arXiv subjects

Z. Berta-Thompson

Publications and source records attributed to Z. Berta-Thompson.

5 recordsLinked to original sources

arXiv2026

A Panchromatic JWST Spectrum of a Giant Starspot on the Fully Convective M-dwarf TOI-3884

TOI-3884 b is a rare super-Neptune transiting a fully convective M dwarf that hosts a persistent giant polar spot. Because the planet occults this active region during every transit, the system offers a unique laboratory to directly probe the stellar surface and spot properties. We present seven James Webb Space Telescope (JWST) transits of TOI-3884 b observed with NIRISS and NIRSpec (spanning 0.6--5.3$\mu$m). While all visits show a recurring spot-crossing signature, each transit exhibits a distinct spot-crossing morphology, enabling us to infer a stellar rotation period of $P$=11.102$\pm$0.003d and tightly constrain the pole-on stellar orientation ($i_{*}$=139.2$\pm$0.3$^{\circ}$, $\lambda_{*}$=31.1$\pm$0.4$^{\circ}$) and spot properties ($R_{\rm{spot}}=0.576^{+0.006}_{-0.005}$R$_{*}$, $\phi_{\rm{spot}}$=-84.69$\pm$0.12$^{\circ}$).We leverage this orbital configuration to measure the first empirical panchromatic spectrum of an M dwarf starspot with JWST, establishing a direct observational benchmark for stellar atmosphere models in the fully convective regime. Comparison with 1D NewEra and SPHINX atmosphere models indicates that the spot is 183$\pm$1K cooler than the photosphere, consistent with previous ground-based measurements and expectations for mid M dwarf spot contrasts. While the models reproduce the observed contrasts at wavelengths longer than 1$\mu$m, they significantly underpredict the contrasts at shorter wavelengths. These results demonstrate that M dwarf stellar atmosphere models may not fully capture the wavelength dependence of stellar contamination in transmission spectra and highlight the importance of empirical spot spectra for robust interpretation of planetary atmospheres, particularly in the optical.

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arXiv2025

Quantifying the Impact of Starspot-Crossing Events on Retrieved Parameters from Transit Lightcurves

Starspot-crossing events (SCEs) in exoplanet transit lightcurves are becoming increasingly common as we focus on cooler host stars and observe higher precision photometric and spectroscopic lightcurves. In this work, we explore how these events affect our retrievals of transit depths and the accuracy with which we can derive spot properties. We inject and recover synthetic SCEs in photometric lightcurves using starry. We find that for high signal-to-noise ratio SCEs, we constrain the spot longitudes tightly (>80% within 1{\deg} of the true value) but degeneracies complicate retrieving spot contrasts, radii, and latitudes (within 17%, 19%, and 9{\deg} respectively). On average, the difference between injected and recovered transit depths is 0.78% or 78.3 ppm. In most (80%) injections, we recover the transit depth to within 0.6%. For transit depths inflated >1.3% by the transit light source effect (TLSE), fitting for a spot crossing improves the transit depth retrieval over masking the SCE in >95% of cases. However, we find that for spots with small contrasts (<5%) and/or covering fractions (<2%), we are likely to overcorrect for the TLSE, recovering a worse transit depth than simply masking. In addition, even when fitted, we find SCEs can inflate the uncertainties on recovered transit depths significantly, especially for JWST-like precisions. Finally, we present a new method, using SCE observables, of carving out the degenerate spot parameter space that provides informative priors for Markov Chain Monte Carlo sampling, demonstrating this technique on a real SCE observed in Kepler-51d's lightcurve.

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arXiv2019

WASP-4b Arrived Early for the TESS Mission

The Transiting Exoplanet Survey Satellite (TESS) recently observed 18 transits of the hot Jupiter WASP-4b. The sequence of transits occurred 81.6 $\pm$ 11.7 seconds earlier than had been predicted, based on data stretching back to 2007. This is unlikely to be the result of a clock error, because TESS observations of other hot Jupiters (WASP-6b, 18b, and 46b) are compatible with a constant period, ruling out an 81.6-second offset at the 6.4$σ$ level. The 1.3-day orbital period of WASP-4b appears to be decreasing at a rate of $\dot{P} = -12.6 \pm 1.2$ milliseconds per year. The apparent period change might be caused by tidal orbital decay or apsidal precession, although both interpretations have shortcomings. The gravitational influence of a third body is another possibility, though at present there is minimal evidence for such a body. Further observations are needed to confirm and understand the timing variation.

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arXiv2019

A Hot Ultraviolet Flare on the M Dwarf Star GJ 674

As part of the Mega MUSCLES Hubble Space Telescope (HST) Treasury program, we obtained time-series ultraviolet spectroscopy of the M2.5V star, GJ~674. During the FUV monitoring observations, the target exhibited several small flares and one large flare (E_FUV = 10^{30.75} ergs) that persisted over the entirety of a HST orbit and had an equivalent duration >30,000 sec, comparable to the highest relative amplitude event previously recorded in the FUV. The flare spectrum exhibited enhanced line emission from chromospheric, transition region, and coronal transitions and a blue FUV continuum with an unprecedented color temperature of T_c ~ 40,000+/-10,000 K. In this paper, we compare the flare FUV continuum emission with parameterizations of radiative hydrodynamic model atmospheres of M star flares. We find that the observed flare continuum can be reproduced using flare models but only with the ad hoc addition of hot, dense emitting component. This observation demonstrates that flares with hot FUV continuum temperatures and significant EUV/FUV energy deposition will continue to be of importance to exoplanet atmospheric chemistry and heating even as the host M dwarfs age beyond their most active evolutionary phases.

astro-ph.SR↗

arXiv2018

Radial velocity follow-up of GJ1132 with HARPS. A precise mass for planet 'b' and the discovery of a second planet

GJ1132 is a nearby red dwarf known to host a transiting Earth-size planet. After its initial detection, we pursued an intense follow-up with the HARPS velocimeter. We now confirm the detection of GJ1132b with radial velocities only. We refined its orbital parameters and, in particular, its mass ($m_b = 1.66\pm0.23 M_\oplus$), density ($ρ_b = 6.3\pm1.3$ g.cm$^{-3}$) and eccentricity ($e_b < 0.22 $; 95\%). We also detect at least one more planet in the system. GJ1132c is a super-Earth with period $P_c = 8.93\pm0.01$ days and minimum mass $m_c \sin i_c = 2.64\pm0.44~M_\oplus$. Receiving about 1.9 times more flux than Earth in our solar system, its equilibrium temperature is that of a temperate planet ($T_{eq}=230-300$ K for albedos $A=0.75-0.00$) and places GJ1132c near the inner edge of the so-called habitable zone. Despite an a priori favourable orientation for the system, $Spitzer$ observations reject most transit configurations, leaving a posterior probability $<1\%$ that GJ1132c transits. GJ1132(d) is a third signal with period $P_d = 177\pm5$ days attributed to either a planet candidate with minimum mass $m_d \sin i_d = 8.4^{+1.7}_{-2.5}~M_\oplus$ or stellar activity. (abridged)

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