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We present estimates of the contribution from 33 nearby (d

We use a sample of 1669 QSOs ($r, Comment: 8 pages, 9 figures, MNRAS accepted

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a emission over the redshift interval z=6-7. In combination with the strong damping wing extending redward of Ly-a in the spectrum of the bright z=7.085 quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in the intergalactic medium (IGM) is still substantially neutral at z~7. Current theoretical models imply HI fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly-a emitter population. We propose that such large neutral fractions are not necessary. Based on a hydrodynamical simulation which reproduces the absorption spectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Ly-a can rise rapidly in average regions of the Universe simply because of the increasing incidence of absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionisation is approached. Our simulations suggest these data do not require a large change in the IGM neutral fraction by several tens of per cent from z=6-7, but may instead be indicative of the rapid decrease in the typical mean free path for ionising photons expected during the final stages of reionisation.

We have analyzed FUSE (905-1187 A) spectra of a sample of 16 local starburst galaxies. These galaxies cover almost three orders of magnitude in star-formation rates and over two orders of magnitude in stellar mass. Absorption features from the stars and interstellar medium are observed in all the spectra. The strongest interstellar absorption features are generally blue-shifted by ~ 50-300 km s–1, implying the almost ubiquitous presence of starburst-driven galactic winds in this sample. The outflow velocites increase with both the star-formation rate and the star-formation rate per unit stellar mass, consistent with a galactic wind, driven by the population of massive stars. We find outflowing coronal-phase gas (T ~105.5 K) detected via the O VI absorption line in nearly every galaxy. The O VI absorption-line profile is optically thin, is generally weak near the galaxy-systemic velocity, and has a higher mean outflow velocity than seen in the lower ionization lines. The relationship between the line width and column density for the O VI absorbing gas is in good agreement with expectations for radiatively cooling and outflowing gas. Such gas will be created in the interaction of the hot out-rushing wind seen in X-ray emission and cool dense ambient material. O VI emission is not generally detected in our sample, suggesting that radiative cooling by the coronal gas is not dynamically significant in draining energy from galactic winds. We find that the measured outflow velocities in the H I and H II phases of the interstellar gas in a given galaxy increase with the strength (equivalent width) of the absorption feature and not with the ionization potential of the species. The strong lines often have profiles consisting of a broad and optically-thick component centered near the galaxy-systemic velocity and weaker but highly blue-shifted absorption. This suggests that the outflowing gas with high velocity has a lower column density than the more quiescent gas, and can only be readily detected in the strongest absorption lines. From direct observations below the Lyman edge in the galaxy rest frame, we find no evidence of Lyman continuum radiation escaping from any of the galaxies in the sample. Moreover, the small escape fraction of light in the center of the strong C II absorption feature confirms the high opacity below the Lyman limit in the neutral ISM. The absolute fraction of escaping Lyman continuum photons is typically

Context. We present the results of a near-infrared survey of the young stellar cluster associated with the IRAS 16571-4029 source. Aims. The main purpose of this survey is to study the cluster members and find the ionizing sources of the associated HII region. Methods. The stellar population was studied by using color–color and color–magnitude diagrams, as well as by analysing the spectral energy distributions in the near- and mid-infrared wavelengths. The extended emission was studied by the construction of contour diagrams, which were compared with near- and mid-infrared images. We computed the corresponding number of Lyman continuum photons (using the integrated Br γ flux density) and compared it with that obtained from the 5 GHz flux density to derive a mean visual extinction. Results. NIR observations in the direction of RCW116B reveal the presence of a young cluster of massive stars coincident with the IRAS 16571-4029 source. These observations, together with published radio data, MSX, and Spitzer images were used to determine some of the physical parameters of the region. We found 102 cluster member candidates in an area of about 3 $\times$ 3 square arcmin, the majority of them showing excess emission in the NIR. We found that IRAS 16571-4029 is formed by multiple infrared sources, all but one are associated with small groups of stars. This suggests that the fragmentation of massive molecular clouds generates the massive sub-clusters. We derived a mean visual extinction of $A_{\rm V}=12.8\pm ^{4.7}_{3.2}$. This result is independent of the assumed distance and agrees with the mean visual extinction $A_{\rm V}=14.4$, as obtained by previous spectroscopic observations of two NIR sources in the direction of the IRAS 16571-4029 source. We also compare the results obtained in this study with those obtained in previous papers in this series finding a very good correlation between the number of cluster members N s and the cluster radius r c . The cluster radius varies from 0.2-0.3 pc (IRAS 15411-5352 and IRAS 16132-5039) until about 1 pc (IRAS 15408-5356). The youngest clusters are those associated with the RCW95 complex (IRAS 15408-5356 and IRAS 15411-5353) with ages in the range 1.5-2 $\times$ 10 6 years, while the sources associated with the RCW106 (IRAS 16132-5039, IRAS 16177-5018) and RCW116B (IRAS 16571-4029) complexes have ages in the range 2.5-3 $\times$ 10 6 years. The oldest of them is the cluster associated with the RCW121 region (IRAS 17149-4029), which has an estimated age of 4.2 $\times$ 10 6 years.

The positions of star-forming complexes (SFCs) in color-luminosity, color-color, and chemical composition-luminosity diagrams are determined by the star-formation regime (history). Taking into account the fraction of Lyman continuum photons that are not absorbed by hydrogen, we find a strong correlation between the observed color indices and the total Lyman continuum flux from the stars in SFCs. The distribution of extragalactic SFCs in a plot of the slope of the initial mass function (IMF) versus the density of stars cannot be distinguished from this distribution for clusters in the Galaxy and the Large Magellanic Cloud, where the IMF slopes were derived directly from star counts.

A thorough analysis of well-studied disk giant H II regions, for which we know the ionizing stellar population, gas metallicity, and Wolf-Rayet population, leads to photoionization models that can only match all observed line intensity ratios ([O III], [O II], [N II], [S II], and [S III] with respect to the intensity of Hβ), as well as the Hβ luminosity and equivalent width if one allows for an important escape of energetic ionizing radiation. For the three regions presented here, the fractions of escaping Lyman continuum photons amount to 10%-73%, and in all cases, the larger fraction of escaping photons has energies of between 13.6 and 24.4 eV. These escaping photons clearly must have an important impact as sources of ionization of the diffuse ionized gas found surrounding many galaxies, as well as of the intergalactic medium.

We report the discovery of Hα emission associated with the redshifted lobe of the M82 superwind extending out to a projected distance of 11 kpc from the disk of M82, which is 3 times farther than previously identified emission-line components associated with the nuclear superwind. The Hα emission can be traced nearly continuously for 10' northwest of M82 out to an emission-line structure (the cap) that lies at a projected distance of 11-12 kpc from the M82 nucleus. The cap has a shell-like morphology, is blueshifted by 50-200 km s-1 relative to the M82 nucleus, and is visible in a ROSAT PSPC image of the region. We consider two possible models for the Hα bright cap: it may be a bow shock formed by the impact of the superwind, either with previously emitted wind material or with ambient intergalactic material and/or tidal debris left over from the interaction between M81 and M82; or it may trace photoionized material illuminated by Lyman continuum photons leaking out of the M82 nuclear region through the hot bipolar cavity produced by the starburst-driven superwind.

We have analyzed the properties of the 20 most radio-luminous UGC starburst galaxies from Condon, Frayer, & Broderick. Near-infrared images, spectra, and optical rotation curves were presented in Smith et al. In this paper, we use these data and published radio data to assess the stellar populations, dust contents, ionizing conditions, and dynamics of the starbursts. Certain properties of the star formation occurring in these galaxies differ from those observed locally. The infrared excesses (IREs) are lower than and span a narrower range of values than those of Galactic H II regions. The starbursts appear to produce a higher proportion of ionizing photons than most Galactic H II regions. Consequently, the initial mass functions (IMFs) of the starbursts may be more strongly biased toward high-mass star formation. The starbursts may also contain fewer old H II regions than the Milky Way. Furthermore, the starburst IRE is likely to be influenced by the presence of large reservoirs of gas that absorb a larger fraction of the Lyman continuum photons. The OB stellar and far-infrared luminosities imply that the upper mass range of the starburst IMF (M > 10 M?) is characterized by a slope of 2.7 ? 0.2. The starburst IMF thus bears a strong similarity to that observed in Magellanic OB associations. Optical line ratios indicate that a range of excitation conditions are present. We conclude that the near-infrared light from many of the starbursts is dominated by a heavily obscured mixture of emission from evolved red stars and young blue stars with small contributions (? 5%) from thermal gas and hot dust, under the assumptions that a Galactic or SMC extinction law can be applied to these systems and that the true reddening curve follows one of the models currently existing in the literature. In some cases, larger amounts of emission from blue stars or hot dust may be required to explain the observed near-infrared colors. The amount of dust emission exceeds that predicted from comparisons with Galactic H II regions. The near-infrared colors of some of the systems may also be influenced by the presence of a low-luminosity active galactic nucleus (AGN). Emission from blue stars and hot dust, if present, dilutes the observed CO index. The activity in the redder, more luminous systems is strongly peaked. The galaxies hosting the starbursts exhibit a wide range of morphological and star-forming properties. While all of the host galaxies are interacting systems, the nuclear separations of the interacting nuclei range from 1 Mpc. The dynamical behavior ranges from relaxed to strongly perturbed. The off-nuclear regions of the galaxies are sites of active star formation and are characterized by a range of excitation conditions. Spatially extended LINER emission is consistent with shock excitation produced by superwinds or galaxy-galaxy collisions. Violent star formation activity occurs over a larger physical scale in the most active starbursts. Systems containing mergers and widely separated nuclei possess similar colors and luminosities. The burst properties are most likely regulated by the internal structures of the interacting galaxies and not the separations of the interacting galaxies.

We present narrowband WFC3-UVIS and -IR images of four externally irradiated protostellar jets in the Carina nebula: HH666, HH901, HH902, and HH1066. These massive jets are unusual because they are bathed in UV radiation from dozens of nearby O stars, but despite the strong incident ionizing radiation, portions of the jet remain neutral. Near-IR [Fe II] images reveal dense, neutral gas that was not seen in previous studies of H-alpha emission. We show that near-IR [Fe II] emitting gas must be self-shielded from Lyman continuum photons, regardless of its excitation mechanism (shocks, FUV radiation, or both). High densities are required for the survival of Fe+ amid the strong Lyman continuum from Tr14, raising estimates of the mass-loss rates by an order of magnitude. Higher jet mass-loss rates require higher accretion rates onto their driving protostars, implying that these jets are driven by intermediate-mass (IM; ~2-8 Msun) stars. Indeed, the IR driving sources of two of these outflows have luminosities that require IM protostars. All four of these HH jets are highly collimated, with opening angles of only a few degrees, similar to those observed in low-mass protostars. We propose that these jets reflect essentially the same outflow phenomenon seen in wide-angle molecular outflows associated with IM and high-mass protostars, but that the collimated atomic jet core is irradiated and rendered observable in Carina's harsh radiative environment. In more quiescent environments, this atomic core remains invisible, and outflows traced by shock-excited molecules in the outflow cavity give the impression that these outflows have a wider opening angle. Thus, the externally irradiated jets in Carina constitute a new view of collimated jets from IM protostars, and offer strong additional evidence that stars up to at least ~8 Msun form by the same accretion mechanisms as low-mass stars., Comment: 15 pages, 10 figures, accepted for publication in MNRAS

We examine self-consistent parameterizations of the high-mass stellar population and resulting feedback, including mechanical, radiative, and chemical feedback, as we understand them locally. To date, it appears that the massive star population follows simple power-law clustering that extends down to individual field OB stars, and the robust stellar IMF seems to have a constant upper-mass limit. These properties result in specific patterns in the HII region LF and ionization of the diffuse, warm ionized medium. The resulting SNe generate superbubbles whose size distribution is also described by a simple power law, and from which a galaxy's porosity parameter is easily derived. A critical star-formation threshold can then be estimated, above which the escape of Lyman continuum photons, hot gas, and nucleosynthetic products is predicted. A first comparison with a large H-alpha sample of galaxies is broadly consistent with this prediction, and suggests that ionizing photons likely escapes from starburst galaxies. The superbubble size distribution also offers a basis for a Simple Inhomogeneous Model for galactic chemical evolution, which is especially relevant to metal-poor systems and instantaneous metallicity distributions. This model offers an alternative interpretation of the Galactic halo metallicity distribution and emphasizes the relative importance of star-formation intensity, in addition to age, in a system's evolution. The fraction of zero-metallicity, Population III stars is easily predicted for any such model. We emphasize that all these phenomena can be modeled in a simple, analytic framework over an extreme range in scale, offering powerful tools for understanding the role of massive stars in the cosmos. (Abridged)

Warm (≈ 104 K), diffuse H+ is a significant component of the interstellar medium within the Galactic disk and lower halo. This gas accounts for about one quarter of the interstellar atomic hydrogen, consumes a large fraction of the interstellar power budget, and appears to be the dominant state of interstellar matter 1 kpc above the midplane. The origin of this ionized gas is not yet established; however, of the known sources of ionization only 0 stars and perhaps supernovae produce enough power to balance the “cooling” rate of the gas. If 0 stars are the source of the ionization, then the interstellar HI, including the extended “Lockman layer”, must have a morphology that allows about 14% of the Lyman continuum photons emitted by the stars to travel hundreds of parsecs within the Galactic disk and up into the lower halo.

Three different types of evidence are presented in favour of the hypothesis that the HII regions in disk galaxies with Hα luminosities greater than a critical value of 1038·6 erg s−1 are density-bounded, and that the escaping Lyman continuum photons from these are the principal ionising agents for the diffuse ISM in disk galaxies. This has important implications for the ionisation of the intergalactic medium, and for computed star formation rates in spirals.

We present high spatial resolution observations of the 6 cm continuum emission of compact H.. regions in well-known sites of massive star formation located in the Small and Large Magellanic Clouds. The observations include N81 in the SMC, and N4A, N83B, N11A, N160A and N159-5 in the LMC. Some of the compact H.. regions are isolated, while others are embedded in more diffuse ionised regions. A description of the radio morphology of the sources, together with comparisons with other observations, is given in detail. The regions cover a wide range in size ( from similar to 0.1 to 7 pc), rms electron density ( from similar to 200 to 6500 cm(-3)), emission measure ( from similar to 3 x 10(5) to 2 x 10(7) pc cm(-6)), ionised gas mass ( from similar to 0.2 to 750 M-.) and rate of Lyman continuum photons ( from similar to 3 x 10(47) to 5 x 10(49) s(-1)). The spectral types determined from the Lyman continuum fluxes are consistent with optical determinations. We have compared these Magellanic Cloud H.. regions with their Galactic counterparts in terms of size, rms electron density and Lyman continuum flux. This comparison shows that their properties relate to each other in the same way as those in Galactic H II regions.

The evolutionary stage between the end of the AGB and PN phases has long been a missing link in our understanding of single-star evolution. As we discussed in Chapter 10, the AGB is terminated by the depletion of the H envelope by mass loss, and this occurs before the onset of carbon detonation. When mass loss reduces the mass of the H envelope (M e ) below a certain value (M e ∼ 10 –3 M ⊙ for a core mass [M c ] of 0.60 M ⊙ , Schonberner 1983), the star will begin to evolve toward the blue side of the H-R diagram. The effective temperature of the star will increase as the remaining H envelope continues to diminish by H-shell burning. This phase will last until the central star is hot enough ( T * ∼ 30,000 K) to ionize the circumstellar nebula. The emergence of recombination lines of H, He, and forbidden lines of metals will make the nebula easily observable in the visible, signaling the beginning of the PN phase. A sketch of the evolutionary tracks of proto-planetary nebulae (PPN) in the H-R diagram is shown in Fig. 14.1. PPN can be defined as the stage of evolution in which their central stars have stopped the large-scale mass loss on the AGB, but have not evolved to be hot enough to emit a sufficient quantity of Lyman continuum photons to ionize the surrounding remnants of the AGB envelope.

We report on the results of Hα imaging of a sample of 13 X-ray luminous early-type galaxies not at the center of clusters. We have detected Hα emission from 9 of these objects, with L H α = 4 x 1038 ÷ 4 x 1040 erg s -1(H 0 = 50km s -1 Mpc -1). The ionized gas is generally extended, but peaked at the nucleus and more concentrated than stars. In some cases it shows filaments, arcs and rings. We compare the properties of the warm phase of the ISM with those of the hot and cold ones, we discuss the possible ionizing mechanisms of the gas, finding that PAGB stars produce enough Lyman continuum photons, and we give an estimate of the mass of the ionized gas.

We have made a multifrequency (6, 3.6, and 2 cm), high-resolution (3"-6"), radio continuum survey of IRAS selected sources from the catalogue of Wouterloot & Brand (1989) to search for and study H II regions in the far outer Galaxy. We identified 31 sources in this catalog with well determined galactocentric distances, and with R approx.. greater than 15 kpc and L(sub FIR) approx.greater than 10(exp 4) solar luminosity, indicating the presence of high-mass star-formation. We have observed 11 of these sources with the Very Large Array (VLA). We observed the sources at 6 and 2 cm using "scaled arrays", making possible a direct and reliable comparison of the data at these two wavelengths for the determination of spectral indices. We detected a total of 12 radio sources, of which 10 have spectral indices consistent with optically-thin free-free emission from H II regions. Combined with previous VLA observations by other investigators, we have data on a total of 15 H II regions at galactocentric distances of 15 to 18.2kpc, among the most remote H II regions found in our Galaxy. The sizes of the H II regions range from approx. less than 0.10 to 2.3 pc. Using the measured fluxes and sizes, we determine the electron densities, emission measures, and excitation parameters of the H II regions, as well as the fluxes of Lyman continuum photons needed to keep the nebulae ionized. The sizes and electron densities are consistent with most of the sources detected in this survey being compact or ultracompact H II regions. Seven of the fifteen H II regions have sizes approx. less than 0.20 pc. Assuming simple pressure-driven expansion of the H II regions, these sizes indicate ages approx. less than 5 x 10(exp 4) yr, or only 1% of the lifetime of an O star, which implies an unlikely overabundance of O stars in the outer Galaxy. Thus, the large number of compact H II regions suggests that the time these regions spend in a compact phase must be much longer than their dynamical expansion times. Five of the fifteen H II regions have cometary shapes; the remainder are spherical or unresolved. Comparison of the radio continuum data with molecular line maps suggests that the cometary shape of the two H II regions in S 127 may be due to pressure confinement of the expanding ionized gas, as in the "blister" or "champagne flow" models of H II regions. Comparison of the radio continuum data with the IRAS far-infrared data indicates that the five most luminous H II regions are consistent with a single 0 or B star exciting a dust-free H II region. Subject headings: stars: formation - ISM: H II regions - ISM: individual objects: S 127 radio continuum: interstellar

Using the Very Large Array with an angular resolution of ~3", we have detected the hydrogen recombination line H92α (ν rest = 8309.38 MHz) from the starburst nuclei in the galaxies NGC 3628 and IC 694 and also from the Seyfert II nucleus in NGC t365. In each case the line-emitting region extends over a few hundred parsecs. The detected lines have peak intensities in the range 0.5-1.5 mJy and widths (FWHM) 200- 400 km s -1 . The line was not detected in three other galaxies (NGC 262, NGC 1068, and NGC 3079) that were observed to a similar sensitivity level. We present a model in which a collection of H II regions in the nuclear region accounts for the observed H92α line. The required number of H II regions, their temperature, electron density, and linear size are constrained by the observed line flux density, line width, continuum spectrum, and size of the line-emitting region. If the temperature of the H II regions is above 5000 K, then electron densities in the range 5-50 × 10 3 cm -3 are permitted by the available constraints. Several hundred H II regions of a few parsecs in size, with a total mass of a few times 105 Msun, are required to account for the observed line flux density. The rate of production of Lyman continuum photons required to maintain the ionization is a few times 10 54 s -1 . Much of the line emission comes from internal stimulated emission due to the continuum generated within the H II regions which account for 5%-30% of the observed total continuum at 5 GHz. Predictions are made for the expected recombination line and continuum flux density as a function of frequency from the nuclear region of the galaxies.

Be stars show enhancement of optical emission up to a few magnitudes. Optical continuum enhancements due to (1) conversion of Lyman continuum photons of the Be star to visual photons in the envelope of the Be star, (2) emission from the H II region formed by X-ray and EUV emission from a compact object in binary motion around the Be star, and (3) emission from an accretion disk around the compact object, are considered. These considerations indicate that large increases of optical emission (above 0.5 mag) would need an ionizing source other than the Be star (for example, an accreting compact object emitting radiation). Specific examples of large increases of optical emission as in the sources Gamma Cas and 4U 0115 + 63 are discussed.

Neither H II regions around nearby B stars nor known white dwarf stars can account for the free electron column densities along two well-defined line segments to the pulsars PSR 0950 + 08 and PSR 0823 + 26. The presence of the ionized gas seems to imply either (1) very long mean free path lengths for the absorption of Lyman continuum photons within the ISM (thereby suggesting a very different morphology for interstellar H I from that of the conventional view); or (2) Lyman continuum luminosities for early B or hot white dwarf stars that are more than an order of magnitude larger than currently accepted values; or (3) an additional and as yet unknown ionization source within the Galactic disk. 54 refs.

The number of Lyman continuum photons emitted from stars with temperatures between 15 000K and 50 000K for several values of the surface gravity are calculated on the basis of Kurucz's new models of stellar atmospheres. Results are compared with previous data.

Using the data of a survey of the northern part of the galactic plane at far infrared wavelengths (71–95 μm and 114–195 μm) made with a balloon born instrument (Gispert, Puget and Serra, 1981) we can compute the far infrared luminosity of the galactic plane as a function of galactic radius. The result is shown in Figure 1. The far infrared radiation is stellar light reemitted by dust, and is a good measure of the total energy output. The ratio of the far infrared luminosity to the amount of interstellar gas gives a measure of the star formation rate per unit mass of interstellar gas. Furthermore one can compute the “infrared excess” (ratio of the infrared luminosity to the energy in Lyman a photons in H II regions). The number of Lyman continuum photons is evaluated by taking the number of Lyman continuum photons from giant H II regions from Smith et al., 1978 and assuming that the fraction of Lyman continuum photons coming from giant H II regions is constant throughout the galaxy.

The nebulosity on the north side of 3C 48 has an optical emission-line spectrum resembling that of an H II region, though there are peculiarities that could be due to an unusual excitation state or to an overabundance of heavy elements. A model is proposed which accounts for the ionization state of the nebulosity in terms of its density and the radiation emerging from the associated QSO. If 3C 48 is at a cosmological distance, the extent of the nebulosity is similar to the size of the extended H I disks around nearby spiral galaxies. Such H I disks would be ionized by QSO events occurring in their nuclei.The observed optical spectrum of the 3C 48 nebulosity is explained in terms of the model. The model predicts that even if nebulosities around QSOs were a common phenomenon, they would not always be easily observable in the optical. The nebulosity on the north side of 3C 48 is inferred to have low density, 3 x 10/sup -2/< or =n3 cm/sup -3/, and cosmic abundances, and to be ionized by the soft X-ray emission from the QSO, 0.1approximately-less-thanh..nu..approximately-less-than1.0 keV. The Lyman continuum photons emitted by the QSO must be absorbed by themore » emissive gas within the nucleus of 3C 48. If this nucleus were optically thin to the Lyman continuum photons, the nebulosity would be highly ionized for densities n

Far-infrared, radio continuum and (C-12)O and (C-13)O line observations are presented of 42 far-infrared sources. The sources range in luminosity from 4000 to 3,000,000 solar luminosities. Most of them are associated with (C-12)O peaks. More than half the sources have associated H2O maser emission, and half possess associated radio continuum emission at a limit of 100 mJy. Eight have radio emission at weaker levels. In many cases, the far-infrared source is smaller than its associated radio source. The difference can be explained in the context of the 'blister' picture of H II regions. One group of sources emits many fewer Lyman continuum photons than expected, considering the far-infrared luminosities. A number of possible reasons for this are examined; the explanation holding that clusters of early type stars rather than single stars excite the far-infrared sources is considered the most reasonable.

Far-infrared maps of W51/G49.5-0.4, W51/G49.4-0.3, and the DR 21 W75 S regions are presented. The simultaneous 50 and 100 micron maps enable a determination of the dust temperature and density structure in these areas. In G49.5-0.4 a luminosity and dust column-density peak at the position of the W51-Main maser is found, suggesting that this is the dominant luminosity source in W51. In G49.4-0.3 three components associated with the three known 2.7 GHz sources are resolved. A comparison of the total far-infrared luminosity with the radio continuum brightness in both G49.5-0.4 and G49.4-0.3 shows that the ratio of Lyman continuum to total luminosity of the exciting stars in not consistent with excitation by a single ZAMS star if most of the Lyman continuum photons are being absorbed bay gas rather than dust. In DR 21 micron far-infrared data and some new 2-20 micron results reveal a region of hot dust associated with the 2 micron H2 emission to the east and west of DR 21. Finally, a new 1.5-100 micron 'protostellar' source 2 deg north of W75 S at the position of a 1 mm continuum peak is reported.

We have detected both the H 76$alpha$ and He 76$alpha$ recombination lines in the Sgr B2 H II region; from a comparison of the integrated power in these lines we infer that the ratio of ionized helium to ionized hydrogen in Sgr B2 is =0.085 + or - 0.015. These observations require that the hydrogen and helium Stromgren spheres be spatially coextensive. Hence if processes intrinsic to the H II region, such as dust absorption of Lyman continuum photons, compete effectively with the gas for the stellar ionizing radiation, then the dust opacity at the helium ionization threshold cannot exceed the dust opacity at the Lyman limit, tau/subd/(504 A) < or = tau/subd/(912 A). The anomalously low He$sup +$/H$sup +$ ratio in Sgr B2 derived from recombination line observations at lower frequencies very likely results from preferential enhancement of the nebular hydrogen lines in the ionized component of the Sgr B2/ molecular cloud interface. (AIP)

Far-infrared observations of the planetary nebula NGC 7027 show that its total infrared flux is 2.4 by 10 to the -10th power W/sq m, approximately one-third of which is emitted in each of the intervals from 1 to 17, 17 to 30, and 30 to 300 microns. Sufficient energy to account for the corresponding infrared luminosity of 23,000 times the solar value at 1.77 kpc is available as diffuse nebular radiation or from moderate competition between dust and gas for Lyman continuum photons. Most of the infrared flux arises from a source comparable in size to the H II region. The flux at wavelengths greater than 17 microns can be attributed to grains with temperatures not exceeding 120 K and total mass such that the gas-to-dust ratio is close to the lower limit allowed by cosmic abundances.