CHARTING THE INTERSTELLAR MAGNETIC FIELD CAUSING THEINTERSTELLAR BOUNDARY EXPLORER(IBEX) RIBBON OF ENERGETIC NEUTRAL ATOMS

Although the continua of radio-loud Active Galactic Nuclei (AGN) are typically dominated by synchrotron radiation over virtually the entire spectrum, it is not clear whether the radio and higher-frequency emission originate in the same or different parts of the jet. Several different radio--optical correlations based on polarization data have been found recently, suggesting that the optical and radio polarization may be closely related, and that the corresponding emission regions may be cospatial (Gabuzda et. al2006, Jorstad et al. 2007, D'Arcangelo et al. 2007) Our joint analysis of optical and VLBA polarization data for a sample of about 40 AGNs shows that, after correction for the inferred VLBA core Faraday rotations, most BL Lac objects and some quasars have aligned VLBA-core and optical polarizations, although many quasars also show no obvious relationship between their VLBA-core and optical polarization angles. This may indicate that not all AGNs have cospatial regions of optical and radio emission in their jets. However, another possibility is that some of the 7mm-2cm VLBA cores have Faraday rotations of the order of several tens of thousand of rad/m^2, which were not properly fit by our three-frequency data due to n*pi ambiguities in the observed polarization angles, leading to incorrect subtraction of the effects of the core Faraday rotation, and so incorrect zero-wavelength radio polarization angles. The possibility of such high core Faraday rotations is supported by the results of the parsec-scale Faraday-rotation studies of Zavala & Taylor (2004) and Jorstad et al. (2007).

Measurements of starlight polarized by aligned interstellar dust grains are used to probe the relation between the orientation of the ambient interstellar magnetic field (ISMF) and the ISMF traced by the ribbons of energetic neutral atoms discovered by the Interstellar Boundary Explorer spacecraft. We utilize polarization data, many acquired specifically for this study, to trace the configuration of the ISMF within 40 pc. A statistical analysis yields a best-fit ISMF orientation, B magpol, aligned with Galactic coordinates ℓ = 42°, b = 49°. Further analysis shows the ISMF is more orderly for “downfield” stars located over 90° from B magpol. The data subset of downfield stars yields an orientation for the nearby ISMF at ecliptic coordinates λ, β ≈ 219° ± 15°, 43° ± 9° (Galactic coordinates l, b ≈ 40°, 56°, ±17°). This best-fit ISMF orientation from polarization data is close to the field direction obtained from ribbon models. This agreement suggests that the ISMF shaping the heliosphere belongs to an extended ordered magnetic field. Extended filamentary structures are found throughout the sky. A previously discovered filament traversing the heliosphere nose region, “Filament A,” extends over 300° of the sky, and crosses the upwind direction of interstellar dust flowing into the heliosphere. Filament A overlaps the locations of the Voyager kilohertz emissions, three quasar intraday variables, cosmic microwave background (CMB) components, and the inflow direction of interstellar grains sampled by Ulysses and Galileo. These features are likely located in the upstream outer heliosheath where ISMF drapes over the heliosphere, suggesting Filament A coincides with a dusty magnetized plasma. A filament 55° long is aligned with a possible shock interface between local interstellar clouds. A dark spot in the CMB is seen within 5° of the filament and within 10° of the downfield ISMF direction. Two large magnetic arcs are centered on the directions of the heliotail. The overlap between CMB components and the aligned dust grains forming Filament A indicates the configuration of dust entrained in the ISMF interacting with the heliosphere provides a measurable foreground to the CMB.

Gigaparsec scale alignments of the quasar optical polarization vectors have been proven to be robust against a scenario of contamination by the Galactic interstellar medium (ISM). This claim has been established by means of optical polarization measurements of the starlight surrounding the lines of sight of the 355 quasars for which reliable optical polarization measurements are available. In this paper, we take advantage of the full-sky and high quality polarization data released by thePlancksatellite to provide an independent, complementary, and up-to-date estimation of the contamination level of the quasar optical polarization data by the Galactic dust. Our analysis reveals signatures of Galactic dust contamination at the two sigma level for about 30 percent of the quasar optical polarization data sample. The remaining 70 percent of the lines of sight do not show Galactic dust contamination above the two sigma level, suggesting low to negligible contamination of the quasar optical polarization signal. We further found arguments suggesting that Galactic thermal dust cannot fully account for the reported quasar optical polarization alignments. Based on the measurements of the ratio of the polarized intensity of the dust in the submillimeter to the degree of linear polarization of the quasar in the optical, we provide a new and independent quality criteria to apply to the quasar optical polarization sample. We argue that, unless correction is applied, such a criterion should be imposed on the data for future investigations in the framework of the cosmological-scale correlations of quasar optical polarization vector orientations that still could compete with the isotropic principle of the cosmological paradigm.

The winds of massive stars create large (>10 pc) bubbles around their progenitors. As these bubbles expand they encounter the interstellar coherent magnetic field which, depending on its strength, can influence the shape of the bubble. We wish to investigate if, and how much, the interstellar magnetic field can contribute to the shape of an expanding circumstellar bubble around a massive star. We use the MPI-AMRVAC code to make magneto-hydrodynamical simulations of bubbles, using a single star model, combined with several different field strengths: B = 5, 10, and 20 muG for the interstellar magnetic field. This covers the typical field strengths of the interstellar magnetic fields found in the galactic disk and bulge. Furthermore, we present two simulations that include both a 5 muG interstellar magnetic field and a 10,000 K interstellar medium and two different ISM densities to demonstrate how the magnetic field can combine with other external factors to influence the morphology of the circumstellar bubbles. Our results show that low magnetic fields, as found in the galactic disk, inhibit the growth of the circumstellar bubbles in the direction perpendicular to the field. As a result, the bubbles become ovoid, rather than spherical. Strong interstellar fields, such as observed for the galactic bulge, can completely stop the expansion of the bubble in the direction perpendicular to the field, leading to the formation of a tube-like bubble. When combined with a warm, high-density ISM the bubble is greatly reduced in size, causing a dramatic change in the evolution of temporary features inside the bubble. The magnetic field of the interstellar medium can affect the shape of circumstellar bubbles. This effect may have consequences for the shape and evolution of circumstellar nebulae and supernova remnants, which are formed within the main wind-blown bubble.

Interstellar linear polarization occurs when starlight passes through elongated dust grains aligned by interstellar magnetic fields. The observed polarization can come from different dust structures along the line of sight (LOS). By combining polarization measurements with stellar distances, we can study the plane-of-sky Galactic magnetic field (GMF) between the observer and the star and separate the contributions of clouds with different GMF properties. We used optical and near-infrared (NIR) polarization data from three regions in the Galactic plane (∣b∣ < 1° and 19 . ° 8 < l < 25 . ° 5) to perform a polarization decomposition across the Galactic arms. A comparison between the optical and NIR data showed an optical-to-NIR polarization ratio of two to three along the LOS and a consistent polarization angle across both wavelengths in all studied regions, within the measurement uncertainties. We applied the Bayesian Inference of Starlight Polarization in one dimension and the Gaussian mixture model methods to decompose the polarization in the three regions. The optical and NIR observations complemented each other, consistently identifying nearby (d ≲ 143 pc), intermediate (0.47 kpc < d < 1.2 kpc), and distant (1.5 kpc < d < 2.5 kpc) polarizing clouds, in agreement with previous findings of the Local Bubble wall, the Local Arm, and Sagittarius Arm dust structures. The results from both polarization decomposition methods agree and complement each other. Polarization tomography revealed significant LOS variations in the plane-of-sky magnetic field orientation in two of the three regions. The relative alignment between the magnetic fields traced by starlight polarization and Planck’s polarized thermal dust emission at 353 GHz reaffirmed these variations.

We report optical and X-ray observations of the AM Her system RE J1844 − 74 using the Anglo-Australian Telescope (AAT) and ROSAT. From previously published data and these new data we derive a new accurate ephemeris for the spin period of the white dwarf. We find we cannot simultaneously fit the green and red polarization data. However, we can fit them separately using different values of the plasma parameter, A. We investigate the possible reasons for this result. Furthermore, we find that the polarization data require long narrow accretion arcs. This is in contrast to results on eclipsing AM Her systems which tend to favour accretion regions that are relatively small. We briefly explore the possible effects that may influence the determination of the size of the accretion region from these different methods. We find evidence that the main accretion region of RE J1844 − 74 has a magnetic field strength ~ 10 MG less than its secondary pole, and that the system inclination is moderately high. Our X-ray data are out of phase with the optical data, consistent with our current view of the accretion flow in AM Her systems.

Interstellar magnetic fields in elliptical galaxies have their origin in stellar fields that accompany normal mass loss from an evolving population of old stars. It is likely that these seed fields are amplified by interstellar turbulence which in turn is driven by stellar mass loss and supernova events. Since the local turbulent velocity is likely to exceed the global velocity of the interstellar cooling flow, magnetic fields are expected to be disordered and this is indicated by numerous observations. Further amplification occurs as these tangled fields are compressed in the inward moving cooling flow of the interstellar gas. Near the centers of galactic cooling flows, where the gas radiates away most of its thermal and gravitational energy, magnetic stresses are expected to dominate. We study here the time-dependent growth of interstellar magnetic fields in elliptical galaxies and demonstrate that fields similar in strength to those observed can be generated solely from natural galactic processes. Although interstellar turbulent velocities and therefore the equipartition field Be are larger near the galactic center, the interstellar field throughout elliptical galaxies can be determined by the outermost turbulent regions in the interstellar gas. This occurs whenever the increase in the field strength due to compression in the galactic cooling flow exceeds the rate of field amplification by local turbulence, i.e., when B > Be. The magnitude of the interstellar field depends on the most distant radius in the galactic interstellar medium at which the turbulent dynamo process can successfully have amplified stellar seed fields. Because of the long hydrodynamic flow times in galactic cooling flows, currently observed magnetic fields may result from periods of turbulent field amplification that occurred in the outer galaxy in the distant past. In our most optimistic turbulent dynamo models in which field reconnection is ignored, tangled interstellar magnetic fields of B ~ 1-10(r/10 kpc)-1.2 ?G are typical; these are consistent with observed fields. However, the sensitivity of the galactic magnetic field to poorly known historical turbulent conditions in the outer galaxy complicates estimates of field strengths that can be produced internally. Internal fields in ellipticals may also result from ancient galactic mergers or from shear turbulence introduced at the boundary between the interstellar gas and ambient cluster gas.

Polarimetry of stars at optical and near-infrared wavelengths is an invaluable tool for tracing interstellar dust and magnetic fields. Recent studies have demonstrated the power of combining stellar polarimetry with distances from the Gaia mission, in order to gain accurate, 3D information on the properties of the interstellar magnetic field and the dust distribution. However, access to optical polarization data is limited, as observations are conducted by different investigators, with different instruments, and are made available in many separate publications. To enable a more widespread accessibility of optical polarimetry for studies of the interstellar medium, we compile a new catalog of stellar polarization measurements. The data are gathered from 81 separate publications spanning two decades since the previous, widely used agglomeration of catalogs by C. Heiles. The compilation contains a total of 55,742 measurements of stellar polarization. We combine this database with stellar distances based on the Gaia Early Data Release 3, thereby providing polarization and distance data for 42,482 unique stars. We provide two separate data products: an extended catalog (containing all polarization measurements) and a unique source catalog (containing a subset of sources excluding duplicate measurements). We propose the use of a common tabular format for the publication of stellar polarization catalogs to facilitate accessibility and increase discoverability in the future.

The highly collimated H-H flows and “jets” from young stellar objects have in some cases be reported to occur parallel to the direction of the interstellar magnetic field. In order to examine the reality and universality of the apparent connection between the H-H flows and the interstellar field, we compared the position angles of all H-H flows for which (1) the flow direction is well known from monochromatic images, and where (2) the local field direction can be derived unambiguously from interstellar polarization data. The result is given in Figure 1. A more detailed description of these results will be published elsewhere (Appenzeller 1989, Proc. 4th IAP Astrophysics Meeting, P. Delache et al. eds., Paris 1989).

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic. The Planck all-sky maps of linearly polarized emission from dust at 353GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter, both in the diffuse ISM and in molecular clouds. The data reveal structures, or ridges, in the intensity map with counterparts in the Stokes Q and/or U maps. We focus on structures at intermediate and high Galactic latitudes with column density from $10^{20}$ to $10^{22}$ cm$^{-2}$. We measure the magnetic field orientation on the plane of the sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are preferentially aligned with the magnetic field measured on the structures. This trend becomes more striking for increasing polarization fraction and decreasing column density. We interpret the increase of alignment with polarization fraction as a consequence of projections effects. The decrease of alignment for high column density is not due to a loss of correlation between the structures and the geometry of the magnetic field. In molecular complexes, we observe structures perpendicular to the magnetic field, which cannot be accounted for by projection effects. We discuss our results in the context of models and MHD simulations, which describe the formation of structures in the magnetized ISM.

Large fractions of metals are missing from the observable gas-phase in the interstellar medium (ISM) because they are incorporated into dust grains. This phenomenon is called dust depletion. It is important to study the depletion of metals into dust grains in the ISM to investigate the origin and evolution of metals and cosmic dust. We characterize the dust depletion of several metals from the Milky Way to distant galaxies. We collected measurements of ISM metal column densities from absorption-line spectroscopy in the literature, and in addition, we determined Ti and Ni column densities from a sample of 70 damped Lyman-α absorbers (DLAs) toward quasars that were observed at high spectral resolution with the Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). We used relative ISM abundances to estimate the dust depletion of 18 metals (C, P, O, Cl, Kr, S, Ge, Mg, Si, Cu, Co, Mn, Cr, Ni, Al, Ti, Zn, and Fe) for different environments (the Milky Way, the Magellanic Clouds, and DLAs toward quasars and towards gamma-ray bursts). We observed overall linear relations between the depletion of each metal and the overall strength of the dust depletion, which we traced with the observed [Zn/Fe]. The slope of these dust depletion sequences correlates with the condensation temperature of the various elements, that is, the more refractory elements show steeper depletion sequences. In the neutral ISM of the Magellanic Clouds, small deviations from linearity are observed as an overabundance of the α-elements Ti, Mg, S, and an underabundance of Mn, including for metal-rich systems. The Ti, Mg, and Mn deviations completely disappear when we assume that all systems in our sample of OB stars observed toward the Magellanic Clouds have an α-element enhancement and Mn underabundance, regardless of their metallicity. This may imply that the Magellanic Clouds have recently been enriched in α-elements, potentially through recent bursts of star formation. We also observe an S overabundance in all local galaxies, which is an effect of ionization due to the contribution of their H ii regions to the measured S ii column densities. The observed strong correlations of the depletion sequences of the metals all the way from low-metallicity quasi-stellar object DLAs to the Milky Way suggest that cosmic dust has a common origin, regardless of the star formation history, which, in contrast, varies significantly between these different galaxies. This supports the importance of grain growth in the ISM as a significant process of dust production.

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.

view Abstract Citations (21) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS R Band Polarimetry of Cygnus OB2: Implications For The Magnetic Field Geometry And Polarization Models Kobulnicky, Henry A. ; Molnar, Lawrence A. ; Jones, Terry Jay Abstract We present new R band polarimetry of 132 members of the Cygnus OB2 association. From these data we have determined the angular coherence length of the polarization position angle and tested two models for interstellar polarization toward a region where the Galactic magnetic field is expected to be primarily along the line of sight. The polarization magnitudes and position angles in our sample decorrelate on size scales of approximately 0.5 deg, compared to 0.2 deg for the optical extinction. This constrains the variations in the magnetic field orientation to sizes less than 8 pc. The distribution of polarization magnitudes and position angles is consistent with a model which describes the polariztion in terms of preferential extinction in small dark clouds threaded by a two-component magnetic field. One component is constant in direction, while the second is random in orientation and is characterized by the amplitude of Alfven waves in the interstellar medium. A similar model, in which the magnitude of the random component has a Gaussian distribution and an orientation evenly distributed through 4 pi steradians, is also consistent with the data. A previously proposed geometrical model involving two slabs of obscuring material with different magnetic field orientations does not seem adequate given these new data. We explore the predictions of each model for the polarization magnitude and position angle as a function of the orientation of the constant component and point our several key differences that might be exploited to further test their potential for describing interstellar polarization. We find the polarization position angles of stars in the vicinity of Cygnus X-3 are approximately orthogonal to the position angle of the elliptical radio scattering reported by Molnar et al. (1989). If the scattering is taking place on the near side of Cyg OB2, the data are consistent with mechanisms that require the major axis of the scattering ellipse to lie perpendicular to the projected magnetic field. Publication: The Astronomical Journal Pub Date: April 1994 DOI: 10.1086/116956 Bibcode: 1994AJ....107.1433K Keywords: Cygnus Constellation; Galactic Radiation; Interstellar Matter; Magnetohydrodynamic Waves; Orion Nebula; Polarization (Waves); Radio Waves; Ubv Spectra; Geomagnetic Latitude; Interstellar Extinction; Interstellar Magnetic Fields; Line Of Sight; Optical Thickness; Solar Longitude; Astrophysics; STARS: MAGNETIC FIELDS; STARS: INDIVIDUAL: CYG OB2 full text sources ADS | data products SIMBAD (167) CDS (1)

This paper presents a review of our current knowledge of interstellar dust. The composition of the interstellar dust is summarized in Table 1. About half of the dust volume consists of amorphous silicates. The other half has to be made up out of a carbonaceous component, such as graphite, amorphous carbon (e.g., soot), and/or organic grain mantles (e.g., mixed polymers). Presently it cannot be decided which of these carbonaceous components dominates the interstellar dust, but future observations which can settle this point are discussed. Some discussion is given of the similarities and differences between graphite and amorphous carbon. Other minor dust components, such as SiC and MgS, are probably also present in the interstellar medium. Inside dense molecular clouds icy grain mantles can be a very important dust component containing up to 40% of the available elemental carbon and oxygen. The evolution of dust in the interstellar medium is described and some important physical processes are outlined. This includes nucleation, condensation and coagulation of Stardust (e.g., silicates, graphite and soot) in the outflows from late-type stars and UV photolysis and transient heating of icy grain mantles forming organic grain mantles in the interstellar medium. The destruction of dust by interstellar shocks is also described. The short destruction timescales which result from analysis of this process form a serious problem for any interstellar dust model based on Stardust alone. Even those models in which the interstellar dust is mainly formed in the interstellar medium may face problems in explaining the measured silicate dust volume. The interrelationship between interstellar and interplanetary dust is briefly described and it is argued that interstellar Polycyclic Aromatic Hydrocarbon molecules (hereafter PAHs) have carried the measured deuterium enhancement of the carbonaceous meteorites into the solar nebula. Finally an unaltered interstellar dust origin for the Ca,Al-rich inclusions in meteorites is rejected. A general description of infrared spectroscopy is given and applied to observations of interstellar icy grain mantles. Recent 5–8µm Observations of compact objects embedded inside dense molecular clouds are described. They show absorption features near 6.0 and 6.85 µm whose shape and peak position vary from source to source. The relatively narrow features observed towards W33 A are identified with the OH and CH deformation modes in H 2O and alcohols (i.e., CH3OH). The much broader features observed towards Mon R2-IRS 2 imply that a more complex array of molecular subgroups are present. The observed band shapes indicate that aldehydes (e.g., H2CO) and possibly ketones (e.g., CH3COCH3) are important grain constituents in the grain mantles along the line of sight towards that source. Mineral identifications for the 6.0 and 6.85 µm absorption features are briefly discussed and it is concluded that minerals do not contribute appreciably to these bands. The identification of each of the molecules proposed to be present in interstellar icy grain mantles is reviewed and critical observations required to confirm some of them are pointed out. The molecular composition of icy grain mantles for several sources is summarized in Table 3. While interstellar icy grain mantles have a variable composition, the simplest spectra imply a composition given approximately by H2O/CH3OH/CO/NH3 ≃ 1/0.66/0.05/0.05.

Voyager 1 (V1) has been observing interstellar magnetic fields (ISMF) for more than one year, from 2012/209 to at least 2013.6. From 2013.0 to 2013.6 the difference between the azimuthal angle of the ISMF and the Parker spiral angle at the latitude 34.6° of V1 was (22 ± 3)° and the corresponding difference of the elevation angle was (0 ± 8)°. During 2012 the deviation from the Parker spiral angle was somewhat smaller. The interstellar magnetic field has a West to East polarity, opposite to the direction of planetary motions. The magnitude of the ISMF varied smoothly in the range 0.38 nT to 0.59 nT with an average strength 0.49nT. The strongest interstellar fields were observed behind a shock at 2012/297 that was preceded by 2.2 KHz plasma oscillations, which implies an interstellar electron density ne = 0.05/cm−3. The ISMF was observed after V1 crossed a current sheet CS0 having the structure of a tangential discontinuity. The inclination of this current sheet is consistent with an interstellar magnetic field draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed earlier in the heliosheath at 2012/167 and 2011/276 with high inclinations (99 +/−10)° and (89 ± 10)°, respectively). The transition from heliosheath to interstellar magnetic fields is related to a two-step increase in the cosmic ray intensity observed by V1 from 2012.30 to 2012.65. The first step-increase began near the end of an unusual away-polarity sector, and it reached a plateau when V1 moved into a toward-polarity sector that ended at CS0. The second step-increase began slowly after V1 crossed CS0, and it ended abruptly at 2012/237.728.