Abstract We report new analyses of spectra of the 3.2–3.3 μ m absorption feature observed in the diffuse interstellar medium toward three Milky Way sources: 2MASS J 17470898 − 2829561 (2M1747) and ...the Quintuplet Cluster, both located in the Galactic center, and Cygnus OB2-12. The 3.2–3.3 μ m interval coincides with the CH-stretching region for compact polycyclic aromatic hydrocarbons (PAHs). We focus on the 2M1747 spectrum. Its published optical depth spectrum contains residual telluric transmission features, which arise from the 0.06 difference in mean airmasses between the observations of the source and its telluric standard star. We corrected the published spectrum by adding the airmass residual optical depth spectrum. The corrected spectrum is well fit by a superposition of four Gaussians. The absorption spectra of the other two sources were also fit by four Gaussians, with similar central wavelengths, widths, and relative peak opacities. We associate the three longer wavelength Gaussians covering the 3.23–3.31 μ m interval with compact PAHs in positive, neutral, and negative charge states. We identify the shortest-wavelength Gaussian, near 3.21 μ m, with irregularly shaped PAHs. Constraints imposed by spectral smoothness on the corrected 2M1747 spectrum, augmented by a PAH cluster formation model for post-asymptotic giant branch stars, suggests that >99% of the PAHs in the diffuse interstellar medium reside in small clusters. This study supports the PAH hypothesis, and it suggests that a family of primarily compact PAHs with a C 66 H 20 (circumvalene) parent is consistent with the observed mid-infrared and ultraviolet interstellar absorption spectrum.
Using a simple relation between the radial expansion velocity of diffuse gas in the Central Molecular Zone (CMZ) of the Galaxy and its distance from Sgr A* we estimate the physical depths within the ...CMZ of Star ι (2MASS J17470898-2829561) and two Sgr B2 far-infrared continuum sources with respect to the location of Sgr A*. To do this we use velocity profiles of infrared absorption lines of H3+ and of far-infrared absorption lines of H2O+, OH+, and 13CH+. The distances to Star ι and to the Sgr B2 sources are found to be ∼90 pc greater than the distance to Sgr A*. Our conclusion that Sgr B2 lies toward the rear of the CMZ is contrary to most previous models in which it has been placed shallower than Sgr A*.
Velocity profiles of a line of \({{\rm{H}}}_{3}^{+}\) at 3.7 μm produced in warm diffuse gas have been observed toward 18 stars in the Central Molecular Zone (CMZ) of the Galaxy. Their ...longitude–velocity diagram indicates that the gas is radially expanding within the CMZ at speeds up to a maximum of ∼150 km s−1. The current momentum and energy in the gas are ∼5 נ108 M ⊙ km s−1 and ∼5 נ1053 erg. The motion is similar to that of the Expanding Molecular Ring discovered in 1972 by Kaifu et al. and by Scoville. We propose that the expanding gas seen in \({{\rm{H}}}_{3}^{+}\) is part of the same phenomenon, despite differences in estimates of density, morphology, and degree of rotation. The outward motion suggests that one or more ejection events occurred near the center of the CMZ (0.5–1) נ106 yr ago, which may be related to creation of the recently observed microwave bubble. These observations revive the circular face-on view of the CMZ proposed in 1972, which fell out of favor after 1991 when Binney et al. proposed that a face-on view of the CMZ would show it to have an elliptical shape, with high eccentricity. While that model may apply on kiloparsec scales, we argue that it is incorrect to apply it to the much smaller CMZ. We discuss the fate of the expanding gas, which appears to be eventual infall into the center, leading to episodes of star formation and violent events associated with accretion onto Sgr A*.
A revised near-infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser and coworkers and Geballe and coworkers, and defined following ...the precepts of the MK process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0-T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion, and accessibility from both the Northern and Southern Hemispheres. The classification of T dwarfs is formally made by the direct comparison of near-infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H sub(2)O and CH sub(4) bands in the 1-2.5 km region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity, and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.
We report observations and analysis of infrared spectra of H3 + and CO lines in the Galactic center, within a few parsecs of the central black hole, Sgr A*. We find a cosmic ray ionization rate ...typically an order of magnitude higher than outside the Galactic center. Notwithstanding, the elevated cosmic ray ionization rate is 4 orders of magnitude too short to match the proton energy spectrum, as inferred from the recent discovery of the TeV γ-ray source in the vicinity of Sgr A*.
We present high-resolution images of NGC 2071-IR in the J, H, and K bands and in the emission at 2.12 m of the v = 1-0 S(1) line of molecular hydrogen. We also present moderate-resolution K-band ...spectra of two young stellar objects, IRS 1 and IRS 3, within NGC 2071-IR, that are candidate sources of one or more of the outflows observed in the region. Two of the eight originally identified infrared point sources in NGC 2071-IR are binaries, and we identify two new sources, one coincident with the radio source Very Large Array-1 and highly reddened. The H2 Q(3)/S(1) line intensity ratios at IRS 1 and IRS 3 yield high and very high extinctions, respectively, to them, as is implied by their near-infrared colors and K-band continuum slopes. The spectra also reveal the presence of hot, dense circumstellar molecular gas in each, suggesting that both are strong candidates for having energetic molecular outflows. We agree with a previous suggestion that IRS 1 is the likely source of an east-west-oriented outflow and conclude that this outflow is probably largely out of the plane of the sky. We also conclude that if IRS 3 is the source of the large-scale northeast-southwest outflow, as has been previously suggested, its jet/wind must precess in order to explain the angular width of that outflow. We discuss the nature of the point sources and their possible contributions, if any, to the complex morphology of the H2 line emission.
We present spatially resolved, near-diffraction-limited 10 km spectra of the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle, the mid-IR imager and spectrometer on the 8.1 m Gemini ...North Telescope. The spectra cover the nucleus and the central 6.0 x 0.4 of the ionization cones at a spatial resolution of approximately 0.4 (-30 pc). The spectra extracted in 0.4 steps along the slit reveal striking variations in continuum slope, silicate feature profile and depth, and fine-structure line fluxes on subarcsecond scales, illustrating in unprecedented detail the complexity of the circumnuclear regions of NGC 1068 at mid-IR wavelengths. A comparison of photometry in various apertures reveals two distinct components: a compact (radius < 15 pc), bright source within the central 0.4 x 0.4 and extended, lower brightness emission. We identify the compact source with the AGN-obscuring torus, and the diffuse component with dust in the ionization cones. While the torus emission dominates the flux observed in the near-IR, the mid-IR flux measured with apertures larger than about 1 is dominated instead by emission from the ionization cones; despite its higher brightness, the torus contributes <30% of the 11.6 km flux in the central 1.2 region. Many previous attempts to determine the torus spectral energy distribution are thus likely to be significantly affected by contamination from the extended emission. The observed spectrum of the compact source is compared with clumpy torus models. The models require most of the clouds to be located within a few parsecs of the central engine, in agreement with recent mid-IR interferometric observations. We also present a UKIRT/CGS4 5 km spectrum covering the R(0)-R(4) lines of the fundamental vibration-rotation band of super(12)CO. None of these lines was detected, and we discuss these nondetections in terms of the filling factor and composition of the nuclear clouds.
ABSTRACT We have investigated the behaviour of three strong near-infrared diffuse interstellar bands (DIBs) at λ13177 Å, λ14680 Å, and λ15272 Å, on a larger sample of sightlines and over a wider ...range of extinctions than previously studied, utilizing spectra from three observatories. We applied two telluric correction techniques to reduce atmospheric contamination and have used Gaussian fits to characterize the DIB profiles and measure equivalent widths. We confirmed strong and approximately linear correlations with reddening of the λ13177 Å, λ14680 Å, and λ15272 Å DIBs, extending them to higher reddening values and strengthening their link to interstellar matter. Modelling of the λ14680 Å DIB profiles revealed intrinsic variations, including line broadening, linked to their formation processes. This effect is particularly pronounced in the Galactic Centre (GC) environment, where multiple diffuse molecular clouds along the line of sight contribute to line broadening. We have detected one new DIB candidate at λ14795 Å on sightlines with high reddening.
We report the detection of emission in the v = 1 − 0 P(1) (3.51629 m) and P(2) (3.60776 m) rovibrational lines of the helium hydride cation (HeH+) from the planetary nebula NGC 7027. These detections ...were obtained with the iSHELL spectrograph on NASA's Infrared Telescope Facility on Maunakea. They confirm the discovery of HeH+ reported recently by Güsten et al., who used the GREAT instrument on the SOFIA airborne observatory to observe its pure rotational J = 1 − 0 transition at 149.137 m. The flux measured for the HeH+ v = 1 − 0 P(1) line is in good agreement with our model for the formation, destruction, and excitation of HeH+ in NGC 7027. The measured strength of the J = 1 − 0 pure rotational line, however, exceeds the model prediction significantly, as does that of the v = 1 − 0 P(2) line, by factors of 2.9 and 2.3 respectively. Possible causes of these discrepancies are discussed. Our observations of NGC 7027, covering the 3.26-3.93 m spectral region, have led to the detection of more than 60 spectral lines including nine rovibrational emissions from CH+. The latter are detected for the first time in an astronomical source.
Abstract
We have obtained images of the center of the star-forming cluster IC 348 with the James Webb Space Telescope and have identified brown dwarf candidates based on their photometry and ...point-like flux profiles. Low-resolution spectroscopy has been performed on four promising candidates, three of which have molecular absorption bands that indicate late spectral types. Among those late-type objects, the brightest is similar to known young L dwarfs while the other two show the so-called 3.4
μ
m feature that has been previously observed in the diffuse interstellar medium and in the atmospheres of Saturn and Titan, which has been attributed to an unidentified aliphatic hydrocarbon. Those two objects also exhibit features between 1.1 and 2.6
μ
m that we identify as the overtone and combination bands for that hydrocarbon. After accounting for the hydrocarbon bands, the remaining spectral features are consistent with youth and inconsistent with field dwarfs. Based on the low extinctions of those objects and the strengths of the overtone and combination bands, we conclude that the hydrocarbon resides in their atmospheres rather than in foreground material. Thus, our detections of the 3.4
μ
m feature are the first in atmospheres outside of the solar system. The presence of this hydrocarbon is not predicted by any atmospheric models of young brown dwarfs. Based on its luminosity and evolutionary models, the faintest new member of IC 348 has an estimated mass of 3–4
M
Jup
, making it a strong contender for the least massive free-floating brown dwarf that has been directly imaged to date.
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