Showing papers by "Charles H. Townes published in 1997"

[C II] 158 Micron Observations of IC 10: Evidence for Hidden Molecular Hydrogen in Irregular Galaxies

Abstract: We have mapped the [C II] 158 μm line over 85 × 65 in the Magellanic irregular galaxy IC 10, thus presenting the first complete [C II] map of an entire low-metallicity galaxy. The total luminosity in the [C II] line in IC 10 is 1.5 × 106 L☉. We discuss the origin of the [C II] emission toward different regions in the galaxy. Overall, about 10% of the [C II] emission can originate in standard H I clouds (n ~ 80, T ~ 100 K), while up to about 10% of the emission can originate in ionized gas, either the low-density warm gas or the denser H II regions. For the two brightest regions, most of the [C II] emission is associated with dense photodissociation regions (PDRs). For several regions, however, the [C II] emission may not be explained by standard PDR models. For these regions, emission solely from the atomic medium can also be precluded because the cooling rate per hydrogen atom would be much greater than the heating rate provided by photoelectric UV heating. We speculate that in these regions the presence of an additional column density of H2, 5 times that observed in H I, is required to explain the [C II] emission. The ambient UV fields present in these regions, combined with the low metallicity, create a situation where small CO cores exist surrounded by a relatively large [C II]-emitting envelope where molecular hydrogen is self-shielded. This additional molecular mass is equivalent to at least 100 times the mass in the CO core that one would derive from the CO integrated intensity alone using the standard CO-to-H2 conversion factor. These [C II] observations may, therefore, make a more reliable inventory of the gas reservoir in dwarf irregular galaxies where use of CO alone may significantly underestimate the molecular mass.

NONSPHERICAL STRUCTURES AND TEMPORAL VARIATIONS IN THE DUST SHELL OF o CETI OBSERVED WITH A LONG BASELINE INTERFEROMETER AT 11 MICRONS

Abstract: Visibility observations at 11 km of o Ceti have been made with the University of California (Berkeley) Infrared Spatial Interferometer during the time period 1988E1995. The observed visibilities change dra- matically from one epoch to another and are not consistent with simple heating or cooling of the dust with change in luminosity as a function of stellar phase. Instead, large temporal variations in the density of dust within a few stellar radii of the photosphere of o Ceti have occurred. Spherically symmetric models of the dust distribution with two dust shells, one within three stellar radii of the photosphere of the star, the other approximately 10 stellar radii from the star, can account reasonably well for the observed changes. Four types of axially symmetric radiative transfer models were also compared with the dataEa spherical shell with an ellipsoidal inner cavity, a disk, a spherical shell with one or two inhomogeneities or clumps, and a set of thin partial shells with a -xed distance between them. Of the four models, only the one with the ellipsoidal inner cavity is excluded. The data were best--tted with the last two models, which emphasize inhomogeneities or clumps. To -t the observed temporal changes in the visibility data, all models must include a change in the densityEincreasing and decreasingEof dust close to the photosphere of the star. The axially symmetric models had clumps placed at distances from the star in agreement with distances of the spherical models. Good -ts to the observed broadband spec- trum of the star were also obtained with these models. Subject headings: circumstellar matter E infrared: stars E stars: individual (o Ceti) E stars: mass loss E stars: variables: long-period variables E techniques: interferometric

The Orion Molecular Clouds OMC-1 and OMC-2 Mapped in the Far-Infrared Fine-Structure Line Emission of C+ and O0

Abstract: We have mapped the Orion Molecular Ridge (OMC-1 and OMC-2) in the fine-structure lines [C II] 158 μm and [O I] 63 and 146 μm. The maps cover a region of 7' × 18' (Δα × Δδ) and are fully sampled toward the Orion A H II region/molecular cloud interface. The emission maximum near the molecular region S6/FIR4 arises most probably from a photodissociation region (PDR) on the surface of that molecular condensation. The PDR has column densities that are most likely explained by an edge-on geometry. For the emitting material, we find a temperature between 300 and 500 K, a number density of 3 × 105 cm-3, and O0 and C+ column densities of 1 × 1019 and 4 × 1018 cm-2, respectively. In the Orion Bar region, the northern edge of the Orion interface region, and M43 west we find somewhat lower values for the temperature, number density, and column densities. Furthermore, the ratio of the oxygen to carbon column density is lower in this region and approaches a value of 1.7. The lower ratio could be due to the edge-on geometry and the lower temperature of these regions. In the Dark Lane region we find evidence for cooler oxygen partly absorbing the oxygen line emission from the warmer background material. For the cooler molecular material covering the warm medium in the Dark Lane, we estimate an atomic oxygen abundance [O]/[H] on the order of a few 10-4. In the northern part of OMC-1 the morphology of the [O I] 63 μm emission is consistent with moderately warm oxygen associated with the dense molecular ridge. From the oxygen emission arising from the ridge, we derive an atomic oxygen abundance [O]/[H] > 10-5. The [O I] line emission in OMC-2 may arise partly from the inner parts of the molecular cloud, but emission arising exclusively from PDRs cannot be excluded.

Nonuniform Dust Outflow Observed around Infrared Object NML Cygni

Abstract: Measurements by the University of California Berkeley Infrared Spatial Interferometer at 11.15 μm have yielded strong evidence for multiple dust shells and/or significant asymmetric dust emission around NML Cyg. New observations reported also include multiple 8-13 μm spectra taken from 1994-1995 and N-band (10.2 μm) photometry from 1980-1992. These and past measurements are analyzed and fitted to a model of the dust distribution around NML Cyg. No spherically symmetric single dust shell model is found consistent with both near- and mid-infrared observations. However, a circularly symmetric maximum entropy reconstruction of the 11 μm brightness distribution suggests a double-shell model for the dust distribution. Such a model, consisting of a geometrically thin shell of intermediate optical depth (τ11 μm ~ 1.9) plus an outer shell (τ11 μm ~ 0.33), is consistent not only with the 11 μm visibility data but also with near-infrared speckle measurements, the broadband spectrum, and the 9.7 μm silicate feature. The outer shell, or large-scale structure, is revealed only by long-baseline interferometry at 11 μm, being too cold (~400 K) to contribute in the near-infrared and having no unambiguous spectral signature in the mid-infrared. The optical constants of Ossenkopf, Henning, & Mathis proved superior to the Draine & Lee (1984) constants in fitting the detailed shape of the silicate feature and broadband spectrum for this object. Recent observations of H2O maser emission around NML Cyg by Richards, Yates, & Cohen (1996) are consistent with the location of the two dust shells and provide further evidence for the two-shell model.

Multiple dust shells and motions around IK Tauri as seen by infrared interferometry

Abstract: A visibility curve of IK Tau has been measured with the ISI, an 11 μm stellar interferometer, over a period of several years. Time variations in 11 μm flux were also measured. The results indicate an approximately periodic distribution of dust shells around the star, with shells separated by 200-250 mas and a diameter of about 200 mas for the innermost shell. Some shell motion has been detected, and if velocities are the same as those measured for CO gas and OH masers, the motion implies that the distance to IK Tau is about 265 pc and that shells have been emitted at times separated by about 12 yr, which is considerably longer than the star's luminosity period of 470 days.

Non-Uniform Outflow Observed around Infrared Object NML Cygni

Abstract: Measurements by the U.C. Berkeley Infrared Spatial Interferometer at 11.15 micron have yielded strong evidence for multiple dust shells and/or significant asymmetric dust emission around NML Cyg. New observations reported also include multiple 8-13 micron spectra taken from 1994-1995 and N band (10.2 micron) photometry from 1980-1992. These and past measurements are analyzed and fitted to a model of the dust distribution around NML Cyg. No spherically symmetric single dust shell model is found consistent with both near- and mid-infrared observations. However, a circularly symmetric maximum entropy reconstruction of the 11 micron brightness distribution suggests a double shell model for the dust distribution. Such a model, consisting of a geometrically thin shell of intermediate optical depth ($\tau_{11 micron} \sim 1.9$) plus an outer shell ($\tau_{11 micron} \sim 0.33$), is consistent not only with the 11 micron visibility data, but also with near-infrared speckle measurements, the broadband spectrum, and the 9.7 micron silicate feature. The outer shell, or large scale structure, is revealed only by long-baseline interferometry at 11 micron, being too cold ($\sim$ 400 K) to contribute in the near-infrared and having no unambiguous spectral signature in the mid-infrared. The optical constants of Ossenkopf, Henning, & Mathis (1992) proved superior to the Draine & Lee (1984) constants in fitting the detailed shape of the silicate feature and broadband spectrum for this object. Recent observations of H$_2$O maser emission around NML Cyg by Richards, Yates, & Cohen (1996) are consistent with the location of the two dust shells and provide further evidence for the two-shell model.

Astronomical masers and lasers

Abstract: A brief account is given of the discovery of the astronomical maser and laser effects in OH radicals and in molecules of water (H2O), carbon monoxide and dioxide (CO and CO2), ammonia (NH3), methyl alcohol (CH3OH), formaldehyde (CH2O), and silicon oxide (SiO). A detailed table is given of all the currently known molecular stimulated-emission lines.

Study of Late Type Stars with the Infrared Spatial Interferometer of the University of California at Berkeley

Abstract: The Infrared Spatial Interferometer (ISI) of the University of California at Berkeley is a two telescopes interferometer operating in the infrared (9–12 microns). Many late type stars and their circumstellar dust shell have been observed and modelled. Several results have been obtained: location of the inner radii of the dust shell, physical conditions in the dust formation zone, optical depth of the shell, diameters of some evolved stars, location of SiO masers by regard to the dust (Danchi et al. 1994a, Greenhill et al. 1995).