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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

A Method to Measure the Transverse Magnetic Field and Orient the Rotational Axis of Stars

Direct measurements of stellar magnetic fields are based on the splitting of spectral lines into polarized Zeeman components. With a few exceptions, Zeeman signatures are hidden in data noise, and a number of methods have been developed to measure the average, over the visible stellar disk, of longitudinal components of the magnetic field. At present, faint stars are only observable via low-resolution spectropolarimetry, which is a method based on the regression of the Stokes V signal against the first derivative of Stokes I . Here, we present an extension of this method to obtain a direct measurement of the transverse component of stellar magnetic fields by the regression of high-resolution Stokes Q and U as a function of the second derivative of Stokes I . We also show that it is possible to determine the orientation in the sky of the rotation axis of a star on the basis of the periodic variability of the transverse component due to its rotation. The method is applied to data, obtained with the Catania Astrophysical Observatory Spectropolarimeter along the rotational period of the well known magnetic star β CrB.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1987

Radiatively driven winds from magnetic, fast-rotating stars - Wolf-Rayet stars?

An analytical procedure is developed to solve the MHD equations for a stellar wind in the strong-magnetic-field, optically thick limit for hot stars. The slow-mode, Alfven, and fast-mode critical points are modified by the radiation terms in the force equation but in a manner that can be easily treated. Once the velocities and distances are known at the critical points, the streamline constants are determined in a straight-forward manner. This shows the structure of the wind with a relatively simple numerical scheme. The magnetic field and a radiation parameter specify the terminal wind velocity. High rotation rates and a modified slow-mode critical point close to the stellar surface determine the high mass-loss rates. Wolf-Rayet stars are modeled with 1000-G fields but require stellar rotational velocities approaching breakup values. The physical conditions that correspond to Wolf-Rayet models are so rare that another mechanism must be operating in these winds.

Nerney, Steven↗
DOE OSTI2017

Unveiling the Role of the Magnetic Field at the Smallest Scales of Star Formation

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of polarized dust emission from the protostellar source Ser-emb 8 at a linear resolution of 140 au. Assuming models of dust-grain alignment hold, the observed polarization pattern gives a projected view of the magnetic field structure in this source. Contrary to expectations based on models of strongly magnetized star formation, the magnetic field in Ser-emb 8 does not exhibit an hourglass morphology. Combining the new ALMA data with previous observational studies, we can connect magnetic field structure from protostellar core (∼80,000 au) to disk (∼100 au) scales. We compare our observations with four magnetohydrodynamic gravo-turbulence simulations made with the AREPO code that have initial conditions ranging from super-Alfvénic (weakly magnetized) to sub-Alfvénic (strongly magnetized). These simulations achieve the spatial dynamic range necessary to resolve the collapse of protostars from the parsec scale of star-forming clouds down to the ∼100 au scale probed by ALMA. Only in the very strongly magnetized simulation do we see both the preservation of the field direction from cloud to disk scales and an hourglass-shaped field at <1000 au scales. We conduct an analysis of the relative orientation of the magnetic field and the density structure in both the Ser-emb 8 ALMA observations and the synthetic observations of the four AREPO simulations. We conclude that the Ser-emb 8 data are most similar to the weakly magnetized simulations, which exhibit random alignment, in contrast to the strongly magnetized simulation, where the magnetic field plays a role in shaping the density structure in the source. In the weak-field case, it is turbulence—not the magnetic field—that shapes the material that forms the protostar, highlighting the dominant role that turbulence can play across many orders of magnitude in spatial scale.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

Magnetic moments and angular momenta of stars and planets

Using published data on magnetic fields, radii, masses, and rotation, we have compiled a data set of magnetic moments mu and angular momenta L for stars and planets. In our subsample of hotter stars (classes A, B, and O), there are 171 objects. In the subsample of cooler stars (classes F, G, K, and M), there are 54 objects. We include 33 white dwarfs, of which 19 are in cataclysmic variables. The pulsar subsample contains 32 pulsars in binaries and 429 isolated pulsars. Som subsamples exhibit significant empirical correlations between log mu and log L. For the hot and cool stars, the correlations are positive. However, the hot-star correlation is significantly shallower than for the cool stars. In the solar system subsample, the correlation has essentially the same slope as for the cool stars, although the magnetic moments are two to three orders of magnitude smaller for the solar system objects at a given L value. For isolated white dwarfs, the correlations are weak or absent. Pulsars and white dwarfs in close binaries show strong negative correlations: the results are quantitatively consistent with magnetically enforced synchronism with the orbital period. When we consider the centers of gravity of the different subsamples of objects, a significant positive correlation appears between log mu and log L.

Arge, C. N.↗
NASA NTRS1980

A magnetic study of spotted UV Ceti flare stars and related late-type dwarfs

A multichannel photoelectric Zeeman analyzer has been used to investigate the magnetic nature of the spotted UV Ceti flare stars. Magnetic observations were obtained on a sample of 19 program objects, of which 5 were currently spotted dKe-dMe stars, 7 were normal dK-dM stars, 7 were UV Ceti flare stars, and 1 was a possible post-T Tauri star. Contrary to most previously published observations and theoretical expectations, no magnetic fields were detected on any of these objects from either the absorption lines or the H-alpha emission line down to an observational uncertainty level of 100-160 gauss (standard deviation).

Vogt, S. S.↗
NASA NTRS1986

Radiatively driven winds from magnetic, fast-rotating stars

An analytical procedure is developed to solve the magnetohydrodynamic equations for the stellar wind problem in the strong-magnetic field, optically thick limit for hot stars. The slow-mode, Alfven, and fast-mode critical points are modified by the radiation terms in the force equation but in a manner that can be treated relatively easily. Once the velocities at the critical points and the distances to the points are known, the streamline constants are determined in a straight-forward manner. This allows the structure of the wind to be elucidated without recourse to complicated computational schemes.

Nerney, S.↗
DOE OSTI2016

The link between turbulence, magnetic fields, filaments, and star formation in the central molecular zone cloud G0.253+0.016

Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However, the turbulence and magnetic fields in molecular clouds near the Galactic center may differ substantially compared to spiral-arm clouds. Here we determine the physical parameters of the central molecular zone (CMZ) cloud G0.253+0.016, its turbulence, magnetic field, and filamentary structure. Using column density maps based on dust-continuum emission observations with ALMA+Herschel, we identify filaments and show that at least one dense core is located along them. We measure the filament width W{sub fil}=0.17±0.08 pc and the sonic scale λ{sub sonic}=0.15±0.11 pc of the turbulence, and find W{sub fil}≈λ{sub sonic}. A strong velocity gradient is seen in the HNCO intensity-weighted velocity maps obtained with ALMA+Mopra. The gradient is likely caused by large-scale shearing of G0.253+0.016, producing a wide double-peaked velocity probability distribution function (PDF). After subtracting the gradient to isolate the turbulent motions, we find a nearly Gaussian velocity PDF typical for turbulence. We measure the total and turbulent velocity dispersion, 8.8±0.2 km s{sup −1} and 3.9±0.1 km s{sup −1}, respectively. Using magnetohydrodynamical turbulence simulations, we find that G0.253+0.016's turbulent magnetic field B{sub turb}=130±50 μG is only ≲1/10 of the ordered field component. Combining these measurements, we reconstruct the dominant turbulence driving mode in G0.253+0.016 and find a driving parameter of b=0.22±0.12, indicating solenoidal (divergence-free) driving. We compare this to spiral-arm clouds, which typically have a significant compressive (curl-free) driving component (b>0.4). Motivated by previous reports of strong shearing motions in the CMZ, we speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this reduces the star-formation rate by a factor of 6.9 compared to typical nearby clouds.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SUPERNOVAE POWERED BY MAGNETARS THAT TRANSFORM INTO BLACK HOLES

Rapidly rotating, strongly magnetized neutron stars (NSs; magnetars) can release their enormous rotational energy via magnetic spin-down, providing a power source for bright transients such as superluminous supernovae (SNe). On the other hand, particularly massive (so-called supramassive) NSs require a minimum rotation rate to support their mass against gravitational collapse, below which the NS collapses to a black hole (BH). We model the light curves (LCs) of SNe powered with magnetars that transform into BHs. Although the peak luminosities can reach high values in the range of superluminous SNe, their post maximum LCs can decline very rapidly because of the sudden loss of the central energy input. Early BH transformation also enhances the shock breakout signal from the magnetar-driven bubble relative to the main SN peak. Our synthetic LCs of SNe powered by magnetars transforming to BHs are consistent with those of some rapidly evolving bright transients recently reported by Arcavi et al.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Vertical equilibrium, energetics, and star formation rates in magnetized galactic disks regulated by momentum feedback from supernovae

Recent hydrodynamic (HD) simulations have shown that galactic disks evolve to reach well-defined statistical equilibrium states. The star formation rate (SFR) self-regulates until energy injection by star formation feedback balances dissipation and cooling in the interstellar medium (ISM), and provides vertical pressure support to balance gravity. In this paper, we extend our previous models to allow for a range of initial magnetic field strengths and configurations, utilizing three-dimensional, magnetohydrodynamic (MHD) simulations. We show that a quasi-steady equilibrium state is established as rapidly for MHD as for HD models unless the initial magnetic field is very strong or very weak, which requires more time to reach saturation. Remarkably, models with initial magnetic energy varying by two orders of magnitude approach the same asymptotic state. In the fully saturated state of the fiducial model, the integrated energy proportions E{sub turb}:E{sub th}:δE{sub mag}: E-bar {sub mag} are 0.35:0.39:0.15:0.11, while the proportions of midplane support P{sub turb}:P{sub th}:δΠ{sub mag}: Π-bar {sub mag} are 0.49:0.18:0.18:0.15. Vertical profiles of total effective pressure satisfy vertical dynamical equilibrium with the total gas weight at all heights. We measure the “feedback yields” η{sub c}≡P{sub c}/Σ{sub SFR} (in suitable units) for each pressure component, finding that η{sub turb}∼3.5−4 and η{sub th}∼1.1−1.4 are the same for MHD as in previous HD simulations, and δη{sub mag}∼1.3−1.5. These yields can be used to predict the equilibrium SFR for a local region in a galaxy based on its observed gas and stellar surface densities and velocity dispersions. As the ISM weight (or dynamical equilibrium pressure) is fixed, an increase in η from turbulent magnetic fields reduces the predicted Σ{sub SFR} by ∼20−30% relative to the HD case.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1983

Observations of magnetic fields on late-type stars

The 'Robinson' method for measuring magnetic fields on solarand late-type stars is reviewed. The results of such measurements for a sample of 29 G and K main-sequence stars are presented. The area covering-factors of magnetic regions are greater in the K dwarfs than in the G dwarfs, but no spectral-type dependence is found for the field strengths, contrary to expectations of some flux-tube models. The dependence of Ca II H and K emission on magnetic fields and T(eff) is consistent with theoretical expections for 'slow-mode' mhd wave-generation rates, but inconsistent with those of other mhd modes. Coronal soft X-ray fluxes correlate well with the magnetic fields, and it is argued that Alfven waves are the likely energy-transport mechanism. Surface magnetic fluxes vary with rotation as V(0.5-1.0 rot) depending on spectral type.

Marcy, G. W.↗
DOE OSTI2015

EXPLAINING THE COEXISTENCE OF LARGE-SCALE AND SMALL-SCALE MAGNETIC FIELDS IN FULLY CONVECTIVE STARS

Despite the lack of a shear-rich tachocline region, low-mass fully convective (FC) stars are capable of generating strong magnetic fields, indicating that a dynamo mechanism fundamentally different from the solar dynamo is at work in these objects. We present a self-consistent three-dimensional model of magnetic field generation in low-mass FC stars. The model utilizes the anelastic magnetohydrodynamic equations to simulate compressible convection in a rotating sphere. A distributed dynamo working in the model spontaneously produces a dipole-dominated surface magnetic field of the observed strength. The interaction of this field with the turbulent convection in outer layers shreds it, producing small-scale fields that carry most of the magnetic flux. The Zeeman–Doppler-Imaging technique applied to synthetic spectropolarimetric data based on our model recovers most of the large-scale field. Our model simultaneously reproduces the morphology and magnitude of the large-scale field as well as the magnitude of the small-scale field observed on low-mass FC stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Origin and Evolution of Magnetic Field in PMS Stars: Influence of Rotation and Structural Changes

During stellar evolution, especially in the pre-main-sequence phase, stellar structure and rotation evolve significantly, causing major changes in the dynamics and global flows of the star. We wish to assess the consequences of these changes on stellar dynamo, internal magnetic field topology, and activity level. To do so, we have performed a series of 3D HD and MHD simulations with the ASH code. We choose five different models characterized by the radius of their radiative zone following an evolutionary track computed by a 1D stellar evolution code. These models characterized stellar evolution from 1 to 50 Myr. By introducing a seed magnetic field in the fully convective model and spreading its evolved state through all four remaining cases, we observe systematic variations in the dynamical properties and magnetic field amplitude and topology of the models. The five MHD simulations develop a strong dynamo field that can reach an equipartition state between the kinetic and magnetic energies and even superequipartition levels in the faster-rotating cases. We find that the magnetic field amplitude increases as it evolves toward the zero-age main sequence. Moreover, the magnetic field topology becomes more complex, with a decreasing axisymmetric component and a nonaxisymmetric one becoming predominant. The dipolar components decrease as the rotation rate and the size of the radiative core increase. The magnetic fields possess a mixed poloidal-toroidal topology with no obvious dominant component. Moreover, the relaxation of the vestige dynamo magnetic field within the radiative core is found to satisfy MHD stability criteria. Hence, it does not experience a global reconfiguration but slowly relaxes by retaining its mixed stable poloidal-toroidal topology.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Effects of neutrino emissivity on the cooling of neutron stars in the presence of a strong magnetic field

One of the most interesting kind of neutron stars are the pulsars, which are highly magnetized neutron stars with fields up to 10{sup 14} G at the surface. The strength of magnetic field in the center of a neutron star remains unknown. According to the scalar virial theorem, magnetic field in the core could be as large as 10{sup 18} G. In this work we study the influence of strong magnetic fields on the cooling of neutron stars coming from direct Urca process. Direct Urca process is an extremely efficient mechanism for cooling a neutron star after its formation. The matter is described using a relativistic mean-field model at zero temperature with eight baryons (baryon octet), electrons and muons. We obtain the relative population of each species of particles as function of baryon density for different magnetic fields. We calculate numerically the cooling of neutron stars for a parametrized magnetic field and compare the results for the case without a magnetic field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Magnetic and gravitational disk-star interactions: an interdependence of PMS stellar rotation rates and spin-orbit misalignments

The presence of giant gaseous planets that reside in close proximity to their host stars, i.e., hot Jupiters, may be a consequence of large-scale radial migration through the protoplanetary nebulae. Within the framework of this picture, significant orbital obliquities characteristic of a substantial fraction of such planets can be attributed to external torques that perturb the natal disks out of alignment with the spin axes of their host stars. Therefore, the acquisition of orbital obliquity likely exhibits sensitive dependence on the physics of disk-star interactions. Here, we analyze the primordial excitation of spin-orbit misalignment of Sun-like stars in light of disk-star angular momentum transfer. We begin by calculating the stellar pre-main-sequence rotational evolution, accounting for spin-up due to gravitational contraction and accretion as well as spin-down due to magnetic star-disk coupling. We devote particular attention to angular momentum transfer by accretion, and show that while generally subdominant to gravitational contraction, this process is largely controlled by the morphology of the stellar magnetic field (that is, specific angular momentum accreted by stars with octupole-dominated surface fields is smaller than that accreted by dipole-dominated stars by an order of magnitude). Subsequently, we examine the secular spin-axis dynamics of disk-bearing stars, accounting for the time-evolution of stellar and disk properties, and demonstrate that misalignments are preferentially excited in systems where stellar rotation is not overwhelmingly rapid. Moreover, we show that the excitation of spin-orbit misalignment occurs impulsively through an encounter with a resonance between the stellar precession frequency and the disk-torquing frequency. Cumulatively, the model developed herein opens up a previously unexplored avenue toward understanding star-disk evolution and its consequences in a unified manner.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1991

Multipolar magnetic fields in neutron stars

The small periods and period derivatives of millisecond pulsars permit interesting upper bounds to be placed on the strength of multipole components in their magnetic fields if the loss of rotational energy may be approximated by vacuum electromagnetic radiation formulas. The surface field strength of millisecond pulsars may be as high as about 10 to the 12th G if the dominant multipole component is l not less than 3. Significant multipole moments may help explain the unusual pulse morphology of millisecond pulsars. If the magnetic fields in gamma-ray burst sources (old neutron stars?) have the same evolutionary history as in millisecond pulsars, the about 10 to the 12th G local fields observed in several gamma-ray bursts must be due to multipole moments with l not less than 3.

Krolik, Julian H.↗
NASA NTRS1988

Superheavy magnetic monopoles and main-sequence stars

The interactions of superheavy monopoles with stars of mass 0.6-25 solar masses are studied. It is shown that captured monopoles cluster near the stellar center, and that their contribution to the total stellar luminosity and their effects on the structure of stars will be unobservable as long as the flux is significantly below the Parker (1970, 1971) bound. It is suggested that all but the strongly magnetic stars will retain a substantial fraction of the monopoles they capture.

Frieman, Joshua A.↗
DOE OSTI2017

New Suns in the Cosmos. IV. The Multifractal Nature of Stellar Magnetic Activity in Kepler Cool Stars

In the present study, we investigate the multifractal nature of a long-cadence time series observed by the Kepler mission for a sample of 34 M dwarf stars and the Sun in its active phase. Using the Multifractal Detrending Moving Average algorithm, which enables the detection of multifractality in nonstationary time series, we define a set of multifractal indices based on the multifractal spectrum profile as a measure of the level of stellar magnetic activity. This set of indices is given by the (A, Δα, C, H)-quartet, where A, Δα, and C are related to geometric features from the multifractal spectrum and the global Hurst exponent H describes the global structure and memorability of time series dynamics. As a test, we measure these indices and compare them with a magnetic index defined as S {sub ph} and verify the degree of correlation among them. First, we apply the Poincaré plot method and find a strong correlation between the 〈S{sub ph}〉 index and one of the descriptors that emerges from this method. As a result, we find that this index is strongly correlated with long-term features of the signal. From the multifractal perspective, the 〈S{sub ph}〉 index is also strongly linked to the geometric properties of the multifractal spectrum except for the H index. Furthermore, our results emphasize that the rotation period of stars is scaled by the H index, which is consistent with Skumanich’s relationship. Finally, our approach suggests that the H index may be related to the evolution of stellar angular momentum and a star’s magnetic properties.

79 ASTRONOMY AND ASTROPHYSICS↗