Continuum Polarization Research Articles

THE ROLE OF THE MAGNETIC FIELD IN THE INTERSTELLAR MEDIUM OF THE POST-STARBURST DWARF IRREGULAR GALAXY NGC 1569

(abridged) NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10 to 40 Myr ago. We present observations that reach surface brightnesses two to eighty times fainter than previous radio continuum observations and the first radio continuum polarization observations. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium. We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Halpha arm at wavelengths shorter than 20cm. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. We derive a total magnetic field strength of 38 microG in the central regions and 10-15 microG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 microG and is strongest in the halo. We find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the interstellar medium in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Halpha bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. The outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC 1569 may play an important role in magnetizing the intergalactic medium.

VLT spectropolarimetry of the optical transient in NGC 300

AIMS: The main goal of this work is to study possible signs of asymmetry in the bright optical transient in NGC300, with the aim of getting independent information on the explosion mechanism, the progenitor star and its circumstellar environment. METHODS: Using VLT-FORS1 we have obtained low-resolution optical linear spectropolarimetry of NGC300 OT2008-1 on two epochs, 48 and 55 days after the discovery, covering the spectral range 3600--9330A. RESULTS: The data show a continuum polarization at a very significant level. At least two separate components are identified. The first is characterized by a strong wavelength dependency and a constant position angle (68.6+/-0.3 degrees), which is parallel to the local spiral arm of the host galaxy. The second shows a completely different position angle (151.3+/-0.4) and displays a mild but statistically significant evolution between the two epochs. While the former is identified as arising in the interstellar dust associated with NGC300, the latter is most likely due to continuum polarization by dust scattering in the circumstellar environment. No line depolarization is detected in correspondence of the most intense emission lines, disfavoring electron scattering as the source of intrinsic polarization. This implies a very small deviation from symmetry in the continuum-forming region. Given the observed level of intrinsic polarization, the transient must be surrounded by a significant amount of dust (>4x10^-5 Msun), asymmetrically distributed within a few thousand AU. This most likely implies that one or more asymmetric outflow episodes took place during the past history of the progenitor.

Determination of the absolute configurations of bicyclo[3.1.0]hexane derivatives via electronic circular dichroism, optical rotation dispersion and vibrational circular dichroism spectroscopy and density functional theory calculations

The electronic circular dichroism (ECD), optical rotation dispersion (ORD), and vibrational circular dichroism (VCD) spectra of a pair of enantiomers, i.e. 3 and 4, of a bicyclo[3.1.0]hexane derivative have been measured in acetonitrile and acetonitrile-d(3), respectively. Extensive conformational searches at the B3LYP/6-311++G** level have been carried out for 3, which has four OH and one N(3) functional groups. For the bicyclo[3.1.0]hexane ring of 3, the boat-like conformers have been found to be much more stable than the chair-like conformers, while the number and the strength of the intramolecular hydrogen bonds have been identified as the dominant factors in the relative stability among the boat-like and among the chair-like conformers. DFT simulations of the ECD, ORD and VCD spectra have been performed for all low energy conformers at the B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ level. Implicit continuum polarization model has been used to account for solvent effects in all these chiroptical measurements. Comparison of the DFT simulations with the experimental data shows that all three chiroptical properties yield the same absolute configuration assignment for . This work demonstrates that using multiple chiroptical spectroscopic methods in combination with DFT calculations allows one to determine absolute configurations with high confidence for chiral carbohydrates and their analogues, which possess a large number of rotatable bonds.

LINEAR POLARIZATION OF FLASH SPECTRUM OBSERVED FROM A TOTAL SOLAR ECLIPSE IN 2008

We measured linear polarization signals of the flash spectrum ranging from 502.5 nm to 528.5 nm after second contact of a total solar eclipse which occurred on 2008 August 1 in China. A large group of spectral lines (especially those lines produced by neutral iron, neutral copper, and as carbon molecules) are found with very high polarization degrees relative to the continuum polarization, and the linear polarization spectrum is more abundantly structured than the flash spectrum itself. According to the observational result, we conclude that coherent scattering and scattering geometry as well as other mechanisms may together play a very important role in producing the high polarization amplitudes. This will help us deepen our understanding of the physical conditions of the solar upper atmospheres and the physical processes occurring there.

MULTIEPOCH OPTICAL SPECTROPOLARIMETRY OF THREE MICROQUASARS, Cyg X-1, LS 5039, AND LS I +61° 303

We present optical spectropolarimetric observations of three microquasars, Cyg X-1, LS 5039, and LS I +61{sup 0} 303. Polarimetry can bring us geometrical information on the innermost part of the circumstellar media. For Cyg X-1, the observed continuum polarization shows sinusoidal time variation ({delta} {approx} 0.2%) correlated with the orbital phase, as indicated by past studies. The constant component of the polarization has significantly changed ({delta} {approx} 0.7%) within a few decades, indicating variation of scattering material surrounding the binary. There is no apparent change of polarization across the H{alpha} emission line in Cyg X-1. For LS 5039, the observed polarization shows no significant orbital variation. The wavelength dependence is almost consistent with the typical interstellar polarization (ISP) and we conclude that the ISP is a dominant component of the observed polarization. For LS I +61{sup 0} 303, the observed polarization is roughly consistent with our previous paper, except for a slight change in the intrinsic polarization and also in the equivalent width of the H{alpha} emission line. These imply a long-term evolution of the Be disk around the mass donor. The position angle of intrinsic polarization at {approx_equal}25{sup 0} was constant from 2005 through 2008. Our observations suggest thatmore » the optical polarization intrinsic to a microquasar with an high mass donor can be explained by the light scattering in the circumstellar matter around the mass donor. A polarization signal from the vicinity of the compact star (including jet and accretion disk) seems absent.« less

Radio properties of CSSs and GPSs

AbstractRadio properties of CSSs/GPSs from continuum (polarization included) and spectral line information are concisely summarized (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

SPECTROPOLARIMETRIC VARIABILITY AND COROTATING STRUCTURE IN HD 92207

We report on low-resolution (R ≈ 3000) spectropolarimetry of the A0 supergiant star HD 92207. This star is well known for significant spectral variability. The source was observed on seven different nights spanning approximately three months in time. With a rotation period of approximately one year, our data cover approximately a quarter of the star's rotational phase. Variability in the continuum polarization level is observed over this period of time. The polarization across the Hα line on any given night is typically different from the degree and position angle of the polarization in the continuum. Interestingly, Hβ is not in emission and does not show polarimetric variability. We associate the changes at Hα as arising in the wind, which is in accordance with the observed changes in the profile shape and equivalent width of Hα along with the polarimetric variability. For the continuum polarization, we explore a spiral shaped wind density enhancement in the equatorial plane of the star, in keeping with the suggestion of Kaufer et al. Variable polarization signatures across Hα are too complex to be explained by this simple model and will require a more intensive polarimetric follow-up study to interpret properly.

Hαspectropolarimetry of RY Tauri and PX Vulpeculae

Aims. To detect line effects using spectropolarimetry in order to find evidence of rotating disks and their respective symmetry axes in T Tauri stars. Methods. We used the IAGPOL imaging polarimeter along with the Eucalyptus-IFU to obtain spectropolarimetric measurements of the T Tauri stars RY Tau (two epochs) and PX Vul (one epoch). Evidence of line effects showing a loop in the Q−U diagram favors a compact rather than an extended source for the line photons in a rotating disk. In addition, the polarization position angle (PA) obtained using the line effect can constrain the symmetry axis of the disk. Results. RY Tau shows a variable Hα double peak in 2004−2005 data. A polarization line effect is evident in the Q−U diagram for both epochs confirming a clockwise rotating disk. A single loop is evident in 2004 changing to a linear excursion plus a loop in 2005. Interestingly, the intrinsic PA calculated using the line effect is consistent between our two epochs (∼167 ◦ ). An alternative intrinsic PA computed from the interstellar polarization-corrected continuum and averaged between 2001−2005 yielded a PA ∼ 137 ◦ .T his last value is closer to perpendicular to the observed disk direction (∼25 ◦ ), as expected from single scattering in an optically thin disk. For PX Vul, we detected spectral variability in Hα along with non-variable continuum polarization when compared with previous data. The Q−U diagram shows a well-defined loop in Hα associated with a counter-clockwise rotating disk. The symmetry axis inferred from the line effect has a PA ∼ 91 ◦ (with an ambiguity of 90 ◦ ). Our results confirm previous evidence that the emission line in T Tauri stars has its origin in a compact source scattered off a rotating accretion disk.

Observing and Modelling Stellar Magnetic Fields

We start by looking at some recent developments in field measurements in middle main sequence and pre-main sequence stars. Then we consider how other magnetic effects such as the Hanle effect and continuum polarisation may be used to detect fields in a variety of stellar environments such as the solar corona and white dwarf atmospheres. Finally we consider how the current body of observational knowledge about magnetic stars may be integrated into a roughly coherent, provisional scenario of field origin and evolution.

Optical Spectropolarimetry and Asphericity of the Type Ic SN 2007gr

We present an optical spectropolarimetric observation of the Type Ic supernova (SN) 2007gr made with the Subaru Telescope at 21 days after maximum brightness (~37 days after the explosion). Nonzero polarization as high as ~3% is observed at the absorption feature of the Ca II IR triplet. The polarization of the continuum light is ~0.5% if we estimate the interstellar polarization (ISP) component by assuming that the continuum polarization has a single polarization angle. This suggests that the axis ratio of the SN photosphere projected on the sky differs from unity by ~10%. The polarization angle at the Ca II absorption is almost aligned with that of the continuum light. These features may be understood in the context of a model in which a bipolar explosion with an oblate photosphere is being viewed from a slightly off-axis direction and explosively synthesized Ca near the polar region obscures the light that originates around the minor axis of the SN photosphere. Given the uncertainty in the ISP, however, the polarization data could also be interpreted with a model with an almost spherically symmetric photosphere and a clumpy Ca II distribution.

DEVIATIONS FROM AXISYMMETRY REVEALED BY LINE POLARIZATION IN THE NORMAL TYPE Ia SUPERNOVA 2004S

We present a single epoch of high signal-to-noise ratio spectropolarimetry of the Type Ia supernova (SN Ia) 2004S taken nine days after maximum light. The flux spectrum is normal, but with the additional presence of high-velocity (HV) line features in both Ca II and Fe II. The object shows continuum polarization at the 0.4% level in the red, a value which appears to be typical of SNe Ia. The continuum data are consistent with a ~10% global asphericity in an axisymmetric geometry. Unlike previous observations of other SNe Ia with HV features, the HV features in SN 2004S show no strong polarimetric signature, though this may be due to the timing of our observations. Instead, the object shows line polarization features (P < 0.5%) that are rotated with respect to the axis of symmetry of the continuum. The line features are visible in Si II, Fe II, and Ca II, and appear to be narrowly confined in velocity space just above the photosphere. These polarization features are a result of compositional inhomogeneities in the ejecta. They may represent newly synthesized elements whose clumpy spatial distribution within the ejecta is distinct from that of the globally aspherical ejecta as a whole.

Observation of near total polarization in the ultrafast laser ablation of Si

We report nearly completely polarized emission from the plasma produced in the femtosecond ablation of Si(111). Pairs of ultrashort laser pulses were focused onto the target in air, and the polarization spectrum was measured as a function of energy, pulse delay, and polarization state of the laser. When the laser was focused on the surface, the fluorescence continuum was strongly polarized, whereas discrete lines appeared as minima in the polarization spectrum. Under this focusing condition, the continuum polarization increased with pulse delay and decreased with pulse energy and fluorescence wavelength, with &amp;gt;95% polarization in the ultraviolet.

Testing Faraday Rotation Estimates of Magnetic Field Strengths toward Dark Clouds

We have carried out OH Zeeman observations toward a position at the edge of a Taurus molecular cloud where a continuum polarization minimum and excessive Faraday rotation measure were reported by Wolleben & Reich in 2004. They had interpreted this result as requiring a line-of-sight magnetic field strength in excess of 20 μG. Our Zeeman measurement failed to detect a field consistent with this result.

LOW-MASS SEYFERT 2 GALAXIES IN THE SLOAN DIGITAL SKY SURVEY

We describe a sample of low-mass Seyfert 2 galaxies selected from the Sloan Digital Sky Survey (SDSS), with a median absolute magnitude of Mg = –19.0 mag. These galaxies are Type 2 counterparts to the Seyfert 1 galaxies with intermediate-mass black holes identified by Greene & Ho. Spectra obtained with the Echellette Spectrograph and Imager at the Keck Observatory are used to determine the central stellar velocity dispersions and to examine the emission-line properties. Overall, the stellar velocity dispersions are low (~40-90 km s–1), and we find 12 objects having σ < 60 km s–1, a range where very few Seyfert 2 galaxies were previously known. The sample follows the correlation between stellar velocity dispersion and FWHM([O III]) seen in more massive Seyfert galaxies, indicating that the narrow-line FWHM values are largely determined by virial motion of gas in the central regions of the host galaxies, but the [O III] emission lines exhibit a higher incidence of redward asymmetries and double-peaked profiles than what is found in typical Seyfert samples. Using estimates of the black hole masses and active galactic nucleus (AGN) bolometric luminosities, we find that these galaxies are typically radiating at a high fraction of their Eddington rate, with a median L bol/L Edd = 0.4. We identify one galaxy, SDSS J110912.40+612346.7, as a Type 2 analog of the nearby dwarf Seyfert 1 galaxy NGC 4395, with a nearly identical narrow-line spectrum and a dwarf spiral host of only Mg = –16.8 mag. The close similarities between these two objects suggest that the obscuring torus of AGN unification models may persist even at the lowest luminosities seen among Seyfert galaxies, below L bol = 1041 ergs s–1. Spectropolarimetry observations of four objects do not reveal any evidence for polarized broad-line emission, but SDSS J110912.40+612346.7 has a continuum polarization significantly in excess of the expected Galactic foreground polarization, a possible indication of scattered light from a hidden nucleus. Forthcoming observations of this sample, including X-ray and mid-infrared spectroscopy, can provide new tests of the obscuring torus model for active galaxies at low luminosities.

Continuum Polarizable Force Field within the Poisson−Boltzmann Framework

We have developed and tested a complete set of nonbonded parameters for a continuum polarizable force field. Our analysis shows that the new continuum polarizable model is consistent with B3LYP/cc-pVTZ in modeling electronic response upon variation of dielectric environment. Comparison with experiment also shows that the new continuum polarizable model is reasonable, with accuracy similar to that of B3LYP/cc-pVTZ in reproduction of dipole moments of selected organic molecules in the gas phase. We have further tested the validity to interchange the Amber van der Waals parameters between the explicit and continuum polarizable force fields with a series of dimers. It can be found that the continuum polarizable model agrees well with MP2/cc-pVTZ, with deviations in dimer binding energies less than 0.9 kcal/mol in the aqueous dielectric environment. Finally, we have optimized atomic cavity radii with respect to experimental solvation free energies of 177 training molecules. To validate the optimized cavity radii, we have tested these parameters against 176 test molecules. It is found that the optimized Poisson-Boltzmann atomic cavity radii transfer well from the training set to the test set, with an overall root-mean-square deviation of 1.30 kcal/mol, an unsigned average error of 1.07 kcal/mol, and a correlation coefficient of 92% for all 353 molecules in both the training and test sets. Given the development documented here, the next natural step is the construction of a full protein/nucleic acid force field within the new continuum polarization framework.

A fully sampled λ21 cm linear polarization survey of the southern sky

Fil: Testori, Juan Carlos. Provincia de Buenos Aires. Gobernacion. Comision de Investigaciones Cientificas. Instituto Argentino de Radioastronomia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto Argentino de Radioastronomia; Argentina. Max Planck Institut fur Radioastronomie; Alemania

The Cool Dark Globule CB68 and Its Associated Protostar: Geometry, Kinematics, Magnetic Vectors, and Pressure Balance

CB68 is a nearby cool dark globule. We observed CB68 near a radial velocity around +5.5 km s-1 in the 12CO, 13CO, and C 18O gas in J = 3-2 covering a 2' × 2' area, and we reanalyzed previous observations of the thermal dust continuum and linear polarization near the same wavelength (850 μm), same angular resolution (14''), and same antenna. We find that the gaseous core has a disklike intensity elongation near a P.A. of 45° with a velocity gradient along that P.A.: a northeast blueshift and a southwest redshift, suggestive of a rotating gas; this gas P.A. is similar to the elongation of the dust emission near a P.A. of 45°. We map the low-velocity blue southeast outflow and red northwest outflow; the CO gas outflow contours are elongated at a P.A. near -45°, similar to the elongation of the polarization percentage contours near -45°. We estimate the Chandrasekhar-Fermi strength of the hourglass-shaped magnetic field to be 120-130 μG in the sky plane. This strength is nearly a factor of 10-20 greater than the Zeeman line-of-sight data (7-10 μG) in the nearby tenuous cloud, outside CB68 proper. We deduce the energy and pressure components in the cool globule and in the hot surrounding region. We find an imbalance in pressure, which we tentatively assign to a neglected halo component (a reservoir of tenuous halo gas still infalling toward the disklike object CB68).

Polarization observations of SNR G156.2+5.7 at $\mathsf{\lambda}$6 cm

Context. G156.2+5.7 is a large supernova remnant (SNR) first discovered in the ROSAT X-ray survey due to its exceptional high X-ray brightness. Subsequently detailed X-ray, optical and Ha studies of this SNR were made, but radio observations are rare because of its low surface brightness and large size. Aims. Radio continuum and polarization images of the SNR G156.2+5.7 at A6 cm are presented for the first time to be discussed in the context with available data to investigate its physical properties. Methods. The SNR G156.2+5.7 was observed with the Urumqi 25 m telescope in radio continuum and linear polarization. The results are analyzed together with 11 cm and 21 cm maps previously obtained with the Effelsberg 100 m telescope and compared with a broad-band X-ray ROSAT image, 100 μm infrared radiation data and a Hα emission map. Results. We obtained an integrated flux density for G156.2+5.7 of S 6 cm = 2.5 ± 0.5 Jy. The spectral index of the integrated emission is a = -0.48 ± 0.08 (S ∼ v α ) between λ74 cm and λ6 cm excluding compact radio sources. We also obtained a spectral index map, which shows little variation between the shells and the central area of the SNR. This is consistent with the constant integrated flux density spectrum. Highly polarized radio emission has been detected from the SNR shell, but also from a central patch, which probably originates in the front part of the spherical SNR shell. We derived the distribution of rotation measure from polarization data at λ1 1 cm and λ6 cm and found RM gradients of opposite direction in the SNR shell. Conclusions. The SNR G156.2+5.7 is unusual by its exceptionally high X-ray brightness and very low surface-brightness in the radio range. The magnetic field is very well ordered along the shell periphery as expected for a compressed ambient magnetic field. A toroidal magnetic field component is indicated by the RM distribution.

Linear and circular polarisation of diffuse interstellar bands

Context: The detection or absence of a polarisation signal in the diffuse interstellar band (DIB) profile can possibly give important clues on the identity of its carrier. For molecular gas-phase carriers the effect of polarisation on its electronic transitions depends, although uncertain how exactly, on its physical properties (e.g. molecular structure). On the other hand, if the carriers are grains, impurities or defects in grain mantles are also expected to show up in the polarisation of DIBs. Aims: Our aim is to detect a polarisation signal in a DIB profile or to derive stricter upper limits on the polarisation efficiency of DIB carriers. Methods: In order to detect and measure the linear and circular polarisation of the DIBs we observe reddened lines of sight showing continuum polarisation; for this study we select the stars HD 21219 , HD 198478 , HD 197770 , HD 183143 and HD 163472 . We use spectropolarimetry in the wavelength range 4480 to 6620 A with the MuSiCoS echelle spectrograph mounted at the Telescope Bernard Lyot. Results: Linear polarisation spectra are constructed in order to search for a polarisation signal in the DIB profiles. No significant change in the linear polarisation degree is found. We obtain a 2sigma polarisation detection limit (per DIB FWHM) of 0.01-0.04% for HD 21219 , 0.04-0.14% for HD 197770 , 0.01-0.14% for HD 183143 and 0.01-0.14% for HD 198478 , for the six investigated DIBs. We derive upper limits for the polarisation efficiency factor f for six strong narrow DIBs; f_max of 0.31, 0.44, 0.45, 0.18, 0.47 and 0.68 for the lambdalambda 5780, 5797, 6196, 6284, 6379 and 6613 DIBs, respectively. The derived detection limits are similar to those derived by Adamson & Whittet (1995), although for more lines of sight and more DIBs. Circular polarisation (Stokes V) spectra of high signal-to-noise have been obtained for the first time for the 5780, 5797, 6196, 6203, 6284, 6376, 6379 and 6613 A DIB profiles. No circular polarisation signal is detected. The 2sigmaV (per 0.1 A) noise level limits are 1.0-2.5% for the DIBs in the line of sight towards HD 197770 and 0.06-0.10% for the DIBs towards HD 163472 . Conclusions: . The lack of polarisation of the DIB profiles is consistent with DIB carriers that are not directly related to solid features, like impurities in grain mantles. However, large (carbonaceous) gas phase molecules remain viable DIB carrier candidates. Based on observation at the Observatoir du Pic du Midi, France with the MuSiCoS spectropolarimeter.

Magnetic Fields in Molecular Clouds

Observations of magnetic fields in molecular clouds are essential for understanding their role in the evolution of dense clouds and in the star formation process. The two extreme-case models of star formation are the weak magnetic field scenario, in which turbulence drives molecular cloud evolution and therefore star formation, and the strong magnetic field scenario, with magnetic fields governing molecular cloud formation and ambipolar diffusion ultimately driving star formation. Continuum and spectral line polarization observations make it possible to infer magnetic field strengths and morphologies, and therefore to test which of the two star formation scenarios dominates. The observational techniques available for such observations – the Zeeman effect, mapping of polarized dust emission, and mapping of linearly polarized line emission – are briefly described, together with how these observations can test star formation theory. Examples of the existing observations and the current state of the testing of magnetically controlled star formation are presented. Finally, the prospects for the future of study of magnetic fields in molecular clouds and their role in the star formation process is discussed.