Periodic Reporting for period 4 - EASY (Ejection Accretion Structures in YSOs (EASY))
Période du rapport: 2022-04-01 au 2023-12-31
As the Solar System was born almost 5 billion years ago, we cannot directly probe how it came into being. Nevertheless by looking at star and planet formation today, we can gain major insights. In particular by studying 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 realise 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 and may in fact be the regulator of stellar and planetary birth.
The youngest stars are known as Young Stellar Objects (YSOs) and they are effectively stellar embryos. The Ejection Accretion Structures in YSOs (EASY) project attempted 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 was to use state-of-the-art facilities that have recently come on stream including the James Webb Space Telescope (JWST). 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.
The youngest stars are known as Young Stellar Objects (YSOs) and they are effectively stellar embryos. The Ejection Accretion Structures in YSOs (EASY) project attempted 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 was to use state-of-the-art facilities that have recently come on stream including the James Webb Space Telescope (JWST). 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.
As with most scientific projects, EASY has produced some expected findings, some hoped-for results, and a few surprises!
One of our most striking achievements was imaging, for the first time, the accretion zone around a young star, TW Hydra, using the ESO Interferometer GRAVITY. We reported in Nature that we not only resolved 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, was in line with what is expected from models and gave 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). In addition, however some electrons are accelerated to relativistic energies by shocks in the outflow in a process similar to what occurs in supernova remnants. These electrons produce synchrotron radiation at low frequencies, the intensity of which is ultimately related to the ambient magnetic field strength. The observation of a downturn at the longest wavelengths gave us a new handle on the outflow's magnetic field strength, something that is poorly known.
Another major finding during the project's lifetime, arose from our Guaranteed Time Observations on JWST. Here we used both the near-infrared camera (NIRCam) and the Mid-Infrared Instrument (MIRI) to study one of the youngest nearby protostars. This resulted not only in high resolution dramatic images that featured as a front-cover on Nature (Main Journal) but also showed unambiguously that the youngest outflows are almost entirely molecular. This result is of major importance for outflow launching models.
Finally the project, in addition to producing a very large number of publications in refereed journals, generated results that were publicly disseminated using a wide variety of platforms including NASA/ESA press releases, national and international newspapers and websites (e.g. The Times of India, Fox News, etc.). The PI and several postdoctoral fellows were interviewed on radio and also contributed to podcasts. Of course EASY results were presented at a number of conferences including the European Astronomical Society Annual Meeting in Krakow, talks in Leiden and many other locations.
One of our most striking achievements was imaging, for the first time, the accretion zone around a young star, TW Hydra, using the ESO Interferometer GRAVITY. We reported in Nature that we not only resolved 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, was in line with what is expected from models and gave 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). In addition, however some electrons are accelerated to relativistic energies by shocks in the outflow in a process similar to what occurs in supernova remnants. These electrons produce synchrotron radiation at low frequencies, the intensity of which is ultimately related to the ambient magnetic field strength. The observation of a downturn at the longest wavelengths gave us a new handle on the outflow's magnetic field strength, something that is poorly known.
Another major finding during the project's lifetime, arose from our Guaranteed Time Observations on JWST. Here we used both the near-infrared camera (NIRCam) and the Mid-Infrared Instrument (MIRI) to study one of the youngest nearby protostars. This resulted not only in high resolution dramatic images that featured as a front-cover on Nature (Main Journal) but also showed unambiguously that the youngest outflows are almost entirely molecular. This result is of major importance for outflow launching models.
Finally the project, in addition to producing a very large number of publications in refereed journals, generated results that were publicly disseminated using a wide variety of platforms including NASA/ESA press releases, national and international newspapers and websites (e.g. The Times of India, Fox News, etc.). The PI and several postdoctoral fellows were interviewed on radio and also contributed to podcasts. Of course EASY results were presented at a number of conferences including the European Astronomical Society Annual Meeting in Krakow, talks in Leiden and many other locations.
As already stated we have already made major inroads by finding a whole new method for measuring magnetic fields in outflows from young stars, imaging accretion onto protostars, and determining the molecular versus ionised/atomic content of outflows using infrared spectroscopy from space. All of these discoveries represent progress beyond the state of the art. Of course, and as normal in science, further questions are raised by our results. For example it is now clear, particularly through our use of LOFAR, that low frequency non-thermal radio emission from shocks is detectable in outflows. This suggests that such shocks can accelerate particles to relativistic velocities, despite the shock velocities being much lower than those of supernova remnants. Such findings are very challenging for standard acceleration models. Moreover, the presence of such particles may have a direct impact on whether accretion disks are ionised 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. High energy particles from outflows could ensure coupling of the magnetic field to the neutral gas and perhaps enhance transport of angular momentum.
There are a number of major achievements of the project not mentioned previously. In particular we have made the first radio images of outflows using the upgraded interferometer e-MERLIN. As radio emission can be used to probe the launching region very close to the source, even when the optical extinction is many orders of magnitude, very tight constraints were obtained for the youngest sources implying jet focusing on scales of several au. As a result of obtaining this legacy data, we were also awarded large amounts of observing time on the VLA to target sources in a multi-frequency approach. One rather striking finding was that one outflow from the binary source L1551 IRS 5 almost vanished in approximately a decade.
SPIRou is only now producing the first magnetic field maps of embedded young stars which we accessed through of our membership of the SPIRou Consortium (supported through our ERC award). Field strengths for these sources appear similar to those of less embedded sources and, more importantly, seem to be dynamo generated. In the case of one known accreting young star (DK Tau) we were able not only to measure the magnetic field of the star and to determine its inner disk is misaligned with respect to its outer disk, but also to detect changes in the magnetic field strength on periods of years.
During the award's final phase, we obtained observations from JWST which in turn led to an enormous number of high profile publications. Aside from the major result on one of the youngest know outflows, Herbig-Haro 211, we were also involved in the discovery of water, carbon dioxide and abundant hydrocarbons, including ices, in the terrestrial planet forming zones in disks around young stars. These discoveries give us a much clearer picture of what conditions are like for planetary formation which we have recently realised begins much earlier than previously thought.
There are a number of major achievements of the project not mentioned previously. In particular we have made the first radio images of outflows using the upgraded interferometer e-MERLIN. As radio emission can be used to probe the launching region very close to the source, even when the optical extinction is many orders of magnitude, very tight constraints were obtained for the youngest sources implying jet focusing on scales of several au. As a result of obtaining this legacy data, we were also awarded large amounts of observing time on the VLA to target sources in a multi-frequency approach. One rather striking finding was that one outflow from the binary source L1551 IRS 5 almost vanished in approximately a decade.
SPIRou is only now producing the first magnetic field maps of embedded young stars which we accessed through of our membership of the SPIRou Consortium (supported through our ERC award). Field strengths for these sources appear similar to those of less embedded sources and, more importantly, seem to be dynamo generated. In the case of one known accreting young star (DK Tau) we were able not only to measure the magnetic field of the star and to determine its inner disk is misaligned with respect to its outer disk, but also to detect changes in the magnetic field strength on periods of years.
During the award's final phase, we obtained observations from JWST which in turn led to an enormous number of high profile publications. Aside from the major result on one of the youngest know outflows, Herbig-Haro 211, we were also involved in the discovery of water, carbon dioxide and abundant hydrocarbons, including ices, in the terrestrial planet forming zones in disks around young stars. These discoveries give us a much clearer picture of what conditions are like for planetary formation which we have recently realised begins much earlier than previously thought.