David M. Harrington

THE SOLAR-SYSTEM-SCALE DISK AROUND AB AURIGAE

The young star AB Aurigae is surrounded by a complex combination of gas-rich and dust-dominated structures. The inner disk, which has not been studied previously at sufficient resolution and imaging dynamic range, seems to contain very little gas inside a radius of least 130 AU from the star. Using adaptive optics coronagraphy and polarimetry, we have imaged the dust in an annulus between 43 and 302 AU from the star, a region never seen before. An azimuthal gap in an annulus of dust at a radius of 102 AU, along with a clearing at closer radii inside this annulus, suggests the formation of at least one small body at an orbital distance of ~100 AU. This structure seems consistent with crude models of mean motion resonances or accumulation of material at two of the Lagrange points relative to the putative object and the star. We also report a low significance detection of a point source in this outer annulus of dust. This source may be an overdensity in the disk due to dust accreting onto an unseen companion. An alternate interpretation suggests that the objects mass is between 5 and 37 times the mass of Jupiter. The results have implications for circumstellar disk dynamics and planet formation.

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 Hi-Resolution Visible spectropolarimeter at a resolution of R = 13, 000 on the 3.67 m Advanced Electro-Optical System telescope. We also have many observations from the ESPaDOnS spectropolarimeter at a resolution of R = 68, 000 on the 3.6 m Canada-France-Hawaii Telescope. In roughly ~2/3 of the so-called windy or disky Herbig Ae/Be stars, the detected Hα 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 Hα 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, ten out of a sample of 30 show a typical broad depolarization effect but four of these ten show weaker effects only visible at high resolution. Another five 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.

Characterization of telescope polarization properties across the visible and near-infrared spectrum - Case study: the Dunn Solar Telescope

Accurate astrophysical polarimetry requires a proper characterization of the polarization properties of the telescope and instrumentation employed to obtain the observations. Determining the telescope and instrument Muller matrix is becoming increasingly difficult with the increase in aperture size, precision requirements and instrument complexity of new and upcoming projects. We have carried out a detailed multi-wavelength characterization of the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak as a case study and explore various possibilites for the determination of its polarimetric properties. We show that the telescope model proposed in this paper is more suitable than that in previous work in that it describes better the wavelength dependence of aluminum-coated mirrors. We explore the adequacy of the degrees of freedom allowed by the model using a novel mathematical formalism. Finally, we investigate the use of polarimeter calibration data taken at different times of the day to characterize the telescope and find that very valuable information on the telescope properties can be obtained in this manner. The results are also consistent with the entrance window polarizer measurements. This general method opens interesting possibilities for the calibration of future large-aperture telescopes and precision polarimetric instrumentation.

Eccentric binaries - Tidal flows and periastron events

Context. A number of binary systems present evidence of enhanced activity around periastron passage, suggesting a connection between tidal interactions and these periastron effects. Aims. 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. Here we focus on the tidal shear energy dissipation. Methods. We derive a mathematical expression for computing the rate of dissipation, u E, of the kinetic energy by the viscous flows that are driven by tidal interactions on the surface layer of a binary star. The method is tested by comparing the results from a grid of model calculations with the analytical predictions of Hut (1981, AA 2008, EAS Pub. Ser., 29, 67). Results. Our results for the dependence of the average (over orbital cycle) energy dissipation, u Eave, on orbital separation are consistent with those of Hut (1981) for model binaries with an orbital separation at periastron rper/R1 > 8, where R1 is the stellar radius. The model also reproduces the predicted pseudo-synchronization angular velocity for moderate eccentricities (e ≤ 0.3). In addition, for circular orbits our approach yields the same scaling of synchronization timescales with orbital separation as given by Zahn (1977, 2008) for convective envelopes. The computations give the distribution of u 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 u E over the surface changes significantly as a function of orbital phase, with small spatial structures appearing after periastron. An exploratory calculation for a highly eccentric binary system with parameters similar to those of δ Sco (e = 0.94, P = 3944.7 d) indicates that u Eave changes by ∼5 orders of magnitude over the 82 days before periastron, suggesting that the sudden and large amplitude variations in surface properties around periastron may, indeed, contribute toward the activity observed around this orbital phase.

Spectropolarimetry of the Hα line in herbig Ae/Be stars

Using the HiVIS spectropolarimeter built for the Haleakala 3.7 m 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 hr of observing. Our sample of five HAeBe stars, AB Aurigae, MWC 480, MWC 120, MWC 158, and HD 58647, all show systematic variations in the linear polarization amplitude and direction as a function of time and wavelength near the Hα line. In all our stars, the Hα 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%-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 that produce polarization changes centered on the emissive core. We speculate that polarized absorption via optical pumping of the intervening gas may be the cause.

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.

The New HiVIS Spectropolarimeter and Spectropolarimetric Calibration of the AEOS Telescope

We have designed, built, and calibrated a new spectropolarimeter for the HiVIS spectrograph (R ∼ 12,000-49,000) on the Advanced Electro-Optical System (AEOS) telescope. We have also conducted a polarization calibration of the telescope and instrument. In this paper, we introduce the design and use of the spectropolarimeter, in addition to a new data-reduction package we have developed, and then discuss the polarization calibration of the spectropolarimeter and the AEOS telescope. We used observations of unpolarized standard stars at many pointings to measure the telescope-induced polarization and compare it with a Zemax model. The telescope induces polarization of 1%-6%, with a strong variation with wavelength and pointing, consistent with the expected altitude and azimuth variation. We then used scattered sunlight as a linearly polarized source to measure the telescopes spectropolarimetric response to linearly polarized light. Finally, we made an all-sky map of the telescopes polarization response to calibrate future spectropolarimetry.

A New Mechanism for Polarizing Light from Obscured Stars

Recent spectropolarimetric observations of Herbig Ae/Be stellar systems show linear polarization variability with wavelength and epoch near their obscured Ha emission. Surprisingly, this polarization is not coincident with the Ha 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.

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 highresolution 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. Subject headings: techniques: polarimetric

Spectropolarimetric Observations of Herbig Ae/Be Stars. I. HiVIS Spectropolarimetric Calibration and Reduction Techniques

Using the High-Resolution Visible Spectrograph (HiVIS) spectropolarimeter built for the Haleakala 3.7 m Advanced Electro-optical System (AEOS) telescope in Hawaii, we are collecting a large number of high precision spectropolarimetric 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 HD 163296. Comparison of this and other HiVIS data with stellar observations from the Intermediate-dispersion Spectroscopic and Imaging System (ISIS) and William-Wehlau (WW) spectropolarimeters in the literature further shows the usefulness of this instrument.