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The detection of main-sequence circumstellar dust systems provides a powerful tool in the study of possible planetary systems similar to our own.
The last few years have seen a rapid growth in observations aimed at identifying intermediate and high-mass star forming sites in a wide range of evolutionary stages ranging from Hot Cores to ultracompact regions, to proto-Ae/Be stars.
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Results of an 18 cm VLBI survey of 31 luminous (L_(fir) greater than 10^(11.25)L dot circle),radio-compact (theta less than approximately 0."25) infrared galaxies are presented. Over half thesample galaxies show high-brightness temperature emission, with T_b greater than 10^5K andstructure on scales of 5 - 150 mas. The median VLBI core power for detected sources is log P_(core)= 22.0(WHz^(-1)0, and the ratio of core to total 1.6 GHz flux density less than S_(core)/S_(total)greater than = 0.12. The limits for non- detected sources are similar, consistent with a picture inwhich all the galaxies have compact cores at a level of a few percent of the total radio flux density. Characteristics of the extended radio structure, infrared properties and optical excitation are not goodindicators of the detectability of VLBI-scale emission. Structural information and energeticconsiderations rule out a single supernova interpretation of the compact emission in these galaxies,though we cannot exclude the possibility of several simultaneous extraordinarily luminous radiosupernovae within the central few hundred pc^3. Our results instead favor the presence of an AGNobscured by starburst-related dust.
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Medium-resolution 3-4 micron spectra have been obtained of three objects, IRAS 04296 + 3429, IRAS 22272 + 5435, and CRL 2688, which are in transition between the asymptotic red giant branch and the planetary nebula phase. All three show unusual 3-micron emission features. The two IRAS objects, already unusual in having 21 micron features, have remarkably strong 3.4-3.5 micron emission relative to the usually dominant 3.3-micron feature, with the spectrum of IRAS 04296 + 3429 being nearly identical to that of IRAS 05341 + 0852 (previously published). CRL 2688's spectrum bears some resemblance to those of the IRAS objects, but its 3.4-3.5 micron emission is not as strong. The relationships between the various emission features are discussed. CRL 2688 may be completing a transition between the '21-micron phase' and the 'normal phase', which is associated with dominant 3.3-micron emission, weak 3.4-3.6 micron features and plateau, and a lack of 21-micron emission.
Infrared Astronomy Satellite (IRAS), ISO, Space Infrared Telescope Facility (SIRTF), WIRE, Deep Near-Infrared Survey (DENIS), and Two Micron All-Sky Survey (2MASS) observations were used to compute the maximum number of observable brown dwarfs for various infrared surveys by combining the maximum possible Oort limit (0.1 'missing' solar mass p/cu c) with all possible brown dwarf mass and age distributions. This approach shows what limits will be placed on the contribution of brown dwarfs to any possible 'missing mass' if no brown dwarfs are observed. I consider brown dwarfs with masses of 0.01-0.08 solar mass and ages of 10(exp 9)-10(exp 10) years. The full range of predicted numbers of brown dwarfs above approx. 6 times the noise of each of the below surveys is: IRAS Point Source Catalog, 0.02-6; IRAS Faint Source Catalog absolute value of b greater than 10 deg, 0.05-16; ISO (2 week 12 micrometer survey), 0.15-80; SIRTF (2 week 12 micrometer survey), 2.50-1600; WIRE (4 month 12 micrometer survey), 21.80-6000; DENIS(half sky) absolute value of b greater than 10 deg, 0.00-2000; and 2MASS(full sky) absolute value of b greater than 10 deg, 0.00-8800. A failure to find brown dwarfs in the IRAS FSC would just barely rule out about half of the mass-age range for Oort limit total masses. A failure to find brown dwarfs in 2MASS/DENIS would rule out roughly the same mass-age range, but would set a limit of 0.1-0.01 times the Oort mass in that mass-age region. No limits would be set for the other half of the mass-age range since both IRAS and 2MASS/DENIS have insufficient sensitivity for brown dwarfs with T less than 750 K. A failure to find brown dwarfs with ISO would rule out almost all of the mass-age range for Oort limit total masses, but would not set a significantly lower limit to the brown dwarf mass limit. A failure to find brown dwarfs with SIRTF or WIRE would rule out the entire mass-age range for Oort limit total masses and set an upper limit of 0.1-0.001 times the Oort mass. To date, about 18% of the IRAS FSC has been searched down to 6 sigma, and no brown dwarfs have been found. This sets a 95% upper limit of 3 in 18% of the sky, or 13 in the entire FSC for absolute value b greater than 10 deg. To begin to set useful limits from 2MASS or DENIS, approximately 400 square degrees needs to be analyzed. To date, only a few square degrees of results from the 2MASS prototype camera have been examined, with no brown dwarfs found so far.
We report the X-ray properties of Bootes void galaxies detected by the ROSAT All-Sky Survey (RASS). By searching the fields of 26 radio and 27 emission-line-selected void galaxies, we have detected nine X-ray-emitting void galaxies at greater than 2.7 sigma confidence level. Five of these nine galaxies are in the IRAS subsample. Two of the IRAS galaxies were previously identified in positional cross-correlation studies of the RASS and IRAS Point Source Catalogue sources. Three of the X-ray-emitting galaxies are active galactic nuclei (AGNs; IRAS 14288+5255, Mrk 845, and IRAS 15195+5050), three are emission-line galaxies (PC 1357+4641, CG 547, and CG 922), and the remaining three are of unknown spectral type (IRAS 14500+4804, CG 637, and MAS 15092+3940). The far-infrared flux levels of the AGN sources imply that most of the observed X-ray emissivity is from starburst activity. We have carried out timing and spectral analysis for the Seyfert 1 galaxy Mrk 845. Poor statistics prevents detailed analysis of the remaining sources. Only two galaxies in our sample, BHI 1514+3819 and FSS 1515+3823, were observed during ROSAT pointed observations resulting in a nondetection at the 1 sigma level.
We have now analyzed a substantial fraction of the IRAS observations of the zodiacal cloud, particularly in the 25 micron waveband. We have developed a gravitational perturbation theory that incorporates the effects of Poynting-Robertson light drag (Gomes and Dermott, 1992). We have also developed a numerical model, the SIMUL mode, that reproduces the exact viewing geometry of the IRAS telescope and calculates the distribution of thermal flux produced by any particular distribution of dust particle orbits (Dermott and Nicholson, 1989). With these tools, and using a distribution of orbits based on those of asteroidal particles with 3.4 micron radii whose orbits decay due to Poynting-Robertson light drag and are perturbed by the planets, we have been able to: (1) account for the inclination and node of the background zodiacal cloud observed by IRAS in the 25 micron waveband; (2) relate the distribution of orbits in the Hirayama asteroid families to the observed shapes of the IRAS solar system dustbands; and (3) show that there is observational evidence in the IRAS data for the transport of asteroidal particles from the main belt to the Earth by Poynting-Robertson light drag.
In order to make the best high resolution images of IRAS data it is necessary to incorporate any knowledge about the instrument into a model: the IRAS model. This is necessary since every remaining systematic effect will be amplified by any high resolution technique into spurious artifacts in the images. The search for random noise is in fact the never-ending quest for better quality results, and can only be obtained by better models. The Dutch high-resolution effort has resulted in HIRAS which drives the MEMSYS5 algorithm. It is specifically designed for IRAS image construction. A detailed description of HIRAS with many results is in preparation. In this paper we emphasize many of the instrumental effects incorporated in the IRAS model, including our improved 100 micron IRAS response functions.
We are conducting an extensive investigation of high-mass (OB) star formation within the dense cores of giant molecular clouds (GMCS) throughout the first Galactic quadrant of the Milky Way using enhanced resolution Infrared Astronomical Satellite (IRAS) images in combination with high-resolution ground-based observations in millimeter wave molecular transitions and radio continuum. As part of this investigation several resolution enhancement algorithms are applied to the IRAS data, including the HIgh RESolution (HIRES) algorithm developed at the IRAS Processing and Analysis Center (IPAC), as well as others ("pixon" image reconstruction). In addition, as part of a related study, we have completed a large survey of the CO emission in the first Galactic quadrant using the 15-element array detector (QUARRY) with the Five College Radio Astronomy Observatory (FCRAO) 14 m antenna, which provides sampling at an angular resolution of 50", comparable to that attained in the reprocessed IRAS data. Both of these data sets are compared with a sample of ultra-compact (UC) H II regions taken from a high-resolution multi-wavelength (6 and 20 cm) radio survey of the Galactic plane using the NRAO Very Large Array (VLA). Selected regions are observed in 1.3 mm continuum, which has proven to be particularly sensitive to the dust column density. Extensive observations of molecular clouds at high resolution in CO, CS and HCN are combined with the reprocessed IRAS high-resolution images to give a more complete picture of the physical conditions and kinematics of high-mass star forming GMCS. Our goals are to study in detail the morphology, structure, and rate of high-mass star formation within GMCs throughout the Galactic disk from the inner edge of the molecular ring to the outer Galaxy.