Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: We describe a 2.5D MHD simulation describing the evolution of cool jets triggered by initial vertical velocity perturbations in the solar chromosphere. We implement random velocity pulses of amplitude 20-50 km/s between 1 Mm and 1.5 Mm, along with various switch-off periods between 50 s and 300 s. The applied vertical velocity pulses create a series of magnetoacoustic shocks steepening above TR. These shocks interact with each other in the inner corona, leading to complex localized velocity fields. The upward propagation of such perturbations creates low-pressure regions behind them, which propel a variety of cool jets and plasma flows. We study the transverse oscillations of a representative cool jet J1 , which moves up to the height of 6.2 Mm above the TR from its origin point. During its evolution, the plasma flows make the spine of jet J1 radially inhomogeneous, which is visible in the density and Alfv\'en speed smoothly varying across the jet. The highly dense J1 supports the propagating transverse wave of period of approximately 195 s with a phase speed of about 125 km/s. In the distance-time map of density, it is manifested as a transverse kink wave. However, the careful investigation of the distance-time maps of the x- and z-components of velocity reveals that these transverse waves are actually the mixed Alfv\'enic modes. The transverse wave shows evidence of damping in the jet. We conclude that the cross-field structuring of the density and characteristic Alfv\'en speed within J1 causes the onset of the resonant conversion and leakage of the wave energy outward to dissipate these transverse oscillations via resonant absorption. The wave energy flux is estimated as approximately of 1.0 x 10^6 ergs cm^{-2} s^{-1}. This energy, if it dissipates through the resonant absorption into the corona where the jet is propagated, is sufficient energy for the localized coronal heating.

Abstract: The fine structure of solar prominences is made by thin threads that outline the magnetic field lines. Observations show that transverse waves of Alfv\'enic nature are ubiquitous in prominence threads. These waves are driven at the photosphere and propagate to prominences suspended in the corona. Heating due to Alfv\'en wave dissipation could be a relevant mechanism in the cool and partially ionized prominence plasma. We explore the construction of 1D equilibrium models of prominence thin threads that satisfy energy balance between radiative losses, thermal conduction, and Alfv\'en wave heating. We assume the presence of a broadband driver at the photosphere that launches Alfv\'en waves towards the prominence. An iterative method is implemented, in which the energy balance equation and the Alfv\'en wave equation are consecutively solved. From the energy balance equation and considering no wave heating initially, we compute the equilibrium profiles along the thread of the temperature, density, ionisation fraction. We use the Alfv\'en wave equation to compute the wave heating rate, which is then put back in the energy balance equation to obtain new equilibrium profiles. The process is repeated until convergence to a self-consistent thread model heated by Alfv\'en waves is achieved. We have obtained equilibrium models composed of a cold and dense thread, a extremely thin PCTR, and an extended coronal region. The length of the cold thread decreases with the temperature at the prominence core and increases with the Alfv\'en wave energy flux. Equilibrium models are not possible for sufficiently large wave energy fluxes when the wave heating rate inside the cold thread becomes larger than radiative losses. The maximum value of the wave energy flux that allows an equilibrium depends on the prominence core temperature. This constrains the existence of equilibria in realistic conditions.

Abstract: The leading spectrographs used for exoplanets' sceince offer online data reduction softwares (DRS) that yield as an ancillary result the full-width at half-maximum (FWHM) of the cross-correlation function (CCF) that is used to estimate the radial velocity of the host star. The FWHM also contains information on the stellar projected rotational velocity vsini We wanted to establish a simple relationship to derive the vsini directly from the FWHM computed by the HARPS-N DRS in the case of slow-rotating solar-like stars. This may also help to recover the stellar inclination i, which in turn affects the exoplanets' parameters. We selected stars with an inclination of the spin axis compatible with 90 deg by looking at exoplanetary transiting systems with known small sky-projected obliquity: for these stars, we can presume that vsini is equal to stellar equatorial velocity veq. We derived their rotational periods from photometric time-series and their radii from SED fitting. This allowed us to recover their veq, which we could compare to the FWHM values of the CCFs obtained both with G2 and K5 spectral type masks. We obtained an empirical relation for each mask, useful for slow rotators (FWHM < 20 km/s). We applied them to 273 exoplanet-host stars observed with HARPS-N, obtaining homogeneous vsini measurements. We compared our results with the literature ones to confirm the reliability of our work, and we found a good agreement with the values found with more sophisticated methods for stars with log g > 3.5. We also tried our relations on HARPS and SOPHIE data, and we conclude that they can be used also on FWHM derived by HARPS DRS with G2 and K5 mask, and they may be adapted to the SOPHIE data as long as the spectra are taken in the high-resolution mode. We were also able to recover or constrain i for 12 objects with no prior vsini estimation.

Abstract: The asteroseismic analysis of stellar power density spectra is often computationally expensive. The models used in the analysis may use several dozen parameters to accurately describe features in the spectra caused by oscillation modes and surface granulation. Many parameters are often highly correlated, making the parameter space difficult to quickly and accurately sample. They are, however, all dependent on a smaller set of parameters, namely the fundamental stellar properties. We aim to leverage this to simplify the process of sampling the model parameter space for the asteroseismic analysis of solar-like oscillators, with an emphasis on mode identification. Using a large set of previous observations, we applied principal component analysis to the sample covariance matrix to select a new basis on which to sample the model parameters. Selecting the subset of basis vectors that explains the majority of the sample variance, we redefine the model parameter prior probability density distributions in terms of a smaller set of latent parameters. We are able to reduce the dimensionality of the sampled parameter space by a factor of two to three. The number of latent parameters needed to accurately model the stellar oscillation spectra cannot be determined exactly but is likely only between four and six. Using two latent parameters, the method is able to describe the bulk features of the oscillation spectrum, while including more latent parameters allows for a frequency precision better than $\approx10\%$ of the small frequency separation for a given target. We find that sampling a lower-rank latent parameter space still allows for accurate mode identification and parameter estimation on solar-like oscillators over a wide range of evolutionary stages. This allows for the potential to increase the complexity of spectrum models without a corresponding increase in computational expense.

Abstract: For existing and future projects dedicated to measuring precise radial velocities (RVs), we have created an open-source, flexible data reduction software to extract RVs from \'echelle spectra via the iodine (I$_2$) absorption cell method. The software, called $pyodine$, is completely written in Python and has been built in a modular structure to allow for easy adaptation to different instruments. We present the fundamental concepts employed by $pyodine$, which build on existing I$_2$ reduction codes, and give an overview of the software's structure. We adapted $pyodine$ to two instruments, Hertzsprung SONG located at Teide Observatory (SONG hereafter) and the Hamilton spectrograph at Lick Observatory (Lick hereafter), and demonstrate the code's flexibility and its performance on spectra from these facilities. Both for SONG and Lick data, the $pyodine$ results generally match the RV precision achieved by the dedicated instrument pipelines. Notably, our code reaches a precision of roughly $0.69 \,m\,s^{-1}$ on a short-term solar time series of SONG spectra, and confirms the planet-induced RV variations of the star HIP~36616 on spectra from SONG and Lick. Using the solar spectra, we also demonstrate the capabilities of our software in extracting velocity time series from single absorption lines. A probable instrumental effect of SONG is still visible in the $pyodine$ RVs, despite being a bit damped as compared to the original results. With $pyodine$ we prove the feasibility of a highly precise, yet instrument-flexible I$_2$ reduction software, and in the future the code will be part of the dedicated data reduction pipelines for the SONG network and the Waltz telescope project in Heidelberg.