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Register a new userDetection of parent H2O and CO2 molecules in the 2.5--5 micron spectrum of comet C/2007 N3 (Lulin) observed with AKARI
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Comet C/2007 N3 (Lulin) was observed with the Japanese infrared satellite AKARI in the near-infrared at a post-perihelion heliocentric distance of 1.7 AU. Observations were performed with the spectroscopic (2.5--5.0 micron) and imaging (2.4, 3.2, and 4.1 micron) modes on 2009 March 30 and 31 UT, respectively. AKARI images of the comet exhibit a sunward crescent-like shape coma and a dust tail extended toward the anti-solar direction. The 4.1 micron image (CO/CO2 and dust grains) shows a distribution different from the 2.4 and 3.2 micron images (H2O and dust grains). The observed spectrum shows distinct bands at 2.66 and 4.26 micron, attributed to H2O and CO2, respectively. This is the fifth comet in which CO2 has been directly detected in the near-infrared spectrum. In addition, CO at 4.67 micron and a broad 3.2--3.6 micron emission band from C-H bearing molecules were detected in the AKARI spectrum. The relative abundance ratios CO2/H2O and CO/H2O derived from the molecular production rates are sim 4%--5% and < 2%, respectively. Comet Lulin belongs to the group that has relatively low abundances of CO and CO2 among the comets observed ever.
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We report optical imaging, optical and near-infrared polarimetry, and Spitzer mid-infrared spectroscopy of comet C/2007 N3 (Lulin). Polarimetric observations were obtained in R (0.676 micron) at phase angles from 0.44 degrees to 21 degrees with simultaneous observations in H (1.65 micron) at 4.0 degrees, exploring the negative branch in polarization. Comet C/2007 N3 (Lulin) shows typical negative polarization in the optical as well as a similar negative branch near-infrared wavelengths. The 10 micron silicate feature is only weakly in emission and according to our thermal models, is consistent with emission from a mixture of silicate and carbon material. We argue that large, low-porosity (akin to Ballistic Particle Cluster Aggregates) rather absorbing aggregate dust particles best explain both the polarimetric and the mid-infrared spectral energy distribution.
We present an analysis of simultaneous X-Ray and UV observations ofcomet C/2007 N3 (Lulin) taken on three days between January 2009 and March 2009 using the Swift observatory. For our X-ray observations, we used basic transforms to account for the movement of the comet to allow the combination of all available data to produce an exposure-corrected image. We fit a simple model to the extracted spectrum and measured an X-ray flux of 4.3+/-1.3 * 10^-13 ergs cm-2 s-1 in the 0.3 to 1.0 keV band. In the UV, we acquired large-aperture photometry and used a coma model to derive water production rates given assumptions regarding the distribution of water and its dissociation into OH molecules about the comets nucleus. We compare and discuss the X-ray and UV morphology of the comet. We show that the peak of the cometary X-ray emission is offset sunward of the UV peak emission, assumed to be the nucleus, by approximately 35,000 km. The offset observed, the shape of X-ray emission and the decrease of the X-ray emission comet-side of the peak, suggested that the comet was indeed collisionally thick to charge exchange, as expected from our measurements of the comets water production rate (6--8 10^28 mol. s-1). The X-ray spectrum is consistent with solar wind charge exchange emission, and the comet most likely interacted with a solar wind depleted of very highly ionised oxygen. We show that the measured X-ray lightcurve can be very well explained by variations in the comets gas production rates, the observing geometry and variations in the solar wind flux.
We observed comet C/2007 N3 (Lulin) twice on UT 28 January 2009, using the UV grism of the Ultraviolet and Optical Telescope (UVOT) on board the Swift Gamma Ray Burst space observatory. Grism spectroscopy provides spatially resolved spectroscopy over large apertures for faint objects. We developed a novel methodology to analyze grism observations of comets, and applied a Haser comet model to extract production rates of OH, CS, NH, CN, C3, C2, and dust. The water production rates retrieved from two visits on this date were $6.7 pm 0.7$ and 7.9 $pm$ 0.7 x 1E28 molecules s-1, respectively. Jets were sought (but not found) in the white-light and `OH images reported here, suggesting that the jets reported by Knight and Schleicher (2009) are unique to CN. Based on the abundances of its carbon-bearing species, comet Lulin is `typical (i.e., not `depleted) in its composition.
We present AKARI 2.5-5um spectra of 145 local luminous infrared galaxies in the Great Observatories All-sky LIRG Survey. In all of the spectra, we measure the line fluxes and EQWs of the polycyclic aromatic hydrocarbon (PAH) at 3.3um and the hydrogen recombination line Br-alpha, with apertures matched to the slit sizes of the Spitzer spectrograph and with an aperture covering ~95% of the total flux in the AKARI 2D spectra. The star formation rates (SFRs) derived from Br-alpha measured in the latter aperture agree well with SFRs(LIR), when the dust extinction correction is adopted based on the 9.7um absorption feature. Together with the Spitzer spectra, we are able to compare the 3.3 and 6.2um PAH features, the two most commonly used near/mid-IR indicators of starburst (SB) or active galactic nucleus (AGN) dominated galaxies. We find that the 3.3 and 6.2um PAH EQWs do not follow a linear correlation and at least 1/3 of galaxies classified as AGN-dominated using 3.3um PAH are classified as starbursts based on 6.2um PAH. These galaxies have a bluer continuum slope than galaxies that are indicated to be SB-dominated by both PAH features. The bluer continuum emission suggests that their continuum is dominated by stellar emission rather than hot dust. We also find that the median Spitzer spectra of these sources are remarkably similar to the pure SB-dominated sources indicated by high PAH EQWs in both 3.3 and 6.2um. We propose a revised SB/AGN diagnostic diagram using 2-5um data. We also use the AKARI and Spitzer spectra to examine the performance of our new diagnostics and to estimate 3.3um PAH fluxes using the JWST photometric bands in 0<z<5. Of the known PAH features and mid-IR high ionization emission lines used as SB/AGN indicators, only the 3.3um PAH feature is observable with JWST at z>3.5, because the rest of the features at longer wavelengths fall outside the JWST wavelength coverage.
We report on the results of systematic infrared 2.5-5 micron spectroscopy of 45 nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.3 using IRC onboard the AKARI satellite. This paper investigates whether the luminosities of these ULIRGs are dominated by starburst activity, or optically elusive buried AGNs are energetically important. Our criteria include the strengths of the 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission features and the optical depths of absorption features at 3.1 micron due to ice-covered dust grains and at 3.4 micron from bare carbonaceous dust grains. Because of the AKARI IRCs spectroscopic capability in the full 2.5-5 micron wavelength range, unaffected by Earths atmosphere, we can apply this energy diagnostic method to ULIRGs at z > 0.15. We estimate the intrinsic luminosities of extended (several kpc), modestly obscured (Av < 15 mag) starburst activity based on the 3.3 micron PAH emission luminosities measured in AKARI IRC slitless spectra, and confirm that such starbursts are energetically unimportant in nearby ULIRGs. In roughly half of the observed ULIRGs classified optically as non-Seyferts, we find signatures of luminous energy sources that produce no PAH emission and/or are more centrally concentrated than the surrounding dust. We interpret these energy sources as buried AGNs. The fraction of ULIRGs with detectable buried AGN signatures increases with increasing infrared luminosity. Our overall results support the scenario that luminous buried AGNs are important in many ULIRGs at z < 0.3 classified optically as non-Seyferts, and that the optical undetectability of such buried AGNs occurs merely because of a large amount of nuclear dust, which can make the sightline of even the lowest dust column density opaque to the ionizing radiation of the AGNs.
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