Prof Patrick Roche

Abstract:

We present 8-13 μm spectropolarimetry, and 12- and 2-μm imaging polarimetry of the southern massive star-forming region G333.6-0.2. Spectropolarimetry measurements show that the polarization observed towards the nebula contains a mixture of both absorptive and emissive polarizations. Model fitting to the spectra indicates that the temperature of the mid-infrared emitting dust grains is generally ∼200 K and the optical depth of the absorbing dust at 9.7 μm is ∼ 1.5. Fits are also made to the polarimetry spectra, which show a reasonably constant peak absorptive polarization (∼3.4 per cent at 43°) across the face of the H II region. This absorptive polarization position angle is consistent with that found by the 2-μm imaging polarimetry (38° ± 6°) and is most likely due to the Galactic magnetic field local to G333.6-0.2. When the absorptive polarization is subtracted from the 12-μm polarization image, the emissive polarization pattern that is intrinsic to the star-forming region is revealed. A probable magnetic field configuration implied by the intrinsic polarization suggests star formation initially influenced by the Galactic magnetic field which is eventually perturbed by the star formation process.

Abstract:

We address the question of whether the distribution of warm-dust compositions in IR-bright Galactic disc PNe (Paper I, Casassus et al.) can be linked to the underlying stellar population. The PNe with warm dust emission represent a homogeneous population, which is presumably young and minimally affected by a possible dependence of PN lifetime on progenitor mass. The sample in Paper I thus allows testing of the predictions of single-star evolution, through a comparison with synthetic distributions and under the assumption that tip-of-the-AGB and PN statistics are similar. We construct a schematic model for AGB evolution (adapted from Groenewegen & de Jong), the free parameters of which are calibrated with the luminosity function (LF) of C stars in the LMC, the initial-final mass relation and the range of PN compositions. The observed metallicity gradient and distribution of star-forming regions with Galactocentric radius (Bronfman et al.) allow us to synthesize the Galactic disc PN progenitor population. We find that the fraction of O-rich PNe, f(O), is a tight constraint on AGB parameters. For our best model, a minimum PN progenitor mass Mmin = 1 M⊙ predicts that about 50 per cent of all young PNe should be O-rich compared with an observed fraction of 22 per cent; thus Mmin = 1.2 M⊙, at a 2σ confidence level (Mmin = 1.3 M⊙ at 1σ). By contrast, current AGB models for single stars can account neither for the continuous range of N enrichment (Leisy & Dennefeld) nor for the observation that the majority of very C-rich PNe have Peimbert type I (Paper I). f(O) is thus an observable quantity much easier to model. The decrease in f(O) with Galactocentric radius as reported in Paper I, is a strong property of the synthetic distribution, independent of Mmin. This trend reflects the sensitivity of the surface temperature of AGB stars and of the core mass at the first thermal pulse to the Galactic metallicity gradient.

Abstract:

We investigate the Galactic disc distribution of a sample of planetary nebulae characterized in terms of their mid-infrared spectral features. The total number of Galactic disc PNe with 8-13 μm spectra is brought up to 74 with the inclusion of 24 new objects, the spectra of which we present for the first time. 54 PNe have clearly identified warm dust emission features, and form a sample that we use to construct the distribution of the C/O chemical balance in Galactic disc PNe. The dust emission features complement the information on the progenitor masses brought by the gas-phase N/O ratios: PNe with unidentified infrared emission bands have the highest N/O ratios, while PNe with the silicate signature have either very high N enrichment or close to none. We find a trend for a decreasing proportion of O-rich PNe towards the third and fourth Galactic quadrants. Two independent distance scales confirm that the proportion of O-rich PNe decreases from 30 ± 9 per cent inside the solar circle to 14 ± 7 per cent outside. PNe with warm dust are also the youngest. PNe with no warm dust are uniformly distributed in C/O and N/O ratios, and do not appear to be confined to C/O ∼ 1. They also have higher 6-cm fluxes, as expected from more evolved PNe. We show that the IRAS fluxes are a good representation of the bolometric flux for compact and IR-bright PNe, which are probably optically thick. Selection of objects with F(12 μm) > 0.5 Jy should probe a good portion of the Galactic disc for these young, dense and compact nebulae, and the dominant selection effects are rooted in the PN catalogues.

Abstract:

By the end of 2001, the two Gemini telescopes will be in full operation, with the UK community having access to about 70 nights per year on each. With the WHT, UKIRT and the AAT, the community has access to over 600 nights per year of 4 m time. It is clear that most programmes will continue to he conducted on our existing telescopes, with Gemini being used only where the programmes cannot be completed on the 4 m instruments. Gemini has the advantages of large collecting area and active optics to ensure that observations are site-limited. However it also has a restricted field of view (<10 arcmin) and, initially will have a restricted instrument suite. The excellent sites and ability to apply aluminium or silver coatings to the telescope mirrors will ensure high throughput. Gemini is well-suited for deep observation over small fields. with the best results obtained over fields where tip-tilt or higher degrees of correction are effective (3 arcmin). The largest gains arise where the full gain of the increased area is realised, in near-diffraction-limited observations, e.g at thermal infrared wavelengths and with adaptive correction in the near-infrared, and in photon-starved applications such as high spectral resolution, faint object spectroscopy. Many programmes carried out with Gemini will rely on supporting observations from 4 m telescopes for target selection and complementary observations such as sample completion, simultaneous measurements in different wavebands etc. In the next decade, the UK will have access of right to almost one half of an 8 m telescope in a world where about a dozen 8-10 m class telescopes will be in operation. We enjoy a much greater fraction of the total 4 m telescopes and it will be imperative that they are used effectively with Gemini in order to ensure that our highest priority scientific programmes are completed efficiently. (C) 2001 Elsevier Science B.V. All rights reserved.