Matthew Kenworthy

Student paper day! I’ve been looking at the light curves that the ASAS-SN survey releases on their transients page - these are light curves that show curious dimmings caused by stellar variability or by something moving in front of the star. One of these light curves, named ASASSN-21js, showed a sudden dimming in 2021, and over the past three years, has faded and then brightened up again. Theo worked on analyzing this light curve with me as a Masters thesis project here in Leiden: after some experimentation, the light curve is exactly matched by a highly tilted and inclined disk with two rings with different transmissions. What’s particularly impressive is that the residuals after the model fit are consistent with noise. This model explains the data almost perfectly.

Using a MCMC fitting package, he was able to determine the confidence intervals on the parameters, with the outermost ring about 1.12 au in radius. Due to the geometry of the eclipse, we only see these two rings - there may be more material interior to this radius, but the star’s motion will not reveal this. What’s more amazing is that the eclipse is still ongoing, finishing only in May 2027!

Okay, so what are the challenges here? First, the star itself is very large star with a mass of 4.5 times that of the Sun. Those are relatively rare, but are seen long distances across the Galaxy. Second, the transverse velocity is less than one kilometre per second! If this is orbiting around this star, then it’s at a distance of over 13000 au from the star! Deep into Oort Cloud territory. This means that the chance of seeing such an eclipse is tiny, around one in ten thousand for all the solar type stars in the ASAS-SN survey size and duration. That’s an uncomfortable problem and we don’t know the answer as to why we’re seeing this now.

What’s tantalizing is that the MCMC fitting also found solutions with a cleared ring gap between the two main rings (squint at around 59200 days to see this additional gap in the rings). It’s compelling, but not confirmed. There is also multi band data coming in as well, so in future research we can see if the material is sub-micron sized or not. More of these unusual systems are being discovered in ASAS-SN, and with other large survey telescopes coming online soon (think the Vera C. Rubin Observatory) many more of these systems might be discovered. Watch this space!

The paper is published in A&A as ASASSN-21js: A multi-year transit of a ringed disc by Pramono, Kenworthy and van Boekel. #astrodon 🔭🪐

Fortunately it hasn’t impacted my friends and relatives directly, but they know something’s not right. They catch me with one eye closed, staring furiously at an otherwise innocent appliance, be it a computer screen, a phone surface, or the kitchen counter illuminated by the light shining through a plastic container. I reluctantly bought a refurbished iPhone 13, because the 8 was dying and I wanted a better camera for photos (Ask me about how I obsessed over comatic aberrations in the blue channel of early 2000’s Canon cameras!) and the phone I received works perfectly…. but I can very clearly see that the front glass plate is bowed in around the edges. Out of the corner of my eye I can see the distortion of distant parallel lines take on the distinctive S shape of a bowl-like distortion, and it distracts me no end. I can’t help myself. It’s not disruptive enough to go and fix it, partially because it gives me inspiration in my daily work too. There have been a few times where I’ve made a casual observation about some optical phenomenon and then had quizzical looks from fellow academics - “What, you haven’t seen the diffraction pattern from a sodium street lamp though a net curtain before?” and I keep my observations to myself. My point is that there is a tremendous amount of wonder in the physical world, if you look for it. Putting on a protective screen on the curved iPhone surface meant that it did not adhere correctly…. but then I could use the pressure my fingers to get the air gap within a few wavelengths of light and treat myself to Newton’s fringes first hand, transient colours under white light. What’s not to love about that?

Last week I was in Bern participating in a Workshop on #exocomets, at the #ISSI on top of the hill behind the University. We discussed chemistry, the number of detections, and what to expect in the next decade or so. I gave a review on detection methods - transit, spectroscopy and maybe direct detection in the not too distant future! I’m interested in setting up ground based observations during the upcoming PLATO mission which will stare at Beta Pictoris for two to three years. This is a unique opportunity to get spectra from the ground during a large exocomet transit towards this famous star.