Evidence for Centrifugal Breakout around the Young Star TIC 234284556

Elsa Palumbo, Caltech 2023

15 July 2021

Figure 1: TESS light curve folded on the stellar rotation period. TIC 234284556's dip evolves throughout Sectors 1 and 27, quickly disappearing between Sectors 27 and 28. A +.04 vertical offset is added for each new rotational period, so that time progresses upwards on the plot.
Young stars, those under ~100 million years old, tend to have very strong magnetic fields. As such, they have frequent stellar flares (sudden flashes of starlight), coronal mass ejections (CMEs; eruptions of plasma which escape from the star), and prominent starspots (dark magnetic spots like sunspots on our own Sun). Perhaps more surprisingly, the high magnetic activity of young stars can also show up in the destination of material from their stellar wind. In our own Solar System, ionized dust and/or gas from the stellar wind escapes the Sun's gravitational field, ending up in the interstellar medium. Around young stars with especially strong and stable magnetic fields, however, material from the stellar wind can be trapped by the star's magnetic field, accumulating in dense clumps of material called magnetospheric clouds. These clouds are co-rotating, meaning that the cloud's orbital period matches the star's rotational period. Since part of the star will be in the cloud's shadow when it eclipses the star, or moves in front of the star from our perspective, these magnetospheric clouds may show up as dips in a plot of how the light we receive from a star (its flux) varies over time.

Stellar winds continually stream out from a star, so material from it will continue to accumulate over time and, eventually, as the magnetic field drags this material along, the centrifugal force on the magnetic field will overwhelm the restoring force. So, in the end, the material must somehow escape from the centrifugal magnetosphere. How this escape occurs has been a subject of debate among astronomers. Stellar theorists, notably Townsend et al. (2005), have predicted a mass-loss mechanism known as centrifugal breakout, in which the ionized cloud becomes so massive that the magnetic loops constraining the material quickly snap. At this point, the previously trapped material would suddenly get expelled, with the magnetic field lines reconnecting immediately afterwards.

However, there's a catch. Because of limited empirical evidence for centrifugal breakout, including a non-detection in the candidate system σ Ori E, some alternative theories have been proposed. Using data from the NASA TESS mission, we have discovered variability in the observed brightness of a young (45-million-year-old) low-mass star, TIC 234284556, which offers strong supporting evidence to the centrifugal breakout narrative. TIC 234284556, which we present in a paper submitted to the AAS Journals, features a dip of variable depth (See Figure 1), indicating that the amount of material that is eclipsing the star changes over time.

Data from TESS shows a dip that is present over a 1 month timescale that then disappears over ~1 day, as shown in Figure 2 below. The dips themselves have nearly the same period as the star's rotation, as seen through starspot variability in the light curve (the big, nearly sinusoidal variability), suggesting that whatever is transiting the star is co-rotating.

Figure 2: A dip disappears in a potential centrifugal breakout event. Note the short timescales involved in the disappearance of the dip, whose location or projected location is identified by the dashed blue lines.
Spectra of the star obtained with the Veloce spectrograph at Siding Spring Observatory in Australia don't show significant variability in the star's hydrogen emission over a rotational period, which suggests the transiting material is opaque and dusty, like would be expected from a magnetospheric cloud, rather than a band of hydrogen plasma. Moreover, TIC 234284556's spectral energy distribution does not show excess emission at infrared wavelengths when compared to a model of a blackbody with a stellar atmosphere, suggesting the dips are unlikely to be caused by a partially dissipated protoplanetary disk, as in the so-called "dipper" stars.

When the dips disappear, we also see a sudden brightening event during the same ~1 day interval (Figure 3) and, based on the timing and the fact that its morphology is much more symmetric than the steep rise and exponential decay of a typical flare, we interpret this as evidence for the magnetic re-connection event that we'd expect to see accompanying breakout as a magnetospheric cloud dissipates.

Figure 3: TESS light curve at the end of Sector 27 (blue), with an observed brightening at the end of the sector. For comparison, the light curve one rotational period earlier is underlaid (gray). Note that this brightening event has a symmetric morphology unlike the steep rise and exponential decline typical of stellar flares. We consider this feature a potential post-breakout magnetic reconnection event.
Additional breakout events can explain two other features of our data: a change in the dip's position relative to the star's rotational signal between data taken ~2 years apart, as shown in the panels labeled "Sector 1" and "Sector 27" in Figure 1 and a reappearance of the dip ~100 days after it first disappeared, observed with ground-based photometry from the LCO global telescope network. Besides offering support support for the centrifugal breakout narrative, this star makes some other interesting contributions. First, since the standard description of centrifugal breakout was created with more massive stars and more powerful stellar winds in mind, TIC 234284556's low-mass makes us wonder whether CMEs, not stellar winds, might be the source of the material producing the dips. We also propose an explanation for the more gradual variations in the dip depth, duration, and shape that we can see in Figure 1. Since magnetic fields only interact with ionized particles, the dip size—a proxy for the amount of trapped material—depends on whether the material can stay ionized after leaving the star's high-temperature surface. We propose that, if the material continues to be heated by stellar flares, which are seen on TIC 234284556 in abundance, the material will continue to stay ionized, and therefore accumulate. In the absence of flares, however, the electrons and positively charged ions trapped by the star's magnetic field will start to recombine. As their presence will no longer be "felt" by a magnetic field when they are no longer charged, that material will gradually stop being constrained and the dip will start to decrease in size.

Finally, this star hints at the possibility of uniting a number of systems that have puzzled astronomers, including PTFO 8-8695, Rik-210, and other young stars observed by K2. In part because TIC 234284556 is far brighter than most of these systems, it is an excellent representative for this class of magnetically active young stars with eclipsing magnetosopheric clouds. Hopefully, TIC 234284556 will continue to give insights into magnetic activity around young stars in the years to come.

To read the full story, check out the paper at https://arxiv.org/abs/2107.05649.