A Global Jet/Circulation Model for Young Stars

Powerful, highly collimated jets, surrounded by bipolar molecular outflows, are commonly observed near Young Stellar Objects (YSOs). In the usual theoretical picture of star formation, a jet is ejected from a magnetized accretion disk, with a molecular outflow being driven either by the jet or by a wider wind coming from the disk. Here, we propose an alternative global model for the flows surrounding YSOs. In addition to a central accretion-ejection engine driving the jet, the molecular outflow is powered by the infalling matter and follows a circulation pattern around the central object without necessarily being entrained by a jet. It is shown that the model produces a heated pressure-driven outflow with magneto-centrifugal acceleration and collimation. We report solutions for the three different parts of this self-similar model, i.e. the jet, the infalling envelope and the circulating matter that eventually forms the molecular outflow. This new picture of the accretion/outflow phase provides a possible explanation for several observed properties of YSO outflows. The most relevant ones are the presence of high mass molecular outflows around massive protostars, and a realistic fraction (typically 0.1) of the accretion flow that goes into the jet.Comment: accepted for publication in Astronomy and Astrophysic

Optical polarimetry of star forming regions

Not availabl

The Numerical Modelling Of Scenarios For The Herbig-Haro Object HH30

The classical T-Tauri star HH30 in Taurus-Auriga exhibits a well-collimated plume of hot, optically-emitting atomic and partially ionised Hydrogen, and also a colder, dense, wide-angle molecular Hydrogen ouflow. Observations suggest HH30 is a binary system system, surrounded by a circumbinary accretion disc. We investigated the propagation and interaction of dual atomic and molecular outflows from HH30, using a series of numerical simulations with parameters informed by observational campaigns. These 3-dimensional models were computed using the established Eulerian astrophysics code ZEUS-MP, with in-house modifications and an enhanced chemistry and cooling module. These simulations assumed off-domain launch and tracked the evolution of the jets over spatial scale of ~ 100 AU, and with a timescale ~ 100 - 200 years. The propagation in this region is of special interest, as this is where the greatest difference between the two scenarios is likely to emerge. Our work here differs from "classical" simulations of jet propagation by virtue of one or both outflow sources moving in an orbit. Two competing scenarios were investigated, in which the morphology of the light-year scale outflow from HH30 is explained by different kinds of motion of the atomic outflow source, and in which the launch site of the molecular outflow differs. In both cases a velocity-pulsed atomic jet emerges from the more massive binary object. In the Orbital scenario, the orbital motion of the primary explains the morphology seen at large scale, while the molecular flow is launched from the secondary partner; in the Precessional scenario, precession of the primary dominates the morphology, while launch of the molecular flow is from the inner edge of the circumbinary disc. The binary orbit and inner depletion zone of the circumbinary disc differs between the scenarios, with the Precessional scenario having a much smaller orbit and correspondingly reduced inner depletion zone. Clearly identifiable structural differences emerge between the simulated models. We compared the effects of the two different kinds of perturbing molecular outflow on the faster atomic jet; position, velocity, line mass per unit length, temperature and other variables, as a function of distance x (AU) from the binary source. Linear and quadratic fit functions were determined to facilitate comparison with observation. These quantify the expected behaviours of the atomic jet in the presence of the two different kinds of molecular flow. Where the fit function domains overlap direct comparisons may be drawn; where 26 < x < 42 AU, the average velocity as a function of distance is Vx(x) = (1.39×10^?1 ±2.15×10^?3)x + (246.82±1.29) km s^?1 in the Precessional model, while in the Orbital model we find Vx(x) = (?3.26 ± 0.26)x + (269.57 ± 6.75) km s^?1. In the region 10 < x < 60 AU, the Precessional model has temperature dependence T(x) = (64.53 ± 12.54)x + (3535 ± 330) K. Whilst in the same region of the Orbital model, T(x) = (401.99 ± 333.19)x + (4258.4 ± 1340.3) K. Synthetic Mass-Velocity Spectra have been generated for our models, to investigate distinguishing features of these spectra in the presence of the two different types of molecular outflow. The shallow-angle spectra matching the aspect angle of HH30 itself are examined and the link between outflow scenario and time variability discussed. Spectra from the same dual outflow systems observed at different aspect angles to the sky plane are given, to provide a means to confirm these senarios in other HH30-like T-Tauri stars. Using code written in-house to calculate emission using rate coefficients for photon production, we generated synthetic observations; spatially resolved images, velocity channel maps and position-velocity diagrams. The morphology of the synthetic images from the two scenarios when compared to HST R-band imaging of HH30 suggests that the Orbital case is unlikely, whilst the Precessional case is supported

Hollow Cylindrical Lobes with a Helical Velocity Field of the L1551 Bipolar Flow

Observations of the structure and the velocity field in the L1551 bipolar flow were made with the 45m telescope at Nobeyama in the 115GHz 12CO J = 1 – 0 line with high spatial resolution. It was found that the bipolar flow lobes have a clear hollow cylindrical structure and show evidence of a helical velocity field. They appear to rotate in the same direction as the CS disk found by Kaifu et al. (1984). The velocity of the flow in the bipolar directions increases with distance up to ∼ 3′ from the central object, IRS 5. These characteristics coincide with those predicted by the magnetodynamic theory proposed by Uchida and Shibata and indicate the essential importance of the magnetic field in producing such flows and also in the star-formation process itself through the enhancement of angular-momentum loss.</jats:p

NASA University Program Management Information System: FY 1995

The University Program Report, Fiscal Year 1995, provides current information and related statistics for grants/contracts/cooperative agreements active during the report period. NASA field centers and certain Headquarters program offices provide funds for those R&D activities in universities which contribute to the mission needs of that particular NASA element. This annual report is one means of documenting the NASA-university relationship, frequently denoted, collectively, as NASA's University Program