Heather Fenwick Johnston
Young, developing stars often exhibit both interesting and erratic behaviour in the form of outbursts (the accretion of stellar material) and starspots (regions of the star that are hotter or colder). Understanding these natural events is vital to furthering our understanding of stellar evolution and planetary development.
The aim of my research was to reconstruct the observations of some interesting Young Stellar Objects (YSOs) by creating a model using synthetic data.
I would choose the synthetic data that matched the observed temperature of the YSO in question and then run through several scenarios of what the spot(s) on that YSO might look like in terms of location, temperature, and size. A Chi-Square Test was then used to analyse which model generated the best fit of the observational data. This would indicate that that model was the most promising candidate for the physical phenomena that occurred during the observational period.
I successfully created a robust model capable of recreating the light curves found in physical observations of YSOs. In particular, it was extremely successful in independently verifying observational findings of the hot spots - powered by the accretion material – exhibited by the young star ASASSN-13db.
Starspots are a distinct form of stellar activity and occur in the photospheric layer. Hot spots are caused by accreting material impacting the stellar surface. Cool spots are caused by the twisting of internal magnetic dynamo.
The aim of my research was to reconstruct the observations of some interesting Young Stellar Objects (YSOs) by creating a model using synthetic data.
There is an illustration of a starspot occurring in the photospheric layer and a citation to reference 1.
Synthetic data matching the observed temperatures of the proposed YSO would be used to run through several potential scenarios of what the spot(s) on that YSO might look like in terms of location, temperature, and size. The models can then be examined statistically.
A Chi-Square Test was then used to analyse the how well the model fits the observational data. This would indicate that the model generated was the most promising candidate for the physical phenomena that occurred during that period.
The figure shows the photometric starspot model of wavelength against flux. The red data is the flux of the star at a constant temperature of 5,250 K. The blue data is the flux of a star with a temperature of 5,250 K, with cool spot coverage of 4,000 K at 35%. The green lines show the model convolved with some standard filters used in Astronomy. This shows the principle function of the model’s capabilities.
The robust model is capable of recreating the light curves found in physical observations of YSOs – in particular, ASASSN-13db. An active star that exhibits both flares of heightened outburst powered by accretion material (~5,250 K) and periods of quiescence (~3,100 K).
The total spot coverage for both models in the figure add up to 40%, consistent with both the literature (citation to reference 2) and my preliminary findings generated by the photometric model. The thick sinusoidal nature of the observed data itself could suggest extended structures from a somewhat stable starspot that fluctuates with accretion rate, similar to EX Lupi (citation to reference 3).
The figure shows epoch (a specific period) C of ASASSN-13db’s observational data plotted alongside the best fitted result achieved from both the 2-spot and 3-spot models. The chi-square (best fit values) is 2.382 for the 2-spot model and 3.824 for the 3-spot model – which means that the 2-spot model matches the observational data the best.
The 2-spot model curve has spots located with 150 degree separation, suggesting dipolar accretion – disk accretion to a rotating star with a dipole magnetic field. In the case of ASASSN-13db the hot spots could be connected to Magnetospheric funnel flows, as shown in the illustration. This indicates dominant accretion-powered hot spot activity, again reiterating literature findings (citation to reference 2).
The illustrated figure, created by me, shows an example of dipolar accretion – 2 accretion spots at opposite sides of the star feeding material onto the stellar surface, creating two distinct hot spot sites.
Reference 1 [1] D.A. Aguilar, Centre for Astrophysics, Harvard (2015).
Reference 2 [2] A. Sicilia-Aguilar et al. Astronomy & Astrophysics. 607, A127 (2017).
Reference 3 [3] A. Sicilia-Aguilar et al. Astronomy & Astrophysics, 580 (2015).