Explosive New Thinking on Stellar Evolution

By Becca SanchezNovember 3, 2016

Until a few years ago, the accepted leading theory of stellar explosions stated that until a star explodes, its lifetime path to explosion is relatively uneventful. But the events behind Supernova 2009ip shook that theory when the star suffered a number of eruptions in 2010 and 2011 — proving it survived the eruption episodes of 2009 — and then experienced an unusual double brightening in 2012. These events led observer Raffaella Margutti, physics and astronomy,  to reconsider the final stages of evolution in massive stars.

Margutti, a faculty member within the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern, studies and models transient astrophysical phenomena including stellar explosions and stellar tidal disruptions. By chance, a team of astronomers that included Margutti was observing what was thought to be a normal supernova explosion when SN 2009ip exploded again.

“This changes all our understanding of stellar evolution,” explains Margutti. The new information produced by SN 2009ip led her to research stellar explosions in terms of understanding how stars explode, why they explode at that particular time of their history, and which kinds of stars produce which kinds of explosions.

Margutti’s research is unique in that her group works with “broad-band observations” from all over the spectrum, from x-rays to gamma rays, “all the way down to radio” — an inclusive approach that researchers have not been using, as they typically specialize in one key field, like radio or optical astronomy.

The supernova community is still segregated into these wavelength domains and “up until now did not really speak to one another” or even use the same units, rendering a cohesive understanding more difficult to achieve. Margutti is bringing a wide scope of understanding to the supernova field.

To do this, she uses a variety of observatories to collect photonic data across the spectrum. They include an entire suite of NASA satellites and ASASSN (All-Sky Automated Survey for Supernovae) telescopes, as well as an orchestra of optical telescopes for spectroscopy and radio antennae. Considering that physical phenomena contribute to different parts of the spectrum, this range of data allows Margutti to better constrain the properties of the explosions, such as their environments and their progenitor stars. Ultimately, this variety of information can provide a more holistic understanding of the relationship between mass and environment to understand stellar evolution.

The next step to include in the investigation is non-photonic mass messengers like neutrinos and gravitational waves with the collaboration of CIERA Director Vicky Kalogera. Kalogera’s research concentrates on information that can be extracted from gravitational wave emissions produced from events that are tightly connected with stellar explosions and mergers of compact objects, like neutron stars and black holes. Specifically, electromagnetic signals expected to accompany these emissions can potentially unveil the precise location of the event and thus a number of important properties, like astrophysical environment. Margutti notes that this collaboration puts Northwestern in a unique position to maximize the scientific reward from the “new era of gravitational wave astronomy.”

There are two theories Margutti proposes. One is the existence of processes in the interior of the star that create instabilities. The second is that these massive stars that explode are not alone but have companions whose presence creates instability.

“The bottom line is our understanding of stellar evolution is not exactly right. These are very different predictions in terms of how this behavior should show up so I am trying to test that … if one of the two theories is right.”

Raffaella MarguttiVicky Kalogera