Measuring Protoplanetary Disk Accretion with H I Pf-beta
  In this work, we introduce the use of H I Pfund-beta (4.6538 micron) as a
tracer of mass accretion from protoplanetary disks onto young stars. Pf-beta
was serendipitously observed in NIRSPEC and CRIRES surveys of CO fundamental
emission, amounting to a sample size of 120 young stars with detected Pf-beta
emission. Using a subsample of disks with previously measured accretion
luminosities, we show that Pf-beta line luminosity is well correlated with
accretion luminosity over a range of at least three orders of magnitude. We use
this correlation to derive accretion luminosities for all 120 targets, 65 of
which are previously unreported in the literature. We also report accretion
rates for 67 targets, 16 previously unmeasured. Our large sample size and our
ability to probe high extinction values allow for relatively unbiased
comparisons between different types of disks. We find that the transitional
disks in our sample have lower than average Pf-beta line luminosities, and thus
accretion luminosities, at a marginally significant level. We also show that
high Pf-beta equivalent width is a signature of transitional disks with high
inner disk gas/dust ratios. In contrast, we find that disks with signatures of
slow disk winds have Pf-beta luminosities comparable to those of other disks in
our sample. Finally, we investigate accretion rates for stage I disks,
including significantly embedded targets. We find that stage I and stage II
disks have statistically indistinguishable Pf-beta line luminosities, implying
similar accretion rates, and that the accretion rates of stage I disks are too
low to be consistent with quiescent accretion. Our results are instead
consistent with both observational and theoretical evidence that stage I
objects experience episodic, rather than quiescent, accretion.
