JWST observations of three long-period AM CVn binaries: detection of the donors and hints of magnetically truncated disks

We present JWST/NIRSpec high-cadence infrared spectroscopy of three long-period, eclipsing AM CVn binaries, Gaia14aae, SRGeJ0453, and ZTFJ1637. These systems have orbital periods of 50-62 minutes and cool donors that are undetectable in the optical. The data cover a wavelength range of 1.6-5.2 $μ$m at resolution $R=1000-2000$. We obtained 150-200 spectra of each system over two orbits, split between the G235M and G395M gratings. All three systems show strong, double-peaked He I emission lines dominated by an accretion disk. These lines are nearly stationary but contain radial velocity (RV) variable sub-components that trace stream-disk interactions. In Gaia14aae and SRGeJ0453, we detect two Na I doublets in emission whose RVs track the irradiated face of the donor, marking the first direct detection of the donors of long-period AM CVns. No absorption lines from the donors are detected, implying that the IR excesses observed in many long-period AM CVns primarily trace disks, not donors. The He I emission profiles in all systems lack high-velocity wings and show no emission beyond $\approx 1500,\rm km,s^{-1}$. The morphology of the disk eclipses and Doppler tomograms are best reproduced by models in which the disk is truncated well outside the white dwarf and only material at $r \gtrsim 0.07,R_{\odot}$ contributes to the disk emission. We interpret this as possible evidence of magnetized white dwarf accretors. For plausible mass transfer rates, the truncation radii imply surface magnetic fields of $B = 30-100$ kG, consistent with recent constraints based on X-ray periodicity. The absence of cyclotron humps out to 5 $μ$m rules out stronger MG-level fields. We make the data from the program publicly available to the community.

💡 Research Summary

This paper presents the first high‑cadence, infrared spectroscopic study of three long‑period (50–62 min) eclipsing AM CVn binaries—Gaia14aae, SRGeJ0453, and ZTFJ1637—using JWST/NIRSpec. The observations cover 1.6–5.2 µm at a resolving power of R≈1000–2000, with 150–200 spectra obtained per target over roughly two orbital cycles, split between the G235M and G395M gratings. All three systems display strong, double‑peaked He I emission lines that dominate the spectra, indicating an accretion‑disk origin. The line cores are essentially stationary, but each contains a low‑amplitude, phase‑dependent radial‑velocity component that traces the stream‑disk impact region, consistent with previous optical Doppler tomography.

A key breakthrough is the detection of Na I emission doublets (near 2.2 µm and 2.33 µm) in Gaia14aae and SRGeJ0453. The radial‑velocity curves of these Na I features follow the motion of the irradiated face of the donor star, providing the first direct spectroscopic detection of the donors in long‑period AM CVns. No corresponding Na I absorption is seen, and no donor signatures appear in the ZTFJ1637 spectra. The lack of donor absorption lines, together with the spectral energy distribution (SED) analysis, implies that the infrared excesses observed in many long‑period AM CVns are dominated by the accretion disk rather than the donor itself.

He I line profiles lack high‑velocity wings; emission does not extend beyond ≈1500 km s⁻¹. Modeling of eclipse morphology and Doppler tomograms shows that only material at radii ≳0.07 R⊙ (≈5×10⁹ cm) contributes to the observed line emission. This suggests that the inner disk is truncated well outside the white dwarf surface. By combining plausible mass‑transfer rates (Ṁ≈10⁻¹⁰ M⊙ yr⁻¹) with the inferred truncation radius, the authors estimate surface magnetic fields of 30–100 kG for the accreting white dwarfs. These values are consistent with independent constraints from X‑ray periodicity reported for Gaia14aae and SRGeJ0453. The absence of cyclotron humps out to 5 µm rules out megagauss‑level fields, supporting the modest‑kG scenario.

The authors also performed a detailed SED decomposition. Using DB white‑dwarf model spectra (based on the temperatures and radii derived from optical fits) they find that the white dwarf contributes roughly half the flux at 1.6 µm but less than 10 % at 5 µm. The remaining infrared flux is supplied by the disk and, at the longest wavelengths, by thermal emission from the donor. However, the donor’s contribution is largely featureless, and its temperature (≈2000 K) is inferred from irradiation models rather than direct spectral lines.

Data reduction employed the Eureka! pipeline with extensive manual flagging of warm pixels, achieving per‑exposure signal‑to‑noise ratios of 2–5 per pixel. The final data set comprises 75–88 calibrated 1‑D spectra per grating for each target, and all raw and processed files have been made publicly available via MAST.

In summary, this work delivers three major advances: (1) the first direct infrared detection of donor stars in long‑period AM CVns via Na I emission, (2) compelling evidence that the accretion disks in these systems are magnetically truncated, implying white‑dwarf surface fields of 30–100 kG, and (3) a clarified picture of the infrared excesses, showing that disks—not donors—dominate the flux beyond ~3 µm. These results have important implications for the evolutionary pathways of AM CVns, for modeling their gravitational‑wave signals detectable by LISA, and for future infrared studies of ultra‑compact binaries.