PRODIGE - envelope to disk with NOEMA: VII. (Complex) organic molecules in the NGC1333 IRAS4B1 outflow: A new laboratory for shock chemistry

Abstract

Shock chemistry is an excellent tool to shed light on the formation and destruction mechanisms of complex organic molecules (COMs). The L1157-mm outflow is the only low-mass protostellar outflow that has extensively been studied in this regard. Using the data taken as part of the PRODIGE (PROtostars & DIsks: Global Evolution) large program, we aim to map COM emission and derive the molecular composition of the protostellar outflow driven by the Class 0 protostar NGC1333 IRAS4B1 to introduce it as a new laboratory to study the impact of shocks on COM chemistry. In addition to typical outflow tracers such as SiO and CO, outflow emission is seen from H2CO, HNCO, and HC3N, as well as from the COMs CH3OH, CH3CN, and CH3CHO, and even from deuterated species such as DCN, D2CO, and CH2DOH. Maps of integrated intensity ratios between CH3OH and DCN, D2CO, and CH3CHO reveal gradients with distance from the protostar. Intensity ratio maps of HC3N and CH3CN with respect to CH3OH peak in the southern lobe where temperatures are highest. Rotational temperatures derived towards two positions, one in each lobe, are found in the range ~50-100 K. Abundances with respect to CH3OH are higher by factors of a few than for the L1157-B1. In conclusion, for the first time, we securely detected the COMs CH3CN, CH3CHO, and CH2DOH in the IRAS 4B1 outflow, serendipitously with limited sensitivity and bandwidth. Targeted observations will enable the discovery of new COMs and a more detailed analysis of their emission. Morphological differences between molecules in the IRAS 4B1 outflow lobes and their relative abundances provide first proof that this outflow is a promising new laboratory for shock chemistry, which will offer crucial information on COM formation and destruction as well as outflow structure and kinematics.

Figures (10)

• Figure 1: Integrated intensity maps towards IRAS 4B1 (yellow star) for SiO 2 -- 1 ($E_u=31$ K; colour) and CH$_3$OH $4_2-3_1$ and DCN 3 -- 2 ($E_u=45$ K and 21 K, respectively; contours). The SiO maps are integrated from $-25$ to 6 km s$^{-1}$ and 6.8 to 47 km s$^{-1}$ and show emission above 10$\sigma$ with $\sigma_\mathrm{blue}=0.42$ K km s$^{-1}$ and $\sigma_\mathrm{red}=0.49$ K km s$^{-1}$. Contours are at $-$30$\sigma$,$-$15$\sigma$, 15$\sigma$, 30$\sigma$, and then increase by a factor of 3 for CH$_3$OH ($\sigma_\mathrm{blue}=0.25$ K km s$^{-1}$ and $\sigma_\mathrm{red}=0.23$ K km s$^{-1}$) and at $-$10$\sigma$, $-$5$\sigma$, 5$\sigma$, 10$\sigma$, and then increase by a factor of 3 for DCN ($\sigma_\mathrm{blue}=0.17$ K km s$^{-1}$ and $\sigma_\mathrm{red}=0.24$ K km s$^{-1}$). Velocity ranges (in km s$^{-1}$) used for the integration of CH$_3$OH and DCN emission are indicated in the top left. The HPBW is shown in the bottom left. Positions R1 and B1 were selected for further spectral-line analysis (Sect. \ref{['ss:analysis']}). Black crosses indicate H$_2$ knots Choi2011.
• Figure 2: Integrated intensity ratios, $(W_\mathrm{mol} / W_\mathrm{CH_3OH})$ / $( W_\mathrm{mol} / W_\mathrm{CH_3OH})_\mathrm{max}$ of various molecular transitions with either CH$_3$OH $10_{2,9}-9_{3,6}$ ($E_u=165$ K; for HC$_3$N and CH$_3$CN) or $5_{1,4}-4_{2,2}$ ($E_u=56$ K; for D$_2$CO, DCN, HNCO, and CH$_3$CHO) towards the IRAS 4B1 outflow. Integrated intensities contain the sum of blue- and redshifted emission (cf. Fig. \ref{['fig:mom0']} and spectra in Fig. \ref{['fig:spectra']}). The ratio is only shown if both molecules are above a 5$\sigma$ threshold, where $\sigma$ is the noise level in the respective map. Another mask with a 1$^{\prime\prime}$ radius around the protostar is applied to the ratios. Black contours show the integrated intensities of CH$_3$OH, starting at $-5\sigma$, 5$\sigma$, and then increasing by a factor of 2, where $\sigma=0.34$ K km s$^{-1}$. The molecule as well as the upper-level energy of the shown transition are in the top left corner. In all panels, the white star marks the position of IRAS 4B1. The HPBW is shown in the bottom left corner and black crosses mark H$_2$ knots Choi2011. Spectroscopic information on the shown transitions are given in Table \ref{['tab:lines']}.
• Figure 3: Column densities (top) and abundances with respect to CH$_3$OH (bottom) towards B1 and R1 in IRAS 4B1 derived in this work and values obtained for L1157-B1 (see Table \ref{['tab:analysis']}). The dashed and dotted black lines indicate equal abundances and a difference of a factor of 5, respectively. Empty bars or symbols with arrows indicate upper limits.
• Figure 4: Cartoon highlighting the main molecular features of the IRAS 4B1 outflow analysed in this work. The main emission morphology of our reference molecule, CH$_3$OH, is outlined in grey, including the main outflow lobes (red and blue arrows) and additional arc-like features (black lines) of currently unknown origin. Peaks of intensity ratios with respect to CH$_3$OH are highlighted in teal and pink, depending on the species (cf. Fig. \ref{['fig:ratios']}). A spectral-line analysis was done towards positions R1 and B1 that yielded rotational temperatures of $T_\mathrm{rot}=60-100$ K (Sect. \ref{['ss:analysis']}). The position of the protostar is indicated in yellow.
• Figure 5: Rotational temperatures derived from two transitions of HC$_3$N and CH$_3$OH assuming LTE. Rest frequencies (in GHz) and upper-level energies of the transitions are written below the molecules. Contour levels are shown in the top right. Markers are the same as in Fig. \ref{['fig:ratios']}. The HPBW is shown in the bottom left corner.