ABSTRACT The aim of this work is to study structure and gas kinematics in the photodissociation regions (PDRs) around the compact H ii regions S235 A and S235 C. We observe the [C ii], [13C ii], and [O i] line emission, using SOFIA/upGREAT, and complement them by data of HCO+ and CO. We use the [13C ii] line to measure the optical depth of the [C ii] emission, and find that the [C ii] line profiles are influenced by self-absorption, while the [13C ii] line remains unaffected by these effects. Hence, for dense PDRs, [13C ii] emission is a better tracer of gas kinematics. The optical depth of the [C ii] line is up to 10 in S235 A. We find an expanding motion of the [C ii]-emitting layer of the PDRs into the front molecular layer in both regions. Comparison of the gas and dust columns shows that gas components visible neither in the [C ii] nor in low-J CO lines may contribute to the total column across S235 A. We test whether the observed properties of the PDRs match the predictions of spherical models of expanding H ii region + PDR + molecular cloud. Integrated intensities of the [13C ii], [C ii], and [O i] lines are well represented by the model, but the models do not reproduce the double-peaked [C ii] line profiles due to an insufficient column density of C+. The model predicts that the [O i] line could be a more reliable tracer of gas kinematics, but the foreground self-absorbing material does not allow using it in the considered regions.

We present results of an extinction-CO line survey of the southeastern part of the California molecular cloud (CMC). Deep, wide-field, near-infrared images were used to construct a sensitive, relatively high resolution (similar to 0.5 arcmin) (NICEST) extinction map of the region. The same region was also surveyed in the (CO)-C-12(2-1), (CO)-C-13(2-1), and (CO)-O-18(2-1) emission lines at the same angular resolution. These data were used to investigate the relation between the molecular gas, traced by CO emission lines, and the dust column density, traced by extinction, on spatial scales of 0.04 pc across the cloud. We found strong spatial variations in the abundances of (CO)-C-13 and (CO)-O-18 that were correlated with variations in gas temperature, consistent with temperature-dependent CO depletion/ desorption on dust grains. The (CO)-C-13-to-(CO)-O-18 abundance ratio was found to increase with decreasing extinction, suggesting selective photodissociation of (CO)-O-18 by the ambient UV radiation field. The effect is particularly pronounced in the vicinity of an embedded cluster where the UV radiation appears to have penetrated deeply (i.e., A(V) less than or similar to 15 mag) into the cloud. We derived the cloud-averaged X-factor to be < X-CO > = 2.53 x 10(20) cm(2)(K km s(-1))(-1), a value somewhat higher than the Milky Way average. On sub-parsec scales we find there is no single empirical value of the (CO)-C-12 X-factor that can characterize the molecular gas in cold (T-k less than or similar to 15 K) cloud regions, with X-CO infinity A(V)(0.74) for A(V) greater than or similar to 3 mag. However, in regions containing relatively hot (Tex greater than or similar to 25 K) molecular gas we find a clear correlation between W((CO)-C-12) and A(V) over a large (3 less than or similar to A(V) less than or similar to 25 mag) range of extinction. This results in a constant X-CO = 1.5 x 10(20) cm(-2)(K km s(-1))(-1) for the hot gas, a lower value than either the average for the CMC or the Milky Way. Overall we find an (inverse) correlation between X-CO and T-ex in the cloud with X-CO infinity T-ex(-0.7). This correlation suggests that the global X-factor of a giant molecular cloud may depend on the relative amounts of hot gas contained within the cloud.

The main objective of this paper is to study the possibility of triggered star formation on the border of the HII region S233, which is formed by a B-star. Using high-resolution spectra we determine the spectral class of the ionizing star as B0.5 V and the radial velocity of the star to be -17.5(1.4) km/s. This value is consistent with the velocity of gas in a wide field across the S233 region, suggesting that the ionizing star was formed from a parent cloud belonging to the S233 region. By studying spatial-kinematic structure of the molecular cloud in the S233 region, we detected an isolated clump of gas producing CO emission red-shifted relative to the parent cloud. In the UKIDSS and WISE images, the clump of gas coincides with the infrared source containing a compact object and bright-rimmed structure. The bright-rimmed structure is perpendicular to the direction of the ionizing star. The compact source coincides in position with IRAS source 05351+3549. All these features indicate a possibility of triggering formation of a next-generation star in the S233 region. Within the framework of a theoretical one-dimensional model we conclude that the "collect-and-collapse" process is not likely to take place in the S233 region. The presence of the bright-rimmed structure and the compact infrared source suggest that the "collapse of the pre-existing clump" process is taking place.

We present spectroscopy of the supernova remnant Cassiopeia A (Cas A) observed at infrared wavelengths from 10 to 40 mu m with the Spitzer Space Telescope and at millimeter wavelengths in (CO)-C-12 and (CO)-C-13 J = 2-1 (230 and 220 GHz) with the Heinrich Hertz Submillimeter Telescope. The IR spectra demonstrate high-velocity features toward a molecular cloud coincident with a region of bright radio continuum emission along the northern shock front of Cas A. The millimeter observations indicate that CO emission is broadened by a factor of two in some clouds toward Cas A, particularly to the south and west. We believe that these features trace interactions between the Cas A shock front and nearby molecular clouds. In addition, some of the molecular clouds that exhibit broadening in CO lie 1'-2' away from the furthest extent of the supernova remnant shock front. We propose that this material may be accelerated by ejecta with velocity significantly larger than the observed free-expansion velocity of the Cas A shock front. These observations may trace cloud interactions with fast-moving outflows such as the bipolar outflow along the southwest to northeast axis of the Cas A supernova remnant, as well as fast-moving knots seen emerging in other directions.

Context. In the dense and cold interiors of starless molecular cloud cores, a number of chemical processes allow for the formation of complex molecules and the deposition of ice layers on dust grains. Dust density and temperature maps of starless cores derived from Herschel continuum observations constrain the physical structure of the cloud cores better than ever before. We use these to model the temporal chemical evolution of starless cores. Aims: We derive molecular abundance profiles for a sample of starless cores. We then analyze these using chemical modeling based on dust temperature and hydrogen density maps derived from Herschel continuum observations. Methods: We observed the $^{12}$CO (2-1), $^{13}$CO (2-1), C$^{18}$O (2-1) and N$_{2}$H$^{+}$ (1-0) transitions towards seven isolated, nearby low-mass starless molecular cloud cores. Using far infrared (FIR) and submillimeter (submm) dust emission maps from the Herschel key program Earliest Phases of Star formation (EPoS) and by applying a ray-tracing technique, we derived the physical structure (density, dust temperature) of these cores. Based on these results we applied time-dependent chemical modeling of the molecular abundances. We modeled the molecular emission profiles with a line-radiative transfer code and compared them to the observed emission profiles. Results: CO is frozen onto the grains in the center of all cores in our sample. The level of CO depletion increases with hydrogen density and ranges from 46% up to more than 95% in the core centers of the three cores with the highest hydrogen density. The average hydrogen density at which 50% of CO is frozen onto the grains is 1.1 {plusmn} 0.4 { imes} 10$^{5}$ cm$^{-3}$. At about this density, the cores typically have the highest relative abundance of N$_{2}$H$^{+}$. The cores with higher central densities show depletion of N$_{2}$H$^{+}$ at levels of 13% to 55%. The chemical ages for the individual species are on average (2 {plusmn} 1) { imes} 10$^{5}$ yr for $^{13}$CO, (6 {plusmn} 3) { imes} 10$^{4}$ yr for C$^{18}$O, and (9 {plusmn} 2) { imes} 10$^{4}$ yr for N$_{2}$H$^{+}$. Chemical modeling indirectly suggests that the gas and dust temperatures decouple in the envelopes and that the dust grains are not yet significantly coagulated. Conclusions: We observationally confirm chemical models of CO-freezeout and nitrogen chemistry. We find clear correlations between the hydrogen density and CO depletion and the emergence of N$_{2}$H$^{+}$. The chemical ages indicate a core lifetime of less than 1 Myr. This work is partially based on observations by the Herschel Space Observatory. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.org

Aims. A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of asymptotic giant branch (AGB) stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models.Methods. In order to constrain the circumstellar HCN abundance distribution a detailed non-local thermodynamic equilibrium (LTE) excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included.Results. The median values for the derived abundances of HCN (with respect to H-2) are 3 x 10(-5), 7 x 10(-7) and 10(-7) for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available.Conclusions. We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars.

Spectropolarimetric observations are presented for 21 AGB stars, 13 proto - planetary nebulae ( PPNs), and two R CrB - type stars. The spectra cover the wavelength range from similar to 4200 to 8400 angstrom with 16 angstrom resolution. Among the AGB stars, 8 of 14 M giants, five of six carbon stars, and zero of one S star showed intrinsic polarization. At least 9 of 13 PPNs exhibited intrinsic polarization, while the R CrB - type stars show intrinsic polarization during fading episodes. There is a statistical correlation between mean polarization, < P >, and IR color, K - [12], among the AGB stars such that redder stars tend to be more polarized. The PPN sample is significantly redder and more polarized, on average, than the AGB stars. This increase in hPi with increased reddening is consistent with an evolutionary sequence in which AGB stars undergo increasing mass loss, with growing asymmetries in the dust distribution as they evolve up and then off the AGB into the short-lived PPN phase. A related trend is found between polarization and mass-loss rate in gas,. M-gas. The detectability of polarization increases with mass-loss rate, and probably all AGB stars losing mass at > 10(-6) M circle dot yr(-1) have detectable polarization. Multiple observations of three polarized AGB stars show that in some cases hPi increases with mV, and in others it decreases. If polarization arises from scattering of starlight off an aysmmetric distribution of grains, then the distribution varies with time. Polarized features are detected in the TiO bands of three M- type Mira variables, in the CN bands of the carbon stars R Lep and V384 Per, and in the Swan bands of C-2 in R CrB and two PPNs. Polarization effects in the molecular bands appear to be more common and the effects are larger in O-rich than C-rich objects.

AbstractWe present the first results from a project to map Giant Molecular Clouds (GMCs) in the 12CO J=2-1, 13CO J=2-1, and 12CO J=3-2 lines using the Heinrich Hertz Submillimeter Telescope (HHT) at the University of Arizona. We mapped nearly 2.5 sq. deg of W3 and 1.0 sq. deg of W51 in the J=2-1 lines. We have begun mapping in the J=3-2 line. We achieve angular resolutions of 33″ and 24″ in the J=2-1 and J=3-2 lines with 1.3 and 0.9 km s−1 resolution.

We measured the spectrum of polarization for three proto - planetary nebulae (PPNs), IRAS 17411 - 2411, IRAS 08005 - 2356, and IRAS 04296+ 3429, and made model calculations with the dust-scattering Monte Carlo code DIRTY and the dust emission code 2Dust. We show that high levels of polarization in these PPNs correlate with extreme asymptotic giant branch (AGB) superwind mass-loss rates in excess of 10(-4) M-. yr(-1). All three objects show evidence for evacuated lobes cleared by collimated fast winds, and two indicate a significant equatorial mass enhancement. Our best-fit models require sharply peaked grain size distributions, suggesting that most of the light is being scattered by grains of a characteristic size in IRAS 17441 - 2411 and IRAS 08005 - 2356. IRAS 17441 - 2411 and IRAS 08005 - 2356 have lobes with wide opening angles, perhaps produced by deflection of a polar jet from an accreting companion by the AGB superwind. Modeling the spectropolarimetry of IRAS 04296+ 3429 indicates a point-symmetric, multipolar morphology in the PPN phase. The modeling of spectropolarimetry and other observations of PPNs provides a powerful way to constrain circumstellar morphology and dust parameters.

We present high-resolution (similar to0."1), very high Strehl ratio (0.97 +/- 0.03) mid-IR adaptive optics (AO) images of the asymptotic giant branch (AGB) star RV Boo utilizing the MMT adaptive secondary AO system. RV Boo was observed at a number of wavelengths over two epochs (9.8 mum in 2003 May and 8.8, 9.8, and 11.7 mum in 2004 February) and appeared slightly extended at all wavelengths. While the extension is very slight at 8.8 and 11.7 mum, the extension is somewhat more pronounced at 9.8 mum. With such high Strehl ratios, we can achieve superresolutions of 0."1 by deconvolving RV Boo with a point-spread function (PSF) derived from an unresolved star. We tentatively resolve RV Boo into a 0."16 FWHM extension at a position angle of 120degrees. At a distance of 390(-100)(+250) pc, this corresponds to a FWHM of 60(-15)(+40) AU. We measure a total flux at 9.8 mum of 145 +/- 24 Jy for the disk and star. Based on a dust thermal emission model for the observed IR spectral energy distribution and the 9.8 mum AO image, we derive a disk dust mass of 1.6 x 10(-6) M-. and an inclination of 30degrees- 45degrees from edge-on. We discuss whether the dust disk observed around RV Boo is an example of the early stages in the formation of asymmetric structure in planetary nebulae.

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.

The current understanding of the planetary nebula NGC 7027 is summarized. New HST/STIS observations are introduced.

We present high resolution (similar to0.1") very high Strehl ratio (0.97+/-0.03) mid-IR images of RV Boo and AC Her utilizing the MMT adaptive secondary AO system. RV Boo was imaged at 9.8 mum; AC Her was imaged at 9.8, 11.7, and 18 mum. At such high Strehls we can achieve super-resolutions of 0.1" by deconvolving RV Boo with a PSF derived from an unresolved star. We resolve RV Boo into a 0.16" FWHM extension at a position angle of 120 degrees. At a distance of 390 pc, this corresponds to a FWHM of similar to60 AU. We measure a total flux at 9.8 pm of 145+/-24 Jy for the disk and star. For AC Her, we find no extension greater than 0.2".

We have obtained high quality polarization spectra of a sample of PPN's and AGB stars for which we performed model calculations with a 3-dimensional Monte Carlo dust scattering and re-emission code to constrain the distribution and properties of the dust grains. We display our current modeling results of two PPN's, the bipolar IRAS 17441-2411 (the Silkworm Nebula) and IRAS 04296+3429, a reflection symmetric PPN surrounding a partially obscured central star. The results indicate that these two PPN's are physically different objects and not simply viewed from different inclinations. Modeling spectropolarimetry clearly provides strong constraints on the dust geometry and grain types that otherwise would not be possible.

We present observations of the bipolar nebula OH 231.8+4.2, made with the Hubble Space Telescope (HST) NICMOS camera, in three wide filter bands (F205W, F160W, and F110W). The images have excellent dynamic range after removal of low-level instrumental artifacts. In the F205W filter (lambda approximate to 2.04 mum), we achieve a peak/rms of greater than 8000 with an angular resolution of 0."20 (FWHM). The combination of high dynamic range and angular resolution confirms previous observations but also reveals new features in the near-IR morphology of the nebula, which at these wavelengths is dominated by scattered light. The northern (approaching) lobe shows well-defined, limb-brightened edges. The central jet splits into two laments and exhibits some curvature between the center of the nebula and the end of the northern lobe. The southern (receding) lobe has a diffuse, occulent appearance without a sharply defined central jet, in contrast to the northern lobe. A sharpened version of the F205W image shows indications of turbulent structures both in the northern lobe and jet and in the southern lobe. A faint cylindrical halo of scattered light shows a sharp increase in surface brightness inside a radius of similar to4" from the center, possibly resulting from a transition from a spherical wind to a disk- or torus-like mass ejection, on the same timescale as the formation of the collimated fast wind seen in CO and HCO(+) images. We calculate the extinction through the nebula from the measured near-IR colors and a silicate grain model. For a simple geometric model of a dense central disk, we estimate a disk mass (gas and dust) in the range 0.03-0.06 M., relatively insensitive to grain size. The circumstance of an asymptotic giant branch (AGB) star with a massive, highly collimated, high-velocity bipolar flow may be understood in terms of the model of Soker & Rappaport if the central star underwent a change in mass-loss properties from a (roughly) spherical AGB wind to equatorially enhanced mass loss beginning similar to(1-3) X 10(3) yr ago. Supposing the presence of a dwarf companion in a suitable orbit, the bipolar nebula then is a consequence of a strong increase in mass loss as the central star evolves close to the tip of the AGB and will soon evolve to higher T(eff) and appear as a more typical proto planetary nebula.

We utilized the unique 6.5 m Multiplie Mirror Telescope deformable secondary adaptive optics (AO) system to produce high-resolution (FWHM = 0."3), very high Strehl mid-infrared (9.8, 11.7, and 18 mum) images of the post-asymptotic giant branch star AC Her. The very high (98% +/- 2%) Strehls achieved with mid-IR AO led naturally to an ultrastable point-spread function (PSF) independent of air mass, seeing, or location on the sky. We find no significant difference between AC Her's morphology and our unresolved PSF calibration stars (mu UMa and alpha Her) at 9.8, 11.7, and 18 mum. Our current observations do not confirm any extended mid-IR structure around AC Her. These observations are in conflict with previously reported Keck (seeing-limited) 11.7 and 18 mum images that suggested the presence of a resolved similar to0."6 edge-on circumbinary disk. We conclude that AC Her has no extended mid-IR structure on scales greater than 0."2 (R < 75 AU). These first results of mid-IR AO science are very R ! 75 encouraging for future high-accuracy mid-IR imaging with this technique.

We present spectra covering the wavelength range 2.28 to 2.36 mum at a resolution of Deltalambda = 0.0007 mum (or R = 3500) for a sample of 24 cool evolved stars. The sample comprises 8 M supergiants, 5 M giants, 3 S stars, 6 carbon stars, and 2 RV Tauri variables. The wavelengths covered include the main parts of the C-12 O-16 v = 2-0 and 3-1 overtone bands, as well as the v = 4-2 and (CO)-C-13 v = 2-0 bandhead regions. CO lines dominate the spectrum for all the stars observed, and at this resolution most of the observed features can be identified with individual CO R- or P-branch lines or blends. The observed transitions arise from a wide range of energy levels extending from the ground state to E/k > 20 000 K. We looked for correlations between the intensities of various CO absorption line features and other stellar properties, including IR colors and mass loss rates. Two useful CO line features are the v = 2-0 R14 line, and the CO v = 2-0 bandhead. The intensity of the 2-0 bandhead shows a trend with K-[12] color such that the reddest stars (K-[12] > 3 mag) exhibit a wide range in 2-0 bandhead depth, while the least reddened have the deepest 2-0 bandheads, with a small range of variation from star to star. Gas mass loss rates for both the AGB stars and the red supergiants in our sample correlate with the K-[12] color, consistent with other studies. The data imply that stars with (M) over dot(gas) < 5 x 10(-7) M-. y(-1) exhibit a much narrower range in the relative strengths of CO 2-0 band features than stars with higher mass loss rates. The range in observed spectral properties implies that there are significant differences in atmospheric structure among the stars in this sample.