- The paper demonstrates that JWST transit spectroscopy can resolve starspot substructure on V1298 Tau, finding umbral temperatures of about 3,264–3,436 K, penumbral temperatures of 4,388–4,659 K, and umbra-to-penumbra area ratios near 0.3.
- The analysis combines JWST/NIRISS transit spectra with LCOGT photometry and shows that a single-temperature spot model fails, while three-component models and MURaM simulations support thermal properties broadly consistent with solar sunspots.
- The results suggest that spot thermal structure remains Sun-like on this highly active 20–30 Myr solar analog, while unresolved spot clusters, uncertain activity cycles, and discrepancies between disk-integrated and occulted spots remain important challenges.
Overview and motivation
This paper presents a starspot characterization of the young (~20–30 Myr) solar analog V1298 Tau using JWST NIRISS/SOSS transit observations of its planets b, c, and d, combined with contemporaneous Las Cumbres Observatory (LCOGT) multi-band photometry. The central result is that the starspot contrast spectra measured from starspot crossing events (SCEs) cannot be explained by a single-temperature spot model; they require decomposition into umbral and penumbral components with temperatures broadly consistent with sunspots. This makes V1298 Tau only the second star other than the Sun for which spot substructure has been characterized, and the first via JWST exoplanet transit spectroscopy.
Observations and data reduction
Two JWST visits were analyzed: Visit 1 (2025 September 6), a transit of planet c, and Visit 2 (2025 September 10–11), simultaneous transits of planets d and b, separated by four days—roughly one stellar rotation period (Prot=2.97 days). The data were reduced with exoTEDRF using an iterative scheme for bad-pixel masking, group-level background subtraction, and scale-achromatic $1/f$ correction. No significant systematic PCA components were identified beyond additional hot pixels. Contemporaneous LCOGT Sinistro g′ and r′ photometry spanning 2025 August 28–September 16 provided rotational modulation context across multiple rotation periods.
Disk-integrated stellar spectrum
Fitting the out-of-transit flux-calibrated spectra with up to three components (photosphere, cool spots, hot faculae/plages) drawn from the NewEra grid strongly favors the three-component model by Bayesian evidence in both visits. The derived properties are consistent between visits: Tphot≈4876±20 K, Tcool≈3400 K, and Thot≈5646 K, with the hot component covering ~30% of the disk. The cool covering fraction decreased from ~19–23% to ~15–18% between visits, consistent with the star being uniformly ~1.6% brighter during Visit 2. A caveat is that the best-fit stellar radius is sensitive to the assumed interstellar extinction, with discrepancies up to 4.5σ within a visit between fixed (AV=0.074) and free extinction cases; free AV tends toward zero, likely degenerate with R⋆.
Broadband light curves and occulted spot geometry
The broadband SCEs were modeled both as Gaussian profiles and with the fleck starspot model, achieving residuals of 117–138 ppm that pass Anderson–Darling normality tests. Fourteen distinct SCEs were identified: six along the chord of planet c in Visit 1 and at least eight more in Visit 2 along the chords of d and b. Correcting for rotational smearing during crossings, the occulted active regions span corrected lengths of roughly 3.7–24.6 $1/f$0, exceeding the largest individual sunspots (~9.5 $1/f$1); the authors note these may be unresolved clusters of smaller spots, and fleck's circular-spot assumption biases radii accordingly. The spots lie within ±33° of the equator, analogous to sunspot latitudes, though detections are limited to transit chords. Occulted spots account for ~27–31% of total spot coverage in Visit 1 but ~64–80% in Visit 2.
Contrast spectra and umbral/penumbral decomposition
The key methodological result is that two-component (photosphere + single-temperature spot) fits fail: they yield photosphere temperatures discrepant by 11σ between visits and inconsistent with the out-of-transit spectrum, overpredict the short-wavelength slope, and are disfavored by ΔBIC ≈ 40 and Bayes factors $1/f$2 of 23.5 and 128.8 relative to three-component models. The three-component model yields:
| Quantity |
Visit 1 |
Visit 2 |
| $1/f$3 |
4877±20 K |
4877±20 K |
| $1/f$4 |
3436$1/f$5 K |
3264$1/f$6 K |
| $1/f$7 |
4659$1/f$8 K |
4388$1/f$9 K |
| g′0 |
0.28g′1 |
0.30g′2 |
These results are robust to substitution of the Phoenix grid for NewEra. The MURaM 3D radiative-MHD spot spectra provide independent support: for Visit 2, the K0 model with g′3 fits well (reduced χ² = 1.4), consistent with the empirical fit within ~0.7σ. The umbral temperature deficits relative to the photosphere (1457±244 K and 1636±163 K) fall squarely within the solar range (972–1872 K), while penumbral deficits (234±99 K and 427±53 K) bracket the solar value (272–372 K). The implication is that spot substructure thermal physics on this pre-main-sequence solar analog resembles that of the modern Sun, even though spot prevalence is far higher.
Comparison to the Sun and EK Draconis
Individual spot contrasts correlate linearly with g′4, as observed for Solar Cycle 22 sunspots. The Visit 2 relation is within 2σ of the solar trend, and a two-dimensional Kolmogorov–Smirnov test cannot reject that Visit 2 measurements derive from the same distribution as sunspots (g′5); Visit 1 is marginally inconsistent (g′6), possibly due to scatter from two high-g′7 outliers. Derived total-spot-to-umbra area ratios (g′8–4.3) overlap the solar range [4.0, 5.9] only partially. Compared to EK Draconis (~50 Myr, Doppler-imaged umbra/penumbra deficits of 990 K and 180 K), V1298 Tau shows somewhat larger deficits, though the authors caution this could reflect measurement or intrinsic differences.
Global spot distribution and radius discrepancies
Jointly fitting the LCOGT g′9/r′0 rotational modulation with the JWST-derived occulted spots fixed requires 5–6 additional large unocculted regions, preferentially near the equator and filling longitudes unprobed by the transits. These likely represent spot clusters or active regions rather than single spots. The paper also shows that decade-long variations in measured r′1 for planets b, c, and d can be reproduced by a sinusoidal spot coverage variation with a hypothesized ~1.5 yr activity cycle via the transit light source effect—a cycle length adopted from empirical scaling relations, not independently constrained, and the forward model predicts an unobserved peak coverage near 75%.
Limitations and open questions
Several caveats bear directly on the results. The fleck light curve model assumes uniform-contrast circular spots even while the contrast analysis resolves umbra and penumbra, which may bias inferred contrasts. Latitudes are poorly constrained by chord symmetry, and spot sizes are degenerate with unresolved clustering. The discrepancy between the disk-integrated spectrum (whose cool component matches the umbra temperature despite penumbrae covering ~70% of spot area) and the contrast-derived properties remains unresolved; a four-component spectral fit is prior-dominated, and reconciling the datasets requires either implausibly high disk-averaged r′2 or an overabundance of umbral pores. Whether the Visit 1 versus Visit 2 contrast offset (~Δα ≈ 0.1) is astrophysical or systematic is undetermined. Finally, the activity cycle length and the true maximum spot coverage remain unconstrained.
Conclusion
This work demonstrates that JWST transit spectroscopy can resolve starspot substructure on stars other than the Sun. For V1298 Tau, umbral and penumbral temperatures, umbral area fractions, and the contrast–area-ratio relation are consistent with sunspot properties, suggesting that while spot coverage evolves strongly with age and activity state, the underlying thermal structure of spots may persist across a Sun-like star's lifetime. Open questions include the physical origin of the missing penumbral contribution to the disk-integrated spectrum, the systematics of visit-to-visit contrast offsets, and the applicability of the technique to other spotted systems observed with JWST.