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Join us every Friday (LASP, 10:30am) for an informal discussion of recent papers that are relevant to solar and stellar physics. With at least three papers claimed, we meet on Friday morning.

Title: New Rotation Period Measurements for Kepler Stars Using Deep Learning: The 100K Sample
Authors: Ilay Kamai, Hagai Perets

Abstract: We propose a new framework to predict stellar properties from light curves. We analyze the light-curve data from the Kepler space mission and develop a novel tool for deriving the stellar rotation periods for main-sequence stars. Using this tool, we provide the largest (108785 stars) and most accurate (an average error of $1.6$ Days) sample of stellar rotations to date. Our model, LightPred, is a novel deep-learning model designed to extract stellar rotation periods from light curves. The model utilizes a dual-branch architecture combining Long Short-Term Memory (LSTM) and Transformer components to capture both temporal and global features within the data. We train LightPred on a dataset of simulated light curves generated using a realistic spot model and enhance its performance through self-supervised contrastive pre-training on Kepler light curves. Our evaluation demonstrates that LightPred outperforms classical methods like the Autocorrelation Function (ACF) in terms of accuracy and robustness. We apply LightPred to the Kepler dataset, generating the largest catalog to date of stellar rotation periods for main-sequence stars. Our analysis reveals a systematic shift towards shorter periods compared to previous studies, suggesting a potential revision of stellar age estimates. We also investigate the impact of stellar activity on period determination and find evidence for a distinct period-activity relation. Additionally, we confirm tidal synchronization in eclipsing binaries with orbital periods shorter than 10 days. Our findings highlight the potential of deep learning in extracting fundamental stellar properties from light curves, opening new avenues for understanding stellar evolution and population demographics.

Title: Measuring stellar surface rotation and activity with the PLATO mission -- I. Strategy and application to simulated light curves
Authors: S.N. Breton, A.F Lanza, S. Messina, I. Pagano, L. Bugnet, E. Corsaro, R.A. García, S. Mathur, A.R.G Santos, S. Aigrain, L. Amard, A.S. Brun, L. Degott, Q. Noraz, D.B. Palakkatharappil, E. Panetier, A. Strugarek, K. Belkacem, M.-J Goupil, R.M. Ouazzani, J. Philidet, C. Renié, O. Roth

Abstract: The Planetary Transits and Oscillations of stars mission (PLATO) will allow us to measure surface rotation and monitor photometric activity of tens of thousands of main sequence solar-type and subgiant stars. This paper is the first of a series dedicated to the preparation of the analysis of stellar surface rotation and photospheric activity with the near-future PLATO data. We describe in this work the strategy that will be implemented in the PLATO pipeline to measure stellar surface rotation, photometric activity, and long-term modulations. The algorithms are applied on both noise-free and noisy simulations of solar-type stars, which include activity cycles, latitudinal differential rotation, and spot evolution. PLATO simulated systematics are included in the noisy light curves. We show that surface rotation periods can be recovered with confidence for most of the stars with only six months of observations and that the {recovery rate} of the analysis significantly improves as additional observations are collected. This means that the first PLATO data release will already provide a substantial set of measurements for this quantity, with a significant refinement on their quality as the instrument obtains longer light curves. Measuring the Schwabe-like magnetic activity cycle during the mission will require that the same field be observed over a significant timescale (more than four years). Nevertheless, PLATO will provide a vast and robust sample of solar-type stars with constraints on the activity-cycle length. Such a sample is lacking from previous missions dedicated to space photometry.

Week of 07/01/2024

Title: The California Legacy Survey V. Chromospheric Activity Cycles in Main Sequence Stars
Authors: Howard Isaacson, Andrew Howard, Benjamin Fulton, Erik Petigura, Lauren Weiss, Stephen Kane, Brad Carter, Corey Beard, Steven Giacalone, Judah Zandt, Joseph Murphy, Fei Dai, Ashley Chontos, Alex Polanski, Malena Rice, Jack Lubin, Casey Brinkman, Ryan Rubenzahl, Sarah Blunt, Samuel Yee, Mason MacDougall, Paul Dalba, Dakotah Tyler, Aida Behmard, Isabel Angelo, Daria Pidhorodetska, Andrew Mayo, Rae Holcomb, Emma Turtelboom, Michelle Hill, Luke Bouma, Jingwen Zhang, Ian Crossfield, Nicholas Saunders

Abstract: We present optical spectroscopy of 710 solar neighborhood stars collected over twenty years to catalog chromospheric activity and search for stellar activity cycles. The California Legacy Survey stars are amenable to exoplanet detection using precise radial velocities, and we present their Ca II H and K time series as a proxy for stellar and chromospheric activity. Using the HIRES spectrometer at Keck Observatory, we measured stellar flux in the cores of the Ca II H and K lines to determine S-values on the Mt. Wilson scale and the log(R'HK) metric, which is comparable across a wide range of spectral types. From the 710 stars, with 52,372 observations, 285 stars are sufficiently sampled to search for stellar activity cycles with periods of 2-25 years, and 138 stars show stellar cycles of varying length and amplitude. S-values can be used to mitigate stellar activity in the detection and characterization of exoplanets. We use them to probe stellar dynamos and to place the Sun's magnetic activity into context among solar neighborhood stars. Using precise stellar parameters and time-averaged activity measurements, we find tightly constrained cycle periods as a function of stellar temperature between log(R'HK) of -4.7 and -4.9, a range of activity in which nearly every star has a periodic cycle. These observations present the largest sample of spectroscopically determined stellar activity cycles to date.

Title: Fine Structure of the Age-Chromospheric Activity Relation in Solar-Type Stars: II. H$α$ Line
Authors: Paulo Santos, Gustavo Mello, Erica Costa-Bhering, Diego Lorenzo-Oliveira, Felipe Almeida-Fernandes, Letícia Dutra-Ferreira, Ignasi Ribas

Abstract: Excess chromospheric emissions within deep photospheric lines are effective proxies of stellar magnetism for FGK stars. This emission decays with stellar age and is a potential determinant of this important stellar quantity. We report absolutely calibrated H$\alpha$ chromospheric fluxes for 511 solar-type stars in a wide interval of precisely determined masses, $[$Fe/H$]$, ages, and evolution states from high S/N, moderately high$-$resolution spectra. The comparison of H$\alpha$ and H+K chromospheric fluxes reveals a metallicity bias (absent from H$\alpha$) affecting Ca II H+K fluxes thereby metal-rich stars with deep line profiles mimic low chromospheric flux levels, and vice versa for metal-poor stars. This bias blurs the age-activity relation, precluding age determinations for old, inactive stars unless mass and $[$Fe/H$]$ are calibrated into the relation. The H+K lines being the most widely studied tool to quantify magnetic activity in FGK stars, care should be exercised in its use whenever wide ranges of mass and $[$Fe/H$]$ are involved. The H$\alpha$ age-activity-mass-metallicity calibration appears to be in line with the theoretical expectation that (other parameters being equal) more massive stars possess narrower convective zones and are less active than less massive stars, while more metal-rich stars have deeper convective zones and appear more active than metal-poorer stars. If regarded statistically in tandem with other age diagnostics, H$\alpha$ chromospheric fluxes may be suitable to constrain ages for FGK stars with acceptable precision.

Title: An improved asteroseismic age of the rapid rotator Altair from TESS data
Authors: Michel Rieutord, Daniel Reese, Joey Mombarg, Stéphane Charpinet

Abstract: Understanding the effects of rotation in stellar evolution is key to modelling early-type stars, half of which have equatorial velocities over 100 km/s. The nearby star Altair is an example of such fast-rotating stars, and furthermore, it has the privilege of being modelled by a detailed 2D concordance model that reproduces most of its observables. The aim of this paper is to include new asteroseismic frequencies to improve our knowledge of Altair, especially its age. We processed images of Altair obtained during July 2022 by the Transiting Exoplanet Survey Satellite using the halo photometry technique to obtain its light curve over this observation period. By analysing the light curve, we derived a set of 22 new frequencies in the oscillation spectrum of Altair and confirmed 12 previously known frequencies. Compared with model predictions, we could associate ten frequencies with ten axisymmetric modes. This identification is based on the modelled visibility of the modes. Moreover, nine of the modelled frequencies can be adjusted to simultaneously match their corresponding observed frequencies, once the core hydrogen mass fraction of the concordance model is set to $X_{\rm core}/X_{\rm ini}\simeq0.972$, with $X_{\rm ini}=0.739$. Using the combined results of a 1D MESA model computing the pre-main sequence and a 2D time-dependent ESTER model computing the main sequence, we find that this core hydrogen abundance sets the age of Altair to 88$\pm$10 Myrs, which is slightly younger than previous estimates.