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

Abstract

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 $\hat{I}¼$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 $\approx 1500,\mathrm{km},{\mathrm{s}}^{-1}$. 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 $\hat{I}¼$m rules out stronger MG-level fields. We make the data from the program publicly available to the community.

18 pages, 15 figures, accepted to OJAp. Data at https://data.caltech.edu/records/jjsjy-37d32