One Last Hurrah: End-of-life Mass Loss in the Large Magellanic Cloud Red Supergiants

Title: Establishing a mass-loss rate relation for red supergiants in the Large Magellanic Cloud

Authors: K. Antoniadis, A.Z. Bonanos, S. de Wit, E. Zapartas, G. Munoz-Sanchez, G. Maravelias

First Author’s Institution: National Observatory of Athens, Greece

Status: Accepted to Astronomy and Astrophysics [open access]

Stars don’t tend to die quietly. In particular, evolved massive stars, known as red supergiants (RSGs), often throw tantrums in their final years, spewing out vast amounts of stellar material into space. These bouts of bad behaviour aren’t just for show, they also have a drastic effect on the evolution and fate of massive stars. However, the precise mechanisms at play behind these stellar fits of rage continue to perplex astronomers. Therefore, the authors of today’s paper set about to better understand these moody stellar giants on the verge of death, using a sample of stars from our galactic neighbour, the Large Magellanic Cloud (LMC). 

Figure 1. SEDs for 2 RSGs in the LMC sample. The top plot shows the SED of an optically thin star (i.e. that is not shrouded by dust), while the bottom image shows a dusty RSG. The orange diamonds are the data points from the different surveys, and the solid black lines give the best-fit model for each data set. The remaining lines (dotted and dashed) are the individual components that contribute to the best-fit model. The physical parameters derived from the best-fit model are listed in the top-right corner, while each star’s coordinates are given in the top-left corner.  Adapted from Figure 5 in the paper.

Drawing their candidates from previous catalogues of RSGs in the LMC, the authors were able to assemble a sample consisting of 2219 stars. The stars in the sample had been observed as part of previous surveys across a variety of wavelengths from the ultraviolet to the mid-infrared. This meant the authors had sufficient data to construct a spectral energy distribution (SED) for each star, which describes how the energy emitted by a star varies with wavelength. The SEDs for a subset of the stars in the RSG sample are shown in Figure 1. 

Data in hand, the authors were able to begin their analysis. To understand how RSGs lose mass as their end approaches, they used the 1-dimensional radiative transfer code DUSTY, which models how light propagates through some medium, in this case the dust and material surrounding the LMC RSGs. 

Their analysis required several assumptions, including a spherically symmetric dust shell that extends out to 104 times the inner radius of the shell, steady-state (unchanging) winds and a density distribution that decays as r-2. Using the SED data, they were able to fit a DUSTY model and from the results calculate the mass-loss rate for each RSG. This indicates how quickly each star spews out material in its terminal years. 

Of course, the authors were interested in understanding how the mass-loss rate varied within the population. Therefore, in Figure 2, the authors show a plot of the mass-loss rate versus luminosity (total energy radiated by a star per second) for the RSGs in their sample. Such plots are commonly created by astronomers and are known as mass-loss rate relations. 

Looking at the plot in Figure 2, it is clear that the lower luminosity RSGs behave somewhat differently to the higher luminosity stars. This is shown by the kink in the relation that occurs at roughly log(L/L) ~ 4.4. At higher luminosities, the relation becomes steeper, implying enhanced mass-loss rates. 

Figure 2. The mass-loss rate relation (mass-loss rate vs luminosity) for the RSGs in the LMC sample. The colour bar gives the optical depth, which is related to how much material surrounds the star, with greater values of log(t_v) indicating dustier RSGs.  The grey triangles indicate upper limits and open circles are stars that have a known spectral type. The dashed line indicates the location of the kink in the relation. The red star is a very dusty RSG, WOH G64. Figure 10 in the paper.

Previous studies had indicated a similar ‘knee’ feature associated with stellar variability. Therefore, the authors examined the variability of the RSGs in the LMC sample. What they found was that for values of log(L/L) greater than 4.4, the RSGs exhibited enhanced variability. As such, they identified variability as a likely culprit behind the enhanced mass-loss rates seen at higher luminosities. 

Finally, the authors compared their derived mass-loss rate to others from the literature. Of note was the Small Magellanic Cloud (SMC), the LMC’s less massive neighbour, which also exhibited a kink in its mass-loss rate relation. However, the SMC turning point occurs at a higher value of log(L/L) ~ 4.6, compared to the LMC value of ~ 4.4. 

One key difference between the SMC and the LMC is metallicity, or what fraction of material in a galaxy is not hydrogen or helium. The SMC is a lower metallicity environment than the LMC, leading the authors to speculate that the compositional difference between the two galactic neighbours gave rise to the distinct mass-loss rate relation turning points.  

Clearly, the RSGs in the LMC needed one last hurrah to let loose before the end. While several factors appear to affect how end-of-life mass loss occurs, metallicity and variability seem to be key players. Performing a similar study of the mass-loss rate relation in other galaxies including Andromeda (M31) and Triangulum (M33) will likely enlighten things, but that’s for another paper, another day, another bite.

Astrobite edited by Jessie Thwaites

Featured image credit: ESA/NASA/JPL-Caltech/STScI via Wikimedia Commons

About Sonja Panjkov

I'm a second-year PhD student at the University of Melbourne. My research focuses on the high-energy emission from the supernova remnants in the Magellanic Clouds. In my spare time, I enjoy hanging out with my cats and going to see live music.

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