Characterizing pulsating low-mass white dwarfs: new asteroseismological constraints from TESS
Abstract
Pulsating low-mass and extremely low-mass white dwarfs (ELMVs; ≲ 0.45 Msun) offer a unique opportunity to probe the internal structure of compact remnants formed in close binary systems. While previous asteroseismic studies focused on small samples, recent discoveries — many of them observed by TESS — now allow for broader exploration. In this work, we present new asteroseismic analyses for several ELMVs, combining light curves from TESS with detailed period-to-period fits. Special attention is given to the hydrogen envelope thickness, which provides key insights into the evolutionary history of these stars.
Full text
Asociación Argentina de Astronomía 63ª Reunión Anual - Octubre 2021 ★Low-mass and extremely low-mass white dwarfs (M*< 0.45M0) are the compact remnants of low-mass stars that underwent intense mass loss on the RGB, typically due to close-binary interaction, avoiding the core He flash and forming He core WDs. Their atmospheres are dominated by hydrogen, H (DA WDs). ★Some show high-order nonradial g-mode pulsations, and are classified as ELMVs: allows constraints on the stellar mass (M*) and other relevant parameters. ★From stellar evolution considerations, in particular, the way mass loss proceeds during binary evolution, low-mass WD stars are expected to exhibit a broad range of H-envelope thicknesses, MH. Systems that undergo stable Roche-lobe overflow typically retain thick H layers, while those formed through a common-envelope phase are expected to end up with thinner envelopes. ★Asteroseismology is currently the only means to assess MH, and thus constitutes a powerful tool to shed light on the origin and internal structure of ELMVs. ★We present an asteroseismological characterization of a set of ELMVs observed by TESS, including detailed period-to-period fits. Their location in the Kiel diagram is also shown, enabling the determination of spectroscopic masses for comparison with the seismological inferences. Context Leila M. Calcaferro (1,2) , Alejandro H. Córsico (1,2) , Murat Uzundag (3), Leandro G. Althaus (1,2) (1) Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina (2) Instituto de Astrofísica La Plata (IALP) CONICET-UNLP, Argentina (3) Institute of Astronomy, KU Leuven, Belgium Characterizing pulsating low-mass white dwarfs: new asteroseismological constraints from TESS 2. Period-to-period fits: Searching for the best match between observed and theoretical periods (assuming l=1,2): We plot the inverse of the quality function, 1/𝜒2: the sequence with the largest 1/𝜒2 function, for each stellar mass. Each point corresponds to the H envelope mass that maximizes 1/𝜒2. 1. TESS Data Acquisition and Analysis: We obtained TESS light curves (LC) from MAST in two cadences: 2-minute (short-cadence) for all targets and 20-second (ultra-short cadence) where available. To identify pulsation modes, we computed Lomb-Scargle periodograms of the LCs and adopted a 0.1% false-alarm probability (FAP) threshold for significance. We performed frequency, amplitude, and phase extraction using the STAR SHADOW12 software (IJspeert et al. 2024). Summary & Future Work ★We analyzed TESS light-curves for a sample of ELMVs. After preprocessing and frequency extraction, we identified a set of pulsation modes for each target. ★Applying period-to-period fits to each star, we found multiple possible solutions. However, it was possible to adopt a representative seismological model for each object by imposing constraints from spectroscopic Teff and its uncertainty. ★For TIC 72637474 and J1112, the adopted model corresponds to the best-fit solution among all possibilities. ★For J1112 and TIC 188087204, the adopted models indicate thin H envelopes. ★The comparison between M spec and Mastero shows mixed agreement: two cases are consistent, while the other two differ by ~ 0.1 Mo, with M spec either underestimating or overestimating M astero . ★Moving forward, we aim to derive photometric and astrometric masses using Gaia data, extending to the ELMV class the multi-method mass comparison framework previously applied in Calcaferro et al. (2024) to other pulsating WDs (DAVs, DBVs, and GW Virs). We also plan to apply this approach to the full sample of known ELMVs. Calcaferro et al. 2024, A&A, 691, A194 Córsico et al. 2019, A&ARv, 27, 7 Hermes et al. 2013, ApJ, 765, 102 TASC9/KASC16 - July 2025 [email protected] IJspeert et al. 2024, A&A, 685, A62 Romero et al. 2022, MNRAS, 511, 1574 3. Kiel diagram. Evolutionary tracks for canonical (solid; Althaus et al. 2013) and thin (dashed; Calcaferro et al. 2018) H-envelope masses. Interpolation between the corresponding tracks for each target leads to estimation of spectroscopic masses: TIC 72637474: Mspec= 0.30 Mo (canonical) | J1112 (TIC 290904838): Mspec= 0.14 Mo (thin) TIC 156064657: Mspec= 0.33 Mo (canonical) | TIC 188087204: Mspec= 0.40 Mo (thin) J1112 TIC188087204 TIC72637474 TIC156064657 Althaus et al. 2013, A&A, 557, A19 Bognár & Sódor 2024, A&A, 684, A76 Calcaferro et al. 2018, A&A, 620, A196 TESS-observed period range: [814-1065] s. Parameters of seismological solution: M*=0.36 M o Teff=10400 K M H=7.9e-4 M * (canonical) TIC 72637474 TIC 72637474 Lomb-Scargle periodograms of four ELMVs (top to bottom: TIC 72637474, J1112 (TIC 290904838), TIC 156064657,and TIC 188087204). The red line in each panel indicates the 0.1% FAP threshold; peaks above are statistically significant. TESS-observed period range: [1884-2259] s Parameters of seismological solution: M*=0.17 M o Teff=8920 K M H=4.7e-6 M * (thin) J1112 600 400 200 800 0 1200 TIC 156064657 TESS-observed period range: [814-1491] s Parameters of seismological solution: M*=0.20M o Teff=10060K M H=3.7e-3M * (canonical) TESS-observed period range: [501-742] s Parameters of seismological solution: M*=0.36M o Teff=9820 K M H=2.4e-4M * (thin) TIC 188087204