The Activity of a Solar-Type Star TOI-1422 and Mass Loss of the Atmosphere of the Planet TOI-1422b

ISSN 1063-7729, Astronomy Reports, 2023, Vol. 67, No. 7, pp. 719–724. © The Author(s), 2023. This article is an open access publication. Russian Text © The Author(s), 2023, published in Astronomicheskii Zhurnal, 2023, Vol. 100, No. 7, pp. 605–610. The Activity of a Solar-Type Star TOI-1422 and Mass Loss of the Atmosphere of the Planet TOI-1422b I. S. Savanov* Institute of Astronomy of the Russian Academy of Sciences, Moscow, Russia *e-mail: Received March 25, 2023; revised April 21, 2023; accepted May 18, 2023 Abstract—The results of the analysis of the activity of a solar type star G2V TOI-1422 are presented and estimates of the loss of atmospheric matter of the planet TOI-1422b are made. The planet can be attributed to the type of hot Neptunes. Compared to other exoplanets of a similar mass range, it is expected that the planet has an extensive gas envelope. According to the long-term photometric survey of Kamogata Wide-field Survey (KWS) the TOI-1422 activity was analyzed and the existence of possible activity cycles of 1650–1680 days and 2450 days was suggested. The probable value of the rotation period P of the star lies in the range of 27 (+19, –8) days, observations of the KWS survey in the V filter indicate the most possible value for the period P = 32 days. Mass loss of the planet’s atmosphere is determined using an approximation formula corresponding to the model of atmospheric loss with an energy restriction. To estimate the flux of XUV photons analytical dependences were used linking the flux values and the parameter log RHK ' . Calculations have shown 8  that the atmosphere matter loss of TOI-1422b is M = 9.4 × 10 g/s; considering the existing errors in determining the parameters of the atmosphere it lies in the range from 6.8 × 108 to 1.4 × 109 g/s, and considering the uncertainty in the value of the chromospheric activity index—from 8.3 × 108 to 1.1 × 109 g/s. Keywords: exoplanets, mass loss DOI: 10.1134/S1063772923070089 1. INTRODUCTION Neptune-type planets are one of the main types of exoplanets and have chemical and physical characteristics that are between rocky planets and gas giant planets. The study of such exoplanets is relevant for understanding both the mechanisms of their formation and models of their evolution [1]. The authors of [2] studied the exoplanet candidate object TOI-1422b, whose transits were detected by the TESS space telescope, with the main goal of confirming its planetary nature and finding its properties. The solar-type parent star TOI-1422 (spectral class G2 V, V = 10.6m ) is located at a distance of 155 pc from us. In [2], additional observations of TOI-1422 were made with the HARPS-N spectrograph for 1.5 years to more accurately determine changes in its radial velocity (RV), which were analyzed together with TESS photometry and the results of other observations (including images with high spatial resolution). The performed analysis made it possible to refine the main parameters (mass, radius, luminosity, etc.) of TOI-1422 and to characterize the properties of TOI-1422b, as well as to suggest the presence of the Neptune-mass planet TOI-1422c in a more distant orbit, which was not detected in the TESS light curves. The inner planet, TOI-1422b, orbits with a period of 12.9972 ± 0.0006 days and has an equilibrium temperature of Teq,b = 867 ± 17 K. According to [2], the planet’s radius is Rb = 3.96+−0.13 0.11 R⊕ , the mass is Mb = 9.0+−2.3 2.0 M ⊕ and, consequently, the density is ρb = 3 0.795+−0.290 0.235 g/cm , i.e., the planet can be attributed to the type of hot Neptunes. Compared to other exoplanets of a similar mass range, TOI-1422b is one of the most “bloated”. The authors of [2] expect that this planet will have an extensive gas envelope surrounding the core with a mass fraction of about 10–25% of the total mass of the planet. The outer planet candidate, TOI-1422c, has an orbital period of 29.29+−0.21 0.20 days, its +2.6 minimum mass, M c sin i , is 11.1−2.3 M ⊕ , the equilibrium temperature is Teq,c = 661 ± 13 K, and therefore, if its type is confirmed, it could be considered another hot Neptune. The first part of this work presents the results of the analysis of TOI-1422 activity manifestations. In the second, the results obtained were used to estimate the loss of matter from the atmosphere of the planet TOI-1422b. 719 720 SAVANOV 2. MANIFESTATIONS OF TOI-1422 ACTIVITY According to the results of [2], the effective temperature of the star is 5840 ± 62 K, the logarithm of the acceleration of gravity log g = 4.41 ± 0.11, the radius is R /R = 1.019 ± 0.14 , the luminosity is L/L = 1.116 ± 0.037 and the mass is M /M  = 0.981 ± 0.06. The age of TOI-1422 is 5.1 ± 3.9 billions of years. The object is identified with the source Gaia EDR3 192 0333 4491 6951 6288, its parallax is π = 6.4418 ± 0.0138 mas. According to the long-term KWS,1 we analyzed the manifestations of TOI-1422 activity. The review presents observations of the star in filters V and Ic . First of all, we analyzed the data for the filter V , which has a longer observation interval, 4142 days (11.3 years) (HJD 245 5777.3–9919.9). A total of 1355 star brightness estimates in the V filter were considered. Presented in Fig. 1 (top), the data clearly indicate the presence of cyclicity in the change in its brightness. Based on the constructed power spectrum for TOI-1422, we can assume the existence of possible activity cycles 1650d and 2450d (middle diagram). The vertical dotted lines represent periods corresponding to annual and double annual variability. The bottom diagram of Fig. 1 shows the power spectrum for the interval of periods 1–140d, which includes the probable rotation period of the star. According to the authors of [2], for TOI-1422, it should be expected that the rotation period P of the star lies within an interval of 27+−19 8 days. On the bottom diagram of Fig. 1, the specified interval is marked with vertical thin dotted lines, and the possible value of P is marked with a thick dotted line. The dashed line corresponds to the maximum amplitude peak in the considered interval ( P = 32d ). Less numerous are the data on the brightness of the object in the filter Ic. There are a total of 1170 estimates of star brightness in this filter. Presented in Fig. 2 (top), the data undoubtedly indicate the presence of a change in its brightness in the considered time interval. We can assume the presence of possible activity cycles of the order of 1680d and 4600d (the latter is unlikely to be real, its magnitude is comparable to the duration of the observation interval) (Fig. 2, middle diagram), and variability on the time scale of the order of 1–140d (Fig. 2, lower diagram). In the time interval characterizing the probable rotation period of the star, one can point to a set of peaks, among which there is no dominant one. Manifestations of details pointing to P = 27d or 32d were not found. Photometric observations of TOI-1422 were also made in 2004, 20 (...truncated)