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Register a new userDust Extinction and Molecular Cloud Structure: L977
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We report results of a near--infrared imaging survey of L977, a dark cloud in Cygnus seen in projection against the plane of the Milky Way. We use measurements of the near--infrared color excess and positions of the 1628 brightest stars in our survey to measure directly dust extinction through the cloud following the method described by Lada et al. (1994). We spatially convolve the individual extinction measurements with a square filter 90 in size to construct a large-scale map of extinction in the cloud. We derive a total mass of M(L977)= (660 +/- 30)(D/500 pc)^2 Msun and, via a comparison of source counts with predictions of a galactic model, estimate a distance to L977 of 500 +/- 100 pc. We find a correlation between the measured dispersion in our extinction determinations and the extinction. We interpret this as evidence for the presence of structure on scales smaller than the 90 resolution of our extinction map. To further investigate the structure of the cloud we construct the frequency distribution of the 1628 individual extinction measurements in the L977 cloud. The shape of the distribution is similar to that of the IC 5146 cloud. Monte Carlo modeling of this distribution suggests that between 2 < Av < 40 mag (or roughly 1 < r < 0.1 pc) the material inside L977 is characterized by a density profile n(r) propto r^(-2). Direct measurement of the radial profile of a portion of the cloud confirms this result. (more...)
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We report observations of the J =(1--0) C18O molecular emission line toward the L977 molecular cloud. To study the correlation between C18O emission and dust extinction we constructed a Gaussian smoothed map of the infrared extinction measured by Alves et al. (1998) at the same angular resolution (50) as our molecular--line observations. This enabled a direct comparison of C18O integrated intensities and column densities with dust extinction over a relatively large range of cloud depth (2 < Av < 30 mag) at 240 positions inside L977. We find a good linear correlation between these two column density tracers for cloud depths corresponding to Av < ~10 magnitudes. For cloud depths above this threshold there is a notable break in the linear correlation. Although either optically thick C18O emission or extremely low (Tex < 5 K) excitation temperatures at high extinctions could produce this departure from linearity, CO depletion in the denser, coldest regions of L977 may be the most likely cause of the break in the observed correlation. We directly derive the C18O abundance in this cloud over a broad range of cloud depths and find it to be virtually the same as that derived for IC 5146 from the data of Lada et al. (1994). In regions of very high extinction (Av > 10 mag), such as dense cores, our results suggest that C18O would be a very poor tracer of mass. Consequently, using C18O as a column density tracer in molecular clouds can lead to a 10 to 30% underestimation of overall cloud mass. We estimate the minimum total column density required to shield C18O from the interstellar radiation field to be 1.6 +/- 0.5 magnitudes of visual extinction.
We examine the three-dimensional structure and dust extinction properties in a ~ 200 pc $times$ 100 pc region in the southwest bar of the Small Magellanic Cloud (SMC). We model a deep Hubble Space Telescope optical color-magnitude diagram (CMD) of red clump and red giant branch stars to infer the dust extinction and galactic structure. We model the distance distribution of the stellar component with a Gaussian and find a centroid distance of 65.2 kpc (distance modulus $mu$ = 19.07 mag) with a FWHM $approx$ 11.3 kpc. This large extent along the line of sight reproduces results from previous studies using variable stars and red clump stars. Additionally, we find an offset between the stellar and dust distributions, with the dust on the near side relative to the stars by 3.22 $^{+1.69}_{-1.44}$ kpc, resulting in a 73% reddened fraction of stars. Modeling the dust layer with a log-normal $A_V$ distribution indicates a mean extinction $langle A_V rangle$ = 0.41 $pm$ 0.09 mag. We also calculate $A_V/N_H$ = 3.2 - 4.2 $times10^{-23}$ mag cm$^2$ H$^{-1}$ which is significantly lower than the Milky Way value but is comparable to previous SMC dust-to-gas ratio measurements. Our results yield the first joint dust extinction and 3D geometry properties in a key region in the SMC. This study demonstrates that CMD modeling can be a powerful tool to simultaneously constrain dust extinction and geometry properties in nearby galaxies.
To elucidate the dust properties in the SMC we have for the first time measured linear polarization in five colors in the optical region of the spectrum for a sample of reddened stars. For two of these stars, for which there were no existing UV spectrophotometric measurements, but for which we measured a relatively large polarization, we have also obtained data from the International Ultraviolet Explorer (IUE) in order to study the extinction. The main results are: (1) the wavelength of maximum polarization, $lambda_{max}$, in the SMC is typically smaller than that in the Galaxy; (2) however, AZV 456, which shows the UV extinction bump, has a $lambda_{max}$ typical of that in the Galaxy, its polarization curve is narrower, its bump is shifted to shorter wavelengths as compared to the Galaxy and its UV extinction does not conform to the Galactic analytical interpolation curve based on the ratio of total to selective extinction; (3) the typical, monotonic SMC extinction curve can be best fit with amorphous carbon and silicate grains; (4) the extinction towards AZV456 may only be explained by assuming a larger gas-to-dust ratio than the observed N(HI)/A(V) value, with a small amount of the available carbon in graphite form; (5) from an analysis of both the extinction and polarization data and our model fits it appears that the SMC has typically smaller grains than those in the Galaxy.
Using near infrared UKIDSS Galactic Plane Survey data, we make extinction measurements to individual stars along the same line of sight as molecular clouds. Using an existing 3D extinction map of the inner Galaxy, that provides line of sight specific extinction-distance relationships, we convert the measured extinction of molecular clouds to a corresponding distance. These distances are derived independently from kinematic methods, typically used to derive distances to molecular clouds, and as such they have no near/far ambiguity. The near/far distance ambiguity has been resolved for 27 clouds and distances have been derived to 20 clouds. The results are found to be in good agreement with kinematic measurements to molecular clouds where the ambiguity has already been resolved, using HI self-absorption techniques.
We present a three dimensional (3D) extinction analysis in the region toward the supernova remnant (SNR) S147 (G180.0-1.7) using multi-band photometric data from the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC), 2MASS and WISE. We isolate a previously unrecognised dust structure likely to be associated with SNR S147. The structure, which we term as S147 dust cloud, is estimated to have a distance $d$ = 1.22 $pm$ 0.21 kpc, consistent with the conjecture that S147 is associated with pulsar PSR J0538 + 2817. The cloud includes several dense clumps of relatively high extinction that locate on the radio shell of S147 and coincide spatially with the CO and gamma-ray emission features. We conclude that the usage of CO measurements to trace the SNR associated MCs is unavoidably limited by the detection threshold, dust depletion, and the difficulty of distance estimates in the outer Galaxy. 3D dust extinction mapping may provide a better way to identify and study SNR-MC interactions.
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