Searching for Magnetic White Dwarfs in LAMOST DR10

Abstract

Magnetic white dwarfs (MWDs) are key to understanding the origin and evolution of magnetic fields in compact stars. While large spectroscopic surveys such as SDSS have greatly expanded the known sample, the potential of LAMOST has not yet been fully explored. Our aim is to identify and characterize isolated MWDs in the LAMOST DR10 database. We cross-matched LAMOST DR10 spectra with white dwarf candidates from Gaia EDR3 and with recent SDSS-based catalogs of MWDs. Zeeman splitting in Balmer and helium absorption lines was used as the primary diagnostic to identify magnetic fields and to estimate their strengths. Reference objects from SDSS catalogs were used to test the detectability of MWDs in LAMOST low-resolution spectra. We identified 63 isolated MWDs in LAMOST DR10, of which 32 are new discoveries. Surface magnetic field strengths were measured from Zeeman splitting, covering a range from a few MG up to several tens of MG. For previously known SDSS MWDs, our LAMOST-based field measurements show mostly agreement with published values. This work demonstrates the capability of LAMOST low-resolution spectroscopy to identify and characterize isolated MWDs. The newly discovered objects expand the known population and provide valuable targets for future high-resolution spectroscopic and polarimetric follow-up studies. Our results highlight the potential of combining LAMOST with Gaia and other large surveys to build a more complete census of MWDs.

Figures (15)

• Figure 1: Theoretical Zeeman Splitting distribution as a function of magnetic field strength for the hydrogen Balmer lines H$\alpha$, H$\beta$, and H$\gamma$ transitions calculated based on the numerical models of Schimeczek2014.
• Figure 2: Several LAMOST isolated MWD spectra with the Zeeman effect on their H$\alpha$, H$\beta$ and H$\gamma$ lines in the linear regimes. spectra were normalized to make better comparisons. As can be seen, the strength of derived magnetic fields are positively correlated to the separation among sub-components of Balmer lines caused by Zeeman-Splitting.
• Figure 3: Composite spectral analysis of J1538+0842. The LAMOST spectrum (black) is compared with an MWD template with a surface field of $B \approx 12$ MG (red dotted line) and an M0 star template (red dashed line). While the MWD template fits the Zeeman-split Balmer lines in the blue ($\lambda < 6500$ Å), the M0 template is required to reproduce the molecular bands in the red ($\lambda > 5500$ Å). On the other hand, the SDSS spectrum of the same target (blue), scaled for comparison, shows no significant M star features in the red.
• Figure 4: The 6'$\times$6' field-of-view of J1538+0842, the star in the cursor is our target MWD, a M type main sequence star is sitting roughly 7 arcsec away from it.
• Figure 5: Hertzsprung–Russell diagram showing the locations of MWDs. Red stars mark SDSS MWDs and blue stars mark LAMOST MWDs. Both groups are distributed uniformly along the WD cooling sequence.