Anders M. Jorgensen

Sloan digital sky survey imaging of low galactic latitude fields: technical summary and data release

The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope have obtained five-band optical-wavelength imaging near the Galactic plane outside of the nominal survey boundaries. These additional data were obtained during commissioning and subsequent testing of the SDSS observing system, and they provide unique wide-area imaging data in regions of high obscuration and star formation, including numerous young stellar objects, Herbig-Haro objects, and young star clusters. Because these data are outside the survey regions in the Galactic caps, they are not part of the standard SDSS data releases. This paper presents imaging data for 832 square degrees of sky (including repeats), in the star-forming regions of Orion, Taurus, and Cygnus. About 470 deg2 are now released to the public, with the remainder to follow at the time of SDSS Data Release 4. The public data in Orion include the star-forming region NGC 2068/NGC 2071/HH 24 and a large part of Barnards loop.

Charge exchange contribution to the decay of the ring current, measured by energetic neutral atoms (ENAs)

In this paper we calculate the contribution of charge exchange to the decay of the ring current. Past works have suggested that charge exchange of ring current protons is primarily responsible for the decay of the ring current during the late recovery phase, but there is still much debate about the fast decay of the early recovery phase. We use energetic neutral atom (ENA) measurements from Polar to calculate the total ENA energy escape. To get the total ENA escape we apply a forward modeling technique, and to estimate the total ring current energy escape we use the Dessler-Parker-Sckopke relationship. We find that during the late recovery phase of the March 10, 1998 storm ENAs with energies greater than 17.5 keV can account for 75% of the estimated energy loss from the ring current. During the fast recovery the measured ENAs can only account for a small portion of the total energy loss. We also find that the lifetime of the trapped ions is significantly shorter during the fast recovery phase than during the late recovery phase, suggesting that different processes are operating during the two phases.

Navy Prototype Optical Interferometer Imaging of Line Emission Regions of beta Lyrae Using Differential Phase Referencing

We present the results of an experiment to image the interacting binary star β Lyrae with data from the Navy Prototype Optical Interferometer using a differential phase technique to correct for the effects of the instrument and atmosphere on the interferometer phases. We take advantage of the fact that the visual primary of β Lyrae and the visibility calibrator we used are both nearly unresolved and nearly centrally symmetric, and consequently have interferometric phases near zero. We used this property to correct for the effects of the instrument and atmosphere on the phases of β Lyrae and to obtain differential phases in the channel containing the Hα emission line. Combining the Hα-channel phases with information about the line strength, we recovered complex visibilities and imaged the Hα emission using standard radio interferometry methods. Our images show the position of the Hα-emitting regions relative to the continuum photocenter as a function of orbital phase, indicating a major axis line of nodes along Ω = 249° ± 4°. The orbit is smaller than previously predicted, a discrepancy that can be alleviated if we assume that the system is at a larger distance, or if the stellar continuum contribution to the Hα channel was underestimated. We do not detect a jet in the Hα images, which may be due to the limited resolution of the observations along the direction perpendicular to the orbital plane. We find that the differential phase results are consistent with those obtained from a more standard analysis using squared visibilities (V 2s) and closure phases, which also indicate an Hα disk radius of 0.6 ± 0.1 mas, and ΔV = 1.30 ± 0.1 and ΔR = 1.20 ± 0.1 mag for the magnitude difference between the stars.

A study of omega bands and Ps6 pulsations on the ground, at low altitude and at geostationary orbit

We investigate the electrodynamic coupling between auroral omega bands and the inner magnetosphere. The goal of this study is to determine the features to which omega bands map in the magnetosphere. To establish the auroral-magnetosphere connection, we appeal to the case study analysis of the data rich event of September 26, 1989. At 6 magnetic local time (MLT), two trains of Ps6 pulsations (ground magnetic signatures of omega bands) were observed to drift over the Canadian Auroral Network For the OPEN Program Unified Study (CANOPUS) chain. At the same time periodic ionospheric flow patterns moved through the collocated Bistatic Auroral Radar System (BARS) field of view. Similar coincident magnetic variations were observed by GOES 6, GOES 7 and SCATHA, all of which had magnetic foot points near the CANOPUS/BARS stations. SCATHA, which was located at 6 MLT, 0.5 R E earthward of GOES 7 observed the 10 min period pulsations, whereas GOES 7 did not. In addition, DMSP F6 and F8 were over-flying the region and observed characteristic precipitation and flow signatures. From this fortunate constellation of ground and space observations, we conclude that auroral omega bands are the electrodynamic signature of a corrugated current sheet (or some similar spatially localized magnetic structure) in the near-Earth geostationary magnetosphere.

Association of energetic neutral atom bursts and magnetospheric substorms

In this paper we present evidence that short-lived bursts of energetic neutral atoms (ENAs) observed with the Comprehensive Energetic Particle and Pitch Angle Distribution/Imaging Proton Spectrometer (CEPPAD/IPS) instrument on the Polar spacecraft are signatures of substorms. The IPS was designed primarily to measure ions in situ, with energies between 17.5 and 1500 keV. However, it has also proven to be a very capable ENA imager in the range 17.5 keV to a couple hundred keV. It was expected that some ENA signatures of the storm time ring current would be observed. Interestingly, IPS also routinely measures weaker, shorter-lived, and more spatially confined bursts of ENAs with duration from a few tens of minutes to a few hours and appearing once or twice a day. One of these bursts was quickly associated with magnetospheric and auroral substorm activity and has been reported in the literature [Henderson et al., 1997]. In this paper we characterize ENA bursts observed from Polar and establish statistically their association with classic substorm signatures (global auroral onsets, electron and ion injections, AL drops, and Pi2 onsets). We conclude that ∼90% of the observed ENA bursts are associated with classic substorms and thus represent a new type of substorm signature.

Improved Coherent Integration through Fringe Model Fitting

Maximizing the signal-to-noise ratio (SNR) of interferometric measurements of faint or low-visibility sources requires coherent integration. Coherent integration requires the use of a fringe-tracking method for determining the appropriate quantities to use for cophasing a large number of short low-SNR measurements. We present a method for tracking fringes during postprocessing. The method is based on fitting a time-dependent model of the fringe pattern to a sequence of data frames in order to determine the phase parameters for the central frame. This approach was chosen in accordance with the philosophy that optimal parameter estimation depends on a accurate model of the data. The model contains two parameters which must be chosen: the order of the time variation, and the number of simultaneous data frames to fit to. We show that this procedure results in a better SNR than a group-delay approach, particularly for low-NV2 data. In one case, the fringe-modeling approach improved coherent SNR by a factor of 2.6 over the group-delay approach. The fringe-modeling algorithm is tested on observations of the star ξ Bootis obtained from the Navy Prototype Optical Interferometer. We also offer suggestions for future improvements to the fringe-tracking model.

A Spectroscopic and Photometric Survey of Stars in the Field of L1457: A New Distance Determination

We present a spectroscopic and photometric survey of a sample of field stars in the region of the molecular cloud L1457. High-quality coud? feed spectra, together with five-band photometry in the Sloan Digital Sky Survey system and near-infrared archival data from the Two Micron All Sky Survey, are used to derive color excesses and distances for the stars. Based on these data, a new distance estimate of 360 ? 30 pc is derived for the cloud, supporting recent results by K. L. Luhman. The data further indicate that the north-south velocity gradient seen in the millimeter-wave CO data is mirrored in a distance gradient, with the northern part of the cloud being closer to us. A second, less opaque, layer of extinction is detected at ~80 pc. This distance is consistent with the earlier distance estimates to the cloud, based on Na I absorption. We identify this layer with the wall of the hot Local Bubble. Hence, the dense cloud is not, as previously thought, associated with the Local Bubble.

Coherent integration results from the NPOI

In this paper we will discuss the current status of coherent integration with the Navy Prototype Optical Interferometer (NPOI). Coherent integration relies on being able to phase reference interferometric measurements, which in turn relies on making measurements at multiple wavelengths. We first discuss the generalized group-delay approach, then the meaning of the resulting complex visibilities and then demonstrate how coherent integration can be used to perform very precision measurement of stellar diameters. The phase of the complex visibility is particularly attractive as a data product because it is not biased in the same way as visibility amplitudes. We discuss the relative SNR of triple-product phases and single-baseline phases. We then demonstrate how singlebaseline phases can be used to make accurate measurements of magnitude differences and separations of binary stars.

Vision: A Six-telescope Fiber-fed Visible Light Beam Combiner for the Navy Precision Optical Interferometer

Visible-light long baseline interferometry holds the promise of advancing a number of important applications in fundamental astronomy, including the direct measurement of the angular diameters and oblateness of stars, and the direct measurement of the orbits of binary and multiple star systems. To advance, the field of visible-light interferometry requires development of instruments capable of combining light from 15 baselines (6 telescopes) simultaneously. The Visible Imaging System for Interferometric Observations at NPOI (VISION) is a new visible light beam combiner for the Navy Precision Optical Interferometer (NPOI) that uses single-mode fibers to coherently combine light from up to six telescopes simultaneously with an image-plane combination scheme. It features a photometric camera for calibrations and spatial filtering from single-mode fibers with two Andor Ixon electron multiplying CCDs. This paper presents the VISION system, results of laboratory tests, and results of commissioning on-sky observations. A new set of corrections have been determined for the power spectrum and bispectrum by taking into account non-Gaussian statistics and read noise present in electron-multipying CCDs to enable measurement of visibilities and closure phases in the VISION post-processing pipeline. The post-processing pipeline has been verified via new on-sky observations of the O-type supergiant binary

Coherent integration: to real time or not to real time? That is the question.

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