Tracing shock type with chemical diagnostics. An application to L1157 (original) (raw)
Related papers
The B1 shock in the L1157 outflow as seen at high spatial resolution
Monthly Notices of the Royal Astronomical Society, 2013
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), CH 3 OH (3 K -2 K ), HC 3 N (16-15) and p-H 2 CO (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 ′′ ) 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 5×10 3 n H2 5×10 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 HC 3 N column density.
The CHESS survey of the L1157-B1 shock: the dissociative jet shock as revealed by Herschel –PACS
Astronomy & Astrophysics, 2012
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 µm spectra of the L1157-B1 shock, showing emission lines from CO, H 2 O, OH, and [O i]. 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-J up CO lines indicates high-excitation conditions (T ex ≃ 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< T kin <800 K and n ≥ 10 5 cm −3 . The CO emission comes from a region of about 7 ′′ located at the rear of the bow shock where the [O i] and OH emission also originate. Comparison with shock models shows that the bright [O i] 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-J up CO emission.
THE CHESS SURVEY OF THE L1157-B1 SHOCK REGION: CO SPECTRAL SIGNATURES OF JET-DRIVEN BOW SHOCKS
The Astrophysical Journal, 2012
The unprecedented sensitivity of Herschel coupled with the high resolution of the HIFI spectrometer permits studies of the intensity-velocity relationship I(v) in molecular outflows, over a higher excitation range than possible up to now. Over the course of the CHESS Key Program, we have observed toward the bright bow shock region L1157-B1, the CO rotational transitions between J = 5-4 and J = 16-15 with HIFI, and the J = 1-0, 2-1, and 3-2 with the IRAM 30 m and the Caltech Submillimeter Observatory telescopes. We find that all the line profiles I CO (v) are well fit by a linear combination of three exponential laws ∝ exp(−|v/v 0 |) with v 0 = 12.5, 4.4, and 2.5kms −1 . The first component dominates the CO emission at J 13, as well as the high-excitation lines of SiO and H 2 O. The second component dominates for 3 J up 10 and the third one for J up 2. We show that these exponentials are the signature of quasi-isothermal shocked gas components: the impact of the jet against the L1157-B1 bow shock (T k ≃ 210 K), the walls of the outflow cavity associated with B1 (T k ≃ 64 K), and the older cavity L1157-B2 (T k ≃ 23 K), respectively. Analysis of the CO line flux in the large-velocity gradient approximation further shows that the emission arises from dense gas (n 10 5 -10 6 cm −3 ) close to LTE up to J = 20. We find that the CO J = 2-1 intensity-velocity relation observed in various other molecular outflows is satisfactorily fit by similar exponential laws, which may hold an important clue to their entrainment process.
The Astrophysical Journal, 2013
We present the first Herschel spectroscopic detections of the [OI]63µm and [CII]158µm fine-structure transitions, and a single para-H 2 O line from the 35 x 15 kpc 2 shocked intergalactic filament in Stephan's Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (> 1000 km s −1 ) luminous [CII] line profiles, as well as fainter [OI]63µm emission. SPIRE FTS observations reveal water emission from the p-H 2 O (1 11 -0 00 ) transition at several positions in the filament, but no other molecular lines. The H 2 O line is narrow, and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [CII]/PAH tot and [CII]/FIR ratios are too large to be explained by normal photo-electric heating in PDRs. HII region excitation or X-ray/Cosmic Ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [CII], [OI] and warm H 2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the IGM is dissipated to small scales and low-velocities, via shocks and turbulent eddies. Low-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H 2 ratios observed. The discovery that [CII] emission can be enhanced, in large-scale turbulent regions in collisional environments has implications for the interpretation of [CII] emission in high-z galaxies.
The CHESS spectral survey of star forming regions: Peering into the protostellar shock L1157-B1
Astronomy and Astrophysics, 2010
We present the first results of the unbiased survey of the L1157-B1 bow shock, obtained with HIFI in the framework of the key program Chemical Herschel surveys of star forming regions (CHESS). The L1157 outflow is driven by a low-mass Class 0 protostar and is considered the prototype of the so-called chemically active outflows. The bright blue-shifted bow shock B1 is the ideal laboratory for studying the link between the hot (∼ 1000-2000 K) component traced by H 2 IR-emission and the cold (∼ 10-20 K) swept-up material. The main aim is to trace the warm gas chemically enriched by the passage of a shock and to infer the excitation conditions in L1157-B1. A total of 27 lines are identified in the 555-636 GHz region, down to an average 3σ level of 30 mK. The emission is dominated by CO(5-4) and H 2 O(1 10 -1 01 ) transitions, as discussed by Lefloch et al. in this volume. Here we report on the identification of lines from NH 3 , H 2 CO, CH 3 OH, CS, HCN, and HCO + . The comparison between the profiles produced by molecules released from dust mantles (NH 3 , H 2 CO, CH 3 OH) and that of H 2 O is consistent with a scenario in which water is also formed in the gas-phase in high-temperature regions where sputtering or grain-grain collisions are not efficient. The high excitation range of the observed tracers allows us to infer, for the first time for these species, the existence of a warm (≥ 200 K) gas component coexisting in the B1 bow structure with the cold and hot gas detected from ground.
2010
Context. 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. Aims. 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. Methods. The CO 5-4 and o-H 2 O 1 10 − 1 01 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.
Broad N2H+ Emission toward the Protostellar Shock L1157-B1
We present the first detection of N 2 H + towards a low-mass protostellar outflow, namely the L1157-B1 shock, at ∼ 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 N 2 H + (1-0) line originates from the dense (≥ 10 5 cm −3 ) gas associated with the large (20 ′′ -25 ′′ ) cavities opened by the protostellar wind. We find a N 2 H + column density of few 10 12 cm −2 corresponding to an abundance of (2-8) × 10 −9 . The N 2 H + 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 (≃ 2000 yr). Modelling of C-shocks confirms that the abundance of N 2 H + is not increased by the passage of the shock. In summary, N 2 H + 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.
The CHESS survey of the L1157-B1 bow-shock: high and low excitation water vapor
Astronomy & Astrophysics, 2014
Context. 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. In particular, water is a powerful tracer of shocked material due its sensitivity to both physical conditions and chemical processes. Aims. As part of the "Chemical Herschel Surveys of Star forming regions" (CHESS) guaranteed time key program, we aim at investigating the physical and chemical conditions of H 2 O in the brightest shock region B1 of the L1157 molecular outflow. Methods. We observed several ortho-and para-H 2 O transitions using HIFI and PACS instruments on board Herschel toward L1157-B1, providing a detailed picture of the kinematics and spatial distribution of the gas. We performed a Large Velocity Gradient (LVG) analysis to derive the physical conditions of H 2 O shocked material, and ultimately obtain its abundance. Results. We detected 13 H 2 O lines with both instruments probing a wide range of excitation conditions. This is the largest data set of water lines observed in a protostellar shock that provide both the kinematics and the spatial information of the emitting gas. PACS maps reveal that H 2 O traces weak and extended emission associated with the outflow identified also with HIFI in the o-H 2 O line at 556.9 GHz, and a compact (∼10 ′′ ) bright, higher-excitation region. The LVG analysis of H 2 O lines in the bow-shock show the presence of two gas components with different excitation conditions: a warm (T kin ≃200-300 K) and dense (n(H 2 )≃(1-3)×10 6 cm −3 ) component with an assumed extent of 10 ′′ and a compact (∼2 ′′ -5 ′′ ) and hot, tenuous (T kin ≃900-1400 K, n(H 2 )≃10 3−4 cm −3 ) gas component, which is needed to account for the line fluxes of high E u transitions. The fractional abundance of the warm and hot H 2 O gas components is estimated to be (0.7-2)×10 −6 and (1-3)×10 −4 , respectively. Finally, we identified an additional component in absorption in the HIFI spectra of H 2 O lines connecting with the ground state level. This absorption probably arises from the photodesorption of icy mantles of a water-enriched layer at the edges of the cloud, driven by the external UV illumination of the interstellar radiation field.
Anatomy of HH 111 from CO Observations: A Bow‐Shock‐driven Molecular Outflow
The Astrophysical Journal, 2007
We present single-dish and interferometric millimeter line observations of the HH 111 outflow and its driving source. The physical conditions of the protostellar core have been determined from the emission of the millimeter line emission of CO and its isotopomers and CS with the IRAM 30m telescope, and the CO J = 7 → 6 line with the Caltech Submm Observatory. The molecular gas emission reveals a small condensation of cold (T = 20−25 K) and dense gas (n(H 2 ) = 3×10 5 cm −3 ). The low-velocity outflowing gas has been mapped with the IRAM Plateau de Bure interferometer. The cold gas is distributed in a hollow cylinder surrounding the optical jet. The formation of this cavity and its kinematics are well accounted for in the frame of outflow gas entrainment by jet bow shocks. Evidence of gas acceleration is found along the cavity walls, correlated with the presence of optical bow shocks. The cavity has been expanding with a mean velocity of 4 km s −1 on a timescale of 8700 yr, similar to the dynamical age of the optical jet. The separation of the inner walls reaches 8 ′′ − 10 ′′ , which matches the transverse size of the wings in the bow shock. CSO observations of the J = 7 → 6 line show evidence of a high-velocity and hot gas component (T = 300−1000 K) with a low filling factor. This emission probably arises from shocked molecular gas in the jet. Observations of the 3 P 2 − 3 P 1 [CI] line are consistent with C-type non-dissociative shocks. Mapping of the high-velocity molecular bullets B1-B3, located beyond the optical jet, with the IRAM PdBI reveals small structures of 3 ′′ × 7 ′′ flattened perpendicular to the flow direction. They are made of cold (T ∼ 30 K), moderate density gas (n(H 2 ) = (0.5 − 1.0) × 10 4 cm −3 ). We find evidence that the bullets are expanding into the low-density surrounding medium. Their properties are consistent with their being shocked gas knots resulting from past time-variable ejections in the jet.