Star-forming regions are often associated with nebulosity. In this study, we investigated infrared diffuse emission in Spitzer IRAC images. The infrared nebula L1527 traces outflows emanating from a low-mass protostar. The nebular color is consistent with the color of a stellar photosphere with large extinction. Nebulae around the HII region W5-East are bright in the infrared. These colors are consistent with the model color of dust containing polycyclic aromatic hydrocarbon (PAH). The strength of ultraviolet irradiation of the nebulae and the small dust fraction were deduced from the infrared colors of the nebulae. We found that the edges of the nebulae are irradiated by strong ultraviolet radiation and have abundant small dust. Dust at the surface of the molecular cloud is thought to be destroyed by ultraviolet radiation from an early-type star.
Dust is one of the most fundamental solid materials in the universe. It is ubiquitous, but dust grains have diverse sizes. It is widely believed that small dust is abundant in the interstellar medium. Interstellar extinction increases with decreasing wavelength in the ultraviolet (UV) and optical wavelengths, with a characteristic bump at 2175 Å [
Polycyclic aromatic hydrocarbon (PAH) is a major component of dust. PAH has many broad emission features in the near- and mid-infrared wavelengths. The strength of the PAH features depends on the number of molecules, degree of ionization, and strength of the UV radiation. [
The fluxes and shapes of PAH features can be obtained using an infrared spectrograph. However, the two-dimensional distribution of PAHs cannot be obtained except by slit-scan observations or the use of a three-dimensional spectrograph. Instead, we investigate the spatial distribution of PAHs by imaging observations. [
We investigate the infrared colors of nebulae irradiated by a nearby star using archival Spitzer IRAC data. By comparison with the model colors, we consider the properties of dust in molecular clouds. In this paper, the phrase “small dust” is used for dust with less than 103 C atoms, which corresponds to a diameter of ~13 Å, as it is used in DL07.
Infrared data were taken from the Spitzer Archive Center. We used the reduced data [post-basic calibrated data (PBCD)] of 3.6 μm images, 4.5 μm images, and 5.8 μm images (PI: Fazio Giovanni). The spatial resolution of the data is about 1.8 arc seconds. We measured the average and standard deviation of the sky region or the HII region adjacent to the nebula and then subtracted the average count from the image. Next, we made three types of mask images. The first mask image was designed to reject low signal-to-noise regions. We made it from the sky-subtracted image by replacing counts more than 10σ above the sky count with one and counts less than 10σ above the sky count to zero. The second mask image was designed for point source rejection. Point sources were identified on the sky-subtracted image by the DAOFIND task in the IRAF software, and the flux of each source was measured by aperture photometry. A mask was created for each source, as a large aperture mask was made for a bright object. The mask regions were set to zero, and the other regions were set to unity. We also made another point source mask image using Sextractor with the SEGMENTATION option in the CHECKIMAGE_TYPE parameter. These three types of mask images were multiplied by the sky-subtracted image so that only diffuse emission appears in the final image.
We investigate the infrared color of a nebula without PAH emission. L1527 is a reflection nebula associated with the low-mass protostar IRAS 04368 + 2557 in the Taurus molecular cloud. It is seen in edge-on geometry with bipolar outflows toward the east and west. Strong UV radiation is not expected from the central star; thus, PAH is not expected to be excited.
diagram distinguishes between a reflection nebula and an emission nebula containing PAH features.
From the figure, the amount of extinction is estimated. The photospheric color of the protostar IRAS 04368 + 2557 is assumed to be (0, 0), the same color as dwarfs and giants. We consider that the distance from the points of the nebular color to the line perpendicular to the extinction vector and through the origin corresponds to the extinction in the path from the central star through the nebula to us.
W5 is an intensively investigated HII region. It is located at a distance of 2.2 kpc in the Perseus arm and constitutes a chain of molecular clouds with W3 and W4.
[
W5 consists of two circular H II regions, W5-East (W5-E) and W5-West. The ionizing star in W5-E is an O7V star, BD +59˚ 0578. We investigate the diffuse emission of the nebulae in the northeast region of W5-E. In this region, two bright rim clouds (BRCs) have been identified [
DL07 calculated the IRAC colors of interstellar dust irradiated by starlight. They considered a mixture of amorphous silicate and graphitic dust. The size distribution of the silicate and graphitic dust follows the power law distributions presented by [
Heating of dust by starlight is considered. Its strength is parameterized by the factors of the interstellar radiation field ( U ISRF , [
The observed colors of the nebulae are consistent with the model colors of
DL07 (
The observed colors of the nebulae are consistent with the model colors of DL07 (
The strength of the UV radiation is estimated from the nebular color. We calculated the mean color of 5 × 5 pixel (3.0” × 3.0”). The observed colors of the nebulae are compared with the model color of DL07. The model-calculated PAH fluxes for single UV strengths are up to 3 × 105 times the interstellar value. We excluded a pixel if its observed color did not match the model color of DL07. We also ignored pixels near the bright stars, which are masked in the data reduction procedure.
the spatial distribution of the small dust fraction in W5-E. The distribution is not uniform in the nebulae. At the inner part of the nebulae, the small dust fraction is small, and at the edge of the nebulae, it is large. This inhomogeneity is especially obvious for BRC 13, a Type B BRC.
The fact that small dust is more abundant at the edge of the nebulae than in the inner part of the nebulae indicates that large dust grains at the surface of the molecular clouds are reduced to small dust grains by UV radiation from the early-type star.
Our claim that dust grains at the surface of the molecular cloud are destroyed by UV radiation emanating from an early-type star is deduced from the nebular color in the flux-calibrated images. To estimate the precise flux of the nebulae, we subtracted the flux of the HII region adjacent to the nebulae. We examined whether the uncertainty in the flux of the HII region challenges our claim. We added or subtracted a flux representing 5 times the standard deviation of the flux of the HII region and then estimated the strength of the UV radiation and fraction of small dust. Even after we added or subtracted the flux, the claim that small dust is abundant at the surface of the molecular cloud remained valid. Diffuse emission appears even in the HII region (see
The overabundance of small dust at the surface of the molecular cloud is deduced from the “blue” infrared colors at the edge of the nebula. Nebulae have different infrared colors not only with different small dust fractions and different UV radiation strengths but also with different amounts of extinction. The blue color at the edge of the nebula is naturally thought to be produced by the low extinction. At the edge of the nebulae, the small dust fraction is as high as 4%, whereas at the inner part of the nebulae, the fraction is as low as 2% and reaches 1.5% locally. The [4.5] - [5.8] color difference between the edge and the region with q = 1.5 % is about 0.2 mag (
The process of dust destruction has been intensively investigated. [
We investigated the infrared colors of nebulae associated with star-forming regions using Spitzer IRAC images.
• The color of the diffuse emission of L1527 is consistent with a photospheric color with large extinction. The central source of the emission is a low-mass protostar, which does not emit strong UV radiation that excites PAHs.
• The nebulae in W5-E facing the HII region are bright in the infrared. Their color is not uniform. A comparison with the model color of small dust emission revealed that the nebulae are bright in PAH emission features. The nebular color indicated that the nebulae facing the HII region are irradiated by strong UV radiation and contain a large population of small dust. We claim that the dust grains at the surface of the molecular cloud are destroyed by UV radiation from an early-type star.
We thank the editor and the referee for their comments. This work is based on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This work was supported by JSPS KAKENHI Grant Number JP24540231.
The authors declare no conflicts of interest regarding the publication of this paper.
Itoh, Y. and Oasa, Y. (2019) Spitzer IRAC Colors of Nebulae Associated with Star-Forming Regions. International Journal of Astronomy and Astrophysics, 9, 39-50. https://doi.org/10.4236/ijaa.2019.91004