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Important observables to reveal the nature of dark energy are the equation ofstate $w$ and its time derivative in units of the Hubble time $w'$. Recently,it is shown that the simplest scalar field models of dark energy (quintessence)occupy rather narrow regions in the $w-w'$ plane. We extend the $w-w'$ plane to$w<-1$ and derive bounds on $w'$ as a function of $w$ for tracker phantom darkenergy. We also derive bounds on tracker k-essence.Comment: 7 pages, 2 figures, errors correcte

physical review. d. particles, fields, gravitation, and cosmology/physical review. d, particles, fields, gravitation, and cosmology

<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>w</mml:mi></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>w</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math>of scalar field models of dark energy

wandw′of scalar field models of dark energy