Determination of the Sun's Offset from the Galactic Plane Using Pulsars

Abstract

We derive the Sun's offset from the local mean Galactic plane($z_\odot$) using the observed $z$ distribution of young pulsars. Pulsar distances are obtained from measurements of annual parallax, HI absorption spectra or associations where available and otherwise from the observed pulsar dispersion and a model for the distribution of free electrons in the Galaxy. We fit the cumulative distribution function for a ${\rm sech}^2(z)$ distribution function, representing an isothermal self-gravitating disk, with uncertainties being estimated using the bootstrap method. We take pulsars having characteristic age $\tau_c<10^{6.5}$~yr and located within 4.5~kpc of the Sun, omitting those within the local spiral arm and those significantly affected by the Galactic warp, and solve for $z_\odot$ and the scale height, $H$, for different cutoffs in $\tau_c$. We compute these quantities using just the independently determined distances, and these together with DM-based distances separately using the YMW16 and NE2001 Galactic electron density models. We find that an age cutoff at $10^{5.75}$~yr with YMW16 DM-distances gives the best results with a minimum uncertainty in $z_\odot$ and an asymptotically stable value for $H$ showing that, at this age and below, the observed pulsar $z$-distribution is dominated by the dispersion in their birth locations. From this sample of 115 pulsars, we obtain $z_\odot=13.4\pm$4.4~pc and $H=56.9\pm$6.5~pc, similar to estimated scale heights for OB stars and open clusters. Consistent results are obtained using the independent-only distances and using the NE2001 model for the DM-based distances.