Thermal Continuum Research Articles

The dust coma of Comet P/Giacobini-Zinner in the infrared.

We present 1-20 micrometers photometry of P/Giacobini-Zinner obtained at the NASA Infrared Telescope Facility, during 1985 June-September (r = 1.57-1.03 AU). A broad, weak 10 micrometers silicate emission feature was detected on August 26.6; a similar weak emission feature could have been hidden in the broadband photometry on other dates. The total scattering and emitting cross section of dust in the inner coma was similar to that in other short-period comets, but a factor of 10 (r = 1.56 AU) to 100 (r = 1.03 AU) lower than the amount of dust in Comet Halley. The thermal emission continuum can be fit with models weighted toward either small or large absorbing grains. The dust production rate near perihelion was approximately 10(5) g/s (small-grain model) to approximately 10(6) g/s (large-grain model). The corresponding dust/gas mass ratio on August 26 was approximately 0.1-1. A silicate-rich heterogeneous grain model with an excess of large particles is compatible with the observed spectrum of Giacobini-Zinner on August 26. Thus, weak or absent silicate emission does not necessarily imply an absence of silicates in the dust, although the abundance of silicate particles < or = 1 micrometer radius must have been lower than in Comet Halley.

NGC 2024: excitation of the dense cores

Measurements of CS and C 34 S J=7-6 toward five of the six millimetre continuum sources in NGC 2024 were used to estimate the excitation temperatures and column densities of warm, dense gas associated with the cores. For estimated CS source sizes of ∼20 arcsec, the calculated excitation temperatures are ≥24 K, consistent with previous studies. A correlation between CS column density and hydrogen column density derived from the thermal dust continuum shows that it is self-consistent to assume internally heated cores and a close relationship between the characteristic temperatures of the CS-traced gas and the dust traced by the millimetre continuum. The nature of the individual sources is also discussed

The wind geometry of the Wolf-Rayet star HD 191765

A time-dependent spectropolarimetric data set of HD 191765 in the wavelength range 3159-7593 A is presented. At all epochs the present observations display a large and strongly wavelength-dependent continuum polarization and reduced levels of polarization across the emission lines. The data imply a significant intrinsic continuum polarization which requires a general deviation of the electron distribution from spherical symmetry. The global shape is quite stable as a function of time; small fluctuations may arise from localized density/temperature changes. The line polarizations are consistent with an axisymmetric wind geometry and ionization stratification. A qualitative model for polarization in a Wolf-Rayet atmosphere is developed. It is argued that the blueward rise of the continuum polarization in HD 191765 can be explained if the density in the wind is high, resulting in a competition of thermal and electron-scattering continuum opacity in the vertical.

Wavelength dependence of the photoassociation of Kr(4p 61S 0)−F(2p 52P) collision pairs

The excitation spectrum for the photoassociation (bound←free absorption) of thermalized pairs of Kr(4p 6 1S 0) and F(2p 5 2P) atoms in Kr/F 2 gas mixtures at 300 K has been recorded in the ultraviolet (208⩽λ⩽250 nm). By photodissociating F 2 in the presence of Kr and subsequently photoexciting Kr—F collision pairs in the ground state thermal continuum with tunable laser radiation, deeply modulated Franck—Condon structure associated with bound←free transitions terminating on the lowest 26 vibrational levels of the B 2Σ ion pair state of the KrF molecule was observed. Since the KrF ground state interaction potential is repulsive, photoassociation occurs in a narrow (Δ R≈0.7 Å) Franck—Condon region and structural information regarding the upper and lower state potentials, as well as the dependence of the electronic transition moment on internuclear separation, can be extracted from the spectrum. The Kr—F system is presented here as an example of the effectiveness of photoassociation as a tool with which one can determine the chemical, structural and optical properties of small excimer molecules that are otherwise difficult to access experimentally.

The molecular structure around HL Tauri

view Abstract Citations (142) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Molecular Structure around HL Tauri Sargent, A. I. ; Beckwith, S. V. W. Abstract High-resolution (2.7 arcsec) observations of (C-13)O (1-0) emission around the young star HL Tauri confirm the presence of circumstellar gas distributed in an elongated structure. Thermal continuum emission, centered on the stellar position, is unresolved (radius 190 AU or less) and suggests a mass equal to or greater than 0.1 solar mass. The (C-13)O extends along PA 160 deg to radii of almost 2000 AU but is less than 380 AU in width. Extended molecular emission is observed only at velocities close to the systemic value, about 6.6 km/s. Higher velocity gas, and most of the system mass, is confined to a core of radius 190 AU or less. Satellite condensations, with masses only a few percent of the main concentration, are also observed; these must be less than 100 AU in size to survive. The general form of the rotation curve is consistent with the gas moving in bound orbits about the central star. A low-velocity molecular feature, emanating from the stellar position along the disk axis, may be a low-velocity neutral flow from the disk itself. Publication: The Astrophysical Journal Pub Date: November 1991 DOI: 10.1086/186207 Bibcode: 1991ApJ...382L..31S Keywords: Molecular Clouds; Shell Stars; Stellar Envelopes; Carbon Monoxide; Pre-Main Sequence Stars; Astrophysics; STARS: CIRCUMSTELLAR SHELLS; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (2)

Location of the thermal continuum source Sagittarius D

view Abstract Citations (15) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Location of the Thermal Continuum Source Sagittarius D Lis, Dariusz C. Abstract Very peculiar kinematics of the dense molecular gas in the inner 500 pc of the Milky Way are discussed on the basis of observations of the CS J = 5-4 and CO J = 3-2 emission toward the Sgr D continuum source. The distribution of the CS emission correlates well with the millimeter and radio continuum emission, which suggests that these tracers all originate in the same region. The central velocity of the CS line is consistent with that of the H91-alpha radio recombination line, which indicates that the H II region is associated with the core. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/186152 Bibcode: 1991ApJ...379L..53L Keywords: Continuous Radiation; Forbidden Bands; Molecular Clouds; Sagittarius Constellation; Thermal Emission; Extragalactic Radio Sources; Galactic Nuclei; Infrared Sources (Astronomy); Milky Way Galaxy; Star Formation; Astrophysics; GALAXIES: NUCLEI; GALAXIES: THE GALAXY; INFRARED: SOURCES; INTERSTELLAR: MOLECULES; RADIO SOURCES: GENERAL full text sources ADS | data products SIMBAD (3)

The thermal continuum in coronal loops: Instability criteria and the influence of perpendicular thermal conduction

The linear MHD spectrum is investigated for cylindrical equilibrium models under typical coronal conditions. Non-ideal effects are included and attention is focussed on the thermal instability and the influence of perpendicular thermal conduction. It is shown that, when thermal conduction across magnetic field lines is neglected, the ‘classic’ Alfven and slow continua are supplemented by a new ‘thermal’ continuum. Surprisingly, the existence of this non-ideal continuous spectrum appears to have been overlooked for a very long time. Unlike the (still purely oscillatory) Alfven continuum modes and the slow continuum modes (overstable or damped), the thermal continuum modes are exponentially growing or decaying in time. As with the Alfven and slow continua, discrete modes may be present above or below the thermal continuum, depending upon the choice of equilibrium parameters. These modes are localized using a simple WKB approach. The knowledge of the thermal subspectrum is then exploited to find necessary and sufficient conditions for instability.

On the existence of a thermal continuum in nonadiabatic magnetohydrodynamic spectra

The linear magnetohydrodynamic (MHD) spectrum of an infinite straight cylinder is investigated, taking nonadiabatic terms in the energy equation into account. It is shown that in nonideal MHD, a new continuum arises. Unlike the ‘‘classical’’ Alfvén and slow continua, this thermal continuum consists of modes that are purely exponentially growing or decaying in time. Thus unstable continua can be encountered.

Effect of Source Morphology on Thermal Radio Continuum Spectra

The thermal radio continuum spectra derived from a selection of nebular models are compared. One class of models arises from three-dimensional distributions of emissivity, while the remainder are specified in a more abstract way. It is shown that the most important distinguishing feature between the models is the proportion of the nebular solid angle occupied by very faint emission. This finding is developed into a measure of nebular clumping. Practical aspects of the modelling are examined by applying a selection of the model spectra to the flux densities observed for the planetary nebula IC418.

The Stellar Association LH 99

The stellar content of this stellar association, related to the supernova remnant N 157B = SNR 0538-691, was up to now unknown, except for three Wolf-Rayet stars and a few red supergiant candidates. A thorough study based on UBV photometry, spectra of 95 stars and nebular spectra will be described elsewhere (see also Schild et al. in these Proceedings).The outstanding properties of this region are :A. the presence of a shell-shaped supernova, which dominates the thermal radio continuum. It is relatively faint in [OUI], strong in [Sil], [Fe 4658] and He II 4686. The expansion velocity reported by Chu and Kennicutt (1988) is 180 km/s. In addition, many extensions and filaments are found all over a wide area, and possibly there has been more than one supernova responsible.

UKIRT observations of the mid-infrared and submillimetre thermal continuum in W75N

Maps in the mid-infrared and submillimetre thermal continuum are presented for the star-forming region W75N. Observations at 10 and 20 μm, with ∼ 5-arcsec resolution, trace warm dust heated by members of the embedded young stellar cluster. The submillimetre continuum emission traces the cool dust in the cloud core surrounding the luminosity sources. The core has been spatially resolved at 350 μm but shows no disc-like structure related to the known molecular outflow on scales larger than ∼ 0.2 pc.

H I emission-absorption experiments toward Galactic H II regions

21 cm atomic hydrogen emission-absorption experiments have been conducted toward 19 Galactic H II regions lying within 1 deg of the Galactic plane. With the exception of one source, absorption has been detected at the hydrogen recombination line velocity of each H II region. 141 absorption features at positive LSR velocities have been identified. If the observed absorption is solely due to cold gas attenuating thermal continuum, these data can provide kinematic distance estimates for the H II regions. Approximately 40 percent of the absorption features are correlated with (C-13)O emission. Nearly 17 percent of the (C-13)O lines appear not to be associated with H I absorption. The derived spin temperatures for these features range from 25 to about 350 K. The measured optical depths are strongly correlated with the spin temperatures. 14 refs.

Primeval and “Rejuvenated” Galaxies

The observational strategy proposed by Ozernoy (1988) to search for primeval galaxies via their close interactions is discussed. The theoretical arguments in favor of early galactic interactions are reviewed; the observational signatures of gas and dust-grain heating by galaxy collisions (e.g., thermal continuum emission peaking in the FIR) are characterized; and a qualitative assessment is given of the prospects for detecting, determining the redshifts of, and imaging primeval or rejuvenated galaxies (older galaxies undergoing starburst activity) using telescope facilities now under development. Also considered are the potential implications of the search results for elliptical galaxies as mergers, gas left in galaxy systems, superwinds, cosmological megamasers, searches for quasars at z greater than 4, ultraluminous starburst galaxies, and peaks in the metallicity distribution function. 29 refs.

Future AGN Research II

The “standard model” of AGN involves a supermassive black hole accreting gas, possibly through a flattened disk. This model enjoys widespread popularity among astronomers, but convincing proof has been elusive. Recent observations of “dormant” black holes in nearby galactic nuclei encourage the idea that black holes lie at the centers of AGN. An understanding of the nature of the accretion flow seems most likely to come from Doppler shifts of spectral features. The thermal continuum from the inner disk offers an intriguing opportunity. Theoreticians should not neglect the study of the thin disks that should exist for some AGN parameters, as these relatively “tame” objects may yield the most reliable observational tests. The broad emission lines may represent the debris of tidally disrupted stars.

Turbulent and Ordered Motions in HII Regions

SummaryThe Fill regions morphology have suggested, from the beginning of this century, the evidence of internal motions, very few gas radial velocities (RV) on the brightest “nebulae” had been obtained, because of obvious technical difficulties. The modern Fabry-Perot interferometers and the new integral Field Spectrograph provide abundant RV fields at various scales in the interstellar gas. New ways in imagery, owing to these new instruments, lead to a better analyse of both, the velocities field and their precise morphology free of stellar, dusty, and non thermal continua. The richness and the guallty of RV data, a better understanding of the expansion phenomena authorize again a rebirth of the turbulence observations of the ionized interstellar gas.

Kinematics of the H II region sharpless 142. III - Molecular line observations and analysis of the IRAS data

The molecular cloud associated with the H II region S142 has been mapped in the J = 1-0 transition lines of (C-12)O and (C-13)O. It is found that the molecular cloud has two components separated by 25 parsecs and has a total mass of about 5600 solar masses. High-resolution observations in these transition lines reveal the presence of condensations in the molecular cloud. An analysis of IRAS data is presented, showing that the IR thermal continuum emission has a larger extent at 100 microns than at 12 microns. The mean dust color temperature is shown to be 169 K for the hot, small grains, and 34 K for the colder, larger grains. It is shown that the total dust mass is about 60 solar masses and the total IR flux is 8.5 + or - 0.8 X 10 to the -10 W/sq m. A gas-to-dust ratio of about 200 is derived for the gas complex S142. A mass of 1000 solar masses is found for the stellar cluster associated with S142. The star formation efficiency for the S142 gas complex is estimated at about 8 percent. 44 references.

A new component of terrestrial radio emission observed from ISEE‐3 and ISEE‐1 in the solar wind

At a time when the AE index was 1300, the radio receivers on board ISEE‐3 (216 RE sunward of Earth) and ISEE‐1 recorded a new type of terrestrial radio emission between 20 and 50 kHz. The radiation had a smooth time profile, with a negative frequency drift; its angular diameter at 47 kHz was at least 170°. This smooth component followed the onset of a TKR burst whose angular diameter was 50° at the same frequency 47 kHz and varied as f−2, i.e., was controlled by interplanetary scattering. Because of its time association with the TKR burst, the source of the smooth radiation must have been in the Earth environment. We suggest that it was produced or leaked into the interplanetary medium very far from Earth in order that the scattering path to ISEE‐3 be long enough (2000 RE) to account for the observed angular diameter. More than 50 such events, clearly distinct from the Earth non thermal continuum, were observed between 1978 and 1983.

The X-ray spectrum of the bright galactic bulge source X1813?14=GX17+2

The spectral analysis of the persistent X-ray flux from the bright galactic bulge X-ray source and an X-ray burster X1813−14=GX17+2 is presented. A model with a single thermal bremsstrahlung continuum plus iron emission line at ∼6.7 keV fits the lower and higher intensity state data reasonably well. The line feature observed here is reproduced by a single emission line at ∼6.7 keV with intrinsic line width less than 0.7 keV. The equivalent width of the line ranges between 52 and 43 eV, depending on the intensity state of the source. This implies that the observed line is mostly due to helium-like iron (Fexxv). The properties of the line suggest that line-emitting matter is located far outside the neutron star.

Observations of infrared emission from a fast-moving knot in Cassiopeia A

Broadband (10 microns) and spectroscopic (8.99, 10.52 and 12.81 microns) scans were made of the metal-rich knots in Cas A using instrumentation at the NASA IR Telescope Facility. The observations were performed to distinguish between thermal continuum emission and other features of the metal-rich knots. A faint, forbidden Si IV line was detected, and upper limits were set on the forbidden Ar III and forbidden Ne II emissions, which are now expected to be completely due to lines that may be accessible to the 12 microns sensor on the IRAS. The implications of the measurements for the interpretation of broadband data on metal-rich remnants are discussed.

A simultaneous X-ray and radio observation of a flare from Algol

An X-ray flare was observed from Algol using the low and medium energy detectors on the European Space Agency's EXOSAT observatory. Spectra obtained during the flare are well fitted by thermal continua while an Fe XXV emission feature was also detected. The strength of this feature indicates a cosmic abundance for iron. The data indicate that the flare occurred in a loop of height approximately 0.25 of the K star radius & with a magnetic field >300 Gauss.