Northern Lobe Research Articles

DEEP IMAGING SURVEYS OF STAR-FORMING CLOUDS. V. NEW HERBIG-HARO SHOCKS AND GIANT OUTFLOWS IN TAURUS

We report the discovery of new Herbig-Haro shocks in the southeastern portion of the Taurus molecular cloud complex. The embedded protostar IRAS04325+2402, located in the eastern end of the B18 dark cloud in the cloud region known as L1535, drives a parsec-scale outflow with a length of at least 134' (5.5?pc) toward P.A. = 17?/197?? making this the largest protostellar outflow found thus far in Taurus. The southern terminus of this flow located about 65' (~2.7?pc) from the IRAS source consists of a large previously discovered shock, HH 434-436. The northern lobe contains three faint shocks with the most distant being HH 696 located 2.8?pc north of the source. Two smaller shocks, HH 699 and HH 703, are found midway between the IRAS source and HH 434-436. HH 703 has a proper motion of 170 ? 30?km s?1 toward P.A. = 195? ? 10?. Additionally, we report the discovery of a dozen new Herbig-Haro objects tracing outflows and protostellar jets in B18 and near IC 2087, at least two of which have also attained parsec-scale dimensions. These observations demonstrate that some low-mass stars forming from the Taurus clouds drive giant flows which can excite shocks in the inter-cloud medium degrees from their sources, providing further evidence for the generation of large-scale mass motions and turbulence by protostellar outflows.

Deep observation of the giant radio lobes of Centaurus A with theFermiLarge Area Telescope

The detection of high energy (HE) {\gamma}-ray emission up to about 3 GeV from the giant lobes of the radio galaxy Centaurus A has been recently reported by the Fermi-LAT Collaboration based on ten months of all-sky survey observations. A data set more than three times larger is used here to study the morphology and photon spectrum of the lobes with higher statistics. The larger data set results in the detection of HE {\gamma}-ray emission (up to about 6 GeV) from the lobes with a significance of more than 10 and 20 {\sigma} for the North and the South lobe, respectively. Based on a detailed spatial analysis and comparison with the associated radio lobes, we report evidence for a substantial extension of the HE {\gamma}-ray emission beyond the WMAP radio image in the case of the Northern lobe of Cen A. We reconstruct the spectral energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data from the same integration region. The implications are discussed in the context of hadronic and leptonic scenarios.

Triggered star formation in the inner filament of Centaurus A

We present recent Hubble Space Telescope observations of the inner filament of Centaurus A, using the new Wide Field Camera 3 (WFC3) F225W, F657N and F814W filters. We find a young stellar population near the south-west tip of the filament. Combining the WFC3 data set with archival Advanced Camera for Surveys (ACS) F606W observations, we are able to constrain the ages of these stars to ≲10 Myr, with best-fitting ages of 1–4 Myr. No further recent star formation is found along the filament. Based on the location and age of this stellar population, and the fact that there is no radio lobe or jet activity near the star formation, we propose an updated explanation for the origin of the inner filament. Sutherland et al. suggested that radio jet-induced shocks can drive the observed optical line emission. We argue that such shocks can naturally arise due to a weak cocoon-driven bow shock (rather than from the radio jet directly), propagating through the diffuse interstellar medium from a location near the inner northern radio lobe. The shock can overrun a molecular cloud, triggering star formation in the dense molecular cores. Ablation and shock heating of the diffuse gas then give rise to the observed optical line and X-ray emission. Deeper X-ray observations should show more diffuse emission along the filament.

Discovery of shocked H2 around OH 231.8+4.2

ABSTRACT We present K-band integral field observations of the circumstellar envelope of the evolved star OH 231.8+4.2. Spatial and spectral information was simultaneously acquired using the SINFONI integral field unit, with adaptive optics, on the Very Large Telescope. The observations reveal the discovery of H2 emission (1) around the centre of the nebula and (2) located in clumps along the western side of the northern lobe, presumably associated with the strong shocks that stimulate the previously reported Hα emission at the same location. An observed H2 1→0/2→1S(1) line ratio of 8.3 ± 1.9 was calculated for the central field, a value consistent with shock excitation.

Multiple responses of magnetotail to the enhancement and fluctuation of solar wind dynamic pressure and the southward turning of interplanetary magnetic field

During the interval from 06:15 to 07:30 UT on 24 August 2005, the Chinese Tan-Ce 1 (TC1) satellite observed the multiple responses of the near-Earth magnetotail to the combined changes in solar wind dynamic pressure and interplanetary magnetic field (IMF). The magnetotail was highly compressed by a strong interplanetary shock because of the dynamic pressure enhancement (similar to 15 nPa), and the large shrinkage of magnetotail made a northern lobe and plasma mantle move inward to the position of the inbound TC1 that was initially in the plasma sheet. Meanwhile, the dynamic pressure fluctuations (similar to 0.5-3 nPa) behind the shock drove the quasi-periodic oscillations of the magnetopause, lobe-mantle boundary, and geomagnetic field at the same frequencies: one dominant frequency was around 3 mHz and the other was around 5 mHz. The quasi-periodic oscillations of the lobe-mantle boundary caused the alternate entries of TC1 into the northern lobe and the plasma mantle. In contrast to a single squeezed or deformed magnetotail by a solar wind discontinuity moving tailward, the compressed and oscillating magnetotail can better indicate the dynamic evolution of magnetotail when solar wind dynamic pressure increases and fluctuates remarkably, and the near-Earth magnetotail is quite sensitive even to some small changes in the solar wind dynamic pressure when it is highly compressed. Furthermore, it is found that a considerable amount of oxygen ions (O(+)) appeared in the lobe after the southward turning of IMF.

Statistical properties of the magnetic field in the Kronian magnetotail lobes and current sheet

[1] We examine the characteristics of the magnetic field in Saturn's magnetotail lobes and current sheet during the Cassini spacecraft's exploration of the magnetotail from day 18 to day 291 of 2006. During this period Cassini reached maximum downtail distances of ∼68 RS, with orbits at the beginning of the survey interval concentrating on the equatorial regions, while later orbits moved to somewhat higher latitudes. We find that the field strength in the lobes falls off as Blobe (nT) = (251 ± 22) × (r (RS))−1.20±0.03. We show that the lobes and current sheet have distinctive magnetic pressure distributions, and we examine the transition of the field from the central current sheet out into the northern and southern lobes.

RADIO AND DEEPCHANDRAOBSERVATIONS OF THE DISTURBED COOL CORE CLUSTER ABELL 133

We present results based on new Chandra and multi-frequency radio observations of the disturbed cool core cluster Abell 133. The diffuse gas has a complex bird-like morphology, with a plume of emission extending from two symmetric wing-like features, and capped with a filamentary radio relic. X-ray observations indicate the presence of either high temperature gas or non-thermal emission in the region of the relic. We find evidence for a weak elliptical X-ray surface brightness edge surrounding the core, consistent with a sloshing cold front. The plume is consistent with having formed due to uplift by a buoyantly rising radio bubble, now seen as the radio relic. Our results are inconsistent with the previous suggestion that the X-ray wings formed due to the passage of a weak shock through the cool core. We instead conclude that the wings are due to X-ray cavities formed by gas displacement by the radio relic. The central cD galaxy contains two small-scale cold gas clumps that are slightly offset from their optical and UV counterparts, suggestive of a galaxy-galaxy merger event. On larger scales, there is evidence for cluster substructure in both optical observations and the X-ray temperature map. We suggest that Abell 133 has recently undergone a merger event with an interloping subgroup, initiating gas sloshing in the core. We show that the additional buoyant force from a passing subcluster can have a significant effect on the rise trajectories of buoyant bubbles, although this effect alone cannot fully explain the morphology of Abell 133. The radio observations reveal a previously unreported background giant radio galaxy at z = 0.293, the northern lobe of which overlies the radio relic in the core of Abell 133. A rough estimate indicates that the contribution of this background lobe to the total radio emission in the region of the relic is modest (< 13%). (Abridged)

THE MAGNETIC FIELD TOWARD THE YOUNG PLANETARY NEBULA K 3-35

K 3-35 is a planetary nebula (PN) where H2O maser emission has been detected, suggesting that it departed from the proto-PNe phase only some decades ago. Interferometric VLA observations of the OH 18 cm transitions in K~3-35 are presented.OH maser emission is detected in all four ground state lines (1612, 1665, 1667, and 1720 MHz). All the masers appear blueshifted with respect to the systemic velocity of the nebula and they have different spatial and kinematic distributions.The OH 1665 and 1720 MHz masers appear spatially coincident with the core of the nebula, while the OH 1612 and 1667 MHz ones exhibit a more extended distribution. We suggest that the 1665 and 1720 masers arise from a region close to the central star, possibly in a torus, while the 1612 and 1667 lines originate mainly from the extended northern lobe of the outflow. It is worth noting that the location and velocity of the OH 1720 MHz maser emission are very similar to those of the H2O masers (coinciding within 0.1" and ~2 km/s, respectively). We suggest that the pumping mechanism in the H2O masers could be produced by the same shock that is exciting the OH 1720 MHz transition. A high degree of circular polarization (>50%) was found to be present in some features of the 1612, 1665, and 1720 MHz emission.For the 1665 MHz transition at ~ +18 km/s the emission with left and right circular polarizations (LCP and RCP) coincide spatially within a region of ~0.03" in diameter.Assuming that these RCP and LCP 1665 features come from a Zeeman pair, we estimate a magnetic field of ~0.9 mG within 150 AU from the 1.3 cm continuum peak. This value is in agreement with a solar-type magnetic field associated with evolved stars.

Geotechnical controls on a steep lateral moraine undergoing paraglacial slope adjustment

Abstract Sustained post ‘Little Ice Age’ retreat of the northern lobe of the Feegletscher, Valais, Switzerland, has exposed lateral moraines that show pronounced oversteepening on the upper proximal slopes, with upper-slope segments displaying angles of up to about 70°. Paraglacial processes have eroded gullies into the upper-slope segments, and associated debris-flow deposits result in lower angles of between 34° and 25° in the mid-slope and slope-foot zones, respectively. In order to assess the geotechnical properties of morainic sediments that permit development of quasi-stable, oversteepened slope segments, a standard suite of geotechnical measures was applied to samples of Feegletscher moraine sediments. Shear box testing yielded angles of friction ranging from 35° for loose samples to 52° for dense samples. Although the heterogeneous nature of moraine deposits makes laboratory testing of the whole size range of in situ sediments impractical, shear box test results imply that in situ upper-slope angles exceed the angles of friction of moraine sediments by 26°–40°. We are unable to replicate angles of friction in shear box tests that correspond to in situ angles of the upper-slope sections measured in the field. However, we suggest that distal dipping mica-schist clasts may play an important role in permitting high-angle slope stability. Quasi-stable storage of glacigenic sediments in high-angle moraine sequences over decadal timescales has implications for understanding the period following deglaciation over which paraglacial reworking and redistribution of sediments may operate.

High-energy particle acceleration and production of ultra-high-energy cosmic rays in the giant lobes of Centaurus A

The nearby radio galaxy Centaurus A is poorly studied at high frequencies with conventional radio telescopes because of its very large angular size, but is one of a very few extragalactic objects to be detected and resolved by the Wilkinson Microwave Anisotropy Probe (WMAP). We have used the five-year WMAP data for Cen A to constrain the high-frequency radio spectra of the 10° giant lobes and to search for spectral changes as a function of position along the lobes. We show that the high-frequency radio spectra of the northern and southern giant lobes are significantly different: the spectrum of the southern lobe steepens monotonically (and is steeper further from the active nucleus) whereas the spectrum of the northern lobe remains consistent with a power law. The inferred differences in the northern and southern giant lobes may be the result of real differences in their high-energy particle acceleration histories, perhaps due to the influence of the northern middle lobe, an intermediate-scale feature which has no detectable southern counterpart. In light of these results, we discuss the prospects for Fermi Gamma-ray Space Telescope detections of inverse-Compton emission from the giant lobes and the lobes' possible role in the production of the ultra-high-energy cosmic rays (UHECR) detected by the Pierre Auger Observatory. We show that the possibility of a Fermi detection depends sensitively on the physical conditions in the giant lobes, with the northern lobe more likely to be detected, and that any emission observed by Fermi is likely to be dominated by photons at the soft end of the Fermi energy band. On the other hand, we argue that the estimated conditions in the giant lobes imply that UHECRs can be accelerated there, with a potentially detectable γ-ray signature at TeV energies.

A MULTI-EPOCHHSTSTUDY OF THE HERBIG-HARO FLOW FROM XZ TAURI

We present nine epochs of Hubble Space Telescope optical imaging of the bipolar outflow from the pre–mainsequence binary XZ Tauri. Our data monitor the system from 1995 to 2005 and include emission line images of the flow. The northern lobe appears to be a succession of bubbles, the outermost of which expanded ballistically from 1995 to 1999 but in 2000 began to deform and decelerate along its forward edge. It reached an extent of 6 �� from the binary in 2005. A larger and fainter southern counterbubble was detected for the first time in deep Advanced Camera for Surveys images from 2004. Traces of shocked emission are seen as far as 20 �� south of the binary. The bubble emission nebulosity has a low excitation overall, as traced by the [Sii]/Hα line ratio, requiring a nearly comoving surrounding medium that has been accelerated by previous ejections or stellar winds. Within the broad bubbles there are compact emission knots whose alignments and proper motions indicate that collimated jets are ejected from each binary component. The jet from the southern component, XZ Tau A, is aligned with the outflow axis of the bubbles and has tangential knot velocities of 70–200 km s −1 . Knots in the northern flow are seen to slow and brighten as they approach the forward edge of the outermost bubble. The knots in the jet from the other star, XZ Tau B, have lower velocities of ∼100 km s −1 . To explain the observations of the outer bubble, we propose that the XZ Tau A stellar jet underwent a large velocity pulse circa 1980. This ejection quickly overtook older, slower-moving ejecta very near the star, producing a ∼70 km s −1 shock in a hot (T ∼ 80,000 K), compact “fireball.” The initial thermal pressure of this gas parcel drove the expansion of a spherical bubble. Subsequent cooling caused the bubble to transition to ballistic expansion, followed by slowing of its forward edge by mass loading from the pre-shock medium. Repeated pulses may explain the multiple bubbles seen in the data. Collimated jets continue to flow through the bubble’s interior, and with the fading of the original fireball they are becoming the primary energizing mechanism for the emission line structures. Future evolution of the flow should see the outer bubble structures fade from view, and the emergence of a more typical Herbig–Haro jet/bow shock morphology. We present a preliminary numerical model of a pulsed jet to illustrate this scenario.

Dynamical response of the magnetotail to changes of the solar wind direction: an MHD modeling perspective

Abstract. We performed global MHD simulations to investigate the magnetotail response to the solar wind directional changes (Vz-variations). These changes, although small, cause significant variations of the neutral sheet shape and location even in the near and middle tail regions. They display a complicated temporal response, in which ~60 to 80% of the final shift of the neutral sheet in Z direction occurs within first 10–15 min (less for faster solar wind), whereas a much longer time (exceeding half hour) is required to reach a new equilibrium. The asymptotic equilibrium shape of the simulated neutral sheet is consistent with predictions of Tsyganenko-Fairfield (2004) empirical model. To visualize a physical origin of the north-south tail motion we compared the values of the total pressure in the northern and southern tail lobes and found a considerable difference (10–15% for only 6° change of the solar wind direction used in the simulation). That difference builds up during the passage of the solar wind directional discontinuity and is responsible for the vertical shift of the neutral sheet, although some pressure difference remains in the near tail even near the new equilibrium. Surprisingly, at a given tailward distance, the response was found to be first initiated in the tail center (the "leader effect"), rather than near the flanks, which can be explained by the wave propagation in the tail, and which may have interesting implications for the substorm triggering studies. The present results have serious implications for the data-based modeling, as they place constraints on the accuracy of tail magnetic configurations to be derived for specific events using data of multi-spacecraft missions, e.g. such as THEMIS.

Plasma convection in the magnetotail lobes: statistical results from Cluster EDI measurements

Abstract. A major part of the plasma in the Earth's magnetotail is populated through transport of plasma from the solar wind via the magnetotail lobes. In this paper, we present a statistical study of plasma convection in the lobes for different directions of the interplanetary magnetic field and for different geomagnetic disturbance levels. The data set used in this study consists of roughly 340 000 one-minute vector measurements of the plasma convection from the Cluster Electron Drift Instrument (EDI) obtained during the period February 2001 to June 2007. The results show that both convection magnitude and direction are largely controlled by the interplanetary magnetic field (IMF). For a southward IMF, there is a strong convection towards the central plasma sheet with convection velocities around 10 km s−1. During periods of northward IMF, the lobe convection is almost stagnant. A By dominated IMF causes a rotation of the convection patterns in the tail with an oppositely directed dawn-dusk component of the convection for the northern and southern lobe. Our results also show that there is an overall persistent duskward component, which is most likely a result of conductivity gradients in the footpoints of the magnetic field lines in the ionosphere.

Petrology and textural classification of the Jericho kimberlite, northern Slave Province, Nunavut, Canada

The paper presents data on petrology, bulk rock and mineral compositions, and textural classification of the Middle Jurassic Jericho kimberlite (Slave craton, Canada). The kimberlite was emplaced as three steep-sided pipes in granite that was overlain by limestones and minor soft sediments. The pipes are infilled with hypabyssal and pyroclastic kimberlites and connected to a satellite pipe by a dyke. The Jericho kimberlite is classified as a Group Ia, lacking groundmass tetraferriphlogopite and containing monticellite pseudomorphs. The kimberlite formed during several consecutive emplacement events of compositionally different batches of kimberlite magma. Core-logging and thin-section observations identified at least two phases of hypabyssal kimberlites and three phases of pyroclastic kimberlites. Hypabyssal kimberlites intruded as a main dyke (HK1) and as late small-volume aphanitic and vesicular dykes. Massive pyroclastic kimberlite (MPK1) predominantly filled the northern and southern lobes of the pipe and formed from magma different from the HK1 magma. The MPK1 magma crystallized Ti-, Fe-, and Cr-rich phlogopite without rims of barian phlogopite, and clinopyroxene and spinel without atoll structures. MPK1 textures, superficially reminiscent of tuffisitic kimberlite, are caused by pervasive contamination by granite xenoliths. The next explosive events filled the central lobe with two varieties of pyroclastic kimberlite: (1) massive and (2) weakly bedded, normally graded pyroclastic kimberlite. The geology of the Jericho pipe differs from the geology of South African or the Prairie kimberlites, but may resemble Lac de Gras pipes, in which deeper erosion removed upper facies of resedimented kimberlites.

Detection of Extended Hot Water in the Outflow from NGC 2071

We report the results of spectroscopic mapping observations carried out toward a ~1 min x 1 min region within the northern lobe of the outflow from NGC 2071 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations covered the 5.2-37 um spectral region and have led to the detection of a number of ionic, atomic, and molecular lines, including fine-structure emission of Si+, Fe+, S++, S, the S(0)-S(7) pure rotational lines of H2, the R(3) and R(4) transitions of HD, and at least 11 transitions of H2O. In addition, the 6.2, 7.4, 7.6, 7.9, 8.6 and 11.3 um PAH emission bands were also observed and several transitions of OH were tentatively detected. Most of the detected line transitions were strong enough to map including, for the first time, three transitions of hot H2O. We find that: (1) the water emission is extended; (2) the extended emission is aligned with the outflow; and, (3) the spatial distribution of the water emission generally follows that observed for H2. Based on the measured line intensities, we derive an HD abundance relative to H2 of 1.1-1.8 10^-5 and an H2O number density of 12-2 cm^3. The H2 density in the water-emitting region is not well constrained by our observations, but is likely between 3 10^4 and 10^6 cm^3, yielding an H2O abundance relative to H2 of between 2 10^-5 and 6 10^-4. Future observations planned for the Herschel Space Observatory should greatly improve the density estimate, and thus our knowledge of the H2O abundance, for the water-emitting regions reported here. Finally, we note a possible departure from the H2O ortho-to-para ratio of 3:1 expected for water formed in hot post-shocked gas, suggesting that a significant fraction of the water vapor we detect may arise from H2O sputtered from cold dust grains.

Petrogenesis of contact-style PGE mineralization in the northern lobe of the Bushveld Complex: comparison of data from the farms Rooipoort, Townlands, Drenthe and Nonnenwerth

In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal % and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca. 400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites, and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics. At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas, notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which precipitated during cooling and crystallization.

The X-ray jet and lobes of PKS 1354+195 (=4C 19.44)

We present a Chandra image of the quasar, jet, and lobes of PKS 1354+195 (=4C 19.44). The radio jet is 18 arcsec long, and appears to be very straight. The length gives many independent spatial resolution elements in the Chandra image while the straightness implies that the geometrical factors are constant along the jet although their values are uncertain. We also have 4 frequency radio images with half to one arcsecond angular resolution, and use HST and Spitzer data to study the broad band spectral energy distributions. The X-ray and radio spectra are both consistent with a spectrum fν∝ν−0.7 for the integrated jet. Using that spectral index, the model of inverse Compton scattering of electrons on the cosmic microwave background (IC/CMB) gives magnetic field strengths and Doppler factors that are relatively constant along the jet. Extended X-ray emission is evident in the direction of the otherwise unseen counter-jet. X-ray emission continues past the radio jet to the South, and is detected within both the southern and northern radio lobes.

Herbig-Haro jets in 3D: the HL/XZ Tauri region

Context. Jets and outflows from young stellar objects (YSOs) can be identified and traced through the distribution and structure of shocked emission-line features. To understand the formation of these regions requires kinematic data at high spectral resolution and with full spatial coverage. Aims. In this work, we investigate the environment of HL/XZ Tau, which contains a compact and very active nest of YSOs. We explore the kinematic properties of the close association of jets in this region and study the interaction of jets with the ambient medium, as well as the outflows with each other. Methods. We present scanning Fabry-Perot interferometry of the HL/XZ Tau region in Ha and [S II] 6716 angstrom emission. We also measure the proper motions of the knots in the outflows, as derived from images obtained in 1997 and 2001, to achieve the full 3D kinematic picture. Results. Radial velocities of the HL Tau jet indicate a fast spine of low excitation surrounded by a slower sheared sheath of higher excitation. Proper motions range from 200-220 km s(-1) in the HL Tau jet and are aligned within 10 degrees of the jet spine. In combination, the proper motions and radial velocities indicate that three outflows in this region may be interacting with each other. Evidence of an outflow associated with LkH alpha 358 is found, and we suggest it is a source of Herbig-Haro (HH) knots that lie to the southeast of HL Tau and HH 265. Conclusions. We conclude that the southern lobe of the XZ Tau wind disrupts the eastern lobe of the collimated outflow from LkHa 358. The jet emerging from HL Tau is deflected by the northern lobe of the wind from XZ Tau. We propose several probable explanations for the unusual structure of the HL Tau jet. It is plausible that the shocks in the jet spine are maintained by the ram pressure of a low-density crosswind from XZ Tau. The crosswind interacts to form a sheath of entrained gas.

Discrete plumbing systems and heterogeneous magma sources of a 24 km 3 off-axis lava field on the western flank of East Pacific Rise, 14 °S.

The largest known mid-ocean-ridge off-axis lava flow field occurs slightly off-axis near 14 °S along the East Pacific Rise (EPR). It comprises at least four volcanologically discrete units. We collected lava samples from 24 sites within the off-axis lava field and 7 sites on the adjacent ridge crest using the Shinkai-6500 submersible. The lava field comprises at least three distinct MORB compositions, all of which are different from the lavas collected from the nearby ridge axis. The east and west cones and the northern lobe of the lava field consist of normal MORB (N-MORB) lavas with a low concentration of incompatible elements and low LILE/HFSE and LREE/HREE ratios. By contrast, the samples from the west plain of the field have a higher concentration of incompatible elements and higher LILE/HFSE and LREE/HREE ratios indicating T-MORB character. The lava samples collected from the summit of the east cone show the highest concentration of LILE elements and LREEs among the lava field (E-MORB lava). The N-MORB of the off-axis lava field are more depleted in incompatible elements than the adjacent EPR axis lavas, possibly reflecting the re-melting of the residual mantle in the off-axis region. The E-MORB lava was probably derived from fertile mantle that did not undergo melting beneath the spreading center. T-MORB, which occupies the main part of the 14 °S lava field, is the product of magma mixing between N-MORB and E-MORB magmas.

Dynamics of the Extratropical Response to a Tropical Atlantic SST Anomaly

Abstract Previous atmospheric general circulation model (AGCM) experiments revealed that atmospheric responses to a tropical Atlantic sea surface temperature anomaly (SSTA) were asymmetric with respect to the sign of the SSTA. A positive SSTA produced a south–north dipole in geopotential heights, much like the North Atlantic Oscillation (NAO), while a negative SSTA yielded an eastward-propagating wave train, with the northern lobe of the NAO absent. Here these height responses are decomposed into components that are symmetric or antisymmetric with respect to the sign of the SSTA. The symmetric, or notionally linear, component is a nearly south–north dipole projecting on the NAO, while the antisymmetric, or notionally nonlinear, component is a different dipole. Experiments with a diagnostic linear baroclinic model (LBM) suggest that both components are maintained primarily by transient-eddy forcing. Dynamical mechanisms for the formation of the two components are explored using the LBM and a nonlinear barotropic vorticity equation model (BVM). Transient-eddy feedback is sufficient to explain the linear response. The NAO-like linear response occurs when the initial heating induces transient-eddy forcing in the exit of the Atlantic jet. The structure of the background absolute vorticity in this region is such that this transient-eddy forcing induces a nearly north–south dipole in anomalous geopotential heights. When the nonlinear self-interaction of this transient-induced low-frequency perturbation is included in the BVM, the dipole axis tilts to the east or west, resulting in a response that is nonlinear about the sign of the forcing.