[Show abstract][Hide abstract] ABSTRACT: One of the most interesting characteristics of the planetary nebula NGC 7293 (the Helix Nebula) is its comet-shaped knots. We have observed one of the knots using the SINFONI imaging spectrometer on the VLT with adaptive optics. The spectra are analysed to obtain the spatial variation of molecular hydrogen line intensities within the knot. The images clearly show the detailed structure, which resembles a tadpole in shape, suggesting that hydrodynamic flows around the knot core create a pressure gradient behind the core. The three-dimensional spectra reveal that the excitation temperature is uniform at approximately 1800 K within the knot. The SINFONI observations help to determine the H2 excitation mechanism in planetary nebulae, as well as the importance of hydrodynamics in shaping the knots.
[Show abstract][Hide abstract] ABSTRACT: We present K-band integral field spectroscopy of the bipolar post-asymptotic giant branch (post-AGB) object IRAS 18276−1431 (OH 17.7−2.0)
using SINFONI on the VLT. This allows us to image both the continuum and molecular features in this object from 1.95 to 2.45
μm with a spatial resolution down to 70 mas and a spectral resolution of ∼5000. We detect a range of H2 rovibrational emission lines which are consistent with shock excitation in regions of dense (∼107 cm−3) gas with shock velocities in the range of 25–30 km s−1. The distribution of H2 emission in the bipolar lobes suggests that a fast wind is impinging on material in the cavity walls and tips. H2 emission is also seen along a line of sight close to the obscured star as well as in the equatorial region to either side
of the stellar position which has the appearance of a ring with radius 0.3 arcsec. This latter feature may be radially cospatial
with the boundary between the AGB and post-AGB winds. The first overtone 12CO bandheads are observed longward of 2.29 μm with the v = 2–0 bandhead prominently in emission. The CO emission has the same
spatial distribution as the K-band continuum and therefore originates from an unresolved central source close to the star. We interpret this as evidence
for ongoing mass-loss in this object. This conclusion is further supported by a rising K-band continuum indicating the presence of warm dust close to the star, possibly down to the condensation radius. The redshifted
scattered peak of the CO bandhead is used to estimate a dust velocity along the bipolar axis of 95 km s−1 for the collimated wind. This places a lower limit of ∼125 yr on the age of the bipolar cavities, meaning that the collimated
fast wind turned on very soon after the cessation of AGB mass-loss.
Preview · Article · Sep 2010 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Knots are commonly found in nearby planetary nebulae (PNe) and star-forming regions. Within PNe, knots are often found to
be associated with the brightest parts of the nebulae and understanding the physics involved in knots may reveal the processes
dominating in PNe. As one of the closest PNe, the Helix Nebula (NGC 7293) is an ideal target to study such small-scale (∼300 au)
structures. We have obtained infrared integral spectroscopy of a comet-shaped knot in the Helix Nebula using the Spectrograph
for INtegral Field Observations in the Near Infrared (SINFONI) on the Very Large Telescope at high spatial resolution (50–125
mas). With spatially resolved 2-μm spectra, we find that the H2 rotational temperature within the cometary knots is uniform. The rotational-vibrational temperature of the cometary knot
(situated in the innermost region of the nebula, 2.5 arcmin away from the central star) is 1800 K, higher than the temperature
seen in the outer regions (5–6 arcmin from the central star) of the nebula (900 K), suggesting that the excitation temperature
varies across the nebula. The obtained intensities are reasonably well fitted with 27 km s−1 C-type shock model. This ambient gas velocity is slightly higher than the observed [He ii] wind velocity of 13 km s−1. The gas excitation can also be reproduced with a photon-dominant region (PDR) model, but this requires an order of magnitude
higher ultraviolet radiation. Both models have limitations, highlighting the need for models that treat both hydrodynamical
physics and the PDR.
Full-text · Article · Sep 2007 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We present near-infrared polarimetric images of the dusty circumstellar envelope (CSE) of IRAS 19306+1407, acquired at the
United Kingdom Infrared Telescope (UKIRT) using the UKIRT 1–5 μm Imager Spectrometer (UIST) in conjunction with the half-waveplate
module IRPOL2. We present additional 450- and 850-μm photometry data obtained with the Submillimetre Common-User Bolometer
Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), as well as archived Hubble Space Telescope (HST) F606W- and F814W-filter images. The CSE structure in polarized flux at J and K bands shows an elongation north of north-east and south of south-west with two bright scattering shoulders north-west and
south-east. These features are not perpendicular to each other and could signify a recent ‘twist’ in the outflow axis. We
model the CSE using an axisymmetric light scattering (als) code to investigate the polarization produced by the CSE, and an axisymmetric radiation transport (dart) code to fit the spectral energy distribution. A good fit was achieved with the als and dart models using silicate grains, 0.1–0.4 μm with a power-law size distribution of a−3.5, and an axisymmetric shell geometry with an equator-to-pole ratio of 7:1. The spectral type of the central star is determined
to be B1i supporting previous suggestions that the object is an early planetary nebula. We have constrained the CSE and interstellar
extinction as 2.0 and 4.2 mag, respectively, and have estimated a distance of 2.7 kpc. At this distance, the stellar luminosity
is ∼4500 L⊙ and the mass of the CSE is ∼0.2 M⊙. We also determine that the mass loss lasted for ∼5300 yr with a mass-loss rate of ∼3.4 × 10−5 M⊙ yr−1.
Preview · Article · Nov 2006 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We use H_2 as a marker to trace the interaction between the older and slower AGB wind and the newer fast wind. Integral field spectroscopy is a tool to probe the interactions of these winds. The measurement of the various line ratios makes it possible to differentiate between the excitation mechanisms. We have obtained K-band observations with UIST+IFU at UKIRT and SINFONI at VLT. The UKIRT observations enable us to locate the areas of emission over an area of ˜ 6 × 3 arcseconds, whilst SINFONI can provide high resolution sub-arcsecond observations. IRAS 19306+1407 is a B0/1 spectral type post-AGB/Young PN with a bipolar outflow and displays a mixture of shocks and fluorescence. It has emission lines that emanate from an elongated bipolar structure and bright arcs. The combination of H_2 and existing polarimetry enables us to analyse the gas and dust around this evolved star.
Preview · Article · Apr 2006 · Proceedings of the International Astronomical Union
[Show abstract][Hide abstract] ABSTRACT: We have carried out JHK polarimetric observations of 11 dusty young stars, by using the polarimeter module IRPOL2 with the near-infrared camera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Our sample targeted systems for which UKIRT-resolvable discs had been predicted by model fits to their spectral energy distributions. Our observations have confirmed the presence of extended polarized emission around TW Hya and around HD 169142. HD 150193 and HD 142666 show the largest polarization values among our sample, but no extended structure was resolved. By combining our observations with Hubble Space Telescope (HST) coronographic data from the literature, we derive the J- and H-band intrinsic polarization radial dependences of the disc of TW Hya. We find the polarizing efficiency of the disc is higher at H than at J, and we confirm that the J- and H-band percentage polarizations are reasonably constant with radius in the region between 0.9 and 1.3 arcsec from the star. We find that the objects for which we have detected extended polarizations are those for which previous modelling has suggested the presence of flared discs, which are predicted to be brighter than flat discs and thus would be easier to detect polarimetrically.
Preview · Article · Jan 2006 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Polarimetry can be used to observe the dusty outer shells of Post-AGB
stars. We can discriminate between the unpolarized light from the
central star and the reflected polarized flux. The polarisation vectors
and polarized flux can give valuable information on the morphology of
the object and the dust grain size distribution. Here we present J and K
waveband polarimetry information and modeling of IRAS 19306+1407. We
have detected scattered light and high degrees of linear polarisation.
In polarized flux we have detected a `dumbbell' structure, possibly
indicating a detached dusty circumstellar envelope (CSE). We have
constructed a light scattering model that includes polarisation to
investigate the CSE structure in detail.