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Research interests

Cepheids

My PhD was a study of southern Cepheid variables with an interest in those in binary or multiple systems with a view to determine their masses. Radial velocities were obtained from high resolution echelle spectroscopy to determine precise radial velocities for many species of lines to measure orbital motion and sample the velocity gradient in Cepheid atmospheres. The precision of the measurements was improved using telluric lines to further calibrate the observed radial velocities, reducing the errors from 1 km/s to 300 m/s. Our data provides a consistent set of data against which other sources of data can be compared.

Amongst the binaries, masses were determined for 2 systems, the 9 day Cepheid S Mus (6.2+/-0.2 M(Sun)) and the 5 day Cepheid V350 Sgr (6.0+/-0.9 M(Sun)). For another 7 Cepheids (Y Car, YZ Car, AX Cir, BP Cir, V636 Sco, W Sgr and T Mon), new or improved orbital solutions were found.

New results presented here include the first orbital solution for AX Cir, a completely revised orbital solution for YZ Car, which established its eccentricity and orbital motion and a new pulsation period (2.39819 days) for BP Cir. The pulsational mass determinations provide further confirmation of the convergence of mass determinations between dynamical and evolutionary methods.

We discussed the line level effects found in our observations, where a number of spectral lines were observed to show departures from the Fe I velocity curve. Line level effects were observed in Hα, Ca II, Ba II, Fe II, Si I, Si II and other species. Most of the Cepheids were observed to show the same progression of line level effects, with the best example being the bright Cepheid β Dor.

The Si II velocities were observed to have the lowest velocity amplitude of the lines observed indicating that they form deeper in the atmosphere than the Fe I lines. The Ca II and Hα lines were all observed to have greater amplitudes of several km/s or more, with the peak of those velocity curves occurring later, as the amplitude increased. These observations are consistent with the lines forming at different depths as a density wave propagates through the atmosphere.

We also present the results of dynamical modelling of a pulsating Cepheid. Using a non-linear radiative hydrodynamic code developed by A. Fokin, we have modelled two Cepheids AX Cir and YZ Car. These models have then been compared with the observations obtained in my thesis.


Proto planetary systems

My MSc was a study of the "CaII absorption in the circumstellar disk of Beta Pictoris and other A-type stars". β Pic was the first star to be discovered with a circumstellar dust disk which was visible at visible wavelengths. We present the results of observations made at Mt John Observatory during spectroscopic campaigns in 1992, 93, 94 and 95, to characterise the behaviour of the Ca II H and K lines and to test the Falling Evaporating Bodies scenario.

Using the method of division of a reference spectrum, both narrow and broad absorption features in both the redshifted and blueshifted sides of the Ca II H and K lines are detected. Lagrange-Henri et al (1996) found that there are 2 velocity regimes and this is confirmed in the Mt John Data. The higher the redshift, the smaller the variability timescales and the smaller the absorption cloud. In contrast the low velocity features tend to be longer lived and to have the deepest absorptions. Significant activity was seen in each of the sets of observations with long-lived absorption features at low velocity almost always present, and it has been found that 1/4 of all features observed are most likely due to more than one FEB.

The effect of stellar rotation is suggested in the data of some of the strong and more variable absorption features. Large numbers of high velocity features are also observed and are seen to vary in timescales no longer than the crossing time for an orbiting body to pass across the stellar disk. This lends further support to the FEB scenario as an explanation for the variable absorption features. The measurement of the filling factors of the clouds of ions indicate that these clouds do indeed cover large fractions of the stellar disk and some of the lines even exhibit pK/pH less than 1, as predicted.

The FEB scenario appears to explain many of the characteristics of the variable absorption features very well, simulations can reproduce many of the absorptions however there are some cases where the FEB scenario fails to adequately explain the observations. The ability for some of the long-lived features to last as long as they are observed to would require either large numbers of bodies on similar orbits crossing the line of sight for many weeks, or that there is some other explanation for the origin of the absorptions.

Contact Details

DDI: +64 3 364-2707
Internal: Extension 6707

Email: orlon.petterson@canterbury.ac.nz

Postal address:
Department of Physics and Astronomy,
College of Science,
University of Canterbury,
Private Bag 4800,
Christchurch 8140
New Zealand

Office:
Room 211a, Physics and Astronomy
Ernest Rutherford Building
Engineering Road
Ilam campus