Thursday, 29 December 2016

JJ's 2016 in review.

So that was a busy year. I’m not sure what critical reflection on all of this I can give beyond this was probably at the limit of what I can achieve/do within a year. I spent way too much time working and not enough with family and friends. But at least with the #NZstars2016 conference out of the way and only the proceedings to sort out my time should become a little less constrained and I can try to get some balance back into my life.

Anyway if you're wondering why I've been so stressed this year, here is a probably incomplete list of everything I've done:


Service (astronomy):
  • Organized an IAU Symposium, #NZstars2016 which was complete success on every level! 177 astronomers in NZ to discuss the science find most interesting. Also equity was at the top of our minds when organizing it so will be writing a guide for how to make an inclusive and equitable meeting soon.
  • In collaboration with Nick Rattenbury advised on the science to Gilda Kirkpartick for Astarons Volume 2 which also featured on Real Housewives of Auckland.
  • Took part in discussions to set up Astro Aotearoa of research astronomers in NZ group.
  • For with the Astronomical Society of Australia to boost links to NZ.
  • Joined the ASA Inclusive Diverse Equitable Astronomy (IDEA) committee.
  • Applied for Pleiades award for Auckland Physics Department in collaboration with the department’s equity working group..
  • Multiple public outreach talks in Te Awamatu, Hamilton and AAS again.
  • Curated @astrotweeps for a week.
  • Started writing this blog for science, sci-fi and more stuff!

Service (University, Academia and wider):

Teaching (as one of people with highest teaching load that is still research active!):.
  • Taught 5 courses: 2x 1st year astro, 2nd year astro, 2nd year electromagnetism and 3rd year astro+particles! This all pushed me to the limit and I don’t think my teaching was as good as it could have been. Especially when 4 of these courses were in the same semester. But somehow scraped through for students thinking that my teaching was okay. This was also in addition to being openly gender diverse too.
  • At same time taking a course in Academic Practice, scoring A- grade. Also replanned and rewrote a design for 2nd year courses. Hopefully setting the format for future success of students in physics and to build on the 1st year redesign.
  • Still supervising my 3 PhD students, two of which published papers this year. Two are on course to submit next year and all their science is so super exciting!
  • Continued running the successful seminar series for physics department PhD students. Where previously there was no formal time all the PhD students would be together. This has led to the students having a great feeling of being a cohort of students.
  • Won the departmental teaching award for all of the above, especially the 2nd year course redesign.

Saturday, 3 September 2016

Science of sci-fi: Does every planet look like home?

It is a fact that today we know a surprising amount out extrasolar planets. The first exoplanets were detected in 1988 and as of August 2016, we know of 3374 exoplanets in 1257 planetary systems with 121 around binary stars. But before we get onto what we know about real planetary systems, what does sci-fi say planets should be like.

In sci-fi’s early days budgets were low and it was cheapest to have a planet of the week episode. While it was possible to make a set or choose a location, like a quarry that looked alien, all the worlds looked very much like Earth and buildings look very much like Earthican architecture. Although some locations could be picked with modern/strange architecture to give the impression of the future and/or alien worlds. Originally shows like Star Trek would have painted backdrops of locations to get around this issue. Of course today with modern computer graphics everything is possible.

An example of this can be seen in Star Trek episodes like “The Paradise Syndrome” where the planet really looks very lifelike, especially with the pine trees! Even modern Dr Who still has Earth like planets as in “New Earth”, although they do say in the episode this is because humanity were looking for an exact replica of Earth.

There are exceptions to this though for example in the Star Trek episode “Whom Gods Destroy” the planet the prison is on has a poisonous atmosphere which humanoids cannot survive in and spacesuits are needed to survive outside. Then in the Dr Who episode “The Impossible Planet” the episode is set on a planet orbiting a black hole. While in Star Wars there is the planet Tatooine which is a planet around a binary star system.

In all these cases we know planets like this, Venus in our own solar system is extremely deadly to life. While the first extrasolar planets discovered were discovered around another type of remnant of a dead star, a neutron star: pulsar planets. While we are now discovering many planets around binary stars.

It is worth noting in passing that some TV series do try to explain away the reason why all life and planets look so human. For example in Star Trek the planets were all seeded with the same DNA by an extinct race that were alone in the Galaxy (see "The Chase"). While in Stargate we are all descended from the Ancients with the Goa'uld having taking slaves from the Earth around the Universe.

When we look at our solar system we see that we only have 8 planets, 4 terrestrial or Earth-like planets and 4 gas-giants. Other than Earth the 3 other terrestrial planets: Mercury, Venus and Mars are all inhospitable. Mercury is a tiny ball or mainly iron with no atmosphere that is close to the Sun. Venus has a thick dense carbon dioxide atmosphere that would bake and crush you due to the temperature and the pressure while Mars is not too cold but has only a rare atmosphere without any oxygen. Only the Earth is habitable with water liquid on its surface. Habitable zones around stars are called “Goldilocks” zones although considering Venus and Mars are in or close to the zone we can see that being in the zone does not ensure humans could survive easily on such a planet.

Although it is possible to live on Mars as anyone who has seen/read “The Martian” knows (or Dan Dare or any of the other multiple books/movies/programs based on Mars). Furthermore we realise that Mars could be terraformed, that is using technology we could engineer the planet so that it becomes more Earth like. Infact such a scheme is the main plot of a computer game UFO: Afterlight and a few different book series. I show below screenshots from the game of how Mars would look “before” and “after”.

Terraforming also features in the TV series Firefly. The backstory to the series being that our solar system was dying so humanity moved to another system with a large number of planets that had to be terraformed. Interestingly only some of the planets are fully terraformed with some of the planets more arid and dry, suitable for the western style stories the show is focused on. [Warning: be warned if you watch Firefly you will never get over the fact there is only one series and a movie. You will spend the rest of your life waiting for the revival or animated episodes that may never be made…]

When we look at other solar systems most of the planets we find are more like the gas giants in our system, Jupiter, Saturn, Uranus and Neptune. However they orbit much closer to the host star so are typically called “hot-Jupiters”. The problem with gas giants is they’re all just gas! There is no solid surface so they are uninhabitable.

One final trope that has happened repeatedly in science fiction is the idea that gas giants have moons, just as our solar system has, and if the gas giants are closer in to their star those moons might become habitable. Star Wars has both the moon “Yavin IV” and the “forest moon of Endor” although the gas giant is never seen in Return of the Jedi, but it is in the “Ewoks” cartoon series. Also many people normally forget but in Avatar, Pandora is orbit around a gas giant called Polyphemus (a very blue looking jupiter clone!). With many other moons visible setting way for the sequels.

We know that moon of gas giants in our solar system are very large and would be considered to be terrestrial planets if they were in orbit around the Sun or another star. There looking for these exomoons are also a possible place where humanity could visit in the future. They’re difficult to find but we will find them eventually and with so many gas giants known in the habitable zone maybe these moons are the greatest place we might find other life forms in the Universe.

Tuesday, 23 August 2016

So how do you make a merging black-hole binary?

So this blog post is something different: it's about my own research. The main thing I do is make computer models of stars in binaries and then compare them to stars, galaxies and other fun things and events in the Universe. I also make as many models and predictions as possible public so other astronomers can use my models in their own studies.

Over the past year I have created my latest set of models which, while not perfect,are still a big improvement on my first go. I was in the process of writing this all up for what I call ”the instrument paper” when I started finding lots of exciting results that I thought I should publish as quickly as possible... which has led to some pretty cool papers by myself and others.

Out of these, the coolest started with the rumour that the LIGO gravitational wave detectors had found something. Gravitational waves come from many sources, but one of the most common is from the merger of binary star systems… of exactly the kind my models can simulate. For a long time I ignored the rumours, but papers by other people in the same field started to appear which suggested the rumours might be worth investigating further.

It wasn't until someone posted a comment on Facebook, quoting an email from someone in the US stating that LIGO had detected the merger of two black-holes, each of around 30 times the mass of the Sun, merging into a single black-hole that I got interested. While a lot of scientists were excited as gravitational waves had been detected for the first time I was excited because black holes are created in stars and we had a new way to determine what the masses of the black holes that are born in stars could be.

It seemed that a lot of researchers thought that these black holes were unusually massive. That is true, and they are more massive than most black holes we have seen in the past, but they’re not too much more massive. Anyway, I like observational data and I like nothing better than comparing it to what my binary models predict. So I started to write a code and made some predictions in the two weeks before the announcement. The cool thing was that we already made the black hole binaries in existing models; we only needed to write a new code to calculate how long it took for the two black holes to merge and this would allow us to see which of our models could produce the observed merger within the age of the Universe.

This work was done in close collaboration with Elizabeth Stanway from the University of Warwick. As she was in the UK and I was in Australia at the time we could take turns working on an interpretation paper with one of us sleeping while the other was hard at work! At the time it was kind of a shot in the dark - the rumours might have been untrue, and we’d have had to write off our hard work as a useful but ultimately futile exercise. Then the announcement was made and we confirmed the masses of the black holes and submitted our paper.

As always we had to go through the peer-review process and we were asked to considerably expand our predictions to include neutron star-neutron star systems as well describe in more detail our code, which had been discussed elsewhere and it will all be in the instrument paper but it was best to also include a description of all the important stuff in this paper. So after a considerable amount of extra work and clarifications to our much improved paper it was finally accepted.

During this time others published very similar papers but with different views and some interesting extensions. Our paper was always about showing that our BPASS code could predict roughly the right rate of black hole mergers and could reproduce GW150914, as well as all the other observations it can predict, where some other similar codes only consider single stars so can’t produce GW150914 type events.

However one really cool and exciting new bit of science we put in was in response to trying to explain why our computer models predicted more massive black holes than some other codes. This is actually a key uncertainty of stellar evolution, how massive is the final remnant formed by the death of a star? The merger of black holes gives us a way to measure this, but it only tells us about the black holes in binary stars. What about single black holes that might been ejected from a binary by a ‘kick’ they received in their natal supernova?

At the same time as GW150914 was being announced, another really quite awesome study (Wyrzykowski et al., 2016) has used gravitational microlensing to find candidate single black holes drifting through our own Galaxy. This technique is widely used to discover planets around other stars, but they also found many possible black holes. What was interesting about the study is they found a number of low mass black holes near the minimum mass that can be created in stars of 3 times the mass of the Sun. Others had suggested these didn’t exist so this was a surprise.

In our models we have a range of possible black hole masses from 3 times the mass of the Sun up to and beyond the masses of GW150914. A really fun thing though is that these single black-holes that were discovered are on average lower masses than those we see in binaries in our Galaxy and those we saw from GW150914 and the other recent similar detections.

The figure below shows the results of the black hole masses. Predictions from our code for single stars are shown in red, black holes in binaries in blue with the solid line representing the mean black hole masses with the dashed lines represent the boundaries within which about 68% of black hole masses must be within. The vertical axis is the black hole masses while the horizontal axis is the “metallicity” of the stellar models - a measure of which generation of stars they are: less metals is an earlier star. The grey shaded region represents the metallicity range of our Galaxy. The black horizontal lines are the masses of the black holes in GW150914. The asterisks are known black holes in binaries while the red and blue points are the masses of the single star and binary star observations.

What this plot tells us is that more massive black holes are more likely to be seen in binaries and lower mass single star black holes are more likely to be unbound in their forming supernova and so seen in isolation. This is quite an interesting finding: more gravitational wave sources and more single black hole detections by gravitational microlensing will tell us a surprising amount about black hole formation.

My favourite plot those is the next one, it is a quite colourful and dramatic plot. Each panel shows the predictions for a different generation of stars in the Universe and the brightest colours indicate where the most common black hole mergers should occur. They range from some of the earliest stars in the Universe (in the upper left panel) to those similar to the ones forming in our own Galaxy (in the lower right). On top of this are the 3 detected black-hole mergers to date of GW150914 (dark blue, top right) LVT151012 (green, middle) and GW151226 (cyan, lower right). Each system is plotted twice as we don’t know which black hole came from which star: the initially more or less massive.

The contours and shading then represent where the masses of the most likely black hole mergers. We can see that GW150914 is only possible at the lower metallicities, while the others are possible in all populations and GW151226 is closer to the typical mass of black hole mergers expected in all the populations.

The one interesting thing is GW151014 is a typical merger at the lowest metallicities which means it might have come from some of the earliest stars to form in the Universe. Although we can’t be certain. To show this we need to do a similar study to some of our fellow astronomers, Belczynski et al., who also modelled in how many stars of different generations were formed at different times through the Universe and how long they would take to merge and so whether they would be observed today. They found either the binary merger was relatively recent or again closer to the formation of the Universe. While this requires lots of assumptions about unknowns in cosmic history, we may try to calculate this with our own models in future.

The key result we wanted to show, and why we wrote the paper, was just that in BPASS our models naturally have these black-hole mergers in. The code does both population and spectral synthesis and it is one of very few spectral synthesis codes that can predict black-hole mergers alongside all our predictions of stellar clusters and galaxy populations. Why? Well the other most common ones assume all stars are single! Only a binary code like ours can get close to the correct black-hole merger rate inferred from LIGO after its first observing run.

We’re starting to find that including binaries makes a key difference to our understanding of the Universe - all the way from distant galaxies to individual stars. Combining the LIGO and lensing results with our BPASS code has added another piece to the jigsaw.