SpeX near-infrared spectroscopic extinction curves in the Milky Way
  Interstellar dust extinction curves provide valuable information about dust
properties, including the composition and size of the dust grains, and are
essential to correct observations for the effects of interstellar dust. In this
work, we measure a representative sample of near-infrared (NIR; 0.8-5.5 $\mu$m)
spectroscopic extinction curves for the first time, enabling us to investigate
the extinction at wavelengths where it is usually only measured in broad
photometric bands. We use IRTF/SpeX spectra of a sample of reddened and
comparison stars to measure 15 extinction curves with the pair method. Our
sample spans A(V) values from 0.78 to 5.65 and R(V) values from 2.43 to 5.33.
We confirm that the NIR extinction curves are well fit by a power law, with
indices and amplitudes differing from sight line to sight line. Our average
diffuse NIR extinction curve can be represented by a single power law with
index $\alpha = 1.7$, but because of the sight line-to-sight line variations,
the shape of any average curve will depend on the parental sample. We find that
most of the variation in our sample can be linked to the ratio of
total-to-selective extinction R(V), a rough measurement of the average dust
grain size. Two sight lines in our sample clearly show the ice extinction
feature at 3 $\mu$m, which can be fitted by a modified Drude profile. We find
tentative ice detections with slightly over 3$\sigma$ significance in two other
sight lines. In our average diffuse extinction curve, we measure a 3$\sigma$
upper limit of A(ice)/A(V) = 0.0021 for this ice feature.
