J. A. Yates

Axisymmetry in protoplanetary nebulae: using imaging polarimetry to investigate envelope structure

We use ground-based imaging polarimetry to detect and image the dusty circumstellar envelopes of a sample of proto-planetary nebulae (PPNe) at near-infrared wavelengths. This technique allows the scattered light from the faint envelope to be separated from the glare of the bright central star and is particularly well suited to this class of object. We detect extended (up to 9 arcsec diameter) circumstellar envelopes around 15 out of 16 sources with a range of morphologies including bipolars and shells. The distribution of scattered light in combination with its polarization (up to 40 per cent) provides unambiguous evidence for axisymmetry in 14 objects showing this to be a common trait of PPNe. We suggest that the range of observed envelope morphologies results from the development of an axisymmetric dust distribution during the superwind phase at the end of the AGB. We identify shells seen in polarized light with scattering from these superwind dust distributions which allows us to provide constraints on the duration of the superwind phase. In one object (IRAS 19475+3119) the circumstellar envelope has a two-armed spiral structure which we suggest results from the interaction of the mass losing star with a binary companion.

Resolved Structure in the Nuclear Region of the Ultraluminous Infrared Galaxy Markarian 273

We have studied the core morphology of the ultraluminous infrared galaxy Mrk 273 by combining a high-resolution adaptive optics near-infrared image with an optical image from the Hubble Space Telescope and interferometric radio continuum data, all at spatial resolutions of 150 mas or better. The near-infrared image reveals that the nucleus has two main components, both of which have radio counterparts. The strongest component (N) shows very similar extended structure in the radio and near-infrared. It has a flat radio spectrum and is resolved into a double-lobed structure (Ne; Nw), with a separation of 90±5 mas (70 pc). A similar structure is detected in the near-infrared. We identify this component as the location of the active nucleus. The second component (SW), strong in the near-infrared but relatively weak in the radio, is located ~1 to the southwest. We interpret this as an obscured starburst region associated with the merger. The radio continuum images show a third, strong, component (SE) that has previously been interpreted as a second nucleus. However, it shows no associated optical or near-infrared emission, which suggests that it is in fact a background source.

Circular polarisation in star-forming regions: Possible implications for homochirality

Our discovery of high degrees of circular polarisation in some star-forming regions provides an attractive mechanism for the origin of homochirality. The largest degrees of circular polarisation, so far observed at near-infrared wavelengths, are thought to arise from the scattering of stellar radiation from aligned dust grains and are calculated to extend down to UV wavelengths. The extent of the region where circularly polarised light (CPL) of a single handedness originates is very large, and it is likely that the whole of a planetary system would see a single handedness of CPL also. We present the observational data, models of the scattering that leads to the production of CPL, and a model for the origin of homochirality. We also discuss briefly future laboratory and space-based experiments.

The simultaneous detection of 22‐ and 321‐GHz H2O maser emission towards the symbiotic Mira R Aquarii

We report high spatial and spectral resolution measurements of masers towards R Aqr and H1−36, both of which are examples of the sub-class of symbiotic stars that contain a long-period Mira-type variable. Our observations have resulted in the first detection of 321-GHz H2O maser action towards a symbiotic Mira — R Aqr. Comparison with simultaneous 22-GHz H2O maser data suggests that the masers do not have the same properties as those in the circumstellar envelopes of field Miras. R Aqrs 22-/321-GHz peak flux density and luminosity ratios are low, as is the line width ratio. Continuum and spectral-line maps indicate that the 22-GHz maser and free–free emission are aligned. Three mechanisms can reproduce the data with varying degrees of success. All three lead naturally to normal levels of maser emission in SiO and 321-GHz H2O and anomalously weak OH and 22-GHz H2O masers. In the most convincing model, UV radiation and a fast wind from the companion remove the Miras envelope of dusty, molecular gas, leaving a relatively small cavity of dense, neutral material within a large, ionized nebula. Excitation temperatures suggest that 321-GHz masers are normally excited close to the Mira whilst 22-GHz masers are more remote; in R Aqr, therefore, the 22-GHz masers do not form under optimum conditions. Instead, we see weak and narrow lines that form closer to the Mira, consistent with our high-resolution maps.

Testing the `clump' model of SiO maser emission

Building on the detection of the J = 7 6 SiO maser emission in both the v = 1 and v = 2 vibrational states towards the symbiotic Mira, R Aquarii, we have used the James Clerk Maxwell Telescope to study the changes in the SiO maser features from R Aqr over a stellar pulsational period. The observations, complemented by contemporaneous data taken at 86GHz, represent a test of the popular thermal-instability clump models of SiO masers. The ‘clump’ model of SiO maser emission considers the SiO masers to be discrete emitting regions which differ from their surroundings in the values of one or more physical variables (SiO abundance for example). We find that our observational data are consistent with a clump model in which the appearance of maser emission in the J = 7 6 transitions coincides with an outward-moving shock impinging on the inner edge of the maser zone.

Near infrared coronagraph images of IRC +10216 - Faint structures at 1-5 from the central star

We present J, H and K band coronagraph images of the circumstellar envelope around IRC +10216 (CW Leo) obtained with a near infrared camera, CIAO and the 8.2 m Subaru telescope. A circular occulting mask of 2 �� in diameter was used to block out the light from the bright central object. The images show 2 collimated radial structures to the NNW and WNW, 2 fan-like structures to the S and NE, respectively, and 3 arc-like structures at a radius of 4 to 5 arcsec from the stellar center. We compare this intermediate size-scale structure to that seen on larger and smaller scales and find evidence for a deviation from spherically symmetric outflow beginning ∼150 years ago. Previous near infrared speckle imaging has revealed a complex clumpy structure on a scale of less than 200 mas, and it is likely that at least some of the radial features seen in our images could be due to shadowing by dust clumps close to the star.

A Model for the Variability of SiO Masers in Mira Variables

Mass loss from M-Miras makes a major contribution to the injection of heavy elements into the interstellar medium and is a key stage in stellar evolution in which a late-type star embarks on its route towards the formation of a white dwarf. Recent VLBI images of SiO maser emission find that SiO masers lie in an incomplete ring around the host M-Mira (Diamond et al. 1994), stellar radius from the photosphere in the case of TX Cam, i.e. in the zone where mass loss is initiated. SiO masers show complicated and time-varying structure. With the new VLBI data and new hydrodynamic pulsation models and maser models, it is now possible to use SiO maser emission to examine critically our understanding of the mass loss mechanism in late-type stars.