Showing papers by "Charles H. Townes published in 2000"

The Berkeley Infrared Spatial Interferometer: A Heterodyne Stellar Interferometer for the Mid-Infrared

Abstract: A detailed description is given of the Infrared Spatial Interferometer (ISI), developed at the Space Sciences Laboratory of the University of California at Berkeley, which is a high spatial resolution interferometer for mid-infrared wavelengths. The instrumentation, its capabilities and performance, data analysis, science program, and future plans are all discussed. The system's use of heterodyne detection, analogous to that of a modern radio interferometer, is also compared with the homodyne or direct methods more commonly encountered in the visible and infrared. The ISI has been operating productively on Mount Wilson for the past 10 years measuring materials immediately surrounding stars and their changes as well as some stellar diameters. The new spectral capabilities described here, a recent increase in baseline length, and the upcoming expansion to a closure-phase imaging array provide important additional types of measurements.

Mid-Infrared Interferometry on Spectral Lines. III. Ammonia and Silane around IRC +10216 and VY Canis Majoris

Abstract: Using the University of California Berkeley Infrared Spatial Interferometer with a radio frequency (RF) filter bank, the first interferometric observations of mid-infrared molecular absorption features of ammonia (NH3) and silane (SiH4) with very high spectral resolution (λ/Δλ ~ 105) were made. Under the assumptions of spherical symmetry and uniform outflow, these new data permitted the molecular stratification around carbon star IRC +10216 and red supergiant VY CMa to be investigated. For IRC +10216, both ammonia and silane were found to form in the dusty outflow significantly beyond both the dust formation and gas acceleration zones. Specifically, ammonia was found to form before silane in a region of decaying gas turbulence (20R*), while the silane is produced in a region of relatively smooth gas flow much farther from the star (80R*). The depletion of gas-phase SiS onto grains soon after dust formation may fuel silane-producing reactions on the grain surfaces. For VY CMa, a combination of interferometric and spectral observations suggest that NH3 is forming near the termination of the gas acceleration phase in a region of high gas turbulence (~40R*).

Mid-infrared interferometry on spectral lines: III. Ammonia and Silane around IRC+10216 and VY CMa

Abstract: Using the U.C. Berkeley Infrared Spatial Interferometer with an RF filterbank, the first interferometric observations of mid-infrared molecular absorption features of ammonia (NH_3) and silane (SiH_4) with very high spectral resolution (R ~ 100000) were made. Under the assumptions of spherical symmetry and uniform outflow, these new data permitted the molecular stratification around carbon star IRC+10216 and red supergiant VY CMa to be investigated. For IRC+10216, both ammonia and silane were found to form in the dusty outflow significantly beyond both the dust formation and gas acceleration zones. Specifically, ammonia was found to form before silane in a region of decaying gas turbulence (>~ 20 R_star), while the silane is produced in a region of relatively smooth gas flow much further from the star (>~ 80 R_star). The depletion of gas-phase SiS onto grains soon after dust formation may fuel silane-producing reactions on the grain surfaces. For VY CMa, a combination of interferometric and spectral observations suggest that NH_3 is forming near the termination of the gas acceleration phase in a region of high gas turbulence (~ 40 R_star).

Mid-Infrared Interferometry on Spectral Lines. II. Continuum (Dust) Emission Around IRC +10216 and VY Canis Majoris

Abstract: The University of California Berkeley Infrared Spatial Interferometer has measured the mid-infrared visibilities of the carbon star IRC +10216 and the red supergiant VY CMa. The dust shells around these sources have been previously shown to be time variable, and these new data are used to probe the evolution of the dust shells on a decade timescale, complementing contemporaneous studies at other wavelengths. Self-consistent, spherically symmetric models at maximum and minimum light both show the inner radius of the IRC +10216 dust shell to be much larger (150 mas) than expected from the dust-condensation temperature, implying that dust production has slowed or stopped in recent years. Apparently, dust does not form every pulsational cycle (638 days), and these mid-infrared results are consistent with recent near-infrared imaging, which indicates little or no new dust production in the last 3 yr. Spherically symmetric models failed to fit recent VY CMa data, implying that emission from the inner dust shell is highly asymmetric and/or time variable.

Near- and mid-infrared subarcsecond structure of the dusty symbiotic star R Aquarii

Abstract: The results of a high-resolution interferometric campaign targeting the symbiotic long-period variable (LPV) R Aqr are reported. With both near-infrared measurements on baselines out to 10 m and mid-infrared data extending to 32 m, we have been able to measure the characteristic sizes of regions from the photosphere of the LPV and its extended molecular atmosphere out to the cooler circumstellar dust shell. The near-infrared data were taken using aperture-masking interferometry on the Keck I telescope and show R Aqr to be partially resolved for wavelengths out to 2.2 μm but with a marked enlargement, possibly due to molecular opacity, at 3.1 μm. Mid-infrared interferometric measurements were obtained with the Univeristy of California, Berkeley, infrared spatial interferometer (ISI) operating at 11.15 μm from 1992 to 1999. Although this data set is somewhat heterogeneous with incomplete coverage of the Fourier plane and sampling of the pulsation cycle, clear changes in the mid-infrared brightness distribution were observed, both as a function of position angle on the sky and as a function of pulsation phase. Spherically symmetric radiative transfer calculations of uniform-outflow dust shell models produce brightness distributions and spectra which partially explain the data; however, limitations to this approximation are noted. Evidence for significant deviation from circular symmetry was found in the mid-infrared and more tentatively at 3.08 μm in the near-infrared; however, no clear detection of binarity or of non-LPV elements in the symbiotic system is reported.

Precision Measurements of the Diameters of α Orionis and ο Ceti at 11 Microns

Abstract: The angular diameters of α Orionis and o Ceti were measured at a wavelength of 11.15 μm using the two-telescope Infrared Spatial Interferometer (ISI). Based on fitting the visibility data to uniform disk models, the diameter of α Orionis is 54.7 ± 0.3 mas and that of o Ceti at phase 0.90 is 47.8 ± 0.5 mas. These diameters are the most precise ever measured in the mid-infrared, due in part to the addition of a 56 m baseline to the ISI, which provided sufficient resolution to observe the first zero in the visibility function of the stellar disks. Moreover, the effects of limb darkening and stellar hot spots are small at these wavelengths. Theoretically estimated limb-darkening effects indicate the actual diameters are approximately 1% ± 0.5% larger than the uniform disk approximation, or 55.2 ± 0.5 and 48.2 ± 0.6 mas for α Orionis and o Ceti, respectively.

Mid-infrared interferometry on spectral lines: II. Continuum (dust) emission around IRC+10216 and VY CMa

Abstract: The U. C. Berkeley Infrared Spatial Interferometer has measured the mid-infrared visibilities of the carbon star IRC+10216 and the red supergiant VY CMa. The dust shells around these sources have been previously shown to be time-variable, and these new data are used to probe the evolution of the dust shells on a decade time-scale, complementing contemporaneous studies at other wavelengths. Self-consistent, spherically-symmetric models at maximum and minimum light both show the inner radius of the IRC+10216 dust shell to be much larger (150 mas) than that expected from the dust condensation temperature, implying that dust production has slowed or stopped in recent years. Apparently, dust does not form every pulsational cycle (638 days), and these mid-infrared results are consistent with recent near-IR imaging which indicates little or no new dust production in the last three years (Tuthill et al 2000). Spherically symmetric models failed to fit recent VY CMa data, implying that emission from the inner dust shell is highly asymmetric and/or time-variable.

Near and mid-IR sub-arcsecond structure of the dusty symbiotic star R Aqr

Abstract: The results of a high-resolution interferometric campaign targeting the symbiotic long-period variable (LPV) R~Aqr are reported. With both near-infrared measurements on baselines out to 10m and mid-infrared data extending to 32m, we have been able to measure the characteristic sizes of regions from the photosphere of the LPV and its extended molecular atmosphere, out to the cooler circumstellar dust shell. The near-infrared data were taken using aperture masking interferometry on the Keck-I telescope and show R~Aqr to be partially resolved for wavelengths out to 2.2 microns but with a marked enlargement, possibly due to molecular opacity, at 3.1 microns. Mid-infrared interferometric measurements were obtained with the U.C. Berkeley Infrared Spatial Interferometer (ISI) operating at 11.15 microns from 1992 to 1999. Although this dataset is somewhat heterogeneous with incomplete coverage of the Fourier plane and sampling of the pulsation cycle, clear changes in the mid-infrared brightness distribution were observed, both as a function of position angle on the sky and as a function of pulsation phase. Spherically symmetric radiative transfer calculations of uniform-outflow dust shell models produce brightness distributions and spectra which partially explain the data, however limitations to this approximation are noted. Evidence for significant deviation from circular symmetry was found in the mid-infrared and more tentatively at 3.08 microns in the near-infrared, however no clear detection of binarity or of non-LPV elements in the symbiotic system is reported.

Interferometric Observations of IRC +10011 and IRC +10420 in the Mid-Infrared

Abstract: The 11 km fringe visibilities of IRC )10011 (\ CIT3) and IRC )10420 have been measured at base- lines ranging from 2 to 14 m with a two-telescope interferometer. The visibility data provide one- dimensional pro—les of these two stars and their immediate surroundings with higher resolution in this spectral range than is presently possible with a single-aperture telescope. Spherically symmetric radiative transfer models suggest that IRC )10011 is surrounded by a dust shell with an inner radius subtending which falls oU in density as r~1.5, slightly slower than the r~2 expected for constant dust out—ow. 0A.033, IRC )10420 appears to have a dust shell of inner radius the density of which drops very slowly 0A.12, compared with that in a constant out—ow. This implies decreasing out—ow or a signi—cant emission in the past. Subject headings: circumstellar matterinfrared: starsstars: AGB and post-AGB ¨ stars: individual (IRC )10011, IRC )10420) ¨ techniques: interferometric

Mid-Infrared Interferometry on Spectral Lines. I. Instrumentation

Abstract: The Infrared Spatial Interferometer at University of California, Berkeley has been outfitted with a filter-bank system to allow interferometric observations of mid-infrared spectral lines with very high spectral resolution (λ/Δλ ~ 105). This paper describes the design, implementation, and performance of the matched 32 channel filter-bank modules, and new spectral-line observations of Mars and IRC +10216 are used to demonstrate their scientific capability. In addition, observing strategies are discussed for accurate calibration of fringe visibilities in spectral lines, despite strong atmospheric fluctuations encountered in the infrared.

Combining high spectral and spatial resolution in the mid-infrared

Abstract: A novel, fast-switching filterbank system has been incorporated into the Infrared Spatial Interferometer to allow high spectral resolution observations of mid-infrared spectral lines, while precisely calibrating atmospheric fluctuations. This system allows the complex visibility in a spectral line to be measured relative to the continuum with high precision, about 1% in visibility amplitude and a few degrees in visibility phase. The high spectral resolving power (R approximately 100000) and spatial resolution (1/10 arcsecond) allow the formation radii of various polyatomic molecules (e.g., ammonia and silane) to be directly measured. Initial results for carbon star IRC + 10216 and red supergiant VY CMa are presented.