On Zero Point Calibration of the Red Giant Branch Tip in the Magellanic
  Clouds
  A zero point calibration of the Red Giant Branch Tip (TRGB) in the $I$-band
is determined from OGLE photometry of the Magellanic Clouds (MCs). It is shown
that TRGB measurements made in star-forming regions, with concomitantly high
quantities of gas and dust, are less precise and biased to fainter magnitudes,
as compared to the same measurements made in quiescent regions. Once these low
accuracy fields are excluded from consideration, the TRGB can be used for the
first time to constrain the three-dimensional plane geometry of the LMC.
Composite CMDs are constructed for the SMC and LMC from only those fields with
well-defined TRGB features, and the highest accuracy TRGB zero point
calibration to date is presented. The $I$-band TRGB magnitude is measured to be
flat over the color range $ 1.45 < (V-I)_0 < 1.95$ mag, with a modest slope
introduced when including metal-rich (up to $(V-I)_0 = 2.2$ mag) Tip stars into
the fit. Both the flat, blue zero point and the shallow slope calibration are
consistent with the canonical value of $-4.05$ mag for the old, metal-poor
TRGB, and would appear to resolve a recent debate in the literature over the
method's absolute calibration.
