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Register a new userVariability of the SiO thermal line emission toward the young L1448-mm outflow
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Izaskun Jimenez-Serra
Publication date
2011
fields
Physics
and research's language is
English
Authors
I. Jimenez-Serra
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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.
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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.
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.
Spectral line survey observations are conducted toward the high-mass protostar candidate NGC 2264 CMM3 in the 4 mm, 3 mm, and 0.8 mm bands with the Nobeyama 45 m telescope and the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. In total, 265 emission lines are detected in the 4 mm and 3 mm bands, and 74 emission lines in the 0.8 mm band. As a result, 36 molecular species and 30 isotopologues are identified. In addition to the fundamental molecular species, many emission lines of carbon-chain molecules such as HC5N, C4H, CCS, and C3S are detected in the 4 mm and 3 mm bands. Deuterated molecular species are also detected with relatively strong intensities. On the other hand, emission lines of complex organic molecules such as HCOOCH3, and CH3OCH3 are found to be weak. For the molecules for which multiple transitions are detected, rotation temperatures are derived to be 7-33 K except for CH3OH. Emission lines with high upper-state energies (Eu > 150 K) are detected for CH3OH, indicating existence of a hot core. In comparison with the chemical composition of the Orion KL, carbon-chain molecules and deuterated molecules are found to be abundant in NGC 2264 CMM3, while sulfur-bearing species and complex organic molecules are deficient. These characteristics indicate chemical youth of NGC 2264 CMM3 in spite of its location at the center of the cluster forming core, NGC 2264 C.
We present new JVLA multi-frequency measurements of a set of stars in transition from the post-AGB to the Planetary Nebula phase monitored in the radio range over several years. Clear variability is found for five sources. Their light curves show increasing and decreasing patterns. New radio observations at high angular resolution are also presented for two sources. Among these is IRAS 18062+2410, whose radio structure is compared to near-infrared images available in the literature. With these new maps, we can estimate inner and outer radii of 0.03$$ and 0.08$$ for the ionised shell, an ionised mass of $3.2times10^{-4}$ M$_odot$, and a density at the inner radius of $7.7times 10^{-5}$ cm$^{-3}$, obtained by modelling the radio shell with the new morphological constraints. The combination of multi-frequency data and, where available, spectral-index maps leads to the detection of spectral indices not due to thermal emission, contrary to what one would expect in planetary nebulae. Our results allow us to hypothesise the existence of a link between radio variability and non-thermal emission mechanisms in the nebulae. This link seems to hold for IRAS 22568+6141 and may generally hold for those nebulae where the radio flux decreases over time.
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.
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