Magnetic fields are usually considered dynamically important in star formation when the dimensionless mass-to-flux ratio is close to, or less than, unity (lambda<~1). We show that, in disk formation, the requirement is far less stringent. This conclusion is drawn from a set of 2D (axisymmetric) simulations of the collapse of rotating, magnetized, singular isothermal cores. We find that a weak field corresponding to 1ambda~100 can begin to disrupt the rotationally supported disk through magnetic braking, by creating regions of rapid, supersonic collapse in the disk. These regions are separated by one or more centrifugal barriers, where the rapid infall is temporarily halted. The number of centrifugal barriers increases with lambda. When lambda>~100, they merge together to form a more or less contiguous, rotationally supported disk. Even though the magnetic field in such a case is extremely weak on the scale of dense cores, it is amplified by collapse and differential rotation, to the extent that its pressure dominates the thermal pressure in both the disk and its surrounding region. For relatively strongly magnetized cores with lambda<~10, the disk formation is suppressed completely, as found previously. A new feature is that the mass accretion is highly episodic, due to reconnection of the accumulated magnetic field lines. For rotationally supported disks to appear during the protostellar mass accretion phase of star formation in dense cores with realistic field strengths, the powerful magnetic brake must be weakened, perhaps through nonideal MHD effects and/or protostellar winds. We discuss the possibility of observing a generic product of the magnetic braking, an extended circumstellar region that is supported by a combination of toroidal magnetic field and rotation - a "magnetogyrosphere".Comment: 62 pages, 20 figures, submitted to Ap

It is established that the formation of rotationally supported disks during the main accretion phase of star formation is suppressed by a moderately strong magnetic field in the ideal MHD limit. Non-ideal MHD effects are expected to weaken the magnetic braking, perhaps allowing the disk to reappear. We concentrate on one such effect, ambipolar diffusion, which enables the field lines to slip relative to the bulk neutral matter. We find that the slippage does not sufficiently weaken the braking to allow rotationally supported disks to form for realistic levels of cloud magnetization and cosmic ray ionization rate; in some cases, the magnetic braking is even enhanced. Only in dense cores with both exceptionally weak fields and unreasonably low ionization rate do such disks start to form in our simulations. We conclude that additional processes, such as Ohmic dissipation or Hall effect, are needed to enable disk formation. Alternatively, the disk may form at late times when the massive envelope that anchors the magnetic brake is dissipated, perhaps by a protostellar wind.

Brown dwarfs are classified as objects which are not massive enough to sustain nuclear fusion of hydrogen, and are distinguished from planets by their ability to burn deuterium. 1 Old (>10 Myr) brown dwarfs are expected to possess short-lived magnetic fields 2 and, since they no longer generate energy from collapse and accretion, weak radio and X-ray emitting coronae. Several efforts have been undertaken in the past to detect chromospheric activity from the brown dwarf LP944−20 at X-ray 1,3 and optical 4,5,6,7 wavelengths, but only recently an X-ray flare from this object was detected. 3 Here we report on the discovery of quiescent and flaring radio emission from this source, which represents the first detection of persistent radio emission from a brown dwarf, with luminosities that are several orders of magnitude larger than predicted from an empirical relation 8,9 between the X-ray and radio luminosities of many stellar types. We show in the context of synchrotron emission, that LP944−20 possesses an unusually weak magnetic field in comparison to active dwarf M stars, 10,11 which might explain the null results from previous optical and X-ray observations of this source, and the deviation from the empirical relations. This paper has been submitted to Nature. You are free to use the results here for the purpose of

We examine a sample of 45 Mg II absorption-selected systems over the redshift range 0.4 to 1.4 in order to better understand the range of physical conditions present in the interstellar and halo gas associated with intermediate redshift galaxies. Mg II and Fe II absorption profiles were observed at a resolution of ≃ 6 km s −1 with HIRES/Keck. Ly α and C IV data were measured in FOS spectra obtained from the Hubble Space Telescope archive (resolution ≃ 230 km s −1 ). We perform a multivariate analysis of W r (Mg II), W r (Fe II), W r (C IV) and W r (Ly α) (rest-frame equivalent widths) and the Mg II kinematic spread. There is a large range of high-to-low ionization properties and kinematics in intermediate redshift absorbers, that we find can be organized into five categories: "Classic", "C IV-deficient", "Single/Weak", "Double", and "DLA/H I-Rich". These categories arise, in part, because there is a strong connection between low-ionization kinematics and the location of an absorber on the W r (C IV)-W r (Mg II) plane. Using photoionization modeling, we infer that in most absorbers a significant fraction of the C IV arises in a phase separate from that giving rise to the Mg II. We show that many of the C IV profiles are resolved in the FOS spectra due to velocity structure in the C IV gas. For 16 systems, the galaxy M K , M B , B − K, and impact parameters are measured. We compare the available absorption-line properties (taken from Churchill et al. 1999, Paper I) to the galaxy properties but find no significant (greater than 3 σ) correlations, although several suggestive trends are apparent. We compare the locations of our intermediate redshift absorbers on the W r (C IV)-W r (Mg II) plane with those of lower and higher redshift data taken from the literature and find evidence for evolution that is connected with the Mg II kinematics seen in HIRES/Keck profiles of Mg II at z > 1.4. We discuss the potential of using the above categorizations of absorbers to understand the evolution in the underlying physical processes giving rise to the gas and governing its ionization phases and kinematics. We also discuss how the observed absorbing gas evolution has interesting parallels with scenarios of galaxy evolution in which mergers and the accretion of "proto-galactic clumps" govern the gas physics and provide reservoirs for elevated star formation rates at high redshift. At intermediate and lower redshifts, the galaxy gaseous components and star formation rates may become interdependent and self-regulatory such that, at z ≤ 1, the kinematics and balance of high and low ionization gas may be related to the presence of star forming regions in the host galaxy.

We present extensive metal-line absorption properties for 45 absorption systems that were selected by their Mg II absorption at redshifts between 0.4 and 1.4. For each system the properties of several chemical species are determined, including a wide range of ionization conditions. In the optical, the absorption systems have been observed at ∼ 6 km s −1 resolution with HIRES/Keck, which covered Mg II, several Fe II transitions, Mg I, and in some cases (depending upon redshift), Ca II, Ti II, Mn II, and Al III. Ultraviolet, lower resolution (∼ 230 km s −1 ) Faint Object Spectrograph data (1600Å-3275Å) were obtained from the Hubble Space Telescope archive. These spectra covered Al II, Al III, Si II, Si III, Si IV, C II, C III, C IV, N V, O VI, and several Lyman series transitions, with coverage dependent upon the absorption system redshift. From these data, we infer that Mg II absorbing galaxies at intermediate redshifts have multiphase gaseous structures.