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Several methods exist to convert near-infrared (NIR) stellar observations into extinction maps. We present a new method based on NIR multiband observations. The method uses a discretised version of the distribution of intrinsic stellar colours. A number of variations of the basic method are tested, and the results are compared to NICER calculations. When photometric errors are large, the results are close to those of NICER method but some advantages can be seen when the distribution of intrinsic colours cannot be described well with a single covariance matrix. A priori information about relative column density variations at sub-beam scales can result in a significant increase in accuracy. The results may be further improved by considering the magnitude dependence of the intrinsic colours. Thus, the new methods are useful mostly when photometric errors are small, the distribution of intrinsic colours is well known, or one has prior knowledge of the small-scale structures.
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We determine the slope of the near infrared extinction power law (A$_{lambda} propto lambda^{-alpha}$) for 8 regions of the Galaxy between l$sim27^{circ}$ and $sim100^{circ}$. UKIDSS Galactic Plane Survey data are compared, in colour-colour space, with Galactic population synthesis model data reddened using a series of power laws and convolved through the UKIDSS filter profiles. Monte Carlo simulations allow us to determine the best fit value of $alpha$ and evaluate the uncertainty. All values are consistent with each other giving an average extinction power law of $alpha$=2.14$^{+0.04}_{-0.05}$. This is much steeper than most laws previously derived in the literature from colour excess ratios, which are typically between 1.6 and 1.8. We show that this discrepancy is due to an inappropriate choice of filter wavelength in conversion from colour excess ratios to $alpha$ and that effective rather than isophotal wavelengths are more appropriate. In addition, curved reddening tracks, which depend on spectral type and filter system, should be used instead of straight vectors.
A precise extinction law is a critical input when interpreting observations of highly reddened sources such as young star clusters and the Galactic Center (GC). We use Hubble Space Telescope observations of a region of moderate extinction and a region of high extinction to measure the optical and near-infrared extinction law (0.8 $mu$m -- 2.2 $mu$m). The moderate extinction region is the young massive cluster Westerlund 1 (Wd1; A$_{Ks} sim$ 0.6 mag), where 453 proper motion-selected main-sequence stars are used to measure the shape of the extinction law. To quantify the shape we define the parameter $mathcal{S}_{1/lambda}$, which behaves similarly to a color excess ratio but is continuous as a function of wavelength. The high extinction region is the GC (A$_{Ks} sim$ 2.5 mag), where 819 red clump stars are used to determine the normalization of the law. The best-fit extinction law is able to reproduce the Wd1 main sequence colors, which previous laws misestimate by 10%-30%. The law is inconsistent with a single power law, even when only the near-infrared filters are considered, and has A$_{F125W}$/A$_{Ks}$ and A$_{F814W}$/A$_{Ks}$ values that are 18% and 24% larger than the commonly used citet{Nishiyama:2009fc} law, respectively. Using the law we recalculate the Wd1 distance to be 3896 $pm$ 328 pc from published observations of eclipsing binary W13. This new extinction law should be used for highly reddened populations in the Milky Way, such as the Quintuplet cluster and Young Nuclear Cluster. A python code is provided to generate the law for future use.
Extinction remains one of the most reliable methods of measuring column density of nearby Galactic interstellar clouds. The current and ongoing near-infrared surveys enable the mapping of extinction over large sky areas. We produce allsky extinction maps using the 2MASS near-infrared survey. We use the NICER and NICEST methods to convert the near-infrared colour excesses to extinction estimates. The results are presented in Healpix format at the resolutions of 3.0, 4.5, and 12.0 arcmin. The main results of this study are the calculated J-band extinction maps. The comparison with earlier large-scale extinction mappings shows good correspondence but also demonstrates the presence of resolution-dependent bias. A large fraction of the bias can be corrected by using the NICEST method. For individual regions, best extinction estimates are obtained by careful analysis of the local stellar population and the use of the highest resolution afforded by the stellar density. However, the uniform allsky maps should still be useful for many global studies and as the first step into the investigation of individual clouds.
Based on the data obtained from the Spitzer/GLIPMSE Legacy Program and the 2MASS project, we derive the extinction in the four IRAC bands, [3.6], [4.5], [5.8] and [8.0] micron, relative to the 2MASS Ks band (at 2.16 micron) for 131 GLIPMSE fields along the Galactic plane within |l|<65 deg, using red giants and red clump giants as tracers. As a whole, the mean extinction in the IRAC bands (normalized to the 2MASS Ks band), A_[3.6]/A_Ks=0.63, A_[4.5]/A_Ks=0.57, A_[5.8]/A_Ks=0.49, A_[8.0]/A_Ks=0.55, exhibits little variation with wavelength (i.e. the extinction is somewhat flat or gray). This is consistent with previous studies and agrees with that predicted from the standard interstellar grain model for R_V=5.5 by Weingartner & Draine (2001). As far as individual sightline is concerned, however, the wavelength dependence of the mid-infrared interstellar extinction A_{lambda}/A_Ks varies from one sightline to another, suggesting that there may not exist a universal IR extinction law. We, for the first time, demonstrate the existence of systematic variations of extinction with Galactic longitude which appears to correlate with the locations of spiral arms as well as with the variation of the far infrared luminosity of interstellar dust.
We present a deep circular polarization image of the Orion BN/KL nebula in the Ks band and correlations of circular polarization, linear polarization, and H-Ks color representing extinction. The image of circular polarization clearly reveals the quadrupolar structure around the massive star IRc2, rather than BN. H-Ks color is well correlated with circular polarization. A simple relation between dichroic extinction, color excess, circular and linear polarization is derived. The observed correlation between the Stokes parameters and the color excess agrees with the derived relation, and suggests a major contribution of dichroic extinction to the production of circular polarization in this region, indicating the wide existence of aligned grains.
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