Mass loss in red giant stars


Postdoctoral astrophysicist Gioia Rau looks at the driving forces of the mass loss process in evolved stars. The winds that carry this mass outward begin in the chromosphere, an outer layer of the atmospheres of red giant and supergiant stars. As an Erwin Schrödinger fellow at NASA’s Goddard Space Flight Center and formerly working at our Faculty, Gioia Rau will now compare spectra, in different colour bands, of a sample of stars observed by three different telescopes in order to unravel the dynamics of the mass loss process.

When the moon obscures the Sun in a total eclipse, the chromospheric layer in the Sun’s atmosphere becomes visible. In the chromosphere the winds of red giants and red supergiant stars begin; from here they drive the mass loss process. In her project “Cool stars winds and chromospheres”, funded by the Austrian Science Fund FWF, Gioia Rau analyses the conditions of the place where evolved stars begin to lose gas and dust, which become over time the seed for new stars.

As Erwin Schrödinger awardee, Gioia Rau wants to find out more about the crucial role of the chromosphere in this process: “Despite recent efforts, details of the mass loss scenario remain mysterious, as well as a detailed understanding of the dynamic regions in the outer atmosphere of these stars.”

The astrophysicist applies her skills in spectroscopy and optical interferometry to analyse the chromospheres of red giants and red supergiant stars in our galaxy. She will compare data from three different telescopes that provide observations from different wavelengths, ranging from ultraviolet (bluer than the eye can see) to infrared radiation (redder than the eye can see).

Towards a theoretical model

At the NASA Goddard Space Flight Center (GSFC) located in Greenbelt, Maryland (USA), the Italian-born scientist will compare data provided by the NASA Hubble Space Telescope (HST), the ESO Very Large Telescope Interferometer (VLTI) in the Chilean Atacama desert, and the brand new James Webb Space Telescope (JWST), which will be launched in 2019: “I will analyse my sample of stars in great detail, and with this multi-wavelength approach applied for the first time simultaneously, I aim to develop a theoretical model within the research group for cool stars at NASA’s GSFC.”

The mass loss in evolved stars is often overlooked, Rau explains: “A fundamental attempt to understand what drives the mass loss mechanism in these stars is needed, given the large number of stars passing through this evolutionary stage.” For example, Rau wants to discover if there is a clear border where the chromosphere ends and the wind begins. Or: what are the velocities of the winds in these stars? Is there a direct relationship between the chromospheric activity and the presence of dust in giant and supergiant stars?

Her two years in the research group of Kenneth Carpenter at NASA/GSFC and the following twelve months return phase at the Department of Astrophysics at the University of Vienna will leverage the joint and complementary expertise in UV spectroscopy, interferometry, and theoretical modeling in Austria and the USA. In the course of her project, Gioia Rau will also collaborate with various experts such as astronomer Markus Wittkowski of the European Southern Observatory (ESO) in Germany and Theo ten Brummelaar, director of the Center for High Angular Resolution Astronomy (CHARA) in Los Angeles.

the Sun's chromosphere
Looking at the Sun's chromosphere - a thin layer of solar atmosphere sandwiched between the visible surface (i.e. photosphere) and its atmosphere, the corona. Hinode, a collaborative mission of the space agencies of Japan, the United States, United Kingdom and Europe, captured these very dynamic pictures of our Sun's chromosphere on Jan. 12, 2007. (Copyright: JAXA/NASA)
Scientist Gioia Rau in front of NASA logo
Erwin Schrödinger fellow Gioia Rau (Copyright: private)