Showing papers by "Michael W. Werner published in 2002"

Spatially Resolved Observations of the Galactic Center Source IRS 21

Abstract: We present diffraction-limited 2-25 μm images obtained with the W. M. Keck 10 m telescopes that spatially resolve the cool source IRS 21, one of a small group of enigmatic objects in the central parsec of our Galaxy that have eluded classification. Modeled as a Gaussian, the azimuthally averaged intensity profile of IRS 21 has a half-width at half-maximum (HWHM) size of 650 ± 80 AU at 2.2 μm and an average HWHM size of 1600 ± 200 AU at mid-infrared wavelengths. These large apparent sizes imply an extended distribution of dust. The mid-infrared color map indicates that IRS 21 is a self-luminous source rather than an externally heated dust clump as originally suggested. The spectral energy distribution has distinct near- and mid-infrared components. A simple radiative transfer code, which simultaneously fits the near- and mid-infrared photometry and intensity profiles, supports a model in which the near-infrared radiation is scattered and extincted light from an embedded central source, while the mid-infrared emission is from thermally reradiating silicate dust. We argue that IRS 21 (and by analogy, the other luminous sources along the Northern Arm) is a massive star experiencing rapid mass loss and plowing through the Northern Arm, thereby generating a bow shock, which is spatially resolved in our observations.

Mid-infrared images of the debris disk around HD 141569

Abstract: We have imaged the circumstellar debris disk around the A0 Ve/B95 Ve star HD 141569 (D = 99 pc), at λ = 125, 179, and 208 μm using the Keck II telescope, and confirm the general morphology from other recently published mid-infrared observations Model fits to an assumed flat radially symmetric dust disk yield an inclination i = 53° ± 5° and a position angle of -6° ± 4° for the tilt axis and indicate the presence of a depression in optical depth by a factor of about 4 within a radial distance of 30 AU (03) from the star Such a depression is suggestive of a density depletion and is consistent with published spectral energy distributions that indicate the absence of a near-infrared excess even though a mid-infrared excess is present Our mid-infrared results, in conjunction with previously published near-infrared scattering images, suggest that the two wavelength regimes are viewing, respectively, the inner and outer parts of a common disk structure, which is dominated by small (probably submicron) grains throughout

Large format long-wavelength GaAs/AlGaAs multi-quantum well infrared detector arrays for astronomy

Abstract: A long-wavelength large format quantum well infrared photodetector (QWIP) focal plane array has been successfully used in a ground based astronomy experiment. QWIP arrays afford greater flexibility than the usual extrinsically doped semiconductor infrared (IR) arrays. The wavelength of the peak response and cutoff can be continuously tailored over a range wide enough to enable light detection at any wavelength range between 6-20 /spl mu/m. The spectral band width of these detectors can be tuned from narrow (/spl Delta//spl lambda///spl lambda/ /spl sim/ 10%) to wide (/spl Delta//spl lambda///spl lambda/ /spl sim/ 40 %) allowing various applications. Also, QWIP device parameters can be optimized to achieve extremely high performances at lower operating temperatures (/spl sim/ 30 K) due to exponential suppression of dark current. Furthermore, QWIPs offer low cost per pixel and highly uniform large format focal plane arrays (FPAs) mainly due to mature GaAs/AlGaAs growth and processing technologies. The other advantages of GaAs/AlGaAs based QWIPs are higher yield, lower 1/f noise and radiation hardness. Recently, we operated an infrared camera with a 256/spl times/256 QWIP array sensitive at 8.5 /spl mu/m at the prime focus of the 5 m Hale telescope, obtaining the images. The remarkable noise stability - and low 1/f noise - of QWIP focal plane arrays enable cameras to operate by modulating the optical signal with a nod period up to 100 s. A 500 s observation on dark sky renders a flat image with little indication of the low spatial frequency structures associated with imperfect sky subtraction or detector drifts.

High-Resolution Mid-Infrared Observations of Very Young Stellar Objects in NGC 1333

Abstract: We observed 22 young stellar objects in the region of NGC 1333 by using the mid-infrared camera MIRLIN. NGC 1333 (in the Perseus OB2 molecular cloud complex) is a relatively well studied region, but not at high spatial resolution in the mid-infrared. MIRLIN's 05 spatial resolution allows us to look for source extension and multiplicity and to place new constraints on spectral energy distributions. We report here new detections of eight objects at mid-IR wavelengths. We find one object, SVS 12, that may be extended or multiple, and we confirm multiplicity in SVS 16. We find a new companion to ASR 107. We are able to classify six objects as Class I, flat spectrum, or II, place strong classification constraints on two objects, and more loosely restrict the classification of eight more objects. These observations will aid in interpretation of planned SIRTF observations of this cluster.

GaAs/AlGaAs based multiquantum-well infrared detector arrays for low-background applications

Abstract: A long-wavelength large format Quantum Well Infrared Photodetector (QWIP) focal plane array has been successfully used in a ground based astronomy experiment. QWIP arrays afford greater flexibility than the usual extrinsically doped semiconductor infrared (IR) arrays. Recently, we operated an infrared camera with a 256x256 QWIP array sensitive at 8.5 μm at the prime focus of the 5-m Hale telescope, obtaining the images. The remarkable noise stability - and low 1/f noise - of QWIP focal plane arrays enable camera to operate by modulating the optical signal with a nod period up to 100 s. A 500 s observation on dark sky renders a flat image with little indication of the low spatial frequency structures associated with imperfect sky substration or detector drifts. At low operating temperatures for low-background irradiance levels, high resistivity of thick barriers in the active region of QWIPs impeded electrons from entering the detector from the opposite electrode. This could lead to a delay in refilling the space-charge buildup, and result in a lower responsitivity at high optical modulation frequencies. In order to overcome this problem we have designed a new detector structure, the blocked intersubband detector (BID) with separate active quantum well region and blocking barrier.

Spatially Resolved Observations of the Galactic Center Source, IRS 21

Abstract: We present diffraction-limited 2-25 micron images obtained with the W. M. Keck 10-m telescopes that spatially resolve the cool source, IRS 21, one of a small group of enigmatic objects in the central parsec of our Galaxy that have eluded classification. Modeled as a Gaussian, the azimuthally-averaged intensity profile of IRS 21 has a half-width half-maximum (HWHM) size of 650+/-80 AU at 2.2 microns and an average HWHM size of 1600+/-200 AU at mid-infrared wavelengths. These large apparent sizes imply an extended distribution of dust. The mid-infrared color map indicates that IRS 21 is a self-luminous source rather than an externally heated dust clump as originally suggested. The spectral energy distribution has distinct near- and mid-infrared components. A simple radiative transfer code, which simultaneously fits the near- and mid- infrared photometry and intensity profiles, supports a model in which the near-infrared radiation is scattered and extincted light from an embedded central source, while the mid-infrared emission is from thermally re-radiating silicate dust. We argue that IRS 21 (and by analogy the other luminous sources along the Northern Arm) is a massive star experiencing rapid mass loss and plowing through the Northern Arm, thereby generating a bow shock, which is spatially resolved in our observations.