Stellar Astrophysics at Canterbury
(Professor John Hearnshaw, Drs Peter Cottrell & William Tobin)
The research which we will undertake as part of our Marsden Fund grant encompasses a broad range of fields in the general area of the astrophysics of stars and involves collaborators from a variety of institutions throughout the world. It seeks to determine some fundamental properties of different types of star through intensive observational programmes using the excellent astronomical facilities that have been developed at Mount John University Observatory and in the Department over the last 20 years. The stars to be investigated include those which are intrinsically changing in brightness or orbiting about a common centre of mass for which we will use instruments to detect these changes in brightness (using photometric techniques) and motion (using spectroscopic techniques).
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At Canterbury we have undertaken a managed development of the telescopes and instruments, which has seen the building and/or acquisition of both a new telescope (the 1m McLellan telescope which was designed and built by the technical staff at Canterbury), two major instruments and 3 digital detectors over the last 10 years. This development also highlights the excellent technical expertise which has been and continues to be available within the Department of Physics and Astronomy. We have also made our facilities available to other researchers within New Zealand and also to a number of astronomical visitors from overseas.
There are four significant aspects which enable our research to target specific types of projects. These are: reasonable access to observing facilities through ownership and close management,
undertaking specific longer-term research projects,
making use of our unique longitude and
southern latitude for solving specific astrophysical problems.
The long-term projects on southern objects, for example the Magellanic Clouds which are above our horizon for the whole year, enable astrophysical problems dependent on time-scales from a few days to several years to be addressed. This is the envy of many researchers worldwide, who have to rely on national facilities on which they can obtain only irregular and brief amounts of telescope time.
Overall Aim of the Research
Our programme will be divided into two major research areas:
- determination of fundamental parameters and
- dynamical processes in astrophysics.
These were in turn divided into a number of smaller projects which we will undertake.
Determination of fundamental parameters
Eclipsing binaries and Cepheid variable stars are two types of star from which significant information about the masses, sizes, surface temperatures and intrinsic brightness of stars can be determined. These parameters are required for a more complete understanding of the structure of stars and ultimately the universe. We will make use of Mount John facilities and various collaborations (both space- and ground-based) that we have developed in the international community.
Dynamical processes in astrophysics
Every object in the universe is in motion and can be studied as part of a dynamic system, whether as an individual star intrinsically varying in brightness or as a member of some larger group of stars. The timescales of these processes vary from seconds to (billions of) years as a consequence of their evolution. The projects in this area are the motions of stars within the gravitational potential of our Galaxy, variability of groups of semi-regular intrinsically bright stars which are near the end of their evolution, and the study of the early stages of the evolution of planetary systems. Studies of the (long-term) variability of stars provides a unique symbiosis between the facilities which we have available and some outstanding problems in the stability of the periods of a number of different types of variable star.
Personnel
As well as the 3 academic staff on the project, there will be 2 fixed-term (3 year) research positions and research support for a number of graduate students. The fixed-term positions include Dr Michael Albrow, who is an Associate Investigator on the Cepheid project, and a yet to be advertised postdoctoral position for the eclipsing binary project. In addition, Dr Karen Pollard will be joining the Astronomy Research Group as the Department will be the host institution for her New Zealand Foundation for Research, Science & Technology Postdoctoral Fellowship. The following is slightly more detailed information for those who feel the need to read on.
There are five projects which comprise the Marsden Fund programme. Below we outline these and include some of the major objectives of each of them. In the interest of keeping this description in a reasonably compact form the detailed methodology and data analysis sections have not been included.
Project #1: Cepheid variable stars
PI: Cottrell & AI: M.D. Albrow[1]
Hypotheses:
That there are significant systematic errors in the determination of distances to and radii of Cepheids by the Baade-Wesselink method stemming from the misinterpretation of radial velocities as representing a simple projection of a star's pulsational motion.
That there are few independent direct observational tests of the theory of stellar evolution.
Objectives:
To obtain radial velocity and line profile information through the acquisition of spectroscopic observations at Mount John University Observatory and elsewhere.
To construct appropriate dynamic stellar envelope and stellar model atmospheres to interpret the radial velocity and line profile observations.
To derive orbits and masses of Cepheids in binary systems as an independent observational test of the theory of stellar evolution.
This project involves collaborations with the Sydney University Stellar Interferometer group lead by Professor John Davis, but with specific collaborations with Dr Andrew Booth and graduate student Melinda Taylor, with Dr Nancy Evans (AXAF project, Harvard University, Boston) and Professor Erica Böhm-Vitense through their access to Hubble Space Telescope observations of the binary Cepheids and through Canterbury PhD student Orlon Petterson.
Project #2: Eclipsing binaries in the Magellanic Clouds
PI: Tobin & AIs: E.F. Guinan[2], J-V. Clausen[3]
Hypotheses:
That the correctness or otherwise of models of stellar structure and evolution with respect to the metallicity parameter can be tested by comparing real stars in the metal-poor Magellanic Clouds with theoretical ones.
That there are few independent direct observational tests of fundamental stellar parameters. *That extragalactic eclipsing binaries can be used as distance indicators.
Objectives:
To work on the one aspect of photometric technique, flat fielding, that can be improved further.
To discover and catalogue additional eclipsing binaries in the Clouds.
To obtain extremely precise and accurate optical light curves of selected eclipsing binaries in the Magellanic Clouds (MC) from Mt John.
To combine these light curves with data obtained by or in collaboration with the associate investigators (UV data: Guinan, with a massive 30 orbits of the Hubble Space Telescope & radial-velocity curves: Clausen, with spectrographs on European Southern Observatory telescopes). From the combined data, to determine fundamental parameters for comparison with stellar structure models at low metallicity (LMC: 40 per cent of solar, SMC: 20 per cent). (The dependence on metallicity is at present untested and is important e.g. for modelling evolution of external galaxies, which vary greatly in metallicity.)
To obtain distance moduli to the MC and, possibly, to probe the controversial depth structure of the SMC.
To probe the interior mass distribution for systems with apsidal motion (e.g. HV982, HV2274).
As well as the collaborators mentioned above, current graduate student John Pritchard and the new post-doctoral position made possible by the Marsden Fund grant make this an excellent nucleus for astronomical research.
Project #3: Long-term stellar variability
PI: Cottrell
Hypothesis:
That there are evolutionary connections between groups of intrinsically luminous evolved stars.
Objectives:
To investigate the evolutionary links between R Coronae Borealis (RCB) stars and RV Tauri stars through photometric, spectroscopic and polarimetric observations.
To monitor and interpret the rapid, large-scale mass loss events in the R Coronae Borealis stars.
This project involves collaborators Drs Warrick Lawson (Australian Defence Force Academy, Canberra, Australia) and Karen Pollard (South African Astronomical Observatory, Cape Town), which have been developed over more than a decade, and the continuing graduate programme at Canterbury, currently through PhD student Ljiljana Skuljan.
Project #4: Moving groups of stars in the Galaxy
PIs: Cottrell, Hearnshaw
Hypothesis:
O.J. Eggen has identified about 750 stars which are putative members of moving groups of stars in the Galaxy, being stars of very similar space motions and presumed to be dissolved star clusters which are now field stars. His hypothesis is that stars in a moving group have a common origin, chemical composition, age and galactic orbits, but are in different stages in their evolution (as a result of different masses).
Objectives:
We propose to demonstrate the reality or otherwise of the moving group hypothesis by determining precise space motions for most of these stars.
If verified, then the concept would provide a unique opportunity of studying stars in different stages of their evolution, and to compare their different spectra given that they have a common initial composition.
This project will be advanced significantly through Canterbury PhD student Jovan Skuljan and the Principal Investigators priority access to the HIPPARCOS satellite (operated by the European Space Agency) data due to be released in early 1997.
Project #5: Beta Pictoris: test of the FEB hypothesis
PI: Tobin & AI: A-M. Lagrange[4]
Hypotheses:
That the unique, deep-southern star beta Pictoris (A5V, V=3.85, d~18pc, declination = -51°) is a system of crucial importance for understanding planet formation.
That the strongly-variable absorption features seen in the spectrum of the star in the ultraviolet (IUE, HST & Mg II, Fe II, Al III) and optical (Ca II H & K) arise from in Falling, Evaporating, comet-like Bodies (the FEB hypothesis) from a surrounding annulus of protoplanetary debris.
That the FEB hypothesis can explain many but not all of the properties of these absorptions.
That the occurrence of Ca II absorptions has increased greatly since b Pictoris first attracted attention in the mid 1980s, and that further observations will yield important new data to characterise the absorption phenomenon more fully and provide a sounder basis for testing and refining the FEB scenario.
That truly simultaneous measurements of the Ca II H and K lines will yield accurate filling factors to confirm (or refute) the finding from a campaign of alternating observations in 1992 that the H line (which has the weaker oscillator strength) can exhibit stronger absorptions than the K line. (This can be explained by the FEB hypothesis as resulting from intensity changes of the underlying photospheric Ca II absorption caused by Doppler shifting produced by the axial rotation of the star.)
Objectives:
To use the recently-acquired large CCD, échelle spectrograph and focal reducer at MJUO to obtain truly simultaneous high-resolution observations of the variable absorptions of the Ca II H & K lines for 2 years.
To use these data to permit refinement and testing of the FEB hypothesis, and hence probably shed additional light on (a) the FEBs themselves and their presumed origin in the outer annulus, and possibly (b) the dynamical interaction of the annulus with planets, which may explain the redshift/blueshift asymmetry e.g. through secular resonances causing streams of FEBs from a particular direction.
[1] Currently a New Zealand Foundation for Research, Science & Technology Postdoctoral Fellow hosted by the Department of Physics & Astronomy at Canterbury, but based at the South African Astronomical Observatory, Cape Town
[2] Villanova University, Department of Astronomy & Astrophysics, United States of America
[3] Copenhagen University Observatory, Denmark
[4] Groupe d'Astrophysique de Grenoble, Laboratoire d'Astrophysique de l'Observatoire de Grenoble, France
Rhondda Sullivan - rhondda.sullivan@canterbury.ac.nz Or...
Rosalie Reilly - rosalie.reilly@canterbury.ac.nz