Thermal Continuum Research Articles

Bulk Comptonization spectra in blazars

We study the time dependent spectra produced via the bulk Compton process by a cold, relativistic shell of plasma moving (and accelerating) along the jet of a blazar, scattering on external photons emitted by the accretion disc and reprocessed in the broad line region. Bulk Comptonization of disc photons is shown to yield a spectral component contributing in the far UV band, and would then be currently unobservable. On the contrary, the bulk Comptonization of broad line photons may yield a significant feature in the soft X-ray band. Such a feature is time-dependent and transient, and dominates over the non thermal continuum only when: a) the dissipation occurs close to, but within, the broad line region; b) other competing processes, like the synchrotron self-Compton emission, yield a negligible flux in the X-ray band. The presence of a bulk Compton component may account for the X-ray properties of high redshift blazars that show a flattening (and possibly a hump) in the soft X-rays, previously interpreted as due to intrinsic absorption. We discuss why the conditions leading to a detectable bulk Compton feature might be met only occasionally in high redshift blazars, concluding that the absence of such a feature in the spectra of most blazars should not be taken as evidence against matter--dominated relativistic jets. The detection of such a component carries key information on the bulk Lorentz factor and kinetic energy associated to (cold) leptons.

Precision Fe Kalpha and Fe Kbeta Line Spectroscopy of the Seyfert 1.9 Galaxy NGC 2992 with Suzaku

We present detailed time-averaged X-ray spectroscopy in the 0.5-10 keV band of the Seyfert 1.9 galaxy NGC 2992 with the Suzaku X-ray Imaging Spectrometers (XIS). The source had a factor approximately 3 higher 2-10 keV flux (approximately 1.2 x l0(exp -11) erg per square cm per s) than the historical minimum and a factor approximately 7 less than the historical maximum. The XIS spectrum of NGC 2992 can be described by several components. There is a primary continuum, modeled as a power-law with a photon index of Gamma = 1.57(sup +0.06) (sup -0.03) that is obscured by a Compton-thin absorber with a column density of 8.01(sup +0.6) (sup -0.5)x l0 (exp 21) per square cm. . There is another, weaker, unabsorbed power-law component (modeled with the same slope as the primary), that is likely to be due to the primary continuum being electron-scattered into our line-of-sight by a region extended on a scale of hundreds of parsecs. We measure the Thomson depth of the scattering zone to be Tau = 0.072 +/- 0.021. An optically-thin thermal continuum emission component, which probably originates in the same extended region, is included in the model and yields a temperature and luminosity of KT = 0.656(sup +0.088) (sup -0.0.61) keV and approximately 1.2 +/- 0.4 x l0 (exp 40) erg per s respectively. We detect an Fe K emission complex which we model with broad and narrow lines and we show that the intensities of the two components are decoupled at a confidence level > 3 sigma. The broad Fe K alpha line has an equivalent width of 118(sup +32) (sup -61) eV and could originate in an accretion disk (with inclination angle greater than approximately 30 deg) around the putative central black hole. The narrow Fe K alpha line has an equivalent width of 1632(sup +47) (sup -26) eV and is unresolved (FWHM < 4630 km per s) and likely originates in distant matter. The absolute flux in the narrow line implies that the column density out of the line-of-sight could be much higher than measured in the line-of-sight, and that the mean (historically-averaged) continuum luminosity responsible for forming the line could be a factor of several higher than that measured from the data. We also detect the Fe K Beta line (corresponding to the narrow Fe K alpha line) with a high signal-to-noise ratio and describe a new robust method to constrain the ionization state of Fe responsible for the Fe K alpha and Fe K Beta lines that does not require any knowledge of possible gravitational and Doppler energy shifts affecting the line energies. For the distant line-emitting matter (e. g. the putative obscuring torus) we deduce that the predominant ionization state is lower than Fe VIII (at 99% confidence), conservatively taking into account residual calibration uncertainties in the XIS energy scale and theoretical and experimental uncertainties in the Fe K fluorescent line energies. From the limits on a possible Compton-reflection continuum it is likely that the narrow Fe K alpha and Fe K Beta lines originate in a Compton-thin structure.

Stellar formation in HI interstellar bubbles around massive stars

Stellar winds from O and WR stars transfer large amounts of mechanical energy and momentum into the interstellar medium. They sweep up and compress the interstellar material, creating interstellar bubbles. These structures are detected as optical ring nebulae, as thermal radio continuum sources, as infrared shells, as neutral gas voids and expanding shells in the HI line emission distribution, and as molecular shells.

Shock‐Wave Heating Model for Chondrule Formation: Prevention of Isotopic Fractionation

Chondrules are considered to have much information on dust particles and processes in the solar nebula. It is naturally expected that protoplanetary disks observed in present star forming regions have similar dust particles and processes, so study of chondrule formation may provide us great information on the formation of the planetary systems. Evaporation during chondrule melting may have resulted in depletion of volatile elements in chondrules. However, no evidence for a large degree of heavy-isotope enrichment has been reported in chondrules. In order to meet this observed constraint, the rapid heating rate at temperatures below the silicate solidus is required to suppress the isotopic fractionation. We have developed a new shock-wave heating model taking into account the radiative transfer of the dust thermal continuum emission and the line emission of gas molecules and calculated the thermal history of chondrules. We have found that optically-thin shock waves for the thermal continuum emission from dust particles can meet the rapid heating constraint, because the dust thermal emission does not keep the dust particles high temperature for a long time in the pre-shock region and dust particles are abruptly heated by the gas drag heating in the post-shock region. We have also derived the upper limit of optical depth of the pre-shock region using the radiative diffusion approximation, above which the rapid heating constraint is not satisfied. It is about 1 - 10.

High-Resolution Studies of the Dense Molecular Cores toward Massive Star-Forming Regions

We present the results of the imaging of dense molecular cores in three massive star-forming regions (IRAS 02461$+$6147, IRAS 03035$+$5819, and IRAS 06058$+$2138), which are associated with luminous infrared sources with $1 \hbox{--} 5 \times 10^{4} \,{{{L}_{\odot}}}$. The C${{18\atop} \mathrm{O}}$ ($J = 1 \hbox{--} 0$) molecular emission from the dense cores and the thermal dust and/or free-free emission at 98 GHz and 110 GHz in the hot core candidates have been imaged with a resolution of $\sim {4{}^{\mathrm {\prime \prime }}}$ using the Nobeyama Millimeter Array. We identified 28 C${{18\atop} \mathrm{O}}$ dense cores, whose mass, effective radius, and line-width range from 2.1 to 29${M}_{\odot}$, from 0.013 to 0.108 pc, and from 0.7 to $2.7 \,\mathrm{km} \,\mathrm{s}^{-1}$, respectively, and identified four thermal dust millimeter continuum sources (MCSs). We divided the C${{18\atop} \mathrm{O}}$ cores into two types, a turbulent core and a non-turbulent core. The non-turbulent cores are similar to the typical cores in the low-mass star-forming regions. The turbulent cores have a higher average $\mathrm{H}_2$ density than those of the non-turbulent cores, and the external pressure of these cores is 100–1000 times higher than that of low-mass star-forming regions. Three of the turbulent cores are associated with massive protostar candidates and the intensity peak of MCS. Massive stars are formed from the turbulent cores which are gravitationally bound. In order to form such a turbulent core a molecular cloud has a large kinetic motion and a large mass.

The Remarkable Mid-Infrared Jet of the Massive Young Stellar Object G35.20-0.74

The young massive stellar object G35.20-0.74 was observed in the mid-infrared using T-ReCS on Gemini South. Previous observations have shown that the near-infrared emission has a fanlike morphology that is consistent with emission from the northern lobe of a bipolar radio jet known to be associated with this source. Mid-infrared observations presented in this Letter show a monopolar jet-like morphology as well, and it is argued that the mid-infrared emission observed is dominated by thermal continuum emission from dust. The mid-infrared emission nearest the central stellar source is believed to be directly heated dust on the walls of the outflow cavity. The hydroxyl, water, and methanol masers associated with G35.20-0.74 are spatially located along these mid-infrared cavity walls. Narrow jet or outflow cavities such as this may also be the locations of the linear distribution of methanol masers that are found in association with massive young stellar objects. The fact that G35.20-0.74 has mid-infrared emission that is dominated by the outflow, rather than disk emission, is a caution to those who consider mid-infrared emission from young stellar objects to come only from circumstellar disks.

Thermal emission spectroscopy (5.2–38 μm) of three Trojan asteroids with the Spitzer Space Telescope: Detection of fine-grained silicates

We present thermal emission spectra (5.2–38 μm) of the Trojan asteroids 624 Hektor, 911 Agamemnon, and 1172 Aneas. The observations used the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. Emissivity spectra are created by dividing the measured Spectral Energy Distribution (SED) by a model of the thermal continuum. We employ the Standard Thermal Model (STM), allowing physical parameters (e.g., radius and albedo) to vary in order to find the best thermal continuum fit to the SED. The best-fit effective radius ( R) and visible geometric albedo ( p v ) for Hektor ( R = 110.0 ± 7.3 , p v = 0.038 −0.017 +0.028 ) and Aneas ( R = 69.1 ± 5.1 , p v = 0.044 −0.014 +0.020 ) agree very well with previous estimates, and for Agamemnon ( R = 71.5 ± 5.2 , p v = 0.062 −0.019 +0.024 ) we find slightly a smaller size and higher albedo than previously derived. Other thermal models (e.g., thermophysical) result in estimates of R and p v that vary a few percent from the STM, but the resulting emissivity spectra are identical. The emissivity spectra of all three asteroids display an emissivity plateau near 10-μm and another broader rise from ∼18 to 28 μm. We interpret these as indications of fine-grained silicates on the surfaces of these asteroids. The emissivity spectra more closely resemble emission spectra from cometary comae than those from solid surfaces and measured in the laboratory for powdered meteorites and regolith analogs. We hypothesize that the coma-like emission from the solid surfaces of trojans may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. These hypotheses need to be tested by further laboratory and theoretical scattering work as well as continued thermal emission observations of asteroids.

Large Proper Motions in the Jet of the High‐Mass YSO Cepheus A HW2

Using high angular resolution (~025-005) Very Large Array (VLA) observations made at 3.6 cm, 1.3 cm, and 7 mm during the period 1991-2004, we report the detection of large proper motions in the components of the radio continuum jet associated with the high-mass young stellar object (YSO) HW2 in the star-forming region Cepheus A. The relative proper motions observed for the two main components of the outflow, moving away from the central source in nearly opposite directions, are of the order of 140 mas yr-1, or ~480 km s-1 at a distance of 725 pc. The proper motions observed in the northeast and southwest lobes are not completely antiparallel, and the central elongated source seems to be changing orientation. We discuss possible scenarios to account for these and other observed characteristics. We also report the detection of a 7 mm compact continuum condensation of emission near the center of the thermal radio continuum jet, which we propose as the location of the exciting star.

An X-Ray Observation of the L1251 Dark Cloud

An X-ray image of the L1251 dark cloud in Cepheus was obtained with the XMM-Newton telescope. More than three dozen sources were detected above a 3 delta limit in X-ray luminosity of L(sub X = 10(exp 29) ergs/s. Among the detections are eight optically visible T Tauri stars, which had been identified in earlier work from their emission at H(alpha). The two strongest X-ray sources have steady luminosities of L(sub X) approx. 10(exp 31) ergs/s and are at the saturation limit for X-ray activity in late-type stars, L(sub X)/L(sub bol) approx. 10(exp -3). X-ray emission was also observed from two CO emission cores in L1251, core C (L1251A) and core E (L1251B). Both regions contain high-velocity molecular gas, bright IRAS sources (Class I protostars), thermal radio sources, and Herbig-Haro (HH) jets. In L1251A strong X-ray emission was discovered in close proximity to the near-inbred and radio source IRSA/VLA 7 and to IRAS 22343+7501. IRSA/VLA 7 thus appears to be the most likely source of the molecular and HH outflows in L1251A. In L1251B X-ray emission was observed from a visible T Tauri star, KP2-44, which is thought to be the driving source for HH 189. Also reported is the tentative detection of X-ray emission from VLA 3, a thermal radio continuum source in L1251B that is closely associated with the extreme Class I protostar IRAS 22376+7455.

A Cool Filament Crossing the Warm Protostar DR 21(OH): Geometry, Kinematics, Magnetic Vectors, and Pressure Balance

Here we made multiconstituent (gas/dust/field), homogeneous maps of a length of a vertical filament including DR 21(OH) proper. We observed at a radial velocity around -3.5 km s-1 the 12CO, 13CO, and C18O gas in the J = 3-2 lines, as well as the thermal dust continuum near 850 ?m and the dust's linear polarization properties, all near the same wavelength (850 ?m) and with the same angular resolution (14'') using the same antenna (James Clerk Maxwell Telescope [JCMT]). We deduce the energy components in the cool filament, and we compute the pressure components there and those in the surrounding region. The vertical filament has a total pressure composed about equally of turbulent energy and magnetic energy. The magnetic field in the vertical filament appears to go across the filament and has an estimated strength of 100-200 ?G in the sky plane. Near the map center, we find a low-velocity outflow from DR 21(OH), with a pattern of blue southeast gas and red northwest gas. For the central DR 21(OH) protostar, our JCMT polarimetric data allow us to infer a median plane-of-sky field of 780 ?G oriented mainly east-west (as seen with the JCMT beam of 14''), which is comparable to the published Zeeman data along the line of sight (-530 ?G).

Review of Spitzer Space Telescope observations of small bodies

The Spitzer Space Telescope (SST), aloft for over two years at time of writing, has so far devoted almost 600 hours of observing time to Solar System science, and small bodies make up a significant fraction of the objects that have been observed. For the first time we now have high accuracy mid-infrared data to study the fundamental mineralogical and physical properties of a large number of objects of different types. In this paper we review some of the exciting recent results derived from SST photometry (in six bands from 3.6 to 70 $\mu$ m) and spectroscopy (from 5 to 40 $\mu$ m) of asteroids and comets. The observations reveal their spectral energy distributions (SEDs), and we discuss three important science goals that can be addressed with these data: (1) finding compositional diagnostics of these objects, (2) determining their bulk thermal properties, and (3) understanding the surface evolution of primitive and icy bodies. We focus primarily on comet-asteroid transition objects, low-albedo asteroids, cometary nuclei, Trojans, Centaurs, and trans-Neptunian objects. In particular, we will show: emissivity features in the SEDs and identification of the compositional sources, the constraints on thermal inertia and infrared beaming through the samples of the thermal continuum, and an intercomparison of albedos across dynamically-related bodies.

Deposing the Cool Corona of KPD 0005+5106

The ROSAT PSPC pulse height spectrum of the peculiar He-rich hot white dwarf KPD 0005+5106 provided a great surprise when first analysed by Fleming, Werner & Barstow (1993). It defied the best non-LTE modelling attempts in terms of photospheric emission from He-dominated atmospheres including C, N and O and was instead interpreted as the first evidence for a coronal plasma around a white dwarf. We show here that a recent high resolution Chandra LETGS spectrum has more structure than expected from a thermal bremsstrahlung continuum and lacks the narrow lines of H-like and He-like C expected from a coronal plasma. Moreover, a coronal model requires a total luminosity more than two orders of magnitude larger than that of the star itself. Instead, the observed 20-80 AA flux is consistent with photospheric models containing trace amounts of heavier elements such as Fe. The soft X-ray flux is highly sensitive to the adopted metal abundance and provides a metal abundance diagnostic. The weak X-ray emission at 1 keV announced by O'Dwyer et al (2003) instead cannot arise from the photosphere and requires alternative explanations. We echo earlier speculation that such emission arises in a shocked wind. Despite the presence of UV-optical O VIII lines from transitions between levels n=7-10, no X-ray O VIII Ly alpha flux is detected. We show that O VIII Lyman photons can be trapped by resonant scattering within the emitting plasma and destroyed by photoelectric absorption.

ISO observations of the Galactic center interstellar medium

We present fine structure and recombination line observations of the ionized gas toward a sample of 18 sources located within 300 pc of the center of the Galaxy (hereafter Galactic center, GC). The sources were selected as molecular clouds located far from thermal continuum sources. The fine structure lines from [Nii] and [Siii] were detected in 16 sources. In 10 sources we even detected the [Oiii] 88 μm line. Several techniques have been used to determine lower and upper limits to the extinction toward each source to correct the observed line fluxes. The derived electron densities of the ionized gas vary from ~100 to ≤30 cm-3. For some sources we were able to derive N, S and Ne abundances. We found that they are similar to those measured in the Hii regions in the 5-kpc ring and in the nuclei of starburst galaxies. The fine structure lines ratios measured for all the sources can be explained by photo-ionization with an effective temperature of the ionizing radiation of 32 000-37 000 K and an ionization parameter, U, of . The highest excitation is found in the Radio Arc region but it does not decrease smoothly with distance. There must be more ionizing sources distributed over the Galactic center than the known clusters of massive stars. Most of the clouds are located far (up to 45 pc for M-0.96+0.13) from the prominent continuum complexes (Sgr C, B ...). However, it is possible that the clouds are ionized by escaped photons from those complexes. The comparison of the effective temperatures of the ionizing radiation to the measured Lyman continuum photon emission rate implies that the clouds are indeed ionized by distant sources. The excitation ratios, effective temperature and ionization parameter measured in the GC are similar to those found in some low excitation starburst galaxies. The [Neiii]/[Neii] line ratios measured in the GC sources are consistent with the results of the Thornely et al. (2000) model for a short burst of massive star formation less than 8 Myr ago. We have also found that the [Neii] 13 μm to far-infrared continuum ratio measured for the GC sources is similar to that of external galaxies, supporting the idea of Sturm et al. ([CITE]) that the far-infrared continuum in Active Galaxies is dominated by dust heated by stellar radiation rather than by the AGN.

Quiescent Dense Gas in Protostellar Clusters: The Ophiuchus A Core

We present combined BIMA interferometer and IRAM 30 m telescope data of N2H+ 1-0 line emission across the nearby dense star-forming core Ophiuchus A at high linear resolution (e.g., ~1000 AU). Six maxima of integrated line intensity are detected, which we designate Oph A-N1 through N6. The N4 and N5 maxima coincide with the starless continuum objects SM1 and SM2, respectively, but the other maxima do not coincide with previously identified objects. In contrast, relatively little N2H+ 1-0 emission coincides with the starless object SM2 and the Class 0 protostar VLA 1623. The FWHM of the N2H+ 1-0 line, ΔV, varies by a factor of ~5 across Oph A. Values of ΔV < 0.3 km s-1 are found in 14 locations in Oph A, but only that associated with N6 is both well defined spatially and larger than the beam size. Centroid velocities of the line, VLSR, vary relatively little, having an rms of only ~0.17 km s-1. Small-scale VLSR gradients of less than 0.5 km s-1 over ~0.01 pc are found near SM1, SM1N, and SM2 but not N6. The low N2H+ abundances of SM2 or VLA 1623 relative to SM1, SM1N, or N6 may reflect relatively greater amounts of N2 adsorption onto dust grains in their colder and probably denser interiors. The low ΔV of N6, i.e., 0.193 km s-1 FWHM, is only marginally larger than the FWHM expected from thermal motions alone, suggesting that turbulent motions in the Oph A core have been reduced dramatically at this location. The nondetection of N6 in previous thermal continuum maps suggests that interesting sites possibly related to star formation may be overlooked in such data.

On the band-to-continuum intensity ratio in the infrared spectra of interstellar carbonaceous dust

Published interpretations of the relative intensity variations of the Unidentified Infrared Bands (UIBs) and their underlying continuum are discussed. An alternative model is proposed, in which a single carrier for both emits a) mostly a continuum when it is electronically excited by photons (visible or UV), or b) exclusively the UIBs, when only chemical energy is deposited by H capture on its surface, inducing only nuclear vibrations. The bands will dominate in atomic H regions but will be overcome by thermal continuum radiation when the ambient field is strong but lacks dissociating photons (900-1100 Angstroms). The model applies to PDRs as well as to limbs of molecular clouds in the ISM and agrees quantitatively with recent satellite observations. It gives indications on atomic H density and UIB intensity provided the ambient radiation field is known. It invokes no chemical, electronic, structural or size change in order to interpret the observed intensity variations.

Exploring the Jet/Accretion Flow Relationship in low Disk Luminosity Sources

Astrophysical jets seem to gain strength disproportionate to the power of their associated accretion flow, making low-luminosity sources ideal targets for studies of the role of outflows. Radio/X-ray correlations have supported the case for a strong relationship between the jets and the hard X-ray emitting regions, and here we explore the strongest scenario where the base of the jets subsumes the role of the corona. The properties of coronae, as inferred from spectral models, are very similar to what is empirically required at jet bases assuming conservation laws hold. We present a few preliminary fits to simultaneous radio and X-ray data sets from our GX 339-4 and Cyg X-1 campaigns. The fits are performed in detector space, and include a jet plus thermal disk continuum model, with added Gaussian line and non-relativistic reflection features similar to the approach of other X-ray models. We find that we can fit the entire radio through X-ray spectrum quite well, with any deviation occurring in the line/reflection region. The results suggest that a jet/corona unification can provide a reasonable description of the data. Future work will benefit from a more complex approach to the disk feedback features.

The Late Appearance of Heiin V445 Puppis

We present a 0.9–2.5 μm spectrum of V445 Puppis obtained about 3 years after maximum brightness. The spectrum shows thermal continuum emission and two narrow, structured emission lines: He I 1.0830 μm and 2.0581 μm. We interpret this to be emission from a hydrogen-deficient, helium-rich, low-temperature nebula surrounded by an optically thick dust shell, probably carbonaceous.

ISO observations of the Galactic center interstellar medium

The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100–200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines ([Oi] 63 and 146 μm, [Cii] 158 μm, [Si ii] 35 μm, [S i] 25 μm and [Fe ii] 26 μm), which are the main coolants of the gas with kinetic temperatures of several hundred K. We also present the full continuum spectra of the dust between 40 and 190 μm. All the clouds exhibit a cold dust component with a temperature of ~15 K. A warmer dust component is also required to fit the spectra. The temperature of this dust component changes between 27 and 42 K from source to source. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 103 cm-3 and an incident far-ultraviolet field 103 times higher than the local interstellar radiation field. The fine structure line emission arises in PDRs in the interface between a diffuse ionized gas component and the dense molecular clouds. The [Cii] 158 μm and [Si ii] 35 μm lines also have an important contribution from the ionized gas component. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10–30% of the total H2 column density with a temperature of ~150 K. We discuss other possible heating mechanisms for the rest the warm molecular gas, such as non-stationary PDRs, X-ray Dominated Regions (XDRs) or the dissipation of supersonic turbulence.

Sharpless 170 and the surrounding interstellar medium

Sharpless 170 is a diffuse H region ionized by a single main sequence O-star located near the periphery of a small dense molecular cloud at a distance of ∼2 kpc. We describe wide-field observations of the region in the radio continuum, in H and CO-lines, and in the far-infrared which delineate the major ionized, atomic, molecular and dust components of the gas affected by the exciting star. From the thermal continuum emission we estimate the mass of ionized gas at ∼350 Mwithin a radius of ∼ 7p c. The H (λ21 cm) and far-infrared observations show an extended low-density atomic component, of ∼1000 M� , within an irregular boundary surrounding the ionized gas of mean radius ∼10 pc. Mean densities in the H and H are similar, in the range 9-16 nucleons cm −3 . CO emission shows a molecular cloud of ∼1150 Mwithin an area 6 pc × 4 pc with densities ∼2000 nucleons cm −3 . A compact infrared component coincides with the cloud. The exciting star is located on the near side of the cloud, just inside the southern periphery. Sh170 is an example of a young H region with the ionized gas, seen in Hα emission, streaming outward in the manner of a champagne flow. Although the observed velocities of the H are close to the mean velocity of the CO cloud, the morphology of the associated atomic hydrogen closely resembles that seen in the surrounds of other young H regions which show clear evidence of expansion of their H. We propose that much of the H is a diffuse dissociation zone beyond the ionization front, in directions from the star within a wide annulus, approximately transverse to the line-of-sight, between the dense photon- bounded region on the far side of the star and the density-bounded ionized flow region on the near side. In this view, much of the associated atomic gas, like the ionized gas, has been eroded from the molecular cloud in a small fraction (≤10%) of the star's main sequence lifetime.

The emissivity of dust grains in spiral galaxies

We use the radiation transfer simulation of Xilouris et al. (1999) to constrain the quantity of dust in three nearby spiral galaxies (NGC 4013, NGC 5907 and NGC 4565). The predicted visual optical depth from the model is compared with the thermal continuum radiation detected from NGC 4013 and NGC 5907 at 850 µm and from NGC 4565 at 1.2 mm. The former is based on SCUBA images of NGC 4013 and NGC 5907, reduced and presented for the first time in this work. The comparison of visual optical depth and 850 µm (1.2 mm) emission yields the emissivity of dust grains in the submillimeter (millimeter) waveband. We infer a value of 1.2 × 10 4 for the emissivity at 850 µm which is a factor 4 higher than the benchmark, semi- empirical model of Draine & Lee (1984). At 1.2 mm our values are a factor 1.5 higher than this model. Our estimates are more closely aligned with recent measurements carried out in the laboratory on amorphous carbon and silicate particulates. A comparison between the distribution of 850 µm (1.2 mm) surface brightness and the intensity levels in the 12 CO(1-0) and 21 cm lines underlines the spatial association between dust detected in the submillimeter/millimeter waveband and molecular gas clouds. We suggest that the relatively high emissivity values that we derive may be attributable to amorphous, fluff yg rains situated in denser gas environments.