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Articles 1 - 8 of 8
Full-Text Articles in Physical Sciences and Mathematics
Dust-Polarization Maps And Interstellar Turbulence, Robert R. Caldwell, Chris Hirata, Marc Kamionkowski
Dust-Polarization Maps And Interstellar Turbulence, Robert R. Caldwell, Chris Hirata, Marc Kamionkowski
Dartmouth Scholarship
Perhaps the most intriguing result of Planck's dust-polarization measurements is the observation that the power in the E-mode polarization is twice that in the B mode, as opposed to pre-Planck expectations of roughly equal dust powers in the E and B modes. Here we show how the E- and B-mode powers depend on the detailed properties of the fluctuations in the magnetized interstellar medium (ISM). These fluctuations can be decomposed into slow, fast, and Alfvén magnetohydrodynamic (MHD) waves, which comprise a complete basis that can be used to describe linear fluctuations of a magnetized fluid. They can …
Absolute Ages And Distances Of 22 Gcs Using Monte Carlo Main-Sequence Fitting, Erin M. O'Malley, Christina Gilligan, Brian Chaboyer
Absolute Ages And Distances Of 22 Gcs Using Monte Carlo Main-Sequence Fitting, Erin M. O'Malley, Christina Gilligan, Brian Chaboyer
Dartmouth Scholarship
The recent Gaia Data Release 1 of stellar parallaxes provides ample opportunity to find metal-poor main-sequence stars with precise parallaxes. We select 21 such stars with parallax uncertainties better than σ π /π ≤ 0.10 and accurate abundance determinations suitable for testing metal-poor stellar evolution models and determining the distance to Galactic globular clusters (GCs). A Monte Carlo analysis was used, taking into account uncertainties in the model construction parameters, to generate stellar models and isochrones to fit to the calibration stars. The isochrones that fit the calibration stars best were then used to determine the distances and ages …
X-Ray And Optical Study Of The Gamma-Ray Source 3fgl J0838.8–2829: Identification Of A Candidate Millisecond Pulsar Binary And An Asynchronous Polar, Jules P. Halpern, Slavko Bogdanov, John R. Thorstensen
X-Ray And Optical Study Of The Gamma-Ray Source 3fgl J0838.8–2829: Identification Of A Candidate Millisecond Pulsar Binary And An Asynchronous Polar, Jules P. Halpern, Slavko Bogdanov, John R. Thorstensen
Dartmouth Scholarship
We observed the field of the Fermi source 3FGL J0838.8−2829 in optical and X-rays, initially motivated by the cataclysmic variable (CV) 1RXS J083842.1−282723 that lies within its error circle. Several X-ray sources first classified as CVs have turned out to be γ-ray emitting millisecond pulsars (MSPs). We find that 1RXS J083842.1−282723 is in fact an unusual CV, a stream-fed asynchronous polar in which accretion switches between magnetic poles (that are ≈120° apart) when the accretion rate is at minimum. High-amplitude X-ray modulation at periods of 94.8 ± 0.4 minutes and 14.7 ± 1.2 hr are seen. The former appears …
A Differential Abundance Analysis Of Very Metal-Poor Stars, Erin M. O'Malley, Andrew Mcwilliam, Brian Chaboyer, Ian Thompson
A Differential Abundance Analysis Of Very Metal-Poor Stars, Erin M. O'Malley, Andrew Mcwilliam, Brian Chaboyer, Ian Thompson
Dartmouth Scholarship
We have performed a differential line-by-line chemical abundance analysis, ultimately relative to the Sun, of nine very metal-poor main-sequence (MS) halo stars, near [Fe/H] = −2 dex. Our abundances range from dex with conservative uncertainties of 0.07 dex. We find an average [α/Fe] = 0.34 ± 0.09 dex, typical of the Milky Way. While our spectroscopic atmosphere parameters provide good agreement with Hubble Space Telescope parallaxes, there is significant disagreement with temperature and gravity parameters indicated by observed colors and theoretical isochrones. Although a systematic underestimate of the stellar temperature by a few hundred degrees could explain this …
The X-Ray And Mid-Infrared Luminosities In Luminous Type 1 Quasars, Chien-Ting J. Chen, Ryan C. Hickox, Andrew D. Goulding, Daniel Stern
The X-Ray And Mid-Infrared Luminosities In Luminous Type 1 Quasars, Chien-Ting J. Chen, Ryan C. Hickox, Andrew D. Goulding, Daniel Stern
Dartmouth Scholarship
Several recent studies have reported different intrinsic correlations between the AGN mid-IR luminosity (LMIR) and the rest-frame 2-10 keV luminosity (LX) for luminous quasars. To understand the origin of the difference in the observed LX−LMIR relations, we study a sample of 3,247 spectroscopically confirmed type 1 AGNs collected from Bo\"{o}tes, XMM-COSMOS, XMM-XXL-North, and the SDSS quasars in the Swift/XRT footprint spanning over four orders of magnitude in luminosity. We carefully examine how different observational constraints impact the observed LX−LMIR relations, including the inclusion of X-ray non-detected objects, possible X-ray absorption …
Sensitivity To A Frequency-Dependent Circular Polarization In An Isotropic Stochastic Gravitational Wave Background, Tristan L. Smith, Robert Caldwell
Sensitivity To A Frequency-Dependent Circular Polarization In An Isotropic Stochastic Gravitational Wave Background, Tristan L. Smith, Robert Caldwell
Dartmouth Scholarship
We calculate the sensitivity to a circular polarization of an isotropic stochastic gravitational wave background (ISGWB) as a function of frequency for ground- and space-based interferometers and observations of the cosmic microwave background. The origin of a circularly polarized ISGWB may be due to exotic primordial physics (i.e., parity violation in the early universe) and may be strongly frequency dependent. We present calculations within a coherent framework which clarifies the basic requirements for sensitivity to circular polarization, in distinction from previous work which focused on each of these techniques separately. We find that the addition of an interferometer with the …
A Multiwavelength Continuum Characterization Of High-Redshift Broad Absorption Line Quasars, D. Tuccillo, G. Bruni, M. A. Dipompeo, M. S. Brotherton
A Multiwavelength Continuum Characterization Of High-Redshift Broad Absorption Line Quasars, D. Tuccillo, G. Bruni, M. A. Dipompeo, M. S. Brotherton
Dartmouth Scholarship
We present the results of a multiwavelength study of a sample of high-redshift radio-loud (RL) broad absorption line (BAL) quasars. This way, we extend to higher redshift previous studies on the radio properties and broad-band optical colours of these objects. We have selected a sample of 22 RL BAL quasars with 3.6 ≤ z ≤ 4.8 cross-correlating the FIRST radio survey with the Sloan Digital Sky Survey (SDSS). Flux densities between 1.25 and 9.5 GHz have been collected with the Jansky Very Large Array and Effelsberg-100 m telescopes for 15 BAL and 14 non-BAL quasars used as a comparison sample. …
Testing Metal-Poor Stellar Models And Isochrones With Hst Parallaxes Of Metal-Poor Stars, B. Chaboyer, E. O'Malley, G. F. Benedict, G. A. Feiden
Testing Metal-Poor Stellar Models And Isochrones With Hst Parallaxes Of Metal-Poor Stars, B. Chaboyer, E. O'Malley, G. F. Benedict, G. A. Feiden
Dartmouth Scholarship
Hubble Space Telescope (HST) fine guidance sensor observations were used to obtain parallaxes of eight metal-poor ([Fe/H] < −1.4) stars. The parallaxes of these stars determined by the new Hipparcos reduction average 17% accuracy, in contrast to our new HST parallaxes, which average 1% accuracy and have errors on the individual parallaxes ranging from 85 to 144 μas. These parallax data were combined with HST Advanced Camera for Surveys photometry in the F606W and F814W filters to obtain the absolute magnitudes of the stars with an accuracy of 0.02–0.03 mag. Six of these stars are on the main sequence (MS) (with −2.7 < [Fe/H] < −1.8) and are suitable for testing metal-poor stellar evolution models and determining the distances to metal-poor globular clusters (GCs). Using the abundances obtained by O'Malley et al., we find that standard stellar models using the VandenBerg & Clem color transformation do a reasonable job of matching five of the MS stars, with HD 54639 ([Fe/H] = −2.5) being anomalous in its location in the color–magnitude diagram. Stellar models and isochrones were generated using a Monte Carlo analysis to take into account uncertainties in the models. Isochrones that fit the parallax stars were used to determine the distances and ages of nine GCs (with −2.4 ≤ [Fe/H] ≤ −1.9). Averaging together the age of all nine clusters led to an absolute age of the oldest, most metal-poor GCs of 12.7 ± 1.0 Gyr, where the quoted uncertainty takes into account the known uncertainties in the stellar models and isochrones, along with the uncertainty in the distance and reddening of the clusters.