Robert L. Hurt

The Two Micron All Sky Survey (2MASS)

Between 1997 June and 2001 February the Two Micron All Sky Survey (2MASS) collected 25.4 Tbytes of raw imagingdatacovering99.998%ofthecelestialsphereinthenear-infraredJ(1.25 � m),H(1.65 � m),andKs(2.16 � m) bandpasses. Observations were conducted from two dedicated 1.3 m diameter telescopes located at Mount Hopkins, Arizona,andCerroTololo,Chile.The7.8sofintegrationtimeaccumulatedforeachpointontheskyandstrictquality control yielded a 10 � point-source detection level of better than 15.8, 15.1, and 14.3 mag at the J, H, and Ks bands, respectively, for virtually the entire sky. Bright source extractions have 1 � photometric uncertainty of <0.03 mag and astrometric accuracy of order 100 mas. Calibration offsets between any two points in the sky are <0.02 mag. The 2MASS All-Sky Data Release includes 4.1 million compressed FITS images covering the entire sky, 471 million source extractions in a Point Source Catalog, and 1.6 million objects identified as extended in an Extended Source Catalog.

The Anatomy of Star Formation in NGC 300

The Spitzer Space Telescope was used to study the mid- to far-infrared properties of NGC 300 and to compare dust emission to Hα to elucidate the heating of the interstellar medium (ISM) and the star formation cycle at scales smaller than 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 μm dust emission with respect to 24 μm dust and to H II regions traced by Hα light. The 8 μm emission highlights the rims of H II regions, and the 24 μm emission is more strongly peaked in star-forming regions than 8 μm. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 μm, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star-forming regions. When averaging over regions larger than ~1 kpc, the ratio of Hα to aromatic feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid- to far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.

Near- and mid-infrared photometry of the pleiades and a new list of substellar candidate members

We make use of new near- and mid-IR photometry of the Pleiades cluster in order to help identify proposed cluster members. We also use the new photometry with previously published photometry to define the single-star main-sequence locus at the age of the Pleiades in a variety of color-magnitude planes. The new near- and mid-IR photometry extend effectively 2 mag deeper than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new set of candidate very low-mass and substellar mass members of the Pleiades in the central square degree of the cluster. We identify 42 new candidate members fainter than K_s = 14 (corresponding to 0.1 M_☉). These candidate members should eventually allow a better estimate of the cluster mass function to be made down to of order 0.04 M_☉. We also use new IRAC data, in particular the images obtained at 8 μm, in order to comment briefly on interstellar dust in and near the Pleiades. We confirm, as expected, that—with one exception—a sample of low-mass stars recently identified as having 24 μm excesses due to debris disks do not have significant excesses at IRAC wavelengths. However, evidence is also presented that several of the Pleiades high-mass stars are found to be impacting with local condensations of the molecular cloud that is passing through the Pleiades at the current epoch.

Distributed circuit elements in impedance spectroscopy: A unified treatment of conductive and dielectric systems

Abstract Distributed elements have a long-established position in the field of impedance spectroscopy (IS). Presented here is a survey of a number of the more common elements, including the new distribution-of-activation-energies element. A unified treatment of the different IS levels (impedance, admittance, complex modulus, and complex dielectric constant) is proposed, enabling standard distributed elements to be used in the fitting of data taken from either conductive or dielectric materials. Various distributed elements are discussed and their responses are shown graphically and compared to one another in both 2-D complex plane projections and 3-D perspective plots.

(C-13)O in IC 342 - Evidence for a spiral density wave in the nucleus

We present high-resolution (5″, ∼40 pc) maps of the 13 CO emission in the nucleus of the nearby spiral galaxy IC 342. 13 CO emission, a tracer of the H 2 density distribution, indicates that the molecular gas in the nucleus takes the form of two spiral arms of total extent 500 pc. Molecular gas constitutes ∼10% of the total dynamical mass within the inner half-kiloparsec, increasing to ∼50% in the inner 100 pc

MOLECULAR GAS PROPERTIES OF THE STARBURST NUCLEUS OF IC 342: HIGH-RESOLUTION 13CO (2¨1) IMAGING

We present a map of the J = 2-1 transition of 13CO in the starburst nucleus of IC 342 made with the Owens Valley Millimeter Array. The interferometric 13CO map allows us to directly compare with 12CO (1-0), 12CO (2-1), and 13CO (1-0) maps of nearly identical (~45) resolution. While all four transitions show similar basic morphology, there are spatial differences between the 12CO and 13CO transitions that show up in ratio maps. In particular, the 13CO (2-1)/13CO (1-0) ratio has a markedly different distribution across the nuclear region than does 12CO (2-1)/12CO (1-0), indicating that 12CO and 13CO trace different components of the molecular gas. These differences are explained if 12CO traces the warm, photodissociation region skins of the clouds and 13CO traces the cooler, interior portions constituting the bulk of the molecular gas. We derive excitation temperatures for the bulk of the cloud mass of ~10-20 K and densities of ~103.7 cm-2 along the molecular minispiral.

PROTOSTARS IN THE ELEPHANT TRUNK NEBULA

The optically dark globule IC 1396A is revealed using Spitzer Space Telescope images at 3.6, 4.5, 5.8, 8, and 24 μm to be infrared-bright and to contain a set of previously unknown protostars. The mid-infrared colors of the 24 μm detected sources indicate several very young (Class I or 0) protostars and a dozen Class II stars. Three of the new sources (IC 1396A:γ, 1396A:δ, and 1396A:) emit over 90% of their bolometric luminosities at wavelengths greater than 3 μm, and they are located within ~0.02 pc of the ionization front at the edge of the globule. Many of the sources have spectra that are still rising at 24 μm. The two previously known young stars LkHα 349a and 349c are both detected, with component c harboring a massive disk and component a being bare. On the order of 5% of the mass of material in the globule is presently in the form of protostars in the 105-106 yr age range. This high star formation rate was likely triggered by radiation from a nearby O star.

A CO Map of the Dwarf Starburst Galaxy NGC 5253

We have mapped CO and 2.6 mm continuum emission in the dwarf galaxy NGC 5253 with the Owens Valley Millimeter Array. CO is detected along the prominent dust lane in the nucleus. The CO emission is extremely weak: NGC 5253 appears to be the most CO-poor starburst known. The CO flux appears to be low by factors of ~100 compared to star-forming regions of similar radio and infrared luminosity in large spiral galaxies and as much as a factor of 10 weaker than those in other metal-poor dwarf galaxies. This unusually weak CO emission, together with the observed morphology of the CO gas and its unusual kinematical properties, lead us to suggest that NGC 5253 has recently accreted gas of extremely low metallicity, significantly lower than that of the galaxy itself.

Aperture synthesis mapping of CO (2-1) in IC 342

We present aperture synthesis maps of CO J=2-1 emission in the central region of the spiral galaxy IC 342. The 3″ resolution maps reveal emission with implied excitation temperatures of 5-30 K. This emission is significantly brighter than the CO (1-0) emission mapped at the same resolution. Since the CO (2-1) emission is likely to be optically thick, the high ratio is probably due to the fact that the CO preferentially traces the outer, warmer cloud layers in externally heated clouds. The locations of the peak values of the CO (2-1)/CO (1-0) intensity ratio appear to occur at the interfaces between regions of high molecular column density and ionized gas

Analysis of dielectric or conductive system frequency response data using the Williams–Watts function

The electrical, optical, and mechanical behavior of many materials, particularly polymers and glasses, have been analyzed using the Kohlrausch–Williams–Watts stretched exponential relaxation function in both the time and frequency domains. This function is currently of considerable experimental and theoretical interest. Unfortunately, no relatively simple and accurate approximation representing the small‐signal frequency response of stretched exponential relaxation has been available. Thus it has been impractical to obtain accurate parameter estimates from fitting of frequency response data or to discriminate well between Williams–Watts response and that of other similar response models. Here we develop such an approximation for both dielectric systems and for intrinsically conducting ones (e.g., defect hopping materials). It is in complex form and allows fitting of both real and imaginary parts of all the data simultaneously (e.g., by complex nonlinear least squares) or of either part separately. For app...