arXiv subjects

D. M. Harrington

Publications and source records attributed to D. M. Harrington.

At least 19 recordsLinked to original sources

arXiv2026

The MuFusE Large-Volume Diamond Anvil Cell for Exploring Muon-Catalyzed Fusion at Higher Pressures and Temperatures

A new large-volume diamond anvil cell (DAC) has been developed for the Muon-catalyzed Fusion ($μ$CF) Experiment (MuFusE), enabling the compression and heating of deuterium-tritium (d-t) mixtures to pressures and temperatures needed to advance $μ$CF research. The MuFusE DAC achieves the large sample volumes necessary for high-precision fusion measurements while integrating cryogenic loading, all-metal sealing, and flexible bellows to maintain a secure environment during cell compression. Combined with remote pneumatic actuation and secondary containment, the DAC safely managed a 25 Ci tritium inventory while providing a clear optical path for in situ measurements of sample pressure and composition via laser spectroscopy. Utilizing 5 mm diameter diamond anvils oriented in the path of a high-intensity muon beam, the apparatus achieved a stable sample volume of 19.2 mm$^3$ at liquid density, pressures up to 933 MPa and temperatures up to 400 K - benchmarks that significantly exceed previously reported limits for static d-t targets.

physics.ins-det↗

arXiv2018

Polarization Modeling and Predictions for DKIST Part 4: Calibration Accuracy Over Field of View, Retardance Spatial Uniformity And Achromat Design Sensitivity

Modern observatories and instruments require optics fabricated at larger sizes with more stringent performance requirements. The Daniel K. Inouye Solar Telescope will be the world's largest solar telescope at 4.0 m aperture delivering a 300 Watt beam and a 5 arc-minute field. Spatial variation of retardance is a limitation to calibration of the full field. Three polarimeters operate seven cameras simultaneously in narrow bandpasses from 380 nm to 1800 nm. The DKIST polarization calibration optics must be 120 mm in diameter at Gregorian focus to pass the beam and operate under high heat load, UV flux and environmental variability. Similar constraints apply to the instrument modulators with large beams near focal planes at F/18 to F/62. We assess how design factors can produce more spatial and spectral errors simulating elliptical retardance caused by polishing errors. We measure over 5deg circular retardance and spectral oscillations over +-2 for optics specified as strictly linear retarders. Spatial variations on scales larger than 10 mm contain 90% of the variation. Different designs can be a factor of 2 more sensitive to polishing errors with dissimilar spatial distributions even when using identical retardance bias values and materials. The calibration for the on axis beam is not impacted once circular retardance is included. Calibration of the full field is limited by spatial retardance variation unless techniques account for this variation. We show calibration retarder variation at amplitudes of 1deg retardance for field angles greater than roughly one arc minute for both quartz and MgF_2 retarders at visible wavelengths with significant variation between the three DKIST calibration retarders. We present polishing error maps to inform new calibration techniques attempting to deliver absolute accuracy of system calibration below effective cross-talk levels of 1deg retardance.

astro-ph.IM↗

arXiv2017

First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved

We report the first direct detection of a strong, 5 kG magnetic field on the surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting transient radio and optical emission bursts modulated by fast rotation. We have detected the surface magnetic field as circularly polarized signatures in the 819 nm sodium lines when an active emission region faced the Earth. Modeling Stokes profiles of these lines reveals the effective temperature of 2800 K and log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on evolutionary tracks as a young brown dwarf with the mass of 55$\pm$4 M$_{\rm J}$ and age of 22$\pm$4 Myr. Its magnetic field is at least 5.1 kG and covers at least 11% of the visible hemisphere. The active region topology recovered using line profile inversions comprises hot plasma loops with a vertical stratification of optical and radio emission sources. These loops rotate with the dwarf in and out of view causing periodic emission bursts. The magnetic field is detected at the base of the loops. This is the first time that we can quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG surface magnetic field and solve the puzzle of their driving mechanism. This is also the coolest known dwarf with such a strong surface magnetic field. The young age of LSR J1835+3259 implies that it may still maintain a disk, which may facilitate bursts via magnetospheric accretion, like in higher-mass T Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs and hot Jupiters.

astro-ph.SR↗

arXiv2011

Deriving Telescope Mueller Matrices Using Daytime Sky Polarization Observations

Telescopes often modify the input polarization of a source so that the measured circular or linear output state of the optical signal can be signficantly different from the input. This mixing, or polarization "cross-talk", is defined by the optical system Mueller matrix. We describe here an efficient method for recovering the input polarization state of the light and the full 4 x 4 Mueller matrix of the telescope with an accuracy of a few percent without external masks or telescope hardware modification. Observations of the bright, highly polarized daytime sky using the Haleakala 3.7m AEOS telescope and a coude spectropolarimeter demonstrate the technique.

astro-ph.IM↗

arXiv2011

Eccentric binaries: Tidal flows and periastron events

A number of binary systems present evidence of enhanced activity around periastron passage, suggesting a connection between tidal interactions and these periastron effects. The aim of this investigation is to study the time-dependent response of a star's surface as it is perturbed by a binary companion. We derive expressions for the rate of dissipation, $\dot{E}$, of the kinetic energy by the viscous flows driven by tidal interactions on the surface layer. The method is tested by comparing the results from a grid of model calculations with the analytical predictions of Hut (1981) and the synchronization timescales of Zahn (1977, 2008). Our results for the orbital cycle averaged energy dissipation on orbital separation are consistent with those of Hut for model binaries with orbital separations at periastron >8 stellar radii. The model also reproduces the predicted pseudo-synchronization angular velocity for moderate eccentricities and the same scaling of synchronization timescales for circular orbits with separation as given by Zahn. The computations gives the distribution of $\dot{E}$ over the stellar surface, and show that it is generally concentrated at the equatorial latitude, with maxima generally located around four clearly defined longitudes, corresponding to the fastest azimuthal velocity perturbations. Maximum amplitudes occur around periastron passage or slightly thereafter for supersynchronously rotating stars. In very eccentric binaries, the distribution of $\dot{E}$ over the surface changes significantly as a function of orbital phase, with small spatial structures appearing after periastron. An exploratory calculation for the highly eccentric binary system delta Sco suggests that the sudden and large amplitude variations in surface properties around periastron may contribute toward the activity observed around this orbital phase.

astro-ph.SR↗

arXiv2010

Precision and Resolution in Stellar Spectropolarimetry

Stellar spectropolarimetry is a relatively new remote sensing tool for exploring stellar atmospheres and circumstellar environments. We present the results of our HiVIS survey and a multi-wavelength ESPaDOnS follow-up campaign showing detectable linear polarization signatures in many lines for most obscured stars. This survey shows polarization at and below 0.1% across many lines are common in stars with often much larger H-alpha signatures. These smaller signatures are near the limit of typical systematic errors in most night-time spectropolarimeters. In an effort to increase our precision and efficiency for detecting small signals we designed and implemented the new HiVIS bi-directionally clocked detector synchronized with the new liquid-crystal polarimeter package. We can now record multiple independent polarized spectra in a single exposure on identical pixels and have demonstrated 10^-4 relative polarimetric precision. The new detector allows for the movement of charge on the device to be synchronized with phase changes in the liquid-crystal variable retarders at rates of >5Hz. It also allows for more efficient observing on bright targets by effectively increasing the pixel well depth. With the new detector, low and high resolution modes and polarization calibrations for the instrument and telescope, we substantially reduce limitations to the precision and accuracy of this new spectropolarimetric tool.

astro-ph.IM↗

arXiv2010

Using Absorptive Linear Polarization Spectroscopy to Understand Imbedded Stars

Sensitive measurements of the linearly polarized spectra of stars can be used to deduce geometric properties of their otherwise unresolved circumstellar environments. This paper describes some of the evidence for optical pumping and absorptive linear polarization and explores some interesting applications of linear spectropolarimetry for obtaining spatial information from imbedded stars.

astro-ph.SR↗

arXiv2010

Achromatizing a liquid-crystal spectropolarimeter: Retardance vs Stokes-based calibration of HiVIS

Astronomical spectropolarimeters can be subject to many sources of systematic error which limit the precision and accuracy of the instrument. We present a calibration method for observing high-resolution polarized spectra using chromatic liquid-crystal variable retarders (LCVRs). These LCVRs allow for polarimetric modulation of the incident light without any moving optics at frequencies >10Hz. We demonstrate a calibration method using pure Stokes input states that enables an achromatization of the system. This Stokes-based deprojection method reproduces input polarization even though highly chromatic instrument effects exist. This process is first demonstrated in a laboratory spectropolarimeter where we characterize the LCVRs and show example deprojections. The process is then implemented the a newly upgraded HiVIS spectropolarimeter on the 3.67m AEOS telescope. The HiVIS spectropolarimeter has also been expanded to include broad-band full-Stokes spectropolarimetry using achromatic wave-plates in addition to the tunable full-Stokes polarimetric mode using LCVRs. These two new polarimetric modes in combination with a new polarimetric calibration unit provide a much more sensitive polarimetric package with greatly reduced systematic error.

astro-ph.IM↗

arXiv2009

Line-profile variability from tidal flows in Alpha Virginis (Spica)

We present the results of high precision, high resolution (R~68000) optical observations of the short-period (4d) eccentric binary system Alpha Virginis (Spica) showing the photospheric line-profile variability that in this system can be attributed to non-radial pulsations driven by tidal effects. Although scant in orbital phase coverage, the data provide S/N>2000 line profiles at full spectral resolution in the wavelength range delta-lambda = 4000--8500 Angstroms, allowing a detailed study of the night-to-night variability as well as changes that occur on ~2 hr timescale. Using an ab initio theoretical calculation, we show that the line-profile variability can arise as a natural consequence of surface flows that are induced by the tidal interaction.

astro-ph.SR↗

arXiv2009

Spectropolarimetric Observations of Herbig Ae/Be Stars. II. Comparison of Spectropolarimetric Surveys: HAeBe, Be and Other Emission-Line Stars

The polarization of light across individual spectral lines contains information about the circumstellar environment on very small spatial scales. We have obtained a large number of high precision, high resolution spectropolarimetric observations of Herbig Ae/Be, Classical Be and other emission-line stars collected on 117 nights of observations with the HiVIS spectropolarimeter at a resolution of R=13000 on the 3.67m AEOS telescope. We also have many observations from the ESPaDOnS spectropolarimeter at a resolution of R=68000 on the 3.6m CFH telescope. In roughly ~2/3 of the so-called "windy" or "disky" Herbig Ae/Be stars, the detected H-alpha linear polarization varies from our typical detection threshold near 0.1% to over 2%. In all but one HAe/Be star the detected polarization effect is not coincident with the H-alpha emission peak but is detected in and around the obvious absorptive part of the line profile. The qu-loops are dominated by the polarization in this absorptive region. In several stars the polarization varies in time mostly in the absorptive component and is not necessarily tied to corresponding variations in intensity. This is a new result not seen at lower resolution. In the Be and emission-line stars, 10 out of a sample of 30 show a typical broad depolarization effect but 4 of these 10 show weaker effects only visible at high resolution. Another 5 of 30 show smaller amplitude, more complex signatures. Six stars of alternate classification showed large amplitude (1-3%) absorptive polarization effects. These detections are largely inconsistent with the traditional disk-scattering and depolarization models.

astro-ph↗

arXiv2008

Ubiquitous H-alpha polarized line profiles: absorptive spectropolarimetric effects and temporal variability in Post-AGB, Herbig Ae/Be and other stellar types

We show here that the absorptive H-alpha polarized line profile previously seen in many Herbig Ae/Be (HAeBe) stars is a nearly ubiquitous feature of other types of embedded or obscured stars. This characteristic 1% linear polarization variation across the absorptive part of the H-alpha line is seen in Post-AGB stars as well as RV-Tau, Delta-Scuti, and other types. Each of these stars shows evidence of obscuration by intervening circumstellar hydrogen gas and the polarization effect is in the absorptive component, consistent with an optical pumping model. We present ESPaDOnS spectropolarimetric observations of 9 post-AGB and RV-Tau types in addition to many multi-epoch HiVIS observations of these targets. We find significant polarization changes across the H-alpha line in 8/9 stars with polarization amplitudes of 0.5% to over 3% (5/6 Post-AGB and 3/3 RV-Tau). In all but one of these, the polarization change is dominated by the absorptive component of the line profile. There is no evidence that subclasses of obscured stars showing stellar pulsations (RV-Tau for Post-AGB stars and Delta-Scuti for Herbig Ae/Be stars) show significant spectropolarimetric differences from the main class. Significant temporal variability is evident from both HiVIS and ESPaDOnS data for several stars presented here: 89 Her, AC Her, SS Lep, MWC 120, AB Aurigae and HD144668. The morphologies and temporal variability are comparable to existing large samples of Herbig Ae/Be and Be type stars. Since Post-AGB stars have circumstellar gas that is very different from Be stars, we discuss these observations in the context of their differing environments.

astro-ph↗

arXiv2008

Stellar spectropolarimetry with HiVIS: Herbig Ae/Be stars, circumstellar environments and optical pumping

The polarization of light across spectral lines contains information about the circumstellar material on small spatial scales. Many models relate the circumstellar environment to observable polarization changes across spectral lines. However, measuring signals at the 0.1% level requires a very careful control of systematic effects. We have recently built a high-resolution spectropolarimeter for the HiVIS spectrograph on the 3.67m AEOS telescope and have obtained a large number of high precision spectropolarimetric observations of Herbig Ae/Be, Classical Be and other stars. The detected linear polarization varies from our typical detection threshold near 0.1% up to 2%. This polarization effect is generally not coincident with the H-alpha emission peak but is detected only in the absorptive part of the line profile and varies with the absorption. These detections are largely inconsistent with the traditional scattering models and inspired a new explanation of their polarization. We developed a new spectropolarimetric model and argue that polarization in absorption is evidence of optical pumping. We argue that, while scattering theory fits many Be and emission-line star observations, this new theory has much more potential to explain polarization-in-absorption as seen in Herbig Ae/Be and other stellar systems. This thesis presents a large spectropolarimetric study that combines new instrumentation, custom processing software, thorough calibrations, cross-instrument comparisons, a massive observing campaign on many targets, comparison of current theories on multiple objects and finally the creation of a new theory.

astro-ph↗

arXiv2008

High Resolution Spectropolarimetry of the H-alpha Line: Obscured Stars and Absorptive Polarization

The near-star environment around obscured stars is very dynamic. Many classes of stars show evidence for winds, disks, inflows and outflows with many phenomena occurring simultaneously. These processes are involved in stellar evolution, star and planet formation, and influence the formation and habitability of planets around host stars. Even for the nearest stars, this region will not be imaged even after the completion of the next generation of telescopes. Other methods for measuring the physical properties of circumstellar material must be developed. The polarization of light across spectral lines is a signature that contains information about the circumstellar material on these small spatial scales. We used the HiVIS (R=13000 to 50000) and ESPaDOnS (R=68000) spectropolarimeters to monitor several classes of stars on over a hundred nights of observing from 2004-2008. In 10/30 classical Be stars, the traditional broad depolarization morphology is reproduced, but with some additional absorptive effects in 4 of these 10 stars. In Herbig Ae/Be stars roughly 2/3 of the stars (14/20) with strong absorptive components (either central or blue-shifted) showed clear spectropolarimetric signatures typically centered on absorptive components of the spectral lines. They were typically 0.3% to 2% with some signatures being variable in time. Post-AGB and RV-Tau type evolved stars showed very strong absorptive polarimetric effects (5/6 PAGB and 4/4 RVTau) very similar to the Herbig Ae/Be stars. These observations were inconsistent with the traditional scattering models and inspired the development of a new explanation of the observed polarization. This new model, based on optical-pumping, has the potential to provide direct measurements of the circumstellar gas properties.

astro-ph↗

arXiv2007

The Near-Star Environment: Spectropolarimetry of Herbig Ae/Be Stars

The near-star environment around young stars is very dynamic with winds, disks, and outflows. These processes are involved in star and planet formation, and influence the formation and habitability of planets around host stars. Even for the closest young stars, this will not be imaged even after the completion of the next generation of telescopes decades from now and other proxies must be used. The polarization of light across individual spectral lines is such a proxy that contains information about the geometry and density of circumstellar material on these small spatial scales. We have recently built a high-resolution spectropolarimeter (R~13000 to 50000) for the HiVIS spectrograph on the 3.67m AEOS telescope. We used this instrument to monitor several young intermediate-mass stars over many nights. These observations show clear spectropolarimetric signatures typically centered on absorptive components of the spectral lines, with some signatures variable in time. The survey also confirms the large spectroscopic variability in these stars on timescales of minutes to months, and shows the dyamic bullets and streamers in the stellar winds. These observations were largely inconsistent with the traditional scattering models and inspired the development of a new explanation of their polarization, based on optical-pumping, that has the potential to provide direct measurements of the circumstellar gas properties.

astro-ph↗

arXiv2007

Spectropolarimetric observations of Herbig Ae/Be Stars I: HiVIS spectropolarimetric calibration and reduction techniques

Using the HiVIS spectropolarimeter built for the Haleakala 3.7m AEOS telescope in Hawaii, we are collecting a large number of high precision spectropolarimetrc observations of stars. In order to precisely measure very small polarization changes, we have performed a number of polarization calibration techniques on the AEOS telescope and HiVIS spectrograph. We have extended our dedicated IDL reduction package and have performed some hardware upgrades to the instrument. We have also used the ESPaDOnS spectropolarimeter on CFHT to verify the HiVIS results with back-to-back observations of MWC 361 and HD163296. Comparision of this and other HiVIS data with stellar observations from the ISIS and WW spectropolarimeters in the literature further shows the usefulness of this instrument.

astro-ph↗

arXiv2007

Spectropolarimetry of the H-alpha line in Herbig Ae/Be stars

Using the HiVIS spectropolarimeter built for the Haleakala 3.7m AEOS telescope, we have obtained a large number of high precision spectropolarimetrc observations (284) of Herbig AeBe stars collected over 53 nights totaling more than 300 hours of observing. Our sample of five HAeBe stars: AB Aurigae, MWC480, MWC120, MWC158 and HD58647, all show systematic variations in the linear polarization amplitude and direction as a function of time and wavelength near the H-alpha line. In all our stars, the H-alpha line profiles show evidence of an intervening disk or outflowing wind, evidenced by strong emission with an absorptive component. The linear polarization varies by 0.2% to 1.5% with the change typically centered in the absorptive part of the line profile. These observations are inconsistent with a simple disk-scattering model or a depolarization model which produce polarization changes centered on the emmissive core. We speculate that polarized absorption via optical pumping of the intervening gas may be the cause.

astro-ph↗

arXiv2007

A New Mechanism for Polarizing Light from Obscured Stars

Recent spectropolarimetric observations of Herbig AeBe stellar systems show linear polarization variability with wavelength and epoch near their obscured H-alpha emission. Surprisingly, this polarization is not coincident with the H-alpha emission peak but is variable near the absorptive part of the line profile. With a new and novel model we show here that this is evidence of optical pumping - anisotropy of the incident radiation that leads to a linear polarization-dependent optical depth within the intervening hydrogen wind or disk cloud. This effect can yield a larger polarization signal than scattering polarization in these systems.

astro-ph↗

arXiv2007

First Direct Detection of Magnetic Fields in Starspots and Stellar Chromospheres

Here we report on the first detection of circular polarization in molecular lines formed in cool magnetic regions (starspots) and in chromospheric emission lines formed in hot plages on the surfaces of active stars. Our survey of G-K-M stars included young main-sequence dwarfs and RS CVn-type giants and subgiants. All stars were found to possess surface magnetic fields producing Stokes V LSD signals in atomic lines of 0.05 to 0.5%. Several stars clearly showed circular polarization in molecular lines of 0.1 to 1%. The molecular Stokes V signal is reminiscent of that observed in sunspots. Chromospheric magnetic fields were detected on most active targets in Stokes V profiles of emission lines with peak polarization up to 2%. The observed molecular circular polarization on M dwarfs indicates single-polarity magnetic fields covering at least 10% of the stellar disk. Smaller signals on K stars imply that their magnetic fields are apparently weaker, more entangled than on M dwarfs, or more diluted by the bright photosphere.

astro-ph↗