Limits on the Dependence of the Fine-Structure Constant on Gravitational Potential from White-Dwarf Spectra

We propose a new probe of the dependence of the fine-structure constant $\alpha$ on a strong gravitational field using metal lines in the spectra of white-dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white-dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits of $\Delta \alpha /\alpha =(4.2\pm 1.6)\times {10}^{-5}$ and $(-6.1\pm 5.8)\times {10}^{-5}$ from FeV and NiV spectra, respectively, at a dimensionless gravitational potential relative to Earth of $\Delta \varphi \approx 5\times {10}^{-5}$. With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to 2 orders of magnitude.

Figure 1

$\Delta \lambda /\lambda$ vs $Q\alpha$ for transitions in FeV (blue circles) and NiV (red squares). The slope of the lines gives $\Delta \alpha /\alpha =(4.2\pm 1.6)\times {10}^{-5}$ and $(-6.1\pm 5.8)\times {10}^{-5}$ for FeV and NiV, respectively. The slope seen in the NiV spectra is likely due to systematics present in the laboratory wavelength measurements rather than indicating a gravitational dependence of $\alpha$.

Figure 2

$\Delta \lambda /\lambda$ vs ${\lambda }_0$ for transitions in FeV (blue circles) and NiV (red squares). The correlation seen here could be due to calibration systematics, or gravitational dependence of $\alpha$ since $Q_{\alpha }$ and ${\lambda }_0$ are (anti)correlated.