Thermal Radio Continuum Research Articles

Metallicity Structure across the Galactic Disk: Radio Observations of H ii Regions

AbstractHii regions are the sites of massive star formation and are the archetypal tracers of spiral arms. Because of their short lifetimes (<10 Myr) their abundances provide a measure of the nuclear processing of many stellar generations. Here we review our ongoing efforts to explore the metallicity structure of the Galactic disk by observing radio recombination line (RRL) and thermal radio continuum emission toward Hii regions. The RRL-to-continuum ratio provides an accurate measure of the electron temperature which is used as a proxy for metallicity. Since collisionally excited lines from metals (e.g., O, C) are the main coolant in Hii regions, the thermal electron temperature is well correlated with metallicity (e.g., [O/H]). We determine Hii region distances from maser parallax measurements when possible; otherwise we use kinematic distances. Such radio diagnostics of Hii regions yield an extinction free tracer to map the metallicity distribution across the entire Galactic disk.

Hα imaging survey of Wolf–Rayet galaxies: morphologies and star formation rates

The Halpha and optical broadband images of 25 nearby Wolf-Rayet (WR) galaxies are presented. The WR galaxies are known to have the presence of a recent ($\le$10 Myr) and massive star formation episode. The photometric Halpha fluxes are estimated, and corrected for extinction and line contamination in the filter pass-bands. The star formation rates (SFRs) are estimated using Halpha images and from the archival data in the far-ultraviolet (FUV), far-infrared (FIR) and 1.4 GHz radio continuum wave-bands. A comparison of SFRs estimated from different wavebands is made after including similar data available in literature for other WR galaxies. The Halpha based SFRs are found to be tightly correlated with SFRs estimated from the FUV data. The correlations also exist with SFRs estimates based on the radio and FIR data. The WR galaxies also follow the radio-FIR correlation known for normal star forming galaxies, although it is seen here that majority of dwarf WR galaxies have radio deficiency. An analysis using ratio of non-thermal to thermal radio continuum and ratio of FUV to Halpha SFR indicates that WR galaxies have lesser non-thermal radio emission compared to normal galaxies, most likely due to lack of supernova from the very young star formation episode in the WR galaxies. The morphologies of 16 galaxies in our sample are highly suggestive of an ongoing tidal interaction or a past merger in these galaxies. This survey strengthens the conclusions obtained from previous similar studies indicating the importance of tidal interactions in triggering star-formation in WR galaxies.

G192.8-1.1: A CANDIDATE OF AN EVOLVED THERMAL COMPOSITE SUPERNOVA REMNANT REIGNITED BY NEARBY MASSIVE STARS

G192.8-1.1 has been known as one of the faintest supernova remnants (SNRs) in the Galax until the radio continuum of G192.8-1.1 is proved to be thermal by Gao et al. (2011). Yet, the nature of G192.8-1.1 has not been fully investigated. Here, we report the possible discovery of faint non-thermal radio continuum components with a spectral index <TEX>${\alpha}{\sim}0.56(S_{\nu}{\propto}{\nu}^{-{\alpha}})$</TEX> around G192.8-1.1, while of the radio continuum emission is thermal. Also, our Arecibo <TEX>$H_I$</TEX> data reveal an <TEX>$H_I$</TEX> shell, expanding with an expansion velocity of <TEX>$20-60km\;s^{-1}$</TEX>, that has an excellent morphological correlation with the radio continuum emission. The estimated physical parameters of the <TEX>$H_I$</TEX> shell and the possible association of non-thermal radio continuum emission with it suggest G192.8-1.1 to be an~0.3 Myr-old SNR. However, the presence of thermal radio continuum implies the presence of early-type stars in the same region. One possibility is that a massive star is ionizing the interior of an old SNR. If it is the case, the electron distribution assumed by the centrally-peaked surface brightness of thermal emission implies that G192.8-1.1 is a "thermal-composite" SNR, rather than a typical shell-type SNR, where the central hot gas that used to be bright in X-rays has cooled down. Therefore, we propose that G192.8-1.1 is an old evolved thermal-composite SNR showing recurring emission in the radio continuum due to a nearby massive star. The infrared image supports that the <TEX>$H_I$</TEX> shell of G192.8-1.1 is currently encountering a nearby star forming region that possibly contains an early type star(s).

Thermal and non-thermal emission in the Cygnus X region

Radio continuum observations detect non-thermal synchrotron and thermal bremsstrahlung radiation. Separation of the two different emission components is crucial to study the properties of diffuse interstellar medium. The Cygnus X region is one of the most complex areas in the radio sky which contains a number of massive stars and HII regions on the diffuse thermal and non-thermal background. More supernova remnants are expected to be discovered. We aim to develop a method which can properly separate the non-thermal and thermal radio continuum emission and apply it to the Cygnus X region. The result can be used to study the properties of different emission components and search for new supernova remnants in the complex. Multi-frequency radio continuum data from large-scale surveys are used to develop a new component separation method. Spectral analysis is done pixel by pixel for the non-thermal synchrotron emission with a realistic spectral index distribution and a fixed spectral index of beta = -2.1 for the thermal bremsstrahlung emission. With the new method, we separate the non-thermal and thermal components of the Cygnus X region at an angular resolution of 9.5arcmin. The thermal emission component is found to comprise 75% of the total continuum emission at 6cm. Thermal diffuse emission, rather than the discrete HII regions, is found to be the major contributor to the entire thermal budget. A smooth non-thermal emission background of 100 mK Tb is found. We successfully make the large-extent known supernova remnants and the HII regions embedded in the complex standing out, but no new large SNRs brighter than Sigma_1GHz = 3.7 x 10^-21 W m^-2 Hz^-1 sr^-1 are found.

MILLIMETER AND SUBMILLIMETER HIGH ANGULAR RESOLUTION INTERFEROMETRIC OBSERVATIONS: DUST IN THE HEART OF IRAS 18162–2048

The GGD27 complex includes the HH 80-81-80N system, which is one of the most powerful molecular outflows associated with a high mass star-forming region observed up to now. This outflow is powered by the star associated with the source IRAS 18162-2048. Here we report the detection of continuum emission at sub-arcsec/arcsec resolution with the Submillimeter Array at 1.36mm and 456microns, respectively. We detected dust emission arising from two compact cores, MM1 and MM2, separated by about 7" (~12000AU in projected distance). MM1 spatially coincides with the powerful thermal radio continuum jet that powers the very extended molecular outflow, while MM2 is associated with the protostar that drives the compact molecular outflow recently found in this region. High angular resolution obervations at 1.36mm show that MM1 is unresolved and that MM2 splits into two subcomponents separated by ~1". The mass of MM1 is about 4Msun and it has a size of <300AU. This is consistent with MM1 being associated with a massive and dense (n(H2)>10^9cm-3) circumstellar dusty disk surrounding a high-mass protostar, which has not developed yet a compact HII region. On the other hand, the masses of the two separate components of MM2 are about 2Msun each. One of these components is a compact core with an intermediate-mass young protostar inside and the other component is probably a pre-stellar core. MM1 is the brigthest source at 1.36mm, while MM2 dominates the emission at 456microns. These are the only (sub)millimeter sources detected in the SMA observations. Hence, it seems that both sources may contribute significantly to the bolometric luminosity of the region. Finally, we argue that the characteristics of these two sources indicate that MM2 is probably in an earlier evolutionary stage than MM1.

GSH 91.5+2–114: A large H i shell in the outer part of the Galaxy

GSH91.5+2-114 is a large HI shell located in the outer Galaxy at a kinematic distance of about 15 kpc. It was first identified in the Canadian Galactic Plane Survey (CGPS) by Pineault et al. (2002) as being possibly associated with objects possessing infrared colors which indicates strong stellar winds. The HI shell has no obvious continuum counterpart in the CGPS radio images at 408 and 1420 MHz or in the IRAS images. We found no evidence for early-type massive stars, most likely as a result of the large extinction that is expected for this large distance. An analysis of the energetics and of the main physical parameters of the HI shell shows that this shell is likely the result of the combined action of the stellar winds and supernova explosions of many stars. We investigate whether a number of slightly extended regions characterized by a thermal radio continuum and located near the periphery of the HI shell could be the result of star formation triggered by the expanding shell.

Revealing the transition from post-AGB stars to planetary nebulae: non-thermal and thermal radio continuum observations

Midcourse Space eXperiment and Infrared Astrorcomical Satellite colour diagnostics as well as OH maser profile characteristics were used to select a sample of post-asymptotic giant branch (pAGB) candidates for a radio continuum detection experiment with the Australia Telescope Compact Array. Seven out of 28 sources, six of which are new detections, show a continuum. A planetary nebula serendipitously detected in the field of an undetected pAGB candidate also reveals radio continuum. The radio continuum properties of these eight sources are described. Almost half have non-thermal emission. DUSTY modelling of the infrared spectral energy distributions (SEDs) of the three strongest detections reveals that they all have central stars with temperatures substantially lower than that required for significant photoionization, leading us to infer that the radio continuum has arisen from wind-shock interactions. This hypothesis is consistent with the detection of non-thermal radio emission in one of these three objects.

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.

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.

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.

NGC 604, the Scaled OB Association (SOBA) Prototype. I. Spatial Distribution of the Different Gas Phases and Attenuation by Dust

We have analyzed HST and ground-based data to characterize the different gas phases and their interaction with the MYC in NGC 604, a GHR in M33. The warm ionized gas is made out of two components: a high-excitation, high-surface brightness H II surface located at the faces of the molecular clouds directly exposed to the ionizing radiation of the central SOBA; and a low-excitation, low-surface brightness halo that extends to much larger distances from the ionizing stars. The cavities created by the winds and SN explosions are filled with X-ray-emitting coronal gas. The nebular lines emitted by the warm gas experience a variable attenuation as a consequence of the dust distribution, which is patchy in the plane of the sky and with clouds interspersed among emission-line sources in the same line of sight. The optical depth at H alpha as measured from the ratio of the thermal radio continuum to H alpha shows a very good correlation with the total CO (1-0) column, indicating that most of the dust resides in the cold molecular phase. The optical depth at H alpha as measured from the ratio of H alpha to H beta also correlates with the CO emission but not as strongly as in the previous case. We analyze the difference between those two measurements and we find that <=11% of the H II gas is hidden behind large-optical-depth molecular clouds. We detect two candidate compact H II regions embedded inside the molecular cloud; both are within short distance of WR/Of stars and one of them is located within 16 pc of a RSG. We estimate the age of the main stellar generation in NGC 604 to be approx. 3 Myr from the ionization structure of the H II region. The size of the main cavity is smaller than the one predicted by extrapolating from single-star wind-blown bubbles.

Imaging MWC 349 from 7 Millimeters to 90 Centimeters

MWC 349A is the brightest radio continuum star in the centimeter domain. The thermal radio continuum emission is believed to originate in an ionized bipolar flow that photoevaporates from the surfaces of a neutral Keplerian disk. In this work we present high angular resolution observations taken with the Very Large Array (VLA) at wavelengths from 7 mm to 90 cm that allow the study of structures over 2 orders of magnitude in size in this object. The 7 mm image shows an intermediate equatorial region ~004 wide, with no free-free emission, that could be the neutral photoevaporating disk around MWC 349A. Combining these data with archival observations at 1.3, 2, 3.6, 6, and 20 cm, we estimate that the flux increases with frequency as ν0.67±0.03 and the angular size decreases with frequency as ν-0.74±0.03, confirming the presence of a biconical thermal wind that expands at constant velocity. We also report the marginal detection of MWC 349A at 90 cm. At the wavelengths of 3.6, 6, and 20 cm, we image and model the interaction zone between the winds of MWC 349A and MWC 349B, supporting the physical association of these components. Finally, by comparing 6 cm data taken in 1982 and 1996 we find evidence of variability in MWC 349A that is interpreted as a decrease of ~2% in the mass-loss rate over the time interval of the observations.

The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625

We present new multifrequency radio continuum imaging of the dwarf starburst galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm reveal global continuum emission dominated by free-free emission, with only mild synchrotron components. Each of the major H II regions is detected; the individual spectral indices are thermal for the youngest regions (showing largest Hα emission) and nonthermal for the oldest. We do not detect any sources that appear to be associated with deeply embedded, dense, young clusters, although we have discovered one low-luminosity, obscured source that has no luminous optical counterpart and resides in the region of highest optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent star formation, these radio properties suggest that the youngest star formation complexes have not yet evolved to the point where their thermal spectra are significantly contaminated by synchrotron emission. The nonthermal components are associated with regions of older star formation that have smaller ionized gas components. These results imply a range of ages for the H II regions and radio components that agrees with our previous resolved stellar population analysis, where an extended burst of star formation has pervaded the disk of NGC 625 over the last ~50 Myr. We compare the nature of radio continuum emission in selected nearby dwarf starburst and Wolf-Rayet galaxies, demonstrating that thermal radio continuum emission appears to be more common in these systems than in typical H II galaxies with less recent star formation and more evolved stellar clusters.

Warm molecular gas, dust and ionized gas in the 500 central pc of the Galaxy

We present infrared and millimeter observations of molecular gas, dust and ionized gas towards a sample of clouds distributed along the 500 central pc of the Galaxy. The clouds were selected to investigate the physical state, in particular the high gas temperatures, of the Galactic center region (GCr) clouds located far from far-infrared of thermal radio continuum sources. We have found that there is ionized gas associated with the molecular gas. The ionizing radiation is hard (~35000 K) but diluted due to the inhomogeneity of the medium. We estimate that ~30 % of the warm molecular gas observed in the GCr clouds is heated by ultra-violet radiation in photo-dissociation regions.

The Origin of X-Ray Emission from a Galactic Center Molecular Cloud: Low-Energy Cosmic-Ray Electrons

The Galactic center region near l ≈ 02 hosts a mixture of nonthermal linear filaments and thermal radio continuum features associated with the radio arc. Chandra observations of this region reveal an X-ray filament and diffuse emission with an extent of roughly 60'' × 2'' and 5' × 3', respectively. The X-ray filament lies at the edge of the nonthermal radio filaments and the dense molecular shell G0.13-0.13 that has an unusually high kinetic temperature ≥70 K. These observations demonstrate that the G0.13-0.13 molecular cloud and the nonthermal radio filaments of the arc are interacting. The diffuse X-ray emission is correlated with the molecular shell and is fitted either by two-temperature (1 and 10 keV) thermal emission or by power-law and 1 keV thermal gas. Fluorescent 6.4 keV line emission is also detected throughout the molecular shell. This cloud coincides within the error circle of a steady unidentified EGRET source, 3EG J1746-2851. We argue that low-energy cosmic-ray electrons produce the power-law continuum by bremsstrahlung and 6.4 keV line emission from the filament and the diffuse cloud with the implication on the origin of the Galactic ridge X-ray emission. The strong 6.4 keV Fe line emission seen from other Galactic center clouds could be produced in a similar fashion rather than via fluorescent emission induced by a transient hard X-ray source in the Galactic center. In addition, heating by ionization induced by low-energy cosmic-ray electrons are also responsible for the high temperature of G0.13-0.13. The gamma-ray source is a result of bremsstrahlung by the high-energy tail of the electron energy distribution.

A Comparison of Ultraviolet Imaging Telescope Far‐Ultraviolet and Hα Star Formation Rates

We have used archival ultraviolet (UV) imaging of 50 nearby star-forming galaxies obtained with the Ultraviolet Imaging Telescope (UIT) to derive integrated near-UV and far-UV magnitudes, and have combined these data with Hα, far-infrared, and thermal radio continuum measurements to explore the consistency of UV and Hα star formation rates (SFRs). In agreement with previous studies, we find that the UV and Hα SFRs are qualitatively consistent, even before corrections for extinction are applied. The uncorrected UV SFRs are systematically lower by a factor of 1.5 (with a factor of 2 scatter) among luminous galaxies with SFR ≳ 1 M☉ yr-1, indicating a higher effective attenuation of the far-UV radiation. Among less luminous galaxies there is no significant offset between the Hα and far-UV SFR scales. This behavior is consistent with that of higher redshift samples observed by Sullivan et al., Glazebrook et al., and Yan et al. for comparable ranges of galaxy luminosities and absolute SFRs. Far-infrared and thermal radio continuum data available for a subset of our sample allow us to estimate the attenuation in the UV and at Hα independently. The UV and Hα attenuations appear to be correlated, and confirm systematically higher attenuations in the UV. Although the galaxies in our sample show modest levels of attenuation (with median values of 0.9 mag at Hα and 1.4 mag at 1550 Å), the range across the sample is large, ~4 mag for Hα and ≳5 mag in the far-UV (1550 Å). This indicates that the application of a single characteristic extinction correction to Hα or UV SFRs is only realistic for large, well-defined and well-studied galaxy samples, and that extinction bias may be important for UV or emission-line-selected samples of star-forming galaxies.

350 Micron Images of Massive Star Formation Regions

We present 350 μm continuum images of 24 massive star formation regions obtained with the Caltech Submillimeter Observatory equipped with the SHARC focal-plane array. At this wavelength the spatial resolution is 11''. The 350 μm emission is compared with thermal radio continuum emission and OH, H2O, and CH3OH masers. Emission at 350 μm is believed to be thermal emission from dust heated by embedded or nearby stars. Compact radio continuum sources are usually present in the mapped 350 μm fields, and more than 60% of the 350 μm peaks coincide with radio continuum peaks. This association lends strong support to the notion that the dust is heated primarily by hot stars. Masers are also a common property of massive star formation regions. Usually OH, H2O, and/or CH3OH masers are found near, but generally not coincident with, the 350 μm peaks. Less than 25% of the 350 μm peaks have no reported masers located close by. In most of the observed regions, the 350 μm dust maps show one or more components surrounded by fainter extended emission. In total, we identify 28 separate 350 μm components. Ten of the 28 components do not have radio continuum counterparts. These are luminous sources and should produce detectable H II regions. It is postulated that these sources may be undergoing such rapid accretion that the infalling matter quenches the H II region very close to the protostar, thereby making the H II region undetectable in free-free emission. Objects in this evolutionary state may well represent the long-sought precursors to ultracompact H II regions and should have the properties of accreting massive protostars (e.g., accretion disks, bipolar molecular outflows, hot shocked gas, infall with spin-up toward the protostar). We suggest that the 10 sources in this category in our sample merit further observational study for these properties. Two-temperature graybody models constrained by the observed infrared spectral energy distributions were calculated to estimate the total mass, luminosity, average dust temperature, and hydrogen column and number densities for each source. The graybody models do not account for possible low-level emission from the coolest dust (<25 K) and therefore may underestimate the total mass. There is, however, no evidence for emission in excess of the models at 1.3 mm, and we conclude that the mass contribution of dust colder than ~25 K cannot be large. Thus, the graybody models shown provide useful average global properties required to understand massive star formation environments.

Infrared observations of the center of the Galaxy

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 52 μm [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 ≈ 60 km/ s) and a broad ( Δv ≈ 150 km/ 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 × 10 21 cm −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 O 2+. 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 ⩾ 10 4 cm −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 0.03 M⊙/year.

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.