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We present observations of CO(3-2) and $^{13}$CO(3-2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The 0.3" resolution images reveal an unusual molecular cloud, "Cloud D1", coincident with the radio-infrared supernebula. The ~6-pc diameter cloud has a linewidth, $\Delta$ v = 21.7 km/s, that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 x 10$^5$ M$_\odot$. The cluster appears to have a top-heavy initial mass function, with $M_{low}$~1-2 M$_\odot$. Cloud D1 is optically thin in CO(3-2) probably because the gas is hot. Molecular gas mass is very uncertain but constitutes < 35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ~1500-2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster., Comment: Revised version, 12 pages, 2 figures. Accepted, ApJ. New version corrects typo in RA coordinate in Table 1 in original Journal article as appears in the Erratum

A local dwarf galaxy, NGC 5253, has a young super star cluster that may provide an example of highly efficient star formation. Here we report the detection and imaging, with the Submillimeter Array, of the J= 3-2 rotational transition of CO at the location of the massive cluster associated with the supernebula. The gas cloud is hot, dense, quiescent, and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by Wolf-Rayet stars within the embedded star cluster. Its star formation efficiency exceeds 50%, ten times higher than clouds in the Milky Way: this cloud is a factory of stars and soot. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy., Comment: Accepted to Nature. 15 pages, 5 figures. Abstract modified from Nature summary paragraph

Aims. We are trying to probe conditions in the youngest super star clusters, those still embedded in dense obscuring clouds. Methods. The hydrogen recombination lines in the radio and infrared can be observed through the obscuration, as the optical and UV lines cannot, and give us the kinematics of the ionized gas. Results. The line profiles of the clusters resemble superpositions of the lines of many very young ultra-compact or hyper-compact HII regions. This can be explained if each OB star is individually embedded in dense material which it is accreting, even as it ionizes. Conclusions. We speculate on what this implies for the status and evolutionary state of cluster stars.

Little is known about the origins of the giant star clusters known as globular clusters. How can hundreds of thousands of stars form simultaneously in a volume only a few light years across the distance of the sun to its nearest neighbor? Radiation pressure and winds from luminous young stars should disperse the star-forming gas and disrupt the formation of the cluster. Globular clusters in our Galaxy cannot provide answers; they are billions of years old. Here we report the measurement of infrared hydrogen recombination lines from a young, forming super star cluster in the dwarf galaxy, NGC 5253. The lines arise in gas heated by a cluster of an estimated million stars, so young that it is still enshrouded in gas and dust, hidden from optical view. We verify that the cluster contains 4000-6000 massive, hot "O" stars. Our discovery that the gases within the cluster are bound by gravity may explain why these windy and luminous O stars have not yet blown away the gases to allow the cluster to emerge from its birth cocoon. Young clusters in "starbursting" galaxies in the local and distant universe may be similarly gravitationally confined and cloaked from view., Comment: Letter to Nature

We report subarcsecond-resolution VLA and Keck mid-infrared imaging of the dwarf starburst galaxy II Zw 40. II Zw 40 contains a bright, compact thermal radio and infrared source with all the characteristics of a collection of dense HII regions ionized by at least 14,000 O stars. The supernebula is revealed to be multiple sources within an envelope of weaker emission. The radio emission is dominated by free-free emission at 2cm, and the spectrum of this emission appears to be rising. This suggests that the free-free emission is optically thick at 2cm, and that the individual HII regions are ~1pc in size. This complex of "supernebulae" dominates the total infrared luminosity of II Zw 40, although the radio source is less than ~150pc in diameter. Multiple super star clusters appear to be forming here, the much larger analogues of large Galactic HII region complexes., Comment: Accepted for publication in The Astronomical Journal (~November 2002), 26 pages and 4 JPEG figures

We have made subarcsecond-resolution images of the central 10" of the Wolf-Rayet dwarf galaxy He 2-10 at 11.7 microns, using the Long Wavelength Spectrometer on the Keck Telescope. The spatial distribution of the infrared emission roughly agrees with that of the rising spectrum radio sources seen by Kobulnicky & Johnson (1999) and confirms that those sources are compact HII regions rather than SNR or other objects. The infrared sources are more extended than the subarcsecond rising spectrum radio sources, although the entire complex is still less than 5" in extent. On sizescales of 1" the infrared and radio emission are in excellent agreement, with each source requiring several hundred to a thousand O stars for excitation. The nebulae lie in a flattened disk-like distribution about 240 by 100 pc and provide all of the flux measured by IRAS for the entire galaxy in the 12 micron band; 30% of the total IRAS flux from the galaxy emanates from one 15-30 pc source. In this galaxy, intense star formation, probably triggered by an accretion event, is confined to a central disk which breaks up into distinct nebulae which presumably mark the sites of young super star clusters., Comment: Accepted for Publication in the Astronomical Journal

ISO-SWS spectroscopy of the WR galaxy NGC5253 is presented, and analysed to provide estimates of its hot young star population. Our approach differs from previous investigations in that we are able to distinguish between the regions in which different infrared fine-structure lines form, using complementary ground-based observations. The high excitation nebular [SIV] emission is formed in a very compact region, which we attribute to the central super-star-nucleus, and lower excitation [NeII] nebular emission originates in the galactic core. We use photo-ionization modelling coupled with the latest theoretical O-star flux distributions to derive effective stellar temperatures and ionization parameters of Teff>38kK, logQ=8.25 for the compact nucleus, with Teff=35kK, logQ<8 for the larger core. Results are supported by more sophisticated calculations using evolutionary synthesis models. We assess the contribution that Wolf-Rayet stars may make to highly ionized nebular lines (e.g. [OIV]). From our Br(alpha) flux, the 2" nucleus contains the equivalent of approximately 1000 O7V star equivalents and the starburst there is 2-3Myr old; the 20" core contains about 2500 O7V star equivalents, with a representative age of 5Myr. The Lyman ionizing flux of the nucleus is equivalent to the 30 Doradus region. These quantities are in good agreement with the observed mid-IR dust luminosity of 7.8x10^8 L(sun) Since this structure of hot clusters embedded in cooler emission may be common in dwarf starbursts, observing a galaxy solely with a large aperture may result in confusion. Neglecting the spatial distribution of nebular emission in NGC5253, implies `global' stellar temperatures (or ages) of 36kK (4.8Myr) and 39kK (2.9 or 4.4Myr) from the observed [NeIII/II] and [SIV/III] line ratios, assuming logQ=8., Comment: 16 pages, 7 figures, uses mn.sty, to appear in MNRAS

We have obtained near-infrared images and mid-infrared spectra of the starburst core of the dwarf Wolf-Rayet galaxy He 2-10. We find that the infrared continuum and emission lines are concentrated in a flattened ellipse 3-4'' or 150 pc across which may show where a recent accretion event has triggered intense star formation. The ionizing radiation from this cluster has an effective temperature of 40,000 K, corresponding to $30M_\odot$ stars, and the starburst is $0.5-1.5 \times 10^7$ years old., Comment: 17 pages Latex, 7 postscript figures, 1 postscript table, accepted to AJ

Gravitational interactions between galaxies are believed to increase star formation activity dramatically, and most of the brightest starburst galaxies show clear signs of recent interactions. However, it is still not known how interaction triggers star formation, nor are there models to relate the type or strength of interaction to the location or amount of star formation. We report on a series of deep H alpha images of interacting and post-interaction galaxies which we took with the purpose of finding the young stars and ionized gas in these objects. We were motivated in part by the hope that by studying the very recently formed stars we could see how the interaction process had affected the star formation. We observed the galaxies through 50 A-wide filters, one on the redshifted H alpha line and one off, and a standard R filter. Depending on the galaxy and conditions, images in the B, V, and I filters were also obtained. The images were recorded with a 4x7 ft. or 17 ft. diameter CCD at the 1-meter telescope of the Wise Observatory in Mitzpe Ramon. The H alpha and continuum images are used, together with observations at other wavelengths, to put together as complete a picture as possible of star formation and interactions in each galaxy. The complete observation set is not yet available for all the galaxies but certain results are already clear. There do not seem to be any correlations between H 1 and H alpha structures. In some H 1 plume galaxies H alpha extensions were seen on the other side of the galaxy from the H 1; in others extensive H alpha filaments have been found but not H 1. The preliminary results agree with the simplest model that interaction-induced star formation will be concentrated in the system center, since that is where the mass ends up.

Observations were obtained at wavelengths from 1.5 to 7.5 microns with beams varying in diameter from 4 to 28 arcsec of infrared hydrogen recombination lines toward the Mon R2 IRS1 H II region. It is found that the data cannot be fitted with the extinction law which characterizes the interstellar medium unless the obscuring matter is clumped on a small scale of not greater than 0.3 arcsec; in which case considerable fluctuations in the amount of extinction on scales smaller than 1 arcsec are expected. The data of Simon et al. (1983) suggest a dip in the extinction about 5 arcsec from the 2-micron and radio continuum peak, and rule out models with uniform dust and clump distributions.

The spatial and velocity distribution of shocked gas in the Orion molecular cloud has been studied in detail through observations of near-infrared and middle-infrared H2 lines. It is argued that the observed structure requires either an asymmetric outflow or an inhomogeneous ambient molecular cloud, and that the second explanation appears the more likely. It is suggested that mild activity by IRc9, such as a T Tauri type wind or FU Orionis type flare, would be consistent with its apparent nature and could have caused the necessary density structure in the cloud.

Maps of the H2 emission from the bipolar nebulae CRL 2688 and CRL 618 are presented, along with a map of the 2.1-micron continuum emission from CRL 2688. The H2 emission is seen in the direction of the lobes of each nebula and not in between the lobes. From the distribution of the H2 and continuum emission, it is argued that the H2 is excited in shock waves produced in the lobes of the visible nebulae by fast winds from the central stars which overtake slower moving material lost in a slow wind during the red-giant phase of each object. These shocks may be similar to those seen in Herbig-Haro objects. The mass-loss rates for the fast winds in each object are estimated from the H2 data, and a brief discussion is given of the origin of these winds.

Observations of infrared hydrogen recombination line emission are presented for several positions within the galaxies NGC 1068, NGC 1097, NGC 2903, and NGC 4151. Emission is seen from the arms and nuclei of these spiral galaxies. The luminosity from the most recent burst of star formation is deduced and compared to the infrared luminosity which accounts for the bulk of the total energy. Young stars (primarily O and B stars) provide most of the luminosity in the arms and nuclei of the normal galaxies NGC 1097 and NGC 2903. The infrared luminosities of the nuclei in the Seyfert galaxies NGC 1068 and NGC 4151 exceed those provided by young stars by factors of approximately 10.

The B-alpha and B-gamma lines of atomic hydrogen have been observed at eight positions near the nucleus of NGC 253 with 6 arcsec spatial resolution. Most if not all of the bolometric luminosity of the central 200 pc of this galaxy is probably produced by O and B stars which have recently formed. The spatial distribution of the line fluxes is similar to that of the 10 microns flux, supporting the hypothesis that the 10 microns radiation is powered by the newly formed stars. The extinction to the ionized gas varies over small distances in this region, explaining the discrepancies between extinction and Lyman continuum luminosity derived separately from visual and infrared line observations.

Observations of the spatial distribution of extinction and excitation temperature toward the molecular hydrogen emission in the Orion molecular cloud OMC-1 are presented. Most, although not all, of the observed structure in the near-infrared line intensities results from variations in the column density of vibrationally excited H2 and is not due to variable extinction or temperature. The extinction toward the center of the emission region is between 1 and 2 mag at 4712/cm, the frequency of the v = 1-0 S(1) line, but increases toward the edges. The lack of emission from the eastern part of the nebula may result from increased extinction in that direction. Variations in the extinction temperature are less than the observational uncertainties of + or - 200 K at all but one position observed. Therefore, the excitation temperature of the hydrogen molecules is probably not a strong function of either the shock velocity or the density of the gas. Observations of the v = 3-2 S(3) line in the direction of strongest emission indicate the presence of gas temperatures about 2700 K and place constraints on the column density of gas which is at higher temperature.

Maps of five H II regions in one or more of the infrared fine-structure lines of Ne II (12.8 microns), Ar III (9.0 microns), and S IV (10.5 microns) have been obtained with angular resolutions ranging from 4 to 7 arcsec. The observations are used to discuss the morphology and excitation of these nebulae. Considerable diversity is found in the structures of the nebulae, probably resulting from differences in their ages and the circumstances of their formation. In all cases, more ionizing luminosity than would be provided by a single dominant ionizing star appears to be required, although uncertainties in the model nebulae make this conclusion uncertain.

The pure rotational S(2) line of H2 at 12.28 microns was sought in 44 positions in the Orion Molecular Cloud with 6-arcsec beams and 35 km/s spectral resolution; and it was detected in 27 positions. The lines are approximately symmetric and have full widths at half-maximum ranging from 100 km/s down to the resolution limit. The distribution of intensities and line shapes is largely consistent with that observed in the 2-micron hydrogen transitions; however, unexpectedly complex line profiles and point-to-point variations in line shapes appear, particularly in the region near IRc9.

The detection of HCN on Jupiter is reported. Three R-branch lines of the nu-2 fundamental of HCN near 13.5 microns were observed in absorption, from which the HCN column density is inferred to be 0.005 cm-am with an uncertainty of a factor of two. If emission from the stratosphere exists, then the derived column density is only a lower limit. It is suggested that the Jovian HCN most likely originates from the photolysis of CH4 and NH3 in the lower stratosphere and upper troposphere. In addition, an upper limit of 0.025 cm-am was established for the column density of HCN on Saturn.

New infrared observations of Ne II, Ar III, and S IV are used in optical observations of other ionization states of the considered elements to evaluate the abundances of neon, argon, and sulfur in 18 planetary nebulae. Attention is also given to one or more of the infrared lines in 18 other nebulae. It is pointed out that S IV was detected in approximately 90% of the observed objects, while Ar III was found in about 80%, and Ne II in roughly one-third. It is noted that optical observations typically include only a limited region of the nebula, while the infrared measurements frequently involve integration over the entire nebular image.

A 1-micron CCD picture and maps of the Ne II and Ar III fine-structure lines have been obtained in a region roughly one arcmin in size centered on the core of the compact H II region G333.6 - 0.2. The two emission-line maps show nearly identical intensity distributions, although much different fluxes. The maps are similar to the 1-micron picture smoothed to their angular resolutions. The infrared line of S IV also was detected and appears to have a spatial intensity distribution similar to that of the other fine-structure lines. The 1-micron picture reveals a small number of objects of nearly stellar appearance in the core, suggesting that G333.6 - 0.2 is powered by a compact cluster of sources. Stellar continuum radiation probably is responsible in part for the low equivalent width of the B-gamma line in the core.

The pure rotational S(2) line of molecular hydrogen at 12.28 microns was looked for in 44 positions in the Orion moleular cloud with 6 in. beams and 35 km/s spectral resolution; it was detected in 27 positions. Emission was observed over a velocity range of + or - 100 km/s. The lines are approximately symmetric, and have full widths at half maximum ranging from 100 km/s down to the resolution limit. The distribution of intensities and line shapes is largely consistent with that seen in the 2 micron hydrogen transitions. However, unexpectedly complex line profiles and point-to-point variations in linear shapes appear, particularly in the region near IRc9.

The pure rotational S(2) transition of molecular hydrogen at 12.28 microns has been detected in emission from the region of vibrational-rotational line emission in Orion. The line was resolved both spectrally and spatially, and the line shapes, widths, and velocities are consistent with those observed in the upsilon = 1-0 transitions. The results put new constraints on the structure, temperature, and dynamics of the region of molecular hydrogen emission.

The 12.8-micron fine-structure line of Ne II has been mapped in the central 25 arcsec of NGC 253 with a 6-arcsec beam and 0.1-0.15-kayser spectral resolution. As in visible studies, both circular and noncircular motions were observed. However, there are major differences between the Ne II velocities and those measured in the optical and radio regions.

Observations of vibration-rotation transitions of silicon monoxide in VY CMa show that the lines originate in accelerating, expanding, and cool (about 600 K) layers of a circumstellar cloud at a distance of approximately 0.15 arcsec from the central star. The central stellar velocity, as estimated from observed SiO P Cygni line profiles, is somewhat redshifted from the midpoint of the maser emission features. Most of the silicon is probably in the form of dust grains. The isotopic ratios of silicon are nearly terrestrial.

Ne II fine-structure emission at 12.8 microns from the galaxy M82 has been spatially and spectrally resolved. This radiation is relatively unaffected by obscuration, scattering, or maser amplification, and should hence provide a more reliable map of the velocity field within the galaxy than do optical or radio observations. A recessional velocity of 215 + or - 20 km/s is found for the galaxy, and a much steeper rotation curve is obtained than that from optical measurements. Furthermore, there is evidence for large-scale noncircular motion of the gas within the inner 150 pc of the nebula. The results are consistent with the idea that the peculiarities of M82 are due to collision with an intergalactic dust cloud as suggested by Solinger, Morrison, and Markert (1977).

Observations of vibration-rotation transitions of silicon monoxide in VY CMa show that the lines originate in accelerating, expanding, and cool (600 K) layers of a circumstellar cloud at a distance of roughly 0.15 minutes from the central star. The central stellar velocity, as estimated from observed SiO P Cygni line profiles, is somewhat redshifted from the midpoint of the maser emission features. Most of the silicon is probably in the form of dust grains. The isotopic ratios of silicon are nearly terrestrial.