Starspot temperature of CoRoT-2 from multiwavelength observations with SPARC4

TL;DR

Starspots on active stars like CoRoT-2 complicate precise exoplanet transit measurements and illuminate stellar magnetic dynamos. The paper combines simultaneous four-band transit photometry from SPARC4 with the ECLIPSE spot-transport model and MCMC fitting to estimate spot temperatures and sizes. Three independent temperature-estimation approaches—blackbody fits, PHOENIX atmosphere models, and a joint multiwavelength fit—yield consistent temperatures around 5.0–5.3 kK and reveal large spot radii, consistent with penumbral-like regions. This methodology improves constraints on starspot properties, aiding exoplanet characterization and stellar activity studies, with implications for future missions such as ARIEL.

Abstract

Measuring starspot temperatures is crucial for understanding stellar magnetic activity, as it affects stellar brightness variations, influences exoplanet transit measurements, and provides constraints on the physical conditions and energy transport in active regions, offering insights into stellar dynamos. Our goal is to determine the temperature of starspots on the active star CoRoT-2 to enhance our understanding of magnetic activity in young, solar-like stars. Multiwavelength observations were conducted using the SPARC4 instrument on the 1.6-m telescope at Pico dos Dias Observatory (Brazil), capturing simultaneous transit data in four photometric bands (g, r, i, and z). The ECLIPSE model, combined with MCMC fitting, was used to model spot characteristics during the planetary transit of CoRoT-2 b. The spot intensities were analyzed considering three different methods: the assumption of blackbody emission, the PHOENIX atmospheric model, and multiwavelength fitting assuming the same spot parameters for all wavelengths. Two starspots were detected in the residuals of the light curve, yielding temperature estimates of 5040 - 5280 K based on the three different methods. These values align more closely with the temperatures of solar penumbrae than with typical umbral temperatures, suggesting relatively moderate magnetic activity. The radius of the spots ranged from 0.34 - 0.61 the planetary radius, or equivalently (38 - 69)$\times10^6$m, much larger than sunspots. This study provides a method to estimate spot temperatures on active stars using multiband photometry, with results indicating penumbral-like temperatures on CoRoT-2. The methodology enhances precision in starspot temperature estimation, beneficial for studies of stellar activity and exoplanet characterization.

Figures (6)

• Figure 1: Light curves of the transit of CoRoT-2 b in the four photometric bands of SPARC4, with the smoothed data shown in green and a model of a spotless star depicted in blue. The bottom panels of each waveband shows the residuals, after subtraction of a spotless star model (blue curve in the top panel). The time at transit center is 2,460,507.10 BJD. The red curves show the result of the model fitting for two spots. The horizontal dashed lines represent a 2$\sigma$ threshold for the signal to be considered due to a spot ($1\sigma = 930, 530, 580,$ and $570$ ppm for the smoothed signal from bands g, r, i, and z, respectively).
• Figure 2: Starspot temperature estimation using blackbody fitting. The red and orange dots represent the observed spot intensities in the four photometric bands (g, r, i, and z), whereas the red and orange curves correspond to the best-fit blackbody spectra, yielding temperatures of $5110\pm140$ K for spot 1 (longitude of $20^\circ$) and $5040\pm190$ K for spot 2 (longitude of $-50^\circ$). The stellar photosphere is assumed to have an effective temperature of 5529 K (green curve).
• Figure 3: Starspot temperature estimation using PHOENIX stellar atmosphere models. The blue curve shows the PHOENIX spectrum for a 5529 K star (log(g) = 4.48, [Fe/H] = -0.04, [$\alpha$/M] = 0.0). The red dots represent the observed spot intensities in the four photometric bands (g, r, i, and z). The red and orange curves correspond to the best-fit PHOENIX spectra, yielding temperatures of $5130\pm120$ K for spot 1 (longitude of $20^\circ$) and $5060\pm200$ K for spot 2 (longitude of $-50^\circ$).
• Figure 4: Results of the MCMC fit to spot at longitude $20^\circ$ in the residuals of the transit light curve for the four photometric bands. The fit parameters of the spot are: radius (in units of planetary radius), intensity, and longitude. The plots show the MCMC samples and the respective posterior distributions of each free parameter.
• Figure 5: Same as Fig. \ref{['fig:spot1']} for the second spot at longitude $-50^\circ$.