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A panel discussion: Nine specialists foresee an expanded frequency range, new kinds of lasers, and many novel applications.

The fields of optics and photonics have experienced dramatic technical advances over the past several decades and have cemented themselves as key enabling technologies across many different industries. This paper explores past milestones, present state of the art, and future perspectives of several different topics, including: lasers, materials, devices, communications, bioimaging, displays, manufacturing, and industry evolution.

Dust shells surrounding the star W Hydrae have been resolved and measured at a wavelength of 11.15 ?m, using the Infrared Spatial Interferometer. Two different models for the star and dust shells are used to fit the data, one for recent data taken over short time spans and another for the combined data of 15 years. Modeling of the data shows the presence of two dust shells with diameters of approximately 100 mas and 250 mas. The inner dust shell is seen to expand over the year 2008. The observations are consistent with a stellar diameter of 50 mas.

We present results from a spectro-interferometric study of the Miras o Cet, R Leo and W Hya obtained with the Keck Aperture Masking Experiment from 1998 Sep to 2002 Jul. The spectrally dispersed visibility data permit fitting with circularly symmetric brightness profiles such as a simple uniform disk. The stellar angular diameter obtained over up to ~ 450 spectral channels spaning the region 1.1-3.8 microns is presented. Use of a simple uniform disk brightness model facilitates comparison between epochs and with existing data and theoretical models. Strong size variations with wavelength were recorded for all stars, probing zones of H2O, CO, OH, and dust formation. Comparison with contemporaneous spectra extracted from our data show a strong anti-correlation between the observed angular diameter and flux. These variations consolidate the notion of a complex stellar atmosphere consisting of molecular shells with time-dependent densities and temperatures. Our findings are compared with existing data and pulsation models. The models were found to reproduce the functional form of the wavelength vs. angular diameter curve well, although some departures are noted in the 2.8-3.5 micron range.

The sizes and shapes of the stars o Ceti and R Leonis have been measured in the mid-infrared. The observations were made using the UC Berkeley Infrared Spatial Interferometer (ISI), and they reveal details about the size, shape and asymmetry of both stars over several epochs in 2006. The star o Ceti appears to be rather symmetric, while the shape of R Leonis appears more consistent with a uniform disk plus a point source that provides approximately 9% additional intensity somewhere in the southern half of the star.

Asymmetries and motions in the dust shell surrounding Mira A (o Ceti) are reported. Measurements were taken with the UC Berkeley Infrared Spatial Interferometer (ISI), a three-element interferometer operating at 11.15 μm. At the time of these observations, it was in a linear, east-west configuration with a maximum baseline of 12 m and thus had a resolution of better than 100 mas. Three years of data (2003-2005) are presented and permit observation of the movement of dust shells over time. Fits are made to the visibility and closure-phase curves, which are then used to create one-dimensional profiles of the dust shells. Asymmetries in the circumstellar dust have been observed, and several possible explanations for these asymmetries are explored.

Three-telescope interferometric observations from the Infrared Spatial Interferometer (ISI) are reported at spatial frequencies that resolve the size and shape of the star Betelgeuse (α Ori) at a wavelength of 11.15 μm with a bandwidth of 0.18 cm-1. The data include closure phase measurements, the first such measurements of a stellar disk in the mid-infrared. The data indicate a clear asymmetry of the stellar surface at these wavelengths. Mechanisms for generating the observed asymmetry via an elliptical shape or a hot spot near the southern limb of the star are discussed.

Measurements and analysis of large-scale turbulent irregularities in the atmosphere are reported. Results were obtained from precision measurement of atmospheric temperature fluctuations at 24 and 70 feet above ground for a variety of wind velocities at the Mount Wilson Observatory. Measurements were made as wind blew air past detectors, at frequencies of 0.01-10 Hz, corresponding to distances in the atmosphere between about 1 and 250 m. Analysis shows that large-scale turbulences fit the Greenwood-Tarazano model well, and notably better than the von Karman model of turbulence.

The unusual source IRC +10216 is the brightest stellar object at mid-infrared wavelengths in the northern hemisphere. Adding to its distinctiveness, the dust around IRC +10216 almost completely enshrouds the star and has an extremely complex distribution. We report the imaging of IRC +10216 at 11.15 μm with three telescopes and the closure phase at two different stellar phases. Three-baseline interferometry data from a linear array of telescopes is used to create a one-dimensional image of the star and circumstellar dust. The two epochs over which data have been taken provide information at different position angles, which yields some insight into the two-dimensional structure of IRC +10216. Specifically, we observe two areas of peaked intensity. The first is 66 ± 4 mas to the west and 160 ± 51 mas to the south of the star, and the second is 227 ± 8 mas to the east and 94 ± 57 mas to the south. These two features can explain most of the observed asymmetry.

Interferometric observations of α Herculis at a wavelength of 11.15 μm over the period 1989-2004 show large visibility variations. These variations are interpreted as an ejection of approximately 10-6 M☉ of material in 1990 that has subsequently expanded and dissipated. The expansion rate is approximately 75 km s-1, much larger than previous observations. No substantial material has been emitted during the subsequent 14 years.

Interferometric observations of six Mira-type stars: R Aqr, CIT 3, χ Cyg, W Aql, R Leo, and U Ori are reported. All measurements were made by UC Berkeley's Infrared Spatial Interferometer (ISI), which is comprised of three 1.65 m telescopes using a heterodyne detection system currently operating at 11.15 μm. All data were taken in a nonredundant east-west linear configuration of telescopes, at a variety of spatial frequencies, with baselines of up to 12 m. By fitting a smooth curve to the closure phase data, as a function of the shortest baseline, the phases of individual visibility measurements can be determined. With curves of the visibility and phase, one-dimensional images are then constructed by an inverse Fourier transform. These images show significant changes in the stars and surrounding dust between the years 2003 and 2004 indicating nonconstant gas emission. They also show significant and varied types of asymmetry, including asymmetries that may be caused by companions, asymmetric stars, or asymmetric dust emission.

International audience; We have measured nonzero closure phases for about 29% of our sample of 56 nearby asymptotic giant branch (AGB) stars, using the three-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 mas. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.

Interferometric measurements of stellar sizes in frequency bands ranging from the near-infrared to longer wavelengthsgivedifferentresults.Variousexplanationshavebeenproposedtoaccountforthesevariationsinapparentsize withwavelength,butnonehave beenentirely consistent.Weproposethat thermal ionizationinthestellar atmosphere and resulting opacity, primarily due to free-free electron-hydrogen collisions, play a significant role. Such an opacity has a quadratic dependence on photon wavelength and produces variations in the opacity of the atmosphere with wavelength, consistent with pertinent measurements. This may be particularly important for Mira-type stars, and two examples, o Ceti and W Hya, are analyzed as examples. For stars that are much smaller or with more concentrated mass, it is not likely to be significant. Subject headingg stars: atmospheres — stars: fundamental parameters — stars: variables: other — techniques: interferometric

Mid-infrared observations of IK Tau have been made at 11.15 μm with the three-telescope Infrared Spatial Interferometer on Mount Wilson and also using individual segments of the Keck telescope for multiple-aperture interferometry on the Keck telescope at 10.7 μm. Both experiments provided closure phase and show temporal variations and asymmetries in the surrounding dust, with a difference of about 15% in intensity between two sides of the star. Asymmetries have been previously observed in the distribution of SiO masers closely surrounding the star. Comparison with earlier interferometric measurements shows substantial reduction in dust surrounding the star over the last decade. Several asymmetric dust models are investigated and simple images constructed.

All the ideas essential to making a laser were known before 1930, but there was no operating laser before 1960. So why didn't the laser come sooner? There are several reasons. One important impedim...

The spatial distributions of dust around four late-type stars measured with the University of California, Berkeley, Infrared Spatial Interferometer (ISI) located at Mount Wilson, California, are described. Used as a heterodyne interferometer at 11.15 μm, the ISI was able to resolve the dust shells around late-type stars U Orionis, χ Cygni, W Aquilae, and IRC +10011 (CIT 3), including their inner radii. Models for dust distribution around these stars have been obtained by fitting the visibility data for both maximum and minimum luminosities and also available mid-infrared spectra of each star. Dust grains are modeled as a mixture of silicates and graphite, and since only two telescopes were used for these observations, the shells are assumed to be spherically symmetric. Visibility curves are shown to change with the luminosity phase of the star, with a larger fraction of the total 11 μm flux emitted from the dust near the star at the maximum than at the minimum phase. Mass-loss rates and estimates of the composition of the dust shell are provided for each star, and dust particle size is determined for IRC +10011 (CIT 3) by comparison of near- and mid-infrared visibilities.

Simultaneous measurements of atmospheric temperature fluctuations at various altitudes and locations at Mount Wilson Observatory provide a quantitative description of local turbulence characteristics. The average rms value of the temperature fluctuations at an altitude of 150 feet is found to be 26% of the rms value at 9 feet, showing a substantial decrease with altitude for temperature fluctuations close to the ground. These rms values are found to be more heavily dependent on wind speed than on time of night, in contrast to prior belief. For measurements made close to the ground, the power spectra of the temperature fluctuations closely fit the Kolmogorov-Taylor prediction of -1.67 at frequencies up to 8 Hz, although at higher elevations some discrepancy is observed: the mean slope at 70 feet is approximately -1.50. The average size of major turbulent eddies is found to depend on wind speed, varying approximately linearly from about 1 to 25 m in both the horizontal and vertical directions for wind speeds between 0 and 8 m s-1. Analysis of the correlation between horizontally separated sensors indicates that Taylor's approximation remains useful on timescales of 10-15 s, after which time the turbulence changes. This suggests that local measurements of temperature fluctuations could provide some appreciable correction to variations in path length in interferometric observations of stars caused by atmospheric turbulence.

The size of the continuum photospheres of α Ori, α Her, R Leo, and χ Cyg have been measured at 11 μm using heterodyne interferometry to accuracies as high as 1%. An assortment of narrow wavelength bands near 11 μm (each having a width ~0.17 cm-1) were used to avoid spectral lines. The resulting apparent diameters for α Ori and α Her are ~30% larger than measured near-infrared sizes, whereas the Mira variables, R Leo and χ Cyg, have 11 μm apparent diameters roughly twice their reported near-infrared sizes. The complexities of interpretation are discussed.

The size and variability of the continuum photosphere of o Ceti have been measured with 11 μm heterodyne interferometry to an accuracy of about 1%. Narrow bandwidths (~0.17 cm-1) were used to avoid spectral lines and measure continuum only. The resulting 11 μm diameter of o Cet is larger than the previously measured visible and near-infrared sizes. In addition, variations in the diameter with phase and a possible elongation have been observed. Visibilities were also measured at wavelengths known to contain strong H2O spectral contamination, giving larger apparent stellar sizes and information on the distribution of hot H2O gas.

Adaptive optics provides a method for improving telescope imaging affected by atmospheric seeing distortions, but the differences in path length fluctuations through the atmosphere to two or more separate telescopes continues to limit the quality of stellar interferometry, and unfortunately is not ameliorated by adaptive optics. Some corrections to such fluctuations can be made by atmospheric density measurements near the ground, particularly since in some cases a substantial fraction of path length fluctuations occur in the atmosphere within 20-30 m of ground level. It is shown that more extensive corrections can be made by measurements of backscattered lidar radiation sent along the direction of the telescopes' observation. It is proposed that Rayleigh or Raman backscattering be used to measure changes in atmospheric density or index of refraction. Rayleigh scattering by molecules provides more intensity than does Raman and can allow path length corrections over distances of a few hundred meters to somewhat more than a kilometer with a fractional accuracy as good as 10-6 of the total atmospheric delay. Such measurements can substantially assist stellar interferometry. Details of how this might be done, and of likely errors and difficulties, are discussed quantitatively.

We have mapped the inner 360'' regions of M51 in the 158micron [CII] line at 55'' spatial resolution using the Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). The emission is peaked at the nucleus, but is detectable over the entire region mapped, which covers much of the optical disk of the galaxy. There are also two strong secondary peaks at ~43% to 70% of the nuclear value located roughly 120'' to the north-east, and south-west of the nucleus. These secondary peaks are at the same distance from the nucleus as the corotation radius of the density wave pattern. The density wave also terminates at this location, and the outlying spiral structure is attributed to material clumping due to the interaction between M51 and NGC5195. This orbit crowding results in cloud-cloud collisions, stimulating star formation, that we see as enhanced [CII] line emission. The [CII] emission at the peaks originates mainly from photodissociation regions (PDRs) formed on the surfaces of molecular clouds that are exposed to OB starlight, so that these [CII] peaks trace star formation peaks in M51. The total mass of [CII] emitting photodissociated gas is ~2.6x10^{8} M_{sun}, or about 2% of the molecular gas as estimated from its CO(1-0) line emission. At the peak [CII] positions, the PDR gas mass to total gas mass fraction is somewhat higher, 3-17%, and at the secondary peaks the mass fraction of the [CII] emitting photodissociated gas can be as high as 72% of the molecular mass.... (continued), Comment: 14 pages, 6 figures, Accepted in ApJ (for higher resolution figures contact the author)

The distribution of dust emitted by the supergiant star NML Cygni has been resolved by interferometry at 11 μm wavelengths at various times over a period of 6 yr. Results show there are two discrete dust shells, which have both moved away from the star approximately the same amount during the 6 yr period. This allows determination of the time between ejection of material forming the two shells to be 65 ± 14 yr. Assuming the radial outflow velocity can be derived from Doppler-measured velocities of masers surrounding the star, its distance can be calculated from the observed angular motion to be 1220 ± 300 pc. This decreases the luminosity of the star by about 1 mag over that deduced from the distance 1900 pc previously assumed.

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.

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).

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.

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.

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.

We review the classical and the quantum aspects of the noise behaviour of heterodyne detection, making connections among quantum noise in heterodyne receivers, the uncertainty principle, and the concept of electromagnetic zero-point fluctuations as a description of that noise.

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

We present observations of NGC 4038/9 in the [C II] 158 μm fine-structure line taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). A fully sampled map of the galaxy pair (without the tidal tails) at 55'' resolution has been obtained. The [C II] emission line is detected from the entire galaxy pair and peaks at the interaction zone. The total [C II] luminosity of the Antennae is L[C II] = 3.7 × 108 L☉, which is about 1% of the far-infrared luminosity observed with IRAS. The main part of the [C II] emission is probably produced by photodissociation regions (PDRs), and a minor fraction may be emitted from H II regions. A small part of the [C II] emission comes from a standard cold neutral medium (CNM); however, for high temperatures (T ~ 100 K) and high densities (nH ~ 200 cm-1) of the CNM, up to about one third of the observed [C II] emission may originate from CNM. From PDR models, we derive densities on the order of ~105 cm-3 and far-UV (FUV) intensities of 460χ0, 500χ0, and 240χ0 for the PDRs in the interaction zone, NGC 4038, and NGC 4039, respectively. However, PDRs with densities on the order of ~102 cm-3 and FUV intensities on the order of ~100χ0 could also explain the observed [C II] emission. The minimum masses in the [C II]-emitting regions in the interaction zone and the nuclei is a few ×107 M☉. A comparison with single-dish CO observations of the Antennae shows a [C II] to CO intensity ratio at the interaction zone that is a factor of 2.6 lower than usually observed in starburst galaxies, but still a factor of about 1.3 to 1.4 higher than at the nuclei of NGC 4038/9. Therefore, no global starburst is taking place in the Antennae. [C II] emission arising partly from confined starburst regions and partly from surrounding quiescent clouds could explain the observed [C II] radiation at the interaction zone and the nuclei. Accordingly, there are small confined regions with high star formation activity in the interaction zone and with a lower star formation activity in the nuclei. This supports the high density and high FUV intensity of the PDRs in the interaction zone and the nuclei.

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.

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.

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.

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.

When in 1959 Cocconi and Morrison first suggested communication with extraterrestrial intelligence and showed it could be done in the microwave region, there were no lasers. In addition, since the hydrogen line at 21 centimeters was the only astronomical line known at that time in the microwave region, it seemed unique and the obvious place to look. Only one year later, however, the laser came along and shortly after that the use of lasers for SETI could also be suggested (Schwartzet al., 1961). And still later, a number of important microwave lines at various frequencies were detected from interstellar clouds, making a choice of frequency less obvious.The first laser was built in 1960. But the laser perhaps came along unnecessarily late; there is no principle of science or technology involved in lasers that was not understood as early as the 1920s. One can find an early discussion by Tolman (1924), for example, about so-called «negative absorption,» produced by molecules in excited states. At that time, the coherence of stimulated radiation was not completely clear, but Tolman said it was probably coherent, even though that it had not yet been mathematically proven. In a few years, it was proven and so all the principles were known. Lasers could indeed have been invented at that time, even though their variety would have been limited because solid state physics and infrared spectroscopy were not very far along.

Since the center of our own galaxy is the closest region of this type, it should provide a proving ground for our understanding of such regions. And fortunately, recent developments in radio, infrared, and x-ray astronomy allow us to look at the center of our galaxy in considerable detail – a privilege not possible in the visible region because of the rather dense dust clouds surrounding it. We find there many complex and interesting phenomena, including evidence for a compact central mass of a few million solar masses from the dynamics of stars and gases. However, its existence has been doubted because the radiant energy coming from the very center is much less than normal theoretical expectations for a black hole of such a mass. This discrepancy has led to speculation whether the velocity measurements could be misleading, whether the concentrated mass is in some form other than the expected black hole, whether the radiation might be taking place in an exotic form, being directed away from us or happening to be low at the moment, or whether our theories of such radiation are as yet incomplete. The general characteristics of the very central region of our galaxy will be reviewed along with the dynamic evidence for a large concentrated mass and comparison of the radiation it emits with that of various theoretical models.

The subject of this review is the central 100 parsecs of our Galaxy, with a strong focus on the central few parsecs. Observations of the electromagnetic spectrum over 13 orders of magnitude in wavelength show a broad range of phenomena involving a number of physical processes. We discuss the stellar and interstellar components, the importance of magnetic and gravitational forces, the evidence for stellar formation and a central massive black hole, and the origin and nature of ionization, outflows and interstellar gas dynamics.

FIR [CII] images of galaxies have been made using the MPE/UCB Far-Infrared Fabry-Perot Imaging Spectrometer (FIFI) on the KAO with 55” resolution. The 158 μm cooling line of ionized carbon shows emission extended over very large scales: peaking in the nucleus and present at least out to the extent of the spiral arms as seen in the CO(1-0) line in the galaxies NGC6946, M83, NGC891 and Centaurus A and often beyond the molecular disks. Evidence for photodissociation of molecular gas over kpc scales in the spiral arms is seen in the nearly face-on galaxies M83 and NGC6946. The [CII] emission present in the disk beyond the extent of molecular gas can be produced in the diffuse HI gas, thereby allowing us to derive properties of the atomic medium, such as temperatures and pressures.

We report velocity resolved maps of the 158 μm [CII] fine structure line, of the 63 μm [OI] fine structure line, and of the 88 μm and 52μm [OIII] fine structure lines toward the center region of the Galaxy. Observations were carried out on board the Kuiper Airborne Observatory (KAO) using the MPE/UCB Far-infrared Imaging Fabry-Perot interferometer (FIFI). The angular resolution was 55″ for the [CII] line and 22″ for the oxygen lines. The [CII] emission, tracing the distribution of dense gas dissociated and partially ionized by far-UV photons, is strongest toward the rotating circumnuclear disk surrounding Sgr A West. A continuous bridge of [CII] line emission connects the Sgr A complex to the thermal radio filaments in the Radio Arc 10' north of the center, thus strongly suggesting a direct physical connection between the two. Outside the nuclear region, there is an anticorrelation between the [CII] intensity and the distribution of dense molecular material. The brightest emission occurs near the edges of the massive Galactic Center molecular clouds. We conclude that these clouds are predominantly ionized by external UV photons, partly from the Sgr A West region and partly from OB stars near the center. Our data indicate a physical connection between the center and the massive interstellar clouds in the surrounding 50 pc. Our 88 μm [OIII] maps of the “sickle” region within the Radio Arc and of the thermal arches approximately follow the thermal radio continuum. The line width varies substantially over the mapped area; we find a narrow (Δv ≈ 60km/s) and a broad (Δv ≈ 150km/s) component, with the broad component being more prominent in the sickle. From the 52 μm 88 μm line ratio we derive a density ≈ 300 cm−3 and a hydrogen column density ≈3 × 1021cm−1 in the [OIII] emitting medium. This rules out shocks or fast winds from stars as excitation mechaniμs. UV excitation, however, could produce the observed amount of O2+. About 10 to 100 O-stars would be required to ionize the gas in the “pistol/sickle” region. The broad linewidth could be due to interaction between the ionized gas and magnetic fields. Our maps of both [OIII] lines toward the circum-nuclear disk show the brightest emission from inside the central, “ionized cavity” in the molecular ring. The high 52 μm/88μm ratio requires a density ⩾ 104cm−3. The derived hydrogen column density, however, is low, which could be understood by a small ionization ratio N o2+ N o0 or a small beam filling factor of the ionized gas. We infer about 10 m⊙ of ionized gas inside the cavity, with no indication of an additional low-density component. The [OI] data, on the other hand, reveal the presence of a large amount of neutral atomic gas inside the central cavity, and associated with the “northern” and “eastern” arms of ionized gas. We estimate about 300 M⊙ of dense neutral hydrogen gas within the 1.5 pc radius of the circumnuclear ring, about a factor 10 more than the amount of ionized gas. The northern arm appears to be a bright, ionized rim at the surface of this neutral gas streamer, which is probably in the plane of the circumnuelear disk and falling from a gap in the 1.5pc ring into the central 0.5pc. We infer a mass inflow rate into the central parsec of 0.03M⊙/year.

The University of California Infrared Spatial Interferometer (ISI) for the 10 μm wavelength region is briefly described and results obtained on 13 prominent stars and on atmospheric phenomena are discussed. The interferometer has two movable telescopes of 1.65 m aperture and operates in principle like a modern radio interferometer, using heterodyne detection, CO 2 laser local oscillators, RF delay lines, and lobe rotation to maintain a fixed-frequency fringe rate. Extensive measurements have been made on atmospheric pathlength or phase fluctuation characteristics. These show substantial deviations from the Kolmogorov-Taylor model, fortunately in a direction favoring adaptive optics, long baselines, and the use of infrared wavelengths. Outer scales as small as about 10m occur under good seeing conditions. Visibility results on the 13 stars show that six of them have dust shells rather far from the star and provide evidence for episodic emission of gas with times between emissions of a number of decades. Other stars also vary with time, but are characterized by cyclic variations typical of long-term variables and more continuous emission with dust formation near the stars at temperatures as high as 1300K.

This lecture is being given as part of an international meeting on astronomical imaging. It is, in fact, the first major meeting of people with backgrounds in both optical and radio imaging, and has attracted 200 people from all around the world. One of those is a man who has been an active researcher in microwave and infrared spectroscopy for over half a century. It is my pleasure to introduce that man, Professor Charles Townes from the Space Science Laboratory, University of California, Berkeley. It is clear that throughout his life Professor Townes has been attracted by a series of fundamental challenges. He was already an acknowledged molecular spectroscopist when, in 1951, to solve the problem of short wavelength oscillators, he conceived a system for using excited ammonia molecules that became the ammonia beam maser oscillator. He followed this in 1958 by publishing a paper with his brother-in-law, Arthur Schawlow, that laid the foundations for the development of the laser. These two activities, flowing as they did from the pursuit of the most fundamental physics, paved the way for some of the key elements of modern communications.

Recent upgrades of the Infrared Spatial Interferometer are described. These provide improved sensitivity, precision and convenience of measurement. Analysis of phase fluctuations within the interferometer as well as in the atmosphere above indicates that popular simple atmospheric models need to be refined. Results of stellar observations are presented in another paper (cf. Danchi et al., in these Proceedings).

Dust surrounding the star V Hydrae has been measured at a 11.15 μm wavelength using the three-telescope Infrared Spatial Interferometer (ISI). The narrowband heterodyne detection system is tuned to a region free of spectral lines due to molecules surrounding the star, and only continuum radiation from the dust and star is measured. Closure-phase data show that the dust is symmetrically distributed around the star. Results obtained in 2006–2007 are well fitted by a model with a Gaussian intensity distribution with HWHM of 49 mas that contributes 0.52 of the total flux and by a uniform disk of radius 287 mas with a total flux fraction of 0.26; the remaining 0.22 of the total flux is due to the star, which is unresolved. Visibility measurements conducted in 1997 show a substantial difference from those of 2006–2007, indicating that the star was surrounded by more dust in 1997.

We present an interferometric study of the continuum surface of the red supergiant star Betelgeuse at 11.15 microns wavelength, using data obtained with the Berkeley Infrared Spatial Interferometer each year between 2006 and 2010. These data allow an investigation of an optically thick layer within 1.4 stellar radii of the photosphere. The layer has an optical depth of ~1 at 11.15 microns, and varies in temperature between 1900 K and 2800 K and in outer radius between 1.16 and 1.36 stellar radii. Electron-hydrogen atom collisions contribute significantly to the opacity of the layer. The layer has a non-uniform intensity distribution that changes between observing epochs. These results indicate that large-scale surface convective activity strongly influences the dynamics of the inner atmosphere of Betelgeuse, and mass-loss processes., 13 pages, 5 figures, in press (ApJ)