John H. Lacy

Galactic center gasdynamics : a one-armed spiral in a Keplerian disk

The inner 3 × 4 pc of the Galaxy have been imaged in the [Ne II] (12.8 μm) line with 2″ and 30 km/s resolution. The morphology of much of the ionized gas can be explained by a one-armed linear (r ∞ θ) spiral. The kinematics of the gas along the spiral can be modeled by approximately circular orbits with a nearly Keplerian rotation curve, requiring a central mass of 2 ± 0.5 × 10 6 M ⊙ . Several posible origins are suggested for the spiral

A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-Infrared Bands of C2H2 and HCN

A spectral survey of IRC+10216 has been carried out in the range 11 to 14 � m with a spectral resolution of about 4 km s −1 . We have identified a forest of lines in six bands of C2H2 involving the vibrational states from the ground to 3ν5 and in two bands of HCN, involving the vibrational states from the ground up to 2ν2. Some of these transitions are observed also in H 13 CCH and H 13 CN. We have estimated the kinetic, vibrational, and rotational temperatures, and the abundances and column densities of C2H2 and HCN between 1 and 300 R∗ (≃ 1.5 × 10 16 cm) by fitting about 300 of these ro-vibrational lines. The envelope can be divided into three regions with approximate boundaries at 0 ′′ .019 (the stellar photosphere), 0 ′′ .1 (the inner dust formation zone), and 0 ′′ .4 (outer dust formation zone). Most of the lines might require a large microturbulence broadening. The derived abundances of C2H2 and HCN increase by factors of 10 and 4, respectively, from the innermost envelope outwards. The derived column densities for both C2H2 and HCN are ≃ 1.6 ×10 19 cm −2 . Vibrational states up to 3000 K above ground are populated, suggesting pumping by near-infrared radiation from the star and innermost envelope. Low rotational levels can be considered under LTE while those with J > 20 −30 are not thermalized. A few lines require special analysis to deal with effects like overlap with lines of other molecules. Subject headings: line: identification — line: profiles — surveys — stars: AGB and post-AGB — stars: carbon — stars: individual (IRC+10216)

Collision of comet Shoemaker-Levy 9 with Jupiter observed by the NASA infrared telescope facility

The National Aeronautics and Space Administration (NASA) Infrared Telescope Facility was used to investigate the collision of comet Shoemaker-Levy 9 with Jupiter from 12 July to 7 August 1994. Strong thermal infrared emission lasting several minutes was observed after the impacts of fragments C, G, and R. All impacts warmed the stratosphere and some the troposphere up to several degrees. The abundance of stratospheric ammonia increased by more than 50 times. Impact-related particles extended up to a level where the atmospheric pressure measured several millibars. The north polar near-infrared aurora brightened by nearly a factor of 5 a week after the impacts.

High-Frequency Measurements of the Spectrum of Sagittarius A*

We report near-simultaneous interferometric measurements of the spectrum of Sagittarius A* over the 5-354 GHz range and single-dish observations that have yielded the first detection of Sgr A* at 850 GHz. We confirm that Sgr A*s spectrum rises more steeply at short millimeter wavelengths than at centimeter wavelengths, leading to a near-millimeter/submillimeter excess that dominates its luminosity. Below 900 GHz, Sgr A*s observed luminosity is 70 ± 30 L. A new upper limit to Sgr A*s 24.3 μm flux, together with a compilation of other extant IR data, imply a far-infrared spectral turnover, which can result from either an intrinsic synchrotron cutoff or excess extinction near Sgr A*. If the former applies, Sgr A*s total synchrotron luminosity is <103 L, while in the latter case it is <3 × 104 L if spherical symmetry also applies.

Thermal spectroscopy of Neptune: The stratospheric temperature, hydrocarbon abundances, and isotopic ratios☆

Abstract Portions of Neptunes disk-averaged spectrum were observed from the NASA/Infrared Telescope Facility in 1989 and 1990 with a cryogenic echelle array spectrometer, with resolution of ν/Δν ∼ 104. A lower-resolution spectrum (Orton et al. 1990, Icarus 85, 257–265) was used in conjunction with the analysis of these spectra to provide a more reliable absolute intensity calibration. Together, these data imply a stratospheric temperature of 168 K near 1–10 μbar pressure. Considering the infrared data, Voyager UVS experiment, and Earth-based stellar occultation results, we adopt an uncertainty of ±10 K. The temperature profile adopted in this study is consistent with the size and shape of the H2 J = 3−1 quadrupole feature which was detected in emission. The maximum mixing ratios of CH4, C2H2, and C2H6 which are consistent with this temperature and its uncertainty are 7.5−5.6+18.6 × 10−4, 5.1−4.3+2.0 × 10−8, and 1.0−0.8+0.2 × 10−6, respectively. These C2H2 and C2H6 abundances are completely consistent with previous measurements, after correcting for different temperature profile assumptions. The combined measurements imply a CH3D/CH4 ratio of 3.6 ± 0.5 × 10−4, from which we deduce D/H = 1.13 ± 0.16 × 10−4; this is enhanced with respect to solar values and consistent with near-infrared high-resolution spectroscopy (deBergh et al. 1990, Astrophys. J. 355, 661–666). High-resolution measurements of 13C12CH6 and 12C2H6 imply that 12C/13C = 78 ± 26, consistent with solar and telluric values.

Discovery of interstellar acetylene

Absorption by interstellar acetylene (C2H2) has been detected in the spectra of three IR sources embedded in molecular clouds. One or more lines in the 13.7 micron nu5 vibration-rotation band were detected toward GL 2591, W3 IRS 5, and OMC-1 IRc2. The observations require C{sub 2}H{sub 2} abundances of about 0.0003 to 0.003 of CO. Such abundances can be explained by models in which the molecular clouds are chemically unevolved or in which gas phase abundances have been enhanced by recent evaporation of grain mantles. 17 refs.

Fabrication and evaluation of an etched infrared diffraction grating

We evaluated the optical performance of an IR echelle grating produced on a silicon wafer with anisotropic etching techniques. We measured the diffraction efficiency of a sample with a 55° blaze angle and 25-µm groove spacing. We also calculated the efficiency for typical triangular and trapezoidal groove profiles of etched gratings. The diffraction efficiency for unpolarized light can be approximately as high as the efficiency of right-angle groove gratings. The great potential of the etched silicon grating lies in its ease of fabrication, its excellent surface quality, and the high reproducibility of the production process. Compact high-resolution diffraction gratings can be produced by etching the grating pattern into the rear side of a transparent prism. When used in internal reflection, this increases the resolving power of the grating by a factor equal to the refractive index of the prism over a front surface grating of the same length.

Detection of low-J pure-rotational emission from H2 in the Orion Bar region : evidence for small-scale clumpiness

The paper presents the results of a mapping of a 10 x 16 arcsec area in the vicinity of the Orion Bar ionization front, in the v = 0 J = 3{minus}1 and J = 4{minus}2 emission lines of molecular hydrogen. This is the first detection of the J = 3{minus}1 line in the interstellar medium and the first detection of any pure-rotational H2 line in a photodissociation region. Since these lines are optically thin and thermalized, and the region is seen roughly edge-on, kinetic gas temperature and molecular column density distributions can be derived. When compared with models of photodissociation regions, the results support the scenario that the emission originates on the warm photodissociated surfaces of dense clumps of molecular gas embedded with a small volume filling factor in a low density cooler atomic medium. 23 refs.

HIGH-RESOLUTION MID-INFRARED SPECTROSCOPY OF NGC 7538 IRS 1: PROBING CHEMISTRY IN A MASSIVE YOUNG STELLAR OBJECT

We present high-resolution (R = 75,000-100,000) mid-infrared spectra of the high-mass embedded young star IRS 1 in the NGC 7538 star-forming region. Absorption lines from many rotational states of C{sub 2}H{sub 2}, {sup 13}C{sup 12}CH{sub 2}, CH{sub 3}, CH{sub 4}, NH{sub 3}, HCN, HNCO, and CS are seen. The gas temperature, column density, covering factor, line width, and Doppler shift for each molecule are derived. All molecules were fit with two velocity components between -54 and -63 km s{sup -1}. We find high column densities ({approx}10{sup 16} cm{sup -2}) for all the observed molecules compared to values previously reported and present new results for CH{sub 3} and HNCO. Several physical and chemical models are considered. The favored model involves a nearly edge-on disk around a massive star. Radiation from dust in the inner disk passes through the disk atmosphere, where large molecular column densities can produce the observed absorption line spectrum.

A gaseous tail ablated from the supergiant IRS 7 near the Galactic center

A 1″ resolution map of the [Ne II] 12.8 μm emission from the M supergiant IRS 7 near the Galactic center has revealed a 5″ long ionized «plume» or «tail» of gas leaving the star, in a direction opposite to the radio point source at the Galactic center. While the stellar envelope and tail are likely ionized by the ultraviolet radiation from the Galactic center, ram pressure is more likely to be responsible for the ablation of the supergiants mass outflow envelope. This ram pressure could arise in one of two sources: a wind from the vicinity of the Galactic center, or the drag caused by the passage of IRS 7 through an ambient medium