P. J. Huggins

The photodissociation of CO in circumstellar envelopes

The CO photodissociation rate for the unshielded ISM is calculated using recent laboratory results which confirm that photodissociation occurs by way of line absorption. A value of 2.0 x 10 to the -10th/s, an order of magnitude higher than the rate used in the past, is obtained. The new rate and a treatment of the radiative transfer and shielding are used to develop a theory for the CO abundance in the circumstellar envelopes of cool, evolved stars, and results are presented on the spatial variation of CO, C, and C(+). It is shown that these distributions play important roles in determining the observational properties of circumstellar envelopes. 29 references.

Imaging the circumstellar envelopes of AGB stars

Aims. We report the results of an exploratory program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered galactic light. The goal is to characterize the morphology of the envelopes as a probe of the mass-loss process. Methods. The observations consist of short exposures with the VLT and longer exposures with 1–2 m telescopes, augmented with archival images from the Hubble Space Telescope. Results. We observed 12 AGB stars and detected the circumstellar envelopes in 7. The detected envelopes have mass loss ratesAims. We report the results of an exploratory program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered galactic light. The goal is to characterize the morphology of the envelopes as a probe of the mass-loss process. Methods. The observations consist of short exposures with the VLT and longer exposures with 1–2 m telescopes, augmented with archival images from the Hubble Space Telescope. Results. We observed 12 AGB stars and detected the circumstellar envelopes in 7. The detected envelopes have mass loss rates >∼5 × 10−6 M yr−1, and they can be seen out to distances >∼1 kpc. The observations provide information on the mass loss history on time scales up to ∼10 000 yr. For the five AGB envelopes in which the circumstellar geometry is well determined by scattered light observations, all except one (OH348.2−19.7) show deviations from spherical symmetry. Two (IRC+10216 and IRC+10011) show roughly spherical envelopes at large radii but asymmetry or bipolarity close to the star; one (AFGL 2514) shows an extended, elliptical envelope, and one (AFGL 3068) shows a spiral pattern. The non-spherical structures are all consistent with the effects of binary interactions. Conclusions. Our observations are in accord with a scenario in which binary companions play a role in shaping planetary nebulae, and show that the circumstellar gas is already partly shaped on the AGB, before evolution to the proto-planetary nebula phase.

The abundance of atomic carbon near the ionization fronts in M17 and S140

We have observed the 492 GHz ground-state line of atomic carbon in the edge-on ionization fronts in M17 and S140. We find that, contrary to expectation, the C_I emission peaks farther into the molecular cloud from the ionization front than does the CO. In fact the peak C_I abundance in M17 occurs more than 60 mag of visual extinction into the cloud from the ionization front. Calculations of the ratio of C_I to CO column densities yield values of 0.1-0.2. These observations do not support chemical models which predict that neutral atomic carbon should be found only near the edges of molecular clouds. Other models are discussed which may explain the observations.

A debris disk around the central star of the Helix Nebula

Excess emission from a point-like source coincident with the central star of the Helix Nebula is detected with Spitzer at 8, 24, and 70 µm. At 24 µm, the central source is superposed on an extended diffuse emission region. While the [O IV] 25.89 µm line contributes to the diffuse emission, a 1035 µm spectrum of the central source shows a strong thermal continuum. The excess emission from the star most likely originates from a dust disk with blackbody temperatures of 90–130 K. Assuming a simple optically thin debris disk model, the dust is distributed in a ring between �35 and �150 AU from the central star, possibly arising from collisions of Kuiper-Belt-like Objects or the break-up of comets from an Oort-like cloud that have survived from the post-main-sequence evolution of the central star. Subject headings: infrared: stars – stars: individual (WD 2226 210) – white dwarfs

Jets and Tori in Proto-Planetary Nebulae

We investigate the time sequence for the appearance of jets and molecular tori in the transition of stars from the asymptotic giant branch to the planetary nebula phase. Jets and tori are prominent features of this evolution, but their origins are uncertain. Using optical and millimeter line kinematics, we determine the ejection history in a sample of well-observed cases. We find that jets and tori develop nearly simultaneously. We also find evidence that jets typically appear slightly later than tori, with a lag time of a few hundred years. These characteristics provide strong evidence that jets and tori are physically related, and they set new constraints on theories of jet formation. The ejection of a discrete torus followed by jets on a short timescale favors the class of models in which a companion interacts with the central star. Models with long timescales, or with jets followed by a torus, are ruled out.

Infrared Imaging and Spectroscopy of the Helix with ISOCAM

We report infrared images of the Helix Nebula centered at 6.9 ?m (LW2 filter) and 15 ?m (LW3 filter) obtained with ISOCAM on board the Infrared Space Observatory (ISO). Three fields were also measured using the ISOCAM circular variable filter (CVF). The CVF data show that the 5-16.6 ?m spectrum is dominated by the pure (v=0-0) rotational lines of molecular hydrogen from the S(7) to the S(2) transitions. The strong S(5) H2 line accounts for most of the emission detected in the LW2 filter. The only atomic lines detected are [Ne II] 12.81 ?m and [Ar III] 8.99 ?m, which are weak, and [Ne III] 15.55 ?m, which is strong and accounts for most of the emission in the LW3 filter. No emission bands or continuum of small dust particles are detected despite the carbon richness of the Helix Nebula. The H2 emission traces the individual cometary globules of the molecular envelope of the nebula, whereas the [Ne III] emission is distributed along this envelope toward the inner regions of the ionized cavity. The intensities of H2 rotational lines are accurately predicted using a rotational temperature of 900?50 K and column densities of ~3?10 -->18 cm-2. The total luminosity in the H2 lines ~4 L? (6% of the star luminosity) is much higher than predicted for photodissociation regions. The significant absence of mid-infrared dust features indicates that in this evolved planetary nebula, the molecular-sized dust particles might have been destroyed by the exposure to the radiation field from the central hot star.

The formation of molecules in protostellar winds

The production and destruction processes for molecules in very fast protostellar winds are analyzed and modeled with a one-dimensional chemical kinetics code. Radial density and temperature distributions suggested by protostellar theory are explored as are a range of mass-loss rates. The efficiency of in situ formation of heavy molecules is found to be high if the wind temperature falls sufficiently rapidly, as indicated by theory. The degree of molecular conversion is a strong function of the mass-loss rate and of density gradients associated with the acceleration and collimation of the wind. Even in cases where essentially all of the heavy atoms are processed into molecules, a significant fraction of atomic hydrogen remains so that hghly molecular, protostellar winds are able to emit the 21-cm line. Although CO has a substantial abundance in most models relevant to very young protostars, high abundances of other molecules such as SiO and H2O signify more complete association characteristic of winds containing regions of very high density. Although the models apply only to regions close to the protostar, they are in qualitative accord with recent observations at much larger distances of both atomic and molecular emission from extremely high-velocity flow. 57 refs.

CO in the cometary globules of the Helix nebula

We report the detection of CO emission from the cometary globules which lie inside the ionized cavity of the Helix nebula. Our observations demonstrate that the globules consist of dense condensations of molecular gas. They yield estimates for the mass (5×10 −6 M ○ .) and density (1×10 5 cm −3 ) of the globules, and provide precise measurements of their radial velocities. The physical properties and kinematics of the globules firmly link the globule system with the massive envelope of molecular gas associated with the helix structure around the central cavity

High-Resolution CO and H2 Molecular Line Imaging of a Cometary Globule in the Helix Nebula

We report high-resolution imaging of a prominent cometary globule in the Helix Nebula in the CO J = 1-0 (2.6 mm) and H2 v = 1-0 S(1) (2.12 μm) lines. The observations confirm that globules consist of dense condensations of molecular gas embedded in the ionized nebula. The head of the globule is seen as a peak in the CO emission with an extremely narrow line width (0.5 km s-1) and is outlined by a limb-brightened surface of H2 emission facing the central star and lying within the photoionized halo. The emission from both molecular species extends into the tail region. The presence of this extended molecular emission provides new constraints on the structure of the tails and on the origin and evolution of the globules.

Multifrequency observations of BL Lacertae

We present 20 years of optical, infrared, and radio monitoring data for BL Lac as well as four simultaneous multifrequency spectra covering the 10^9-10^(18) Hz range. Although there is no time delay between the optical and infrared variability, the high-frequency radio variations precede lower frequency variations, but only by weeks. The optical variability precedes the radio variability by a few years. The structure function for the radio variations is nearly that of shot noise for time scales less than 600 days, in contrast to the optical variation, which is similar to flicker noise. These results indicate that, although there are fundamental differences between the optical and radio emitting regions, they are related, possibly by the propagation of shocks between regions. The multifrequency spectra show that the power per logarithmic bandwidth has a well-defined peak in the near-infrared and a sharp cutoff in the optical-ultraviolet region. This cutoff is like those seen in a few other blazars and is attributed to synchrotron losses that prevent particle acceleration from exceeding a critical energy. The X-ray continuum is not smoothly connected to the optical-ultraviolet emission and has a flatter slope, similar to that of the infrared-millimeter region. In addition, the X-ray emission varied in the same sense as the infrared-millimeter emission but opposite that of the optical-ultraviolet emission. These X-ray properties are those expected from the synchrotron-self-Compton process. The best model suggests that the plasma radiating at ~ 10^(11.5) Hz has a size of ~ 10^(-2·5) pc, a Doppler parameter δ>= 2-3, and a magnetic field of B = 2-40 G.