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Ejection Accretion Structures in YSOs (EASY)

Periodic Reporting for period 2 - EASY (Ejection Accretion Structures in YSOs (EASY))

Reporting period: 2019-04-01 to 2020-09-30

Amongst some of the fundamental questions that we all look to answer are: how did the Universe begin, is there life elsewhere beyond the Earth, and how did the Sun and its planets form? While the Solar System was born almost 5 billion years ago, and much of the processes that gave rise to its birth are difficult to unravel, we can find out clues by looking at star and planet formation today. On average one new star is born in the Milky Way every year. The process of course takes much longer but by looking at stars in different stages of evolution, up to a few million years old, we can glimpse snapshots of the formation process.
While we expect stars to have to build up mass while forming, it came as a surprise several decades ago to realize that star formation is accompanied not only be accretion (the build- up of matter) but also by matter being ejected in the form of supersonic jets and outflows. It would appear that such outflows are fundamental to the star formation process as they remove angular momentum (spin) from the system which would otherwise prevent accretion but also by disrupting the surrounding cloud and thereby limiting accretion. Outflows in a sense may be the regulator of stellar and planetary birth.

The youngest stars are known as Young Stellar Objects (YSOs) and are effectively stellar embryos as they are not yet fully-fledged stars. The Ejection Accretion Structures in YSOs (EASY) project is attempting to understand the link between accretion and outflows, how outflows influence their environment, and also how outflows may affect planetary formation, particularly in the so-called terrestrial zone where Earth-like rocky planets form around new stars. The approach taken by the project is to use state-of-the-art observational facilities that have just come on stream or, in the case of the James Webb Space Telescope (JWST), just about to come on stream. This includes facilities such as the GRAVITY instrument on the European Southern Observatory (ESO) facility in Chile, to probe young stars with the highest spatial resolution possible, the LOFAR, VLA and e-MERLIN radio telescopes to examine how outflows are focused as they are ejected, the specially designed infrared spectrometer SPIRou, to probe their magnetic fields, and the Mid-Infrared Instrument (MIRI) on JWST to look at outflows from the youngest embedded stars at very early stages with unprecedented resolution. Underpinning our observational programs, which are largely derived from guaranteed time on these various instruments, we also have a sophisticated modelling effort that relies on modern statistical methods, e.g. when imaging using GRAVITY, or understanding the transport of radiation within ionized gases, e.g. LOFAR and e-MERLIN.
As with most scientific projects, EASY has produced some expected findings, some hoped-for results and a few surprises!

Perhaps our most striking achievement is imaging, for the first time, the accretion zone around a young star, TW Hydra. In a forthcoming paper in Nature (Main Journal) we not only resolve the star itself but also material crashing onto its surface in the form of hot gas. The size of the observed accretion zone, and even the size of the star itself, is in line with what is expected from models and gives us great confidence that we are on the right track in understanding the processes involved.

A totally surprising finding was the discovery of a turnover in the radio emission from outflows at the lowest (LOFAR) radio frequencies. The bulk of the radio emission from outflows comes from thermal particles (electrons) as their motion is slowed down by ions. In addition, however our group were amongst the first to realize that some electrons are accelerated to relativistic energies by shocks in outflows in a process similar to what occurs for example in supernova remnants albeit at much lower energies. These electrons produce synchrotron radiation and exploring how this radiation is produce opens up a new window on understanding young stars. In particular synchrotron radiation is polarized and its intensity and polarization direction are ultimately related to the ambient magnetic field strength and direction respectively. Not only however have we observed synchrotron radiation but also a downturn at the longest wavelengths due to the mixture of thermal and relativistic particles present. This in turn gives us another handle on the magnetic field which, in the case of outflows, is poorly known.

Turning to accretion onto young stars, as stated elsewhere in this report, accretion occurs through a disk surrounding the young star. These disks eventually evolve into planetary systems in most cases and are normally traced using millimetre emission from their dust particles. Dust, although a strong emitter in the infrared and at longer wavelengths, constitutes only a tiny fraction of the disk’s mass. Most of the disk is in the form of gas. To trace the gas, we use molecules that have a strong dipolar moment such as CO and NH_3. A particularly important discovery made by the EASY Team using GRAVITY is imaging the gas close to a massive star for the first time. This gas was inside the so-called dust sublimation radius where dust is destroyed due to the intense radiation.

In addition to the above results, we have started to receive radio observations of outflows from young stars using the new e-MERLIN array centred on Jodrell Bank. This data is in the C-band (approximately 5 GHz) and the images surpass any achieved to date in terms of spatial resolution and sensitivity. Moreover radio emission is not obscured by the dust that surrounds young stars, allowing us to peer into the “central engine”.
As stated above we have already made major inroads by finding a whole new method of measuring magnetic fields in outflows from young stars, imaging accretion onto protostars for the first time, and gas in their accretion disks within the dust sublimation zone. All of these represent progress beyond the state of the art. It is also clear now that we are increasingly detecting low frequency non-thermal radio emission from shocks in young stellar object (YSO) outflows. This suggests that such shocks can accelerate particles to relativistic velocities, despite the shock velocities being much lower than those of supernova remnants. The presence of such particles may have a direct impact on whether accretion disks are ionized at a few au from their star, since galactic cosmic rays, particularly low energy ones, are excluded from the vicinity of the disk by the star’s strong magnetosphere. This in turn ensures coupling of the magnetic field to the neutral gas, disk turbulence and hence transport of angular momentum.

There are a number of hoped for achievements. In particular we have the first radio images from our e-MERLIN legacy program which are of amazingly high quality in terms of signal to noise but also in terms of their spatial resolution. These data will no doubt provide very tight constraints on the launching mechanism for outflows as they directly reveal the ionized jet component that is closely coupled to the magnetic field. As a result of obtaining this legacy data, we have in addition recently been awarded large amounts of observing time on the VLA to target the same sources in a multi-frequency approach.

SPIRou is only now producing the first magnetic field maps of embedded young stars which we are accessing by virtue of our membership of the SPIRou Consortium (supported through this ERC award). Again, the quality appears to be high and we are optimistic that in early 2021 we shall have measurements not only of the magnetic field strength but also its topology for a number of embedded, accreting young stars. Currently we have no idea whether such fields are stronger and/or more complex than those seen in more evolved, optically visible young stars.

During the first period of this award, we have finalized our Guaranteed Time Program on JWST with the intention of mapping a number of outflows from embedded protostars using the Mid-Infrared Instrument (MIRI) Integral Field Unit (IFU) covering the entire spectrum from approximately 5 microns to 28 microns with unprecedented spatial resolution (approximately 30 hours of JWST time). In addition, we shall map the entire blue-shifted outflow of Herbig-Haro 211 using the MIRI IFU, as well as image it in various filters with NIRCAM, and observe important features with NIRSPEC (requiring 20 hours of JWST approximately). Assuming JWST is launched in March 2021, we will expect to get some of this data towards the end of 2021.