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KRONOS II: Solar-like Umbra and Penumbra Properties on the Young Sun V1298~Tau

Using JWST NIRISS/SOSS transit observations of the young solar analog V1298 Tau, researchers characterized the thermal properties of 14 starspot crossings, revealing that these spots possess distinct umbra and penumbra components with temperatures and area ratios remarkably similar to those of sunspots on the mature Sun, suggesting that the fundamental substructure of surface heterogeneities on Sun-like stars remains consistent throughout their lifetimes.

Original authors: Matthew M. Murphy, Adina D. Feinstein, Meir E. Schochet, Benjamin V. Rackham, Luis Welbanks, Darryl Z. Seligman, Girish M. Duvvuri, Eva-Maria Ahrer, John H. Livingston, Saugata Barat, Jean-Michel Dése

Published 2026-06-16

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Matthew M. Murphy, Adina D. Feinstein, Meir E. Schochet, Benjamin V. Rackham, Luis Welbanks, Darryl Z. Seligman, Girish M. Duvvuri, Eva-Maria Ahrer, John H. Livingston, Saugata Barat, Jean-Michel Désert, Rafael Luque, Catriona Murray, Matthew C. Nixon, Sydney Petz, Michael Radica, Hinna Shivkumar

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun as a giant, glowing orange. Now, imagine that orange isn't perfectly smooth; it has dark, cooler patches on its skin, kind of like freckles or bruises. On our own Sun, we call these "sunspots." For a long time, we've only been able to see these spots clearly on our own Sun because it's so close. For all other stars, they are just tiny, distant points of light, making it impossible to see their "freckles" in detail.

This paper is about a team of astronomers using the James Webb Space Telescope (JWST) to finally get a close-up look at the "freckles" of a young star called V1298 Tau. This star is a "baby" version of our Sun, only about 20 to 30 million years old (our Sun is 4.6 billion). Because it's young, it's much more active and covered in spots than our current Sun.

Here is how they did it and what they found, explained simply:

The "Shadow Puppet" Trick

Usually, to see a spot on a star, you have to wait for the star to rotate and bring the spot into view. But this team had a special trick up their sleeves. They were watching three planets orbiting V1298 Tau.

Imagine holding a flashlight (the star) and a marble (the planet) in front of it. As the marble moves across the flashlight beam, it blocks the light. But if the flashlight has a dark smudge on it, and the marble passes over that smudge, the total light doesn't drop as much as it should. In fact, it actually gets a tiny bit brighter for a split second because the marble is blocking a dark spot instead of the bright light around it.

The astronomers watched these planets cross the face of V1298 Tau. When a planet crossed a "freckle" (a starspot), it created a little bump in the data. By studying these bumps, they could map where the spots were and what they looked like.

The "Two-Tone" Freckle

For a long time, scientists thought starspots were just one uniform dark color. But our Sun's spots are actually more complex. They have a very dark center called the umbra (like the deep center of a bruise) and a lighter, fuzzy ring around it called the penumbra (like the fading edge of the bruise).

The big discovery in this paper is that V1298 Tau's spots have this same two-tone structure.

By looking at the light through different colors (wavelengths), the team realized that a simple "one-color" model didn't fit the data. They had to use a model that included both the dark center and the lighter ring.

The Center (Umbra): Is very cold, about 3,200–3,400 degrees.
The Ring (Penumbra): Is warmer, about 4,400–4,600 degrees.
The Star's Skin (Photosphere): Is the hottest, at about 4,880 degrees.

This is the first time anyone has seen this specific "center-and-ring" structure on a star other than our Sun, and it's the first time JWST has helped us see it.

A Baby Sun that Acts Like an Old Sun

The most surprising part of the story is the comparison to our own Sun.

The Size: The spots on this baby star are huge. Some are as big as 24 Earths lined up! Our Sun's biggest spots are only about 9 Earths wide.
The Temperature: Even though the spots are huge and the star is young, the temperature difference between the spot and the star's surface is almost exactly the same as it is on our modern, older Sun.

Think of it like this: If you look at a toddler and an adult, they might have different amounts of acne (the toddler has more), but the actual "pimple" looks and feels the same on both of them. This suggests that the physics of how these spots form doesn't change as a star gets older; only the number of spots changes.

The Global Map

The team didn't just look at the spots the planets crossed. They also used ground-based telescopes to watch the whole star rotate over several weeks. This allowed them to map the "invisible" spots that the planets didn't cross. They found that the star is covered in a lot of activity, with large groups of spots swirling around the equator, much like a stormy weather system.

The Bottom Line

This paper shows that even though V1298 Tau is a wild, young, and spot-covered star, the tiny details of its spots (the dark centers and lighter rings) are surprisingly similar to the spots on our own Sun today. It proves that the James Webb Space Telescope is a powerful tool for not just finding planets, but for taking a "microscope" to the surface of distant stars, revealing that the rules of stellar weather might be the same for stars of all ages.

Technical Summary: KRONOS II: Solar-like Umbra and Penumbra Properties on the Young Sun V1298 Tau

Problem and Motivation
Young stars exhibit significantly higher magnetic activity than their main-sequence counterparts, manifesting as strong global magnetic fields, enhanced flare rates, and photometric variability driven by starspots. While starspot crossing events (SCEs)—where a transiting planet occults a starspot—have been observed for two decades, previous constraints on spot properties were limited by degeneracies in light curve modeling and a lack of multi-wavelength data. Specifically, the internal substructure of starspots (umbrae and penumbrae) has been spatially resolved only for the Sun and, more recently, EK Draconis via Doppler imaging. For the vast majority of stars, the total number, sizes, locations, and thermal contrasts of spots remain unconstrained. This paper addresses the challenge of characterizing the substructure of starspots on a young solar analog using high-precision, multi-wavelength transit observations.

Methodology
The authors analyzed JWST NIRISS/SOSS transit observations of the young (20–30 Myr) solar analog V1298 Tau and its transiting planets (b, c, and d) taken during two separate visits in September 2025.

Data Reduction: The team utilized the exoTEDRF pipeline to reduce time-series spectroscopy (0.8–2.8 µm) and combined this with ground-based multi-band photometry ( $g'$ and $r'$ ) from the Las Cumbres Observatory (LCOGT) to model global stellar heterogeneities.
Spectral Analysis: Out-of-transit stellar spectra were fitted using the NewEra grid with 1-, 2-, and 3-component models (photosphere, cool spots, hot faculae) to determine global covering fractions and temperatures.
Light Curve Modeling: Transit light curves were modeled using two approaches: Gaussian profiles for SCEs and the fleck model, which treats spots as circular regions with specific radii, positions, and contrast.
Contrast Spectra Derivation: By measuring the amplitude of SCEs as a function of wavelength, the authors derived starspot contrast spectra.
Thermal Modeling: The contrast spectra were fitted against stellar model grids to test 2-component (photosphere + single spot temperature) versus 3-component (photosphere + umbra + penumbra) models. The 3-component model included parameters for photospheric temperature ( $T_{phot}$ ), umbral temperature ( $T_{umb}$ ), penumbral temperature ( $T_{pen}$ ), and the umbral-to-penumbral area ratio ( $A_u/A_p$ ).
Global Context: The occulted spot distribution derived from JWST was combined with LCOGT rotational light curves using the fleck framework to infer the presence and distribution of unocculted active regions.

Key Results

Spot Substructure Resolution: The observed contrast spectra (0.8–2.8 µm) could not be reproduced by 2-component models. A 3-component model including distinct umbral and penumbral components was statistically required (Bayes factors > 12) to explain the data.
Derived Temperatures: The analysis yielded a photospheric temperature of $T_{phot} = 4876 \pm 20$ $T_{p h o t} = 4876 \pm 20$ K. The starspots were resolved into:
- Umbrae: $T_{umb} = 3265–3436$ K.
- Penumbrae: $T_{pen} = 4388–4659$ K.
- The umbral-to-penumbral area ratio ( $A_u/A_p$ ) was found to be $\sim 28–31\%$ .
Solar Analogy: The derived temperature deficits ( $\Delta T_{umb} \approx 1457–1636$ K; $\Delta T_{pen} \approx 234–427$ K) and the linear relationship between spot contrast and the $A_u/A_p$ ratio are consistent with observations of sunspots on the modern Sun. This suggests that the physical mechanisms driving spot substructures on young solar analogs are similar to those on the Sun, despite the higher overall spot coverage on young stars.
Global Spot Distribution: Combining JWST and LCOGT data revealed that while the transit chords crossed at least 14 distinct spots, the global photometric variability required the presence of at least 5 additional large, unocculted active regions. The total spot coverage fraction was estimated to be $\sim 15–23\%$ , with a significant portion being unocculted.
Radius Discrepancies: The authors demonstrated that the historically observed variations in the radii of V1298 Tau's planets (between TESS and K2 epochs) can be explained by a time-variable starspot coverage fraction inducing a transit light source effect, potentially driven by a $\sim 1.5$ -year activity cycle.

Significance and Claims
The paper claims that V1298 Tau is only the second star (after the Sun) for which spot substructures (umbrae and penumbrae) have been characterized, and the first to do so using JWST exoplanet transit observations. The authors assert that these results demonstrate that JWST transit spectroscopy is a viable and powerful method for resolving starspot substructure on stars other than the Sun.

Furthermore, the study suggests that while the prevalence of starspots is significantly higher on young stars, the thermal and geometric properties of the spots themselves (specifically the temperature contrasts and the relationship between umbra and penumbra) may remain consistent throughout a star's lifetime. This finding implies that the underlying physical mechanisms driving magnetic activity and spot formation do not fundamentally change between the pre-main-sequence and main-sequence phases for solar analogs. The authors conclude that these measurements provide crucial empirical constraints for stellar models and highlight the importance of accounting for spot substructure when interpreting exoplanet transmission spectra to avoid biases.

The "Shadow Puppet" Trick

The "Two-Tone" Freckle

A Baby Sun that Acts Like an Old Sun

The Global Map

The Bottom Line

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