Dust formation by supernovae and asymptotic giant branch stars

The main visible components of galaxies are stars and the interstellar medium (ISM). The ISM is filled with gas and dust, with dust taking only less than 1% of the total mass. Despite dust occupying such a small fraction of mass, its unique properties make it important for astronomy: dust can absorb and scatter star-light in ultra-violet and optical wavelengths and re-emit the absorbed energy in infrared and submillimetre wavelengths. This dust turns harmful UV photons to soft infrared and submillimetre light, thus protecting molecules, and providing a cradle for star formation. Empirically, it is known that galaxies that undergo extensive star-formation tend to be dusty, and emit more energy at infrared and submillimetre wavelengths. Yet, it is still unknown why these galaxies are so dusty in the first place.

To form dust, atoms such as carbon, silicon, oxygen and iron, need to meet with other atoms. Consecutive meeting and bonding of atoms makes dust grow, ending up with a dust grain composed of hundreds and thousands of refractory elements. The reaction is efficient where gas is reasonably hot and dense, and where many refractory elements are available. The surroundings of asymptotic giant branch (AGB) stars, which are in the late stage of stellar evolution for 1-8 solar mass stars, provides an ideal place to make dust, as there is dense gas with some newly synthesised elements by the stars. Older textbooks usually describe the dust content of the ISM originating from AGB stars.

A decade of advances in observational astronomy has challenged this traditional description of dust origin. Many high-redshift galaxies, located deep in space and observed less than one billion years old after the big bang, have been found to be much dustier than expected if dust is produced by AGB stars only. Hence, there must be some other objects to make a lot of dust within these galaxies.

Theories have proposed that supernovae (SNe) can be an additional and significant dust source that makes galaxies dusty. SNe are the enormous explosion of massive stars at the end their lives, hence ejecting elements synthesised in stars into space. An explosion of SN 1987A was detected in the neighbouring galaxy, the Large Magellanic Cloud. The explosion was initially recorded in optical light, and neutrinos were also detected. Twenty-five years after I and colleagues found that the remnant of SN 1987A is filled with cold dust and molecules. Our observations with the Herschel Space Observatory and the Atacama Millemetre/submillemtre Array (ALMA) began to reveal that the supernova remnant has became cool, but is still dense enough for dust to form. The estimated mass of dust is about 0.5 solar mass, which is enough dust mass to make young galaxies dusty.

A deficit of dust input has been suggested not only in high-redshift galaxies, but also in neighbouring galaxies. Of course, one possibility is that SNe are significant sources of dust in these galaxies as well. But an alternative view is that galaxies have acquired a large mass of dust in the past, but that they are now in a quiescent phase of dust formation, as there are not so many dust-forming stars at present. Combining new techniques introduced by the Spitzer and GAIA missions we can detangle these issues.

My project aims to quantify the dust mass formed in supernovae and seeks to determine the physical and chemical process that lead to the formation in stars. In parallel, the project aims to elucidate what galaxy properties can have an impact on dust formation from stars, or vice-versa. The project will lead the way so we can build a full picture of the origin and evolution of dust in the Milky Way, nearby galaxies and high-redshift galaxies. The project will design the observing programs for future space missions JWST and SPICA. The outcome of this project has a potential to impact astrophysical processes involving dust, such as star-formation, stellar evolution and galaxy evolution.