White dwarfs and stellar evolution

The Sun

Professor Boris Gänsicke, from the Department of Physics at the University of Warwick, UK, met with SEQ at NAM 2019 to discuss evolved planetary systems around white dwarfs.

White dwarfs and stellar evolution

One of the keynote speakers at the Royal Astronomical Society National Astronomy Meeting 2019 (NAM 2019), which SciTech Europa Quarterly attended in Lancaster, UK, was Professor Boris Gänsicke, from the Department of Physics at the University of Warwick, UK. He is an astrophysicist interested in the formation, evolution, and demise of stars and their planetary systems. His scientific contributions include model atmospheres for neutron stars, observational population studies of close binary stars, and measuring the bulk abundances of exo-asteroids, and he has been supported by an ERC Advanced Grant. He is leading surveys within the DESI, WEAVE and SDSS-V collaborations that

will obtain follow-up spectroscopy of several 100,000 white dwarfs and nearby stars identified with Gaia, transforming our insight into the current and past stellar and planetary populations in the solar neighbourhood.

He met with SEQ at the event – which the largest annual astronomy and space science event in the UK and will see leading scientists from the UK and around the world present the latest cutting-edge research, and which incorporates the RAS National Astronomy Meeting (NAM), and includes the annual meetings of the UK Solar Physics (UKSP) and Magnetosphere Ionosphere Solar-Terrestrial (MIST) groups – to discuss the topic of his keynote address (evolved planetary systems around white dwarfs) as well as a few other interesting areas.

Why is it important to study evolved planetary systems around white dwarfs? What do compact binaries teach us about the previous stellar lifecycle and the physical processes governing their evolution?

White dwarfs are the remnants of most stars in the Universe, and so the study of white dwarfs ties into many different areas of astrophysics. Using very similar techniques, you can study planetary systems as well as supernova, which we use for cosmology, for example.

The study of evolved planetary systems around white dwarfs is interesting for at least two reasons. One is that over the past 20 years it has become clear that most white dwarfs probably have remnants of planetary systems. Indeed, many of them are accrediting smaller planetary bodies that have been disrupted and we can thus use those white dwarfs to measure the composition of extra solar planetary bodies. This is very similar to the way that we use meteorites to learn about the composition of the solar system.

The second is that white dwarfs come from stars that were typically more massive than the Sun –one-and-a-half to two solar masses – and so the study of white dwarfs provides us with some insight into the architectures of planetary systems around those more massive stars which, when they are in the main sequence, are very hard to probe for planets.

Given that exotic interacting binaries with collapsed stars are among the most powerful energy sources in the Universe, how would you like to see them being studied moving forwards?

Perhaps half of all stars are binaries, with some of them interacting; they make all kinds of exotic and exciting things such as type Ia supernovae, which are routinely used as distance indicators on cosmological scales. Indeed, the 2011 Nobel Prize for Physics was awarded to Adam Riess, Brian Schmidt, and Saul Perlmutter for the discovery of dark energy, and that was purely based on observations on type Ia supernova.

While astronomers generally agree that type Ia supernovae are white dwarfs that explode after being force-fed by another star, we still do not know what kinds of binary stars lead to those explosions, and if there are multiple pathways. That Is bad news if you want to do supernova cosmology, as you may, at some point, be limited by the fact that they are diverse, and you don’t know how that diversity will affect your results. More generally, compact binaries make all sorts of exotic things, such as black hole binaries, neutron star binaries, millisecond pulsars, and so on, and we don’t have a good grasp of the evolution of those binaries’ boundaries because it involves several very complex phases.

The first one is quite often the common envelope, where there are two stars living together in a close binary system, and once the more massive one expands into a red giant, it engulfs the other star. That phase is terrifically difficult to model because of the very large range of geometric and time scales involved. After that, you have angular momentum loss from the system, which we really don’t understand very well and, in addition, the physics of accretion discs, i.e. the structures through which mass flows from one star to the other, are also not well understood. This is therefore an important area of study, with plenty of work left to be done.

Are there missions yet in place or proposed that have the potential to fill some of these knowledge gaps?

On the neutron star side, we need X-ray machines. Here, Newton and Chandra are, if you like, the work horses, while Athena represents the next generation.

However, it is perhaps better to study white dwarfs than neutron stars because you are able to measure, accurately, many more of their properties, such as masses, radii, abundances, magnetic fields, and so on. While this area is perhaps a little less ‘sexy’ than the study of black holes and neutron stars, we come up with hard numbers, and we have many more of these systems, which are brighter and closer to Earth. For those of us working in this field, however, the problem is that Hubble is our workhorse for ultraviolet spectroscopy, and its life is now coming to an end.

Indeed, we use the Hubble’s Cosmic Origins Spectrograph instrument a lot to obtain ultraviolet spectroscopy of white dwarfs. That is a fantastic instrument, but it its detector is slowly degrading and, once Hubble as a whole comes to the end of its life, there is no space mission capable of delivering ultraviolet spectroscopy (and you need to go above the Earth’s atmosphere, as it blocks the ultraviolet radiation) in the pipeline until the mid- to late-2030s. That is a huge concern as there is a chance that the art of ultra-violet spectroscopy will be lost.

Is there data from Hubble that will continue to enable this kind of science in the interim?

We do have various programmes that will keep us busy for the next couple of years, but these won’t last for the full 25 year gap. This field of astronomy is relatively small, and as it crosses over several disciplines it doesn’t have the same kind of strong community support that areas such as X-ray astronomy has.

Will the emerging field of multi-messenger astronomy and the relevant projects and missions such as the CTA have a bearing on your work moving forwards?

While CTA is a facility that we probably won’t use, just three years ago we discovered the equivalent to a neutron star pulsar, a strongly magnetic white dwarf that is spinning very rapidly. We see synchrotron pulses every two minutes from this star, and we have speculated that it may have some very high energy emissions, and so the new multi-messenger telescopes etc. could be of use here.

Are there any other missions or telescopes etc. that you are looking forward to?

The European Southern Observatory (ESO) has ESPRESSO (the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) which is a high-resolution spectrograph and can combine the light from all four Very Large Telescopes (VLTs). That will be extremely powerful, feeding the light from four, eight metre telescopes into one high resolution spectrograph which will allow us to go quite a bit fainter than we have been able to go in previous studies – which is ideal to observe white dwarfs, as they are very small stars, and therefore not very luminous. This is exciting as we may be able to use ESPRESSO, and/or other radio velocity studies of white dwarfs, to potentially detect the radial-velocity variation of white dwarfs being pulled by planets.

Joining up the light from the four VLTs for ESPRESSO is undergoing a phase of science verification this summer and we will try to start observing white dwarfs soon after that. However, we will need to have patience because the orbital periods of planets around white dwarfs are very likely long, maybe years, so will require many measurements.

How important has Gaia been to your work?

Gaia was a game changer; the second data release has revolutionised what we do. Everything that we do now uses Gaia, and while the three-dimensional maps of the sky that we have now are great, they will sharpen up with each future data release. As Gaia will probably go for another five years past nominal mission, so the data will ever become more accurate and will resolve more and more fine details in the properties of white dwarfs.

From Gaia’s data we produced a Hertzsprung-Russell diagram, which sorts stars by their temperature in one direction, and their luminosity in the other. In that diagram, most stars sit on a diagonal line, which is called the main sequence (and our Sun sits in that sequence). When stars become white dwarfs, they evolve to an almost parallel sequence but, because they are much smaller, they are also much fainter. A white dwarf with the same temperature as the Sun, ~5800K, is a factor of 10,000 fainter than the Sun. That is why the study of white dwarfs has been lagging behind other areas of astronomy – simply because they are so faint and are therefore hard to find. But if you have precise distances, and you find stars that have a colour similar to that of the Sun, but are much, much fainter, then you know that these are white dwarfs.

Because white dwarfs no longer generate energy via fusion, they simply evolve on their sequence in the Hertzsprung-Russell diagram to lower temperatures, and lower luminosities – we call that the cooling sequence. We have studied the cooling sequence of white dwarfs revealed by Gaia, and have spotted some structure in there, which we interpret as the signature of white dwarf crystallisation. That is, as the white dwarf cools, its core eventually crystallises, releasing latent heat (think freezing ice cubes), which slows down the cooling process – and leads to an over-density of white dwarfs where they undergo crystallisation.

This was the first time that effect had been detected, and that was achieved by using just the Gaia data.

In terms of new facilities, I am now looking forward to the big multi object spectroscopic surveys: DESI, WEAVE, SDSS-V and 4MOST. With Gaia we have been able to find white dwarfs, but we now need to get spectroscopy for them to determine their atmosphere compositions, masses, ages, magnetic fields etc. Because of the scale of this project, ~200.000 stars, this is not something we could do on a conventional telescope. We therefore piggyback on these big surveys, which are primarily designed for either cosmology or galactic archaeology, and put a few of the optical fibres on the white dwarfs. In this way, over the next 5-10 years, we will get spectroscopy for most of the white dwarfs that Gaia has identified. By doing so, we can then identify the compact binaries and evolved planetary systems, and many other types of interesting systems.

Another exciting result that came out of the Gaia data, and which links back to type Ia supernovae, is that we have found that a handful white dwarfs which were once members of a binary containing a second white dwarf onto which they dumped some of their mass. This other white dwarf then eventually exploded, at which point the mass-losing star flew away with a velocity of hundreds to thousands of kilometres per second. Understanding this, one of my colleagues decided to use the Gaia data to find these fast supernova-surviving companion stars. Gaia provided him with the distance and a tangential velocity, which he could turn into space velocity, and so he was able to select stars that are blue (as most white dwarfs are moderately hot) and move at such high speeds. I joined his effort and within six hours of the data release, he had a list of candidates. Then, using different telescopes, within 24 hours we had discovered three stars which had survived a thermonuclear supernova. We also found white dwarfs with very odd compositions that we think are the partially burnt remnants of thermonuclear supernovae, in other words, which failed to disrupt totally.

That is a new field which is extremely exciting because finding both examples of the white dwarfs that donate mass, as well as those that ignited in a thermonuclear explosion, will help us to understand the pathways towards supernovae Ia and related phenomena. At the moment, this is a very small number of stars, but Gaia and the spectroscopic surveys will grow that sample in the future.


Professor Boris Gänsicke

Department of Physics

University of Warwick


Tweet @warwickuni


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