Luke David Keller

Panoramic Views of the Cygnus Loop

We present a complete atlas of the Cygnus Loop supernova remnant in the light of [O III] (λ5007), Hα, and [S II] (λλ6717, 6731). We include low-resolution (25) global maps and smaller fields at 6 resolution from observations using the Prime Focus Corrector on the 0.8 m telescope at McDonald Observatory. Despite its shell-like appearance, the Cygnus Loop is not a current example of a Sedov-Taylor blast wave. Rather, the optical emission traces interactions of the supernova blast wave with clumps of gas. The surrounding interstellar medium forms the walls of a cavity through which the blast wave now propagates, including a nearly complete shell in which nonradiative filaments are detected. We identify nonradiative shocks around half the perimeter of the Cygnus Loop, and they trace a circle of radius R = 14 (19 pc) in the spherical cavity walls. The Cygnus Loop blast wave is not breaking out of a dense cloud but is instead running into confining walls. Modification of the shock velocity and gas temperature due to interaction of the blast wave with the surrounding medium introduces errors in estimates of the age of this supernova remnant. The optical emission of radiative shocks arises only where the blast wave encounters inhomogeneities in the ambient medium; it is not a consequence of gradual evolution to a global radiative phase. Distance measurements that rely on this uniform blast wave evolution are uncertain, but the radiative shocks can be used as distance indicators because of the spherical symmetry of the surrounding medium. The interstellar medium dominates not only the appearance of the Cygnus Loop but also the continued evolution of the blast wave. If this is a typical example of a supernova remnant, then global models of the interstellar medium must account for such significant blast wave deceleration.

Fabrication and testing of chemically micromachined silicon echelle gratings

We have fabricated large, coarsely ruled, echelle patterns on silicon wafers by using photolithography and chemical-etching techniques. The grating patterns consist of 142-microm-wide, V-shaped grooves with an opening angle of 70.6 degrees, blazed at 54.7 degrees. We present a detailed description of our grating-fabrication techniques and the results of extensive testing. We have measured peak diffraction efficiencies of 70% at lambda = 632.8 nm and conclude that the gratings produced by our method are of sufficient quality for use in high-resolution spectrographs in the visible and near IR (lambda approximately = 500-5000 nm).

H2 emission as a tracer of molecular hydrogen: Large-scale observations of Orion

We have detected extremely extended (greater than 1.5 deg, or 12 pc) near-infrared H2 line emission from the Orion A molecular cloud. We have mapped emission in the 1.601 micrometer(s) upsilon = 6 - 4 Q(1) and 2.121 micrometer(s) upsilon = 1 - 0 S(1) lines of H2 along a approx. 2 deg R.A. cut and from a 6 x 6 region near theta(sup 1) Ori C. The surface brightness of the extended H2 line emission is 10(exp -6) to 10(exp -5) ergs/s/sq. cm/sr. Based on the distribution and relative strengths of the H2 lines, we conclude that UV fluorescene is most likely the dominant H2 emission mechanism in the outer parts of the Orion cloud. Shock-heated gas does not make a major contribution to the H2 emission in this region. The fluorescent component of the total H2 upsilon = 1 - 0 S(1) luminosity from Orion is 30-40 solar luminosity. Molecular hydrogen excited by UV radiation from nearby OB stars contributes 98%-99% of the global H2 line emission from the Orion molecular cloud, even though this cloud has a powerful shock-excited H2 source in its core. The ability to detect large-scale H2 directly opens up new possibilities for the study of molecular clouds.

Micromachined silicon diffraction gratings for infrared spectroscopy

Micromachined silicon gratings offer two great advantages to astronomical spectroscopy in the IR: (1) Photolithographic processing techniques permit the production of gratings with much larger groove constants than are possible with conventional wavelength coverage, despite the relatively small format of IR arrays. (2) One can use anisotropic etching to form gratings on dielectric wedges. By illuminating the grating through the dielectric, we can achieve higher spectral resolution for a given grating size or a smaller grating for a given desired resolution. We discuss the technical challenges involved in micromachining large grating grooves over large areas while holding positional accuracy to very tight tolerances. Manufacturing issues include material choices, surface preparation, and chemical and physical effects during processing. We also discuss our program for evaluation of the finished products, show result of measurements we have made on front-surface and immersion devices, and use these result to assess the potential of these devices for real-world astronomical applications.

NEAR-INFRARED MOLECULAR HYDROGEN EMISSION FROM THE CENTRAL REGIONS OF GALAXIES: REGULATED PHYSICAL CONDITIONS IN THE INTERSTELLAR MEDIUM

The central regions of many interacting and early-type spiral galaxies are actively forming stars. This process affects the physical and chemical properties of the local interstellar medium, as well as the evolution of the galaxies. We observed near-infrared H2 emission lines: v ¼ 1 0 S(1), 3–2 S(3), 1–0 S(0), and 2–1 S(1) from the central � 1 kpc regions of the archetypical starburst galaxies M82 and NGC 253 and the less dramatic but still vigorously star-forming galaxies NGC 6946 and IC 342. Like the far-infrared continuum luminosity, the nearinfrared H2 emission luminosity can directly trace the amount of star formation activity because the H2 emission lines arise from the interaction between hot and young stars and nearby neutral clouds. The observed H2 line ratios show that both thermal excitation and nonthermal excitation are responsible for the emission lines but that the great majority of the near-infrared H2 line emission in these galaxies arises from energy states excited by ultraviolet fluorescence. The derived physical conditions, e.g., far-ultraviolet radiation field and gas density, from [C ii ]a nd [O i] lines and far-infrared continuum observations when used as inputs to photodissociation models also explain the luminosity of the observed H2 1–0S(1) line. The ratio of the H2 1–0S(1) line to far-IR continuum luminosity is remarkably constant over a broad range of galaxy luminosities: LH2 =LFIR ’ 10 � 5 , in normal late-type galaxies (including the Galactic center), in nearby starburst galaxies, and in luminous IR galaxies (LIRGs: LFIR >10 11 L� ). Examining this constant ratio in the context of photodissociation region models, we conclude that it implies that the strength of the incident UV field on typical molecular clouds follows the gas density at the cloud surface. Subject headings: galaxies: individual(M82,NGC 253,NGC6946,IC 342) — galaxies: ISM — galaxies:spiral — galaxies: starburst — infrared: ISM — ISM: lines and bands

Design for a near-infrared immersion echelle spectrograph: breaking the R=100,000 barrier from 1.5 to 5 μm

We have designed a near IR spectrograph, sensitive in the 1.5-5 micrometers range, that uses a silicon immersion echelle grating. The cross-dispersed design demonstrates that immersion echelles allow compact spectrographs which have excellent spectral coverage and very high resolving power. Our instrument will have continuous spectral coverage over a 5.7 percent passband at 2.3 micrometers or a 7.6 percent passband at 4.6 micrometers and resolving power ranging from R equals 87,000 at 4.6 micrometers to 109,000 at 2.3 micrometers . We discuss design issues that are unique to spectrographs using silicon immersion echelle gratings in the near IR such as grating parameters, geometry, and the mechanical and thermal properties of large pieces of single crystal silicon.

H[TINF]2[/TINF] Emission from the Inner 400 Parsecs of the Galaxy

We have mapped the H2 v = 1 → 0 S(1) (λ = 2.1215 μm) emission line along a 400 pc-long strip and in a 50 pc region in the Galactic center. There is H2 emission throughout the surveyed region. The typical dereddened (AK = 2.5 mag) H2 v = 1 → 0 S(1) surface brightness, ~3 × 10-5 ergs s-1 cm-2 sr-1, is similar to the surface brightness in large-scale photon-dominated regions in the Galactic disk. We investigate two possible excitation mechanisms for the H2 emission, UV excitation by photons from OB stars and shock waves, and conclude that UV excitation is more likely. The total H2 v = 1 → 0 S(1) luminosity in the inner 400 pc region of the Galaxy is 8000 L☉. The ratio of the H2 to far-IR luminosity in the inner 400 pc of the Galaxy agrees with that in starburst galaxies and ultraluminous IR-bright galaxies.

H2 Emission from the Inner 400 Parsecs of the Galaxy

We have mapped the H2 v = 1 → 0 S(1) (λ = 2.1215 μm) emission line along a 400 pc-long strip and in a 50 pc region in the Galactic center. There is H2 emission throughout the surveyed region. The typical dereddened (AK = 2.5 mag) H2 v = 1 → 0 S(1) surface brightness, ~3 × 10-5 ergs s-1 cm-2 sr-1, is similar to the surface brightness in large-scale photon-dominated regions in the Galactic disk. We investigate two possible excitation mechanisms for the H2 emission, UV excitation by photons from OB stars and shock waves, and conclude that UV excitation is more likely. The total H2 v = 1 → 0 S(1) luminosity in the inner 400 pc region of the Galaxy is 8000 L☉. The ratio of the H2 to far-IR luminosity in the inner 400 pc of the Galaxy agrees with that in starburst galaxies and ultraluminous IR-bright galaxies.

SpitzerIRS Spectroscopy ofIRAS‐discovered Debris Disks

We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS) 5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60 μm excess. The spectra of five objects possess spectral features that are well-modeled using micron-sized grains and silicates with crystalline mass fractions 0%-80%, consistent with T Tauri and Herbig AeBe stars. With the exception of η Crv, these objects are young with ages ≤50 Myr. Our fits require the presence of a cool blackbody continuum, Tgr = 80-200 K, in addition to hot, amorphous, and crystalline silicates, Tgr = 290-600 K, suggesting that multiple parent body belts are present in some debris disks, analogous to the asteroid and Kuiper belts in our solar system. The spectra for the majority of objects are featureless, suggesting that the emitting grains probably have radii a > 10 μm. We have modeled the excess continua using a continuous disk with a uniform surface density distribution, expected if Poynting-Robertson and stellar wind drag are the dominant grain removal processes, and using a single-temperature blackbody, expected if the dust is located in a narrow ring around the star. The IRS spectra of many objects are better modeled with a single-temperature blackbody, suggesting that the disks possess inner holes. The distribution of grain temperatures, based on our blackbody fits, peaks at Tgr = 110-120 K. Since the timescale for ice sublimation of micron-sized grains with Tgr > 110 K is a fraction of a Myr, the lack of warmer material may be explained if the grains are icy. If planets dynamically clear the central portions of debris disks, then the frequency of planets around other stars is probably high. We estimate that the majority of debris disk systems possess parent body masses, MPB < 1 M⊕. The low inferred parent body masses suggest that planet formation is an efficient process.

Molecular Hydrogen Emission from the Inner 400 Parsecs of the Galaxy

We have mapped the H2 v = 1 → 0 S(1) (λ = 2.1215 μm) emission line along a 400 pc-long strip and in a 50 pc region in the Galactic center. There is H2 emission throughout the surveyed region. The typical dereddened (AK = 2.5 mag) H2 v = 1 → 0 S(1) surface brightness, ~3 × 10-5 ergs s-1 cm-2 sr-1, is similar to the surface brightness in large-scale photon-dominated regions in the Galactic disk. We investigate two possible excitation mechanisms for the H2 emission, UV excitation by photons from OB stars and shock waves, and conclude that UV excitation is more likely. The total H2 v = 1 → 0 S(1) luminosity in the inner 400 pc region of the Galaxy is 8000 L☉. The ratio of the H2 to far-IR luminosity in the inner 400 pc of the Galaxy agrees with that in starburst galaxies and ultraluminous IR-bright galaxies.