SPITZER OBSERVATIONS OF DUST EMISSION FROM H ii REGIONS IN THE LARGE MAGELLANIC CLOUD

We have selected two pairs of H ii regions to study their spatially resolved dust properties. The first pair consists of young H ii regions, in which massive stars have not cleared a large central cavity via fast stellar winds and supernova explosions. The Hα surface brightness peaks near the central OB association. Such H ii regions are often referred to as "classical" H ii regions. For this type we have chosen N63 and N180, also known as DEM L243 and DEM L323, respectively.

N63 encompasses the OB association LH 83 (Lucke & Hodge 1970). The stellar content of LH 83 has been studied both spectroscopically and photometrically by Oey (1996). As seen in Figure 1, the H ii region appears amorphous with a few bright patches. The most massive star in this region has exploded and produced the SNR N63A (Mathewson et al. 1983; Chu 1997; Warren et al. 2003), marked by the X-ray contour over the [S ii] image in Figure 1. N63 is also classified as a classical H ii region by Slater et al. (2011).

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N180 is photoionized by the OB association LH 117. The massive star content of LH 117 has been reported by Massey et al. (1989). Figure 1 shows a bright compact H ii region with some filamentary structure along its periphery. The internal kinematics of this H ii region show local expansion around some massive stars, but no large-scale expansion or SNR shocks have been detected (Nazé et al. 2001). Both this paper and Slater et al. (2011) classify N180 as a classical H ii region; however, we exclude the faint southeast extension that is photoionized by the OB association LH 118 because the lack of dense gas near the ionizing stars implies a more evolved state.

The second pair consists of superbubbles, in which OB associations have cleared a central cavity and swept the interstellar gas into a shell structure. We have selected two superbubbles, N70 and N144, also known as DEM L301 and DEM L199, respectively.

N70 is a superbubble around the OB association LH 114. The massive stars in LH 114 have been studied by Oey (1996). This superbubble has essentially thermal radio emission (Dopita et al. 1981) but shows detectable diffuse X-ray emission, indicating interactions with a recent supernova explosion (Chu & Mac Low 1990). Slater et al. (2011) also classify N70 as a superbubble.

N144 is a superbubble blown by the OB association LH 58. The massive stars of LH 58 have been studied in detail by Garmany et al. (1994). We have classified N144 as a superbubble, in contrast to the classical H ii region classification by Slater et al. (2011), because N144 shows a limb-brightened morphology and an expanding shell structure in high-dispersion, long-slit echelle spectra in the Hα line. No SNRs have been reported in N144. Massive young stellar objects (YSOs) have been found in N144 (Gruendl & Chu 2009), indicating on-going star formation.

The emission from stars, gas, and dust in these four H ii regions can be seen in the multi-wavelength images presented in Figure 1 (two classical H ii regions) and Figure 2 (two superbubbles).

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Multi-wavelength data sets were used in this work. First, we used optical emission-line images of the H ii regions to examine the spatial distribution and spectral variations of the ionized gas and to diagnose the ionization fronts and interstellar shocks. We then used the observed stellar content of these regions to estimate the interstellar radiation field; we adopted spectroscopically determined stellar types available in the literature, and for the rest we assessed stellar types and masses using optical photometric data. Finally, we used archival Spitzer observations to extract the SEDs of the dust emission for further modeling and analysis. These data sets are described below.

2.2.1. Nebular Emission-line Images

2.2.2. Spectroscopic Classifications of Massive Stars

2.2.3. Magellanic Clouds Photometric Survey

2.2.4. Spitzer Space Telescope Observations

To estimate the masses and luminosities of stars without spectroscopic classifications, we used the photometric measurements from MCPS and Massey et al. (1989) and compared them against stellar evolutionary tracks from Lejeune & Schaerer (2001) for initial masses between 5 and 120 M_☉. Along these evolutionary tracks, the effective mass, luminosity, temperature, and predicted UBVRI photometry are tabulated for time steps of Δ log t = 0.05 dex starting at 10³ yr. We adopted the evolutionary tracks for a metallicity Z = 0.4 Z_☉, which is close to the LMC metallicity.

The extinction to individual stars is unknown, but the stars of interest are blue; therefore, we opted to use the Wesenheit extinction-free magnitude W ≡ V − DM − (B − V)A_V/E(B − V) (Madore 1982) and the Johnson Q reddening-free color Q ≡ (U − B) − (B − V)E(U − B)/E(B − V) (Johnson & Morgan 1953), where DM = 18.5 is the distance modulus of the LMC, E(U − B)/E(B − V) = 0.72 mag for blue stars, and A_V/E(B − V) = 3.1 mag from the canonical extinction law.

We assigned masses and luminosities to our candidates by plotting their extinction-free parameters (W and Q) in a CMD alongside the evolutionary tracks. As with the MCPS data, the evolutionary tracks were expressed in extinction-free parameters for direct comparison. The evolutionary tracks represent stars of masses 5, 7, 10, 12, 15, 20, 25, 40, 60, 85, and 120 M_☉. The closest evolutionary track to a star gives an estimate for its mass, and its position along the track determines its luminosity. We remove stars lower than 7 M_☉ by matching them to the 5 M_☉ track. Our final analysis includes stars with estimated masses ≳ 7 M_☉. The exclusion of lower mass stars is justified because for an unevolved population with the Salpeter (1955) initial mass function (IMF), only ∼4% of the total radiation is contributed by stars <7 M_☉. Indeed, spectroscopic surveys (Massey et al. 1989; Garmany et al. 1994; Oey 1996) have shown that our regions have IMF slopes similar to or steeper than the slope of the Salpeter (1955) IMF. Moreover, these surveys show that our regions are relatively unevolved since they still have massive O-stars (O3–4 in N70, N180, and N144 and an O7 in N63).

A sample CMD can be found in Figure 3, which shows evolutionary tracks and stars in N63; the rest of the CMDs are available in the online version of this paper. With approximated luminosities for the massive star candidates, the radiation field could be quantified. For both N63 and N70, the highest mass stars as ascertained by the evolutionary tracks (and have no spectroscopic classification) were 25 M_☉. N144 and N180, on the other hand, included several stars at 40 and 60 M_☉ that were not observed in spectroscopic surveys. The masses of these stars are uncertain, but this should not affect the calculation of the radiation field incident on our subregions (described in the next section) by more than a factor of two.

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View extended figure (4 panels)

Based on the fitted evolutionary tracks, 146, 208, 682, and 345 massive stars (M_☉ ≳ 7) determined from photometry exist across the H ii regions N63, N70, N144, and N180, respectively. The luminosities of these stars were calculated from their positions along their mass evolutionary tracks. In addition to the photometric sample, 16, 23, 42, and 21 spectroscopic sources exist in N63, N70, N144, and N180, respectively, whose masses and luminosities were estimated directly from their spectral types (Johnson 1966; de Jager & Nieuwenhuijzen 1987; Cox 2000; Smith et al. 2002). Both photometric and spectroscopic luminosities were used in the radiation field calculation.

The line-of-sight locations of the stars are unknown, so it is impossible to compute the three-dimensional radiation field. As an approximation, we produced a two-dimensional radiation field image by adding the fluxes of all the stars about each H ii region using an inverse square law with effective distance $d_{\mathrm{eff}} = \sqrt{2}d_{\mathrm{proj}}$, where d_proj is the projected distance to a star. These images are shown in the rightmost panels of Figures 4(a)–7(a). The radiation flux for various parts of each H ii can be calculated to approximate the total incident power on each subregion for comparison with exhibited dust properties. The calculated interstellar radiation field (ISRF) presented for each subregion is the median ISRF, which allows direct comparison to the methodology discussed in Section 4.

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In order to investigate how spectral properties of dust emission vary radially throughout each H ii region, we (1) defined radial subregions based on ionized features in Hα images, (2) filtered out contamination due to individual stars in the IRAC bands, (3) calculated background-subtracted fluxes in each subregion, (4) established SEDs for each subregion for further modeling, and (5) constructed color–color diagrams (CCDs) for comparisons among the subregions.

For each H ii region (N63, N180, N70, and N144), we used MOSAIC Hα images to examine the distribution of ionized gas in the region. We then placed circular apertures (subregions) with a 40''-diameter (10 pc at the distance of the LMC) in a pattern bisecting the stellar content to sample the dust emission radially for each H ii region. In N63, N70, and N144 we used a cross pattern (evenly spaced in each of the four cardinal directions), and for N180 we used a single cut from the northeast to the southwest in order to avoid the diffuse H ii region around the OB association LH118 to the southeast. For each H ii region, the subregions extend beyond the bright Hα emission. To estimate the local foreground and background emission, we placed an additional subregion well outside the ionized region at a location with low flux in all the Spitzer bands and devoid of massive stars as identified in Section 3.1. The locations of the subregions are marked in the top middle panel of Figures 4–7.

To reduce the contributions of bright stars to the IR emission, we used a box median filter with size 42 × 42 (7 × 7 pixels) to remove point sources from images in the IRAC bands. In the median-filtered images, several of the brightest remaining objects can be identified as extended background galaxies. As a test, SEDs were computed for subregions using both median-filtered and unaltered images for N63. The uncertainty estimates (the method of calculating these are discussed below) indeed show ∼25% improvement when the contamination by individual stars has been median-filtered out. The MIPS images were not median-filtered, as stellar emission is negligible compared to dust emission at these long wavelengths.

The flux density of each subregion was calculated by measuring the median flux value of the pixels within each subregion (using the median-filtered IRAC images and unaltered MIPS images) and multiplying by the total pixel count. A background subregion's flux density was then subtracted from each subregion's flux density to determine the local emission. The median was chosen rather than the mean because large ranges of fluxes with non-Gaussian distributions were often present. To assess the approximate uncertainty of each measurement, we used the first and third quartile of the flux distribution within the subregion as estimates for the lower and upper uncertainty estimates, respectively. Since the resolution and data-sampling of Spitzer is wavelength-dependent, the number of pixels which formed each flux measurement varies with band. A subregion in each of the four IRAC bands contained 3409 pixels, while the MIPS 24, 70, and 160 μm bands had 197, 49, and 5 pixels, respectively. Therefore, the quartile error-bars show the spread of subregional fluxes for all bands except MIPS 160 μm (however the 160 μm error bars do not grossly affect SED fitting). The flux densities from the seven Spitzer mid-IR bands were then combined to form an SED for each subregion. The photometric flux density measurements of each subregion for all four H ii regions are shown in Table 1.

Table 1. Photometric Flux Density Measurements^a

H ii	Subregion	3.6 μm	4.5 μm	5.8 μm	8.0 μm	24 μm	70 μm	160 μm
Region		(mJy)	(mJy)	(mJy)	(mJy)	(mJy)	(mJy)	(mJy)
N63	1	$13.4\,^{+31.1}_{-7.9}$	$12.9\,^{+29.4}_{-7.2}$	$48.1\,^{+145.6}_{-27.0}$	$119.1\,^{+316.3}_{-75.6}$	$933\,^{+804.9}_{-406.0}$	$8513\,^{+6715.9}_{-3565.4}$	$3186\,^{+436.8}_{-637.3}$
	2	$3.4\,^{+1.2}_{-0.6}$	$3.5\,^{+0.9}_{-0.5}$	$10.7\,^{+2.4}_{-1.7}$	$22.5\,^{+4.7}_{-3.7}$	$121\,^{+24.7}_{-25.9}$	$1372\,^{+351.8}_{-306.9}$	$1956\,^{+52.3}_{-87.3}$
	3	$2.2\,^{+0.6}_{-0.5}$	$2.3\,^{+0.3}_{-0.3}$	$8.4\,^{+2.9}_{-1.5}$	$18.3\,^{+7.5}_{-4.1}$	$72\,^{+9.4}_{-5.3}$	$920\,^{+70.1}_{-48.3}$	$1397\,^{+40.0}_{-42.6}$
	4	$2.3\,^{+0.8}_{-0.6}$	$2.4\,^{+0.5}_{-0.4}$	$8.2\,^{+3.8}_{-1.8}$	$19.1\,^{+9.8}_{-5.9}$	$51\,^{+10.5}_{-6.2}$	$702\,^{+39.1}_{-67.3}$	$1014\,^{+6.3}_{-18.0}$
	5	$0.8\,^{+0.3}_{-0.2}$	$1.2\,^{+0.2}_{-0.2}$	$3.7\,^{+1.1}_{-1.1}$	$7.5\,^{+1.4}_{-1.3}$	$21\,^{+3.4}_{-2.4}$	$426\,^{+39.0}_{-15.9}$	$635\,^{+15.5}_{-7.5}$
	6	$0.9\,^{+0.7}_{-0.5}$	$0.9\,^{+0.4}_{-0.4}$	$3.7\,^{+1.5}_{-1.0}$	$6.1\,^{+1.6}_{-1.4}$	$13\,^{+1.6}_{-1.8}$	$283\,^{+20.2}_{-20.4}$	$564\,^{+38.4}_{-26.2}$
	7	$0.4\,^{+0.3}_{-0.3}$	$0.3\,^{+0.3}_{-0.2}$	$1.7\,^{+0.9}_{-0.7}$	$1.6\,^{+2.5}_{-1.4}$	$8\,^{+2.9}_{-2.2}$	$322\,^{+72.0}_{-60.9}$	$740\,^{+43.4}_{-167.8}$
	8	$3.2\,^{+0.7}_{-0.6}$	$3.4\,^{+0.6}_{-0.5}$	$13.9\,^{+3.5}_{-2.4}$	$31.5\,^{+7.2}_{-5.5}$	$481\,^{+46.9}_{-42.8}$	$2704\,^{+589.7}_{-407.9}$	$3040\,^{+394.7}_{-413.9}$
	9	$3.4\,^{+5.2}_{-1.5}$	$2.4\,^{+4.2}_{-1.0}$	$16.7\,^{+21.0}_{-5.9}$	$42.2\,^{+53.3}_{-15.3}$	$70\,^{+67.0}_{-31.4}$	$1481\,^{+692.0}_{-619.2}$	$2718\,^{+104.8}_{-274.0}$
	10	$2.0\,^{+3.4}_{-1.6}$	$1.2\,^{+1.6}_{-1.0}$	$8.1\,^{+16.8}_{-5.1}$	$16.5\,^{+47.5}_{-13.5}$	$19\,^{+20.4}_{-12.5}$	$182\,^{+125.3}_{-90.7}$	$1171\,^{+331.7}_{-407.6}$
	11	$0.3\,^{+0.5}_{-0.2}$	$0.4\,^{+0.4}_{-0.3}$	$1.0\,^{+1.0}_{-0.9}$	$-0.6\,^{+1.3}_{-1.0}$	$2\,^{+1.1}_{-1.1}$	$-15\,^{+23.7}_{-11.4}$	$197\,^{+39.5}_{-64.5}$
	12	$-0.0\,^{+0.3}_{-0.2}$	$0.0\,^{+0.3}_{-0.3}$	$-0.1\,^{+1.0}_{-0.9}$	$-2.6\,^{+0.7}_{-0.6}$	$0\,^{+0.9}_{-0.8}$	$-60\,^{+18.2}_{-16.3}$	$3\,^{+11.2}_{-29.1}$
	13	$4.7\,^{+1.1}_{-0.8}$	$5.3\,^{+1.2}_{-0.7}$	$16.2\,^{+3.5}_{-2.5}$	$36.7\,^{+8.5}_{-6.8}$	$253\,^{+51.7}_{-36.9}$	$2301\,^{+79.9}_{-425.0}$	$2505\,^{+43.7}_{-93.9}$
	14	$3.2\,^{+1.2}_{-1.0}$	$3.2\,^{+0.9}_{-0.7}$	$14.3\,^{+4.9}_{-3.2}$	$32.2\,^{+12.1}_{-7.9}$	$97\,^{+21.4}_{-14.5}$	$1286\,^{+135.0}_{-71.2}$	$2245\,^{+50.3}_{-60.9}$
	15	$4.3\,^{+1.6}_{-1.2}$	$3.3\,^{+0.9}_{-0.8}$	$21.7\,^{+6.3}_{-4.9}$	$53.0\,^{+18.2}_{-11.1}$	$77\,^{+9.7}_{-10.4}$	$1045\,^{+76.2}_{-80.4}$	$2016\,^{+78.2}_{-263.2}$
	16	$2.3\,^{+0.6}_{-0.5}$	$2.3\,^{+0.6}_{-0.4}$	$11.8\,^{+2.5}_{-2.2}$	$26.6\,^{+7.3}_{-3.9}$	$47\,^{+7.9}_{-6.8}$	$1035\,^{+63.4}_{-69.2}$	$1550\,^{+9.3}_{-181.5}$
	17	$2.2\,^{+1.1}_{-0.9}$	$2.1\,^{+1.1}_{-0.9}$	$9.8\,^{+3.9}_{-3.3}$	$21.6\,^{+10.6}_{-9.1}$	$30\,^{+16.6}_{-9.5}$	$849\,^{+161.5}_{-185.1}$	$828\,^{+17.7}_{-22.0}$
	18	$1.4\,^{+0.7}_{-0.4}$	$1.9\,^{+0.7}_{-0.4}$	$4.8\,^{+1.7}_{-1.7}$	$7.9\,^{+2.5}_{-2.5}$	$130\,^{+101.0}_{-41.2}$	$1442\,^{+882.0}_{-529.3}$	$1401\,^{+291.7}_{-159.8}$
	19	$1.4\,^{+0.4}_{-0.3}$	$1.7\,^{+0.4}_{-0.3}$	$4.4\,^{+1.8}_{-1.5}$	$8.3\,^{+4.6}_{-2.5}$	$44\,^{+5.8}_{-6.7}$	$572\,^{+35.5}_{-11.8}$	$1141\,^{+20.9}_{-32.3}$
	20	$1.7\,^{+0.4}_{-0.4}$	$1.7\,^{+0.4}_{-0.4}$	$6.8\,^{+1.3}_{-1.6}$	$16.6\,^{+3.4}_{-2.7}$	$36\,^{+2.3}_{-3.0}$	$596\,^{+55.8}_{-48.0}$	$1116\,^{+45.6}_{-18.7}$
	21	$1.2\,^{+0.4}_{-0.3}$	$1.1\,^{+0.3}_{-0.2}$	$5.8\,^{+1.6}_{-1.3}$	$13.4\,^{+3.9}_{-2.1}$	$23\,^{+2.2}_{-2.2}$	$448\,^{+10.9}_{-15.0}$	$1171\,^{+31.1}_{-44.3}$
	22	$1.9\,^{+0.4}_{-0.4}$	$1.5\,^{+0.3}_{-0.3}$	$10.7\,^{+2.0}_{-2.6}$	$28.1\,^{+5.2}_{-6.3}$	$32\,^{+3.2}_{-5.2}$	$471\,^{+41.6}_{-37.8}$	$1229\,^{+49.1}_{-4.5}$
N180	1	$15.2\,^{+5.4}_{-3.9}$	$13.9\,^{+3.2}_{-2.1}$	$38.6\,^{+20.3}_{-9.9}$	$97.3\,^{+51.9}_{-31.1}$	$495\,^{+113.3}_{-59.4}$	$6103\,^{+1642.5}_{-1140.2}$	$6340\,^{+55.4}_{-636.6}$
	2	$9.7\,^{+2.1}_{-1.3}$	$10.7\,^{+1.6}_{-1.1}$	$26.3\,^{+4.8}_{-4.4}$	$61.2\,^{+11.9}_{-10.2}$	$329\,^{+40.7}_{-26.4}$	$3952\,^{+499.4}_{-386.6}$	$4919\,^{+795.3}_{-237.4}$
	3	$10.2\,^{+1.5}_{-1.4}$	$12.1\,^{+1.1}_{-0.9}$	$29.5\,^{+7.9}_{-5.4}$	$76.8\,^{+19.9}_{-16.4}$	$362\,^{+35.1}_{-24.0}$	$3569\,^{+162.7}_{-322.4}$	$4994\,^{+52.4}_{-454.9}$
	4	$10.8\,^{+2.2}_{-1.7}$	$12.3\,^{+1.6}_{-1.0}$	$30.5\,^{+20.5}_{-6.0}$	$78.9\,^{+55.8}_{-17.8}$	$297\,^{+31.8}_{-41.3}$	$3962\,^{+451.1}_{-500.3}$	$5154\,^{+443.8}_{-102.1}$
	5	$8.6\,^{+4.4}_{-3.0}$	$6.8\,^{+3.2}_{-1.6}$	$45.0\,^{+21.3}_{-13.8}$	$116.8\,^{+55.3}_{-39.3}$	$129\,^{+53.3}_{-35.2}$	$2504\,^{+900.2}_{-544.9}$	$4364\,^{+97.1}_{-13.6}$
	6	$6.1\,^{+2.3}_{-1.9}$	$4.1\,^{+1.2}_{-1.1}$	$33.1\,^{+9.2}_{-10.8}$	$87.3\,^{+24.4}_{-27.9}$	$69\,^{+12.1}_{-20.0}$	$1299\,^{+198.7}_{-273.6}$	$4271\,^{+133.6}_{-712.3}$
	7	$3.0\,^{+0.8}_{-0.6}$	$2.2\,^{+0.5}_{-0.4}$	$17.9\,^{+1.6}_{-1.3}$	$43.6\,^{+4.3}_{-3.2}$	$33\,^{+1.8}_{-2.2}$	$717\,^{+26.1}_{-32.6}$	$2418\,^{+138.9}_{-31.3}$
	8	$5.0\,^{+1.3}_{-1.2}$	$2.9\,^{+0.8}_{-0.7}$	$27.5\,^{+5.7}_{-6.2}$	$69.9\,^{+17.6}_{-17.2}$	$32\,^{+5.9}_{-7.5}$	$642\,^{+70.1}_{-95.2}$	$1984\,^{+229.1}_{-37.5}$
	9	$3.2\,^{+1.2}_{-0.7}$	$2.0\,^{+0.7}_{-0.5}$	$18.1\,^{+3.0}_{-2.5}$	$46.3\,^{+5.6}_{-5.7}$	$25\,^{+2.5}_{-2.6}$	$484\,^{+22.7}_{-23.1}$	$2037\,^{+86.0}_{-165.4}$
	10	$3.3\,^{+2.2}_{-1.0}$	$2.2\,^{+1.5}_{-0.7}$	$14.8\,^{+2.5}_{-2.4}$	$34.2\,^{+6.5}_{-5.8}$	$20\,^{+3.3}_{-2.7}$	$327\,^{+37.6}_{-19.6}$	$2182\,^{+0.7}_{-214.1}$
	11	$4.0\,^{+1.2}_{-0.8}$	$2.1\,^{+0.8}_{-0.5}$	$20.2\,^{+4.0}_{-2.9}$	$50.9\,^{+9.7}_{-7.7}$	$25\,^{+1.7}_{-2.2}$	$324\,^{+25.3}_{-25.2}$	$1750\,^{+98.0}_{-4.6}$
	12	$3.6\,^{+1.8}_{-1.0}$	$2.1\,^{+1.1}_{-0.7}$	$17.6\,^{+2.0}_{-2.3}$	$42.6\,^{+3.9}_{-4.2}$	$20\,^{+1.6}_{-1.5}$	$243\,^{+25.3}_{-25.2}$	$1794\,^{+32.6}_{-605.0}$
	13	$2.1\,^{+0.8}_{-0.4}$	$1.1\,^{+0.5}_{-0.3}$	$15.6\,^{+1.8}_{-1.5}$	$41.8\,^{+3.5}_{-2.3}$	$21\,^{+3.6}_{-1.8}$	$241\,^{+22.9}_{-43.1}$	$1377\,^{+32.7}_{-4.6}$
	14	$11.2\,^{+2.6}_{-2.0}$	$10.6\,^{+2.0}_{-1.6}$	$38.6\,^{+8.3}_{-4.5}$	$98.5\,^{+21.2}_{-10.0}$	$315\,^{+79.6}_{-48.6}$	$5091\,^{+713.4}_{-285.3}$	$5127\,^{+275.8}_{-237.4}$
	15	$8.6\,^{+4.9}_{-1.9}$	$8.4\,^{+2.6}_{-1.4}$	$26.1\,^{+9.5}_{-5.0}$	$61.9\,^{+22.8}_{-10.9}$	$163\,^{+32.9}_{-26.1}$	$3201\,^{+576.4}_{-353.7}$	$3542\,^{+908.0}_{-518.0}$
	16	$2.2\,^{+0.8}_{-0.5}$	$2.6\,^{+0.6}_{-0.5}$	$10.7\,^{+2.0}_{-2.2}$	$25.6\,^{+3.8}_{-5.4}$	$56\,^{+13.6}_{-10.3}$	$1043\,^{+327.4}_{-229.3}$	$1732\,^{+167.7}_{-306.7}$
	17	$1.6\,^{+0.6}_{-0.4}$	$1.8\,^{+0.4}_{-0.3}$	$6.1\,^{+1.4}_{-1.1}$	$14.2\,^{+4.0}_{-2.6}$	$31\,^{+2.5}_{-4.1}$	$702\,^{+26.9}_{-54.0}$	$1078\,^{+48.5}_{-88.8}$
	18	$0.9\,^{+0.8}_{-0.4}$	$1.1\,^{+0.5}_{-0.4}$	$3.3\,^{+1.9}_{-1.4}$	$7.2\,^{+2.9}_{-3.6}$	$7\,^{+2.3}_{-2.8}$	$294\,^{+103.0}_{-11.9}$	$528\,^{+194.2}_{-176.1}$
	19	$0.2\,^{+0.5}_{-0.3}$	$0.3\,^{+0.4}_{-0.3}$	$0.8\,^{+0.8}_{-0.9}$	$0.5\,^{+1.4}_{-1.4}$	$1\,^{+1.0}_{-1.0}$	$118\,^{+37.9}_{-30.5}$	$175\,^{+10.7}_{-14.9}$
	20	$-0.1\,^{+0.3}_{-0.3}$	$0.0\,^{+0.3}_{-0.3}$	$1.9\,^{+0.8}_{-1.1}$	$3.8\,^{+1.6}_{-1.9}$	$3\,^{+1.6}_{-1.3}$	$99\,^{+6.9}_{-11.0}$	$55\,^{+131.3}_{-1.1}$
N70	1	$2.6\,^{+0.9}_{-0.4}$	$2.0\,^{+0.6}_{-0.3}$	$12.4\,^{+2.9}_{-1.3}$	$31.1\,^{+7.4}_{-2.5}$	$29\,^{+11.3}_{-5.0}$	$766\,^{+122.0}_{-102.2}$	$1744\,^{+78.0}_{-251.5}$
	2	$3.2\,^{+1.6}_{-1.1}$	$2.7\,^{+0.9}_{-0.6}$	$10.5\,^{+7.0}_{-3.8}$	$22.9\,^{+18.8}_{-8.5}$	$33\,^{+3.7}_{-4.5}$	$696\,^{+106.3}_{-88.2}$	$1088\,^{+174.6}_{-81.3}$
	3	$1.5\,^{+0.6}_{-0.3}$	$1.8\,^{+0.4}_{-0.5}$	$5.5\,^{+1.0}_{-0.9}$	$13.0\,^{+1.8}_{-2.2}$	$33\,^{+21.5}_{-8.9}$	$519\,^{+88.3}_{-74.7}$	$983\,^{+41.6}_{-158.8}$
	4	$1.8\,^{+1.2}_{-0.5}$	$1.5\,^{+0.6}_{-0.4}$	$8.0\,^{+4.1}_{-1.6}$	$20.4\,^{+9.6}_{-4.1}$	$21\,^{+6.3}_{-1.9}$	$365\,^{+51.1}_{-72.9}$	$464\,^{+113.7}_{-24.7}$
	5	$0.5\,^{+0.5}_{-0.3}$	$0.8\,^{+0.4}_{-0.3}$	$2.1\,^{+1.0}_{-0.7}$	$4.1\,^{+2.5}_{-1.4}$	$8\,^{+1.3}_{-1.4}$	$191\,^{+15.2}_{-18.0}$	$345\,^{+9.6}_{-29.0}$
	6	$1.1\,^{+1.0}_{-0.8}$	$1.9\,^{+1.4}_{-1.0}$	$1.7\,^{+1.4}_{-1.2}$	$2.9\,^{+1.8}_{-2.3}$	$9\,^{+2.1}_{-4.5}$	$196\,^{+21.9}_{-56.5}$	$27\,^{+25.1}_{-25.7}$
	7	$0.1\,^{+0.4}_{-0.2}$	$0.1\,^{+0.3}_{-0.2}$	$-0.3\,^{+0.9}_{-0.9}$	$-1.2\,^{+1.1}_{-0.8}$	$0\,^{+1.4}_{-1.3}$	$-15\,^{+30.4}_{-16.9}$	$-103\,^{+27.4}_{-0.3}$
	8	$0.3\,^{+0.5}_{-0.3}$	$0.2\,^{+0.4}_{-0.3}$	$-0.8\,^{+1.0}_{-0.9}$	$-1.7\,^{+0.9}_{-0.9}$	$0\,^{+1.1}_{-1.2}$	$-31\,^{+20.0}_{-11.3}$	$-109\,^{+22.7}_{-1.2}$
	9	$1.3\,^{+0.3}_{-0.3}$	$1.0\,^{+0.3}_{-0.2}$	$7.0\,^{+1.3}_{-1.3}$	$17.4\,^{+2.8}_{-2.7}$	$15\,^{+2.1}_{-1.8}$	$368\,^{+51.8}_{-15.8}$	$992\,^{+20.2}_{-28.5}$
	10	$1.0\,^{+0.5}_{-0.3}$	$1.0\,^{+0.4}_{-0.3}$	$5.0\,^{+1.4}_{-1.0}$	$12.4\,^{+2.9}_{-2.2}$	$10\,^{+1.2}_{-1.2}$	$258\,^{+23.2}_{-14.3}$	$649\,^{+6.4}_{-11.6}$
	11	$1.3\,^{+0.4}_{-0.3}$	$1.3\,^{+0.4}_{-0.3}$	$4.9\,^{+0.9}_{-0.8}$	$12.2\,^{+1.4}_{-1.2}$	$10\,^{+1.1}_{-1.2}$	$212\,^{+14.0}_{-22.2}$	$493\,^{+9.9}_{-18.7}$
	12	$1.0\,^{+0.4}_{-0.4}$	$0.7\,^{+0.4}_{-0.3}$	$3.5\,^{+1.6}_{-1.1}$	$8.6\,^{+4.2}_{-2.8}$	$8\,^{+2.8}_{-2.4}$	$194\,^{+30.3}_{-25.4}$	$434\,^{+10.8}_{-50.7}$
	13	$0.6\,^{+0.7}_{-0.4}$	$0.3\,^{+0.4}_{-0.2}$	$0.7\,^{+0.7}_{-0.7}$	$1.4\,^{+1.2}_{-1.0}$	$2\,^{+0.8}_{-1.1}$	$78\,^{+25.1}_{-11.9}$	$217\,^{+16.5}_{-35.3}$
	14	$0.3\,^{+0.4}_{-0.2}$	$0.3\,^{+0.4}_{-0.3}$	$-0.1\,^{+0.8}_{-0.8}$	$-0.3\,^{+0.6}_{-0.6}$	$3\,^{+1.6}_{-2.0}$	$72\,^{+12.7}_{-16.9}$	$165\,^{+17.3}_{-4.0}$
	15	$0.0\,^{+0.3}_{-0.2}$	$0.0\,^{+0.3}_{-0.2}$	$0.4\,^{+0.9}_{-0.8}$	$0.9\,^{+2.0}_{-1.4}$	$2\,^{+1.4}_{-1.4}$	$109\,^{+15.2}_{-19.7}$	$125\,^{+14.9}_{-1.3}$
	16	$4.1\,^{+2.3}_{-1.3}$	$2.7\,^{+1.0}_{-0.7}$	$22.2\,^{+10.4}_{-5.8}$	$56.0\,^{+26.7}_{-15.2}$	$60\,^{+23.9}_{-15.7}$	$975\,^{+256.4}_{-114.8}$	$2442\,^{+111.6}_{-338.8}$
	17	$3.3\,^{+1.1}_{-0.7}$	$3.0\,^{+0.7}_{-0.7}$	$11.7\,^{+2.6}_{-1.5}$	$24.8\,^{+7.1}_{-3.2}$	$42\,^{+13.2}_{-10.4}$	$719\,^{+116.4}_{-82.9}$	$1680\,^{+52.0}_{-90.2}$
	18	$2.8\,^{+0.7}_{-0.5}$	$1.8\,^{+0.5}_{-0.5}$	$11.9\,^{+1.7}_{-2.2}$	$27.6\,^{+3.6}_{-6.3}$	$33\,^{+3.4}_{-9.6}$	$598\,^{+85.1}_{-77.5}$	$1109\,^{+270.3}_{-151.8}$
	19	$2.1\,^{+1.1}_{-0.7}$	$1.4\,^{+0.7}_{-0.4}$	$6.0\,^{+1.6}_{-1.6}$	$8.9\,^{+2.3}_{-2.5}$	$13\,^{+2.1}_{-2.4}$	$233\,^{+52.3}_{-52.9}$	$624\,^{+5.2}_{-113.2}$
	20	$1.1\,^{+0.7}_{-0.3}$	$1.4\,^{+0.5}_{-0.4}$	$1.9\,^{+1.0}_{-0.8}$	$2.0\,^{+1.1}_{-1.2}$	$7\,^{+1.4}_{-1.3}$	$127\,^{+15.5}_{-15.0}$	$136\,^{+61.5}_{-7.1}$
	21	$0.2\,^{+0.4}_{-0.2}$	$0.2\,^{+0.4}_{-0.2}$	$0.2\,^{+0.8}_{-0.8}$	$-1.5\,^{+1.1}_{-0.9}$	$1\,^{+1.1}_{-1.0}$	$43\,^{+17.4}_{-14.1}$	$65\,^{+54.5}_{-111.5}$
	22	$0.8\,^{+0.6}_{-0.3}$	$1.2\,^{+0.7}_{-0.5}$	$1.2\,^{+0.9}_{-0.8}$	$0.2\,^{+2.0}_{-1.1}$	$2\,^{+1.6}_{-1.2}$	$44\,^{+9.3}_{-8.1}$	$-49\,^{+8.0}_{-0.0}$
	23	$1.3\,^{+0.4}_{-0.3}$	$1.1\,^{+0.3}_{-0.3}$	$6.2\,^{+1.2}_{-1.1}$	$14.1\,^{+3.7}_{-2.5}$	$12\,^{+3.5}_{-2.3}$	$330\,^{+65.9}_{-55.7}$	$1072\,^{+37.5}_{-144.9}$
	24	$1.1\,^{+0.3}_{-0.3}$	$1.0\,^{+0.4}_{-0.2}$	$4.7\,^{+0.8}_{-1.0}$	$10.9\,^{+1.7}_{-1.7}$	$9\,^{+1.2}_{-1.3}$	$232\,^{+14.6}_{-15.4}$	$622\,^{+19.9}_{-22.1}$
	25	$1.3\,^{+0.4}_{-0.3}$	$1.1\,^{+0.3}_{-0.3}$	$6.0\,^{+0.8}_{-1.4}$	$14.6\,^{+2.4}_{-2.1}$	$12\,^{+1.8}_{-1.8}$	$267\,^{+31.9}_{-28.8}$	$582\,^{+11.3}_{-10.0}$
	26	$2.1\,^{+0.7}_{-0.5}$	$2.0\,^{+0.6}_{-0.5}$	$6.6\,^{+1.9}_{-1.0}$	$15.6\,^{+3.7}_{-2.0}$	$18\,^{+2.5}_{-1.8}$	$359\,^{+33.5}_{-38.5}$	$728\,^{+28.3}_{-1.9}$
	27	$1.2\,^{+0.4}_{-0.3}$	$0.9\,^{+0.4}_{-0.2}$	$5.8\,^{+0.8}_{-0.9}$	$14.0\,^{+1.3}_{-1.8}$	$13\,^{+1.0}_{-1.3}$	$322\,^{+20.6}_{-26.6}$	$745\,^{+11.2}_{-81.0}$
	28	$1.6\,^{+0.3}_{-0.4}$	$1.0\,^{+0.3}_{-0.2}$	$9.7\,^{+2.0}_{-1.8}$	$25.4\,^{+5.1}_{-4.9}$	$22\,^{+4.1}_{-4.3}$	$419\,^{+21.2}_{-23.2}$	$738\,^{+7.5}_{-48.9}$
	29	$2.2\,^{+0.4}_{-0.3}$	$1.1\,^{+0.3}_{-0.2}$	$13.7\,^{+2.4}_{-2.1}$	$39.1\,^{+4.8}_{-5.1}$	$28\,^{+3.1}_{-3.9}$	$422\,^{+27.0}_{-30.1}$	$881\,^{+49.6}_{-3.2}$
N144	1	$30.2\,^{+35.4}_{-11.7}$	$40.6\,^{+40.8}_{-16.4}$	$153.4\,^{+183.5}_{-49.0}$	$343.5\,^{+480.9}_{-140.6}$	$3388\,^{+15462.1}_{-1686.1}$	$25438\,^{+34447.6}_{-7628.3}$	$6879\,^{+1613.1}_{-401.5}$
	2	$17.1\,^{+10.8}_{-4.7}$	$16.1\,^{+7.6}_{-4.0}$	$52.2\,^{+24.8}_{-12.5}$	$114.3\,^{+68.2}_{-28.8}$	$687\,^{+379.6}_{-107.4}$	$10217\,^{+2895.9}_{-2311.5}$	$9491\,^{+577.9}_{-293.9}$
	3	$30.2\,^{+14.8}_{-9.6}$	$25.3\,^{+10.1}_{-5.2}$	$124.9\,^{+58.7}_{-41.2}$	$321.2\,^{+148.3}_{-113.8}$	$1135\,^{+995.6}_{-187.2}$	$14521\,^{+4831.1}_{-1751.8}$	$12412\,^{+106.0}_{-2452.4}$
	4	$19.9\,^{+8.2}_{-4.6}$	$15.3\,^{+5.4}_{-2.3}$	$88.7\,^{+36.6}_{-17.7}$	$233.0\,^{+106.3}_{-49.3}$	$514\,^{+258.1}_{-94.1}$	$10066\,^{+1378.3}_{-1687.3}$	$6761\,^{+704.3}_{-246.6}$
	5	$12.1\,^{+5.4}_{-3.8}$	$10.3\,^{+3.8}_{-2.4}$	$53.4\,^{+24.5}_{-15.0}$	$136.3\,^{+65.3}_{-39.6}$	$265\,^{+34.8}_{-41.6}$	$4191\,^{+417.5}_{-287.1}$	$4914\,^{+107.3}_{-54.4}$
	6	$8.0\,^{+2.4}_{-1.5}$	$6.5\,^{+1.2}_{-0.8}$	$40.4\,^{+10.2}_{-5.3}$	$104.3\,^{+27.3}_{-13.5}$	$192\,^{+15.1}_{-21.3}$	$4035\,^{+460.1}_{-261.6}$	$4610\,^{+26.0}_{-55.7}$
	7	$7.6\,^{+2.5}_{-1.4}$	$5.8\,^{+1.7}_{-0.9}$	$39.1\,^{+7.6}_{-6.7}$	$98.5\,^{+19.4}_{-21.3}$	$115\,^{+24.8}_{-18.5}$	$2824\,^{+455.2}_{-530.0}$	$2669\,^{+274.2}_{-81.9}$
	8	$1.8\,^{+1.8}_{-1.0}$	$1.5\,^{+1.1}_{-0.9}$	$13.5\,^{+2.9}_{-4.3}$	$29.6\,^{+7.6}_{-8.1}$	$44\,^{+9.2}_{-8.5}$	$1244\,^{+56.7}_{-76.2}$	$1761\,^{+68.1}_{-23.8}$
	9	$2.1\,^{+1.4}_{-1.0}$	$1.6\,^{+0.8}_{-0.6}$	$8.7\,^{+2.7}_{-2.7}$	$21.2\,^{+6.7}_{-5.9}$	$34\,^{+10.2}_{-9.2}$	$996\,^{+230.7}_{-169.1}$	$1081\,^{+41.4}_{-96.0}$
	10	$6.6\,^{+3.5}_{-2.7}$	$6.0\,^{+2.6}_{-2.5}$	$40.1\,^{+18.0}_{-22.8}$	$77.2\,^{+44.6}_{-47.2}$	$507\,^{+410.1}_{-188.4}$	$13181\,^{+4492.3}_{-7120.1}$	$8156\,^{+2646.3}_{-3300.4}$
	11	$1.7\,^{+0.8}_{-0.7}$	$1.5\,^{+0.5}_{-0.4}$	$10.5\,^{+2.2}_{-2.3}$	$16.5\,^{+6.3}_{-4.6}$	$103\,^{+25.2}_{-20.4}$	$1450\,^{+68.9}_{-144.1}$	$2821\,^{+483.0}_{-68.5}$
	12	$1.9\,^{+1.1}_{-0.6}$	$2.5\,^{+0.6}_{-0.7}$	$8.6\,^{+1.7}_{-1.7}$	$17.3\,^{+3.8}_{-3.8}$	$72\,^{+7.6}_{-6.7}$	$1023\,^{+58.9}_{-35.5}$	$1431\,^{+323.2}_{-12.7}$
	13	$1.2\,^{+1.2}_{-0.7}$	$1.6\,^{+0.8}_{-0.6}$	$2.7\,^{+2.6}_{-1.1}$	$7.0\,^{+4.9}_{-2.7}$	$55\,^{+5.5}_{-3.3}$	$725\,^{+81.4}_{-43.8}$	$888\,^{+116.8}_{-1.0}$
	14	$0.9\,^{+1.1}_{-0.6}$	$1.1\,^{+0.7}_{-0.4}$	$2.2\,^{+1.7}_{-1.8}$	$4.4\,^{+3.6}_{-1.9}$	$37\,^{+6.4}_{-2.8}$	$437\,^{+19.9}_{-53.0}$	$690\,^{+23.5}_{-3.9}$
	15	$0.6\,^{+0.7}_{-0.4}$	$0.9\,^{+0.5}_{-0.3}$	$0.7\,^{+1.7}_{-1.2}$	$3.2\,^{+3.1}_{-1.9}$	$30\,^{+2.1}_{-3.0}$	$287\,^{+39.5}_{-31.7}$	$474\,^{+41.3}_{-54.1}$
	16	$-0.5\,^{+0.5}_{-0.3}$	$-0.2\,^{+0.4}_{-0.3}$	$-2.1\,^{+0.9}_{-0.7}$	$-3.0\,^{+0.8}_{-0.7}$	$12\,^{+2.3}_{-2.6}$	$75\,^{+20.6}_{-18.4}$	$290\,^{+17.1}_{-15.5}$
	17	$-0.4\,^{+0.6}_{-0.4}$	$-0.3\,^{+0.5}_{-0.3}$	$-2.2\,^{+0.7}_{-0.8}$	$-3.4\,^{+0.7}_{-0.8}$	$4\,^{+1.2}_{-1.4}$	$-27\,^{+15.1}_{-21.3}$	$111\,^{+46.5}_{-10.8}$
	18	$10.0\,^{+2.6}_{-1.6}$	$9.6\,^{+1.6}_{-1.1}$	$49.4\,^{+9.9}_{-11.0}$	$119.8\,^{+29.8}_{-30.6}$	$502\,^{+58.4}_{-53.3}$	$5938\,^{+836.1}_{-410.4}$	$5769\,^{+654.0}_{-208.8}$
	19	$10.8\,^{+4.9}_{-2.3}$	$8.9\,^{+2.8}_{-1.6}$	$49.9\,^{+12.5}_{-8.6}$	$130.1\,^{+30.3}_{-22.6}$	$303\,^{+59.1}_{-45.4}$	$3504\,^{+781.3}_{-531.5}$	$3510\,^{+193.1}_{-119.4}$
	20	$9.6\,^{+2.9}_{-2.4}$	$7.9\,^{+2.1}_{-1.5}$	$34.9\,^{+8.0}_{-7.8}$	$86.1\,^{+17.0}_{-19.3}$	$183\,^{+15.9}_{-21.0}$	$2009\,^{+166.5}_{-165.5}$	$2349\,^{+134.9}_{-779.9}$
	21	$1.6\,^{+0.8}_{-0.6}$	$1.5\,^{+0.6}_{-0.5}$	$7.6\,^{+2.1}_{-1.7}$	$16.4\,^{+5.2}_{-5.1}$	$51\,^{+13.9}_{-11.6}$	$782\,^{+229.4}_{-172.9}$	$1315\,^{+115.2}_{-246.4}$
	22	$1.1\,^{+0.8}_{-0.5}$	$0.8\,^{+0.6}_{-0.3}$	$5.2\,^{+1.8}_{-1.5}$	$13.0\,^{+4.7}_{-4.5}$	$31\,^{+4.4}_{-5.6}$	$501\,^{+75.8}_{-81.1}$	$611\,^{+185.6}_{-120.2}$
	23	$0.3\,^{+0.6}_{-0.4}$	$0.0\,^{+0.5}_{-0.3}$	$1.9\,^{+1.3}_{-1.1}$	$3.0\,^{+2.2}_{-1.3}$	$10\,^{+4.7}_{-3.0}$	$221\,^{+54.9}_{-44.3}$	$406\,^{+27.0}_{-49.6}$
	24	$10.0\,^{+3.9}_{-2.0}$	$10.1\,^{+3.0}_{-2.3}$	$31.7\,^{+9.8}_{-5.9}$	$47.3\,^{+20.1}_{-17.6}$	$503\,^{+223.3}_{-54.9}$	$6364\,^{+6282.2}_{-3030.7}$	$4376\,^{+294.3}_{-555.0}$
	25	$8.9\,^{+6.1}_{-3.5}$	$7.7\,^{+4.1}_{-3.0}$	$15.1\,^{+6.4}_{-6.0}$	$18.5\,^{+9.1}_{-8.7}$	$172\,^{+17.3}_{-38.3}$	$1544\,^{+219.9}_{-186.6}$	$2791\,^{+105.3}_{-743.9}$
	26	$3.9\,^{+1.8}_{-1.1}$	$3.7\,^{+1.3}_{-0.7}$	$8.0\,^{+7.0}_{-3.3}$	$9.5\,^{+18.5}_{-6.0}$	$98\,^{+23.2}_{-12.2}$	$852\,^{+200.8}_{-56.6}$	$1111\,^{+107.4}_{-60.1}$
	27	$3.3\,^{+1.5}_{-0.9}$	$3.1\,^{+1.0}_{-0.6}$	$6.6\,^{+2.9}_{-1.4}$	$8.3\,^{+5.3}_{-2.0}$	$76\,^{+11.6}_{-8.3}$	$748\,^{+133.6}_{-98.0}$	$1049\,^{+2.0}_{-85.1}$
	28	$1.8\,^{+2.6}_{-0.5}$	$1.7\,^{+1.6}_{-0.4}$	$6.0\,^{+5.5}_{-1.8}$	$5.9\,^{+16.0}_{-2.7}$	$46\,^{+11.5}_{-7.7}$	$514\,^{+125.3}_{-33.8}$	$907\,^{+20.8}_{-25.9}$
	29	$2.2\,^{+1.3}_{-0.6}$	$2.1\,^{+1.0}_{-0.7}$	$7.5\,^{+2.6}_{-2.5}$	$18.1\,^{+5.4}_{-6.0}$	$44\,^{+6.9}_{-7.5}$	$598\,^{+28.2}_{-52.1}$	$1108\,^{+69.2}_{-48.0}$
	30	$3.6\,^{+1.6}_{-0.9}$	$3.4\,^{+0.7}_{-0.6}$	$13.5\,^{+4.3}_{-2.9}$	$34.1\,^{+13.4}_{-8.2}$	$59\,^{+8.2}_{-5.5}$	$884\,^{+71.4}_{-76.3}$	$1663\,^{+21.0}_{-160.4}$
	31	$3.0\,^{+1.2}_{-1.1}$	$2.3\,^{+0.8}_{-0.5}$	$8.2\,^{+4.0}_{-2.0}$	$20.9\,^{+7.4}_{-4.8}$	$46\,^{+6.8}_{-5.9}$	$931\,^{+63.4}_{-57.6}$	$1455\,^{+99.5}_{-56.6}$

Note. ^aThe flux densities are measured via 40'' diameter circular apertures and are background subtracted. Negative flux values indicate that the aperture fluxes are similar to the background fluxes.

Download table as:  ASCIITypeset images: 1 2

Our SEDs, as derived from median-filtered images, were also compared to SEDs made from SAGE's smoothed and point-source-subtracted residual images (Meixner et al. 2006). Subregion fluxes for the median-filtered images typically differed from the residual images by less than 10%, though for N144, which has the most stars, fluxes differed by about 15%. Therefore, the general shape of the SEDs for both the median-filtered images and SAGE residual images were very similar. The point-source subtractions in the SAGE residual images sometimes caused artifacts within the subregions and were often either under- or over-subtracted. For these reasons, we preferred our median-filtered images, though similar results would be obtained if we used the residual images.

Finally, we used these photometric measurements to construct CCDs for each H ii region to search for SED slope changes and color groupings among the subregions. The color magnitudes were calculated based on the Vega zero-magnitude flux densities: 280.9 ± 4.1, 179.7 ± 2.6, 115.0 ± 1.7, and 64.13 ± 0.94 Jy for the 3.6, 4.5, 5.8, and 8.0 μm bands respectively (Reach et al. 2005) and 7.17 ± 0.11 Jy for 24 μm (Rieke et al. 2008). CCDs including 70 and 160 μm measurements were not included due to lack of distinct spectral features seen in SEDs, low pixel count sampled, and lack of zero-magnitude flux at these wavelengths in the literature. CCDs for each regions can be seen in panels (c) and (d) of Figures 4–7.

In order to study the dust properties in each subregion, we simulated the SEDs using the dust emission modeling tool DustEM² (Compiègne et al. 2011). The DustEM code allows an arbitrary combination of various grain types as input and produces a model SED based on the incident radiation field strength and grain physics in the optically thin limit. We adopted the Désert et al. (1990) dust model implemented in DustEM which includes stochastic heating of grains and consists of three dust grain populations: polycyclic aromatic hydrocarbons (PAHs), VSGs composed of carbonaceous material, and big grains (BGs) made of astronomical silicates. The free parameters in the SED fitting include the mass column density of each dust population and the ISRF parameter U, as scaled from the solar neighborhood ISRF described by Mathis et al. (1983). With U as a free parameter rather than temperature, the model inherently assumes that radiation is the dominant component for heating the dust grains instead of other processes (e.g., shocks), which is likely the case for our H ii regions.

The model SEDs were integrated over the filter transmission³ of each band to get photometric fluxes. The model fluxes were compared with the observed fluxes, and the smallest χ² value was used to determine the best-fit model. As the 3.6 and 4.5 μm flux densities are potentially contaminated by starlight, hydrogen Brα line, and/or molecular hydrogen lines (Flagey et al. 2006; Churchwell et al. 2004; De Buizer & Vacca 2010), they were excluded in the SED fitting (more on this below). We note that the value found for U is primarily constrained by the band flux ratio of 70 and 160 μm, and the PAHs and VSG mass column densities are most sensitive to IRAC bands and 24 μm band, respectively.

Table 2 summarizes the modeling results. The best-fit dust composition is given as Y_PAH/Y_DUST and Y_VSG/Y_BG, in which Y is the dust mass abundance for a given grain type. The visual extinction, A_V, was derived from the modeled extinction curve and is an indication of the dust column density. The median and range of the BG equilibrium temperatures, T_eq, are also listed in the table and are highly dependent on the value of the modeled U. T_eq values in each region typically peak toward the center and decreases radially. We do not list the fits for a subregion in the following situations: (1) if one of the 70 μm and 160 μm bands used for a fit has a negative background subtracted flux (U strongly depends on these bands, and, without U, the other free parameters usually are not well-constrained), (2) at least three of the four fitted parameters are less than or equal to their corresponding error bars, and (3) the case of N70 subregion 6, which had an unphysical ISRF fit of U = 285.0 ± 69.0 due to a large 160 μm error bar.

Table 2. Dust Properties from SED Modeling^a

H ii	Subregion	Ub	YPAH/YDUST	YVSG/YBG	YVSG/YDUST	Teq	AVc
Region			(× 10−2)	(× 10−2)	(× 10−2)	(K)	(mag)
N63	1	...	...	...	...	...	...
	2	9.0 ± 3.7	4.6 ± 2.0	34.9 ± 20.4	24.7 ± 14.7	${26.6}^{+0.7}_{-2.4}$	0.37 ± 0.06
	3	10.4 ± 1.1	5.3 ± 1.2	22.1 ± 5.1	17.1 ± 3.9	${27.2}^{+0.8}_{-2.5}$	0.23 ± 0.01
	4	15.5 ± 1.7	4.8 ± 1.9	25.3 ± 6.6	19.2 ± 5.0	${29.2}^{+0.8}_{-2.7}$	0.13 ± 0.01
	5	8.9 ± 0.8	4.5 ± 0.8	13.2 ± 3.5	11.1 ± 2.9	${26.5}^{+0.7}_{-2.4}$	0.12 ± 0.01
	6	5.7 ± 0.5	4.8 ± 1.1	8.9 ± 2.8	7.8 ± 2.4	${24.6}^{+0.7}_{-2.2}$	0.15 ± 0.01
	7	5.0 ± 1.0	1.2 ± 1.0	5.3 ± 3.2	5.0 ± 3.0	${24.0}^{+0.7}_{-2.2}$	0.21 ± 0.03
	8	...	...	...	...	...
	9	9.7 ± 4.9	5.2 ± 3.6	9.1 ± 12.0	7.9 ± 10.5	${26.9}^{+0.7}_{-2.4}$	0.48 ± 0.08
	10	1.1 ± 0.4	9.6 ± 9.0	3.7 ± 16.5	3.3 ± 14.4	${18.6}^{+0.5}_{-1.6}$	1.13 ± 0.39
	11	...	...	...	...	...
	12	...	...	...	...	...	...
	13	12.6 ± 2.7	5.6 ± 1.7	63.0 ± 27.2	36.5 ± 15.3	${28.2}^{+0.8}_{-2.6}$	0.36 ± 0.07
	14	7.4 ± 1.0	5.6 ± 1.5	19.3 ± 7.1	15.2 ± 5.6	${25.7}^{+0.7}_{-2.3}$	0.47 ± 0.04
	15	10.8 ± 1.9	10.9 ± 3.0	14.3 ± 6.3	11.2 ± 4.8	${27.5}^{+0.8}_{-2.5}$	0.26 ± 0.03
	16	9.5 ± 0.9	6.7 ± 1.2	8.4 ± 3.2	7.2 ± 2.7	${26.8}^{+0.7}_{-2.4}$	0.29 ± 0.01
	17	22.9 ± 7.9	6.6 ± 3.2	13.5 ± 9.7	11.1 ± 8.0	${31.3}^{+0.9}_{-2.9}$	0.08 ± 0.01
	18	...	...	...	...	...
	19	4.5 ± 0.4	3.7 ± 1.1	22.5 ± 5.9	17.7 ± 4.7	${23.6}^{+0.6}_{-2.1}$	0.35 ± 0.02
	20	5.8 ± 0.6	6.4 ± 1.1	13.1 ± 2.9	10.8 ± 2.4	${24.6}^{+0.7}_{-2.2}$	0.30 ± 0.01
	21	3.9 ± 0.2	5.6 ± 1.0	6.9 ± 1.9	6.1 ± 1.7	${23.0}^{+0.6}_{-2.1}$	0.42 ± 0.02
	22	5.0 ± 0.5	6.6 ± 1.6	11.2 ± 3.5	9.4 ± 2.9	${24.0}^{+0.7}_{-2.2}$	0.35 ± 0.01
N180	1	14.6 ± 6.6	5.3 ± 3.0	41.4 ± 25.4	27.7 ± 17.4	${28.9}^{+0.8}_{-2.6}$	0.83 ± 0.16
	2	9.2 ± 2.2	4.9 ± 1.5	39.8 ± 14.5	27.1 ± 9.7	${26.7}^{+0.7}_{-2.4}$	0.91 ± 0.12
	3	7.7 ± 1.1	6.5 ± 1.3	45.3 ± 9.5	29.1 ± 6.2	${25.9}^{+0.7}_{-2.3}$	1.02 ± 0.08
	4	10.8 ± 2.3	5.8 ± 2.0	27.0 ± 8.9	20.0 ± 6.7	${27.4}^{+0.8}_{-2.5}$	0.85 ± 0.08
	5	33.5 ± 9.2	1.8 ± 1.9	16.7 ± 6.5	14.0 ± 5.5	${33.5}^{+0.9}_{-3.1}$	0.33 ± 0.03
	6	3.4 ± 0.5	10.0 ± 2.4	0.2 ± 3.2	0.2 ± 2.8	${22.5}^{+0.6}_{-2.0}$	1.66 ± 0.14
	7	3.3 ± 0.1	9.0 ± 0.5	0.0 ± 0.0	0.0 ± 0.0	${22.4}^{+0.6}_{-2.0}$	0.96 ± 0.03
	8	3.7 ± 0.3	12.3 ± 2.1	0.0 ± 0.0	0.0 ± 0.0	${22.8}^{+0.6}_{-2.0}$	0.74 ± 0.04
	9	2.5 ± 0.1	9.8 ± 1.0	0.0 ± 0.0	0.0 ± 0.0	${21.2}^{+0.6}_{-1.9}$	1.02 ± 0.06
	10	14.5 ± 0.5	0.0 ± 0.0	16.2 ± 1.2	14.0 ± 1.1	${28.9}^{+0.8}_{-2.6}$	0.21 ± 0.01
	11	5.1 ± 0.3	0.9 ± 2.5	10.4 ± 3.2	9.4 ± 2.9	${24.1}^{+0.7}_{-2.2}$	0.36 ± 0.02
	12	1.2 ± 0.0	10.4 ± 1.1	0.0 ± 0.0	0.0 ± 0.0	${18.8}^{+0.5}_{-1.7}$	1.62 ± 0.12
	13	4.1 ± 0.3	6.1 ± 0.9	7.1 ± 1.9	6.2 ± 1.6	${23.2}^{+0.6}_{-2.1}$	0.42 ± 0.01
	14	18.0 ± 3.2	6.0 ± 1.4	22.5 ± 9.0	17.3 ± 6.9	${30.0}^{+0.8}_{-2.7}$	0.60 ± 0.07
	15	15.6 ± 5.6	5.8 ± 2.7	14.8 ± 9.5	12.2 ± 7.6	${29.3}^{+0.8}_{-2.7}$	0.47 ± 0.11
	16	7.4 ± 2.6	6.1 ± 2.4	11.8 ± 7.6	9.9 ± 6.4	${25.7}^{+0.7}_{-2.3}$	0.38 ± 0.06
	17	9.0 ± 0.9	5.2 ± 1.0	9.5 ± 2.6	8.2 ± 2.2	${26.6}^{+0.7}_{-2.4}$	0.21 ± 0.02
	18	8.0 ± 2.9	5.4 ± 3.5	1.8 ± 3.8	1.6 ± 3.6	${26.0}^{+0.7}_{-2.4}$	0.11 ± 0.04
	19	11.0 ± 3.7	0.7 ± 2.3	1.7 ± 3.9	1.6 ± 3.8	${27.5}^{+0.8}_{-2.5}$	0.03 ± 0.01
	20	62.6 ± 24.7	10.1 ± 6.9	0.0 ± 0.0	0.0 ± 0.0	${37.4}^{+1.1}_{-3.5}$	0.01 ± 0.01
N70	1	5.5 ± 0.8	7.8 ± 1.5	1.7 ± 2.8	1.6 ± 2.5	${24.4}^{+0.7}_{-2.2}$	0.48 ± 0.04
	2	9.1 ± 1.9	8.4 ± 3.6	6.6 ± 5.0	5.7 ± 4.3	${26.6}^{+0.7}_{-2.4}$	0.21 ± 0.03
	3	5.4 ± 1.3	5.9 ± 1.7	15.4 ± 12.5	12.6 ± 10.2	${24.3}^{+0.7}_{-2.2}$	0.27 ± 0.04
	4	13.3 ± 3.6	13.5 ± 5.0	5.1 ± 5.7	4.2 ± 4.7	${28.5}^{+0.8}_{-2.6}$	0.07 ± 0.01
	5	6.9 ± 0.8	5.0 ± 1.9	8.4 ± 3.7	7.3 ± 3.2	${25.4}^{+0.7}_{-2.3}$	0.08 ± 0.01
	6	...	...	...	...	...	...
	7	...	...	...	...	...	...
	8	...	...	...	...	...	...
	9	4.4 ± 0.3	8.1 ± 1.1	1.1 ± 1.7	1.0 ± 1.5	${23.5}^{+0.6}_{-2.1}$	0.32 ± 0.01
	10	7.8 ± 0.5	0.0 ± 1.0	17.4 ± 2.7	14.8 ± 2.3	${25.9}^{+0.7}_{-2.3}$	0.12 ± 0.01
	11	5.3 ± 0.4	10.4 ± 1.2	1.8 ± 1.8	1.6 ± 1.6	${24.3}^{+0.7}_{-2.2}$	0.14 ± 0.01
	12	5.5 ± 0.8	8.4 ± 2.7	3.0 ± 4.7	2.7 ± 4.2	${24.4}^{+0.7}_{-2.2}$	0.12 ± 0.01
	13	3.8 ± 0.7	2.7 ± 2.2	4.7 ± 4.2	4.3 ± 3.9	${22.9}^{+0.6}_{-2.0}$	0.08 ± 0.01
	14	4.2 ± 1.0	0.0 ± 0.0	12.8 ± 9.7	11.3 ± 8.7	${23.3}^{+0.6}_{-2.1}$	0.05 ± 0.01
	15	15.8 ± 3.7	1.5 ± 3.6	6.6 ± 6.8	6.1 ± 6.3	${29.3}^{+0.8}_{-2.7}$	0.02 ± 0.01
	16	4.5 ± 0.8	10.5 ± 3.1	4.1 ± 6.3	3.5 ± 5.5	${23.5}^{+0.6}_{-2.1}$	0.79 ± 0.09
	17	6.7 ± 1.0	5.8 ± 1.2	9.9 ± 5.4	8.5 ± 4.7	${25.3}^{+0.7}_{-2.3}$	0.37 ± 0.03
	18	7.0 ± 1.6	10.3 ± 3.3	5.3 ± 4.2	4.5 ± 3.5	${25.4}^{+0.7}_{-2.3}$	0.26 ± 0.05
	19	3.7 ± 0.7	7.6 ± 2.1	6.5 ± 3.8	5.6 ± 3.3	${22.8}^{+0.6}_{-2.0}$	0.23 ± 0.01
	20	16.1 ± 5.0	4.7 ± 3.1	20.0 ± 11.4	15.9 ± 8.8	${29.4}^{+0.8}_{-2.7}$	0.02 ± 0.01
	21	...	...	...	...	...	...
	22	...	...	...	...	...	...
	23	3.5 ± 0.4	6.9 ± 1.4	0.5 ± 2.1	0.4 ± 1.9	${22.5}^{+0.6}_{-2.0}$	0.41 ± 0.03
	24	4.5 ± 0.3	8.1 ± 1.1	0.5 ± 1.6	0.5 ± 1.5	${23.6}^{+0.6}_{-2.1}$	0.20 ± 0.01
	25	6.0 ± 0.6	10.4 ± 1.6	1.3 ± 2.3	1.1 ± 2.0	${24.8}^{+0.7}_{-2.2}$	0.15 ± 0.01
	26	5.8 ± 0.6	8.7 ± 1.5	5.7 ± 2.6	4.9 ± 2.3	${24.6}^{+0.7}_{-2.2}$	0.19 ± 0.01
	27	5.3 ± 0.4	8.2 ± 1.0	2.2 ± 1.3	2.0 ± 1.2	${24.3}^{+0.7}_{-2.2}$	0.21 ± 0.01
	28	8.2 ± 0.6	12.4 ± 1.9	1.8 ± 3.6	1.6 ± 3.1	${26.1}^{+0.7}_{-2.4}$	0.16 ± 0.01
	29	5.5 ± 0.5	12.4 ± 1.8	10.4 ± 4.0	8.2 ± 3.2	${24.4}^{+0.7}_{-2.2}$	0.24 ± 0.01
N144	1	...	...	...	...	...	...
	2	20.5 ± 10.2	3.9 ± 2.4	32.7 ± 25.0	23.7 ± 18.3	${30.7}^{+0.9}_{-2.8}$	0.99 ± 0.23
	3	25.2 ± 10.3	7.6 ± 4.4	35.9 ± 29.7	24.4 ± 19.8	${31.8}^{+0.9}_{-2.9}$	1.12 ± 0.33
	4	46.6 ± 14.4	7.4 ± 3.1	13.9 ± 9.8	11.3 ± 7.9	${35.5}^{+1.0}_{-3.3}$	0.45 ± 0.08
	5	13.8 ± 1.9	9.2 ± 2.7	15.7 ± 6.3	12.3 ± 4.9	${28.6}^{+0.8}_{-2.6}$	0.71 ± 0.05
	6	18.4 ± 2.3	6.5 ± 1.2	11.0 ± 2.9	9.3 ± 2.5	${30.1}^{+0.8}_{-2.7}$	0.55 ± 0.02
	7	23.1 ± 6.5	9.8 ± 3.2	4.6 ± 4.3	4.0 ± 3.7	${31.3}^{+0.9}_{-2.9}$	0.29 ± 0.03
	8	11.5 ± 1.0	4.6 ± 1.3	11.8 ± 3.6	10.0 ± 3.1	${27.8}^{+0.8}_{-2.5}$	0.27 ± 0.01
	9	17.6 ± 5.3	6.0 ± 2.3	6.2 ± 5.3	5.5 ± 4.7	${29.9}^{+0.8}_{-2.7}$	0.14 ± 0.01
	10	...	...	...	...	...	...
	11	15.0 ± 3.3	3.3 ± 1.2	29.0 ± 12.6	21.8 ± 9.3	${29.1}^{+0.8}_{-2.6}$	0.22 ± 0.04
	12	14.1 ± 1.7	4.9 ± 1.0	24.9 ± 5.6	19.0 ± 4.2	${28.8}^{+0.8}_{-2.6}$	0.17 ± 0.01
	13	13.1 ± 1.9	2.8 ± 1.2	32.1 ± 6.7	23.6 ± 5.0	${28.4}^{+0.8}_{-2.6}$	0.12 ± 0.01
	14	23.6 ± 2.5	0.0 ± 0.0	57.8 ± 10.1	36.6 ± 6.2	${31.5}^{+0.9}_{-2.9}$	0.05 ± 0.01
	15	4.8 ± 1.3	2.6 ± 2.1	53.4 ± 19.9	33.9 ± 12.5	${23.8}^{+0.7}_{-2.1}$	0.14 ± 0.02
	16	1.8 ± 0.5	0.0 ± 0.0	23.1 ± 12.1	18.8 ± 10.0	${20.2}^{+0.5}_{-1.8}$	0.18 ± 0.02
	17	...	...	...	...	...	...
	18	17.4 ± 4.1	7.1 ± 2.2	42.5 ± 15.4	27.7 ± 9.9	${29.8}^{+0.8}_{-2.7}$	0.67 ± 0.09
	19	17.1 ± 6.1	11.6 ± 4.7	31.3 ± 17.7	21.1 ± 12.0	${29.7}^{+0.8}_{-2.7}$	0.43 ± 0.07
	20	12.2 ± 3.3	12.8 ± 4.2	29.3 ± 12.8	19.8 ± 8.2	${28.0}^{+0.8}_{-2.5}$	0.36 ± 0.06
	21	6.4 ± 2.3	5.6 ± 2.5	18.2 ± 12.0	14.5 ± 9.6	${25.1}^{+0.7}_{-2.3}$	0.32 ± 0.06
	22	12.3 ± 5.0	7.3 ± 4.1	18.3 ± 13.1	14.4 ± 9.8	${28.1}^{+0.8}_{-2.5}$	0.09 ± 0.03
	23	6.1 ± 1.7	3.1 ± 1.9	13.0 ± 8.8	11.1 ± 7.6	${24.8}^{+0.7}_{-2.2}$	0.10 ± 0.01
	24	...	...	...	...	...	...
	25	7.9 ± 2.3	3.4 ± 1.7	39.9 ± 18.4	27.5 ± 12.5	${26.0}^{+0.7}_{-2.3}$	0.49 ± 0.09
	26	18.9 ± 5.4	4.4 ± 3.2	47.4 ± 23.2	30.7 ± 14.6	${30.3}^{+0.8}_{-2.8}$	0.10 ± 0.02
	27	20.8 ± 4.2	2.7 ± 1.0	29.7 ± 9.4	22.2 ± 7.1	${30.8}^{+0.9}_{-2.8}$	0.11 ± 0.01
	28	19.6 ± 3.2	1.3 ± 2.1	29.4 ± 9.7	22.5 ± 7.4	${30.5}^{+0.8}_{-2.8}$	0.09 ± 0.01
	29	5.3 ± 0.6	7.2 ± 1.9	18.1 ± 6.9	14.2 ± 5.4	${24.2}^{+0.7}_{-2.2}$	0.31 ± 0.03
	30	10.6 ± 1.3	6.4 ± 1.9	13.6 ± 4.2	11.2 ± 3.5	${27.4}^{+0.8}_{-2.5}$	0.25 ± 0.02
	31	8.4 ± 0.9	5.5 ± 1.4	11.1 ± 3.7	9.5 ± 3.1	${26.3}^{+0.7}_{-2.4}$	0.29 ± 0.02

Notes. ^aUsing the Levenberg–Marquardt minimization method implemented in IDL (http://www.physics.wisc.edu/~craigm/idl/fitting.html). ^bISRF scaled from the solar neighborhood ISRF described in Mathis et al. (1983). ^cThe errors in the fitted A_V is approximated to be at least 0.01.

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An example of the dust emission model is shown in Figure 8 for N180 subregion 2 (high Y_VSG/Y_BG) and subregion 6 (high Y_PAH/Y_DUST). The model emission contributed by three grain populations are presented in dashed or dotted lines, with the total emission in the solid blue line. While the model SED reproduced the photometric flux densities included in the SED fitting, it did not predict 3.6 and 4.5 μm well. In fact, for most locations of our four regions, the 4.5 μm dust emission produced by the Désert et al. (1990) model is much lower than observed. To explain the 3.6 and 4.5 μm discrepancy, we tested more complicated PAHs models from Draine & Li (2007) and Compiègne et al. (2011) (also implemented in DustEM), which have both neutral and ionized PAHs. Although the usual small discrepancy (less than a factor of 3) at 3.6 μm can be explained by adjusting the PAH ionization fraction, the 4.5 μm dust model flux is still underpredicted. Thus, for our dust modeling, we excluded the 3.6 and 4.5 μm bands from our SED fits. Due to the lack of 4–5 μm spectroscopic observations to resolve the line and continuum emission, any further interpretation is beyond the scope of this work. Therefore, we have five fixed parameters (two IRAC and three MIPS bands) and four fitted parameters (Y_PAH, Y_VSG, Y_BG, and U).

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We note that the Désert et al. (1990) modeling of VSGs differs from many recent dust models (e.g., Draine & Li 2007; Compiègne et al. 2010). VSGs in newer models are typically modeled by multiple distributions of different grain types with sizes ranging from ∼3–400 Å. However, due to the limited number of bands, our data are best modeled by one VSG component, which, in the Désert et al. (1990) model, consists of "medium-sized" VSGs that have radii between 12 and 150 Å. This grain is highly dependent on whether or not an SED has a 24 μm excess. With only three grain types, we also note that BG and VSG are typically anti-correlated; thus, the change in the fraction of Y_VSG/Y_BG may be somewhat over-exaggerated.

5.1.1. N63

5.1.2. N180

5.2.1. N70

5.2.2. N144

As seen in N180 and N144, the most obvious parameter that changes radially from the central OB association is the PAH mass fraction due to the presence of a (giant) molecular cloud. In order to further test if such features are genuine, we picked a molecular cloud to the northwest of N180 that is not associated with photoionized gas (i.e., no Hα emission, Figure 5(a)). The subregions in this molecular cloud are plotted as red points in the CCDs of N180 (Figures 5(c) and (d)). These red points are in the same vicinity in the CCD as subregions 8–13 in N180, indicating a similar composition of PAH mass fraction in both molecular clouds.

Even on the outskirts of this northwest cloud, the red points stay in the same general vicinity in the CCD; however, N180 subregions 5–7 show transitory colors between the central OB association and the molecular cloud. For this northwest cloud, it is possible that PAHs are only excited on the surface of the cloud due to starlight and thus are not modified radially. N180, on the other hand, has coexistence of molecular gas and ionized gas, and intense radiation fields break down the PAHs. Thus, the destruction of PAHs by OB stars is likely to lead to changes in PAHs radially throughout an H ii region.

6.1.1. PAHs

Radiation is needed to excite PAHs; however, PAHs also can be destroyed in the presence of a strong radiation field (e.g., Madden et al. 2006; Lebouteiller et al. 2007), primarily due to high energy photons (e.g., Aitken & Roche 1985; Voit 1992). Moreover, the location of PAH and H₂ emission are strongly correlated in cool-PDRs, such as the filaments in the Horsehead Nebula (Habart et al. 2005; Compiègne et al. 2007) and ρ Ophiuchus (Habart et al. 2003). However, the locations become anti-correlated with higher radiation field, as seen in the Orion bar (Tielens et al. 1993) and in Monoceros R2 (Berné et al. 2009).

The results in this paper are consistent with these studies. We find that PAH emission is enhanced toward locations of molecular clouds except where the radiation is high. We also find that PAH emission is diminished at locations that are optically thin to UV radiation.

Since our regions are relatively young (no known SNRs in N144 and N180, one known in N63, and a history of one or multiple in N70), the radiation field is dominated by massive stars that produce harsh photons. Although high energy photons are the main mechanism for destroying PAHs rather than the radiation field, we will assume that the stellar content of the regions follow the same initial mass function and have similar spectral shape in their radiation field and attempt to provide an estimate of the typical U required for significant PAH destruction. In particular, we focus on N144 and N180 since N63 lacks large-scale CO observations and the dust emission from N70 is thought to come from the surrounding shell. For these two regions, Figure 9 shows both Y_PAH/Y_DUST and the color [8.0]–[24.0] versus the radiation field as ascertained by massive stars (U). We only show subregions when at least half the region contains a 2σ Magellanic Mopra Assessment (MAGMA; Wong et al. 2011) CO detection. Note that higher values of the color [8.0]–[24.0] indicates less PAH emission. For both panels, subregions for N180 indicate that PAHs are less coincident with CO molecular gas for high U, particularly above U ≈ 30. N144 subregions have points in the same general vicinity as N180, but an obvious correlation between PAHs and radiation field is not apparent. This could be due to the lack of data (particularly at low values of U) or physical differences between N144 and N180.

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Our calculations of the radiation field may be highly uncertain since they can be quite dissimilar (sometimes by about an order of magnitude) to the fitted results. However, the fitted results of U are typically below those of the massive stars, so U ≈ 30 is a fair estimate of an upper limit of the typical radiation field needed to destroy PAHs in N180. Though the U limit of N180 may not be representative of other molecular clouds, we note that maps in Galametz et al. (2013) also show a reduction at PAHs with similar modeled U values in the LMC classical H ii region N159, with a drastic depletion in the fraction of PAHs for U ≳ 100.

6.1.2. VSGs and BGs

In all of our H ii regions except N70 (whose dust is uniformly distributed about its shell), Y_VSG/Y_BG is highest at locations where the radiation field is the strongest, as evident in Figure 10. We stress that the VSG population is highly dependent on each SED's 24 μm excess and is typically anti-correlated with the BG population.

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Grain populations typically change due to both destruction (i.e., dust changes back to gas) or disruption (large grains break into smaller grains). Jones et al. (1996) showed that fast shocks (>150 km s⁻¹) likely destroy grains via sputtering, while the velocity requirement for shattering BGs with VSGs depends on the dust species, with velocities ranging from ∼1–3 km s⁻¹. In our regions, dust-dust collisions are probably uncommon due to low densities (n_H ∼ 100 cm⁻³). However, O-stars are known to have wind-blown bubbles expanding at velocities of 10–15 km s⁻¹ (with observations of such bubbles in N180, Nazé et al. 2001), which could lead to grain shattering.

Other possibilities of an increased Y_VSG include dust coagulation and dust erosion. Dust coagulation of the smallest grains into slightly larger VSGs could increase the 24 μm emission, but the dust in our regions are not in environments conducive to coagulation (see, e.g., Köhler et al. 2012). Bernard et al. (2008) suggested erosion of BGs into VSGs likely explains the LMC's 70 μm excess in neutral/ionized regions. Moreover, dust erosion is thought to be increased during star formation due to increased dust processing (Paradis et al. 2011). The central OB associations of our H ii regions have a rich history of previous star formation, which could certainly lead to more dust processing/erosion than the areas analyzed at the macro-scale (several 100 pc; Bernard et al. 2008), leading to the increased 24 μm emission. Thus, at scales of ∼10 pc, BGs near OB associations may have gone through substantial erosion that has caused this increase of VSGs.

On the other hand, Pilleri et al. (2012) saw a definite decrease of "evaporating" VSGs with increasing radiation field. The sources they analyzed typically had a higher radiation field (U ≳ 100) than our regions, though we still see an increase in VSGs from U = 100 to U = 1000. However, their dust model of VSGs follows more closely to the smallest grains of Li & Draine (2001) while ours follows Désert et al. (1990). Since Pilleri et al. (2012) modeled VSGs that are more similar to PAHs and thus have an emission spectrum at much shorter wavelengths, these VSGs are not comparable to the VSGs modeled in this paper.

We find that N70, the most evolved H ii region, likely has its dust mass located in the shell. However, major differences in dust properties between classical H ii regions and superbubbles are not obvious with this small sample size. Relaño et al. (2013) analyzes the SEDs of 119 H ii regions in M33, categorizing them in a likely evolutionary sequence (filled, mixed, shell, and clear shell morphologies). They found that each category had similar SED features, though differed slightly between each classification. Specifically, they found that the FIR peak is located at longer wavelengths for older regions (shells and clear shells), indicating that colder dust is likely present. Uniform cold dust temperatures are expected since gas has been pushed far away from the central heating source. This result is consistent with our analysis of N70, which shows a uniform, low fraction of Y_VSG/Y_BG compared to our less evolved H ii regions that do not have the same shell geometry.

Changes in dust populations are commonly studied in SNRs due to their dynamic properties. Fast shocks (≳ 150 km s⁻¹) allow dust to reach high velocities that may cause their destruction via catastrophic shattering or sputtering (e.g., Jones et al. 1996; Micelotta et al. 2010), though VSGs and PAHs can also be created through low-velocity shattering (Jones et al. 1996). While PAHs can survive or even be produced in slow shocks, fast shocks are generally considered to destroy all PAHs in a region except in circumstances where they can be recreated or protected by their environment (e.g., Micelotta et al. 2010; Seok et al. 2012). Additionally, harsh photons provide another mechanism of destroying the smallest grains; the radiation field in SNRs can vary drastically, though it is typically about 10–100 times the local ISRF (e.g., Andersen et al. 2011). With these different mechanisms for creation and destruction of dust, it is important to observe and model a wide range of SNRs with different shock velocities, densities, and radiation fields to test theoretical models. Moreover, dust properties of SNRs can be compared to our H ii regions to provide a diagnostic of how dust changes in varying radiation fields in the absence of large shock velocities.

The Spitzer IR Spectrograph (IRS) has been used to study a large number of SNRs. Dust produced in supernova ejecta has been detected in young (<300 yr) SNRs (Rho et al. 2009), and PAHs are detected in SNRs (e.g., Tappe et al. 2006). Using a large spectral survey of 14 Galactic SNRs, Andersen et al. (2011) used DustEM (also using the Désert et al. 1990 dust models) to model the contributions of emission from BGs, VSGs, and PAHs. They found that the ratio of VSGs to BGs is 2–3 times larger in SNRs than the plane of the Galaxy, and they attribute dust shattering as the mechanism for creating the surplus of VSGs. Two of the 14 SNRs had relatively high radiation fields; fits of these regions had relatively low PAHs and virtually no VSGs. Andersen et al. (2011) suggested that these regions have high shock velocities and low densities, which can lead to destruction of VSGs and PAHs through sputtering.

With high radiation, we find that the VSG population can increase and PAHs are generally destroyed; however, fast shocks can destroy both grains. Unlike SNR regions, we find that VSG population increases where there is higher U, and there are no known fast shocks in our regions. Indeed, while observations from XMM Newton show X-ray emission from N70, there is no X-ray emission toward N180 and N144, and for N63, X-ray emission is only associated with the known SNR N63A (S. L. Snowden et al., in preparation). It has been suggested that N70 has recently been heated by an interior SNR (Chu & Mac Low 1990; Rodríguez-González et al. 2011; N. Zhang et al. 2014). If shocks are the main mechanism that are heating the grains in N70 rather than the ISRF, our DustEM modeling would be invalid (since U is our free parameter rather than temperature). Regardless, the shape of the SEDs of each subregion is the same with a dip at 24 μm, which agree with our DustEM fit of a uniform, low abundance of VSG. Therefore, it is most likely that a past history of fast shocks has destroyed the population of VSGs, and no mechanism has been available for creation of new VSGs (due to, e.g., the lack of centrally located gas since it has been blown away).