THE TWO REGIMES OF PHOTOSPHERIC MOTIONS IN α HYDRA

Abstract

High-resolution spectroscopic observations of α Hya were acquired between 2003 and 2010. Analysis of line shifts, differential shifts, line widths, and line bisectors points to two regimes of velocity fields in the photosphere of α Hya: (1) normal granulation embedded in (2) large convection cells. Variations occur on a wide range of timescales, from several years on down. Radial velocity variations, which are irregular and span 786 m s–1, have a distribution consistent with a true mean rise velocity of the large cells of ~725 m s–1 and a dispersion of ~220 m s–1. The distribution of granulation velocities, as measured from the widths of spectral lines, shows only small variations, consistent with the two regime concepts. On the multi-year timescale, radial velocity changes, small temperature variations (~10 K), and small line-width variations (0.8%) track each other, possibly with phase shifts. The granulation velocity gradient for α Hya is about half as large as the Sun's and no variation with time was seen, implying that any variation in velocity gradient from one large cell to the next must be less than a few percent. The asymmetry in the granulation velocity distribution, as specified in the flux deficit, is smaller than expected for α Hya's position in the HR diagram and appears to be variable.