FM 16: Stellar Behemoths - Red Supergiants across the local Universe

Start Date: 
Monday, August 3, 2015
End Date: 
Wednesday, August 5, 2015
Co-Chairs of SOC: 
  • Rolf-Peter Kudritzki (University of Hawaii)

Astrophysical context

Red Supergiants (RSGs) are an evolved state of stars with masses greater than 8Msun. They are the largest stars known, with radii up to 1000x larger than the Sun. The majority of single massive stars will spend their entire post main-sequence life in the RSG phase, exploding as hydrogen-rich supernovae (SNe) around 1Myr after leaving the main-sequence. The mass-loss rates of these stars rival those of any other class of massive star (up to 1e-4 Msun/yr, e.g. Decin et al. 2006), with the mass ejected in the RSG phase dwarfing that lost during the entire main sequence.

Their extremely high luminosities peak in the near-infrared, allowing them to be detected at distances of over 10Mpc and through many magnitudes of optical extinction. This makes them powerful tools with which to study their host galaxies' structure, chemical evolution and star-formation rate. They are also bright enough to have been identified as direct progenitors of distant supernovae, with current sample sizes now large enough such that statistical conclusions about the fate of massive stars can be drawn (Smartt et al. 2009).

Despite their significance in the massive star life-cycle, they are arguably the least well-understood class of massive stars, with even basic properties such as their temperatures still being hotly debated. This is largely due to historical reasons: massive star research has tended to focus on hot stars, such as O stars and Wolf-Rayets. However, the analysis and modelling of RSGs requires very different expertise, such as the treatment of convection and molecular opacities. These tools have been developed primarily by the low-mass star community for studying objects such as Solar-type stars and red giants. It is only very recently, when the importance and diagnostic potential of RSGs has been widely recognised, that the two communities have begun to work together, arguably only really beginning with the meeting “Hot and Cool: Bridging Gaps in Massive Stellar Evolution”.

Recent progress and prospects

The next 12 months sees the commissioning of brand-new instrumentation and facilities which will provide new windows into the nature of RSGs. Multi-object spectrographs such as MOSFIRE and KMOS will for the first time permit quantitative spectroscopy of large samples of RSGs in external galaxies. Meanwhile, ALMA will provide the first detailed observations of RSG gas-phase mass-loss beyond the Solar neighbourhood by measuring molecular emission lines in their winds. Infrared long-baseline interferometry of RSGs is already a well-developed field of its own, enabling us to not only detect inhomogeneous structures in the atmosphere and surface of RSGs but also spatially resolve the gas dynamics in the atmosphere - just as in Solar physics (e.g. Ohnaka et al. 2013).

In parallel, considerable effort is being put into developing models of the dynamic atmospheres of these stars. Hydrodynamical simulations of convection in RSGs now allow us to study 3-D effects on their emergent spectra (Chiavassa et al. 2011), while corrections to the strengths of spectral lines due to non-LTE effects are beginning to be included, vital for the use of RSGs as abundance tracers (Bergemann et al. 2013).

Moving forward, RSGs will be the brightest (and hence most distant) stars detected by GAIA, and will therefore provide the deepest tracers of the Milky Way’s structure and chemical evolution. A detailed understanding of these stars is essential to maximize the potential of this revolutionary mission.

The Proposed Meeting

At the 2015 IAU-GA we aim to bring together the leading researchers in the fields of massive stars, cool stars, and supernovae, to discuss the latest observational and theoretical advances, and to encourage and stimulate further cross-disciplinary collaboration. The fruition of the various technical and theoretical projects described here, the impending launch of GAIA, and the 7 years since the last similar meeting (“Hot and Cool” in Pasadena, 2008) make such a meeting timely.

We have identified 6 key themes covering the physics of these stars, their evolutionary connections to other massive stars and supernovae, and their use as probes of other areas of astrophysics. Each of these themes will have a session assigned to them, discussed in greater detail in the next Section. Attributing roughly 0.5 days to each session, we propose a focus meeting lasting 3 days.