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Register a new userSpectroscopic Diagnostics of the Mid-Infrared Features of the Dark Globule, DC 314.8-5.1, with the Spitzer Space Telescope
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We present a detailed analysis of the mid-infrared spectra obtained from the Spitzer Space Telescope of the dark globule, DC 314.8-5.1, which is at the onset of low-mass star formation. The cloud has a serendipitous association with a B-type field star, which illuminates a reflection nebula in the cloud, allowing us to investigate infrared characteristics not otherwise discernible in such systems until a later evolutionary stage. We focus specifically on the polycyclic aromatic hydrocarbon (PAH) emission features prevalent throughout the mid-infrared range. We find that the intensity profiles do not obey any single unique scaling, for example a monotonic decrease related to the decreasing starlight toward the clouds central regions. We note that a diversity in trends over distance is also present in the intensity profiles of the molecular (H$_2$) and atomic (Ar, Ne, and S) emission lines, which are however much less prominent in the spectrum when compared with the PAH features. All in all, our analysis reveals that (i) there is a stratification in dust sizes within the reflection nebula, with larger grains dominating the PAH emission at the outskirts of the system, and (ii) the ionization level within the reflection nebula is fairly constant, and as such independent on the amount of the ionizing UV continuum from the neighbouring star, (iii) the intensity ratios of the prominent PAH features, do not follow correlations established for the reflection nebulae with active star formation.
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We measured the mid-infrared (MIR) extinction using Spitzer photometry and spectroscopy (3.6--37 micron) for a sample of Milky Way sightlines (mostly) having measured ultraviolet extinction curves. We used the pair method to determine the MIR extinction that we then fit with a power law for the continuum and modified Drude profiles for the silicate features. We derived 16 extinction curves having a range of A(V) (1.8-5.5) and R(V) values (2.4-4.3). Our sample includes two dense sightlines that have 3 micron ice feature detections and weak 2175 A bumps. The average A(lambda)/A(V) diffuse sightline extinction curve we calculate is lower than most previous literature measurements. This agrees better with literature diffuse dust grain models, though it is somewhat higher. The 10 micron silicate feature does not correlate with the 2175 A bump, for the first time providing direct observational confirmation that these two features arise from different grain populations. The strength of the 10 micron silicate feature varies by $sim$2.5 and is not correlated with A(V) or R(V). It is well fit by a modified Drude profile with strong correlations seen between the central wavelength, width, and asymmetry. We do not detect other features with limits in A(lambda)/A(V) units of 0.0026 (5--10 micron), 0.004 (10--20 micron), and 0.008 (20-40 micron). We find that the standard prescription of estimating R(V) from C times E(K_s-V)/E(B-V) has C = -1.14 and a scatter of $sim$7%. Using the IRAC 5.6 micron band instead of K_s gives C = -1.03 and the least scatter of $sim$3%.
We present the ultraviolet-optical-infrared spectral energy distribution of the low inclination novalike cataclysmic variable V592 Cassiopeiae, including new mid-infrared observations from 3.5-24 microns obtained with the Spitzer Space Telescope. At wavelengths shortward of 8 microns, the spectral energy distribution of V592 Cas is dominated by the steady state accretion disk, but there is flux density in excess of the summed stellar components and accretion disk at longer wavelengths. Reproducing the observed spectral energy distribution from ultraviolet to mid-infrared wavelengths can be accomplished by including a circumbinary disk composed of cool dust, with a maximum inner edge temperature of ~500 K. The total mass of circumbinary dust in V592 Cas (~10^21 g) is similar to that found from recent studies of infrared excess in magnetic CVs, and is too small to have a significant effect on the long-term secular evolution of the cataclysmic variable. The existence of circumbinary dust in V592 Cas is possibly linked to the presence of a wind outflow in this system, which can provide the necessary raw materials to replenish the circumbinary disk on relatively short timescales, and/or could be a remnant from the common envelope phase early in the formation history of the system.
In this paper I summarize the science motivations, as well as a few mid-infrared spectroscopic methods used to identify the principal mechanisms of energy production in dust enshrouded galactic nuclei. The development of the various techniques is briefly discussed. Emphasis is given to the use of the data which are becoming available with the infrared spectrograph (IRS) on Spitzer, as well as the results which have been obtained by IRS over the past two years.
The aromatic infrared bands (AIBs) observed in the mid infrared spectrum are attributed to Polycyclic Aromatic Hydrocarbons (PAHs). We observe the NGC 7023-North West (NW) PDR in the mid-infrared (10 - 19.5 micron) using the Infrared Spectrometer (IRS), on board Spitzer. Clear variations are observed in the spectra, most notably the ratio of the 11.0 to 11.2 micron bands, the peak position of the 11.2 and 12.0 micron bands, and the degree of asymmetry of the 11.2 micron band. The observed variations appear to change as a function of position within the PDR. We aim to explain these variations by a change in the abundances of the emitting components of the PDR. A Blind Signal Separation (BSS) method, i.e. a Non-Negative Matrix Factorization algorithm is applied to separate the observed spectrum into components. Using the NASA Ames PAH IR Spectroscopic Database, these extracted signals are fit. The observed signals alone were also fit using the database and these components are compared to the BSS components. Three component signals were extracted from the observation using BSS. We attribute the three signals to ionized PAHs, neutral PAHs, and Very Small Grains (VSGs). The fit of the BSS extracted spectra with the PAH database further confirms the attribution to ionized and neutral PAHs and provides confidence in both methods for producing reliable results. The 11.0 micron feature is attributed to PAH cations while the 11.2 micron band is attributed to neutral PAHs. The VSG signal shows a characteristically asymmetric broad feature at 11.3 micron with an extended red wing. By combining the NASA Ames PAH IR Spectroscopic Database fit with the BSS method, the independent results of each method can be confirmed and some limitations of each method are overcome.
We identify 17 possible 8.0 micron-selected counterparts to the submillimeter galaxies in the CUDSS 14-hour field, derived from deep imaging carried out with the IRAC and MIPS instruments aboard the Spitzer Space Telescope. Ten of the 17 counterparts are not the same as those previously identified at shorter wavelengths. We argue that 8.0 micron selection offers a better means for identifying counterparts to submillimeter galaxies than near-infrared or optical selection. Based on the panchromatic SEDs, most counterparts appear to be powered by ongoing star formation. Power-law fits to the SEDs suggest that five objects in the 8.0 micron-selected counterpart sample harbor dominant AGNs; a sixth object is identified as a possible AGN. The 3.6 to 8.0 micron colors of the infrared-selected counterparts are significantly redder than the general IRAC galaxy population in the CUDSS 14-hour field.
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