Engineering topics
Greenhouse, M. A.
Publications and source records attributed to Greenhouse, M. A..
Spectroscopy using the Hadamard Transform V2
The IRMOS (infrared multiobject spectrometer) is an imaging dispersive spectrometer, with a micromirror array to select desired objects. In standard operation, the mirrors are "opened" in patterns such that the resulting spectra do not overlap on the detector. The IRMOS can also be operated in a Hadamard mode, in which the spectra are allowed to overlap, but are modulated by opening the mirrors in many combinations. This mode enables the entire field of view to be observed with the same sensitivity as in the standard mode if the uncertainty is dominated by the detector read noise. We explain the concept and discuss the benefits with an example observation of the Orion Trapezium using the 2.1 m telescope at Kitt Peak National Observatory.
Milli-Arcsecond Infrared Observations of the Nova RS Ophiuchi
We report on infrared interferometric measurements of the 2006 outburst of Nova RS Oph in H, K, and N bands. These measurements, conducted using the Infrared and Optical Telescope Array, the Palomar Testbed Interferometer, and the Keck Interferometer, show a small variation in the 3 mas size if this resolved source during the first 65 days of the outburst. Additionally, using interferometric closure phases, these observations show an unambiguous departure from point symmetry of the object. We also describe simultaneous N-band observations of the nova in two separate milli-arcsecond spatial regimes on day four in which we detect line emission indicating onset of the optically thin phase. These data represent the first science from the Keck Interferometer's nulling mode.
High-Resolution N-Band Observations of the Nova RS Ophiuchi with the Keck Interferometer Nuller
We report new observations of the nova RS Ophiuchi (RS Oph) using the Keck Interferometer Nulling Instrument, approximately 3.8 days following the most recent outburst that occurred on 2006 February 12. The Keck Interferometer Nuller (KIN) operates in K-band from 8 to 12.5 pm in a nulling mode, which means that the central broad-band interference fringe is a dark fringe - with an angular width of 25 mas at mid band - rather than the bright fringe used ill a conventional optical interferometer. In this mode the stellar light itself is suppressed by the destructive fringe, effectively enhancing the contrast of the circumstellar material located near the star. By subsequently shifting the neighboring bright fringe onto the center of the source brightness distribution and integrating, a second spatial regime dominated by light from the central portion of the source is almost simultaneously sampled. The nulling technique is the sparse aperture equivalent of the conventional corongraphic technique used in filled aperture telescopes. By fitting the unique KIK inner and outer spatial regime data, we have obtained an angular size of the mid-infrared continuum of 6.2, 4.0. or 5.4 mas for a disk profile, gaussian profile (fwhm), and shell profile respectively. The data show evidence of enhanced neutral atomic hydrogen emission located in the inner spatial regime relative to the outer regime. There is also evidence of a 9.7 micron silicate feature seen outside of this region. Importantly, we see spectral lines excited by the nova flash in the outer region before the blast wave reaches these regions. These lines are from neutral, weakly excited atoms which support the following interpretation. We discuss the present results in terms of a unifying model of the system that includes an increase in density in the plane of the orbit of the two stars created by a spiral shock wave caused by the motion of the stars through the cool wind of the red giant star. These data show the power and potential of the nulling technique which has been developed for the detection of Earth-like planets around nearby stars for the Terrestrial Planet Finder Mission and Darwin missions.
Simulation of Aluminum Micro-mirrors for Space Applications at Cryogenic Temperatures
Closed form and finite element models are developed to predict the device response of aluminum electrostatic torsion micro-mirrors fabricated on silicon substrate for space applications at operating temperatures of 30K. Initially, closed form expressions for electrostatic pressure arid mechanical restoring torque are used to predict the pull-in and release voltages at room temperature. Subsequently, a detailed mechanical finite element model is developed to predict stresses and vertical beam deflection induced by the electrostatic and thermal loads. An incremental and iterative solution method is used in conjunction with the nonlinear finite element model and closed form electrostatic equations to solve. the coupled electro-thermo-mechanical problem. The simulation results are compared with experimental measurements at room temperature of fabricated micro-mirror devices.
A Near IR Fabry-Perot Interferometer for Wide Field, Low Resolution Hyperspectral Imaging on the Next Generation Space Telescope
We discuss work in progress on a near-infrared tunable bandpass filter for the Goddard baseline wide field camera concept of the Next Generation Space Telescope (NGST) Integrated Science Instrument Module (ISIM). This filter, the Demonstration Unit for Low Order Cryogenic Etalon (DULCE), is designed to demonstrate a high efficiency scanning Fabry-Perot etalon operating in interference orders 1 - 4 at 30K with a high stability DSP based servo control system. DULCE is currently the only available tunable filter for lower order cryogenic operation in the near infrared. In this application, scanning etalons will illuminate the focal plane arrays with a single order of interference to enable wide field lower resolution hyperspectral imaging over a wide range of redshifts. We discuss why tunable filters are an important instrument component in future space-based observatories.
Implications of the ISO LWS spectrum of the prototypical ultraluminous galaxy: ARP 220
The low resolution far infrared spectrum of the galaxy Arp 220, obtained with the low wavelength spectrometer (LWS) onboard the Infrared Space Observatory (ISO), is presented. The spectrum is dominated by the OH, H2O, CH, NH3 and O I absorption lines. The upper limits on the far infrared fine structure lines indicate a softer radiation in Arp 220 than in starburst galaxies.
Photoionization modeling of the LWS fine-structure lines in IR bright galaxies
The long wavelength spectrometer (LWS) fine structure line spectra from infrared luminous galaxies were modeled using stellar evolutionary synthesis models combined with photoionization and photodissociation region models. The calculations were carried out by using the computational code CLOUDY. Starburst and active galactic nuclei models are presented. The effects of dust in the ionized region are examined.
The temporal evolution of the 4-14 micron spectrum of V1974 Cygni (Nova Cygni 1992)
We present 4-14 micron spectra of the ONeMg nova V1974 Cygni (Nova Cygni 1992) obtained during 1992 May on the NASA Kuiper Airborne Observatory (KAO) and from the NASA 1.52 m infrared telescope at Mount Lemmon, Arizona in 1992 October/November with the HIFOGS mid-infrared spectrometer. The spectra at both epochs showed continuum emission from thermal bremstrahlung (free-free radiation) with emission lines from hydrogen and (Ne II), (Ar III), and Ne VI). During May, approximately = 80 days after outburst, the dominant emission lines in the mid-infrared spectra were a blend of three lines (Pf-alpha, Hu-beta, H11-7) near 7.5 microns and (Ne II) at 12.8 microns. By October (approximately 160 days later), the hydrogen emission had virtually disappeared, the (Ne II) 12.8 micron line had weakened considerably, and a pronounced (Ne VI) emission line had appeared at 7.6 microns. This behavior confirms that V1974 Cyg is similar to the prototypical slow ONeMg 'neon nova', Nova QU Vulpeculae (1984 No. 2). The remarkable evolution of the spectrum suggests that the ionization conditions changed drastically between 1992 May and 1992 October. We find that the ejecta of V1974 Cyg were overabundant in neon with respect to silicon by a factor of approximately 10 relative to the solar photosphere. We present new model calculations of infrared sodium forbidden line emission from (Na III) 7.319 microns (Na IV) 9.039 microns, and (Na IV) 21.29 microns that can be compared with recent model predictions of sodium synthesis in ONeMg nova outbursts. We conclude that sodium abundances in ONeMg novae can be determined by observations of infrared coronal lines of sodium that are accssible to the Infrared Space Observatory (ISO) and instruments at the NASA Infrared Telescope Facility (IRTF).
The peculiar infrared temporal development of Nova Vulpeculae 1987 (QV Vulpeculae)
The paper reports 1.25-19.5-micron IR photometry and optical/IR spectroscopy of Nova QV Vul (1987) from November 1987 through September 1989. The measurements show that an optically thick carbon dust shell formed within 83 d of the outburst, and that the spectral signatures of four types of astrophysical grains appeared at various times during a 2-yr period following the eruption. Carbon, SiC, and hydrocarbons formed first; oxygen-rich silicates formed later. It is suggested that the carbon dust components formed in fast-moving polar flumes, and that the silicates formed in a slow-moving equatorial ring. Mass estimates from the IR photometry and optical spectroscopy confirm that grain condensation in both the slow and fast ejecta of QV Vul is consistent with constraints established by previous observations of other dusty novae. It is concluded that the condensible elements in these grains were present in approximately solar abundance.
The (32)S/(33)S abundance as a function of galactocentric radius in the Milky Way
Astration of heavy elements by the stars of the Milky Way forms a fossil record which may preserve spacial distribution of the mass function for the stars in the galaxy. Sulfur is among the last common element for which the relative abundance of its various isotopes have yet to be completely measured within our galaxy. Explosive oxygen burning in massive stars is thought to be the process which dominates sulfur production within stars. There models predict that the various isotopes (S-32, S-33, S-34) are formed in relative abundance which depend strongly upon the mass of the parent star. This relative abundance is thought to be unaffected by subsequent stellar procesing since all important sinks of sulfur destroy it without regard for isotopic form. Hence the spacial variation of the mass function (MF) can be studied by measuring the abundance variation of sulfur isotopes in the galaxy provided that the product yields for these isotopes are known accurately as a function of stellar mass.