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Register a new userIdentification of H$_2$CCC as a diffuse interstellar band carrier
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We present strong evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450,AA are caused by the $B,^1$B$_1$,$leftarrow$,$X,^1$A$_1$ transition of H$_2$CCC (l-C$_3$H$_2$). The large widths of the bands are due to the short lifetime of the $B,^1$B$_1$ electronic state. The bands are predicted from absorption measurements in a neon matrix and observed by cavity ring-down in the gas phase and show exact matches to the profiles and wavelengths of the two broad DIBs. The strength of the 5450,AA DIB leads to a l-C$_3$H$_2$ column density of $sim5times10^{14}$ cm$^{-2}$ towards HD,183143 and $sim2times10^{14}$,cm$^{-2}$ to HD,206267. Despite similar values of $E$($B-V$), the 4881 and 5450,AA DIBs in HD,204827 are less than one third their strength in HD,183143, while the column density of interstellar C$_3$ is unusually high for HD,204827 but undetectable for HD,183143. This can be understood if C$_3$ has been depleted by hydrogenation to species such as l-C$_3$H$_2$ towards HD,183143. There are also three rotationally resolved sets of triplets of l-C$_3$H$_2$ in the 6150$-$6330,AA region. Simulations, based on the derived spectroscopic constants and convolved with the expected instrumental and interstellar line broadening, show credible coincidences with sharp, weak DIBs for the two observable sets of triplets. The region of the third set is too obscured by the $alpha$-band of telluric O$_2$.
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We aim to make use of the measurements from the Giraffe Inner Bulge Survey (GIBS) and the Gaia$-$ESO survey (GES) to study the kinematics and distance of the carrier of DIB$,lambda$8620, as well as other properties. We successfully detected and measured DIB$,lambda$8620 in 760 of 4117 GES spectra. Combined with the DIBs measured in GIBS spectra, we confirmed a tight relation between EW and $E(J-K_{rm S})$ as well as $A_{rm V}$, with similar fitting coefficients to those found by previous works. With a more accurate sample and the consideration of the solar motion, the rest-frame wavelength of DIB$,lambda$8620 was redetermined as 8620.83 r{A}, with a mean fit error of 0.36 r{A}. We studied the kinematics of the DIB carriers by tracing their median radial velocities in each field in the local standard of rest ($V_{rm LSR}$) and into the galactocentric frame ($V_{rm GC}$), respectively, as a function of the Galactic longitudes. Based on the median $V_{rm LSR}$ and two Galactic rotation models, we obtained valid kinematic distances of the DIB carriers for nine GIBS and ten GES fields. We also found a linear relation between the DIB$,lambda$8620 measured in this work and the near-infrared DIB in APOGEE spectra at $1.5273,mu m$. We demonstrate that the DIB carriers can be located much closer to the observer than the background stars based on the following arguments: (i) qualitatively, the carriers occupy in the Galactic longitude$-$velocity diagram typical rotation velocities of stars in the local Galactic disk, while the background stars in the GIBS survey are mainly located in the Galactic bulge; (ii) quantitatively, all the derived kinematic distances of the DIB carriers are smaller than the median distances to background stars in each field.
We present observations which probe the small-scale structure of the interstellar medium using diffuse interstellar bands (DIBs). Towards HD 168075/6 in the Eagle Nebula, significant differences in DIB absorption are found between the two lines of sight, which are separated by 0.25 pc, and {lambda}5797 exhibits a velocity shift. Similar data are presented for four stars in the {mu} Sgr system. We also present a search for variations in DIB absorption towards {kappa} Vel, where the atomic lines are known to vary on scales of ~10 AU. Observations separated by ~9 yr yielded no evidence for changes in DIB absorption strength over this scale, but do reveal an unusual DIB spectrum.
The chemical pathways linking the small organic molecules commonly observed in molecular clouds to the large, complex, polycyclic species long-suspected to be carriers of the ubiquitous unidentified infrared emission bands remain unclear. To investigate whether the formation of mono- and poly-cyclic molecules observed in cold cores could form via the bottom-up reaction of ubiquitous carbon-chain species with, e.g. atomic hydrogen, a search is made for possible intermediates in data taken as part of the GOTHAM (GBT Observations of TMC-1 Hunting for Aromatic Molecules) project. Markov-Chain Monte Carlo (MCMC) Source Models were run to obtain column densities and excitation temperatures. Astrochemical models were run to examine possible formation routes, including a novel grain-surface pathway involving the hydrogenation of C$_6$N and HC$_6$N, as well as purely gas-phase reactions between C$_3$N and both propyne (CH$_3$CCH) and allene (CH$_2$CCH$_2$), as well as via the reaction CN + H$_2$CCCHCCH. We report the first detection of cyanoacetyleneallene (H$_2$CCCHC$_3$N) in space toward the TMC-1 cold cloud using the Robert C. Byrd 100 m Green Bank Telescope (GBT). Cyanoacetyleneallene may represent an intermediate between less-saturated carbon-chains, such as the cyanopolyynes, that are characteristic of cold cores and the more recently-discovered cyclic species like cyanocyclopentadiene. Results from our models show that the gas-phase allene-based formation route in particular produces abundances of H$_2$CCCHC$_3$N that match the column density of $2times10^{11}$ cm$^{-2}$ obtained from the MCMC Source Model, and that the grain-surface route yields large abundances on ices that could potentially be important as precursors for cyclic molecules.
In this paper, we present new data with interstellar C2 (Phillips bands A-X), from observations made with the Ultraviolet-Visual Echelle Spectrograph of the European Southern Observatory. We have determined the interstellar column densities and excitation temperatures of C2 for nine Galactic lines. For seven of these, C2 has never been observed before, so in this case the still small sample of interstellar clouds (26 lines of sight), where a detailed analysis of C2 excitation has been made, has increased significantly. This paper is a continuation of previous works where interstellar molecules (C2 and diffuse interstellar bands) have been analysed. Because the sample of interstellar clouds with C2 has increased, we can show that the width and shape of the profiles of some diffuse interstellar bands (6196 and 5797 A) apparently depend on the gas kinetic and rotational temperatures of C2; the profiles are broader because of the higher values of the gas kinetic and rotational temperatures of C2. There are also diffuse interstellar bands (4964 and 5850 A) for which this effect does not exist.
We report the first detection of chloronium, H$_2$Cl$^+$, in the interstellar medium, using the HIFI instrument aboard the emph{Herschel} Space Observatory. The $2_{12}-1_{01}$ lines of ortho-H$_2^{35}$Cl$^+$ and ortho-H$_2^{37}$Cl$^+$ are detected in absorption towards NGC~6334I, and the $1_{11}-0_{00}$ transition of para-H$_2^{35}$Cl$^+$ is detected in absorption towards NGC~6334I and Sgr~B2(S). The H$_2$Cl$^+$ column densities are compared to those of the chemically-related species HCl. The derived HCl/H$_2$Cl$^+$ column density ratios, $sim$1--10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H$_2$Cl$^+$ column densities, in excess of $10^{13}$~cm$^{-2}$, are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.
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