X-wind Model Research Articles

A high-resolution radio study of the L1551 IRS 5 and L1551 NE jets

Using observations with e-MERLIN and the Karl G. Jansky Very Large Array (VLA), together with archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), we obtain high-resolution radio images of two binary YSOs: L1551 IRS 5 and L1551 NE, covering a wide range of frequencies from 5 to 336 GHz, and resolving emission from the radio jet on scales of only ∼15 au. By comparing these observations to those from a previous epoch, it is shown that there is a high degree of variability in the free-free emission from the jets of these sources. In particular, the northern component of L1551 IRS 5 shows a remarkable decline in flux density of a factor of ∼5, suggesting that the free-free emission of this source has almost disappeared. By fitting the spectra of the sources, the ionised mass-loss rates of the jets were derived and it was shown that there is significant variability of up to a factor of ∼6 on timescales of ∼20 years. Using radiative transfer modelling, we also obtained a model image for the jet of the southern component of L1551 IRS 5 to help study the inner region of the ionised high-density jet. The findings favour the X-wind model launched from a very small innermost region.

ALMA and VLBA views on the outflow associated with an O-type protostar in G26.50+0.28

Protostellar jets and outflows are essential ingredients of the star formation process. A better understanding of this phenomenon is important in its own right as well as for many fundamental aspects of star formation. Jets and outflows associated with O-type protostars are rarely studied with observations reaching the close vicinity of the protostars. In this work, we report high-resolution ALMA and VLBA observations to reveal a clear and consistent picture of an outflow associated with an O-type protostar candidate in the G26.50+0.28 region. These observations reveal, for the first time, a collimated jet located in the middle of the outflow cavity. The jet is found to be perpendicular to an elongated disk/toroid and its velocity gradient. The collimated jet appears to show a small amplitude (α≈0°.06) counterclockwise precession, when looking along the blueshifted jet axis from the strongest continuum source MM1, with a precession length of 0.22 pc. The inclination of the jet is likely to be very low (≈8°), which makes it a promising target to study its transverse morphologies and kinematics. However, no clear evidence of jet rotation is found in the ALMA and VLBA observations. The three-dimensional velocities of the water maser spots appear to show the same absolute speed with respect to different opening angles, suggesting the jet winds may be launched in a relatively small region. This favors the X-wind model, that is, jets are launched in a small area near the inner disk edge.

A Unified Model for Bipolar Outflows from Young Stars: The Interplay of Magnetized Wide-angle Winds and Isothermal Toroids

Abstract Large-scale morphology and time evolution are investigated for the unified model of bipolar outflows outlined in Shang et al. (2006), where an outflow forms by a radially directed, wide-angle magnetized wind interacting with magnetized isothermal toroids in various quasistatic states. The primary wide-angle wind is toroidally magnetized and maintains a cylindrically stratified density profile, mimicking the asymptotic solution of the cold X-wind model. We explore the interplay between the toroidally magnetized primary wind and the surrounding toroids threaded by poloidal magnetic fields and examine how the jet and shell morphology and fine structures within them vary with the physical parameters. The variation in flatness of the density distribution in the ambient isothermal toroids helps shape the varieties of lobe morphology and collimation. The presence of a stronger ambient poloidal field helps shape the outflow, forming a magnetic cocoon and nested multilayered cavities surrounding the wind-filled lobe, which is most evident in the more open configurations of the ambient toroids. The wind–toroid interface is prone to substantial shear and thus unstable to the Kelvin–Helmholtz instability. Magnetic forces in the compressed toroidally magnetized high-velocity wind can generate vorticity, leading to nonlinear patterns within the extended magnetized mixing layers. Magnetic disturbances generated by the interplay could modulate lobe shapes, density, and velocities, giving rise to visual impressions of thicker and rugged shells and apparent episodic distribution of matter. The system maintains a quasi-self-similar evolution in time, which serves as a proxy for understanding the underlying physical mechanisms driving it.

RW Aur A FROM THE X-WIND POINT OF VIEW: GENERAL FEATURES

In this paper, the RW Aur A microjet is studied from the point of view of X-wind models. The archived HST/STIS spectra of optical forbidden lines [O I], [S II], and [N II] from RW Aur A, taken in Cycle 8 with seven parallel slits along the jet axis, spaced at 0".07 apart, were analyzed. Images, position-velocity diagrams, and line ratios among the species were constructed, and compared with synthetic observations generated by selected solutions of the X-wind. Prominent features arising in a steady state X-wind could be identified within the convolved images, full-widths at half maxima and high-velocity peaks on both of the redshifted and blueshifted jets. The well-known asymmetric velocity profiles of the opposite jets are built into the selected models. We discuss model selections within the existing uncertainties of stellar parameters and inclination angle of the system. In this framework, the mass-loss rates that were inferred to be decreasing along the jet axis in the literature are the results of slowly decreasing excitation conditions and electron density profiles. Despite the apparent asymmetry in terminal velocities, line intensities and mass-loss rates, the average linear momenta from the opposite sides of the jet are actually balanced. These previously hard-to-explain features of the asymmetric RW Aur A jet system now find a different but self-consistent interpretation within the X-wind framework.

TESTING DISK-LOCKING IN NGC 2264

We test analytic predictions from different models of magnetospheric accretion, which invoke disk-locking, using stellar and accretion parameters derived from models of low resolution optical spectra of 36 T Tauri stars (TTSs) in NGC 2264 (age~3 Myrs). Little evidence is found for models that assume purely dipolar field geometries; however, strong support is found in the data for a modified version of the X-wind model (Shu et al. 1994) which allows for non-dipolar field geometries. The trapped flux concept in the X-wind model is key to making the analytic predictions which appear supported in the data. By extension, our analysis provides support for the outflows predicted by the X-wind as these also originate in the trapped flux region. In addition, we find no support in the data for accretion powered stellar winds from young stars. By comparing the analysis presented here of NGC 2264 with a similar analysis of stars in Taurus (age~1-2 Myr), we find evidence that the equilibrium interaction between the magnetic field and accretion disk in TTS systems evolves as the stars grow older, perhaps as the result of evolution of the stellar magnetic field geometry. We compare the accretion rates we derive with accretion rates based on U-band excess, finding good agreement. In addition, we use our accretion parameters to determine the relationship between accretion and H-beta luminosity, again finding good agreement with previously published results; however, we also find that care must be used when applying this relationship due to strong chromospheric emission in young stars which can lead to erroneous results in some cases.

A CRITICAL EXAMINATION OF THE X-WIND MODEL FOR CHONDRULE AND CALCIUM-RICH, ALUMINUM-RICH INCLUSION FORMATION AND RADIONUCLIDE PRODUCTION

Meteoritic data, especially regarding chondrules and calcium-rich, aluminum-rich inclusions (CAIs), and isotopic evidence for short-lived radionuclides (SLRs) in the solar nebula, potentially can constrain how planetary systems form. Intepretation of these data demands an astrophysical model, and the "X-wind" model of Shu et al. (1996) and collaborators has been advanced to explain the origin of chondrules, CAIs and SLRs. It posits that chondrules and CAIs were thermally processed < 0.1 AU from the protostar, then flung by a magnetocentrifugal outflow to the 2-3 AU region to be incorporated into chondrites. Here we critically examine key assumptions and predictions of the X-wind model. We find a number of internal inconsistencies: theory and observation show no solid material exists at 0.1 AU; particles at 0.1 AU cannot escape being accreted into the star; particles at 0.1 AU will collide at speeds high enough to destroy them; thermal sputtering will prevent growth of particles; and launching of particles in magnetocentrifugal outflows is not modeled, and may not be possible. We also identify a number of incorrect predictions of the X-wind model: the oxygen fugacity where CAIs form is orders of magnitude too oxidizing; chondrule cooling rates are orders of magnitude lower than those experienced by barred olivine chondrules; chondrule-matrix complementarity is not predicted; and the SLRs are not produced in their observed proportions. We conclude that the X-wind model is not relevant to chondrule and CAI formation and SLR production. We discuss more plausible models for chondrule and CAI formation and SLR production.

EXTREME ACTIVE MOLECULAR JETS IN L1448C

The protostellar jet driven by L1448C was observed in the SiO J=8-7 and CO J=3-2 lines and 350 GHz dust continuum at ~1" resolution with the Submillimeter Array (SMA). A narrow jet from the northern source L1448C(N) was observed in the SiO and the high-velocity CO. The jet consists of a chain of emission knots with an inter-knot spacing of ~2" (500 AU) and a semi-periodic velocity variation. The innermost pair of knots, which are significant in the SiO map but barely seen in the CO, are located at ~1" (250 AU) from the central source, L1448C(N). Since the dynamical time scale for the innermost pair is only ~10 yr, SiO may have been formed in the protostellar wind through the gas-phase reaction, or been formed on the dust grain and directly released into the gas phase by means of shocks. It is found that the jet is extremely active with a mechanical luminosity of ~7 L_sun, which is comparable to the bolometric luminosity of the central source (7.5 L_sun). The mass accretion rate onto the protostar derived from the mass-loss rate is ~10^{-5} M_sun/yr. Such a high mass accretion rate suggests that the mass and the age of the central star are 0.03-0.09 M_sun and (4-12)x10^3 yr, respectively, implying that the central star is in the very early stage of protostellar evolution. The low-velocity CO emission delineates two V-shaped shells with a common apex at L1448C(N). The kinematics of these shells are reproduced by the model of a wide opening angle wind. The co-existence of the highly-collimated jets and the wide-opening angle shells can be explained by the unified X-wind model" in which highly-collimated jet components correspond to the on-axis density enhancement of the wide-opening angle wind. The CO $J$=3--2 map also revealed the second outflow driven by the southern source L1448C(S) located at ~8.3" (2000 AU) from L1448C(N).

Ne II FINE-STRUCTURE LINE EMISSION FROM THE OUTFLOWS OF YOUNG STELLAR OBJECTS

The flux and line shape of the fine-structure transitions of \NeII\ and \NeIII\ at 12.8 and 15.55\,$\mu$m and of the forbidden transitions of \OI\ $\lambda6300$ are calculated for young stellar objects with a range of mass-loss rates and X-ray luminosities using the X-wind model of jets and the associated wide-angle winds. For moderate and high accretion rates, the calculated \NeII\ line luminosity is comparable to or much larger than produced in X-ray irradiated disk models. All of the line luminosities correlate well with the main parameter in the X-wind model, the mass-loss rate, and also with the assumed X-ray luminosity --- and with one another. The line shapes of an approaching jet are broad and have strong blue-shifted peaks near the effective terminal velocity of the jet. They serve as a characteristic and testable aspect of jet production of the neon fine-structure lines and the \OI\ forbidden transitions.

The chemical relationship between chondrules and matrix and the chondrule matrix complementarity

Chondrules and matrix are the two major components of carbonaceous chondrites (CC). The Mg/Si ratios of chondrules and matrices in CV, CR, CO and CM chondrites are complementary: chondrules have super CI chondritic and matrices sub CI chondritic Mg/Si ratios. Bulk samples of CC have always CI chondritic Mg/Si ratios, indicating that chondrules and matrix are chemically connected. Such a chemical relationship places important constraints on the chondrule formation process and the chemical evolution of the protoplanetary disk. The chondrule matrix complementarity excludes separate origins and later mixing of chondrules and matrix. Redistribution of Fe and Mg between chondrules and matrix or leaching of Si from chondrule mesostasis into the matrix on the parent body can be excluded. Both, chondrules and matrix must have formed from the same chemical reservoir. The Mg/Si complementarity was probably established in the nebula before parent body accretion by the extraction of a Mg-rich component from a CI chondritic reservoir. This component then served as precursor for chondrules, leaving behind Mg-depleted material, the parent material of matrix. This interpretation excludes all chondrule forming processes that involve a separate origin for chondrules and matrix, such as, for example, the X-wind model. The existence of this complementarity in CV, CR, CO and CM chondrites also suggests that chondrules and matrix are formed by similar processes.

Disc-planet interactions in subkeplerian discs

One class of protoplanetary disc models, the X-wind model, predicts strongly subkeplerian orbital gas velocities, a configuration that can be sustained by magnetic tension. We investigate disc-planet interactions in these subkeplerian discs, focusing on orbital migration for low-mass planets and gap formation for high-mass planets. We use linear calculations and nonlinear hydrodynamical simulations to measure the torque and look at gap formation. In both cases, the subkeplerian nature of the disc is treated as a fixed external constraint. We show that, depending on the degree to which the disc is subkeplerian, the torque on low-mass planets varies between the usual Type I torque and the one-sided outer Lindblad torque, which is also negative but an order of magnitude stronger. In strongly subkeplerian discs, corotation effects can be ignored, making migration fast and inward. Gap formation near the planet's orbit is more difficult in such discs, since there are no resonances close to the planet accommodating angular momentum transport. In stead, the location of the gap is shifted inwards with respect to the planet, leaving the planet on the outside of a surface density depression. Depending on the degree to which a protoplanetary disc is subkeplerian, disc-planet interactions can be very different from the usual Keplerian picture, making these discs in general more hazardous for young planets.

Origin and chronology of chondritic components: A review

Mineralogical observations, chemical and oxygen–isotope compositions, absolute 207Pb– 206Pb ages and short-lived isotope systematics ( 7Be– 7Li, 10Be– 10B, 26Al– 26Mg, 36Cl– 36S, 41Ca– 41K, 53Mn– 53Cr, 60Fe– 60Ni, 182Hf– 182W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides ( 7Be, 10Be, and 36Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of 26Al, 41Ca, and 53Mn. 10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology. 41Ca– 41K, and 60Fe– 60Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen–isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ∼4567–4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1–4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an 16O-rich (Δ 17O ∼ −24 ± 2‰) nebular gas. The 26Al-poor [( 26Al/ 27Al) 0 < 1 × 10 −5], 16O-rich (Δ 17O ∼ −24 ± 2‰) CAIs – FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene–hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites—may have formed prior to injection and/or homogenization of 26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are 16O-depleted (Δ 17O > −10‰) and have ( 26Al/ 27Al) 0 similar to those in chondrules (<1 × 10 −5). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1–4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of 16O-poor (Δ 17O > −5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5–1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1–4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ∼4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.

Magnetocentrifugally Driven Flows from Young Stars and Disks. VI. Accretion with a Multipole Stellar Field

Previous analyses of magnetospheric accretion and outflow in classical T Tauri stars (CTTSs), within the context of both the X-wind model and other theoretical scenarios, have assumed a dipolar geometry for the stellar magnetic field if it were not perturbed by the presence of an accreting, electrically conducting disk. However, CTTS surveys reveal that accretion hot spots cover a small fraction of the stellar surface, and that the net field polarization on the stellar surface is small. Both facts imply that the magnetic field generated by the star has a complex non-dipolar structure. To address this discrepancy between theory and observations, we re-examine X-wind theory without the dipole constraint. Using simple physical arguments based on the concept of trapped flux, we show that a dipole configuration is in fact not essential. Independent of the precise geometry of the stellar magnetosphere, the requirement for a certain level of trapped flux predicts a definite relationship among various CTTS observables. Moreover, superposition of multipole stellar fields naturally yield small observed hot-spot covering fractions and small net surface polarizations. The generalized X-wind picture remains viable under these conditions, with the outflow from a small annulus near the inner disk edge little affected by the modified geometry, but with inflow highly dependent on the details of how the emergent stellar flux is linked and trapped by the inner disk regions. Our model is consistent with data, including recent spectropolarimetric measurements of the hot spot sizes and field strengths in V2129 Oph and BP Tau.

Chronology of the Solar System's Oldest Solids

Determining the origins of our solar system and, by proxy, other planetary systems, depends on knowing accurately and precisely the timing and tempo of the transformation of the disk of gas and dust to the solids that formed the planets. Relative ages based on the short-lived nuclide 26Al indicate that high-temperature calcium-aluminum inclusions (CAIs) formed before lower temperature chondrules but these ages are heavily dependant on a model of homogeneous distribution of 26Al within the protoplanetary disk. The competing X-wind model argues for heterogeneous distribution of 26Al due to its formation by intra-solar system irradiation such that this system would have no chronological significance. We report a207Pb-206Pb isochron age of 4565.45 ± 0.45 Myr for chondrules from the CV chondrite Allende, an age that is 1.66 ± 0.48 Myr younger than the accepted Pb-Pb age for CAIs from this chondrite group. This age offset is in excellent agreement with the relative ages determined using the 26Al-26Mg system, an observation that supports a supernova origin for 26Al and, importantly, the chronological significance of the 26Al-26Mg system in general. This is consistent with an early and brief CAI-forming event followed by recurrent chondrule formation throughout the life span of the protoplanetary disk. The paucity of old chondrules in chondrite meteorites may reflect their early incorporation into the parent bodies of differentiated meteorites after CAIs were effectively removed from the innermost regions of the protoplanetary disk. Lastly, the agreement between the absolute and relative chronology of CAIs and chondrules requires a solar system age younger than ~4567.5 Myr

Constraints for chondrule formation from Ca–Al distribution in carbonaceous chondrites

Chondritic meteorites and their components formed in the protoplanetary disk surrounding the nascent sun. We show here that the two volumetrically dominating components of carbonaceous chondrites, chondrules and matrix did not form independently. They must have been derived from a single, common source. We analyzed Ca and Al in chondrules and matrix of the CV type carbonaceous chondrites Allende and Y-86751. The Ca/Al-ratios of chondrules and matrix of both chondrites are complementary, but in case of Allende chondrules have sub-chondritic and matrix super-chondritic Ca/Al-ratios and in case of Y-86751 chondrules have super-chondritic and matrix sub-chondritic Ca/Al-ratios. This rules out the redistribution of Ca between chondrules and matrix during parent body alteration. Tiny spinel grains in the matrix produce the high Al in the matrix of Y-86751. In Allende these spinels were most probably included in chondrules. The most plausible explanation for this Ca- and Al-distribution in the same type of chondrite is that both chondrules and matrix formed from the same chemical reservoir. Tiny differences in nebular conditions during formation of these two meteorites must have led to the observed differences. These are severe constraints for all models of chondrule formation. Any model involving separate formation of chondrules and matrix, such as the X-wind model can be excluded.

Adaptive Optics Spectroscopy of the [Feii] Outflows from HL Tauri and RW Aurigae

We present new results of [Fe II] λ1.644 μm spectroscopy toward the jets from HL Tau and RW Aur carried out with the Subaru Telescope combined with the adaptive optics system. We observed the regions within 2''-3'' from the stars with the subarcsecond resolutions of 05 and 02 for HL Tau and RW Aur, respectively. In addition to the strong high-velocity component (HVC) extended along each jet, we detected a blueshifted low-velocity component (LVC) seen as a wing or shoulder of the HVC at each stellar position. The position velocity diagrams of the two objects show a characteristic similar to those of the cold disk wind and X-wind models in that the [Fe II] line width is broad close to the stellar position and narrower at the extended jet. A closer comparison suggests, however, that the disk wind model tends to have too large a line width at the HVC, while the X-wind model has excess redshifted emission at the stellar position. The narrow velocity width with symmetric line profiles of the observed HVC supports an X-wind-type model, while the LVC, located away from the star, favors the presence of a disk wind. The [Fe II] emission shows a gap of 08 for HL Tau and a marked drop of Y ~ -02 for RW Aur between the redshifted jet and the star, which indicate optically thick disks of ~160 and <40 AU in radius, respectively. Part of the Br12 emission of HL Tau originates from the jet itself because its normalized line profile shows a signigicantly large deviation from the normalized continuum in spatial profile.

A Unified Model for Bipolar Outflows from Young Stars

We develop a unified model for molecular outflows in star formation. The model incorporates essential features expected of the primary wind, which is thought to be driven magnetocentrifugally from close to the central stellar object, and the ambient core material shaped by anisotropic magnetic support. The primary wind is modeled as a toroidally magnetized fast outflow moving radially away from the origin, with an angle-dependent density distribution: a dense axial jet surrounded by a more tenuous wide-angle wind, as expected in the X-wind model. If dynamically significant magnetic fields are present, the star-forming core will settle faster along the field lines than across, forming a toroid-like structure. We approximate the structure with a singular isothermal toroid whose density distribution can be obtained analytically. The interaction of the laterally stratified wind and the ambient toroid is followed using the Zeus2D magnetohydrodynamics (MHD) code. We find that the lobes produced by the interaction resemble many systematics observed in molecular outflows from very young stars, ranging from Class 0 to I sources. In particular, both the dense axial jet and the wide-angle wind participate in the wind-ambient interaction. In our model, the jet- and wind-driven pictures of molecular outflows are unified. We discuss the observational implications of the unified picture, including the possibility of detecting the primary jet/wind directly.

The Irradiation Origin of Beryllium Radioisotopes and Other Short‐lived Radionuclides

Two explanations exist for the short-lived radionuclides (T1/2 ≤ 5 Myr) present in the solar system when the calcium-aluminum-rich inclusions (CAIs) first formed. They originated either from the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, 7Be is then the key discriminant, since it can be made only by irradiation. Using the same irradiation model developed earlier by our group, we calculate the yield of 7Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if 7Be and 10Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux must be of order F ~ 2 × 1010 cm-2 s-1. The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, 26Al, 36Cl, and 53Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high 3He abundance). The decoupling of the 26Al and 10Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low 3He abundance) are considered. The yields of 41Ca are compatible with an initial solar system value inferred from the measured initial 41Ca/40Ca ratio and an estimate of the thermal metamorphism time (from Young et al.), alleviating the need for two-layer proto-CAIs. Finally, we show that the presence of supernova-produced 60Fe in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin.

Controlling the collimation and rotation of hydromagnetic disc winds

We present a comprehensive set of axisymmetric, time-dependent simulations of jets from Keplerian discs whose mass loading as a function of disc radius is systematically changed. For a reasonable model for the density structure and injection speed of the underlying accretion disc, mass loading is determined by the radial structure of the magnetic field of the disc. We vary this structure by using four different magnetic field configurations, ranging from the Ouyed-Pudritz 'potential' configuration, to the increasingly more steeply falling Blandford-Payne and Pelletier-Pudritz models, and ending with a quite steeply raked configuration that bears similarities to the Shu X-wind model. We find that the radial distribution of the mass load has a profound effect on the rotational profile of the underlying jet as well as the degree of collimation of its outflow velocity and magnetic field lines. These four models have systematic differences in the power-law rotation profiles of jet material far from the source: v Φ(r) oc r a ranging over -0.46 ≥ a ≥ -0.76. We show analytically and confirm by our simulations that the collimation of a jet depends on its radial current distribution, which in turn is prescribed by the mass load. Models with steeply descending mass loads have strong toroidal fields, and these collimate to cylinders (this includes the Ouyed-Pudritz and Blandford-Payne outflows). On the other hand, the more gradually descending mass load profiles (the PP92 and monopolar distributions) have weaker toroidal fields and these result in wide-angle outflows with parabolic collimation. We also present detailed structural information about jets such as their radial profiles of jet density, toroidal magnetic field and poloidal jet speed, as well as an analysis of the bulk energetics of our different simulations. Our results are in excellent agreement with the predictions of asymptotic collimation for axisymmetric, stationary jets.

Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived 10Be and for the possible presence of the short-lived nuclide 7Be in the early solar system

The concentrations and isotopic compositions of lithium, beryllium, and boron, analyzed in situ by ion microprobe in 66 spots of a type B1 Ca–Al-rich inclusion (CAI 3529-41) from the Allende meteorite, are reported. Large variations are observed for both the Li and the B isotopic ratios with 7Li/ 6Li ranging from 9.2 ± 0.22 to 12.22 ± 0.43 (a ≈250‰ range in δ 7Li values) and 10B/ 11B ranging from 0.2468 ± 0.0057 to 0.4189 ± 0.0493 (a 410‰ range in δ 11B values). The very low Li concentrations (<1 ppb) observed in several anorthite and fassaite grains require that a correction for the contribution of spallogenic Li produced during irradiation of the Allende meteoroid by galactic cosmic rays (GCR) be made (after this correction 7Li/ 6Li ranges from 9.2 ± 0.22 to 13.44 ± 0.56, i.e., a ≈350‰ range in δ 7Li values). In 3529-41, the 10B/ 11B ratios are positively correlated with 9Be/ 11B in a manner indicating the in situ decay of short-lived 10Be (half-life = 1.5 Ma) with a 10Be/ 9Be ratio at the time of formation of the CAI of 8.8 ± 0.6 × 10 −4, which is in agreement with previous findings [McKeegan, K.D., Chaussidon, M., Robert, F., 2000. Incorporation of short-lived 10Be in a calcium–aluminum-rich inclusion from the Allende meteorite. Science 289, 1334–1337]. The present detailed investigation demonstrates that only minor perturbations of the 10Be– 10B system are present in 3529-41, contrary to the 26Al/ 26Mg system for which numerous examples of isotopic redistribution following crystallization were observed [Podosek, F.A., Zinner, E.K., MacPherson, G.J., Lundberg, L.L., Brannon, J.C., Fahey, A.J., 1991. Correlated study of initial 87Sr/ 86Sr and Al–Mg systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite. Geochim. Cosmochim. Acta 55, 1083–1110]. Petrographically based criteria were developed to identify within the 66 analyzed spots in 3529-41, those where post-magmatic perturbation of the Li and Be distributions occurred. Li and Be concentrations measured in different analytical spots are compared with those predicted by using experimentally determined partition coefficients according to a model of closed-system crystallization of the CAI melt. These criteria show that 56% of the spots in melilite, 38% in anorthite, and 8% in fassaite suffered post-crystallization perturbations of Li and/or Be distributions. In the remaining spots, which do not show obvious indication of redistribution of Li or Be, the 7Li/ 6Li isotopic variations (corrected for GCR exposure) are positively correlated with 9Be/ 6Li suggesting the in situ decay of now-extinct 7Be. The derived isochron implies that at the time of its formation, the CAI melt had a 7Be/ 9Be ratio of 0.0061 ± 0.0013 and a 7Li/ 6Li ratio of 11.49 ± 0.13. In contrast, all the spots in 3529-41, which do show evidence for post-magmatic redistribution of Li and Be, have relatively constant 7Li/ 6Li, averaging 11.72 ± 0.56, which is consistent with mass balance calculations for Li isotopic homogenization in the CAI after the decay of 7Be. The incorporation of live 7Be in 3529-41 requires, because of the very short half-life of this nuclide (53 days), that it be produced essentially contemporaneously with the formation of the CAI. Therefore, the irradiation processes responsible for production of 7Be must have occurred within the solar accretion disk. Calculations developed in the framework of the x-wind model [Gounelle, M., Shu, F.H., Shang, H., Glassgold, A.E., Rehm, E.K., Lee, T., 2004. The origin of short-lived radionuclides and early Solar System irradiation (abstract). Lunar Planet. Sci. 35, 1829] reproduce the 7Be and 10Be abundances observed in 3529-41. The correlated presence of 7Be and 10Be in 3529-41 is thus a strong argument that 10Be, which is observed rather ubiquitously in CAIs, is also a product of irradiation in the early solar system, as might be a significant fraction of other short-lived radionuclides observed in early solar system materials.

Do comets have chondrules and CAIs? Evidence from the Leonid meteors

Abstract— Chondrules, silicate spheres typically 0.1 to 1 mm in diameter, are the most abundant constituents in the most common meteorites falling on Earth, the ordinary chondrites. In addition, many primitive meteorites have calcium‐aluminum‐rich inclusions (CAIs). The question of whether comets have chondrules or CAIs is relevant to understanding what the interior of a comet is like and what a cometary meteorite might be like. In addition, one prominent model for forming chondrules and CAIs, the X‐wind model, predicts their presence in comets, while most other models do not. At present, the best way to search for chondrules and CAIs in comets is through meteor showers derived from comets, in particular, the Leonid meteor shower. Evidence potentially could be found in the overall mass distribution of the shower, in chemical analyses of meteors, or in light curves. There is no evidence for a chondrule abundance in the Leonid meteors similar to that found in chondritic meteorites. There is intriguing evidence for chondrule‐ or CAI‐sized objects in a small fraction of the light curves, but further work is required to generate a definitive test.