The local stellar velocity field via vector spherical harmonics

Abstract

We analyze the local field of stellar tangential velocities for a sample of $42339$ non-binary Hipparcos stars with accurate parallaxes, using a vector spherical harmonic formalism. We derive simple relations between the parameters of the classical linear model (Ogorodnikov-Milne) of the local systemic field and low-degree terms of the general vector harmonic decomposition. Taking advantage of these relationships we determine the solar velocity with respect to the local stars of $(V_X,V_Y,V_Z)=(10.5,18.5,7.3)\pm 0.1$ \kms. The Oort's parameters determined by a straightforward least-squares adjustment in vector spherical harmonics, are $A=14.0\pm 1.4$, $B=-13.1\pm 1.2$, $K=1.1\pm 1.8$, and $C=-2.9\pm 1.4$ \kmspc. We find a few statistically significant higher degree harmonic terms, which do not correspond to any parameters in the classical linear model. One of them, a third-degree electric harmonic, is tentatively explained as the response to a negative linear gradient of rotation velocity with distance from the Galactic plane, which we estimate at $\sim -20$ \kmspc. The most unexpected and unexplained term within the Ogorodnikov-Milne model is the first-degree magnetic harmonic representing a rigid rotation of the stellar field about the axis $-Y$ pointing opposite to the direction of rotation. This harmonic comes out with a statistically robust coefficient $6.2\pm 0.9$ \kmspc, and is also present in the velocity field of more distant stars. The ensuing upward vertical motion of stars in the general direction of the Galactic center and the downward motion in the anticenter direction are opposite to the vector field expected from the stationary Galactic warp model.

accepted in AJ

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• A new kinematic model of the Galaxy: analysis of the stellar velocity field from Gaia Data Release 3
• Kinematics of the Milky Way from the Gaia EDR3 red giants and sub-giants
• Towards a robust estimation of orientation parameters between ICRF and $Gaia$ celestial reference frames
• Stability, chaos and entrapment of stars in very wide pairs
• The Science of Fundamental Catalogs
• Radio-Optical Reference Catalog, version 1
• Differential proper motion spin of the Hipparcos and Gaia celestial reference frames
• Functional principal component analysis of radio-optical reference frame tie