A Photometric and Spectroscopic investigation of 11 TESS eclipsing contact binaries

By cross-matching the eclipsing binary catalog provided by Prsa et al. (2022) with LAMOST medium resolution spectra, we obtained 11 targets. Combining light and radial velocity curves analysis, we have derived accurate physical parameters for these 11 targets. The results indicate that there are 3 deep contact binaries, 3 moderate ones, and 5 shallow ones. Among them, 3 targets exhibit the O’Connell effect, which is attributed to the presence of star-spot on the component’s surface. One target is a low-mass ratio deep contact binary and may be contact binary merging candidates. The evolutionary status of these 11 targets was studied using the mass-luminosity and mass-radius relation diagrams. Based on the O-C (Observed minus Calculated) analysis of 10 targets, we found that the orbital periods of 5 contact binaries show a long-term decreasing trend, likely due to the combined effects of mass transfer between the two components and loss of angular momentum. Meanwhile, the orbital periods of the other 4 stars are continuously increasing, which is attributed to mass transfer. Besides, the O-C curves of 3 targets show clear periodic changes, which might result from the Applegate mechanism or the light travel time effect.

💡 Research Summary

This paper presents a comprehensive photometric and spectroscopic study of eleven eclipsing contact binary systems identified by cross‑matching the TESS eclipsing binary catalog of Prša et al. (2022) with medium‑resolution spectra from the LAMOST survey. The authors selected targets that have a signal‑to‑noise ratio greater than 10 and at least six LAMOST observations, resulting in a sample of 11 objects spanning orbital periods from ~0.22 to 0.73 days.

TESS 2‑minute cadence light curves were retrieved with the lightkurve package, phase‑folded using the catalog ephemerides, and binned to 1000 points per light curve. Radial velocities were derived from the blue arm (4950–5350 Å) of the LAMOST medium‑resolution spectra. The authors employed PHOENIX synthetic spectra as templates and measured velocities via cross‑correlation functions (CCF). Peak positions of the CCFs were extracted with the GaussPy+ algorithm, providing high‑precision RV measurements for both components.

Simultaneous modeling of the light and radial‑velocity curves was performed with the Wilson–Devinney (W‑D) code in mode 3 (contact configuration). Primary temperatures were taken as the mean of Gaia, TESS, and LAMOST determinations, with the standard deviation used as the temperature uncertainty. Adjustable parameters included orbital inclination (i), secondary temperature (T₂), semi‑major axis (a), systemic velocity (Vγ), primary surface potential (Ω₁), primary luminosity (L₁), and third‑light contribution (ℓ₃). Gravity‑darkening coefficients and bolometric albedos were fixed according to the standard values for stars hotter or cooler than 7200 K (Lucy 1967; Rucinski 1969).

The resulting physical parameters (Table 3–4) reveal mass ratios (q = M₂/M₁) ranging from 0.14 to 0.62, primary masses from 0.74 M⊙ to 2.53 M⊙, and fill‑out factors (f) from 11 % to 94 %. According to the fill‑out factor, three systems are classified as deep contact (f > 50 %), three as moderate contact (20 % < f < 50 %), and five as shallow contact (f < 20 %). Notably, TIC 20212631 exhibits a very low mass ratio (q ≈ 0.14) and a relatively high fill‑out factor (f ≈ 24 %), marking it as a potential merger candidate.

Three binaries (TIC 20212631, TIC 198410119, TIC 367683204) display the O’Connell effect—unequal maxima in the light curve. The authors modelled this asymmetry by introducing cool or hot star‑spots on one component, specifying spot latitude (λ), angular radius (rₛ), and temperature factor, thereby reproducing the observed light‑curve distortion.

Period changes were investigated through O‑C (Observed minus Calculated) diagrams for ten systems. Five binaries show a long‑term decreasing period, interpreted as the combined effect of mass transfer from the more massive to the less massive component and angular‑momentum loss (AML) via magnetic braking or stellar winds. Four systems exhibit a secular period increase, consistent with conservative mass transfer from the less massive to the more massive star. Three binaries present clear cyclic variations superimposed on the secular trend; the authors discuss two plausible mechanisms: the Applegate magnetic activity cycle, which can modulate the quadrupole moment of a component, and the Light‑Travel Time Effect (LTTE) caused by an unseen tertiary companion.

Evolutionary status was assessed by placing the components on mass‑luminosity (M–L) and mass‑radius (M–R) diagrams. Most systems lie close to the main‑sequence locus, with some primaries slightly evolved (larger radii for a given mass). The low‑q deep contact system aligns with theoretical predictions for binaries approaching coalescence, while the remaining systems appear to be in various stages of mass‑transfer driven evolution.

In summary, the paper demonstrates the power of combining high‑precision TESS photometry with LAMOST medium‑resolution spectroscopy to derive accurate orbital and stellar parameters for contact binaries. It provides observational evidence for the O’Connell effect, quantifies secular period changes linked to mass transfer and AML, and identifies candidates for magnetic activity cycles or tertiary companions. The authors suggest that future high‑resolution spectroscopy and long‑baseline photometric monitoring will refine mass‑transfer rates, spot evolution, and third‑body detection, thereby improving theoretical models of contact binary evolution and merger pathways.