A Quantitative Comparison of SMC, LMC, and Milky Way UV to NIR Extinction Curves

TL;DR

This study conducts a comprehensive, quantitative comparison of all known UV–NIR extinction curves in the Small and Large Magellanic Clouds against Milky Way extinction behavior parameterized by the $R_V$-dependent CCM relation. By compiling archival IUE data, new STIS UV spectra, and optical/NIR photometry (including 2MASS and DENIS) and deriving $R_V$ and $N(HI)$ for 24 sightlines, the authors fit FM90 parameters and examine the curves in $A(\,\lambda)/A(V)$ space. They find that only a minority of LMC curves align with the CCM model, while all SMC Bar curves deviate, and that the CCM relation effectively bounds the ensemble of Magellanic Cloud curves, indicating a continuum of dust properties between the Milky Way and the Magellanic Clouds. The work highlights environmental effects on dust, suggests a possible multiparameter extinction framework beyond CCM (potentially involving $N(HI)/A(V)$), and underscores biases from sampling OB-star sightlines in active star-forming regions.

Abstract

We present an exhaustive, quantitative comparison of all of the known extinction curves in the Small and Large Magellanic Clouds (SMC and LMC) with our understanding of the general behavior of Milky Way extinction curves. The R_V dependent CCM relationship and the sample of extinction curves used to derive this relationship is used to describe the general behavior of Milky Way extinction curves. The ultraviolet portion of the SMC and LMC extinction curves are derived from archival IUE data, except for one new SMC extinction curve which was measured using HST/STIS observations. The optical extinction curves are derived from new (for the SMC) and literature UBVRI photometry (for the LMC). The near-infrared extinction curves are calculated mainly from 2MASS photometry supplemented with DENIS and new JHK photometry. For each extinction curve, we give R_V = A(V)/E(B-V) and N(HI) values which probe the same dust column as the extinction curve. We compare the properties of the SMC and LMC extinction curves with the CCM relationship three different ways: each curve by itself, the behavior of extinction at different wavelengths with R_V, and behavior of the extinction curve FM fit parameters with R_V. As has been found previously, we find that a small number of LMC extinction curves are consistent with the CCM relationship, but majority of the LMC and all of the SMC curves do not follow the CCM relationship. For the first time, we find that the CCM relationship seems to form a bound on the properties of all of the LMC and SMC extinction curves. This result strengthens the picture of dust extinction curves exhibit a continuum of properties between those found in the Milky Way and the SMC Bar. (abridged)

Figures (10)

• Figure 1: The STIS spectra of AzV 23 and 404 are plotted in (a). The resulting extinction curve for AzV 23 is shown in (b). The region near Ly$\alpha$ has been corrected for H1 absorption.
• Figure 2: The SMC extinction curves are plotted. The best fit FM90 curve (solid line) and its uncertainties are plotted (dashed lines). The CCM curve for the measured $R_V$ value (dotted line) and its uncertainties (dot-dashed lines) are plotted.
• Figure 3: The LMC extinction curves in the LMC-2 sample are plotted. The best fit FM90 curve (solid line) and its uncertainties are plotted (dashed lines). The CCM curve for the measured $R_V$ value (dotted line) and its uncertainties (dot-dashed lines) are plotted.
• Figure 4: The LMC extinction curves in the LMC-average sample are plotted. The best fit FM90 curve (solid line) and its uncertainties are plotted (dashed lines). The CCM curve for the measured $R_V$ value (dotted line) and its uncertainties (dot-dashed lines) are plotted.
• Figure 5: The ratio spectra for the AzV 23/404 pair (a) and Sk -69 108/Sk -67 78 pair (b) are shown (solid line) with the best fit H1 profile (dot-dashed line). The ratio spectrum divided by the best fit model Ly$\alpha$ profile is shown as a dashed line. The vertical dotted lines give the center of the Ly$\alpha$ line for the SMC (a) and LMC (b) velocities and $\pm 12$Å region which is excluded from the fit. The dotted line gives a nominal continuum.