Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: Wide binaries play a crucial role in analyzing the birth environment of stars and the dynamical evolution of clusters. When wide binaries are located at greater distances, their companions may overlap in the observed images, becoming indistinguishable and resulting in unresolved wide binaries, which are difficult to detect using traditional methods. Utilizing deep learning, we present a method to identify unresolved wide binaries by analyzing the point-spread function (PSF) morphology of telescopes. Our trained model demonstrates exceptional performance in differentiating between single stars and unresolved binaries with separations ranging from 0.1 to 2 physical pixels, where the PSF FWHM is ~2 pixels, achieving an accuracy of 97.2% for simulated data from the Chinese Space Station Telescope. We subsequently tested our method on photometric data of NGC 6121 observed by the Hubble Space Telescope. The trained model attained an accuracy of 96.5% and identified 18 wide binary candidates with separations between 7 and 140 au. The majority of these wide binary candidates are situated outside the core radius of NGC 6121, suggesting that they are likely first-generation stars, which is in general agreement with the results of Monte Carlo simulations. Our PSF-based method shows great promise in detecting unresolved wide binaries and is well suited for observations from space-based telescopes with stable PSF. In the future, we aim to apply our PSF-based method to next-generation surveys such as the China Space Station Optical Survey, where a larger-field-of-view telescope will be capable of identifying a greater number of such wide binaries.

Abstract: We extend a magnetic braking (MB) model, which has been used earlier to address the evolution of cataclysmic variables, to address the spin period $P_\mathrm{spin}$ evolution of fully convective M dwarf (FCMD) stars. The MB mechanism is an $\alpha-\Omega$ dynamo, which leads to stellar winds that carry away angular momentum. We model our MB torque such that the FCMDs experience a MB torque, approximately scaling as $P_\mathrm{spin}^{-1}$ at shorter periods, before transitioning into a Skumanich-type MB torque, scaling as $P_\mathrm{spin}^{-3}$. We also implement a parametrized reduction in the wind mass loss owing to the entrapment of winds in dead zones. We choose a set of initial conditions and vary the two free parameters in our model to find a good match of our spin trajectories with open clusters containing FCMDs such as NGC2547, Pleiades, NGC2516 and Praesepe. We find that our model can explain the long spin periods of field stars and that a spread in spin distribution persists till over 3 Gyr. An advantage of our model is in relating physically motivated estimations of the magnetic field strength and stellar wind to properties of the stellar dynamo, which other models often remain agnostic about. We track the spin dependence of the wind mass losses, Alfv\'en radii and surface magnetic fields and find good agreement with observations. We discuss the implications of our results on the effect of the host FCMD on any orbiting exoplanets and our plans to extend this model to explain solar-like stars in the future.

Abstract: Nonlinear pulsation modeling of classical variable stars is among the first topics which were developed at the beginning of the computational era. Various developments were made, and many questions were answered in the past 60 years, and the models became more complex, describing the genuinely 3D convection in a single dimension. Despite its successes, the recent public availability of the MESA Radial Stellar Pulsations (MESA RSP) module and the emerging results from multidimensional codes made clear that the 8 free convective parameters, unique to these models, together with the underlying physical models need calibration. This could be done by comparing them against multi-dimensional codes, but before that, it is important to scrutinize the free parameters of the 1D codes using observations. This is a follow-up work of our previous calibration on the convective parameters of the Budapest-Florida and MESA RSP pulsation codes for RRab stars. In this paper, we extend the previous calibration to the RRc stars and the RR Lyrae stars in general. We found that correlations of some of the parameters are present in RRc stars as well but have a different nature, while high-temperature RRc stars' pulsation properties are very sensitive to the chosen parameter sets.

Abstract: The optical spectra of novae are characterized by emission lines from the hydrogen Balmer series and either Fe II or He/N, leading to their traditional classification into two spectral classes: "Fe II" and "He/N". For decades, the origins of these spectral features were discussed in the literature in the contexts of different bodies of gas or changes in the opacity of the ejecta, particularly associated with studies by R. E. Williams and S. N. Shore. Here, we revisit these major studies with dedicated, modern data sets, covering the evolution of several novae from early rise to peak all the way to the nebular phase. Our data confirm previous suggestions in the literature that the "Fe II" and "He/N" spectral classes are phases in the spectroscopic evolution of novae driven primarily by changes in the opacity, ionization, and density of the ejecta, and most if not all novae go through at least three spectroscopic phases as their eruptions evolve: an early He/N (phase 1; observed during the early rise to visible peak and characterized by P Cygni lines of He I, N II, and N III), then an Fe II (phase 2; observed near visible peak and characterized by P Cygni lines of Fe II and O I), and then a later He/N (phase 3; observed during the decline and characterized by emission lines of He I. He II, N II, and N III), before entering the nebular phase. This spectral evolution seems to be ubiquitous across novae, regardless of their speed class; however the duration of each of these phase differs based on the speed class of the nova.

Abstract: We present the Keck Infrared Transient Survey (KITS), a NASA Key Strategic Mission Support program to obtain near-infrared (NIR) spectra of astrophysical transients of all types, and its first data release, consisting of 105 NIR spectra of 50 transients. Such a data set is essential as we enter a new era of IR astronomy with the James Webb Space Telescope (JWST) and the upcoming Nancy Grace Roman Space Telescope (Roman). NIR spectral templates will be essential to search JWST images for stellar explosions of the first stars and to plan an effective Roma} SN Ia cosmology survey, both key science objectives for mission success. Between 2022 February and 2023 July, we systematically obtained 274 NIR spectra of 146 astronomical transients, representing a significant increase in the number of available NIR spectra in the literature. The first data release includes data from the 2022A semester. We systematically observed three samples: a flux-limited sample that includes all transients $<$17 mag in a red optical band (usually ZTF r or ATLAS o bands); a volume-limited sample including all transients within redshift $z < 0.01$ ($D \approx 50$ Mpc); and an SN Ia sample targeting objects at phases and light-curve parameters that had scant existing NIR data in the literature. The flux-limited sample is 39% complete (60% excluding SNe Ia), while the volume-limited sample is 54% complete and is 79% complete to $z = 0.005$. All completeness numbers will rise with the inclusion of data from other telescopes in future data releases. Transient classes observed include common Type Ia and core-collapse supernovae, tidal disruption events (TDEs), luminous red novae, and the newly categorized hydrogen-free/helium-poor interacting Type Icn supernovae. We describe our observing procedures and data reduction using Pypeit, which requires minimal human interaction to ensure reproducibility.

Abstract: Monitoring mass accretion onto substellar objects provides insights into the geometry of the accretion flows. We use the Lulin One-meter Telescope to monitor H$\alpha$ emission from FU Tau B, a $\sim$19 $M_{\rm Jup}$ brown-dwarf companion at 5.7" (719 au) from the host star, for six consecutive nights. This is the longest continuous H$\alpha$ monitoring for a substellar companion near the deuterium-burning limit. We aim to investigate if accretion near the planetary regime could be rotationally modulated as suggested by magnetospheric accretion models. We find tentative evidence that H$\alpha$ mildly varies on hourly and daily timescales, though our sensitivity is not sufficient to definitively establish any rotational modulation. No burst-like events are detected, implying that accretion onto FU Tau B is overall stable during the time baseline and sampling windows over which it was observed. The primary star FU Tau A also exhibits H$\alpha$ variations over timescales from minutes to days. This program highlights the potential of monitoring accretion onto substellar objects with small telescopes.