Whence the Interstellar Magnetic Field Shaping the Heliosphere?

We present sub-arcsecond 7.5$-$13 $\mu$m imaging- and spectro-polarimetric observations of NGC 1068 using CanariCam on the 10.4-m Gran Telescopio CANARIAS. At all wavelengths, we find: (1) A 90 $\times$ 60 pc extended polarized feature in the northern ionization cone, with a uniform $\sim$44$^{\circ}$ polarization angle. Its polarization arises from dust and gas emission in the ionization cone, heated by the active nucleus and jet, and further extinguished by aligned dust grains in the host galaxy. The polarization spectrum of the jet-molecular cloud interaction at $\sim$24 pc from the core is highly polarized, and does not show a silicate feature, suggesting that the dust grains are different from those in the interstellar medium. (2) A southern polarized feature at $\sim$9.6 pc from the core. Its polarization arises from a dust emission component extinguished by a large concentration of dust in the galaxy disc. We cannot distinguish between dust emission from magnetically aligned dust grains directly heated by the jet close to the core, and aligned dust grains in the dusty obscuring material surrounding the central engine. Silicate-like grains reproduce the polarized dust emission in this feature, suggesting different dust compositions in both ionization cones. (3) An upper limit of polarization degree of 0.3 per cent in the core. Based on our polarization model, the expected polarization of the obscuring dusty material is $\lesssim$0.1 per cent in the 8$-$13 $\mu$m wavelength range. This low polarization may be arising from the passage of radiation through aligned dust grains in the shielded edges of the clumps.

We present the first polarimetric observations of a circumstellar disk in the far-infrared wavelength range. We report flux and linear polarization measurements of the young stellar object HL Tau in the bands A ( um ), C ( um ), D ( um ), and E ( um ) with the High-resolution Airborne Wideband Camera-plus (HAWC+) on board of the Stratospheric Observatory for Infrared Astronomy (SOFIA). The orientation of the polarization vectors is strongly wavelength-dependent and can be attributed to different wavelength-dependent polarization mechanisms in the disk and its local environment. In bands A, C, and D ( um to um ), the orientation of the polarization is roughly consistent with a value of at the maximum emission. Hereby, the magnetic field direction is close to that of the spin axis of the disk. In contrast, in band E ( um ), the orientation is nearly parallel to the minor axis of the projection of the inclined disk. Based on a viscous accretion disk model combined with a surrounding envelope, we performed polarized three-dimensional Monte Carlo radiative transfer simulations. In particular, we considered polarization due to emission and absorption by aligned dust grains, and polarization due to scattering of the thermal reemission (self-scattering). At wavelengths of um um and um we were able to reproduce the observed orientation of the polarization vectors. Here, the origin of polarization is consistent with polarized emission by aligned non-spherical dust grains. In contrast, at a wavelength of um the polarization pattern could not be fully matched, however, applying self-scattering and assuming dust grain radii up to um we were able to reproduce the flip in the orientation of polarization. We conclude that the polarization is caused by dichroic emission of aligned dust grains in the envelope, while at longer wavelengths, the envelope becomes transparent and the polarization is dominated by self-scattering in the disk.

The low density interstellar medium (ISM) close to the Sun and inside of the heliosphere provides a unique laboratory for studying interstellar dust grains. Grain characteristics in the nearby ISM are obtained from observations of interstellar gas and dust inside of the heliosphere and the interstellar gas towards nearby stars. Comparison between the gas composition and solar abundances suggests that grains are dominated by olivines and possibly some form of iron oxide. Measurements of the interstellar Ne/O ratio by the Interstellar Boundary Explorer spacecraft indicate that a high fraction of interstellar oxygen in the ISM must be depleted onto dust grains. Local interstellar abundances are consistent with grain destruction in ~150 km s−1 interstellar shocks, provided that the carbonaceous component is hydrogenated amorphous carbon and carbon abundances are correct. Variations in relative abundances of refractories in gas suggest variations in the history of grain destruction in nearby ISM. The large observed grains, > 1 μm, may indicate a nearby reservoir of denser ISM. Theoretical three-dimensional models of the interaction between interstellar dust grains and the solar wind predict that plumes of ~0.18 μm dust grains form around the heliosphere.

Starlight linearly polarized by aligned interstellar dust grains provides the necessary data for tracing the structure of the very local interstellar magnetic field (ISMF). Two methods have been developed to recover the ISMF direction from polarized starlight, using data from an ongoing polarization survey. Both methods rely on the probability distribution function for polarized light. Method 1 calculates the ISMF direction from polarization position angles regardless of the data accuracy, while Method 2 relies on high-probability data points. The ISMF direction Bibex recovered by Method 1 corresponds to the closest ISMF to the heliosphere, traced by the center of the IBEX Ribbon arc. Method 2 reveals a new direction for the more distant ISMF, Bnew, toward l=41.1° ± 4.1° and b= 25.8° ± 3.0°, which differs by 30.4° ± 5.6° from the IBEX ISMF direction. Polarizations of filament stars that are located within 25° of a pole of Bnew, where background polarizations would be minimal, show the highest statistical probabilities of tracing the filament ISMF. The IBEX ISMF direction orders the kinematics of interstellar clouds within 15 pc, and Bnew must therefore dominate beyond 15 pc. These new data are consistent with the location of the Sun in the rim of an expanding superbubble shell associated with the evolved Loop I superbubble.

Continuum polarization over the UV-to-microwave range is due to dichroic extinction (or emission) by asymmetric, aligned dust grains. Scattering can also be an important source of polarization, especially at short wavelengths. Because of both grain alignment and scattering physics, the wavelength dependence of the polarization, generally, traces the size of the aligned grains. Similarly because of the differing wavelength dependencies of dichroic extinction and scattering polarization, the two can generally be reliably separated. Ultraviolet (UV) polarimetry therefore provides a unique probe of the smallest dust grains (diameter< 0.09~upmu text{m}), their mineralogy and interaction with the environment. However, the current observational status of interstellar UV polarization is very poor with less than 30 lines of sight probed. With the modern, quantitative and well-tested, theory of interstellar grain alignment now available, we have the opportunity to advance the understanding of the interstellar medium (ISM) by executing a systematic study of the UV polarization in the ISM of the Milky Way and near-by galaxies. The Polstar mission will provide the sensitivity and observing time needed to carry out such a program (probing hundreds of stars in the Milky Way and dozens of stars in the LMC/SMC), addressing questions of dust composition as a function of size and location, radiation- and magnetic-field characteristics as well as unveiling the carrier of the 2175 Å extinction feature. In addition, using high-resolution UV line spectroscopy Polstar will search for and probe the alignment of, and polarization from, aligned atoms and ions - so called “Ground State Alignment”, a potentially powerful new probe of magnetic fields in the diffuse ISM.

The heliosphere is a “bubble” of plasma that forms around the Sun through a pressure balance between the outflowing solar wind and the interstellar medium. The Sun is currently traversing the local interstellar medium at a relative velocity of approximately 26 km s−1. Due to the Sun’s motion, interstellar dust grains present in the interstellar medium are transported through the heliosphere’s boundary, from the upwind direction.Dust grains in a space environment are subject to a variety of charging mechanisms, which result in an overall equilibrium charge on their surface. In the interstellar medium and in the solar wind, the primary charging mechanisms are plasma collection, secondary electron emission, and photoelectric emission. The charge acquired by a dust grain depends on several factors, including the size, composition, and structure of the dust grain itself, as well as on the characteristics of the surrounding environment.The trajectories of charged dust grains are influenced by the magnetic field in the environment they are moving through due to the emerging Lorentz force. When approaching the heliosphere, the interstellar magnetic field starts to get disturbed by the solar wind (heliospheric) magnetic field. The amount of trajectory deflection an inflowing interstellar dust grain experiences depends on its charge-to-mass ratio. Consequently, not all interstellar dust grains enter the solar system.We discuss the dust charging with a particular focus on the influence of the space environment conditions that are expected at different locations throughout the heliosphere, including the boundary regions and including short-term and long-term variations of the environmental conditions due to the solar activity. Using these results, we show the influence of heliospheric properties on the dust grain trajectories at the heliospheric interface in specific. The results will help to understand the physical processes occurring at the boundary of the heliosphere.

In recent years, interstellar dust has been detected by spacecraft within a few AU from the Sun. The first analysis of observed data revealed that certain properties such as size distribution and average mass of interstellar dust grains found close to the Sun differ from those of classical interstellar grains. In particular, detected grains seem to be heavier at small distances from the Sun than in the interstellar space. It was concluded that the characteristics of interstellar dust grains are modified by some process as they approach the vicinity of the Sun. In this paper for the first time we simulate the dynamics of dust grains in the interstellar medium and the heliosphere and demonstrate that they experience a filtration process in the region of the heliopause. The main result of this process is the exclusion of small submicron‐sized grains from the incoming stream. We estimate the cutoff range for the process to lie between 0.1 and 0.2 μm, which corresponds to 10−16 – 10−17 kg dust grains. Although this result is consistent with satellite observations, we show that it alone does not explain interstellar grain data that were collected in situ in the interplanetary space.

The in situ detection of interstellar dust grains in the solar system by the dust instruments on‐board the Ulysses and Galileo spacecraft as well as the recent measurements of hyperbolic radar meteors give information on the properties of the interstellar solid particle population in the solar vicinity. Especially the distribution of grain masses is indicative of growth and destruction mechanisms that govern the grain evolution in the interstellar medium. The mass of an impacting dust grain is derived from its impact velocity and the amount of plasma generated by the impact. Because the initial velocity and the dynamics of interstellar particles in the solar system are well known, we use an approximated theoretical instead of the measured impact velocity to derive the mass of interstellar grains from the Ulysses and Galileo in situ data. The revised mass distributions are steeper and thus contain less large grains than the ones that use measured impact velocities, but large grains still contribute significantly to the overall mass of the detected grains. The flux of interstellar grains with masses &gt; 10−14 kg is determined to be 1 × 10−6 m−2 s−1. The comparison of radar data with the extrapolation of the Ulysses and Galileo mass distribution indicates that the very large (m &gt; 10−10 kg) hyperbolic meteoroids detected by the radar are not kinematically related to the interstellar dust population detected by the spacecraft.

In the interstellar medium (ISM), molecular hydrogen is expected to form almost exclusively on the surfaces of dust grains. Due to that molecule's large formation energy (–4.5 eV), several dynamical effects are likely associated with the process, including the alignment of asymmetric dust grains with the ambient magnetic field. Such aligned dust grains are, in turn, believed to cause the broadband optical/infrared polarization observed in the ISM. Here, we present the first observational evidence for grain alignment driven by H2 formation, by showing that the polarization of the light from stars behind the reflection nebula IC 63 appears to correlate with the intensity of H2 fluorescence. While our results strongly suggest a role for Purcell rockets in grain alignment, additional observations are needed to conclusively confirm their role. By showing a direct connection between H2 formation and a probe of the dust characteristics, these results also provide one of the first direct confirmations of the grain-surface formation of H2. We compare our observations to ab initio modeling based on Radiative Torque Alignment (RAT) theory.

view Abstract Citations (40) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar polarization: magnetite dust. Shapiro, P. R. Abstract The polarization and extinction properties of magnetite dust grains are calculated based on indirect evidence that magnetite grains might be the interstellar polarizers. The extinction and phase-lag cross sections of magnetite grains are calculated using a computer method for nonspherical grains. From these, the linear and circular polarization which would be produced by a medium of aligned magnetite dust grains is determined. It is found that flat platelets of magnetite can reproduce the wavelength dependence of the polarization observations. Given the predictions by the Purcell (1974) 'pinwheel' mechanism of perfect alignment for such grains, it is shown that magnetite grains can produce all the observed polarization at visual wavelength by consuming roughly 16 percent of the cosmic abundance of iron atoms in the interstellar medium, while contributing roughly 4 percent of the total visual extinction. With less than perfect alignment, more iron must be put into the magnetite grains, resulting in more magnetite extinction. A general property of interstellar polarization is also discussed based upon the Kramers-Kronig dispersion relations. An infrared observational test of this property is proposed. Publication: The Astrophysical Journal Pub Date: October 1975 DOI: 10.1086/153868 Bibcode: 1975ApJ...201..151S Keywords: Cosmic Dust; Interstellar Matter; Magnetite; Polarization Characteristics; Absorption Cross Sections; Circular Polarization; Interstellar Gas; Interstellar Magnetic Fields; Light (Visible Radiation); Optical Polarization; Scattering Cross Sections; Stellar Atmospheres; Astrophysics full text sources ADS |

The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, Rg/d = 94. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10-14 to 10-13 g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of CMa, give Rg/d = 427 for assumed solar reference abundances and Rg/d = 551 for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward λ Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H0 indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s-1. If solar reference abundances are correct, the low Rg/d value toward λ Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.

First-principles models of the formation of ${\text{H}}_{2}$ on interstellar media carbonaceous grains are usually concerned with processes occurring on ideal graphenic surfaces. Until now these models are unable to explain the formation of molecular hydrogen due to the presence of absorption barriers that cannot be overcome at the low temperatures of the interstellar media. We propose an approach emphasizing the role of specific topological defects for molecular hydrogen catalysis at interstellar dust grain models. Using the nudged elastic band method combined with density-functional techniques, we obtain the full catalytic cycle for the formation of the ${\text{H}}_{2}$ molecule on complex carbon topologies involving the presence of pentagonal rings and C adatoms. Depending on structures, reaction paths can be barrierless or have adsorption barriers as low as ${10}^{\ensuremath{-}3}--{10}^{\ensuremath{-}2}\text{ }\text{eV}$, which might be easily overcome at the temperatures of the interstellar medium. Such low adsorption barriers indicate that specific carbon grains topological defects are preferential sites for the molecular hydrogen formation in the interstellar medium.

view Abstract Citations (2) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spin-related magnetism of interstellar grains. Srnka, L. J. ; De, B. R. Abstract The magnetic dipole moments and internal magnetic fields due to the spin of electrically charged elongated nonmagnetic interstellar grains in kinetic equilibrium with their surroundings are computed for the grain-size range from 0.01 to 1.0 micron. It is shown that the induced magnetic moments and internal magnetic fields of charged spinning nonmagnetic grains of arbitrary composition and prolate spheroidal shape can be appreciable, possibly even exceeding 0.01 emu/cu cm for 0.01-micron grains. The results indicate that virtually all grains smaller than 0.1 micron in mean diameter, and all elongated grains smaller than about 1 micron in length, are immersed in local magnetic fields due to spin that are much larger than the ambient galactic field. Some implications of this effect are discussed in relation to the polarization of starlight by aligned dust grains and the primordial remanent magnetization found in primitive carbonaceous chondrites. Publication: The Astrophysical Journal Pub Date: October 1978 DOI: 10.1086/156504 Bibcode: 1978ApJ...225..422S Keywords: Interstellar Magnetic Fields; Interstellar Matter; Magnetic Dipoles; Particle Spin; Magnetic Moments; Polarization Characteristics; Spin Dynamics; Astrophysics; Grains:Interstellar Matter; Interstellar Matter:Magnetic Fields full text sources ADS |

Recent Planck results have shown that radiation from the cosmic microwave background passes through foregrounds in which aligned dust grains produce polarized dust emission, even in regions of the sky with the lowest level of dust emission. One of the most commonly used ways to remove the dust foreground is to extrapolate the polarized dust emission signal from frequencies where it dominates (e.g. ∼350 GHz) to frequencies commonly targeted by cosmic microwave background experiments (e.g. ∼150 GHz). In this Letter, we describe an interstellar medium effect that can lead to decorrelation of the dust emission polarization pattern between different frequencies due to multiple contributions along the line of sight. Using a simple 2-cloud model we show that there are two conditions under which this decorrelation can be large: (a) the ratio of polarized intensities between the two clouds changes between the two frequencies; (b) the magnetic fields between the two clouds contributing along a line of sight are significantly misaligned. In such cases, the 350 GHz polarized sky map is not predictive of that at 150 GHz. We propose a possible correction for this effect, using information from optopolarimetric surveys of dichroicly absorbed starlight.

The Sun is currently traversing the local interstellar medium at a relative velocity of approximately 26 km s−1. Due to the Sun’s motion, interstellar dust grains from the interstellar medium are transported trough the heliosphere’s boundary, from the upwind direction.Dust grains in a space environment are subject to a variety of charging mechanisms, which result in an overall equilibrium charge on their surface. In the interstellar medium and in the solar wind, the primary charging mechanisms are plasma collection, secondary electron emission, and photoelectric emission. The charge acquired by a dust grain depends on a number of factors, including the size, composition, and structure of the dust grain itself, as well as on the characteristics of the surrounding environment.The magnetic field that the dust grains encounter when approaching the heliosphere, starts to change into the heliospheric magnetic field. Hence, the motion of individual grains near the heliospheric interface changes due to the influence of the Lorentz forces. The amount of trajectory deflection depends on the charge-to-mass ratio of a grain. Consequently, not all interstellar dust grains enter the solar system.We discuss the dust charging with a particular focus on the influence of the space environment conditions that are expected at different locations throughout the heliosphere, including its boundary regions and including short-term and long-term variations of space environment conditions due to solar activity. These are necessary to calculate the dust trajectories in particular at the heliospheric interface. The results will help to explain the physical processes occurring at the boundary of the heliosphere.