Kathleen E. Kraemer

A Uniform Database of 2.4-45.4 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer*

We present a complete set of all valid SWS full-scan 2.4-45.4 μm spectra processed and renormalized in as uniform a manner as possible. The processing produces a single spectrum for each observation from the 288 individual spectral segments, which are the most processed form available from the ISO archive. The spectra, and the programs used to create them, are available to the community on-line.

Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features*

A study of all full-scan spectra of optically thin oxygen-rich circumstellar dust shells in the database produced by the Short Wavelength Spectrometer on ISO reveals that the strength of several infrared spectral features correlates with the strength of the 13 ?m dust feature. These correlated features include dust features at 19.8 and 28.1 ?m and the bands produced by warm carbon dioxide molecules (the strongest of which are at 13.9, 15.0, and 16.2 ?m). The database does not provide any evidence for a correlation of the 13 ?m feature with a dust feature at 32 ?m, and it is more likely that a weak emission feature at 16.8 ?m arises from carbon dioxide gas rather than dust. The correlated dust features at 13, 20, and 28 ?m tend to be stronger with respect to the total dust emission in semiregular and irregular variables associated with the asymptotic giant branch than in Mira variables or supergiants. This family of dust features also tends to be stronger in systems with lower infrared excesses and thus lower mass-loss rates. We hypothesize that the dust features arise from crystalline forms of alumina (13 ?m) and silicates (20 and 28 ?m).

Classification of 2.4-45.2 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer*

The Infrared Space Observatory observed over 900 objects with the Short Wavelength Spectrometer in full-grating-scan mode (2.4-45.2 micron). We have developed a comprehensive system of spectral classification using these data. Sources are assigned to groups based on the overall shape of the spectral energy distribution (SED). The groups include naked stars, dusty stars, warm dust shells, cool dust shells, very red sources, and sources with emission lines but no detected continuum. These groups are further divided into subgroups based on spectral features that shape the SED such as silicate or carbon-rich dust emission, silicate absorption, ice absorption, and fine-structure or recombination lines. Caveats regarding the data and data reduction, and biases intrinsic to the database, are discussed. We also examine how the subgroups relate to the evolution of sources to and from the main sequence and how this classification scheme relates to previous systems.

HIGH-RESOLUTION IMAGING OF PHOTODISSOCIATION REGIONS IN NGC 6334

We have used the SPIREX telescope to conduct a wide-—eld thermal infrared imaging study of the star formation complex NGC 6334 in the southern Galactic plane. We imaged a 30@ region along the main star-forming ridge of NGC 6334 with pixel scale through broadband —lters for L (3.5 km) and 0A.6 M (4.8 km) and through narrowband —lters for the v 1¨0 Q-branch (2.42 km), polycyclic aromatic H 2 hydrocarbon (PAH) (3.3 km), and Bra (4.05 km) lines. The images reveal the spectacular, complex struc- ture of the photodissociation regions (PDRs) that pervade the region, with enhanced line emission around each of the seven sites of massive star formation along the ridge. Bubbles and loops of PAH emission, typically 1¨1.5 pc across, have been carved out of the parent molecular cloud by the intense UV radiation from the massive stars and surround H II regions (seen in Bra) typically 0.2¨0.3 pc across. The PAH emission regions coincide with both (C II) 158 km line emission, indicating that the PAHs are excited in PDR gas, and extensive emission, which therefore must be —uorescent. However, the tex- H 2 tures of the emission regions in PAH and are diUerent. This is attributable to variations in the physi- H 2 cal environment in which the gas is excited. Several compact reddened objects are observed; these are likely to be massive protostars. Subject headings: infrared: generalinfrared: ISM: lines and bandsISM: molecules ¨ ISM: structurestars: formationtelescopes

Molecular Gas in the NGC 6334 Star Formation Region

We present millimeter- and centimeter-wave spectroscopic observations of the southern massive star formation region NGC 6334. The cloud has been mapped in several transitions of CO,13CO, CS, and NH3. The molecular gas shows a complex structure of filaments, in which the massive star formation occurs, and bubbles, some of which contain photodissociated gas. There is an anticorrelation between the presence of dense gas and the 6 cm radio flux: the hottest stars, with the hardest FUV radiation, have dispersed the dense gas from which they formed, whereas the cooler stars have not yet been able to do so. There is a velocity gradient along the star-forming ridge such that the radial velocity peaks in the center of the ridge. Several blueshifted emission features were discovered, one of which was identified with the 3-kpc arm of the Galaxy. Excitation model calculations were used to determine the physical conditions of the molecular gas in NGC 6334. The average kinetic temperature, hydrogen volume, and column densities at the continuum sources are as follows: -->img1.gif = 56 ± 11 K, log -->img2.gif -->img3.gif (cm-3) = 3.5 ± 0.3, and -->img4.gif -->img3.gif (1022 cm-2) = 7 ± 4, respectively. The properties of the molecular gas are compared to those in other massive star-forming clouds to determine that NGC 6334 is representative of massive star-forming regions in the Galaxy and can therefore be used to test the predictions of the theoretical models of photodissociation regions. The properties of the individual sites of star formation in the cloud are also discussed.

The Mid-Infrared Properties of Three Star-forming Sites in NGC 6334

To investigate their dust properties, we have imaged three sites of massive star formation in the giant H II region/star-forming cloud NGC 6334 with the MIRAC2 instrument. We obtained high-resolution (1) continuum images at 12.5 and 20.6 μm toward each region, which were compared with observations of the radio and near-infrared (near-IR) continuum emission. Both compact sources and extended emission were found at all three star-forming sites. The detected sources span a wide range of evolutionary states in this highly complex star-forming cloud. The infrared sources near NGC 6334 I were resolved into at least four subsources. One such source is substantially colder, denser, and more optically thick than the other mid-IR sources in the region and may be at the earliest stages of stellar formation. Another may be a torus or disk of dust and gas surrounding an embedded B star. NGC 6334 I was also imaged at additional wavelengths (8.8, 9.8, and 11.7 μm) to search for silicate absorption. Only at the Hxa0II region is there a deep silicate absorption feature from foreground dust. Toward the NGC 6334 IV, warm dust is associated with both the inner portions of the massive molecular torus or disk and with the bipolar continuum lobes. A compact mid-IR source, associated with the near-IR and radio source [HHS87] IRS 20, is cooler and more optically thick than the dust emission associated with the H II region. Toward NGC 6334 V, four embedded sources were found, including one previously unidentified object. This newly identified compact object, associated with a dust temperature peak and a radio source, is probably in a more advanced stage of star formation than the other compact mid-IR sources in NGC 6334.

Photodissociation Regions and H II Regions in NGC 6334

Using the VLA at 8.485 GHz, we have imaged the southern portion of the star-forming ridge of molecular gas in NGC 6334. The diffuse radio source G351.20+0.70, discovered by Moran et al. (1990), is now resolved into a roughly spherical shell of radius ~1 (0.5 pc). The distribution of molecular gas (traced by CO emission), of photodissociated gas (traced by [C II] 158 μm emission), and of ionized gas (traced by radio continuum emission), is precisely that expected for a photodissociation region—the ionized gas lies on the interior of the shell, the photodissociated gas just outside the ionized gas, and the molecular gas just outside the photodissociated gas. We also detected faint radio counterparts to the strong infrared sources NGC 6334 IV IRS 20 and NGC 6334 V. If these objects are zero-age main-sequence stars, they produce far less radio free-free emission than would be expected for the observed infrared flux. Some possible explanations for this discrepancy are the following: (1) the radio free-free emission is optically thick, (2) the stellar ionizing radiation is obscured by dust, (3) the objects are not single OB stars but very compact clusters of later type stars, or (4) the objects are protostars. For both NGC 6334 V and NGC 6334 IV IRS 20, the radio spectrum for the unresolved sources is inconsistent with optically thick free-free emission or dust obscuration from a homogeneous H II region. The radio spectral index for NGC 6334 IV IRS 20 is consistent with the value of 0.6 expected for an optically thick H II region for a star undergoing mass loss, but that of NGC 6334 V is not. Because the IR sources in NGC 6334 V are very compact (0.02 pc), the stellar volume densities for a cluster of later-type stars would be unreasonably large. The objects in NGC 6334 V are probably protostars.

[O I] 63 Micron Absorption in NGC 6334

The [O I] 63 μm transition has been imaged around five far-infrared (FIR) and radio continuum sources in the southern massive star formation region NGC 6334. The [O I] 63 μm line is found in absorption toward the FIR continuum source NGC 6334V. This is only the second case in which the [O I] 63 μm line has been seen in absorption against a continuum source. From the depth of the absorption line, the minimum column density of oxygen is calculated to be N(O0) 5 × 1018 cm-2. This amount of oxygen is consistent with [O I] 63 μm absorption due to atomic gas in the foreground molecular cloud. The [O I] 63 μm line is found in emission toward the other four sources observed: NGC 6334, sources A, C, D, and E. Single-component photodissociation region (PDR) models suggest densities of n ~ 104 cm-3 for these sources, based on previously observed [O I] 145 μm and [C II] 158 μm intensities. However, unphysically large far-ultraviolet (FUV) fields are implied for three of the sources, particularly for NGC 6334A. Neither one- nor two-component photodissociation region models can explain the anomalously low [O I] 63 μm intensity toward NGC 6334A nor the absorption toward NGC 6334V. We suggest that self-absorption of the [O I] 63 μm line, such as has been suggested toward DR 21, is suppressing the observed [O I] 63 μm intensity. This underestimate leads to an overestimate of the derived FUV field strengths throughout NGC 6334. The discovery of several more star-forming sites in which the [O I] 63 μm is in absorption or is self-absorbed implies that this line is not always a reliable PDR diagnostic because the PDR models do not treat the radiative transfer through the molecular cloud.

DUST CHARACTERISTICS OF MASSIVE STAR-FORMING SITES IN THE MID-INFRARED

Four massive star-forming regions were imaged in the mid-infrared with the MIRAC3 instrument: W51 IRS 2, Mon R2, DR 21, and S140. We obtained high spatial resolution (~1) images at several wavelengths from 7.8 to 13.2 μm with the circular variable filter, as well as narrow-band continuum images at 12.5 and 20.6 μm toward each region. In each massive star-forming region, one or more sources show deep silicate absorption. For at least two of the massive star-forming regions, W51 IRS 2 and Mon R2, the absorbing material is highly localized and may be circumstellar material in disks or shells. The silicate absorption occurs at least as often around massive young stars as around young stars of lower mass (which are more often observed). The estimated optical depths of the silicate features are consistent with those predicted by radiative transfer models toward ultracompact H II regions, but substantially higher than observed toward T Tauri stars and other low-mass young stellar objects. There is no consistent correspondence between silicate absorption and either the dust color temperature or the 12.5 μm opacity. In W51 IRS 2, the two previously known mid-infrared sources have been resolved into at least six subsources. Infrared counterparts are newly reported for two radio-continuum sources in S140. Also, new mid-infrared sources have been detected in both W51 IRS 2 and S140. We suggest that the infrared source in the southwest of DR 21 may not be self-luminous, but may instead be heated by the three nearby radio continuum sources. The gas density in the ring at Mon R2 supports the blister scenario for the IRS 1 H II region.

A 2000 M☉ Rotating Molecular Disk around NGC 6334A

We present millimeter and centimeter wave spectroscopic observations of the H II region NGC 6334A. We have mapped the source in several transitions of CO, CS, and NH3. The molecular emission shows a distinct flattened structure in the east-west direction. This structure is probably a thick molecular disk or torus (2.2 × 0.9 pc) responsible for the bipolarity of the near-infrared (NIR) and radio continuum emission which extends in two lobes to the north and south of the shell-like H II region. The molecular disk is rotating from west to east (ω ≈ 2.4 km s-1 pc-1) about an axis approximately parallel to the radio and NIR emission lobes. By assuming virial equilibrium, we find that the molecular disk contains ~2000 M☉. Single-component gas excitation model calculations show that the molecular gas in the disk is warmer and denser (Tk ≈ 60 K, n ≈ 3000 cm-3) than the gas to the north and south (Tk ≈ 50 K, n ≈ 400 cm-3). High resolution (~5) NH3 (3, 3) images of NGC 6334A reveal several small (~0.1 pc) clumps, one of which lies southwest of the radio continuum shell, and is spatially coincident with a near-infrared source, IRS 20. A second NH3 clump is coincident with an H2O maser and the center of a molecular outflow. The dense gas tracers, CS J = 5 → 4 and 7 → 6, peak near IRS 20 and the H2O maser, not at NGC 6334A. IRS 20 has a substantial far-infrared (FIR) luminosity LFIR ~ 105 L☉, which indicates the presence of an O 7.5 star but has no detected radio continuum (F6 cm < 0.02 Jy). The combination of dense gas, a large FIR luminosity and a lack of radio continuum can best be explained if IRS 20 is a protostar. A third clump of NH3 emission lies to the west of IRS 20 but is not associated with any other molecular or continuum features. The star formation activity in the region has moved west of NGC 6334A to IRS 20 and the H2O maser position. We suggest that NGC 6334A, IRS 20, and the H2O maser spot are part of a protocluster of stars which is condensing from the massive molecular disk. The similarity between the structure around NGC 6334A and other large (r ~ 1 pc), massive (M ~ 103 M☉), rotating disks (K3-50A and G10.6-0.4) suggests that this may be a common mechanism by which open clusters form.