Emmy Noether Application

More documents

Recommendations

Info

Figure 6: Shows the structure of the same protostellar region at two different scales. The initially homogeneous magnetic field lines (indicated as yellow streamlines) are wound around the rotation axis and pinched towards the centre. At large scales (left panel) the outflow (red velocity isosurfaces) is driven by magnetic pressure and a magnetic tower configuration is build up (Lynden-Bell, 2003). At small scales the onset of a magneto-centrifugally driven jet can be observed. The protostellar disc is shown as a gray density isosurface. Applied to massive star formation, this configuration will help radiation to escape through cavities punched by such outflows and jets which in turn could relax the radiation pressure limiting accretion onto the central star Krumholz et al. (2005b). Images taken from Banerjee & Pudritz (2006). Outflows and Jets, Magnetic fields Another outstanding problem of massive star formation is the influence and backreactions from outflows. Outflows and jets are frequently observed around young stars and connected to their formation through disc accretion (e.g., Bally et al., 2007; Arce et al., 2007). The launching of these jets and outflows are often linked to magnetic fields where the plasma is magneto-centrifugally expelled (Blandford & Payne, 1982; Pudritz & Norman, 1983; Fendt & Camenzind, 1996; Pudritz et al., 2007) or lifted off the disc plane by magnetic pressure (Lynden-Bell, 2003). In Banerjee & Pudritz (2006) we studied the self-consistent launching of jets and outflows from collapsing, magnetised cloud cores. We used again the FLASH code which also solves the magneto-hydrodynamic (MHD) equations describing the evolution of a magnetised compressible fluid (i.e. plasma). For this investigation we used again initial cloud cores modelled on the low-mass Barnard 68 Bok globule whose density distribution follows closely a Bonnor-Ebert-type profile (Alves et al., 2001). Initially the slightly rotating cloud core is threaded with a uniform, weak magnetic field (few micro Gauss) which is aligned with the rotation axis of the sphere. After an initial phase during which a substantial amount of angular momentum is removed by magnetic braking (Mouschovias & Paleologou, 1980) the supercritical cloud core collapses under its own weight. In this initial phase the magnetic field is wound around the rotation axis and a toroidal magnetic field component is build up. Because the magnetic field lines are frozen in in the ideal MHD case they follow the condensing plasma and get pinched. This compression of the magnetic field and further winding due the ever faster spinning disc eventually results in a configuration where magnetic forces overcome the gravitational forces and material is lifted off the disc plane. In the inner core region cooling becomes inefficient and the core starts to heat up. High velocity material falling onto the warm, slowly contracting core shocks and heats up even more. Typically we observe two or more shock fronts above an below the disc plane (see 10
Figure 7: Shows cuts through the density structure of a collapsing massive (M ∼ 170M⊙), magnetised cloud core at two different scales. Similar to the low-mass case (see Figure 6) a large scale outflow and a jet is launched if magnetic fields are present (the 2D magnetic flow lines are shown in green). This indicates that that outflows and jets are also expected around massive stars, at least in the very early phase. Again, the outflows punch cavities in the infalling envelope through which radiation from the young massive star could escape. Images taken from Banerjee & Pudritz (2007). also Banerjee et al., 2004). One distinguishes two different magnetically driven launching mechanisms. In a magnetic tower type configuration (e.g., Lynden-Bell, 2003) it is the magnetic pressure from the dynamically build up toroidal field component which lifts the gas off the disc plane. In such a situation a magnetic ’bubble’ is inflating the inner region of such a configuration. Here the magnetic field with its strong toroidal component act like a spring which tries to expand. Such a configuration can be seen in the left panel of Figure 6. For every rotational turn of the disc in which the magnetic field is anchored the tower grows by a certain amount. The subsequent outflow has a relative low speed of about 0.4km sec −1 . The onset of this outflow and the magnetic tower configuration is associated with the first shock fronts which appear above the disc. In the shock fronts, beneath which infalling gas is essentially stalled the external ram pressure and internal thermal pressure are equal. The additional magnetic pressure in the inner region lifts the gas and the shock fronts upwards. Further into the gravitational potential where the magnetic field is more pinched and wound the outflow is driven by magneto-centrifugal forces: The gas plasma is attached to the magnetic field lines like beads on a wire. It is known that centrifugal forces can launch disc winds if the magnetic field lines have an angle of more than 30 ◦ with the rotation axis (Blandford & Payne, 1982; Pudritz & Norman, 1983). Such a configuration can be seen the right panel of Figure 6 where the magnetic structure appears to have a pinched hourglass shape. Clearly the angle between the vertical axis and the magnetic field lines is larger than 30 ◦ . Here the wind speeds are higher than in the pressure driven case of the magnetic tower. A stronger, i.e. more compressed, magnetic field and a larger disc rotation expels the gas with velocities up to 2km sec −1 . We like to note here that we are looking at the very onset of the jet launching where the protostellar disc is not yet Keplerian and no central star has formed, i.e. the disc is still much more massive than the central object. We expect that this mechanism will accelerate the gas to much higher velocities indicative for jets from young stellar objects (YSO jets) when the disc has settled to a Keplerian state and the central protostar 11
Page 1: Proposal for an Emmy Noether Resear
Page 4 and 5: closely with the star formation gro
Page 6 and 7: a dense and clustered environment,
Page 8 and 9: Figure 2: Shows the evolution of th
Page 10 and 11: 2400 AU 150 AU Figure 4: Shows the
Page 14 and 15: has formed. Recent direct detection
Page 16 and 17: they are observed around YSOs, the
Page 18 and 19: • What is the influence of outflo
Page 20 and 21: Second year At the end of the first
Page 22 and 23: intensity field. Such approximation
Page 24 and 25: • Modelling of jets and outflows
Page 26 and 27: winds and disc-driven outflows. How
Page 28 and 29: 4. Requested Funding 4.1 Personnel
Page 30 and 31: the cost without travel would be ab
Page 32 and 33: 8. Attachments a) Curricula vitae b
Page 34 and 35: Goldsmith, P. F. 2001, ApJ, 557, 73

Emmy Noether Application

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?