IUCAA Preprints

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    Experimental and theoretical study on the infrared spectroscopy of astrophysically relevant PAH derivatives 2- and 9-vinylanthracene
    (2012-07-03) Maurya, Anju; Rastogi, Shantanu; Rouill´, Ga¨el; et.al
    We propose to evaluate the contribution of polycyclic aromatic hydrocarbon molecules that carry side groups to the mid-infrared emission spectra. Within this framework, the IR absorption spectra of 2- and 9-vinylanthracene were measured in Ar matrices at 12 K and in CsI and polyethylene pellets at room temperature. The laboratory spectra were analyzed with the support of simulations based on the density functional theory. For each PAH molecule, eight IR spectra were computed by combining the B3LYP functional with as many different basis sets, namely 4-31G, 4-31G(d), 6-31G, 6-311G, 6-31G(d), 6-31G(d,p), 6-31+G(d,p), and 6-31++G(d,p). The comparison of the theoretical spectra with the laboratory data allowed us to determine the most suitable combinations for modeling the IR spectra of neutral PAH molecules that carry a vinyl side group. It was concluded from the examples of 2- and 9-VA that the optimum basis set is 6-31G unless a steric interaction has to be taken into account, in which case the optimum basis set is 6-31G(d). Thus, in the presence of such an interaction, the use of d-type polarization functions is recommended. We discuss the possibility for neutral vinyl-substituted PAHs to contribute to the mid-infrared emission spectra and find that their specific features do not match with the mid-infrared aromatic emission band
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    Suggestions for an interstellar cyclopropene search
    (2001-07-05) Sharma, A. K.; Chandra, S.
    Following tentative detection of cyclopropene (C3H4) in Sgr B2 through its transition 322−221, several attempts to con rm the presence of cyclopropene in astronomical objects (including Sgr B2 itself) have been made. We suggest that cyclopropene may be observed in astronomical objects through its transition 220−221 at 3.67218 GHz, in absorption, even against the cosmic 2.7 K background, in a region having low density and low kinetic temperature.
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    Suggesions for an interstellar C5H2 search
    (2011-07-06) Chandra, Suresh; Shinde, S. A.
    Laboratory detection of four isomers of C5H2 molecule have been reported by Travers et al. (1997), McCarthy et al. (1997), and Gottlieb et al. (1998). They suggested for detection of the ring-chain isomer of C5H2 (c-C5H2)incosmic objects, as it is the most stable one in comparison to the others. Two transitions 313−212 and 303−202 at 19.147 GHz and 19.606 GHz, respectively, of c-C5H2 have been detected in TMC-1. We suggest that the c-C5H2 may be identified in cool cosmic objects through its transition 313−404 at 4.3 GHz in absorption against the cosmic microwave background. Since in absence of availability of the collisional rates, we have used scaled values for them, we have checked the sensitivity of the lines on the rates by enhancing the rate for the transitions with ∆ka = 0 by a factor of 10. Though the transitions are not found sensitive, our results may be treated as qualitative in nature. This absorption line may play an important role for identification of c-C5H2 in cosmic objects.
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    Multiwavelength investigation of the temperature of the cold neutral medium
    (2005-11-01) Roy, Nirupam; Chengalur, Jayaram; Srianand, R.
    We present measurements of the H i spin temperatures (Ts) of the Cold Neutral Medium (CNM) towards radio sources that are closely aligned with stars for which published H2 ortho-para temperatures (T01) are available from UV observations. Our sample consists of 18 radio sources close to 16 nearby stars. The transverse separation of the lines of sight of corresponding the UV and radio observations varies from 0.1 to 12.0 pc at the distance of the star. The ultraviolet (UV) measurements do not have velocity information, so we use the velocities of low ionization species (e.g Na i/K i/C i) observed towards these same stars to make a plausible identification of the CNM corresponding to the H2 absorption. We then find that T01 and Ts match within observational uncertainties for lines-of-sight with H2 column density above 1015.8 cm−2, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM Ts tracks the kinetic temperature due to collisions and that T01 is driven towards the kinetic temperature by proton exchange reactions.