Nifte: The near infrared faint-object telescope experiment

The high sensitivity of large format InSb arrays can be used to obtain deep images of the sky at 3–5 m. In this spectral range cool or highly redshifted objects (e.g. brown dwarfs and protogalaxies) which are not visible at shorter wavelengths may be observed. Sensitivity at these wavelengths in ground-based observations is severely limited by the thermal flux from the telescope and from the earth's atmosphere. The Near Infrared Faint-Object Telescope Experiment (NIFTE), a 50 cm cooled rocket-borne telescope combined with large format, high performance InSb arrays, can reach a limiting flux < 1 Jy (1) over a large field-of-view in a single flight. In comparison, ISO will require days of observation to reach a sensitivity more than one order of magnitude worse over a similar area of the sky. The deep 3–5 m images obtained by the rocket-borne telescope will assist in determining the nature of faint red objects detected by ground-based telescopes at 2 m, and by ISO at wavelengths longer than 5 m.     

Imaging observations of radio bursts at meter-decameter wavelengths

In this article, we review some of the recent results obtained with imaging observations of the Sun at meter-decameter wavelengths, using the Clark Lake multifrequency radioheliograph. We briefly discuss the use of imaging observations to study the large scale structure of the upper corona. We discuss non-flare associated type II/type IV bursts associated with a coronal streamer disruption event associated with a slow (100 Km/s) CME. We discuss meter-decameter microbursts, which occur at coronal heights, often without any surface activity. Finally, we discuss a correlated type III burst whose emission originates almost simultaneously from two widely separated ( 10⁵ Km) locations.     

Solar observations with a millimeter-wavelength Array

Rapid developments in the techniques of interferometry at millimeter wavelengths now permit the use of telescope arrays similar to the Very Large Array at microwave wavelengths. These new arrays represent improvements of orders of magnitude in the spatial resolution and sensitivity of millimeter observations of the Sun, and will allow us to map the solar chromosphere at high spatial resolution and to study solar radio burst sources at millimeter wavelengths with high spatial and temporal resolution. Here we discuss the emission mechanisms at millimeter wavelengths and the phenomena which we expect will be the focus of such studies. We show that the flare observations study the most energetic electrons produced in solar flares, and can be used to constrain models for electron acceleration. We discuss the advantages and disadvantages of millimeter interferometry, and in particular focus on the use of and techniques for arrays of small numbers of telescopes.Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991.     

Synthetic images of proto-planetary disks around young stars

We present synthetic images of accretion disks around young stars computed from a model where the disk's vertical structure is solved assuming hydrostatic equilibrium. The disk's brightness results from three emission processes: (1) the reprocessing of stellar photons in the optically thick disk's regions; (2) the scattering of stellar photons in the optically thin parts of the disk; and (3) the thermal emission of the disk due to viscous energy dissipation during the accretion process.We discuss the relative importance of these emission processes at wavelengths ranging from 1.2 to 20m.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Microwave and Soft X-ray Study of Solar Active Region Evolution

We have studied the properties and evolution of several active regions observed at multiple wavelengths over a period of about 10 days. We have used simultaneous microwave (1.5 and 17 GHz) and soft X-ray measurements made with the Very Large Array (VLA), the Nobeyama Radio Heliograph (NRH) and the Soft X-ray Telescope (SXT) on board the Yohkoh spacecraft, as well as photospheric magnetograms from KPNO. This is the first detailed comparison between observations at radio wavelengths differing by one order of magnitude. We have performed morphological and quantitative studies of active region properties by making inter-comparison between observations at different wavelengths and tracking the day-to-day variations. We have found good general agreement between the 1.5 and 17 GHz radio maps and the soft X-rays images. The 17 GHz emission is consistent with thermal bremsstrahlung (free-free) emission from electrons at coronal temperatures plus a small component coming from plasma at lower temperatures. We did not find any systematic limb darkening of the microwave emission from active regions. We discuss the difference between the observed microwave brightness temperature and the one expected from X-ray data and in terms of emission of a low temperature plasma at the transition region level. We found a coronal optical thickness of 10^-3 and 1 for radiation at 17 and 1.5 GHz, respectively. We have also estimated the typical coronal values of emission measure ( 5 × 10²⁸ cm^-5), electron temperature ( 4.5 × 10⁶6 K) and density ( 1.2 × 10⁹ cm₃). Assuming that the emission mechanism at 17 GHz is due to thermal free-free emission, we calculated the magnetic field in the source region using the observed degree of polarization. From the degree of polarization, we infer that the 17 GHz radiation is confined to the low-lying inner loop system of the active region. We also extrapolated the photospheric magnetic field distribution to the coronal level and found it to be in good agreement with the coronal magnetic field distribution obtained from microwave observations.     

Dust models confronted with observations

We critically discuss the two major models for interstellar dust grains developed by Greenberg and Mathis and their co-workers. So far, knowledge about the dust has been obtained mainly from extinction and polarization measurements in the UV, visual and near IR. Analysis of dust emission at longer wavelengths generally requires an evaluation of the radiative transfer. Such a procedure is necessary to assess optical grain properties at long wavelengths and also to interpret the emission from the different kinds of IR objects. The 10 m silicate resonance and the submm dust absorptivity are presented as two examples. We then briefly summarize the implications which follow from the existence of extremely small dust grains.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Neutral Hydrogen Column Depths in Prominences Using EUV Absorption Features

Observations of prominence regions in hot coronal lines (106 K) at wavelengths below the hydrogen Lyman absorption limit show what appear to be absorption features. Other authors have suggested that these observed features may be due to H and He continuum absorption. But there has, as yet, been no conclusive evidence that this is indeed the case. In this paper we present new Solar and Heliospheric Observatory (SOHO) observations that allow us to address this problem in a quantitative manner. We find that continuum absorption is the best explanation for the absorption observed in imaging data from the Coronal Diagnostic Spectrometer (CDS) on board SOHO. Furthermore, we discuss a new technique to measure the column depth of neutral hydrogen in a prominence, and use it to obtain estimates of the prominence filling factors as well. We calculate the column depth of neutral hydrogen, H1018 cm–2, and the filling factor, f0.3.     

Limb observations of the 12.32-micron Mg I emission line during the 1994 annular eclipse

We determined the limb profile of the extremely Zeeman-sensitive emission line of Mg i at 12.32 m (811.58 cm–1) during the May 1994 annular eclipse, using the 3.5-m ARC telescope at the Apache Point site on Sacramento Peak, New Mexico. Spectra were recorded at 0.1 cm–1 spectral resolution and 1 s time resolution using a cryogenic grating spectrometer. The time derivatives of the observed line energy and continuum intensity were used to infer high-resolution profiles of the solar limb. Data were obtained at second contact only, since clouds prevented observations at third contact. We find that the emission line energy peaks very close to the 12 m continuum limb. This agrees with our result from the 1991 total eclipse over Mauna Kea, and also with non-LTE radiative transfer theory for this line, which predicts an upper-photospheric origin. However, in 1991, line emission remained observable as high as 2000 km above the continuum limb, whereas the 1994 data show observable emission to only 500 km. This difference greatly exceeds any applicable errors, or sensitivity differences in either data set, and must be attributed to spatial and/or temporal inhomogeneities in the solar limb emission of this line. We discuss possible causes of these inhomogeneities, and implications for observations at far-IR and submillimeter wavelengths.     

Flares From the Giant ar 9433 on 24 April 2001

We present H CCD observations of three small-to-medium-size two-ribbon flares observed in the giant AR 9433 on 24 April 2001. Flare observations at other associated wavelengths (e.g., soft X-rays (SXR), hard X-rays (HXR), microwaves (MW)) obtained from archives are also presented and compared. We have tested the Neupert effect for the most energetic third flare. The flare observations are in agreement with the thick-target model. In the case of this flare the HXR emitting electrons appears to be the heating source of SXR and H emissions. The flares are also studied in EUV and UV emissions using TRACE data. We discuss the complexity of the magnetic field using SOHO/MDI magnetograms. The flares are observed to occur in both (f/p) polarity regions in highly sheared magnetic field with emerging flux regions and MMFs.     

Coronal structures observed at metric wavelengths with the Nançay radioheliograph

We present and discuss two-dimensional maps of the Sun at 169 MHz, obtained with the Nançay radioheliograph used as an Earth rotation aperture synthesis instrument. The maps have been computed on the basis of about 6 hr of one-dimensional observations by the east-west and the north-south arrays of the radioheliograph and have a resolution of 1.5 by 4.2 for a solar declination near 23°. In addition to a broad background component, the maps show several features both brighter and darker than the background. Some of the bright features are sources of noise storm continua, as evidenced by their positions relative to active regions and by the occurrence of type I bursts. Weaker emission regions are apparently associated with neutral lines of the photospheric magnetic field. We found no sources associated with extended quiescent filaments. Some of the depressions on the maps correspond to coronal holes both in the equatorial region and near the poles, while the more shallow ones may be arch regions with low electron temperature and/or emission measure. The distribution of brightness temperature at a height of 0.15 solar radii above the photospheric limb shows a gross similarity with coronal green line observations. The present results indicate that the notion of the slowly varying component at metric wavelengths may have to be reexamined, since sources of different nature may have been grouped in this component in the past.On leave from the section of Astrophysics, Astronomy and Mechanics, Department of Physics, University of Athens.     

The future of large-format InSb arrays for astronomy

Introduced to astronomy less than a decade ago, infrared detector arrays have radically and forever changed astronomer's observational techniques, instruments, and telescope designs. This paper first examines the array technology development that caused this change, with emphasis on the 1- to 5-m band. Technology trends and projections for further development within the infrared industry are analyzed to assist astronomers in planning future instruments. This technology development is predicted to result in arrays with lower noise, greater well depth, response to a wider band of wavelengths, and, above all, much larger formats than arrays currently in use.     

New 8–13 μm Si:Ga/DRO hybrid arrays for very large telescopes

Since 1984, the CEA-LETI-LIR (Infrared Laboratory) has been involved in development of Si:Ga/DRO hybrid detector arrays dedicated to imaging of astronomical sources in the 8–13 m range. Successively, 32×32 element arrays were successfully manufactured for the ISOCAM camera and 64×64 arrays were fabricated for ground-based imaging. The latter detectors have been integrated in 3 cameras (C10, CAMIRAS and TIMMI for ESO) and have led to excellent astrophysical results since 1990.To equip instruments to be mounted on very large telescopes such as the European VLT at ESO, manufacture of new arrays has been undertaken and is currently under way at CEA-LETI-LIR. These new arrays will have a 128×192 format and will present the outstanding essential feature that their storage capacity will be able to be changed according to observation conditions (2 capacities will be implemented in the pixel).The main characteristics of these new detectors will be presented in this paper.     

Automated Identification of Coronal Holes from Synoptic EUV Maps

Coronal holes (CHs) are regions of open magnetic field lines in the solar corona and the source of the fast solar wind. Understanding the evolution of coronal holes is critical for solar magnetism as well as for accurate space weather forecasts. We study the extreme ultraviolet (EUV) synoptic maps at three wavelengths (195 Å/193 Å, 171 Å and 304 Å) measured by the Solar and Heliospheric Observatory/Extreme Ultraviolet Imaging Telescope (SOHO/EIT) and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) instruments. The two datasets are first homogenized by scaling the SDO/AIA data to the SOHO/EIT level by means of histogram equalization. We then develop a novel automated method to identify CHs from these homogenized maps by determining the intensity threshold of CH regions separately for each synoptic map. This is done by identifying the best location and size of an image segment, which optimally contains portions of coronal holes and the surrounding quiet Sun allowing us to detect the momentary intensity threshold. Our method is thus able to adjust itself to the changing scale size of coronal holes and to temporally varying intensities. To make full use of the information in the three wavelengths we construct a composite CH distribution, which is more robust than distributions based on one wavelength. Using the composite CH dataset we discuss the temporal evolution of CHs during the Solar Cycles 23 and 24.     

Images of Gradual Millimeter Emission and Multi-Wavelength Observations of the 17 august 1994 Solar Flare

We present a comprehensive analysis of the 17 August 1994 flare, the first flare imaged at millimeter (86 GHz) wavelengths. The temporal evolution of this flare displays a prominent impulsive peak shortly after 01:02 UT, observed in hard X-rays and at microwave frequencies, followed by a gradual decay phase. The gradual phase was also detected at 86 GHz. Soft X-ray images show a compact emitting region (20), which is resolved into two sources: a footpoint and a loop top source. Nonthermal emissions at microwave and hard X-ray wavelengths are analyzed and the accelerated electron spectrum is calculated. This energy spectrum derived from the microwave and hard X-ray observations suggests that these emissions were created by the same electron population. The millimeter emission during the gradual phase is thermal bremsstrahlung originating mostly from the top of the flaring loop. The soft X-rays and the millimeter flux density from the footpoint source are only consistent with the presence of a multi-temperature plasma at the footpoint.     

Sub-arcsecond 10 micron imaging of newly-formed stars

We describe initial results of a program to image massive newly-formed stars with sub-arc second spatial resolution. We discuss high-precision diffraction-limited size measurements at =10 m made using the 3 m Lick telescope. The point-spread function has FWHM 0.7; deconvolution yields a spatial resolution of 0.35. We find that the core component of one such object, LkH 101, is unresolved at these scales, and we are able to set a 95%-confidence upper limit of 270 AU for the diameter of the circumstellar dust shell. This places the dust at the same radial scale as a strong ionized stellar wind region seen at radio wavelengths. Our observations, when combined with published spectral observations, rule out an optically thick circumstellar disk but allow a radially thin, anisotropic distribution of dust, or alternatively an isotropic distribution of dust with a narrow range of large grain sizes.     

Bipolar molecular outflows

Summary Bipolar molecular outflows are a ubiquitous phenomenon in the process of star formation. We review the main observational properties of the outflows around young stellar objects, highlighting the recent wealth of information provided by the new generation of large radiotelescopes operating at millimeter wavelengths (in particular the IRAM 30-m, the NRO 45-m, and the JCMT 15-m dishes). The observations of outflows containing molecular, jet-like flows and bullets are discussed in detail, as they provide key information for understanding origin and evolution of the outflows. We also discuss a number of closely related issues: the evidence for dense shocked gas associated with the flows, the interaction of the outflows with the ambient dense cores, the evolutionary status of the sources driving the outflows, the properties of circumstellar disks, and theoretical models of the origin of the neutral winds. All these areas are important for developing a plausible scenario for the formation and evolution of the bipolar molecular outflows.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991.     

UV and IR auroral emission model for the outer planets: Jupiter and Saturn comparison

Planetary aurora display the dynamic behavior of the plasma gas surrounding a planet. The outer planetary aurora are most often observed in the ultraviolet (UV) and the infrared (IR) wavelengths. How the emissions in these different wavelengths are connected with the background physical conditions are not yet well understood. Here we investigate the sensitivity of UV and IR emissions to the incident precipitating auroral electrons and the background atmospheric temperature, and compare the results obtained for Jupiter and Saturn. We develop a model which estimates UV and IR emission rates accounting for UV absorption by hydrocarbons, ion chemistry, and non-LTE effects. Parameterization equations are applied to estimate the ionization and excitation profiles in the H₂ atmosphere caused by auroral electron precipitation. The dependences of UV and IR emissions on electron flux are found to be similar at Jupiter and Saturn. However, the dependences of the emissions on electron energy are different at the two planets, especially for low energy (<10 keV) electrons; the UV and IR emissions both decrease with decreasing electron energy, but this effect in the IR is less at Saturn than at Jupiter. The temperature sensitivity of the IR emission is also greater at Saturn than at Jupiter. These dependences are interpreted as results of non-LTE effects on the atmospheric temperature and density profiles. The different dependences of the UV and IR emissions on temperature and electron energy at Saturn may explain the different appearance of polar emissions observed at UV and IR wavelengths, and the differences from those observed at Jupiter. These results lead to the prediction that the differences between the IR and UV aurora at Saturn may be more significant than those at Jupiter. We consider in particular the occurrence of bright polar infrared emissions at Saturn and quantitatively estimate the conditions for such IR-only emissions to appear.     

Analysis of infrared photometry of algol

Infrared (JHKLM) photometry of Per, due to Zeilik (1980), has been analysed by computer-based optimization methods. The data have been used primarily to find empirical mean flux ratios at known IR wavelengths for the two photospheres of the eclipsing pair. These flux ratios have then been compared with values predicted by various model atmosphere calculations. The ratio of effective temperatures can be given asT _B/T _A=0.42±0.02, but there remains some small discrepancy between the empirical results and model calculations at the longest wavelengths (KLM). The possibility of IR-excess at longer wavelengths is considered, but the evidence is marginal.     

Lyman-alpha line intensity as a solar activity index in the far ultraviolet range

The relations between variations of far UV (FUV) emission in 115–210 nm waveband and L 121.6 nm and F_10.7 are studied. The changes of FUV flux are found to lag changes of F_10.7 - as a rule for 1 day. It is shown that such a difference may be caused by two factors: 1) differences between the rates of decrease of local sources' (active regions) brightness in FUV and 10.7 cm; 2) differences between limb-darkening curves for different wavelengths. One may expect the fluxes at different wavelengths to exhibit phase shifts of one relative to another. Cross-correlation analysis reveals no time-delay between emission fluxes within the FUV waveband, in spite of different laws for limb-brightening (darkening) for different spectral intervals. The absence of a phase delay can be caused by relatively small contribution of active regions to the flux of the whole Sun at these wavelengths. Thus the Lyman-alpha line intensity variation reflects variations of Solar FUV emission more precisely than F_10.7. Therefore, using the L intensity for flux intensity calculations of other FUV wavelengths is preferable to using the F_10.7 index.     

10 and 20 Micron imaging with arrays

We discuss imaging with arrays in the thermal IR. Aspects of the design and performance of the Golden Gopher, an infrared array camera are presented. This instrument operates in a high-background environment, for ground-based astronomical imaging from 5 to 27 m. It is built around a 20×64 element Si:As Impurity Band Conduction (IBC) device manufactured by GenCorp Aerojet Electronic Systems Division, and has a noise-equivalent flux density (NEFD) of 23.5mJy min ^-1/2 arcsec^-2 at =10m, =1m, on the Mt. Lemmon 1.5m telescope. We present and discuss a sample of the data. In addition we discuss the design and expected performance of the Long Wavelength Spectrometer which is now under construction for the Keck telescope.