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Description
The detection of earth-mass exoplanets in the habitable zone around solar-mass stars using the radial velocity technique requires extremely high- precision of the order of 10cm/s. This puts the required noise floor below the intrinsic variability of even relatively inactive stars, like the Sun. One such variable is convective blueshift varying temporally, spatially and between spectral lines. We develop a novel approach to measure convective blueshift and determine the strength of convective blueshift for 810 stars observed by the HARPS spectrograph, spanning spectral types from late-F, G, K to early-M. We derive a model to infer blueshift velocity for lines of any depth in later-type stars of any effective temperature. Using a custom list of spectral lines, covering a wide range of absorption depths, we create a model for the line-core shift as a function of line depth, commonly known as the third signature of granulation. For this we utilize an extremely high-resolution solar spectrum (R~1.000.000) to empirically account for the non-linear nature of the third signature. The solar third signature is then scaled to all 810 stars. Through this we obtain a measure of the convective blueshift relative to the Sun as a function of stellar effective temperature. We confirm the general correlation of increasing convective blueshift with effective temperature and establish a tight, cubic relation between the two that strongly increases for stars above ~5800K. For stars between ~4100K and ~4700K we show for the first time a plateau in convective shift and a possible onset of a plateau for stars above 6000K. Stars below ~4000K show neither blue or red shift. We provide a table listing expected blueshift velocities for each spectral subtype in the data set to quickly access the intrinsic noise floor through convective blueshift for the RV technique.
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