I SPI with My Little Eye… a Planetary Magnetic Field?

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Title: Flares, Rotation, Activity Cycles and a Magnetic Star-Planet Interaction Hypothesis for the Far Ultraviolet Emission of GJ 436
Authors: R. O. Parke Loyd et al.
First Author’s Institution: Eureka Scientific
Status: Published in AJ

You Go, GJ 436

GJ 436 is an old, fairly typical dwarf star orbited by a short-period Neptune-sized planet, GJ 436b. GJ 436b is a pretty famous planet among those who research gas giants with orbits faster than that of Mercury around the Sun. It has become a high-interest target for JWST with the hopes of characterizing its hot atmosphere. The type, intensity, duration, and frequency of stellar activity all impact the observations and physical evolution of planetary atmospheres. This motivates stellar astronomers, including the authors of today’s article, to understand what stellar activity looks like for stars like GJ 436.

Stellar activity can take many forms, as listed in the title of today’s article: flares, stellar rotation, stellar activity cycles, and magnetic star-planet interactions.

  • Flares are bursts of activity that typically occur on short timescales (minutes to hours).
  • Stellar rotation refers to the changing view of the surface of a star as it spins, usually over the course of days to weeks.
  • A star’s activity cycle is a long-term (several years) shift between low and high amounts of activity.
  • Magnetic star–planet interactions, or SPI, are a proposed type of activity that can occur when a planet orbits close enough to its host star that its magnetic field interacts with the star’s magnetic field.

All forms of activity lead to changes in a star’s spectrum — the amount and energy of light emitted — which can then lead to changes in an orbiting planet’s atmosphere.

Help Me, Hubble Space Telescope, You’re My Only Hope

Today’s authors focus specifically on the far-ultraviolet portion of GJ 436’s spectrum. The far ultraviolet covers wavelengths between 1150 and 1450 Angstroms — highly energetic light that would give you one heck of a sunburn if Earth’s atmosphere did not protect us from it. The far ultraviolet is important because it drives the creation or destruction of atmospheric molecules (i.e., atmospheric photochemistry). This means that stellar and planetary astronomers are interested in how the far ultraviolet varies with stellar activity. To observe this region of the electromagnetic spectrum, one needs the Hubble Space Telescope, as it is currently the only observatory with the ability to observe ultraviolet light at high resolution.

The authors compiled all far-ultraviolet observations of GJ 436 taken with Hubble’s Cosmic Origins Spectrograph. This includes three separate groups of observations that span a total of 5.5 years, which allows the authors to look for stellar activity on short and long timescales. Figure 1 is an average spectrum of GJ 436. The authors added up all the far-ultraviolet light, excluding the gray regions shown in Figure 1, to see how GJ 436’s output at those wavelengths changed over the 5.5 years of observations. The emission line behavior over time was analyzed separately.

Average spectrum of GJ 436

Figure 1: Average spectrum of GJ 436 observed by Hubble with the Cosmic Origins Spectrograph. Gray regions mark the areas of contamination from Earth’s reflected light. Each strong emission line is labeled. [Loyd et al. 2023]

A Flare for the Magnetic

Emission lines like those of silicon and nitrogen are sensitive to flaring behavior, so the authors identified all fourteen flares present in the data by looking at the total light in all of the far-ultraviolet emission lines over time. The authors calculated the durations and energies of each flare, included in Table 1 in today’s article. The flares were then removed from the data so the authors could look for the other forms of variability.

The authors used visible-light observations from Fairborn Observatory’s Automatic Photoelectric Telescope to measure the periods of the star’s rotation and the star’s activity cycle. Using the periods measured from the visible-light observations, the authors fit sine waves to the far-ultraviolet light curves for stellar rotation and activity cycle. They allowed the amplitudes and phases of those sine waves to vary so they could find the best fit. They also included a potential third sine wave with a period equal to the orbital period of the planet to search for any existing magnetic star–planet interactions.

Results and the Bigger Picture

Unfortunately, no magnetic star–planet interactions were directly detected in the far-ultraviolet data presented in this work, though the authors were able to place an upper limit on the planetary magnetic field of 10 Gauss (Earth’s magnetic field strength is around 0.5 Gauss). However, the team did detect a bunch of frequent, low-energy flares. Stars like GJ 436 typically exhibit more energetic flares, so the existence of these lower-energy flares may be a hint of magnetic star–planet interactions! More work on star–planet interaction signatures and disentangling them from plain ol’ stellar activity is needed to tease out the answer.

The variability from all of the forms of activity explored in today’s article were compared to each other, as shown in Figure 2. The stellar activity cycle dominates GJ 436’s far-ultraviolet variability, followed by flares and noise. This means that the largest changes in GJ 436’s far-ultraviolet emission come from the star’s activity cycle.

variability amplitude of different forms of activity in different regions of the far-ultraviolet spectrum

Figure 2: The variability amplitude — or the contribution to the change in the far-ultraviolet emission — for each form of activity investigated by today’s article, separated by each region of the far-ultraviolet spectrum. [Loyd et al. 2023]

GJ 436 is a typical planet-hosting star — meaning that how it behaves is representative of most planet-hosting stars. The results from today’s article show that most older planet-hosting stars have ultraviolet emission that is likely dominated by their activity cycles. This means that exoplanet astronomers can interpret current exoplanet observations knowing the planets likely experienced this history within their stellar environment, contributing to their atmospheric evolution.

Original astrobite edited by Archana Aravindan.

About the author, Keighley Rockcliffe:

Keighley is a PhD candidate at Dartmouth College, which resides on unceded Abenaki land. She studies young exoplanet atmospheres with Dr. Elisabeth Newton. She firmly believes in making science a more inclusive space for all humans, especially those traditionally excluded and oppressed. Keighley loves to meet and support people, so please reach out to chat!