Cyanide and isocyanide abundances in the cold, dark cloud TMC-1

We have observed emission from HCN, H13CN, HC15N, HN13C, H15NC, HC3N, CH3CN, and possibly CH3NC, and determined an upper limit for NH2CN, toward the cold, dark cloud TMC-1. The abundance ratio [HNC]/[HCN] = 1.55 +/- 0.16 is at least a factor approximately 4 and approximately 100 greater than that observed toward the giant molecular clouds DR 21(OH) and Orion KL, respectively. In contrast, for the corresponding methylated isomers we obtain [CH3NC]/CH3CN] < or approximately 0.1. We also find [NH2CN]/[CH3CN] < or approximately 0.1 and [HC3N]/[CH3CN] = 30 +/- 10. We find no evidence for anomalous hyperfine ratios for H13CN, indicating that the ratios for HCN (cf. recent work of Walmsley et al.) are the result of self-absorption by cold foreground gas.     

First astronomical detection of the cumulene carbon chain molecule H2C6 in TMC-1

The cumulene carbenes are important components of hydrocarbon chemistry in low-mass star-forming cores. Here we report the first astronomical detection of the long-chain cumulene carbene H2C6 in the interstellar cloud TMC-1, from observations of two of its rotational transitions: J(K,K') = 7(1,7) --> 6(1,6) at 18.8 GHz and 8(1,8) --> 7(1,7) at 21.5 GHz, using NASA's Deep Space Network 70 m antenna at Goldstone, California. In addition we also observed the shorter cumulene carbene H2C4 at the same position. The fractional abundance of H2C6 relative to H2 is about 4.7 x 10(-11) and that of H2C4 is about 4.1 x 10(-9). The abundance of H2C6 is in fairly good agreement with gas-phase chemical models for young molecular cloud cores, but the abundance of H2C4 is significantly larger than predicted.     

Detection of a new interstellar molecule, H2CN

We have detected a new interstellar molecule, H2CN (methylene amidogen), in the cold, dark molecular cloud TMC-l. The column density of H2CN is estimated to be approximately 1.5 x 10(11) cm-2 by assuming an excitation temperature of 5 K. This column density corresponds to a fractional abundance relative to H2 of approximately 1.5 x 10(-11). This value is more than three orders of magnitude less than the abundance of the related molecule HCN in TMC-1. We also report a tentative detection of H2CN in Sgr B2(N). The formation mechanism of H2CN is discussed. Our detection of the H2CN molecule may suggest the existence of a new series of carbon-chain molecules, CH2CnN (n = 0, 1, 2,...).     

Origin of oort cloud comets in the interstellar space

Comets must form a major part of the interstellar medium. The solar system provides a flux of comets into the interstellar space and there is no reason to suspect that many other stars and their surrounding cometary systems would not make a similar contribution. Occasionally interstellar comets must pass through the inner solar system, but Whipple (1975) considers it unlikely that such a comet is among the known cases of apparently hyperbolic comets. Even so the upper limit for the density of unobserved interstellar comets is relatively high.In addition, we must consider the possibility that comets are a genuine component of interstellar medium, and that the Oort Cloud is merely a captured part of it (McCrea, 1975). Here we review various dynamical possibilities of two-way exchange of comet populations between the Solar System and the interstellar medium. We describe ways in which a traditional Oort Cloud (Oort, 1950) could be captured from the interstellar medium. However, we note that the so called Kuiper belt (Kuiper, 1951) of comets cannot arise through this process. Therefore we have to ask how necessary the concept of the yet unobserved Kuiper belt is for the theory of short period comets.There has been considerable debate about the question whether short period comets can be understood as a captured population of the Oort Cloud of comets or whether an additional source has to be postulated. The problem is made difficult by the long integration times of comet orbits through the age of the Solar System. It would be better to have an accurate treatment of comet-planet encounters in a statistical sense, in the form of cross sections, and to carry out Monte Carlo studies. Here we describe the plan of action and initial results of the work to derive cross sections by carrying out large numbers of comet — planet encounters and by deriving approximate analytic expressions for them. Initially comets follow parabolic orbits of arbitrary inclination and perihelion distance; cross sections are derived for obtaining orbits of given energy and inclination after the encounter. The results are used in subsequent work to make evolutionary models of the comet population.     

Identification of the interstellar cyanomethyl radical (CH2CN) in the molecular clouds TMC-1 and Sagittarius B2

We report the astronomical identification of the cyanomethyl radical, CH2CN, the heaviest nonlinear molecular radical to be identified in interstellar clouds. The complex fine and hyperfine structures of the lowest rotational transitions at about 20.12 and 40.24 GHz are resolved in TMC-1, where the abundance appears to be about 5 x 10(-9) relative to that of H2. This is significantly greater than the observed abundance of CH3CN (methyl cyanide) in TMC-1. In Sgr B2 the hyperfine structure is blended in the higher frequency transitions at 40, 80, and 100 GHz, although the spin-rotation doubling is clearly evident. Preliminary searches in other sources indicate that the distribution of CH2CN is similar to that for such carbon chain species as HC3N or C4H.     

A study of C3HD in cold interstellar clouds

We have detected the 1(10)-1(01) transition of C3HD at 19.418 GHz at twelve positions in cold, dark clouds and resolved the D hyperfine components in two sources (L1498 and TMC-1C) well enough to derive values for the D quadrupole coupling constants. Simultaneous observations of C3H2 in each source yield relative integrated line intensities in the range 0.10-0.18, from which we derive relative [C3HD]/[C3H2] abundances in the range 0.05-0.15. These are among the highest deuteration ratios yet observed. Within the limits of the observational and modeling uncertainties it is possible to explain the derived [C3HD]/[C3H2] ratios by ion-molecule chemistry if [e-] approximately 3 x 10(-7).     

On the detection of the linear C5 molecule in the interstellar medium

An upper limit of the column density of the C₅ linear molecule in translucent interstellar clouds is estimated from high-resolution ( R =80 000) and very high signal-to-noise ratio (∼1000) echelle spectra. It is 10¹² cm⁻² per E ( B − V )=1 (two orders of magnitude lower than that of C₂).     

Laboratory detection of a new interstellar free radical CH2CN(2B1)

An asymmetric-top free radical CH2CN, which as a 2B1 ground state, was detected for the first time by laboratory microwave spectroscopy. The radical was produced in a free-space absorption cell by a DC glow discharge in pure CH3CN gas. About 60 fine-structure components were observed for the N = 11-10 to 14-13 a-type rotational transitions in the frequency region of 220-260 GHz, and many hyperfine resolved components for the N = 4-3 and 5-4 transitions in the 80 and 100 GHz regions, respectively. The molecular constants, including the rotational constants, centrifugal distortion constants, and spin-rotation coupling constants with centrifugal distortion correction terms were determined from the fine-structure resolved transitions, and the hyperfine coupling constants due to the hydrogen and nitrogen nuclei were obtained from the low-N transitions. As a result we assigned U100602 and U80484 from Sgr B2, and U40240 and U20120 from TMC-1, to the N = 5-4, 4-3, 2-1, and 1-0 transitions with K-1 = 0 of the CH2CN radical.     

Detection of interstellar hydrogen sulfide in cold, dark clouds

We have detected interstellar hydrogen sulfide (H2S) toward the cold, dark clouds L134N and TMC 1. We derive total column densities of approximately 2.6 x 10(13) cm-2 and approximately 7.0 x 10(12) cm-2 at the SO peak of L134N and at the NH3 peak of TMC 1, respectively. Since the expected gas phase reactions leading to the formation of H2S are thought to be endothermic, grain surface reactions may play a major role in the synthesis of this species in cold, dark clouds. If the carbon abundance is high and grain surface reactions are the dominant formation route, H2CS would be expected to form instead of H2S, and the abundances of H2CS have been observed to be high where those of H2S are low in L134N and TMC 1.     

On the contribution of grain surface reactions to interstellar molecule densities

Some recent arguments concerning the relevance of grain surface reactions to interstellar molecule formation are shown to be without foundation. It is probable that surface reactions, gas phase ion-molecule and neutral reactions all contribute to interstellar molecular densities.     

On the detection of interstellar homonuclear diatomic molecules

The pure rotational spectrum of homonuclear diatomic molecules in the interstellar medium is strongly forbidden, and no such spectrum has been detected. In regions of high excitation, vibrational emission may occur, as is widely detected in the case of H₂ in interstellar shocks and photon-dominated regions. However, it is of considerable interest to know the abundance of homonuclear diatomics in quiescent regions. We propose that vibrational emission from homonuclear diatomic molecules in cold clouds may be detectable, where the excitation is mainly through collisions with non-thermal electrons arising from the cosmic-ray ionization of H₂. As an example, we estimate the intensity of emission from N₂ in cold, dark interstellar clouds. We show that such emission is at the limit of detectability with current technology. Other excitation mechanisms may also contribute and enhance this emission.     

Far UV extinction and the interstellar 4430 Å band in the large magellanic cloud

The central depths of the interstellar 4430 Å band and far UV extinction for a sample of reddened LMC members are presented here.     

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2−41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST /STIS spectra of a high-velocity interstellar cloud ( v _LSR=+80 km s⁻¹) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s⁻¹ (FWHM). The high-velocity cloud absorption is observed in the lines of C  ii , O  i , Si  ii , Si  iii , Si  iv and S  iii . Limits can be placed on the amount of S  ii and Fe  ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C  ii and O  i , suggests that this high-velocity gas is warm ( T _k∼10³–10⁴ K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si  iv . This sightline also intercepts two other high-velocity clouds that produce weak absorption features at v _LSR=+113 and +130 km s⁻¹ in the STIS spectra.     

Energy ejection in the collapse of a cold spherical self-gravitating cloud

When an open system of classical point particles interacting by Newtonian gravity collapses and relaxes violently, an arbitrary amount of energy may, in principle, be carried away by particles which escape to infinity. We investigate here, using numerical simulations, how this released energy and other related quantities (notably the binding energy and size of the virialized structure) depend on the initial conditions, for the one-parameter family of starting configurations given by randomly distributing N cold particles in a spherical volume. Previous studies have established that the minimal size reached by the system scales approximately as   N ^1/3  , a behaviour which follows trivially when the growth of perturbations (which regularize the singularity of the cold collapse in the   N  →∞  limit) is assumed to be unaffected by the boundaries. Our study shows that the energy ejected grows approximately in proportion to   N ^1/3  , while the fraction of the initial mass ejected grows only very slowly with N , approximately logarithmically, in the range of N simulated. We examine in detail the mechanism of this mass and energy ejection, showing explicitly that it arises from the interplay of the growth of perturbations with the finite size of the system. A net lag of particles compared to their uniform spherical collapse trajectories develops first at the boundaries and then propagates into the volume during the collapse. Particles in the outer shells are then ejected as they scatter through the time-dependent potential of an already re-expanding central core. Using modified initial configurations, we explore the importance of fluctuations at different scales and discreteness (i.e. non-Vlasov) effects in the dynamics.     

Upper limits for the ethyl-cyanide abundances in TMC-1 and L134N: Chemical implications

We have sought interstellar ethyl-cyanide via its 2₀₂–1₀₁ transition towards two cold, dark clouds and report upper limits of the total column densities of 3×10¹²cm^–2 and 2×10¹²cm^–2 for TMC-1 and L134N, respectively. We also observed the 2₀₂–1₀₁ transition of vinyl cyanide previously identified in TMC-1 by Matthews and Sears (1983b). The detection of vinyl cyanide and the non-detection of ethyl cyanide in TMC-1 are consistent with gas phase ion-molecule chemical models, and there is thus no necessity of invoking grain surface synthesis for vinyl cyanide in cold clouds.