Bright Long Secondary Period Stars for Follow-up Observations

Long secondary period (LSP) variable stars are a subclass of long-period variables (LPV) that exhibit additional long-term variability alongside pulsations. Despite being observed in over 30% of LPVs, the reason behind the LSP phenomenon is still debated. The most favoured explanation, supported by recent growing evidence, is binarity, where the pulsating giant star has a substellar-mass companion. To further test this hypothesis, it is important to identify bright LSP variables, for which high-quality spectroscopic and interferometric observations can be obtained more easily. Motivated by the absence of a catalog of bright nearby LSPs, we searched the All Sky Automated Survey (ASAS) data in the $V$-band magnitude range 5.5-14 mag, and for declinations $< +28^\circ$. The resulting catalog contains 23 LSPs, 13 of which are new discoveries. We compare our catalog with the LSP lists available in the literature.

💡 Research Summary

The paper presents a systematic search for bright long‑secondary‑period (LSP) variable stars using the All‑Sky Automated Survey (ASAS) V‑band photometry. LSPs are an enigmatic subclass of long‑period variables (LPVs) that show an additional, much longer variability component (typically ~10 × the primary pulsation period) in more than one‑third of LPVs. While several mechanisms have been proposed (non‑radial pulsations, convective modes, dust obscuration), recent evidence increasingly favors a binary scenario: a red giant orbited by a low‑mass (often substellar) companion that induces brightness modulation via orbital motion, ellipsoidal distortion, or variable dust obscuration. Testing this hypothesis requires bright, nearby LSP targets amenable to high‑resolution spectroscopy, radial‑velocity monitoring, and interferometric imaging—capabilities that are limited for the majority of known LSPs, which are typically faint and distant (e.g., OGLE or MACHO detections in the Magellanic Clouds or Galactic bulge).

To fill this gap, the authors queried the Tycho‑2 catalog for red giants with V < 8 mag, B–V > 0.5 mag, and declination < +28°, yielding 16 381 candidates. ASAS light curves (27 632 total, because some stars were observed by both the southern and northern stations) were extracted, retaining only A‑grade quality measurements. Period analysis was performed with the FNPEAKS code over a frequency range 0.0005–0.1 day⁻¹ (periods 2000–10 days). Visual inspection of both raw and phase‑folded light curves identified 101 possible LSP candidates. For each, the primary pulsation period (P_S) was removed, and the residuals were re‑searched for a dominant long period (P_L). The authors required that the LSP amplitude exceed the pulsation amplitude to avoid confusion with semi‑regular or ellipsoidal variables. After a second round of visual scrutiny, 23 stars displayed robust LSP signatures. Their periods were refined with the TATRY multiharmonic analysis of variance (AOV) algorithm.

The final catalog lists 23 bright LSP stars, of which 10 were previously known (e.g., BM Eri, S Lep, Y Hya) and 13 are newly identified. For each object the paper provides: ASAS identifier, coordinates, median V magnitude, V‑band amplitude (computed from the 5‑th and 95‑th flux percentiles), LSP period (P_L), pulsation period (P_S), Gaia DR3 parallax and distance (with RUWE quality indicator), and 2MASS JHK_s photometry. The sample spans V ≈ 6–8 mag and distances from ~300 pc to >1 kpc, all located in the southern sky, making them optimal for facilities such as VLT, ALMA, CHARA, and VLTI.

The authors cross‑matched the sample with Gaia DR3, 2MASS, the International Variable Star Index (VSX), and SIMBAD, confirming counterparts for all objects. They discuss recent supporting evidence for the binary hypothesis: OGLE+NEOWISE detections of infrared secondary eclipses consistent with dusty clouds orbiting a companion; ALMA high‑resolution imaging revealing asymmetric dust morphologies that hydrodynamic models attribute to binary interaction; and radial‑velocity plus astrometric analyses of Betelgeuse (α Ori) that show a coherent LSP signal indicative of a low‑mass companion. Together with the bright sample presented here, these results suggest that many LSPs are indeed manifestations of red giants with faint companions and associated dust structures.

All data products—including the machine‑readable Table 1, individual light‑curve files (HJD‑2450000, V magnitude, uncertainty), and a compressed archive—are made publicly available via the authors’ website. This open‑access approach enables the community to conduct follow‑up studies: high‑resolution spectroscopy to measure radial‑velocity curves, interferometric imaging to resolve companion‑induced asymmetries, and multi‑wavelength monitoring to characterize dust emission and obscuration cycles.

In conclusion, the paper delivers the first homogeneous catalog of bright, nearby LSP variables, providing a valuable target list for detailed observational campaigns aimed at unraveling the long‑standing mystery of LSP variability. The catalog’s combination of photometric precision, well‑determined periods, and ancillary Gaia/2MASS information positions it as a cornerstone resource for testing the binary‑companion hypothesis and for advancing our understanding of late‑stage stellar evolution in red giants.