The main thrust of my research concerns evolved stars and their mass loss. Mass loss is of particular interest as it is the process by which the interstellar medium is enriched with material that has been processed in nuclear reactions during the star’s life to produce new elements. For low- and intermediate-mass stars like our Sun, this is dominated by the AGB phase, while for massive stars, which will eventually explode as supernovae, the Red Supergiant phase is most critical. I study both of these phases using a combination of optical, infrared and sub-mm observations, which I compare to physical models of their emission.
Dust in evolved stars
In both AGB stars and red supergiants, the outflow is driven by radiation pressure on dust, the efficiency of which is determined by the optical properties of the dust grains. As a result, the size and composition of the dust is a key factor in mass loss.
Grain sizes can be measured directly and efficiently by observing light that has scattered off the grains and become polarised in the process. By observing several wavelengths we can directly compare to models of the expected polarisation to understand what ranges of grain sizes would produce it. Using this approach, I measured the size of the grains in the outflow of the extreme red supergiant VY CMa using the instrument SPHERE (designed to detect extrasolar planets) at ESO’s Very Large Telescope. These observations showed that the grains are in the right range of sizes for the outflow to be driven by radiation pressure that comes from the scattering of photons, rather than their absorption; this mechanism had been suggested to fix issues in our understanding of wind driving in oxygen-rich AGB stars, which are lower-mass cousins of red supergiants.
I then followed up on this work on VY CMa by studying the somewhat similar source, VX Sgr. VX Sgr is another evolved massive star, although there is some debate about whether it is a red supergiant or a super-AGB star. We used SPHERE to observe the inner outflow of VX Sgr at high angular resolution and high contrast. This revealed an extended, elongated envelope that is strongly polarised, which is consistent with a nearly-face on disk. Such a geometry had been suggested previously from radio observations of masers. We also find indications of grain sizes similar to those in VY CMa, although we were not able to perform the same detailed measurement of their sizes.
The Nearby Evolved Stars Survey
As evolved stars eject their envelopes, they also eject the new elements that have been formed by nuclear fusion during their lifetimes. This drives the chemical evolution of galaxies, determining the composition of future generations of stars and planets, but also plays a critical role in the future evolution of the stars themselves. For more massive stars, the amount of mass they lose determines whether they will end their lives as supernovae, and if so what kind. In low- and intermediate-mass stars, on the other hand, the rate of mass loss determines how long they spend as AGB stars, which has consequences for how much they become enriched with nucleosynthesis products, how massive the white dwarfs they leave behind are, and how much enriched material they return to interstellar space. On top of this, evolved stars are a major source of interstellar dust; although dust makes up a mere 1% of the baryonic mass of the interstellar medium, it is crucial for a number of physical and chemical processes, as it dominates the opacity budget, converting optical and ultraviolet photons from stars into infrared radiation, and provides surfaces which can catalyse key chemical reactions.
To address these issues, I am leading the Nearby Evolved Stars Survey, which aims to perform a complete census of the gas and dust mass-loss from evolved stars within the immediate vicinity of the Sun. We are observing a sample of 850 evolved stars in the sub-millimetre wavelength range; this range contains a number of carbon monoxide emission lines which are reliable tracers of the outflowing gas, while the continuum emission is sensitive to the coldest dust in the outer outflows, which is the oldest dust. NESS began with observations at the James Clerk Maxwell Telescope on Maunakea in Hawaii. We have subsequently had further observations approved on the Atacama Pathfinder Experiment (which provides similar capabilities to the JCMT but in the southern hemisphere), the Nobeyama 45-metre radio telescope, IRAM 30-metre telescope, and Atacama Large Millimetre/submillimetre Array (ALMA).
With these observations we will measure the total amount of gas and dust that is being returned to the interstellar medium by the stars in our own galaxy. This will allow us to improve predictions for stars in other galaxies to understand how their chemical composition evolves over cosmic time. We are also shedding light on the properties of dust in the sub-mm, and measuring important properties like the dust-to-gas ratio and the abundances of different carbon isotopes.
Post-AGB stars and their envelopes
When sun-like stars start to evolve off the asymptotic giant branch, their surfaces begin to heat up and generate more ionising UV photons. When they enter this phase, they are referred to as post-AGB stars.
A significant population of post-AGB stars appear to be surrounded by dense, dusty discs. These are similar to the protoplanetary discs that surround young stars, except that this is the end, rather than the start, of the star’s life. We think that these discs form when a companion star gets close enough to interact with the AGB star, and the transfer of angular momentum from the companion star to the outer layers of the AGB star ejects these outer layers, while the gravitational pull of the companion draws the ejected material into a disc in the same plane that the companion orbits in.
These discs allow material to sit at high densities for much longer that normally occurs in AGB outflows. As a result, many of the physical processes that occur in protoplanetary discs also occur in these post-AGB discs. This includes grain growth to ~millimetre sizes, which we showed was a ubiquitous process in post-AGB discs with a collection of archival data from Herschel as well as new observations from the Submillimeter Array. The dust grains in these discs are much more similar to those in protoplanetary discs than interstellar dust. However, while protoplanetary discs may survive for millions of years, these post-AGB discs may survive for only a few thousand years, or in the longest-live cases maybe as long as one hundred thousand years. This places strong constraints on the timescale for grains to grow from sub-micron sizes to millimetre sizes, with potentially important consequences for our understanding of planet formation.
PNe and NEAT
Peter Scicluna 