Solar and Stellar Astrophysics (astro-ph.SR)

Linear and non-linear models for large-amplitude radial pulsation in faint blue stars (BLAPs)

2025-03-01T00:00:00Z

Linear and non-linear models for large-amplitude radial pulsation in faint blue stars (BLAPs) Jeffery, C. S. The recent discovery of large-amplitude pulsations in faint blue stars (BLAPs) provides both challenges for stellar pulsation theory and opportunities to explore the late evolution of low-mass stars. This paper explores the radial-mode stability of stars across parameter space occupied by BLAPs. Models are constructed for homogeneous stellar envelopes and are agnostic of evolution. Linear non-adiabatic models demonstrate the major requirement for pulsations to be enrichment of iron and nickel in the driving zone to a few times the solar abundance. There is no constraint on mass. Non-linear models demonstrate that BLAP pulsations will be of large amplitude and will show strong shocks at minimum radius. A variety of light-curve shapes are found across the BLAP instability strip, accounting for the variety observed. Linearised period relations are derived from the non-linear models. The phase of maximum luminosity relative to minimum radius is correlated with effective temperature ( T_eff), preceding for cool stars and following for hot stars, and split if close to minimum radius. In both linear and non-linear cases, most models pulsate in the fundamental mode (F). First-overtone (1H) pulsations are excited on the low luminosity blue side of the instability region and become more prevalent at higher mass. The period ratio P_1H/P_F = 0.81 contrasts with the classical Cepheid value (0.70 - 0.75). The transition from F to 1H mode pulsations follows a period-mass relation; the F-mode pulsators adjacent to the transition show a reverse shock. At high T_eff some non-linear models show unstable overtone modes up to 5H and multi-mode behaviour. The linear and non-linear analyses concur on the red-edge of the instability region, but the non-linear blue edge is hotter.

Stellar X-Ray Variability and Planetary Evolution in the DS Tucanae System

2025-02-01T00:00:00Z

Stellar X-Ray Variability and Planetary Evolution in the DS Tucanae System King, George W.; Corrales, Lía R.; Bourrier, Vincent; Dos Santos, Leonardo A.; Doyle, Lauren; Lavie, Baptiste; Ramsay, Gavin; Wheatley, Peter J. We present an analysis of four Chandra observations of the 45 Myr old DS Tuc binary system. We observed X-ray variability of both stars on timescales from hours to months, including two strong X-ray flares from star A. The implied flaring rates are in agreement with past observations made with XMM-Newton, though these rates remain imprecise due to the relatively short total observation time. We find a clear, monotonic decline in the quiescent level of the star by a factor of 1.8 across 8 months, suggesting stellar variability that might be due to an activity cycle. If proven through future observations, DS Tuc A would be the youngest star for which a coronal activity cycle has been confirmed. The variation in our flux measurements across the four visits is also consistent with the scatter in empirical stellar X-ray relationships with Rossby number. In simulations of the possible evolution of the currently super-Neptune-sized planet DS Tuc A b, we find a range of scenarios for the planet once it reaches a typical field age of 5 Gyr, from Neptune size down to a completely stripped super-Earth. Improved constraints on the planet's mass in the future would significantly narrow these possibilities. We advocate for further Chandra observations to better constrain the variability of this important system.

X-Shooting ULLYSES: Massive stars at low metallicity: X. Physical parameters and feedback of massive stars in the LMC N11 B star-forming region

2025-03-01T00:00:00Z

X-Shooting ULLYSES: Massive stars at low metallicity: X. Physical parameters and feedback of massive stars in the LMC N11 B star-forming region Gómez-González, V. M. A.; Oskinova, L. M.; Hamann, W. -R.; Todt, H.; Pauli, D.; Reyero Serantes, S.; Bernini-Peron, M.; Sander, A. A. C.; Ramachandran, V.; Vink, J. S.; Crowther, P. A.; Berlanas, S. R.; ud-Doula, A.; Gormaz-Matamala, A. C.; Kehrig, C.; Kuiper, R.; Leitherer, C.; Mahy, L.; McLeod, A. F.; Mehner, A.; Morrell, N.; Shenar, T.; Telford, O. G.; van Loon, J. Th.; Tramper, F.; Wofford, A. Massive stars drive the ionization and mechanical feedback within young star-forming regions. The Large Magellanic Cloud (LMC) is an ideal galaxy for studying individual massive stars and quantifying their feedback contribution to the environment. We analyze eight exemplary targets in LMC N11 B from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program using novel spectra from HST (COS and STIS) in the UV, and from VLT (X-shooter) in the optical. We model the spectra of early to late O-type stars using state-of-the-art PoWR atmosphere models. We determine the stellar and wind parameters (e.g., T_⋆, log g, L_⋆, Ṁ, and v_∞) of the analyzed objects, chemical abundances (C, N, and O), ionizing and mechanical feedback (Q_H, Q_HeI, Q_{He II}, and L_mec), and X-rays. We report ages of 2–4.5 Myr and masses of 30–60 M_⊙ for the analyzed stars in N11 B, which are consistent with a scenario of sequential star formation. We note that the observed wind-momentum–luminosity relation is consistent with theoretical predictions. We detect nitrogen enrichment by up to a factor of seven in most of the stars. However, we do not find a correlation between nitrogen enrichment and projected rotational velocity. Finally, based on their spectral type, we estimate the total ionizing photons injected from the O-type stars in N11 B into its environment. We report log (Σ Q_H) = 50.5 ph s^‑1, log (Σ Q_{He I}) = 49.6 ph s^‑1, and log (Σ Q_{He II})= 44.4 ph s^‑1, consistent with the total ionizing budget in N11.

Discovery of Metal-poor and Distant Pre–Main Sequence Candidates in WLM with JWST

2025-02-01T00:00:00Z

Discovery of Metal-poor and Distant Pre–Main Sequence Candidates in WLM with JWST Kalari, Venu M.; Salinas, Ricardo; Andersen, Morten; De Marchi, Guido; Rubio, Monica; Vink, Jorick S.; Zinnecker, Hans We present the discovery of 12 metal-poor and distant pre–main sequence (PMS) candidates in the dwarf irregular galaxy Wolf–Lundmark–Melotte ∼968 kpc away, at a present-day metallicity of [Fe/H] ∼ –0.9. These candidates have masses between 1.25 and 5 M_⊙, with ages <10 Myr, and exhibit significant near-infrared excesses at 2.5 and 4.3 μm. They are concentrated within a cluster roughly 10 pc (2″) across, situated in the H II region [HM95]-9. These are the most distant and metal-poor PMS stars known, and they can offer new quantitative insights into star formation at low metallicities.