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- Wouter Vlemmings
Argelander-Institut für Astronomie
Auf dem Hügel 71,
- wouter (at) astro.uni-bonn.de
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Evolved star and starformation magnetic fields
The formation, evolution and death of stars are of fundamental importance in astrophysics. Although significant advances have been made in understanding the physics of star-formation and stellar winds, many important questions remain unanswered. Several of these concern the role of the magnetic field, which is known to be ubiquitous throughout the universe, occurring around planets as well as in the intergalactic medium. Because the formation of stars and planets as well as stellar death occur mostly in optically obscured regions, (sub-)millimeter and radio wavelengths are the optimal wavelengths for studying such regions in detail (e.g., Genzel 1992 in The Galactic Interstellar Medium, eds. D. Pfenniger & P. Bartholdi). Polarization observations at these wavelengths are the only way to probe the strength and structure of magnetic fields in, for instance, proto-planetary disks and the bipolar outflows of young and old stars. Only now instruments are being built that will be capable of combining high angular resolution with polarization capabilities at these wavelengths, and the project described here will use these instruments to illuminate many poorly understood physical processes occurring during star-formation and during the last stages of stellar evolution.
The mass-loss and subsequent evolution of late-type stars:Although the Asymptotic Giant Branch (AGB) and post-AGB phases are a key aspect in the cosmic cycle of matter, they are still poorly understood phases of low-mass stellar evolution. They encompass a short time in which a star loses a large amount of mass. The mass-loss is one of the main contributors to the enrichments of the interstellar medium in dust and heavy elements, and its timescale and yield control the chemical evolution of galaxies (Habing & Olofsson 2004, Asymptotic Giant Branch Stars). Mass loss is a complex process involving pulsations and radiation pressure and possibly magnetic fields (e.g. Woitke 2006, A&A 460, L9). Studies of the dynamics of circumstellar envelopes and mass loss are almost exclusively possible through the observations of masers and (sub-)millimeter molecular lines.
After the AGB phase, the stellar envelopes undergo a major modification as they evolve to Planetary Nebulae (PNe). The standard assumption is that the initial slow AGB mass loss quickly changes into a fast superwind, generating shocks and accelerating the surrounding envelope (Kwok et al. 1978, ApJ 219, L125). It is during this phase that the typically spherical circumstellar envelope evolves into a Planetary Nebula. The formation mechanism for the commonly observed a-spherical shapes of PNe is still a matter of fierce debate. Current theories favor interaction with a binary companion or massive planet that help maintain a strong magnetic field capable of shaping the outflow (e.g. Nordhaus et al. 2007, MNRAS 376, 599). However, the known fraction binary systems cannot explain the large number of aspherical PNe. Since the shaping mechanism is likely related to the mass-loss mechanism, a better understanding of the shaping mechanism is crucial. Several observations indicate that already during the early AGB phase enhanced mass loss in the equatorial plane creates a non-uniform envelope (e.g. Murakawa et al. 2003, MNRAS, 344, 1). Observations of the magnetic field in the circumstellar envelopes of evolved stars have revealed that magnetic fields are dynamically important and could be one of the dominant mechanisms shaping the circumstellar outflow (Vlemmings et al. 2005, A&A 434, 1029; Vlemmings et al. 2006, Nature 440, 58). This is further supported by measurements of strong magnetic fields around the central stars of a number of PNe (O'Toole et al. 2005, A&A 437, 227). During the here-described project the magnetic field in the envelopes of AGB and post-AGB stars will be studied further through VLBI maser observations and interferometric sub-millimeter polarization observations. In addition, high angular resolution observations with ALMA and other millimeter interferometers of the warm dust around these stars, can reveal the equatorial density enhancements or even disks that might be responsible for shaping the stellar outflow.
A magnetically collimated jet around the evolved star W43A, revealed by H2O maser polarization VLBI observations (Vlemmings et al. 2006, Nature 440,58). Panel a and b indicate the measured linear polarization and panels c and d the derived magnetic field direction. Panel d also includes the measured magnetic field strength. The ellipses in panel c, d and e are the confining toroidal magnetic field along the jet.
Magnetic fields around proto-stars:Molecular line observations can yield kinematic information as well as magnetic field measurements throughout star-forming regions. Submillimeter molecular lines and submm to cm wavelength maser lines are particularly well suited, because the high densities and temperatures in star-forming molecular clouds give rise to emission from a large number of high excitation lines and numerous maser lines (e.g. van Dishoeck & Hogerheijde, 1999 in Origins of Stars & Planetary Systems, ed. C.J. Lada & N. Kylafis; Elitzur, 1992 Astronomical Masers). Additionally, the continuum emission from cold and warm dust increases steeply at shorter wavelengths, making submm polarization observations the best method for studying in detail the magnetic environment close to proto-stellar cores.
In low-mass star-formation, magnetic fields are thought to regulate the accretion onto the proto-star from a proto-stellar disk, and to collimate the outflow, which is essential in removing excess angular momentum from the system (e.g. Konigl & Pudritz, 2000 in Protostars and Planets IV). Their role in regulating the initial collapse within star forming clouds is still unclear (e.g. McKee & Ostriker, 2007, ARAA 45, 565). While the interstellar medium is strongly magnetized, stars are typically weakly magnetized. One of the classic problems of star formation is the transition between the phases where magnetic energy dominates the support to that where gravitational collapse can proceed. One way to remove magnetic flux is through ambipolar diffusion. Depending on the fractional ionization of the gas and thereby the coupling between neutral and ionized gas components, the magnetic field structure is expected to deform during the cloud collapse, and theory predicts an hour-glass shaped magnetic field structure pinched toward the protostar (e.g., Stahler et al. 2005, The formation of stars). This pinched magnetic field morphology is also required in the classical jet-launching scenario, where the gas is ejected from the accretion disk along the bent magnetic field lines. While recent observations of linearly polarized dust continuum emission from NGC1333 IRAS4 have clearly shown the pinched hour-glass shaped magnetic field morphology toward that protostellar system (Girart et al. 2006, Science 313, 812), one has not been able to conclude whether ambipolar diffusion is necessary for the initiation of gravitational collapse.
The magnetic field structure in the deeply embedded protobinary, NGC1333-IRAS4A, derived from dust polarization observations (red vectors) with the SMA (Girart et al. Science 2006, 313, 812) shown on top of a submillimeter dust continuum map. The "hourglass pinching" of the polarization pattern in the core is a "smoking gun" pointing at the importance of magnetic fields in resisting gravity during the collapse of a protostellar core.
Current theory indicates that this strongly depends on the magnitude of the initial magnetic energy density relative to the gravitational and turbulent energy density (e.g. Krasnopolsky & Gammie 2005, ApJ 635, 1126; Vazquez-Semadeni et al. 2005, ApJ 618, 344).
Massive star-formation has been suggested to be a scaled up version of low-mass star-formation, though the ratio between the different energy densities is likely significantly different. Compared to low-mass star-formation, there are additional challenges in identifying the mechanisms responsible for the formation of massive stars, particularly as they are less numerous and thus often more distant. There are currently two competing models that try to explain massive star formation: Core accretion (e.g. McKee & Tan, 2003 ApJ 585, 850), where the star forms from a massive core that does not undergo significant fragmentation, and Competitive accretion (e.g. Bonnell et al. 2003 MNRAS 343, 413) where low- and high-mass stars are formed in a cluster due to fragmentation during gravitational collapse and the growth of the fragments occurs due to competitive accretion. A third 'Coalescence' model has been proposed to dominate in regions of high stellar density, where massive stars form through the merging of lower mass stars (e.g. Bally & Zinnecker, 2005 AJ 129 2281).
While feedback processes and turbulence will likely play an important role in the energetic regions forming massive stars, the quest to measure magnetic fields is equally important in massive and in low-mass star forming regions. Massive stars are fully radiative and hence are typically not expected to have significant magnetic fields. However, molecular outflows are known to be as ubiquitous during high- as during low-mass star-formation (e.g. Rodriguez & Bo 1998, RMxAA 34, 13). If the launching mechanisms of these outflows are of similar nature, strong magnetic fields must also be present in dense massive cores (e.g., Arce et al. 2007, Protostars and Planets V, 245), especially as magneto-hydrodynamically driven jets might be needed to aid the removal of angular momentum and relieve the radiation pressure. The current theoretical models for massive star formation now implement magnetic fields in their codes to study their effect on fragmentation, accretion and feedback (Krumholz & Bonnell, 2008 "Models for the formation of massive stars" in "Structure formation in the Universe").
For the research of this project, we will use new and existing radio and (sub-)millimeter instruments to provide direct evidence for magnetic collimation during star-formation, both by producing high angular resolution polarized continuum images of proto-stellar disks and outflows and by using observations of the Zeeman effect on molecular lines such as CN. Additionally, maser observations will be used to probe the magnetic field on even smaller scales (< 1AU) (e.g. Vlemmings et al. 2006, A&A 448, 597; Bartkiewicz et al. 2005, MNRAS 361, 623), where masers are believed to probe the disks, jets and outflows.