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 $μ$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.

Figures (15)

• Figure 1: Phase-averaged spectra of all three sources, ordered by increasing orbital period. We combine data from the G235M and G395M observations and plot the predicted spectrum of the WD accretor in black. The WDs are predicted to contribute about half the flux at 1.6 $\mu$m, but less than 10% at 5 $\mu$m. All three sources have strong emission lines.
• Figure 2: Comparison of phase-averaged spectra (black) to spectra taken during the eclipse of the WD (red). Cyan shows the difference; i.e., the eclipsed component. The in-eclipse spectra are dominated by emission lines, since the donor does not block the whole disk. The eclipsed component, which is dominated by the WD, is nearly featureless. It displays a central spike in several He I lines, which may trace emission on or near the accreting WD.
• Figure 3: Light curves of the three targets at $1.6-2.3\,{\mu}\rm m$ (blue) and $4-5\,{\mu}\rm m$ (red). The WD and disk are eclipsed at phase 1. None of the targets show a secondary eclipse at phase 0.5. The primary eclipses are shallower at longer wavelengths, reflecting the fact that the WDs contribute a smaller fraction of the light there. The broad "wings" of the eclipse trace the occultation of the disk by the donor. In Gaia14aae and ZTFJ1637, the primary eclipse becomes broader at longer wavelengths. This implies that cooler regions in the outer disk dominate at longer wavelengths, while hotter regions closer to the WD dominate at shorter wavelengths.
• Figure 4: Trailed spectra in the G235M grating. Each 46-second exposure is shown as a single row, with time increasing from the bottom to the top. The eclipse of the WD by the donor occurs at phase 0 and 1. All three objects have spectra dominated by He I emission lines, the strongest of which are saturated in the adopted color scale. Most, but not all, of the lines are double-peaked. In Gaia14aae and SRGeJ0453, two Na I lines are also detectable in emission. Unlike the He lines, these are RV-variable and likely trace the irradiated face of the donor (Figure \ref{['fig:trailed_Na']}).
• Figure 5: Trailed spectra in the G395M grating. Compared to the shorter-wavelength G235M observations, these data reveal a broader eclipse of the disk, which darkens the continuum for $\approx 15\%$ of the orbit. All the identified lines are due to He I.