the emitting gas are Tkin = 90 K and nH2 = 4 x 10^5 cm-3, respectively. The CH3OH and CH3CHO abundance ratio towards B1b is 190, varying by less than a factor 3 throughout the whole B0-B1 structure. Conclusions: The comparison of astrochemical model predictions with the observed methanol and acetaldehyde spatial distribution does not allow to distinguish whether acetaldehyde is formed on the grain mantles or rather on the gas-phase, as its gas-phase formation, dominated by the reaction of ethyl radical (CH3CH2) with atomic oxygen, is very fast. Observations of acetaldehyde in younger shocks, e.g. 10^2 yr old, or/and of the ethyl radical, whose frequencies are not presently available, are necessary to settle the issue.
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Aims: A unique environment to study how interstellar Complex Organic Molecules (iCOMs) can be formed is the shocked gas along low-mass protostellar outflows, as the dust mantles composition is sputtered into the gas phase. The chemical richness in these environments has been so far studied only in the L1157 blue shifted outflow. Methods: To understand if the L1157-B1 case is unique, we imaged the NGC 1333 IRAS 4A outflows using the NOEMA (NOrthern Extended Millimeter Array) interferometer as part of the IRAM SOLIS (Seeds Of Life in Space) Large Program and compared the observations with the GRAINOBLE+ gas phase astrochemical model. Results: Several iCOMs were detected in the IRAS 4A outflows: methanol (CH$_3$OH), acetaldehyde (CH$_3$CHO), formamide (NH$_2$CHO) and dimethyl ether (CH$_3$OCH$_3$), all sampling upper excitation energy up to $sim$30 K. We found a significant chemical differentiation between the IRAS 4A1 outflow, showing a richer molecular content, and the IRAS 4A2 one. The CH$_3$OH/CH$_3$CHO abundance ratio is lower by a factor $sim$4 in the former; furthermore the ratio in both outflows is lower by a factor $sim$10 with respect to hot corinos values. Conclusions: After L1157-B1, IRAS 4A outflow is now the second outflow to show an evident chemical complexity. Given that CH$_3$OH is a grain surface species, GRAINOBLE+ reproduced our observations assuming acetaldehyde formation in gas phase by the reaction of ethyl radical (CH$_3$CH$_2$) with atomic oxygen. Moreover, the chemical differentiation between the two outflows suggests that the IRAS 4A1 outflow is likely younger than the IRAS 4A2 one. Further investigation is needed to constrain the age of the outflow and observations of even younger shocks are necessary and future spectroscopic studies on CH$_3$CH$_2$ are needed to be able to observe this species and provide strong constraints on the CH$_3$CHO formation.
Towards the pre-stellar core L1544, the methanol (CH$_3$OH) emission forms an asymmetric ring around the core centre, where CH$_3$OH is mostly in solid form, with a clear peak 4000~au to the north-east of the dust continuum peak. As part of the NOEMA Large Project SOLIS (Seeds of Life in Space), the CH$_3$OH peak has been spatially resolved to study its kinematics and physical structure and to investigate the cause behind the local enhancement. We find that methanol emission is distributed in a ridge parallel to the main axis of the dense core. The centroid velocity increases by about 0.2~km~s$^{-1}$ and the velocity dispersion increases from subsonic to transonic towards the central zone of the core, where the velocity field also shows complex structure. This could be indication of gentle accretion of material onto the core or interaction of two filaments, producing a slow shock. We measure the rotational temperature and show that methanol is in local thermodynamic equilibrium (LTE) only close to the dust peak, where it is significantly depleted. The CH$_3$OH column density, $N_{tot}({rm CH_3OH})$, profile has been derived with non-LTE radiative transfer modelling and compared with chemical models of a static core. The measured $N_{tot}({rm CH_3OH})$ profile is consistent with model predictions, but the total column densities are one order of magnitude lower than those predicted by models, suggesting that the efficiency of reactive desorption or atomic hydrogen tunnelling adopted in the model may be overestimated; or that an evolutionary model is needed to better reproduce methanol abundance.
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