We discovered a very rare kind of star – in fact there are only 25 stars of its type known to exist. The fact that we found it is amazing, but the way we found it was even more interesting. We caught it red handed during an FUor outburst – a period of intense flaring where the star can become several magnitudes brighter than it was during quiescence and appear to change to an earlier spectral type.
This happens due to a preocess called episodic accretion. When a young stellar object (YSO) is evolving towards the main sequence, it is still absorbing material from a disc of gas and dust that is oribiting it. Instead of accreting this material at a constant rate, it’s thought that material accretes in bursts. This is thought to be triggered by either instabilities that form within the disc, or the influence of some outside body. Either way, better understanding this process is vital for gleaning a better understanding of both star and planet formation.
When this accretion is triggered, the star appears to become much brighter and hotter than it was previously. This process has been observed 13 times before, but thanks to the Gaia and NEOWISE mission, for the first time, we were able to watch the process as it was happening at both optical and infrared wavelengths. This is the part where I come in. Gaia is continually scanning the sky in order to measure the brightnesses and positions of stars within our galaxy. As of the last data release, it has measured precise positions for nearly 1.7 billion stars. One foruntate product of this mission is that Gaia records the brightnesses of stars at regular intervals, and the processing software can tell when a particular star brightens incredibly accuractly. When the brightness of a star noticably changes, the Gaia Data Processing Consortium (DPAC) push them into a public feed of photometric science alerts, which we regularly monitor for new YSOs.
Gaia17bpi was published 23rd June 2017 after a rise of over 2 magnitudes in the Gaia G band. This is definitely a young star, but the problem is that, at least a the time, there were no known YSOs to match against. So a direct crossmatch with a catalogue of YSOs would have missed this FUor outburst. This is a small testiment to how poor the census of YSOs still is. However, we leveraged the fact that stars form in clusters to our advantage. As well as a direct crossmatch, we also perform a ‘vicinity match’ which does a match to find YSOs within 2 arcmin of the alert coordinates. If there is something that is rising in magnitude within the vicinity of other YSOs, we want to know about it. After it was caught in the act, I handed it off to Carlos Contreras-Peña and Lynne Hillenbrand who did all who did all the hard follow up work with the rest of the collaboration and yielded some truly exciting results.
In particular, the interesting thing about this target is that the NEOWISE mission managed to observe it 1.4 years before the Gaia alert was published. These observations show a rise in brightness before the optical outburst. This means that the flare actually started out about 1AU away from the star in a much cooler part of the accretion disc and worked it’s way in over a couple years, heating up and emitting increasingly bluer radiation as it went. Palomar and Keck were also able to get spectroscopy of the source to confirm the temperature structure of the system and the youthfulness of the star. Thanks to the wide array of observations on offer we were able to pin down the timeline of events, and the fact that this is indeed a FUor; as opposed to other brightening events such as a cataclysmic variable.
This study helps us pin down some interesting ongoing questions about star formation, particularly the frequency and nature of FUor outbursts. These events are one of the linchpins in making star accretion timescales check out and cementing our understanding of star formation. Furthermore, understanding the protoplanetary environment around potentially exoplanet hosting stars is crucial for being able to develop more refined models for how planets form – especially in their treatment of such flaring events.
If you’d like to read more about the work, I’ll link the paper and some press releases below.
- The paper on the ArXiv: https://arxiv.org/abs/1812.06640
- The University of Exeter press release: https://www.exeter.ac.uk/news/research/title_697534_en.html
- The Caltech press release: http://www.caltech.edu/news/young-star-caught-fit-growth-84666
Image credit: Caltech/T. Pyle (IPAC)
The paper is now officially published in the Astrophysical Journal. If you’d like to see the final refereed journal article, you can find it here: http://iopscience.iop.org/article/10.3847/1538-4357/aaf414