Simulation of a gravitationally unstable disc (NCSA/NASA/A. Boley)
My research involves investigating the physics and chemistry at work within circumstellar discs. Stars like our Sun are created when clouds of gas and dust collapse under their own gravity. As the cloud collapses, the rotation rate of the cloud increases to conserve angular momentum. This causes the cloud to flatten out, creating a circumstellar disc around the central young star.
Planets can then go on to form within these discs (so they are also referred to as protoplanetary discs). There are several theories as to exactly how planets might form, but a set of these theories involves the gas in the disc collapsing, much like the cloud that formed the star did. This 'top down' planet formation scenario is most likely when circumstellar discs are very young and contain much mass. In such cases, the disc can become gravitationally unstable, causing rotating spiral waves to appear.
Using computer models, I discovered that these spiral waves can heat the disc sufficiently to alter the chemistry occuring there. Therefore, observations of molecules that trace this spiral structure, using instruments such as the Atacama Large Millimetre Array (ALMA) will show if this process occurs in the youngest circumstellar discs, and enable us to learn much about the process of planet formation. For more information, please see my publications or my thesis.
Evolution of the disc & elements from Evans et al. (2015)
After this initial study, we performed a similar analysis on a disc that may form planets around a star similar to our Sun. Following the chemistry over a much longer timescale, we found that even these lower mass discs can have their chemistry altered by the relatively weak shocks that are caused by the instabilities in the disc. In fact, permenant changes to the chemical composition of the disc can occur when these dynamic effects are considered. Therefore, these effects may need to be taken into account when considering the chemical evolution of more evolved discs.
Artist's impression of the disc around Elias 2-27 (A. Smith IoA Cambridge; Meru et al. 2017)
Seeing these types of discs 'in the wild' was an exciting prospect, but relied on theoretical predictions and simulated observations. However, that changed in 2016 when Perez et al. published wonderful observations of the young system Elias 2-27, showing two prominent spiral arms in the disc. This immediately excited us, as spiral structure is a hallmark signature of gravitational instability. However, it can also be caused by other phenomena, such as a planet within the disc. So, we set about trying to characterise and quantify what types of discs could reproduce the morphology seen in the observations. This involved a series of hydrodynamic and radiative transfer models, along with simulated interferometric observations - not an easy task. After analysing over 70 of these simulations, we found that BOTH a gravitationally unstable disc and a disc with a giant planet were able to reproduce the observations. While this was initially a little disappointing, we then realised that the mass of the planet required to excite the spirals was huge - much larger than anything that could be made via the tradiational method of planet formation by core accretion. So, it looks like Elias 2-27 could either be one of the first observations of a self gravitating disc, or a disc that has very recently undergone fragmentation. Future observations, particuarly near infrared imaging, will be able to disentangle precisely what is happening in the system once and for all.
Chemical evolution of the disc from Ilee et al. (2017)
In addition to displaying large scale spiral structure, gravitationally unstable discs are also prone to fragmentation. This process can create planetary mass objects within massive discs, and has been suggested as a possible formation mechanism for gas giants. I have investigated the chemistry in a similar disc but one in which four planetary fragements are formed. These fragments, which we named 'John', 'Paul', 'George' and 'Ringo' (don't ask), were between 3 and 10 times the mass of Jupiter. I was able to follow the chemical evolution of all of the material as it formed these fragments, leading to some interesting discoveries. I identified chemical species that were very abundant in the fragments – including molecules like water (CO), ammonia (NH3) and sulphur monoxide (SO2); species that were also abundant in the surrounding disc material and spiral arms – like carbon monoxide (CO), methane (CH4) and formaldehyde (H2CO); and species that seemed to trace the material surrounding the fragments – like the formyl ion (HCO+). In addition, I found that the molecular 'snow lines' in the disc — the locations at which molcules in the gas phase freeze out into ices on dust grains — were significantly affected by the dynamic evolution of the disc, and the fragments developed their own snowlines (see the video to the left).
The fragments can be relatively cold when they first form. This 'cold start' phenomena is very interesting, because it persists on timescales that are comparable to the timescales over which dust can be efficiently settled to the cores of the fragments. It is therefore plausible that this settling could occur before the ices of these grains have evaporated, changing the elemental ratios of the dust and gas that goes on to form planets. Therefore, the atmospheric composition of planets formed in this way should not necessarily follow the bulk chemical composition of the disc from which it formed.
For more information, please see my publications.
Artist's impression of a disc around a massive young star with an inset of CO bandhead emission (ESO/L. Calcada, Ilee et al. 2013)
Stars more massive than our Sun are difficult to image directly during their early stages of formation, because these stages occur very rapidly. This means the entire formation period occurs while the star is still shrouded in the parent cloud that formed it. Due to this, and the fact that most massive young stars are located far from Earth, we know very little about how massive stars form.
However, we can look at these massive young stellar objects (or MYSOs) using infrared wavelengths of light, which are able to penetrate the thick cloud surrounding them. Using a very high resolution spectrograph (CRIRES) mounted on the Very Large Telescope (VLT), I examined emission from the CO molecule that is excited in very warm and dense gas, the type of environment we expect very close to the forming massive star. Modelling this CO 'bandhead' emission shows us that this warm gas is emitted from small discs, suggesting massive stars form in a similar way to their low-mass counterparts. Excitingly, this work was discussed in a review talk at on massive star formation at Protostars & Planets VI. For more information on this topic, you can look at my publications or my thesis.
SMA line and continuum observations of G11.91-0.61 MM1 showing the bipolar molecular outflow, and the gas velocity from the CH3CN K=3 transition, consistent with infall and a rotating Keplerian disc (Ilee et al. 2016).
By going to even longer wavelengths, it is possible to investigate the colder material surrounding these massive young stars, which makes up the majority of the mass. Using the Submillimetre Array (SMA) in Hawaii, we were able to investigate the immediate enviroment of a particular massive young star - G11.92-0.61 MM1.
Using the emission of CH3CN - methyl cyanide - we were able to examine the gas close to the centre of this young protostar. I found that it was rotating in a Keplerian manner (that is, similar to the planets in our Solar System, and also discs around lower mass stars). We were then able to calculate the mass enclosed by the disc, which was between 30 and 60 times the mass of our Sun. By looking more closely at the dust emission, we were able to calculate that the disc is likely 2-3 times as massive as our Sun. Thus, if the remaining mass is entirely due to the central object, then G11.92-0.61 MM1 is one of the most massive protostars discovered to-date.
Such an extreme system begs the question - what would the disc look like? While we couldn't resolved the disc directly with our SMA data, we were able to model it. Using our observations as a basis, we could assess how likely it would be for this disc to fragment into smaller objects, along with a handful of other recently observed discs around massive stars. Therefore, MM1 should be a good target for observing substructure and possible companion formation around high mass young stars.
MM1 a & b as observed with ALMA in 1.3mm dust emission (green) overlaid with the velocity (blue/red) of the CH3CN v8=1 emission, showing the Keplerian rotation of the circumstellar disc (Ilee et al. 2018b).
Based on this, I targeted the G11.92 MM1 system with the ALMA in Cycle 4. ALMA allowed us to observe MM1 in much greater detail than with the SMA, and image the close circumstellar environment for the first time. The observations revealed a wealth of detail in the system. MM1 divides into two main sources, MM1a and MM1b. MM1a weighs in at approximately 40 times the mass of our Sun, and appears to be the source of the large-scale bipolar outflow. It is surrounded by a rotating Keplerian disc. Just beyond the disc lies MM1b, which was measured to weigh about half the mass of our Sun. The analysis suggests MM1b is a bound object in orbit around MM1a. Based on their extreme difference in mass, and our model predictions, the MM1a & b system appears to be one of the first observed examples of binary star formation via disc fragmentation around a massive young star. I will be targeting MM1a & b with the most extended configurations of ALMA in 2019 in order to further characterise these exciting objects.
The Herbig Ae/Be star IRAS 10082-5647 and below it an image showing the long wavelength coverage of VLT/X-Shooter (ESA/Hubble & NASA)
Herbig Ae/Be stars are young stellar objects with masses in between low mass young stars (or T Tauri stars) and high mass young stars (the MYSOs mentioned above). As we consider stars of larger mass, their interior structure changes, and may make it more difficult to generate magnetic fields. These magnetic fields are important because they regulate how the young stars accrete material from surrounding circumstellar discs.
In order to investigate the inner regions of these discs, we used another spectrograph mounted on the VLT called X-Shooter. This instrument provides a very large wavelength coverage, from the UV and blue regions of the spectrum right across to the near-infrared (see right). Collecting all of this information in 'one shot' is very useful, as any effects of source variability can be ignored.
Using our CO model, we showed that the CO bandhead emission can be well represented by small scale gaseous discs around the stars. We also examined the relationship between the CO bandheads mentioned above, and an atomic hydrogen emission line - Brackett γ (produced by even higher temperature processes) and found that these emission lines seem to be correlated, despite appearing to originate in different enviroments in the disc.
In a seperate study, the X-Shooter data mentioned above were used to determine a set of consistent stellar parameters and accretion rates for the ninty objects in the entire sample. This is the largest and most complete set of stellar parameters ever determined for Herbig Ae/Be stars. In addition, various differences between Ae and Be stars were found, suggesting the latter may be accreting in a different manner to low mass stars. For more information on this topic, you can look at my publications
The DIANA (or DIsc ANAlysis) project was a European Framework Seven (FP7) SPACE initiative to conduct a systematic collection, and coherent analysis of observational data from protoplanetary discs. The project collected public and proprietary multi-wavelength data for protoplanetary discs (e.g. Spitzer, Herschel, XMM, HST, VLT, JCMT, ALMA, eMERLIN). Large amounts of such data exist, but are currently seriously underutilised. The team reached an unprecedented level of completeness concerning the modelling of these data sets. The enabled the inclusion of important physical processes such as astrochemistry, gas heating & cooling, dust evolution, continuum & line radiative transfer and non-LTE treatments.
In order to consistently model many objects and multiwavelength datasets, we required a set of standard assumptions about the dust and gas content of disc models. In particular, these assumptions concerned the shape of the disc, the properties of the dust, the opacities considered, the chemical reactions and initial element abundances, and the heating and cooling processes that are taken in to account. In addition to using these assumptions to model the data, we are also releasing our findings to the general community, so that others may make use of them. For more information and to see the models, please visit the DIANA webpages.
In March 2016, I co-chaired the Protoplanetary Discussions conference, held in Edinburgh, UK. The rationale of the conference was to invite observers from all wavelength regimes to share their latest results with modellers from the thermo-chemical and hydrodynamic communities in order to foster collaboration across these often separate fields. In addition to the standard format of invited and contributed talks, we also arranged specialised discussion sessions, allowing participants to interact in closer and more collaborative ways. Further information can be found on the main conference webpages.
As a result of two of these discussion sessions, we wrote a overview paper concerning the forthcoming grand challenges of protoplanetary disc modelling - the inclusion of dust and gas hydrodynamics, magnetic fields, radiation transfer and chemical evolution in a self-consistent way. Rather than being a tradiational review, we preview the challenges faced with incorporating these often distinct, but fundamentally interconnected topics. You can view the paper here.