The Axial Rotation and Variable Macroturbulence of RR Lyrae and Red Horizontal Branch Stars. (arXiv:1902.09908v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Preston_G/0/1/0/all/0/1">George W. Preston</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sneden_C/0/1/0/all/0/1">Christopher Sneden</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chadid_M/0/1/0/all/0/1">Merieme Chadid</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Thompson_I/0/1/0/all/0/1">Ian B. Thompson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shectman_S/0/1/0/all/0/1">Stephen A. Shectman</a>
We have derived relations between full-width-half-maxima and equivalent
widths of metallic absorption lines in the spectra of RR~Lyrae stars to
estimate new upper limits on the axial equatorial rotational velocities of
RR~Lyrae and metal-poor red horizontal branch stars (RHB). We also have derived
the variations of RR~Lyrae macroturbulent velocities during the pulsation
cycles. In RRab cycles the line widths are dominated by phase-dependent
convolutions of axial rotation and macroturbulence, which we designate as
V_macrot. The behavior of V_macrot is remarkably uniform among the RRab stars,
but the behavior of V_macrot among RRc stars varies strongly from star to star.
The RRab stars exhibit an upper limit on V_macrot of 5 +/- 1 km/s with weak
evidence of an anti-correlation with T_eff. The RRc minima range from 2 to 12
km/s. The abrupt decline in large rotations with decreasing T_eff at the blue
boundary of the instability strip and the apparently smooth continuous
variation among the RRab and RHB stars suggests that HB stars gain/lose surface
angular momentum on time scales short compared to HB lifetimes. V_macrot values
for our metal-poor RHB stars agree well with those derived by Fourier analysis
of an independent but less metal-poor sample of Carney et al. (2008); they
conform qualitatively to the expectations of Tanner et al. (2013). A general
conclusion of our investigation is that surface angular momentum as measured by
V_rot*sini is not a reliable indicator of total stellar angular momentum
anywhere along the HB.
We have derived relations between full-width-half-maxima and equivalent
widths of metallic absorption lines in the spectra of RR~Lyrae stars to
estimate new upper limits on the axial equatorial rotational velocities of
RR~Lyrae and metal-poor red horizontal branch stars (RHB). We also have derived
the variations of RR~Lyrae macroturbulent velocities during the pulsation
cycles. In RRab cycles the line widths are dominated by phase-dependent
convolutions of axial rotation and macroturbulence, which we designate as
V_macrot. The behavior of V_macrot is remarkably uniform among the RRab stars,
but the behavior of V_macrot among RRc stars varies strongly from star to star.
The RRab stars exhibit an upper limit on V_macrot of 5 +/- 1 km/s with weak
evidence of an anti-correlation with T_eff. The RRc minima range from 2 to 12
km/s. The abrupt decline in large rotations with decreasing T_eff at the blue
boundary of the instability strip and the apparently smooth continuous
variation among the RRab and RHB stars suggests that HB stars gain/lose surface
angular momentum on time scales short compared to HB lifetimes. V_macrot values
for our metal-poor RHB stars agree well with those derived by Fourier analysis
of an independent but less metal-poor sample of Carney et al. (2008); they
conform qualitatively to the expectations of Tanner et al. (2013). A general
conclusion of our investigation is that surface angular momentum as measured by
V_rot*sini is not a reliable indicator of total stellar angular momentum
anywhere along the HB.
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