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Register a new userBroad N2H+ emission towards the protostellar shock L1157-B1
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We present the first detection of N2H+ towards a low-mass protostellar outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar cocoon. The detection was obtained with the IRAM 30-m antenna. We observed emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the emission coupled with the HIFI CHESS multiline CO observations leads to the conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5 cm-3) gas associated with the large (20-25 arcsec) cavities opened by the protostellar wind. We find a N2H+ column density of few 10^12 cm-2 corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that the abundance of N2H+ is not increased by the passage of the shock. In summary, N2H+ is a fossil record of the pre-shock gas, formed when the density of the gas was around 10^4 cm-3, and then further compressed and accelerated by the shock.
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The outflow driven by the low-mass class 0 protostar L1157 is the prototype of the so-called chemically active outflows. The bright bowshock B1 in the southern outflow lobe is a privileged testbed of magneto-hydrodynamical (MHD) shock models, for which dynamical and chemical processes are strongly interdependent. We present the first results of the unbiased spectral survey of the L1157-B1 bowshock, obtained in the framework of the key program Chemical Herschel Surveys of Star Forming Regions (CHESS). The main aim is to trace the warm and chemically enriched gas and to infer the excitation conditions in the shock region. The CO 5-4 and H2O lines have been detected at high-spectral resolution in the unbiased spectral survey of the HIFI-Band 1b spectral window (555-636 GHz), presented by Codella et al. in this volume. Complementary ground-based observations in the submm window help establish the origin of the emission detected in the main-beam of HIFI, and the physical conditions in the shock.}{Both lines exhibit broad wings, which extend to velocities much higher than reported up to now. We find that the molecular emission arises from two regions with distinct physical conditions: an extended, warm (100K), dense (3e5 cm-3) component at low-velocity, which dominates the water line flux in Band~1; a secondary component in a small region of B1 (a few arcsec) associated with high-velocity, hot (> 400 K) gas of moderate density ((1.0-3.0)e4 cm-3), which appears to dominate the flux of the water line at 179mu observed with PACS. The water abundance is enhanced by two orders of magnitude between the low- and the high-velocity component, from 8e-7 up to 8e-5. The properties of the high-velocity component agree well with the predictions of steady-state C-shock models.
We present high spatial resolution (750 AU at 250 pc) maps of the B1 shock in the blue lobe of the L1157 outflow in four lines: CS (3-2), CH3OH (3_K-2_K), HC3N (16-15) and p-H2CO (2_02-3_01). The combined analysis of the morphology and spectral profiles has shown that the highest velocity gas is confined in a few compact (~ 5 arcsec) bullets while the lowest velocity gas traces the wall of the gas cavity excavated by the shock expansion. A large velocity gradient model applied to the CS (3-2) and (2-1) lines provides an upper limit of 10^6 cm^-3 to the averaged gas density in B1 and a range of 5x10^3< n(H2)< 5x10^5 cm^-3 for the density of the high velocity bullets. The origin of the bullets is still uncertain: they could be the result of local instabilities produced by the interaction of the jet with the ambient medium or could be clump already present in the ambient medium that are excited and accelerated by the expanding outflow. The column densities of the observed species can be reproduced qualitatively by the presence in B1 of a C-type shock and only models where the gas reaches temperatures of at least 4000 K can reproduce the observed HC3N column density.
Outflows generated by protostars heavily affect the kinematics and chemistry of the hosting molecular cloud through strong shocks that enhance the abundance of some molecules. L1157 is the prototype of chemically active outflows, and a strong shock, called B1, is taking place in its blue lobe between the precessing jet and the hosting cloud. We present the Herschel-PACS 55--210 micron spectra of the L1157-B1 shock, showing emission lines from CO, H2O, OH, and [OI]. The spatial resolution of the PACS spectrometer allows us to map the warm gas traced by far-infrared (FIR) lines with unprecedented detail. The rotational diagram of the high-Jup CO lines indicates high-excitation conditions (Tex ~ 210 +/- 10 K). We used a radiative transfer code to model the hot CO gas emission observed with PACS and in the CO (13-12) and (10-9) lines measured by Herschel-HIFI. We derive 200<Tkin<800 K and n>10^5 cm-3. The CO emission comes from a region of about 7 arcsec located at the rear of the bow shock where the [OI] and OH emission also originate. Comparison with shock models shows that the bright [OI] and OH emissions trace a dissociative J-type shock, which is also supported by a previous detection of [FeII] at the same position. The inferred mass-flux is consistent with the reverse shock where the jet is impacting on the L1157-B1 bow shock. The same shock may contribute significantly to the high-Jup CO emission.
Molecular outflows powered by young protostars strongly affect the kinematics and chemistry of the natal molecular cloud through strong shocks resulting in substantial modifications of the abundance of several species. As part of the Chemical Herschel Surveys of Star forming regions guaranteed time key program, we aim at investigating the physical and chemical conditions of H20 in the brightest shock region B1 of the L1157 molecular outflow. We observed several ortho- and para-H2O transitions using HIFI and PACS instruments on board Herschel, providing a detailed picture of the kinematics and spatial distribution of the gas. We performed a LVG analysis to derive the physical conditions of H2O shocked material, and ultimately obtain its abundance. We detected 13 H2O lines probing a wide range of excitation conditions. PACS maps reveal that H2O traces weak and extended emission associated with the outflow identified also with HIFI in the o-H2O line at 556.9 GHz, and a compact (~10) bright, higher-excitation region. The LVG analysis of H2O lines in the bow-shock show the presence of two gas components with different excitation conditions: a warm (Tkin~200-300 K) and dense (n(H2)~(1-3)x10^6 cm-3) component with an assumed extent of 10 and a compact (~2-5) and hot, tenuous (Tkin~900-1400 K, n(H2)~10^3-10^4 cm-3) gas component, which is needed to account for the line fluxes of high Eu transitions. The fractional abundance of the warm and hot H2O gas components is estimated to be (0.7-2)x10^{-6} and (1-3)x10^{-4}, respectively. Finally, we identified an additional component in absorption in the HIFI spectra of H2O lines connecting with the ground state level, probably arising from the photodesorption of icy mantles of a water-enriched layer at the edges of the cloud.
We present here a systematic search for cyanopolyynes in the shock region L1157-B1 and its associated protostar L1157-mm in the framework of the Large Program Astrochemical Surveys At IRAM (ASAI), dedicated to chemical surveys of solar-type star forming regions with the IRAM 30m telescope. Observations of the millimeter windows between 72 and 272 GHz permitted the detection of HC$_3$N and its $^{13}$C isotopologues, and HC$_5$N (for the first time in a protostellar shock region). In the shock, analysis of the line profiles shows that the emission arises from the outflow cavities associated with L1157-B1 and L1157-B2. Molecular abundances and excitation conditions were obtained from analysis of the Spectral Line Energy Distributions under the assumption of Local Thermodynamical Equilibrium or using a radiative transfer code in the Large Velocity Gradient approximation. Towards L1157mm, the HC$_3$N emission arises from the cold envelope ($T_{rot}=10$ K) and a higher-excitation region ($T_{rot}$= $31$ K) of smaller extent around the protostar. We did not find any evidence of $^{13}$C or D fractionation enrichment towards L1157-B1. We obtain a relative abundance ratio HC$_3$N/HC$_5$N of 3.3 in the shocked gas. We find an increase by a factor of 30 of the HC$_3$N abundance between the envelope of L1157-mm and the shock region itself. Altogether, these results are consistent with a scenario in which the bulk of HC$_3$N was produced by means of gas phase reactions in the passage of the shock. This scenario is supported by the predictions of a parametric shock code coupled with the chemical model UCL_CHEM.
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