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The impact of rotation on the stochastic excitation of stellar acoustic modes in solar-like pulsators

Bessila, L., Deckx van Ruys, A., Buriasco, V., Mathis, S., Bugnet, L., García, R. A., Mathur, S., Astronomy and Astrophysics

Context. Recent observational results from asteroseismic studies show that an important fraction of solar-like stars do not present detectable stochastically excited acoustic oscillations. This non-detectability seems to correlate with a high rotation rate in the convective envelope and a high surface magnetic activity. At the same time, the properties of stellar convection are affected by rotation and magnetism. Aims. We investigate the role of rotation in the excitation of acoustic modes in the convective envelope of solar-like stars, to evaluate its impact on the energy injected in the oscillations. Methods. We derived theoretical prescriptions for the excitation of acoustic waves in the convective envelope of rotating solar-like stars. We adopted the rotating mixing-length Theory to model the influence of rotation on convection. We used the MESA stellar evolution code and the GYRE stellar oscillation code to estimate the power injected in the oscillations from our theoretical prescriptions. Results. We demonstrate that the power injected in the acoustic modes is insensitive to rotation if a Gaussian time-correlation function is assumed, while it can decrease by up to 60% for a Lorentzian time-correlation function, for a 20 Ω_⊙ rotation rate. We show that the modification of the excitation rate by rotation depends not only on the rotation rate but also on the radial and angular orders of the considered oscillation mode. This result can allow for better constraints on the properties of stellar convection by studying observationally acoustic mode excitation. Conclusions. These results demonstrate how important it is to take into account the modification of stellar convection by rotation when evaluating the amplitude of the stellar oscillations it stochastically excites. They open the path for understanding the large variety of observed acoustic-mode amplitudes at the surface of solar-like stars as a function of surface rotation rates.

Investigating disc-corona interaction in axisymmetric accretion disc models

Zhang, Lixin, Xue, Li, Luo, Jingyi, Li, Chengzhi, Astronomy and Astrophysics

Context. The interaction between the accretion disc and its corona plays a critical role in the energy balance and emission mechanisms in astrophysical systems such as active galactic nuclei and X-ray binaries. However, the detailed physics of disc-corona interactions, including the mechanisms driving disc evaporation and the impact of the accretion rate and viscosity, remain poorly understood. Aims. Our study aims to extend the well-known disc evaporation model to investigate the disc-corona interaction in a 2D axisymmetric, time-dependent hydrodynamic model, focusing on the effects of the viscosity and accretion rate, and their influence on disc evaporation, luminosity, and corona formation. Methods. We developed a hydrodynamic model consisting of a thin accretion disc, a corona, and a vacuum region. Our model was implemented in Athena++, with the gas-vacuum interface tracking algorithm to handle the vacuum regions. We performed simulations incorporating turbulent viscosity, thermal conduction, bremsstrahlung cooling, and artificial disc cooling, starting from an adiabatic state to explore the disc-corona interaction. Results. We demonstrate the presence of acoustic shock heating. We find that viscosity dominates the intensity of disc evaporation, that the accretion rate primarily determines the disc truncation radius and the disc luminosity, and that there may be a positive correlation between the corona luminosity and the evaporation intensity. We find the warm gas required by the warm corona model. We also compare our results with observations and simulations, and estimate the y parameters to explore the potential effects of Compton cooling as well as the potential effects of the warm corona.

The TOI-1117 multiplanetary system: 3 sub-Neptunes, 1 in both the Neptunian Desert and Radius Valley

Lockley, Isobel S., Armstrong, David J., Fernández Fernández, Jorge, Millholland, Sarah, Knierim, Henrik, Delgado Mena, Elisa, Sousa, Sergio, Collins, Karen A., Watkins, Cristilyn N., Howell, Steve B., Adibekyan, Vardan, Helled, Ravit, Ziegler, Carl, Bayliss, Daniel, Briceño, César, Castro-González, Amadeo, Clark, Catherine A., Collins, Kevin I., Christiansen, Jessie L., Cui, Kaiming, Diaz, Rodrigo, Jenkins, Jon M., Fetzner Keniger, Marcelo A., Kunimoto, Michelle, Law, Nicholas, Lillo-Box, Jorge, Littlefield, Colin, Mann, Andrew W., Mitchell, Morgan A., Nielsen, Louise D., Rodrigues, José, Rowden, Pam, Santos, Nuno C., Seager, Sara, Wheatley, Peter J., Winn, Joshua, Monthly Notices of the Royal Astronomical Society

We present the discovery of three sub-Neptune planets around TOI-1117, a Sun-like star with mass

$0.97\pm 0.02\,{\mathrm M}_{\odot }$

, radius

$1.05\pm 0.03\,{\mathrm {R}}_{\odot }$

, age

$4.42\pm 1.50$

Gyr, and effective temperature

$5635\pm 62$

K. Light curves from Transiting Exoplanet Survey Satellite and Las Cumbres Observatory Global Telescope show a transiting sub-Neptune with a 2.23-d period, mass

$M_b=8.90_{-0.96}^{+0.95}\,\mathrm{ M}_{\rm{\oplus }}$

and radius

$R_b=2.46_{-0.12}^{+0.13}\,\mathrm{ R}_{\rm{\oplus }}$

. This is a rare 'hot Neptune' that falls within the parameter spaces known as the 'Neptunian Desert' and the 'Radius Valley'. Two more planetary signals are detected in HARPS (High Accuracy Radial velocity Planetary Searcher) radial velocities, revealing two non-transiting planets with minimum masses

$M_c=7.46_{-1.62}^{+1.43}\,\mathrm{ M}_{\rm{\oplus }}$

and

$M_d=9.06_{-1.78}^{+2.07}\,\mathrm{ M}_{\rm{\oplus }}$

, and periods of

$4.579\pm 0.004$

and

$8.67\pm 0.01$

d. The eccentricities were poorly constrained by the HARPS data, with upper limits

$e_b=0.11$

$e_c=0.29$

, and

$e_d=0.24$

. However, dynamical simulations of the TOI-1117 system, suggest that the orbits must be nearly circular to be stable. The simulations also show that TOI-1117 b and c are likely to be in a near 2:1 resonance. The multiplanet nature of TOI-1117 makes it a more complex case for formation theories of the Neptunian Desert and Radius Valley, as current theories such as high-eccentricity migration are too turbulent to produce a stable, non-eccentric, and multiplanet system. Moreover, analysis of TOI-1117 b's photoevaporation history found rocky core and H/He atmosphere models to be inconsistent with observations, whilst water-rich scenarios were favoured.

On the Origin of the Short-lived Cocoon in 3C 84: Powered by Tidal Disruption Events?

Kawakatu, Nozomu, Kino, Motoki, Wada, Keiichi, The Astrophysical Journal

We evaluated the jet power and the density of ambient matter in 3C 84 by using the momentum balance along the jet axis and the transonic condition for the cocoons observed at two different scales (approximately 1 and 6 pc). For the inner cocoon, we precisely determined the ratio of jet power to ambient density L_j/n_a to be (0.3─0.7) × 10⁴³ erg s⁻¹ cm³. Similarly, for the outer cocoon, we found that this value is more than an order of magnitude larger at (0.9─3.7) × 10⁴⁴ erg s⁻¹ cm³. This indicates that the outer cocoon is formed by a powerful jet that propagates through an ambient density of 20─300 cm⁻³ with a jet power of 10^45−46.5 erg s⁻¹. On the other hand, the inner cocoon is formed by a weaker jet with a power of 10⁴³⁻⁴⁴ erg s⁻¹, propagating through a relatively low-density environment of 6─20 cm⁻³. These results suggest that (1) with respect to the difference in n_a, it appears to support the hypothesis that the inner cocoon, recently formed about 10 yr ago, is expanding in the low-density environment created by the jet emitted about 25─50 yr ago; (2) to achieve the short-lived and high L_j that generated the outer cocoon, a large mass-accretion rate is required over a short period to activate the jet. These may imply an extreme accretion event driven by the tidal disruption events of massive stars and/or the disk instability.

A candidate field for deep imaging of the Epoch of Reionization observed with MWA

Zhang, Xueying, Zheng, Qian, Wu, Linhui, Guo, Quan, Duchesne, Stefan W., He, Mengfan, Shan, Huanyuan, Wu, Xiang-ping, Johnston-Hollitt, Melanie, Zhao, Feiyu, Ma, Qingyuan, Monthly Notices of the Royal Astronomical Society

Deep imaging of structures from the Cosmic Dawn (CD) and the Epoch of Reionization (EoR) in five targeted fields is one of the highest priority scientific objectives for the Square Kilometre Array (SKA). Selecting 'quiet' fields, which allow deep imaging, is critical for future SKA CD/EoR observations. Pre-observations using existing radio facilities will help estimate the computational capabilities required for optimal data quality and refine data reduction techniques. In this study, we utilize data from the Murchison Widefield Array Phase II extended array for a selected field to study the properties of foregrounds. We conduct deep imaging across two frequency bands: 72─103 and 200─231 MHz. We identify up to 2576 radio sources within a 5-degree radius of the image centre (at RA (J2000) 8

$^h$

, Dec (J2000) 5

$^\circ$

), achieving approximately 80 per cent completeness at 7.7 mJy and 90 per cent at 10.4 mJy for 216 MHz, with a total integration time of 4.43 h and an average RMS of 1.80 mJy. Additionally, we apply a foreground removal algorithm using Principal Component Analysis (PCA) and calculate the angular power spectra of the residual images. Our results indicate that nearly all resolved radio sources can be successfully removed using PCA, leading to a reduction in foreground power. However, the angular power spectra of the residual map remains over an order of magnitude higher than the theoretically predicted CD/EoR 21 cm signal. Further improvements in data reduction and foreground subtraction techniques will be necessary to enhance these results.