Revealing the Secrets of Mimosa - A Southern Cross Star

Each night we look up into the southern sky and gaze with tender affection at the constellation we often associate with part of our modern national identity - The Southern Cross. Historically, many Aboriginal and Torres Strait Island peoples have been gazing upon its stars for the last 65,000 years - having played an important role in science and culture over that period. It is by far the most recognisable constellation in the Southern Hemisphere, adorning the flags of Australia, Brazil, New Zealand, Papua New Guinea and Samoa.  

Now, groundbreaking new research has studied the interior of the largest star in the constellation Beta Crucis - the star that lives on the left-hand side of the Cross’ horizontal arm - and has determined its age and mass. The study found that the blue gas giant is more than 14 times as massive as our sun, and only 11-million years old - comparatively young for a star of that size and type.

So how did they do it? The international team, led by Dr Daniel Cotton from The Australian National University and Monterey Institute for Research in Astronomy in the USA, combined asteroseismology and polarimetry to confirm a 40-year old prediction that polarisations in a star’s light-induced by pulses beneath its surface could be utilised in asteroseismology to determine its mass, age and radius. 

The team collected a swathe of data gathered from some of the most advanced instruments in the world, including light intensity measurements from the orbiting Transiting Exoplanet Survey Satellite (TESS) and Wide-field Infrared Explorer (WIRE), polarimetry from the Anglo-Australian Telescope at Siding Spring and Western Sydney University’s Penrith Observatory, and over 13-years of spectroscopic data from the European Southern Observatory.

Asteroseismology is the study of the seismic waves within stars to determine their internal structure - something that cannot be done with external observations, such as brightness and surface temperature, alone. These oscillations are observed as measurable regular changes in a star’s light. The incoming light peaks and troughs at regular intervals, creating regular patterns which can be extrapolated up to approximate the star’s internal structure.   

Until recently, the seismic activity of massive stars on the brink of collapse, like Beta-Crucis, are difficult to observe because the oscillation patterns are slow and are not the typically recognisable patterns one would expect to observe. This makes the process to determine the internal structures of these massive stars difficult to undertake. In the case of Beta-Crucis, not even decades of spectroscopic data collected by the ESO yielded any oscillation patterns that could be used for asteroseismic analysis.

This is where polarimetry comes in to save the day. Polarimetry is the measurement of the average orientation of light waves with respect to their direction of travel. In 1979, non-radial pulsations could produce polarisation signals that had asymmetrical distribution of peaks and troughs. Non-radial pulsations can be considered as the star’s surface pulsating due to internal seismic activity in a manner that does not maintain its spherical shape - some parts of the surface, others move inwards. These variations are small relative to the overall size of the star - so are not representative of stars of odd shapes and sizes.  

Compare this to radial pulsations - which is what is typically observed in asteroseismology - which see the star's surface pulse uniformly with its spherical shape. The oscillations these pulses produce in the star’s light is seen in irregular shapes in the light’s waveform. Polarimetry was the best chance astronomers had at observing these irregular pulsations - however, only now is the equipment precise enough to measure such signals from massive stars.

16 months and 307 linear polarimetric observations later the team discovered 11 modes of oscillation, which is six more than what was previously known for Beta-Crucis. From these modes, they were able to make an in-depth asteroseismic analysis of Beta-Crucis’ age and structure. They determined the star was approximately 11-million years old, and more than 14-times the mass of Sol. The star rotates every 13-17 days (our’s rotates every 27 days or so), around an axis tilted 46 degrees towards us. These results mean that Beta Crucis is the largest star to ever undergo an asteroseismic analysis.

What does this mean moving forward? It now means intricate analysis of main-sequence stars and how they evolve can be made. This can then be applied to stars close to going supernova. The data gained from these analyses could significantly improve the understanding of the evolution of galaxies.

However, currently, the process is intensive and challenging; and it requires collaboration across multiple sites and deep analysis of the collected data. All this takes time - but now that it has been done, the process will certainly improve as more stars are observed and analysed.

I spoke to the head researcher of this project, Dr Daniel Cotton, about this exciting development.  

Why was Beta-Crucis selected in particular, and were there features of this star that you and the team wanted to explore?

Beta Crucis was our first test target. Prof. Conny Aerts gave myself and Jeremy Bailey a list of half a dozen beta Cephei stars that were bright and had large-amplitude pulsations. We chose to start with beta Crucis because it was the brightest one and because it had one of the earliest spectral types (i.e. it was one of the hottest). Hot stars have a lot of electron scattering in their atmospheres, which leads to higher polarisations.

Beta Cephei stars in general are some of the hottest and heaviest pulsating stars. Asteroseismology has produced a lot of good information on lighter stars, but heavier stars are harder puzzles to solve. Beta Crucis is now the heaviest star with an age determined by asteroseismology, so I think it's fair to say that its mass made it particularly interesting from a scientific point of view.

How would these results differ from a red giant or supergiant star, like Betelguese or even Gamma Crucius?

Red giants have pulsation characteristics similar to stars like the Sun – called Solar-like oscillations. There are a lot more resonant frequencies, which have shorter periods, and they, in general, have simpler patterns (they are lower-order modes).

The difference between hot stars and stars like our Sun is that hot stars have a convective core and outer radiative zone – heat is transferred in the core through mixing of fluids in the core and by photons outside that – whereas in Sun-like stars it is the opposite. Red giants and Sun-like stars excite oscillations in their outer convective zone which then become global oscillations – in this way they are similar. 

What was the process involved in coordinating the data collection and analysis to yield these results?

Well, there's an interesting story there, actually. The first set of test observations we made, we only got to make because the work-horse instrument on the AAT (the 3.9m Anglo-Australian Telescope) was out of commission. Our polarimeter is a visitor instrument at the AAT, which we maintain ourselves, hence there is less work for the AAT staff to do when it's on. So, because we'd been generating good science with it, the Director Chris Lidman asked us to fill in. So, we packed everything up on one day's notice and drove the 6 hours to Coonabarabran from Sydney the next day.

In the first half-night of beta Crucis observations, it was clear that something interesting was going on. So, we were able to make a full proposal at the next call, and Dr Derek Buzasi came out to help us observe during the next observing run. Derek had specifically proposed that NASA's TESS mission observe beta Crucis for the purpose of asteroseismology. Conny sent Derek the archival spectroscopy from ESO's telescopes, I gave him the polarimetry we collected at the AAT, and he obtained the TESS data via NASA. He then conducted a joint frequency analysis on these three data sets looking for common oscillations.

With the frequencies identified, there was a lot of back and forth between myself, Derek and Conny as we applied two different oscillation mode identification strategies and then synched the results up.

Can you describe the technique that you and the team underwent to obtain these results?

The technique is probably best described in the paper, but a summary might go something like this:

• Spectroscopy can be used to determine the modes of oscillations in stars by looking at the shape of aggregate line profiles associated with each oscillation. However, there is a lot of ambiguity here, and you end up with lots of degenerate solutions.

• We obtain contemporary photometry and polarimetry. The difference in phase and amplitude between these signals also allows us to make a mode determination.

• We combine the results from spectroscopy and polarimetry/photometry to remove the degeneracy and come up with unique assignments for as many modes as possible.

• We can then use mathematical models and neural networks to reverse engineer the inside structure of the star – it's kind of like using the sound a flute makes to work out its size and what it is made out of. Of course, we already have some idea of what elements are inside a star, so we're just trying to work out how big the core is, how much helium is in it, and stuff like that. Once we know that we can say how much of the star's fuel is used up, and thus how old it is.

What were your expectations coming into the project?

I was hopeful we would find something. At this stage we'd used our very precise polarimeter to make two big discoveries: we'd measured reflection from stars in binary systems, and we measured the flattening of stars as a result of rapid rotation. Polarimetry is really good for measuring asymmetry in stellar atmospheres, that was the case with the rapid rotators, and in principle, pulsation should be just like that. So, in one way this was an extension of work we were already doing. I wasn't sure how big a signal to expect though. One paper from 1980 suggested it might be really big, but I was a bit dubious about that – I thought someone else would have been making routine measurements by now if the signals were really that big. It turns out the largest signal we found was about 10 times smaller than the one suggested in the paper.

I should say this wasn't something we proposed ahead of time to a funding agency. And in fact, we made our first breakthrough when I had about 3 months to run on my current contract. We had a grant to fund my position after that, but it wasn't successful (nor was one the next year). The initial result was too promising to just let go though, so I kept working on it without a job. For the next 2 years, I had no research position. For a while, I just kept working on this while doing casual teaching work, but that wasn't sustainable financially. Also, the university I was with at the time wasn't very supportive of this continuing, fortunately, WSU and USQ were and offered me Adjunct positions. After a while, I took a job as a Technician and Night Assistant at the AAT. The research was pursued in my spare time while working there before the Monterey Institute for Research in Astronomy (MIRA) hired me this year.

So, while I wasn't sure what to expect initially, the first observations convinced me that we had something, and the excitement of my collaborators on the project sustained me to keep pursuing it.

How do you see this technique used in the future, and are you (and the team) targeting any other particular stars?

We're already looking at more stars, including a few others from the initial test list that we started looking at around the same time as beta Crucis. There are probably a dozen beta Cephei type pulsating stars bright enough for our current instrumentation. We want to collect data on heavy stars with a range of masses and with a range of ages. One of my collaborators on this project, Dr May Pedersen, recently published a paper that looked at SPBs (slowly pulsating B stars) that are kind of the lighter cousins of beta Cephei types. She found that they were a lot more varied than initially expected, so beta Cepheids might be too. That's the type of thing we want to find out because that will help us understand the evolution of hot massive stars. It's these types of stars that become supernovae and black holes. These are responsible for the heavy elements in the Galaxy that give our Solar system and others like it such a rich variety. How they live and die has a big influence on how the Galaxy evolves in terms of its elemental composition, and that's ultimately what we're interested in with this work.