Star formation rate and magnetic fields in spiral galaxies

We have constructed a dynamo model for the magnetic field in spiral galaxies that takes into account the differences in star formation rates in different galaxies. The difficulty in constructing the model is that the star formation rate does not enter directly into the equations of magnetohydrodynamics, which include only the root-mean-square velocity of the interstellar gas, its density, and the half-thickness of the ionized gas disk. We propose a parametrization of these quantities that relates them to the star formation rate and investigate our model in terms of the so-called no-z approximation, which neglects the details of the magnetic field structure in a direction perpendicular to the galactic disk. The influence of the star formation rate on the galactic dynamo is a threshold one. This influence is small at moderate star formation rates and significant only at very high star formation rates. If the starburst intensity reaches some critical level (exceeding that in the Milky Way by an order of magnitude), then the large-scale magnetic field is destroyed and it is restored only after completion of the starburst. We provide a list of galaxies that exhibit a fairly high star formation rate and that can be interesting to study their magnetic fields.     

What Drives Star Formation?

Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. Observations of magnetic fields can distinguish between these two models because of different predictions in three areas: (1) magnetic field morphology, (2) the scaling of field strength with density and non-thermal velocities, and (3) the mass to magnetic flux ratio, M/Φ. We first discuss the techniques and limitations of methods for observing magnetic fields in star formation regions, then describe results for the L1544 prestellar core as an exemplar of the observational results. Application of the three tests leads to the following conclusions. The observational data show that both magnetic fields and turbulence are important in molecular cloud physics. Field lines are generally regular rather than chaotic, implying strong field strengths. But fields are not aligned with the minor axes of oblate spheroidal clouds, suggesting that turbulence is important. Field strengths appear to scale with non-thermal velocity widths, suggesting a significant turbulent support of clouds. Giant Molecular Clouds (GMCs) require mass accumulation over sufficiently large volumes that they would likely have an approximately critical M/Φ. Yet H I clouds are observed to be highly subcritical. If self-gravitating (molecular) clouds form with the subcritical M/Φ of H I clouds, the molecular clouds will be subcritical. However, the observations of molecular cloud cores suggest that they are approximately critical, with no direct evidence for subcritical molecular clouds or cloud envelopes. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.     

Inefficient star formation: the combined effects of magnetic fields and radiative feedback

We investigate the effects of magnetic fields and radiative protostellar feedback on the star formation process using self-gravitating radiation magnetohydrodynamical calculations. We present results from a series of calculations of the collapse of  50 M_⊙  molecular clouds with various magnetic field strengths and with and without radiative transfer. We find that both magnetic fields and radiation have a dramatic impact on star formation, though the two effects are in many ways complementary. Magnetic fields primarily provide support on large scales to low-density gas, whereas radiation is found to strongly suppress small-scale fragmentation by increasing the temperature in the high-density material near the protostars. With strong magnetic fields and radiative feedback, the net result is an inefficient star formation process with a star formation rate of  ≲10  per cent per free-fall time that approaches the observed rate, although we have only been able to follow the calculations for 1/3 of a free-fall time beyond the onset of star formation.     

Star formation and galactic dynamo model with helicity fluxes

We propose a possible scenario of large-scale magnetic field evolution for galaxies with star formation. An important point affecting the results of our calculations is a parametrization of dynamo-governing quantities. In comparison with previous works, we have reconsidered the views of how star formation affects the stationary magnetic field strength, viscosity, and other parameters important for galactic dynamos. The calculations have been performed by taking into account the magnetic helicity fluxes, which introduce an additional nonlinearity into the model and change the regime of galactic dynamo action. We have confirmed the previously suggested idea that for weak star formation its influence on the magnetic field strength is minor and the relationship between them clearly manifests itself only when the star formation rate reaches a certain threshold value. In this case, on the one hand, the threshold lowers-this effect manifests itself at a star formation surface density greater than that in the Milky Way by a factor of 5. On the other hand, intense star formation can cause both a monotonic decay of the large-scale magnetic field and its oscillations near some value.     

Gas phase processes affecting galactic evolution

Gas processes affecting star formation are reviewed with an emphasis on gravitational and magnetic instabilities as a source of turbulence. Gravitational instabilities are pervasive in a multi-phase medium, even for sub-threshold column densities, suggesting that only an ISM with a pure-warm phase can stop star formation. The instabilities generate turbulence, and this turbulence influences the structure and timing of star formation through its effect on the gas distribution and density. The final trigger for star formation is usually direct compression by another star or cluster. The star formation rate is apparently independent of the detailed mechanisms for star formation, and determined primarily by the total mass of gas in a dense form. If the density distribution function is a log-normal, as suggested by turbulence simulations, then this dense gas mass can be calculated and the star formation rate determined from first principles. The results suggest that only 10^-4 of the ISM mass actively participates in the star formation process and that this fraction does so because its density is larger than 10⁵ cm^-3, at which point several key processes affecting dynamical equilibrium begin to break down. This revised version was published online in August 2006 with corrections to the Cover Date.     

New results of magnetic field measurements for BP Tau

We present measurements of the longitudinal magnetic field component B _‖ of the young star BP Tau in the He I 5876 emission line formation region, i.e., in the accretion flow near the stellar surface. The values obtained (?1.7 kG and ?1.0 kG in 2000 and 2001, respectively) agree with the results of similar measurements by other authors. At the same time, we show that the previously obtained field strength at the magnetic pole, B _p, and the inclination of the magnetic axis to the rotation axis, β, are untrustworthy. In our opinion, based on the B _‖ measurements available to date, it is not possible to conclude whether the star’s magnetic field is a dipole one or has a more complex configuration and to solve the question of whether this field is stationary. However, we argue that at least in the He I 5876 line formation region, the star’s magnetic field is not stationary and can be restructured in a time of the order of several hours. Nonstationary small-scale magnetic fields of active regions on the stellar surface and/or magnetospheric field line reconnection due to the twisting of these field lines as the star rotates could be responsible for the short-term magnetic field variability. It seems highly likely that there are no strictly periodic variations in brightness and emission line profiles in BP Tau due to the irregular restructuring of the star’s magnetic field.     

The effect of magnetic fields on star cluster formation

We examine the effect of magnetic fields on star cluster formation by performing simulations following the self-gravitating collapse of a turbulent molecular cloud to form stars in ideal magnetohydrodynamics. The collapse of the cloud is computed for global mass-to-flux ratios of  ∞, 20, 10, 5  and 3, i.e. using both weak and strong magnetic fields. Whilst even at very low strengths the magnetic field is able to significantly influence the star formation process, for magnetic fields with plasma  β < 1  the results are substantially different to the hydrodynamic case. In these cases we find large-scale magnetically supported voids imprinted in the cloud structure; anisotropic turbulent motions and column density striations aligned with the magnetic field lines, both of which have recently been observed in the Taurus molecular cloud. We also find strongly suppressed accretion in the magnetized runs, leading to up to a 75 per cent reduction in the amount of mass converted into stars over the course of the calculations and a more quiescent mode of star formation. There is also some indication that the relative formation efficiency of brown dwarfs is lower in the strongly magnetized runs due to a reduction in the importance of protostellar ejections.     

Magnetic field evolution in neutron stars

Neutron stars contain persistent, ordered magnetic fields that are the strongest known in the Universe. However, their magnetic fluxes are similar to those in magnetic A and B stars and white dwarfs, suggesting that flux conservation during gravitational collapse may play an important role in establishing the field, although it might also be modified substantially by early convection, differential rotation, and magnetic instabilities. The equilibrium field configuration, established within hours (at most) of the formation of the star, is likely to be roughly axisymmetric, involving both poloidal and toroidal components. The stable stratification of the neutron star matter (due to its radial composition gradient) probably plays a crucial role in holding this magnetic structure inside the star. The field can evolve on long time scales by processes that overcome the stable stratification, such as weak interactions changing the relative abundances and ambipolar diffusion of charged particles with respect to neutrons. These processes become more effective for stronger magnetic fields, thus naturally explaining the magnetic energy dissipation expected in magnetars, at the same time as the longer-lived, weaker fields in classical and millisecond pulsars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies of accretion flows around rotating black holes – I. Particle dynamics in a pseudo-Kerr potential

We study the formation of the absorption features, called the cyclotron–annihilation lines, in the γ-spectra of the neutron stars (pulsars), owing to the fundamental quantum-electrodynamic effect of the one–photon pair creation in magnetized vacuum. As a result, we substantiate a new method for the determination of the neutron star magnetic fields B based on measuring the interval between the main annihilation and the first cyclotron–annihilation absorption lines. It is found that these lines may be easily resolved, and, consequently, the method is surely applicable if the following conditions are satisfied. (i) A γ-source has to be compact enough and located near a star, but not close to its magnetic poles. For instance, it may be a disc in the plane of a star magnetic equator with latitudinal angular width less than     and radial extent up to 25 per cent of the star radius. (ii) The source is to produce detectable γ-radiation at large angles ≳60° to the local magnetic field. Being situated in a closed field line region and having a broad radiation pattern, such a source is not what is usually considered in the context of the polar cap and outer gap models of the pulsar γ-emission dealing with open field lines only. (iii) Magnetic field strength must lie in a certain narrow interval with the centre at  ∼(3–4) × 10¹²  G. Its width depends on the star orientation and disc radial extend and in the most favourable case is about 20–30 per cent of its lower boundary. Finally, the influence of the star rotation on this method employment is considered and new possibilities arising from forthcoming polarization observations are briefly discussed.     

Magnetic Neutron Stars. I. Static Field Configurations

The paper is the first in a series dealing with the structure of magnetic and rotating neutron stars including general relativistic effects. The geometry of fossile magnetic fields frozen in the highly conductive neutron star matter in a non-rotating (or weakly rotating) star is studied. § 2 treats the general poloidal field in a vacuum outside the star. The geometry of magnetic fields within the star — whose matter is governed by a barytropic equation of state — is restricted by the condition that the magnetic force density should be curl-free to maintain equilibrium (§ 3). Numerical results are obtained for a poloidal and a toroidal dipole field (§ 4).     

On the role of magnetic fields in star formation

Magnetic fields are observed in star forming regions. However simulations of the late stages of star formation that do not include magnetic fields provide a good fit to the properties of young stars including the initial mass function (IMF) and the multiplicity. We argue here that the simulations that do include magnetic fields are unable to capture the correct physics, in particular the high value of the magnetic Prandtl number, and the low value of the magnetic diffusivity. The artificially high (numerical and uncontrolled) magnetic diffusivity leads to a large magnetic flux pervading the star forming region. We argue further that in reality the dynamics of high magnetic Prandtl number turbulence may lead to local regions of magnetic energy dissipation through reconnection, meaning that the regions of molecular clouds which are forming stars might be essentially free of magnetic fields. Thus the simulations that ignore magnetic fields on the scales on which the properties of stellar masses, stellar multiplicities and planet-forming discs are determined, may be closer to reality than those which include magnetic fields, but can only do so in an unrealistic parameter regime.     

The physics of strong magnetic fields in neutron stars

In this paper we present a new result, namely that the primal magnetic field of the collapsed core during a supernova explosion will, as a result of the conservation of magnetic flux, receive a massive boost to more than 90 times its original value by the Pauli paramagnetization of the highly degenerate relativistic electron gas just after the formation of the neutron star. Thus, the observed super-strong magnetic field of neutron stars may originate from the induced Pauli paramagnetization of the highly degenerate relativistic electron gas in the interior of the neutron star. We therefore have an apparently natural explanation for the surface magnetic field of a neutron star.     

The effect of magnetic fields on the formation of circumstellar discs around young stars

We present first results of our simulations of magnetic fields in the formation of single and binary stars using a recently developed method for incorporating Magnetohydrodynamics (MHD) into the Smoothed Particle Hydrodynamics (SPH) method. An overview of the method is presented before discussing the effect of magnetic fields on the formation of circumstellar discs around young stars. We find that the presence of magnetic fields during the disc formation process can lead to significantly smaller and less massive discs which are much less prone to gravitational instability. Similarly in the case of binary star formation we find that magnetic fields, overall, suppress fragmentation. However these effects are found to be largely driven by magnetic pressure. The relative importance of magnetic tension is dependent on the orientation of the field with respect to the rotation axis, but can, with the right orientation, lead to a dilution of the magnetic pressure-driven suppression of fragmentation.     

On Hoyle's theory of the origin of the solar system

A quantitative re-formulation of Hoyle's theory on the formation of the solar system is attempted, according to a three-dimensional scheme based on the assumption that the original magnetic field of the star is a dipole field. This allows us to obtain analytic expressions for the main paraeters describing the different phases of the process. The protostar is assumed to evolve in gravitational contraction along the Hayashi track, along which, for a given value of the radiusR ₀ depending on the total angular momentum, matter begins to be shed at the equator as a consequence of centrifugal instability. However, owing to the geometry of the dipole field and to Hoyle's assumption that, for a star with a convective envelope, the sign of the inward magnetic pressure determines whether the magnetic lines do wind up or not, it turns out that the magnetic coupling between the star and the disk formed at the equator starts only when the radius of the contracting star has reached the value ofR _s=4/5R ₀; and that the shedding of matter stops for a radius valueR d, depending on the strength of the magnetic field. One is thus able to calculate the total quantity of matter emitted at the solar equator and the distance reached by the rings thus formed as functions of the radius of the star, of the initial values of the magnetic field, of the total angular momentum and of the structural factors of the star. The quantitative results are discussed in order to see whether it is possible to deduce the main characteristics of the disk, from which the solar system should have originated, for reasonable values of these main parameters.     

Forming stars on a viscous time-scale: the key to exponential stellar profiles in disc galaxies?

We argue for implementing star formation on a viscous time-scale in hydrodynamical simulations of disc galaxy formation and evolution. Modelling two-dimensional isolated disc galaxies with the Bhatnagar–Gross–Krook (BGK) hydrocode, we verify the analytic claim of various authors that if the characteristic time-scale for star formation is equal to the viscous time-scale in discs, the resulting stellar profile is exponential on several scalelengths whatever the initial gas and dark matter profile. This casts new light on both numerical and semi-analytical disc formation simulations that either (a) commence star formation in an already exponential gaseous disc, (b) begin a disc simulation with conditions known to lead to an exponential, i.e. the collapse of a spherically symmetric nearly uniform sphere of gas in solid-body rotation under the assumption of specific angular momentum conservation, or (c) in simulations performed in a hierarchical context, tune their feedback processes to delay disc formation until the dark matter haloes are slowly evolving and without much substructure so that the gas has the chance to collapse under conditions known to give exponentials. In such models, star formation follows a Schmidt-like law, which for lack of a suitable time-scale, resorts to an efficiency parameter. With star formation prescribed on a viscous time-scale, however, we find gas and star fractions after ∼12 Gyr that are consistent with observations without having to invoke a 'fudge factor' for star formation. Our results strongly suggest that despite our gap in understanding the exact link between star formation and viscosity, the viscous time-scale is indeed the natural time-scale for star formation.     

Polarization measurements of molecular lines

ALMA observations of circular (Zeeman) and linear (Goldreich–Kylafis) polarization in spectral lines will significantly enhance sensitivity and angular resolution over currently available data, and should lead to major breakthroughs in our understanding of the role of magnetic fields in the star formation process. This work was partially supported by NSF grants AST 0540459 and 0606822.     

Turbulent dynamo in asymptotic giant branch stars

Using recent results on the operation of turbulent dynamos, we show that a turbulent dynamo may amplify a large-scale magnetic field in the envelopes of asymptotic giant branch (AGB) stars. We propose that a slow rotation of the AGB envelope can fix the symmetry axis, leading to the formation of an axisymmetric magnetic field structure. Unlike solar-type αω dynamos, the rotation has only a small role in amplifying the toroidal component of the magnetic field; instead of an αω dynamo we propose an α ² ω . The magnetic field may reach a value of     , where B _e is the equipartition (between the turbulent and magnetic energy densities) magnetic field. The large-scale magnetic field is strong enough for the formation of magnetic cool spots on the AGB stellar surface. The spots may regulate dust formation, and hence the mass-loss rate, leading to axisymmetric mass loss and the formation of elliptical planetary nebulae (PNe). Despite its role in forming cool spots, the large-scale magnetic field is too weak to play a dynamic role and directly influence the wind from the AGB star, as required by some models. We discuss other possible problems in models where the magnetic field plays a dynamic role in shaping the AGB winds, and argue that they cannot explain the formation of non-spherical PNe.     

Magnetic field of CP stars in the Ori OB1 association. I. HD35456, HD35881, HD36313 A,HD36526

We present the results of magnetic field measurements of four chemically peculiar (CP) stars, the members of the Orion stellar association OB1. Observations were carried out with the circular polarization analyzer at the Main Stellar Spectrograph of the 6-m telescope. All the studied stars refer to the subtype of Bp stars with weak helium lines. Canadian astronomer E. F. Borra detected a magnetic field in three of them (HD35456, HD36313, and HD36526) from the Balmer line magnetometer observations. HD35881 was observed for the first time for the purpose to search for a magnetic field. We obtained the following results: HD35456 is a magnetic star with longitudinal field variation range from +300 to +650 G and a period of 4.9506 days; HD35881 is possible a new magnetic star, the longitudinal component of which varies from?1 to +1 kGwith a period of 0.6998 days, however, a small number of lines broadened by rotation does not allow us to conduct measurements more accurately; HD36313 is a binary star with the components similar in brightness, the primary component is a magnetic star with broad lines, the magnetic field of the secondary component (the star with narrow lines) was not detected. Measurements in the Hβ hydrogen line showed the variations of the longitudinal component from ?1.5 to +2 kG with a period of 1.17862 days; a strong longitudinal field was detected in HD36526 (from 0 to +3000 G) varying with a rotation period of the star of 3.081 days. In all the cases, we observe considerable discrepancies with the data on magnetic fields of these objects obtained earlier.     

The Magnetic Environments of Young Stellar Objects

We report on early results of submillimetre polarimetric measurements towards a sample of young stellar objects. The results allow us to infer the magnetic field structure and show a variety of configurations, providing evidence for axial, helical and pinched (i.e. ‘hourglass’) magnetic field configurations. We find that in some cases the field is curved over large scales, implying that it is influenced by the gas kinematics in the local environment, and that at these scales at least, the magnetic field plays a passive rôle in the star formation process.     

Effects of external magnetic field on the outflows of an accretion disc

The aim of this work is to study the effects of an external magnetic field generated by a magnetized compact star on the outflows of its accretion disc. For this purpose, we solve a set of magneto-hydrodynamic (MHD) equations for an accretion disc in spherical coordinates to consider the disc structure along the θ-direction. We also consider the magnetic field of a compact star beyond its surface as a dipolar field, producing a toroidal magnetic field inside the disc. We convert the equations to a set of ordinary differential equations (ODEs) as a function of the θ only by applying self-similar assumptions in the radial direction. Then, this set of equations is solved under symmetrical boundary conditions in the equatorial plane to obtain the velocity field. The results are considered in the gas-pressure-dominated (GPD) region and radiation-pressure-dominated (RPD) region as well. The dipolar field of the compact stars can significantly enhance the speed of outflows. It also can change the structure of the disc. The results of this work would be useful in the study of X-ray binaries, the origin of ultra-relativistic outflows, and jet formation around the compact stars.