John H. Bieging

The Extended Environment of M17: A Star Formation History

M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy. It features a bright H II region erupting as a blister from the side of a giant molecular cloud (GMC). Combining photometry from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) with complementary infrared (IR) surveys, we identify candidate young stellar objects (YSOs) throughout a 15 ? 1? field that includes the M17 complex. The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors. Removing contaminants, we produce a highly reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex. We fit model spectral energy distributions to these sources and constrain their physical properties. Extrapolating the mass function of 62 intermediate-mass YSOs (M > 3?M ?), we estimate that >1000 stars are in the process of forming in the extended outer regions of M17. The remaining 34 candidate YSOs are found in a 0.17?deg2 field containing the well-studied M17 H II region and photodissociation region (PDR), where bright diffuse mid-IR emission drastically reduces the sensitivity of the GLIMPSE point-source detections. By inspecting IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim of a large shell structure ~05 in diameter (~20 pc at 2.1?kpc). We present maps of 12CO and 13CO (J = 2 ? 1) emission observed with the Heinrich Hertz Telescope. The CO emission shows that the shell is a coherent, kinematic structure associated with M17, centered at v = 19?km s?1. The shell is an extended bubble outlining the PDR of a faint, diffuse H II region several Myr old. We identify a group of candidate ionizing stars within the bubble. YSOs in our catalog are concentrated around the bubble rim, providing evidence that massive star formation has been triggered by the expansion of the bubble. The formation of the massive cluster ionizing the M17 H II region itself may have been similarly triggered. We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex.

The distribution of molecules in the circumstellar envelope of IRC +10216: HC3N, C3N, and SiS

We present aperture synthesis images made with the BIMA mm interferometer of molecular emission lines of HC 3 N, C 3 N, and SiS at ∼109 GHz toward the carbon star IRC +10216. The images have angular resolutions of 5″-7″ and velocity resolution of ∼1 km/s. The data are compared with model calculations using a newly-developed statistical equilibrium code which predicts the brightness distribution and spectrum for linear molecules in a spherical expanding circumstellar envelope. Excitation by IR photons and collisions is included. The model calculations are used to derive the abundance of HC 3 N and SiS as a function of distance from the star. For C 3 N, an LTE calculation is used to derive the abundance distribution

Revealing the Photodissociation Region: HST/NICMOS Imaging of NGC 7027

We report results from a Hubble Space Telescope (HST) and Near-Infrared Camera and Multiobject Spectrometer (NICMOS) program to study the distribution of hot neutral (molecular hydrogen) and ionized circumstellar material in the young planetary nebulae NGC 7027. HST/NICMOS provided very high spatial resolution imaging in line and continuum emission, and the stability and large dynamic range needed for investigating detailed structures in the circumstellar material. We present dramatic new images of NGC 7027 that have led to a new understanding of the structure in this important planetary nebula. The central star is clearly revealed, providing near-infrared fluxes that are used to directly determine the stellar temperature very accurately (T = 198,000 K). It is found that the photodissociation layer as revealed by near-infrared molecular hydrogen emission is very thin (ΔR ~ 6 × 1015 cm) and is biconical in shape. The interface region is structured and filamentary, suggesting the existence of hydrodynamic instabilities. We discuss evidence for the presence of one or more highly collimated, off-axis jets that might be present in NGC 7027. The NICMOS data are combined with earlier Hubble Space Telescope data to provide a complete picture of NGC 7027 using the highest spatial resolution data to date. The evolutionary future of NGC 7027 is discussed.

Observational Constraints on Submillimeter Dust Opacity

Infrared extinction maps and submillimeter dust continuum maps are powerful probes of the density structure in the envelope of star-forming cores. We make a direct comparison between infrared and submillimeter dust continuum observations of the low-mass Class 0 core, B335, to constrain the ratio of submillimeter to infrared opacity (κ_(smm)/κ_(ir)) and the submillimeter opacity power-law index (κ ∝ λ–β). Using the average value of theoretical dust opacity models at 2.2 μm, we constrain the dust opacity at 850 and 450 μm. Using new dust continuum models based upon the broken power-law density structure derived from interferometric observations of B335 and the infall model derived from molecular line observations of B335, we find that the opacity ratios are ^κ_(850)_κ_(2.2) = (3.21 - 4.80)^(+0.44)_(-0.30) x 10^(-4) ^κ_(450)_κ(2.0) = (12.8-24.8)^(+2.4)_(-1.3) x 10^(-4) with a submillimeter opacity power-law index of β_(smm) = (2.18-2.58)^(+0.30)_(–0.30). The range of quoted values is determined from the uncertainty in the physical model for B335. For an average 2.2 μm opacity of 3800 ± 700 cm^2 g^(–1), we find a dust opacity at 850 and 450 μm of κ_(850) = (1.18-1.77)^9+0.36)_(–0.24) and κ_(450) = (4.72-9.13)^(+1.9)_(–0.98) cm^2 g^(–1) of dust. These opacities are from (0.65-0.97)κ^(OH5)_(850) of the widely used theoretical opacities of Ossenkopf and Henning for coagulated ice grains with thin mantles at 850 μm.

Infrared Echoes near the Supernova Remnant Cassiopeia A

Two images of Cassiopeia A obtained at 24 micrometers with the Spitzer Space Telescope over a 1-year time interval show moving structures outside the shell of the supernova remnant to a distance of more than 20 arc minutes. Individual features exhibit apparent motions of 10 to 20 arc seconds per year, independently confirmed by near-infrared observations. The observed tangential velocities are at roughly the speed of light. It is likely that the moving structures are infrared echoes, in which interstellar dust is heated by the explosion and by flares from the compact object near the center of the remnant.

Mid-Infrared (8-21 micron) Imaging of Proto-Planetary Nebulae

We present mid-infrared (8-21 μm) images of thermal dust emission from two proto-planetary nebulae (PPNs), IRAS 07134+1005 and IRAS 22272+5435, which show a strong 21 μm emission feature. Both of the sources are well resolved and show evidence for axial symmetry. From our images, we calculate temperature and optical depth maps and estimate the abundance of the 11 μm and 21 μm feature carriers. In both sources, the dust temperatures range from ~160-200 K. The optical depths in IRAS 07134 are about a factor of 3 lower than those in IRAS 22272, but the emission is optically thin in both sources. Our analyses of the feature-to-continuum ratios suggests that 0.5%-5% of the carbon in these objects may be in the form of large PAH molecules. We construct optically thin, axially symmetric cylindrical shell models to simulate the observed mid-IR morphologies and spectra, and calculate nebular masses of 0.26 M☉ for IRAS 07134 and 0.42 M☉ for IRAS 22272. Although the mid-IR emission primarily comes from warm (T ≈ 190 K) dust, our models require a significant cooler dust (T ≈ 80 K) component to fit the observed mid- and far-IR spectral energy distributions.

Heavy-Metal Chemistry in Proto-Planetary Nebulae: Detection of MgNC, NaCN, and AlF toward CRL 2688

The metal-containing molecules AlF, MgNC, and NaCN have been detected toward CRL 2688, a circumstellar envelope in the proto-planetary nebula (PPN) phase. These measurements are the first detections of such species in a source other than the carbon star IRC +10216. Three transitions of MgNC were observed, each of which consists of two spin-rotation components, and five lines of NaCN were measured; both detections were made in the 2 mm wavelength region using the NRAO 12 m telescope. Three transitions of AlF were observed as well at 2 and 1.2 mm with the IRAM 30 m antenna. All three species appear to trace the AGB wind, not the high-velocity outflows characteristic of post-AGB mass loss. Rotational temperature analysis suggests that MgNC emission is from cooler gas than AlF and NaCN, indicating an outer shell distribution for this molecule, as is found in IRC +10216. AlF and NaCN appear to be confined to the inner envelope of CRL 2688. The column density obtained for MgNC in this source assuming a shell-like distribution is Ntot ~ 4 × 1012 cm-2, corresponding to a fractional abundance, relative to H2, of f ~ 4 × 10-9. This abundance is about a factor of 10 less than that in IRC +10216. For NaCN, the column density and fractional abundance in CRL 2688 are Ntot ~ 0.7-3 × 1014 cm-2 and f ~ 3-5 × 10-8, comparable to what has been measured for IRC +10216. In the case of AlF, the column density toward CRL 2688 was determined to be Ntot ~ 0.9-3 × 1013 cm-2, resulting in f ~ 4-5 × 10-9. In IRC +10216, f(AlF) is 1-2 × 10-7. These data suggest that the chemistry in the outer envelope of CRL 2688 has altered abundances for species like MgNC, but inner shell molecules like NaCN may remain intact over a longer timescale. While the abundance of AlF in CRL 2688 is less than that of 19F in the solar system, in IRC +10216, the AlF abundance exceeds the fluorine solar value. Hence, observations of this molecule suggest that 19F is produced in thermal pulses in the AGB phase.

SUBMILLIMETER- AND MILLIMETER-WAVELENGTH OBSERVATIONS OF SiO AND HCN IN CIRCUMSTELLAR ENVELOPES OF AGB STARS

We report molecular line observations with the Heinrich Hertz Submillimeter Telescope of a sample of 30 asymptotic giant branch stars with spectral types M, S, and C. Measured lines include SiO J = 5-4, 8-7, 10-9, and HCN J = 3-2 and 4-3 transitions, which arise from energy levels ranging from 26 to 115 K above ground. The observed transitions were detected in almost all stars observed, regardless of spectral type. The HCN J = 3-2 and 4-3 lines in the M stars are bright compared with previous observations of the J = 1-0 line. We calculated emergent spectra for model circumstellar envelopes and compare these with the observed line intensity ratios. We conclude that (1) the HCN line intensity ratios for the M stars are inconsistent with chemical models in which HCN is produced via photochemistry in the outer circumstellar envelope. HCN must be formed by a nonequilibrium chemical process in the inner envelope or extended stellar atmosphere. (2) The HCN/SiO intensity ratios of lines with similar excitation energies clearly separate the carbon stars from the M and S stars. The M and S stars show a trend of increasing HCN/SiO intensity ratios with increasing mass-loss rate. (3) These data support the idea that pulsation-driven shocks result in the formation of organic molecules like HCN in the envelopes of M stars. Observations of these molecules could give useful constraints on hydrodynamic models for stellar mass loss driven by pulsational shocks in the stellar atmosphere. We detected emission from vibrationally excited HCN in the v = (0,11c,0), J = 3-2 and 4-3 transitions toward four carbon stars. In one star, V Cyg, the lines are strong and narrow, and are most likely a result of maser amplification.

CO J = 2-1 and 4-3 Observations of Proto-planetary Nebulae: Time-variable Mass Loss

Observations made with the Heinrich Hertz Telescope of CO millimeter and submillimeter emission toward a sample of 22 proto-planetary nebula (PPN) candidates resulted in detections of 12 sources in the CO J = 2-1 line. Of these 12, seven sources were also detected in the J = 4-3 line. These 4-3 transitions are the highest yet observed in all but one of these PPNs. Statistical equilibrium/radiative transfer models were calculated for the CO emission in the circumstellar envelopes (CSEs), assuming various power-law density distributions. These models were compared with the intensity and profile shape of the observed spectra. For the region of the CSE probed by CO emission, the density laws must be steeper than inverse squared and are consistent with power laws between ρ ∝ r-3 and r-4. These radial density distributions imply that the mass loss was not constant but increased during the last part of the asymptotic giant branch (AGB) phase. Mass-loss rates at the end of the AGB for the three best-constrained sources are found to be 7.7 × 10-5 M☉ yr-1 (IRAS 22272+5435), 2.3 × 10-5 M☉ yr-1 (IRAS 07134+1005), and 1.3 × 10-5 M☉ yr-1 (IRAS 17436+5003) for the case of ρ ∝ r-3. These time-varying mass-loss rates can be integrated to calculate the enclosed envelope masses ejected in the past ~10,000 yr. The ejected envelope masses close to the star lie in the range 0.02-0.30 M☉; these values are consistent with theoretical models, which indicate that <20% of the stellar mass loss occurs in the last 10,000 years of the AGB. These results are in contrast to some recent dust studies based on infrared emission, however, in which much higher envelope masses are determined. The density laws, mass-loss rates, and enclosed envelope masses that we derive furnish important constraints for evolutionary models of stars in the late AGB and during the transition to the planetary nebula phase.

Spitzer and HHT Observations of Bok Globule B335: Isolated Star Formation Efficiency and Cloud Structure*

We present infrared and millimeter observations of Barnard 335, the prototypical isolated Bok globule with an embedded protostar. Using Spitzer data we measure the source luminosity accurately; we also constrain the density profile of the innermost globule material near the protostar using the observation of an 8.0 � m shadow. Heinrich Hertz Telescope (HHT) observations of 12 CO 2Y1 confirm the detection of a flattened molecular core with diameter � 10,000 AU and the same orientation as the circumstellar disk (� 100 to 200 AU in diameter). This structure is probably the same as that generating the 8.0 � m shadow and is expected from theoretical simulations of collapsing embedded protostars. We estimate the mass of the protostar to be only � 5% of the mass of the parent globule. Subject headingg infrared: ISM — ISM: globules — ISM: individual (Barnard 335) — stars: formation