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Register a new userGrain Evolution across the Shocks in the L1448-mm Outflow
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Added by
Izaskun Jimenez-Serra
Publication date
2005
fields
Physics
and research's language is
English
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The recent detection of shock-precursors toward the very young L1448-mm outflow offers us the possibility to study the grain chemistry during the first stages of the shock evolution, constraining the molecules ejected from grains and the species formed in gas phase. Observations of key molecules in the grain chemistry such as SiO, CH3OH, SO, CS, H2S, OCS, and SO2 toward this outflow are presented. The line profiles and the derived abundances show three distinct velocity regimes that trace the shock evolution: the preshock, the shock-precursor and the postshock gas. The SiO, CH3OH, SO, and CS abundances are enhanced with respect to the quiescent gas by 1 order of magnitude in the shock-precursor component, and by 3 orders of magnitude in the postshock gas. The derived SiO and CH3OH abundances are consistent with the recent ejection of these molecules from grains. Since H2S is only enhanced in the shock-precursor component, and OCS and SO2 are undetected, SO and CS are the most abundant sulfur-bearing species in the grain mantles of L1448-mm. The ejection of mainly SO and CS rather than H2S or OCS from grains, suggests that the sulfur chemistry will depend on the chemical history of the grain mantles in outflows and hot cores.
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The detection of narrow SiO thermal emission toward young outflows has been proposed to be a signature of the magnetic precursor of C-shocks. Recent modeling of the SiO emission across C-shocks predicts variations in the SiO line intensity and line shape at the precursor and intermediate-velocity regimes in only few years. We present high-angular resolution (3.8x3.3) images of the thermal SiO J=2-1 emission toward the L1448-mm outflow in two epochs (November 2004-February 2005, March-April 2009). Several SiO condensations have appeared at intermediate velocities (20-50 km/s) toward the red-shifted lobe of the outflow since 2005. Toward one of the condensations (clump D), systematic differences of the dirty beams between 2005 and 2009 could be responsible for the SiO variability. At higher velocities (50-80 km/s), SiO could also have experienced changes in its intensity. We propose that the SiO variability toward L1448-mm is due to a real SiO enhancement by young C-shocks at the internal working surface between the jet and the ambient gas. For the precursor regime (5.2-9.2 km/s), several narrow and faint SiO components are detected. Narrow SiO tends to be compact, transient and shows elongated (bow-shock) morphologies perpendicular to the jet. We speculate that these features are associated with the precursor of C-shocks appearing at the interface of the new SiO components seen at intermediate velocities.
Context: L1448-mm is the prototype of a low-mass Class 0 protostar driving a high-velocity jet. Given its bright H2O spectra observed with ISO, L1448-mm is an ideal laboratory to observe heavy water (HDO) emission. Aims: Our aim is to image the HDO emission in the protostar surroundings, the possible occurrence of HDO emission also investigating off L1448-mm, towards the molecular outflow. Methods: We carried out observations of L1448-mm in the HDO(1_10-1_11) line at 80.6 GHz, an excellent tracer of HDO column density, with the IRAM Plateau de Bure Interferometer. Results: We image for the first time HDO emission around L1448-mm. The HDO structure reveals a main clump at velocities close to the ambient one towards the the continuum peak that is caused by the dust heated by the protostar. In addition, the HDO map shows tentative weaker emission at about 2000 AU from the protostar towards the south, which is possibly associated with the walls of the outflow cavity opened by the protostellar wind. Conclusions: Using an LVG code, modelling the density and temperature profile of the hot-corino, and adopting a gas temperature of 100 K and a density of 1.5 10^8 cm^-3, we derive a beam diluted HDO column density of about 7 10^13 cm^-2, corresponding to a HDO abundance of about 4 10^-7. In addition, the present map supports the scenario where HDO can be efficiently produced in shocked regions and not uniquely in hot corinos heated by the newly born star.
As part of the WISH (Water In Star-forming regions with Herschel) key project, we report on the observations of several ortho- and para-H2O lines performed with the HIFI instrument towards two bright shock spots (R4 and B2) along the outflow driven by the L1448 low-mass proto-stellar system, located in the Perseus cloud. These data are used to identify the physical conditions giving rise to the H2O emission and infer any dependence with velocity. These observations provide evidence that the observed water lines probe a warm (T_kin~400-600 K) and very dense (n 10^6 - 10^7 cm^-3) gas, not traced by other molecules, such as low-J CO and SiO, but rather traced by mid-IR H2 emission. In particular, H2O shows strong differences with SiO in the excitation conditions and in the line profiles in the two observed shocked positions, pointing to chemical variations across the various velocity regimes and chemical evolution in the different shock spots. Physical and kinematical differences can be seen at the two shocked positions. At the R4 position, two velocity components with different excitation can be distinguished, with the component at higher velocity (R4-HV) being less extended and less dense than the low velocity component (R4-LV). H2O column densities of about 2 10^13 and 4 10^14 cm^-2 have been derived for the R4-LV and the R4-HV components, respectively. The conditions inferred for the B2 position are similar to those of the R4-HV component, with H2O column density in the range 10^14 - 5 10^14 cm^-2, corresponding to H2O/H2 abundances in the range 0.5 - 1 10^-5. The observed line ratios and the derived physical conditions seem to be more consistent with excitation in a low velocity J-type shock with large compression rather than in a stationary C-shock, although none of these stationary models seems able to reproduce all the characteristics of the observed emission.
The isocyanic acid (HNCO) presents an extended distribution in the centers of the Milky Way and the spiral galaxy IC342. Based on the morphology of the emission and the HNCO abundance with respect to H2, several authors made the hypothesis that HNCO could be a good tracer of interstellar shocks. Here we test this hypothesis by observing a well-known Galactic source where the chemistry is dominated by shocks. We have observed several transitions of HNCO towards L1157-mm and two positions (B1 and B2) in the blue lobe of the molecular outflow. The HNCO line profiles exhibit the same characteristics of other well-known shock tracers like CH3OH, H2CO, SO or SO2. HNCO, together with SO2 and OCS, are the only three molecules detected so far whose emission is much more intense in B2 than in B1, making these species valuable probes of chemical differences along the outflow. The HNCO abundance with respect to H2 is 0.4-1.8 10^-8 in B1 and 0.3-1 10^-7 in B2. These abundances are the highest ever measured, and imply an increment with respect to L1157-mm of a factor up to 83, demonstrating that this molecule is actually a good shock tracer. Our results probe that shocks can actually produce the HNCO abundance measured in galactic nuclei and even higher ones. We propose that the gas phase abundance of HNCO is due both to grain mantles erosion by the shock waves and by neutral-neutral reactions in gas phase involving CN and O2. The observed anticorrelation of CN and HNCO fluxes supports this scenario. The observed similarities of the HNCO emission and the sulfured molecules may arise due to formation pathways involving also O2.
During the formation of a star, material is ejected along powerful jets that impact the ambient material. This outflow regulates star formation by e.g. inducing turbulence and heating the surrounding gas. Understanding the associated shocks is therefore essential to the study of star formation. We present comparisons of shock models with CO, H2, and SiO observations in a pure shock position in the BHR71 bipolar outflow. These comparisons provide an insight into the shock and pre-shock characteristics, and allow us to understand the energetic and chemical feedback of star formation on Galactic scales. New CO (Jup = 16, 11, 7, 6, 4, 3) observations from the shocked regions with the SOFIA and APEX telescopes are presented and combined with earlier H2 and SiO data (from the Spitzer and APEX telescopes). The integrated intensities are compared to a grid of models that were obtained from a magneto-hydrodynamical shock code which calculates the dynamical and chemical structure of these regions combined with a radiative transfer module based on the large velocity gradient approximation. The CO emission leads us to update the conclusions of our previous shock analysis: pre-shock densities of 1e4 cm-3 and shock velocities around 20-25 km s-1 are still constrained, but older ages are inferred ( 4000 years). We evaluate the contribution of shocks to the excitation of CO around forming stars. The SiO observations are compatible with a scenario where less than 4% of the pre-shock SiO belongs to the grain mantles. We infer outflow parameters: a mass of 1.8x1e-2 Msun was measured in our beam, in which a momentum of 0.4 Msun km s-1 is dissipated, for an energy of 4.2x1e43erg. We analyse the energetics of the outflow species by species. Comparing our results with previous studies highlights their dependence on the method: H2 observations only are not sufficient to evaluate the mass of outflows.
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