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).
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.
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
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