Polarization Survey Research Articles

Shock-front compression of the magnetic field in the Canis Majoris R1 star-formation region

view Abstract Citations (16) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Shock-Front Compression of the Magnetic Field in the Canis Major R1 Star Formation Region Vrba, F. J. ; Baierlein, R. ; Herbst, W. Abstract Results are presented from a linear polarization survey at optical wavelengths of over 140 stars in the direction of the CMa R1 star-formation region; 26 of these are clearly associated with nebulosity within the area. The observations were obtained in order to test the argument of Herbst et al. (1978) that star formation in CMa R1 is driven by a shock wave from a nearby supernova (Herbs and Assousa, 1977 and 1978). The polarizations are found to be consistent with a simple model of the compression by a supernova-induced spherical shock front of an initially uniform interstellar magnetic field. The polarization vectors are inconsistent with a scenario of quiescent cloud collapse along magnetic-field lines. Multicolor polarimetry of the nebular stars provides evidence of grain growth toward increasing cloud optical depth, characterized by a ratio of total-to-selective extinction of R = 3.0 at E(B-V) = 0.23, increasing to R = 4.2 at E(B-V) = 0.7. Publication: The Astrophysical Journal Pub Date: June 1987 DOI: 10.1086/165269 Bibcode: 1987ApJ...317..207V Keywords: Interstellar Magnetic Fields; Shock Fronts; Star Formation; Supernova Remnants; Magnetohydrodynamic Stability; Nebulae; Polarization Characteristics; Star Clusters; Astrophysics; CLUSTERS: ASSOCIATIONS; NEBULAE: SUPERNOVA REMNANTS; POLARIZATION; SHOCK WAVES; STARS: FORMATION full text sources ADS | data products SIMBAD (2)

A bright optical synchrotron counterpart of the western hot SPOT in Pictor A

A B = 19.5 mag bright, highly polarized (P > 30%) object was detected close to the western hot spot in Pictor A during an optical polarization survey of radio hot spots in classical double radio sources. The unresolved source exhibits a featureless continuum between 400 and 800 nm and is identified as the optical couterpart of the radio hot spot. It is surrounded by optical filaments aligned roughly perpendicular to the source axis. The hot spot is also marginally detected in an Einstein IPC frame.

An Atlas of Supernova Remnant Magnetic Fields

Observations of polarised radio emISSIOn from supernova remnants are reviewed and maps presented for the projected magnetic fields deduced from these observations. It is fairly clear that the fields in the young objects are radial and that there is a strong bias towards finding these objects in polarisation surveys. Well-defined tangential fields can be seen only in older remnants which are located well off the galactic plane.

Polarization measurements of some T Tauri stars

T Tauri stars show linear polarization typically between 1–3%. A two band linear polarization survey of some T Tauri star was reported earlier by Bastien (1982). Most of the stars show pronounced time variability in polarization and position angle (Bastien, 1980; 1982). Wavelength dependence of polarization is important in determining the specific mechanism(s) producing polarization. For a systematic study of polarization in T Tauri stars, we have taken up an observing programme to measure linear polarization of some stars in Taurus-Auriga region. Polarization measurements of 9 T Tauri stars are reported here. Observations were made on January 8–11, 1984 with MINIPOL (Frecker and Serkowski, 1976) on 61“ telescope of University of Arizona.

Staring at the Poles of the Sky

W hen choosing locations for telescopes, astronomers in the Northern Hemisphere look for the most southerly locations with good seeing conditions. In the Southern Hemisphere, they tend to seek the most northerly locations. They want to see as much of the sky as they can from one place, and the closer they can get to the equator, the more they can see. Commenting on this situation, a Canadian astronomer once remarked that observing from a far northern location Edmonton, Alberta, was the specific place in mind one sees only a few things, but one sees them for most of the time. However, this drawback can become an advantage for astronomers interested in seeing very faint objects. A photoelectronic recorder such as a chargecoupled device staring at the same part of the sky for hundreds or even thousands of hours could build up images and spectra of very faint objects, down to 27th magnitude. Jacques M. Beckers, Roger L. Davies and Patrick 0. Seitzer of the National Optical Astronomy Observatories (NOAO) in Tucson, Ariz., propose setting up a dedicated telescope to do just that, to stare steadily at either the North or South Pole. In a paper presented at the recent meeting in Ames, Iowa, of the American Astronomical Society, they call the project DEEPS for Deep Polar Spectroscopic Survey The poles are the obvious target for such long-period imaging. Each pole is visible all night, every night, from the middle or high latitudes of its hemisphere. The poles do not change position as the earth rotates, so a telescope pointed at a pole does not need a complex tracking mechanism to follow its target through the sky Objects near the pole do trace out circular orbits around the pole as the earth rotates, but the DEEPS telescope could compensate for this by rotating around its own axis by an amount calculated to offset the earth's rotation a much simpler motion than the two-dimensional tracking that other telescopes must do to follow objects in other locations.

The Pleiades cluster. III - Polarization, reddening, and the unusual distribution of interstellar matter

view Abstract Citations (59) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Pleiades Cluster. III. Polarization, Reddening, and the Unusual Distribution of Interstellar Matter Breger, Michel Abstract A polarization survey of Pleiades cluster members and nonmembers has been carried out together with a determination of interstellar reddening for cluster members. No correlations of stellar position angles of polarization with nebular filament directions could be found. Light from the eastern part of the cluster passes through only uniform interstellar foreground material. Light from the northwestern part passes through the foreground material in addition to a patchy intracluster cloud. Depolarization occurs due to different magnetic field directions in the two clouds. The northwestern region coincides with that of a 21 cm H I maximum and largest reported CH+ absorption. The high reddening of ≡0.30 mag and polarization ≡2% in much of the southwestern region coincides with the position of the molecular cloud detected by Cohen in CO. Publication: The Astrophysical Journal Pub Date: October 1986 DOI: 10.1086/164602 Bibcode: 1986ApJ...309..311B Keywords: Interstellar Extinction; Interstellar Matter; Nebulae; Pleiades Cluster; Polarimetry; Star Clusters; Hydrogen Clouds; Molecular Clouds; Radio Sources (Astronomy); Astrophysics; CLUSTERS: OPEN; INTERSTELLAR: MATTER; NEBULAE: INDIVIDUAL NAME: PLEIADES; POLARIZATION full text sources ADS | data products SIMBAD (192) Related Materials (3) Part 1: 1984A&A...137..145B Part 2: 1985A&A...143..455B Part 4: 1987ApJ...319..754B

On electromagnetic coupling “removal” from induced polarization surveys

A formulation is presented for the mutual impedance of a four-electrode array on the surface of a homogeneous half-space model of the earth characterized by a complex resistivity. The frequency is sufficiently low that displacement currents in the air are negligible. Using such a representation, the mutual impedance or apparent complex resistivity is computed from convergent series expansions over the frequency range from 0.1 to 1000 Hz. We also show the corresponding pure electromagnetic coupling, calculated for the same geometry but under the assumption that the earth is non-polarizable (i.e., the resistivity is real and frequency-independent). Finally we demonstrate that the actual or measured complex resistivity can be corrected for the electromagnetic coupling by a simple subtractive process provided the frequency is sufficiently low. But in general the electrochemical effects, causing the resistivity to be complex and frequency-dependent, are inextricably intertwined with the electromagnetic effects. Thus we conclude that any sweeping claims to correct induced polarization in the field, by various electromagnetic coupling “removal” schemes, should be viewed with concern.

A simple parametric model for the electromagnetic response of an anomalous body in a host medium

A simple approximate representation of the spectral response of an arbitrary kind of electromagnetic (EM) prospecting system to a small conductive target in a conductive environment has been derived. The representation contains the direct response from the layered host medium and the first‐order effects of eddy current induction, current channeling, magnetic induction, and the coupling between eddy current and magnetic inductions in the anomalous body, as modified by the host medium. The only significant computational task in the representation is evaluation of a few Green’s functions for the host medium. As a guide to establishing proper approximations, a fundamental study of integral equations is presented. Very simple solutions for the secondary or scattering sources which represent the EM effect of the body are obtained for a few basic cases. Equations for more general cases are complicated by additional terms in the Green’s functions which represent ac interaction between scattering sources and the host medium, the effects of layering in the host, or interactions between the different types of scattering sources. Through a supplementary study of the response of a conductive disk embedded in a conductive host to an axisymmetric field, the ac interaction between the host medium and scattering sources in the disk is shown to be relatively unimportant. Hence, interaction with layering in the host is also minimal. Green’s functions in the integral equation can consequently be simplified. The representation as a whole has been tested by comparing its predictions of the spectral form of a response with two published data sets, responses of a plate model in a conductive host. The fits for targets of moderate and large size, relative to the scale of the particular experiment, range from excellent to good qualitative resemblance. The representation should find application in making expensive numerical or analog model studies more useful and understandable to the field geophysicist, in converting real and model data between the time‐ and frequency‐domain formats, in constructing practical inverse algorithms, and in predicting the form of EM effects in induced polarization and magnetometric resistivity surveys.

A linear polarization survey of southern T Tauri stars

The linear polarization survey of T Tauri stars brighter than about 13th magnitude and north of - 30 deg declination which was given in Bastein (1982) has been extended to cover the southern hemisphere. New measurements of stars already observed in the first part of the survey are also given. Polarization variability is confirmed to be a common phenomenon in T Tauri stars. The rapid polarization variations which are observed in some stars are best explained by changes in the illumination of the dust. The absence of correlations between polarization and many other properties is reported in a sample of 85 stars. Correlations have been found between polarization and (1) infrared excesses, (2) He I, and (3) forbidden O I equivalent widths. A detailed analysis of stars for which the wavelength dependence was measured is presented.

Spontaneous polarization associated with porphyry sulfide mineralization

In the years 1978 through 1981, many spontaneous polarization (SP) surveys of porphyry sulfide deposits and prospects were done in the western United States. Effective depth of exploration using SP was found to exceed 1 km for large porphyry sulfide orebodies. The relationship between depth of erosion since time of emplacement and the spontaneous polarization observed on the surface, together with the effects of weathering, are presented from field surveys over known deposits. Successful use of the spontaneous polarization technique in porphyry sulfide exploration led to an investigation of the causative mechanism. The field data do not support the electrochemical model first proposed in Sato and Mooney (1960). The measured potential appears to be simply the oxidation potential ΔE between a reference electrode outside the mineralization and a roving electrode within the mineralization. The ΔE measured in the field is dominated by the difference in Eh (ΔEh) potential between the two electrodes, but it is not equivalent due to the unavoidable inclusion of other potentials in the field measurement. Without a wire connecting the electrodes, current between the sulfide mineralization and the surrounding country rock is sensibly zero, as evidenced by the persistence of the sulfides through geologic time.

The Goonumbla porphyry copper deposits, New South Wales

Significant porphyry copper-gold deposits have been discovered at Goonumbla near Parkes in central New South Wales, Australia. Exploratory drilling to date has defined a resource in excess of 250 million metric tons of mineralization with mean grades of 0.70 percent Cu and 0.28 g/metric ton Au. This mineralization is contained within the three largest deposits of nine so far located, all of which occur toward the top of a calc-alkaline volcanic pile of Middle Silurian age.The host volcanics range from trachyte to andesite in composition and have been deposited in the central part of what is believed to have been a large collapse caldera. Vertical pipelike bodies of vein and disseminated bornite, chalcopyrite, and chalcocite mineralization are associated with small irregular quartz monzonite porphyry intrusions. These porphyries have been derived as late-stage quartz-rich differentiates of a more mafic monzonitic magma and have migrated upward along and close to the contacts of larger, earlier mafic intrusions. Pervasive pink K-feldspar flooding of the volcanics with lesser development of secondary biotite is indicative of the intense potassic alteration which has accompanied the deposition of sulfides. Quartz vein stockwork systems are invariably developed at or near the porphyry-andesite contacts and contain above-average abundances of copper, gold, and silver.The mineralizing hydrothermal solutions were low in sulfur content with the result that pyrite is almost totally absent from the central parts of the mineral bodies. Subsequent oxidation and secondary enrichment, although present, have not played an important role in the ore-forming process.Bed-rock geochemical sampling for copper and gold supported by ground magnetic and induced polarization surveys have proven to be the most effective exploration methods for detecting the soil-covered mineralization.The Goonumbla deposits are high-level volcanic to subvolcanic examples of the porphyry copper type and have been formed in a continental margin-arc environment in the eastern side of the Bogan Gate trough.

An evaluation of electrical methods applied to petroleum exploration in Australia

The use of electrical prospecting methods for petroleum exploration has been under study for the past few years. Bipole-dipole and dipole-dipole induced polarization (IP) surveys have been tested over a number of oil and gas fields and prospects in the Denison Trough, and the Bowen, Cooper and Surat Basins. Self potential (SP) methods have also been tested over a number of prospects; the details of a computer-aided data acquisition system are the subject of another publication. Definite resistivity anomalies are seen over the fields. These are discussed with the induced polarization (chargeability) data. The data from two SP surveys show that significant negative anomalies occur above hydrocarbon deposits. From this work it appears that the most definitive parameter to measure is the apparent resistivity, although measurement of the chargeability parameter is worthwhile because anomalies in both often correlate with hydrocarbon-bearing structures. The significance of the correlation between anomalies in both SP and IP data and hydrocarbon accumulations necessitates a description of the processes that produce the anomalies. The SP anomalies are caused by large scale redox (oxidation-reduction) cells formed in a plume above trapped hydrocarbons. These active cells have slightly altered the lithology above hydrocarbon traps; the mechanism is electrochemical in nature, using energy from the hydrocarbons that escape past the imperfect cap rock.

Regions of Anomalously Large Telluric Interference in IP: The Cobar Syndrome

Regional differences are observed in the severity of telluric noise interference in induced polarization surveys. In some areas, of which Cobar (New South Wales) is an extreme example, surveys must often be postponed or confined to particular times of day. Using simple resistivity and magnetotelluric models, and typical magnetic micropulsation spectra, we show how this arises. The situation is worst when a very resistive basement is overlain by a thick conductive cover. In that case the IP signal is small on account of the conductive cover, while telluric voltages are large because of the resistive basement. For typical IP frequencies, and for the values of overburden resistivity found at Cobar, noise to signal ratio is shown to be greatest for overburden thicknesses of 200?300 m. This compares with the 80?150 m thicknesses actually observed. Noise to signal ratio is relatively insensitive to dipole length for a given separation of dipole centres.

On the distribution of interstellar matter around the sun

Available data on interstellar line or EUV continuum absorption attributable to the local (r equal to or less than 100 pc) interstellar medium are presented and analyzed as a whole for clues leading to a deeper understanding of the spatial distribution and physical properties of this material. By using these data in conjunction with upper limits provided by polarization and color excess surveys and relating everything to the neutral-hydrogen column density, it is shown that the data are consistent with a simple model that assumes the relatively pervasive presence of a tenuous (nH = 0.07 per cu cm), possibly warm (T = 1,000-10,000 K) gas extending uniformly out to considerable distances from the sun in most directions. The only region manifestly clear of this material lies in the 200 deg to 270 deg galactic longitude range in both high and low latitudes. Strong density discontinuities are clearly observable at r approximately 100-150 pc in the general directions of the Sco-Cen and Per associations and a weaker discontinuity much closer to the sun running along a ridge oriented parallel to the line connecting their centers. Beyond these fronts, the space density of interstellar matter must fall to very low values that extend out to several hundred parsecs or so from the sun. These results are not incompatible with H I, 21-cm, and soft X-ray all-sky surveys. The implication of these findings for the theory of the interstellar medium and EUV visibility is briefly discussed.

The optical polarization properties of 'normal' quasars

view Abstract Citations (145) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The optical polarization properties of "normal" quasars. Stockman, H. S. ; Moore, R. L. ; Angel, J. R. P. Abstract In 1977, a linear polarization survey of bright QSOs from the catalog of Burbidge et al. (1977) was begun. Stockman and Angel (1978) and Stockman (1978) have reported preliminary results of this survey. Since these reports, the bright QSO survey has been completed and an extended survey of fainter QSOs with known variability and/or spectral indices has been conducted. This paper presents the final report of the results of the surveys. The selection criteria for the bright QSO survey are defined, the observational techniques are described, and the polarization data for the entire sample are presented. An analysis of the data is performed, taking into account the probability distribution of polarization, the variability of polarization, and the color dependence of polarization. Models for the origin of polarization are also discussed. Publication: The Astrophysical Journal Pub Date: April 1984 DOI: 10.1086/161912 Bibcode: 1984ApJ...279..485S Keywords: Optical Polarization; Polarimetry; Quasars; Astronomical Models; Galactic Radiation; Probability Distribution Functions; Radio Galaxies; Seyfert Galaxies; Astrophysics full text sources ADS | data products SIMBAD (142) NED (133)

The H-R diagram of O-type subdwarfs

A spectroscopic survey by Drilling (1983) of non-emission OB+ stars in the Case-Hamburg survey and its extension to b = ±30° for 1 = ±60° has turned up 12 hot, relatively bright O-type subdwarfs (V<13). Drilling's survey covers more than 13 times the area of the south galactic polar survey of Slettebak and Brundage (1971), but has revealed only the hottest and/or hydrogen-poor objects because only the weakest-lined 0-and B-type stars were observed. We have observed all of these stars in the large-aperture, low-resolution mode of the IUE satellite. A full description of their spectral appearance is given elsewhere (Schönberner and Drilling, 1984). The spectra were corrected for interstellar absorption using the Seaton (1979) reddening law to nullify the 2200 å interstellar feature, and in all cases EB-V < 0.25.

Discovery of a mineralized breccia pipe using gradient array induced polarization

The Promontorio breccia pipe located in southern Sonora, Mexico, has been mined intermittently for silver since the turn of the century. Mineralization occurs as sulfides in the matrix of the breccia. Two to five percent pyrite also occurs in the premineral andesite that the breccia pipe has intruded. Geologic mapping led to the discovery of a small, unmineralized breccia in andesite 200 m west of Promontorio. The discovery of this breccia encouraged the search for other hidden mineralized breccias in areas obscured by postmineral alluvium. Exploration of the covered areas was carried out with an induced polarization (IP) and resistivity survey after a test line indicated that a distinct IP anomaly occurs over the Promontorio breccia pipe. The gradient array was chosen as the most effective electrode configuration to explore the covered areas because it can provide rapid detailed IP coverage necessary to locate small breccia pipes. Several anomalous IP trends were located. The most interesting one has an apparent response of 60 to 90 msec in low‐resistivity rocks similar to Promontorio. Additional information provided by dipole‐dipole profiles indicates that this anomalous zone is close to surface and about 20 m wide with a vertical to steep dip. Trenching and drilling confirmed the presence of a 20-m wide elongate breccia with sulfide mineralization similar to Promontorio. Silver grades were marginal for making ore in 1977.

The Hat Creek coal deposit: A geophysical case history

The Hat Creek Valley encloses one of the world's thickest known coal-deposits. The coal response to magnetics, electromagnetics, induced polarization, various downhole logging techniques and gravity surveys are examined. The magnetic method was used for field mapping of geological units and was particularly useful in detecting volcanic rocks and burned zones. Electromagnetic methods detected conductive fault zones with varying degrees of success, while an induced polarization survey gave poor results because of the scarcity and habit of polarizable minerals in the deposit. Downhole logging techniques were used to correlate geologic units between drillholes. The gravity method directly detected the low density coal and yielded one of the world's largest recorded gravity anomalies over a body of economic importance. Analysis of the gravity data indicates a faulted coal deposit of variable width and depth, extending continuously for 19 km. Coal resources are estimated at 1.5–2.2 · 10 10 tonne (17–24 billion ton).

Geophysical investigation for Copper-Sulphide ore at Dhusa, Nepal

Self-Potential, Induced Polarisation and Magnetic Surveys conducted in an anomalous zone delineated by preliminary to advanced stages of geochemical exploration showed good correlation with the geochemical anomaly. SP and IP survey conducted have a very limited depth of investigation. Qualitative interpretation of SP curves clearly indicated the existence of two faults, whereas combined results of SP and IP showed weathered and "deep­ seated" (30m) polarizable body. The magnetic survey did not give any significant result.
 Three bore holes drilled in the best geophysical, geochemical anomaly zone intersected mineralization of chalcopyrite chalcocite and pyrite with an average grade of 0.17% Cu over 38m true thickness. The post drilling IP and resistivity sounding conducted at 10 points along a line passing through one of the drill holes indicated the possibility of better mineralization further down dip. The mineralization is expected to be present in the form of lenses.

Polarization in NGC 7789 and the membership of blue stragglers

view Abstract Citations (17) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polarization in NGC 7789 and the membership of blue stragglers. Breger, M. Abstract It is pointed out that a full explanation concerning the existence of blue stragglers is not yet available. In stellar clusters the blue stragglers can be recognized by their near-main-sequence positions above the cluster turnoff point. The galactic cluster NGC 7789 selected for the present investigation contains over 30 blue straggler candidates. Cluster membership is an important consideration since nonmembers can be expected to show temperatures and apparent magnitudes different from those of the cluster members and to appear like blue stragglers. An interstellar polarization survey of NGC 7789 has been used to determine cluster membership. The division of stars into members and nonmembers on the basis of polarization considerations provides results which are in good agreement with proper-motion divisions in a study by McNamara and Solomon (1981). Sixteen blue stragglers are found to be cluster members, while 18 candidates are nonmembers. Publication: The Astrophysical Journal Pub Date: December 1982 DOI: 10.1086/160494 Bibcode: 1982ApJ...263..199B Keywords: Galactic Clusters; Polarized Radiation; Stellar Color; Stellar Evolution; Stellar Motions; Stellar Spectrophotometry; Electrophotometry; Hertzsprung-Russell Diagram; Linear Polarization; Stellar Spectra; Stellar Temperature; Ubv Spectra; Astrophysics full text sources ADS | data products SIMBAD (1) GCPD (1)