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Register a new userMapping the Interstellar Magnetic Field Around the Heliosphere with Polarized Starlight
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Added by
Priscilla Chapman Frisch
Publication date
2018
fields
Physics
and research's language is
English
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Starlight that becomes linearly polarized by magnetically aligned dust grains provides a viable diagnostic of the interstellar magnetic field (ISMF). A survey is underway to map the local ISMF using data collected at eight observatories in both hemispheres. Two approaches are used to obtain the magnetic structure: statistically evaluating magnetic field directions traced by multiple polarization position angles, and least-squares fits that provide the dipole component of the magnetic field. We find that the magnetic field in the circumheliospheric interstellar medium (CHM), which drives winds of interstellar gas and dust through the heliosphere, drapes over the heliopause and influences polarization measurements. We discover a polarization band that can be described with a great circle that traverses the heliosphere nose and ecliptic poles. A gap in the band appears in a region coinciding both with the highest heliosheath pressure, found by IBEX, and the center of the Loop I superbubble. The least-squares analysis finds a magnetic dipole component of the polarization band with the axis oriented toward the ecliptic poles. The filament of dust around the heliosphere and the warm helium breeze flowing through the heliosphere trace the same magnetic field directions. Regions along the polarization band near the heliosphere nose have magnetic field orientations within 15 degrees of sightlines. Regions in the IBEX ribbon have field directions within 40 degrees of the plane of the sky. Several spatially coherent magnetic filaments are within 15 pc. Most of the low frequency radio emissions detected by the two Voyager spacecraft follow the polarization band. The geometry of the polarization band is compared to the Local Interstellar Cloud, the Cetus Ripple, the BICEP2 low opacity region, Ice Cube IC59 galactic cosmic ray data, and Cassini results.
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The local interstellar magnetic field affects both the heliosphere and the surrounding cluster of interstellar clouds (CLIC). Measurements of linearly polarized starlight provide the only test of the magnetic field threading the CLIC. Polarization measurements of the CLIC magnetic field show multiple local magnetic structures, one of which is aligned with the magnetic field traced by the center of the ribbon of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX). Comparisons between the bulk motion of the CLIC through the local standard of rest, the magnetic field direction, the geometric center of Loop I, and the polarized dust bridge extending from the heliosphere toward the North Polar Spur direction all suggest that the CLIC is part of the rim region of the Loop I superbubble.
Measurements of the velocity of interstellar HeI inside of the heliosphere have been conducted over the past forty years. These historical data suggest that the ecliptic longitude of the direction of the interstellar flow has increased at an average rate of about 0.19 degrees per year over time. Possible astronomical explanations for these short-term variations in the interstellar gas entering the heliosphere are presented.
We map the distribution and properties of the Milky Ways interstellar medium as traced by diffuse interstellar bands (DIBs) detected in near-infrared stellar spectra from the SDSS-III/APOGEE survey. Focusing exclusively on the strongest DIB in the H-band, at ~1.527 microns, we present a projected map of the DIB absorption field in the Galactic plane, using a set of about 60,000 sightlines that reach up to 15 kpc from the Sun and probe up to 30 magnitudes of visual extinction. The strength of this DIB is linearly correlated with dust reddening over three orders of magnitude in both DIB equivalent width (W_DIB) and extinction, with a power law index of 1.01 +/- 0.01, a mean relationship of W_DIB/A_V = 0.1 Angstrom mag^-1, and a dispersion of ~0.05 Angstrom mag^-1 at extinctions characteristic of the Galactic midplane. These properties establish this DIB as a powerful, independent probe of dust extinction over a wide range of A_V values. The subset of about 14,000 robustly detected DIB features have an exponential W_DIB distribution. We empirically determine the intrinsic rest wavelength of this transition to be lambda_0 = 15,272.42 Angstrom, and then calculate absolute radial velocities of the carrier, which display the kinematical signature of the rotating Galactic disk. We probe the DIB carrier distribution in three dimensions and show that it can be characterized by an exponential disk model with a scaleheight of about 100 pc and a scalelength of about 5 kpc. Finally, we show that the DIB distribution also traces large-scale Galactic structures, including the central long bar and the warp of the outer disk.
We investigate the linear polarization produced by interstellar dust aligned by the magnetic field in the solar neighborhood (d< 50 pc). We also look for intrinsic effects from circumstellar processes, specifically in terms of polarization variability and wavelength dependence. We aim to detect and map dust clouds which give rise to statistically significant amounts of polarization of the starlight passing through the cloud, and to determine the interstellar magnetic field direction from the position angle of the observed polarization. High-precision broad-band (BVR) polarization observations are made of 361 stars in spectral classes F to G, in the magnitude range 4-9, with detection sensitivity at the level of or better than 10E-5 (0.001 %). Statistically significant (>3 sigma) polarization is found in 115 stars, and > 2 sigma detection in 178 stars, out of the total sample of 361 stars. Polarization maps based on these data show filament-like patterns of polarization position angles which are related to both the heliosphere geometry, the kinematics of nearby clouds, and the Interstellar Boundary EXplorer (IBEX) ribbon magnetic field. From long-term multiple observations, a number (18) of stars show evidence of intrinsic variability at the 10E-5 level. This can be attributed to circumstellar effects (e.g., debris disks and chromospheric activity). The star HD 101805 shows a peculiar wavelength dependence, indicating size distribution of scattering particles different from that of a typical interstellar medium.
With the use of the data from archives, we studied the correlations between the equivalent widths of four diffuse interstellar bands (4430$r{A}$, 5780$r{A}$, 5797$r{A}$, 6284$r{A}$) and properties of the target stars (colour excess values, distances and Galactic coordinates). Many different plots of the diffuse interstellar bands and their maps were produced and further analysed. There appears to be a structure in the plot of equivalent widths of 5780$r{A}$ DIB (and 6284$r{A}$ DIB) against the Galactic $x$-coordinate. The structure is well defined below $sim150$ m$r{A}$ and within $|x|<250$ pc, peaking around $x=170$ pc. We argue that the origin of this structure is not a statistical fluctuation. Splitting the data in the Galactic longitude into several subregions improves or lowers the well known linear relation between the equivalent widths and the colour excess, which was expected. However, some of the lines of sight display drastically different behaviour. The region within $150^circ<l<200^circ$ shows scatter in the correlation plots with the colour excess for all of the four bands with correlation coefficients $textrm{R}<0.58$. We suspect that the variation of physical conditions in the nearby molecular clouds could be responsible. Finally, the area $250^circ<l<300^circ$ displays (from the statistical point of view) significantly lower values of equivalent widths than the other regions -- this tells us that there is either a significant underabundance of carriers (when compared with the other regions) or that this has to be a result of an observational bias.
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