3,254 research outputs found
Using Red Clump Stars to Decompose the Galactic Magnetic Field with Distance
A new method for measuring the large-scale structure of the Galactic magnetic
field is presented. The Galactic magnetic field has been probed through the
Galactic disk with near-infrared starlight polarimetry, however the distance to
each background star is unknown. Using red clump stars as near-infrared
standard candles, this work presents the first attempt to decompose the line of
sight structure of the sky-projected Galactic magnetic field. Two example
lines-of-sight are decomposed: toward a field with many red clump stars and
toward a field with few red clump stars. A continuous estimate of magnetic
field orientation over several kiloparsecs of distance is possible in the field
with many red clump stars, while only discrete estimates are possible in the
sparse example. toward the Outer Galaxy, there is a continuous field
orientation with distance that shows evidence of perturbation by the Galactic
warp. toward the Inner Galaxy, evidence for a large-scale change in the
magnetic field geometry is consistent with models of magnetic field reversals,
independently derived from Faraday rotation studies. A photo-polarimetric
method for identifying candidate intrinsically polarized stars is also
presented. The future application of this method to large regions of the sky
will begin the process of mapping the Galactic magnetic field in a way never
before possible.Comment: 11 pages, 8 figures, 2 tables, accepted for publication in The
Astronomical Journa
H II Region Driven Galactic Bubbles And Their Relationship To The Galactic Magnetic Field
The relative alignments of mid-infrared traced Galactic bubbles are compared to the orientation of the mean Galactic magnetic field in the disk. The orientations of bubbles in the northern Galactic plane were measured and are consistent with random orientations-no preferential alignment with respect to the Galactic disk was found. A subsample of H II region driven Galactic bubbles was identified, and as a single population they show random orientations. When this subsample was further divided into subthermal and suprathermal H II regions, based on hydrogen radio recombination linewidths, the subthermal H II regions showed a marginal deviation from random orientations, but the suprathermal H II regions showed significant alignment with the Galactic plane. The mean orientation of the Galactic disk magnetic field was characterized using new near-infrared starlight polarimetry and the suprathermal H II regions were found to preferentially align with the disk magnetic field. If suprathermal linewidths are associated with younger H II regions, then the evolution of young H II regions is significantly affected by the Galactic magnetic field. As H II regions age, they cease to be strongly linked to the Galactic magnetic field, as surrounding density variations come to dominate their morphological evolution. From the new observations, the ratios of magnetic-to-ram pressures in the expanding ionization fronts were estimated for younger H II regions.NSF AST 06-07500, 09-07790NASAW. M. Keck FoundationAstronom
A Lack of Resolved Near-Infrared Polarization Across the Face of M51
The galaxy M51 was observed using the Mimir instrument on the Perkins
telescope to constrain the resolved H-band (1.6 m) polarization across the
galaxy. These observations place an upper limit of on the -band
polarization across the face of M51, at 0.6 arcsecond pixel sampling. Even with
smoothing to coarser angular resolutions, to reduce polarization uncertainty,
the -band polarization remains undetected. The polarization upper limit at
-band, when combined with previous resolved optical polarimetry, rules out a
Serkowski-like polarization dependence on wavelength. Other polarization
mechanisms cannot account for the observed polarization ratio () across the face of M51.Comment: 4 pages, 2 figures, Accepted for publication in ApJ
Magnetic fields in the milky way: near-infrared polarimetry
Thesis (Ph.D.)--Boston UniversityAstronomers have a limited understanding of the large-scale structure of the Galactic magnetic field and its role in the evolution of the interstellar medium (ISM). This understanding derives primarily from Faraday rotation and synchrotron observations which do not probe the cool, dusty ISM. To advance our knowledge of the Galactic magnetic field, this dissertation reports on the application of a different method, near-infrared (NIR) polarization of background starlight, to place new observational constraints on the nature of the Galactic magnetic field and to study the field's role in the evolution of interstellar material.
A radiative transfer computer code was developed to predict all-sky starlight polarization observations. Starlight polarimetry predictions were made for several different dynamo-driven magnetic field geometries, assuming that magnetically-aligned interstellar dust grains polarize background starlight. New NIR starlight polarimetry measurements in the outer Galaxy were tested against these predictions. These observations favor disk-symmetric magnetic fields while rejecting disk-antisymmetric magnetic fields. This result contradicts some previous interpretations of all-sky, radio Faraday rotation measurements. The Galactic magnetic pitch angle is constrained to p = -6 ± 2°.
The physical orientations of Galactic HII regions, traced by mid-infrared emission, are compared to the large-scale, disk-symmetric Galactic magnetic field geometry derived above. Hydrogen recombination line spectra towards these same objects revealed that many possessed turbulent linewidths. If fluid turbulence decays with time, then it may be used as a relative age indicator. A trend is seen between magnetic alignment and the degree of turbulence in the HII region. This result leads to the development of an observationally-driven HII region magnetic evolutionary sequence.
Resolved polarimetry across the face of the galaxy M51 was measured for comparison with the internal, edge-on view of the Milky Way seen from Earth. Strong upper limits ( < 0.05% at a resolution of 0.6 arcseconds) were placed on the degree of NIR polarization across the face of M51. These results were combined with resolved optical polarimetry measurements from the literature. Normal polarization mechanisms cannot explain the observed polarization dependence on wavelength
Prediction and Simulator Verification of Roll/Lateral Adverse Aeroservoelastic Rotorcraft–Pilot Couplings
The involuntary interaction of a pilot with an aircraft can be described as pilot-assisted oscillations. Such
phenomena are usually only addressed late in the design process when they manifest themselves during ground/flight
testing. Methods to be able to predict such phenomena as early as possible are therefore useful. This work describes a
technique to predict the adverse aeroservoelastic rotorcraft–pilot couplings, specifically between a rotorcraft’s roll
motion and the resultant involuntary pilot lateral cyclic motion. By coupling linear vehicle aeroservoelastic models
and experimentally identified pilot biodynamic models, pilot-assisted oscillations and no-pilot-assisted oscillation
conditions have been numerically predicted for a soft-in-plane hingeless helicopter with a lightly damped regressive
lead–lag mode that strongly interacts with the roll modeat a frequency within the biodynamic band of the pilots. These
predictions have then been verified using real-time flight-simulation experiments. The absence of any similar adverse
couplings experienced while using only rigid-body models in the flight simulator verified that the observed
phenomena were indeed aeroelastic in nature. The excellent agreement between the numerical predictions and the
observed experimental results indicates that the techniques developed in this paper can be used to highlight the
proneness of new or existing designs to pilot-assisted oscillation
Testing Galactic Magnetic Field Models using Near-Infrared Polarimetry
This work combines new observations of NIR starlight linear polarimetry with
previously simulated observations in order to constrain dynamo models of the
Galactic magnetic field. Polarimetric observations were obtained with the Mimir
instrument on the Perkins Telescope in Flagstaff, AZ, along a line of constant
Galactic longitude (\ell = 150\circ) with 17 pointings of the 10' \times 10'
field of view between -75\circ < b < 10\circ, with more frequent pointings
towards the Galactic midplane. A total of 10,962 stars were photometrically
measured and 1,116 had usable polarizations. The observed distribution of
polarization position angles with Galactic latitude and the cumulative
distribution function of the measured polarizations are compared to predicted
values. While the predictions lack the effects of turbulence and are therefore
idealized, this comparison allows significant rejection of A0-type magnetic
field models. S0 and disk-even halo-odd magnetic field geometries are also
rejected by the observations, but at lower significance. New predictions of
spiral-type, axisymmetric magnetic fields, when combined with these new NIR
observations, constrain the Galactic magnetic field spiral pitch angle to
-6\circ \pm 2\circ.Comment: 11 pages, 10 figures, Accepted for publication in Ap
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