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Lina Borg

Publications and source records attributed to Lina Borg.

3 recordsLinked to original sources

Detecting Solar-Like Oscillations in the Highest Mass TESS Giants

Red-giant asteroseismology yields precise stellar parameters, making it a powerful tool for studying stellar structure and evolution, as demonstrated by the Kepler mission. However, due to Kepler's limited field of view, it primarily sampled the more populous low-mass red giants found outside of the Galactic plane, leading to limited detections of red giants above $\rm 3\ M_{\odot}$. Here we use the all-sky TESS data to isolate 227 intermediate-mass candidates from large catalogs with a pre-selection based on photometric and spectroscopic data. We optimize TESS light curves using a boutique light curve detrending method with custom apertures. Compared to the MIT Quick Look Pipeline, this yields a 12% average increase in the power-to-background ratio within the oscillation envelope, even in the heavily crowded Galactic plane. We detect solar-like oscillations in 98 targets, including 43 with $\rm M_* > 3\ M_{\odot}$. Our sample also includes 10 stars having masses greater than $5\ \rm{M}_{\odot}$, among the highest-mass solar-like oscillators detected to date. From our detections, we find that the APOGEE DR19 spectroscopic $\log g$ is systematically larger by, on average, 0.23 dex compared to the seismic $\log g$. This offset is possibly due to the lack of intermediate-mass giants observed by Kepler, which was used to calibrate the spectroscopic $\log g$ in the APOGEE pipeline. Extending the same pre-selection criteria to TESS targets with Gaia XP spectroscopic parameters identifies up to 37,000 candidate intermediate-mass solar-like oscillators for follow-up and population studies.

astro-ph.SR

Hidden worlds: A non-transiting candidate planet in the Neptunian desert around the solar-type pulsator KIC 9139163

Close-in substellar companions experience strong tidal and magnetic interactions with their host stars and are therefore subject to fast orbital evolution. The solar-type pulsator KIC 9139163 exhibits in its light curve a stable 0.6-day modulation for which the best explanation is the presence of a close-in non-transiting companion that we therefore attempt to characterise. We combine Kepler and TESS photometric data with spectroscopic observations obtained with HARPS-N. The analysis of the radial velocities obtained with HARPS-N provides a companion mass $M_p \sin i = 7.3 \pm 1.4 \, \mathrm{M}_{\oplus}$. We infer a planetary radius of $2.43 \pm 0.14 \, \mathrm{R}_\oplus$, which, combined with the measured mass and retrieved inclination, implies a bulk density consistent with a hot water-rich world. This places the non-transiting companion candidate of KIC 9139163 within the Neptunian desert, a regime where planets are expected either to have lost their primordial hydrogen/helium envelopes or to harbour metal-enriched atmospheres. We further detect significant variations in amplitude between the Kepler and TESS phase curves, obtained six years apart, as well as a secular increase in amplitude over the Kepler baseline. Our fit favours a model with two distinct longitudinal cloud offsets over a single-offset scenario. Both datasets indicate a moderate-to-high geometric albedo and low-to-moderate heat redistribution. The opposite phase offsets observed in the Kepler and TESS datasets suggest a time-variable longitudinal brightness distribution. While making KIC 9139163 an interesting candidate for future ground follow-ups, it also suggests that searching for other non-transiting planets around fast stellar rotators in space-borne photometric surveys might provide new insights into the physics of the Neptunian planets located in the desert. [shortened]

astro-ph.EP

TOI-1438: A rare system with two short-period sub-Neptunes and a tentative long-period Jupiter-like planet orbiting a K0V star

We present the detection and characterisation of the TOI-1438 multi-planet system discovered by TESS. We collected a series of follow-up observations including high-spectral resolution observations with HARPS-N over a period of five years. Our modelling shows that the K0V star hosts two transiting sub-Neptunes with Rb = 3.04 +/- 0.19 RE, Rc = 2.75 +/- 0.14 RE, Mb = 9.4 +/- 1.8 ME, and Mc = 10.6 +/- 2.1 ME. The orbital periods of planets b and c are 5.1 and 9.4 days, respectively, corresponding to instellations of 145 +/- 10 and 65 +/- 4 FE. The bulk densities are 1.8 +/- 0.5 and 2.9 +/- 0.7 g cm-3, respectively, suggesting a volatile-rich interior composition. We computed a set of planet interior structure models. Planet b presents a high-metallicity envelope that can accommodate up to 2.5 % in H/He in mass, while planet c cannot have more than 0.2 % as H/He in mass. For any composition of the core considered (Fe-rock or ice-rock), both planets would require a volatile-rich envelope. In addition to the two planets, the radial velocity (RV) data clearly reveal a third signal, likely coming from a non-transiting planet, with an orbital period of 7.6 +1.6 -2.4 years and a radial velocity semi-amplitude of 35+3-5 m s-1. Our best fit model finds a minimum mass of 2.1 +/- 0.3 MJ and an eccentricity of 0.25+0.08-0.11. However, several RV activity indicators also show strong signals at similar periods, suggesting this signal might (partly) originate from stellar activity. More data over a longer period of time are needed to conclusively determine the nature of this signal. If it is confirmed as a triple-planet system, TOI-1438 would be one of the few detected systems to date characterised by an architecture with two small, short-period planets and one massive, long-period planet, where the inner and outer systems are separated by an orbital period ratio of the order of a few hundred.

astro-ph.EP