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In this work, we introduce the use of Hi Pfund β (Pfβ; 4.6538μm) as a tracer of mass accretion from protoplanetary disks onto young stars. Pfβ was serendipitously observed in NIRSPEC and CRIRES surveys of CO fundamental emission, amounting to a sample size of 120 young stars with detected Pfβ emission. Using a subsample of disks with previously measured accretion luminosities, we show that Pfβ 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. The conversion from accretion luminosity to accretion rate is limited by the availability of stellar mass and radius measurements; nevertheless, 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 Pfβ line luminosities, and thus accretion luminosities, at a marginally significant level. We also show that high Pfβ 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 Pfβ 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 Pfβ 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. [ABSTRACT FROM AUTHOR]

We report the detection of a unique CO2 ice band toward the deeply embedded, low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared Spectrograph on board the Spitzer Space Telescope, reveals a 15.2μmCO2 ice bending mode profile that cannot be modeled with the same ice structure typically found toward other protostars.We develop a modified CO2 ice profile decomposition, including the addition of new high-quality laboratory spectra of pure, crystalline CO2 ice. Using this model, we find that 87%-92% of the CO2 is sequestered as spherical, CO2-rich mantles, while typical interstellar ices show evidence of irregularly shaped, hydrogen-rich mantles.We propose that (1) the nearly complete absence of unprocessed ices along the line of sight is due to the flattened envelope structure of HOPS-68, which lacks cold absorbing material in its outer envelope, and possesses an extreme concentration of material within its inner (10 AU) envelope region and (2) an energetic event led to the evaporation of inner envelope ices, followed by cooling and re-condensation, explaining the sequestration of spherical, CO2 ice mantles in a hydrogen-poor mixture. The mechanism responsible for the sublimation could be either a transient accretion event or shocks in the interaction region between the protostellar outflow and envelope. The proposed scenario is consistent with the rarity of the observed CO2 ice profile, the formation of nearly pure CO2 ice, and the production of spherical ice mantles. HOPS-68 may therefore provide a unique window into the protostellar feedback process, as outflows and heating shape the physical and chemical structure of protostellar envelopes and molecular clouds. [ABSTRACT FROM AUTHOR]