Discovery of SiCSi in IRC+10216: A missing link between gas and dust carriers of SiC bonds

We report the discovery in space of a disilicon species, SiCSi, from observations between 80 and 350 GHz with the IRAM 30m radio telescope. Owing to the close coordination between laboratory experiments and astrophysics, 112 lines have now been detected in the carbon-rich star CWLeo. The derived frequencies yield improved rotational and centrifugal distortion constants up to sixth order. From the line profiles and interferometric maps with the Submillimeter Array, the bulk of the SiCSi emis- sion arises from a region of 6 arcseconds in radius. The derived abundance is comparable to that of SiC2. As expected from chemical equilibrium calculations, SiCSi and SiC2 are the most abundant species harboring a SiC bond in the dust formation zone and certainly both play a key role in the formation of SiC dust grains.

Discovery of Time Variation of the Intensity of Molecular Lines in IRC+10216 in The Submillimeter and Far Infrared Domains

We report on the discovery of strong intensity variations in the high rotational lines of abundant molecular species towards the archetypical circumstellar envelope of IRC+10216. The observations have been carried out with the HIFI instrument on board textit{Herschel}thanks{textit{Herschel} is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA} and with the IRAMthanks{This work was based on observations carried out with the IRAM 30-meter telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain)} 30-m telescope. They cover several observing periods spreading over 3 years. The line intensity variations for molecules produced in the external layers of the envelope most probably result from time variations in the infrared pumping rates. We analyze the main implications this discovery has on the interpretation of molecular line emission in the envelopes of Mira-type stars. Radiative transfer calculations have to take into account both the time variability of infrared pumping and the possible variation of the dust and gas temperatures with stellar phase in order to reproduce the observation of molecular lines at different epochs. The effect of gas temperature variations with stellar phase could be particularly important for lines produced in the innermost regions of the envelope. Each layer of the circumstellar envelope sees the stellar light radiation with a different lag time (phase). Our results show that this effect must be included in the models. The sub-mm and FIR lines of AGB stars cannot anymore be considered as safe intensity calibrators.

Tentative Detection of the Nitrosylium Ion in Space

We report the tentative detection in space of the nitrosylium ion, NO$^+$. The observations were performed towards the cold dense core Barnard 1-b. The identification of the NO$^+$ $J$=2--1 line is supported by new laboratory measurements of NO$^+$ rotational lines up to the $J$=8--7 transition (953207.189,MHz), which leads to an improved set of molecular constants: $B_0 = 59597.1379(62)$,MHz, $D_0 = 169.428(65)$,kHz, and $eQq_0(textrm{N}) = -6.72(15)$,MHz. The profile of the feature assigned to NO$^+$ exhibits two velocity components at 6.5 and 7.5 km s$^{-1}$, with column densities of $1.5 times 10^{12}$ and $6.5times10^{11}$ cm$^{-2}$, respectively. New observations of NO and HNO, also reported here, allow to estimate the following abundance ratios: $X$(NO)/$X$(NO$^+$)$simeq511$, and $X$(HNO)/$X$(NO$^+$)$simeq1$. This latter value provides important constraints on the formation and destruction processes of HNO. The chemistry of NO$^+$ and other related nitrogen-bearing species is investigated by the means of a time-dependent gas phase model which includes an updated chemical network according to recent experimental studies. The predicted abundance for NO$^+$ and NO is found to be consistent with the observations. However, that of HNO relative to NO is too high. No satisfactory chemical paths have been found to explain the observed low abundance of HNO. HSCN and HNCS are also reported here with an abundance ratio of $simeq1$. Finally, we have searched for NNO, NO$_2$, HNNO$^+$, and NNOH$^+$, but only upper limits have been obtained for their column density, except for the latter for which we report a tentative 3-$sigma$ detection.

Laboratory and Astronomical Discovery of HydroMagnesium Isocyanide

We report on the detection of hydromagnesium isocyanide, HMgNC, in the laboratory and in the carbon rich evolved star IRC+10216. The J=1-0 and J=2-1 lines were observed in our microwave laboratory equipment in Valladolid with a spectral accuracy of 3,KHz. The hyperfine structure produced by the Nitrogen atom was resolved for both transitions. The derived rotational constants from the laboratory data are $B_0$=5481.4333(6),MHz, $D_0$=2.90(8),KHz, and $eQq(N)$=-2.200(2),MHz. The predicted frequencies for the rotational transitions of HMgNC in the millimeter domain have an accuracy of 0.2-0.7,MHz. Four rotational lines of this species, J=8-7, J=10-9, J=12-11 and J=13-12, have been detected towards IRC+10216. The differences between observed and calculated frequencies are $<$0.5,MHz. The rotational constants derived from space frequencies are $B_0$=5481.49(3),MHz and $D_0$=3.2(1),KHz, i.e., identical to the laboratory ones. A merged fit to the laboratory and space frequencies provides $B_0$=5481.4336(4),MHz and $D_0$=2.94(5),KHz. We have derived a column density for HMgNC of (6$pm$2)$times10^{11}$,cm$^{-2}$. From the observed line profiles the molecule have to be produced produced in the layer where other metal-isocyanides have been already found in this source. The abundance ratio between MgNC and its hydrogenated variety, HMgNC, is $simeq$20.