Chemistry in cometary comae.

Significant gas-phase chemistry occurs in the comae of bright comets, as is demonstrated here for the case of Comet Hale-Bopp. The abundance ratio of the two isomers, hydrogen cyanide and hydrogen isocyanide, is shown to vary with heliocentric distance in a way that is consistent with production of HNC by ion-molecule chemistry initiated by the photoionization of water. Likewise, the first maps of emission from HCO+ show an abundance and an extended distribution that are consistent with the same chemical model.

Chemical processing in the coma as the source of cometary HNC.

The discovery of hydrogen isocyanide (HNC) in comet Hyakutake with an abundance (relative to hydrogen cyanide, HCN) similar to that seen in dense interstellar clouds raised the possibility that these molecules might be surviving interstellar material. The preservation of material from the Sun's parent molecular cloud would provide important constraints on the processes that took place in the protostellar nebula. But another possibility is that HNC is produced by photochemical processes in the coma, which means that its abundance could not be used as a direct constraint on conditions in the early Solar System. Here we show that the HNC/HCN ratio determined for comet Hale-Bopp varied with heliocentric distance in a way that matches the predictions of models of gas-phase chemical production of HNC in the coma, but cannot be explained if the HNC molecules were coming from the comet's nucleus. We conclude that HNC forms mainly by chemical reactions in the coma, and that such reactions need to be considered when attempting to deduce the composition of the nucleus from observations of the coma.

HCO+ imaging of comet Hale-Bopp (C/1995 O1).

The HCO+ J = 1-0 rotational transition at 89.189 GHz has been mapped in comet Hale-Bopp (C/1995 O1) over a total of 38 individual days spanning the period 1997 March 10-June 20 with the Five College Radio Astronomy Observatory 14 m antenna. HCO+ is detectable over an extended region of the comet, with the peak emission commonly located 50,000-100,000 km in the antisolar direction. Maps made throughout the apparition show significant variability in the structure of the HCO+ coma, sometimes on timescales of several hours. The HCO+ brightness is usually depressed at the nucleus position, and on some occasions, the emission is spread into a ring around the position of the nucleus. Individual spectra within the maps display broad (approximately 4 km s-1) lines redshifted by 1-2 km s-1 or more from the nominal velocity of the nucleus, with the redshift typically increasing in the antisolar direction. The spectra and maps may be generally explained by models in which the ions are accelerated tailward at a rate on the order of 10 cm s-2, provided that HCO+ is destroyed within 50,000-100,000 km of the nucleus.

Chemical and physical gradients along the OMC-1 ridge.

We present a survey of the distribution of 20 chemical and isotopic molecular species along the central ridge of the Orion molecular cloud from 6' north to 6' south of BN-KL observed with the QUARRY focal plane array on the FCRAO 14 m telescope, which provides an angular resolution of approximately 50" in the 3 mm wavelength region. We use standard tools of multivariate analysis for a systematic investigation of the similarities and differences among the maps of integrated intensities of the 32 lines observed. The maps fall in three broad classes: first, those strongly peaked toward BN-KL; second, those having rather flat distributions along the ridge; and third, those with a clear north-south gradient or contrast. We identify six positions or regions where we calculate relative abundances. Line velocities and line widths indicate that the optically thin lines generally trace the same volume of dense gas, except in the molecular bar, where C18O, C34S, H13CO+, CN, C2H, SO, and C3H2 have velocities characteristic of the bar itself, whereas the emission from other detected species is dominated by the background cloud. The strongest abundance variations in our data are the well-known enhancements seen in HCN, CH3OH, HC3N, and SO toward BN-KL and, less strongly, toward the Orion-South outflow 1'.3S. The principal result of this study is that along the extended quiescent ridge the chemical abundances, within factors of 3-4, exhibit an impressive degree of uniformity. The northern part of the ridge has a chemistry closest to that found in quiescent dense clouds. While temperature and density are similar around the northern radical-ion peak near 3'.5N and in the southern core near 4'.2S, some abundances, in particular, those of the ions HCO+ and N2H+, are significantly lower toward 4'.2S. The areas near 4.'2S and the molecular bar itself around (1'.7E, 2'.4S) stand out with peculiar and similar properties probably caused by stronger UV fields penetrating deeper into the clumpy molecular gas. This leads to higher electron abundances and thereby reduced abundances of the ions, as well as a lack of complex molecules.

A survey of the chemical properties of the M17 and Cepheus A cloud cores.

We present the results of a systematic survey of the chemical properties of two giant molecular cloud (GMC) cores in M17 and Cepheus A. In all, we have mapped the emission from 32 molecular transitions of 13 molecules and seven isotopic variants over a 4' x 5' region in each core. Each map includes known sites of massive star formation, as well as the more extended quiescent material. In M17 most molecules have emission peaks away from the H II region/molecular cloud interface, while two species, HC3N and CH3C2H, deviate from this structure with sharp maxima closer to this interface. In Cepheus A the core is influenced by a compact high-velocity molecular outflow and a more extended low-velocity flow. The molecular emission distributions in this source are generally quite similar, with most molecules peaking near the center of the core to the east of the compact H II region HW 2. A few molecules, SO, CH3OH, H13CN, and C18O, have more extended emission. Only two molecules, CO and HCO+, appear to trace the high- and low-velocity outflows; all other species are tracing the quiescent core. We have used the results of previous studies of the density and temperature of the dense gas in the same cloud cores to derive accurate abundances relative to CO for several positions in each core. The principal result is that the chemical composition of all the cores we have surveyed (which include OMC-1 as well as M17 and Cepheus A) show remarkable similarity, both within a given core and among the cores. This suggests that the chemical processes are similar in quiescent GMC core material. In M17 the lack of variation of molecular abundances is remarkable because the radiation field and the gas temperature are known to vary appreciably throughout the surveyed region, suggesting that the bulk of the emission arises from gas that is well shielded from radiation.

The HNC/HCN ratio in comets.

The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4-3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced in significant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus.

Collisional quenching of OH radio emission from comet Hale-Bopp.

Observations of comets in the 18-cm OH transitions offer a means to probe gas production, kinematics, and OH excitation in comets. We present initial results of OH observations of comet Hale-Bopp obtained with the NRAO 43 m antenna located in Greenbank, WV. Maps of the emission provide strong constraints on the amount of quenching of the inversion of the OH ground state A-doublet in the coma. Analysis of the total radio OH flux and maps of its radial brightness distribution indicate a quenched region on the order of approximately 500,000 km during March and April 1997. This large value is generally consistent with previous observations of radio OH quenching in lower production rate comets when the high production rate of comet Hale-Bopp is considered.

HCO+ in the coma of comet Hale-Bopp.

Maps of comet C/1995 O1 (Hale-Bopp) in the millimeter-wave emission of the ion HCO+ revealed a local minimum near the nucleus position, with a maximum about 100,000 km in the antisolar direction. These observed features of the HCO+ emission require a low abundance of HCO+ due to enhanced destruction in the inner coma of the comet, within a region of low electron temperature (Te). To set constraints on the formation of HCO+ in the coma, as well as the location and magnitude of the transition to higher Te, the data are compared with the results of ion-molecule chemistry models.

A study of the physics and chemistry of TMC-1.

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

Spectroscopic evidence for interstellar ices in comet Hyakutake.

Volatile compounds in comets are the most pristine materials surviving from the time of formation of the Solar System, and thus potentially provide information about conditions that prevailed in the primitive solar nebula. Moreover, comets may have supplied a substantial fraction of the volatiles on the terrestrial planets, perhaps including organic compounds that played a role in the origin of life on Earth. Here we report the detection of hydrogen isocyanide (HNC) in comet Hyakutake. The abundance of HNC relative to hydrogen cyanide (HCN) is very similar to that observed in quiescent interstellar molecular clouds, and quite different from the equilibrium ratio expected in the outermost solar nebula, where comets are thought to form. Such a departure from equilibrium has long been considered a hallmark of gas-phase chemical processing in the interstellar medium, suggesting that interstellar gases have been incorporated into the comet's nucleus, perhaps as ices frozen onto interstellar grains. If this interpretation is correct, our results should provide constraints on the temperature of the solar nebula, and the subsequent chemical processes that occurred in the region where comets formed.

1300 micron continuum and C18O line mapping of giant molecular cloud cores. II. W3, NGC 2264, NGC 6334I, RHO Ophiuchi and S140.

In this paper we present nearly simultaneous 1300 microns continuum and J = 2-1 C18O maps of the cores of five molecular clouds, W3, NGC 2264, NGC 6334I, rho Oph, and S140. The purpose of this experiment was to compare these two column density tracers. We find that dust continuum and C18O emission are equally effective tracers of column density in molecular cloud cores and give a good indication of cloud structure. When the maps are analyzed in terms of the quantity q = Q/[a rho RX(C18O)], we find that q does not vary by much more than an order of magnitude either within objects or from object to object, implying that nominal dust parameters of absorption efficiency, radius, and gas-to-dust ratio and CO abundance are on average correct in a variety of sources. We did detect source-to-source variations in q. This variation could be either in the dust-to-CO number density ratio or in grain parameters. These variations are not well correlated with total source luminosity, average or typical temperature, or total column density. The best example of this variation appears to be rho Oph where q is about a factor of 7 lower than is typically found. Our approach is analogous to the study of the A nu to CO ratio and is probably equivalent to extending this study to large A nu if the same grains are responsible for both optical opacity and far-infrared to millimeter-wave emission. There is no fundamental reason to expect A nu/NCO or q to be constant and, in fact, we have found that it is not constant in even a small source sample.

Observations of HCN in comet P/Halley.

We present observations of the HCN J = 1-0 rotational transition at 3.4 mm wavelength in comet P/Halley. The data were obtained during a total of 56 individual observing sessions between November 1985 and May 1986 and represent the first time that a cometary parent molecule has been so extensively monitored. The HCN production rate is well correlated with the total visual magnitude of the comet, and comparison of the HCN production to the total gas production of the comet indicates that it is a relatively minor constituent with 0.1% the abundance of H2O. Comparison of HCN and CN production suggests that HCN is a major parent molecule of CN, but probably not the sole parent. HCN spectra obtained by binning the data with heliocentric distance show that the line width, and thus the parent outflow velocity, increases with decreasing heliocentric distance, and that there is a tendency for the lines to be blue shifted due to anisotropic outgassing from the nucleus. Finally, there is evidence of day-to-day time variability in the total HCN emission and in the hyperfine ratios. The time variation of the total emission is consistent with the known time variable behavior of the comet, and detailed comparisons to optical data, where possible, confirm this interpretation. However, non-LTE values of the hyperfine ratios are not consistent with theoretical modeling of the excitation of these transitions.

1300 micron continuum and C18O line mapping of the giant molecular cloud cores in Orion, W49, and W51.

We present observations of the 1300 micron continuum emission and the C18O spectral-line emission from three well-studied giant molecular cloud cores: Orion, W49, and W51. The observations were obtained at the Five College Radio Astronomy Observatory, and they provide a means to examine the consistency of these two methods to trace the column density structure of molecular clouds. We find a good general correlation between the 1300 micron continuum, which traces the column density of dust, and the C18O J = 2 --> 1 line emission, which traces the column density of molecular gas, when the effects of source temperature are taken into consideration. Moreover, nominal values for the gas and dust abundances and the dust properties reproduce the observed continuum-to-line ratios. Thus, no strong C18O abundance gradients within sources has been found, and it appears that either the C18O emission lines or the submillimeter dust emission may be used to derive the mass column density within molecular clouds accurately.

High-resolution images of the L1551 bipolar outflow: evidence for an expanding, accelerated shell.

Approximately one-half of the L1551 bipolar outflow was mapped in the J = 1-0 transition of 12CO using the FCRAO 14 m telescope. The data were obtained by heavily oversampling over the beam area and then were reconstructed using a maximum entropy algorithm to obtain images of the high-velocity gas with an angular resolution of approximately 20". The outflow exhibits a striking shell-like structure with the lowest velocity out-flowing gas found along the limb of the outflow, and the highest velocity outflowing gas found along the axis of the outflow. A smooth transition is found between low-velocity emission on the periphery and high-velocity gas in the center. Our data can be modeled by molecular material located in a thin expanding shell which is accelerating away from IRS 5.

Search for molecular oxygen in dense interstellar clouds.

We have carried out a search for the 234 GHz N = 2 --> 0, J = 1 --> 1 transition of 16O18O using the 13.7 m FCRAO radio telescope. No emission was detected toward six giant molecular clouds. Observations of the 220 GHz J = 2 --> 1 transition of C18O yield column densities for this species 1-3 x 10(16) cm-2; the resulting limits on the [O2]/[CO] ratio lie between <0.5 and <4. According to various chemical models, the ratio of molecular oxygen to carbon monoxide is primarily sensitive to the age of a cloud and to its carbon to oxygen ratio. For ages exceeding 3 x 10(6) yr and total carbon-to-oxygen ratio < 1, [O2]/[CO] can approach unity. Our best limits can be interpreted as indicating that the observed clouds are not chemically "mature" or that [carbon]/[oxygen] > 1. However, significant exploitation of molecular oxygen as a tracer of cloud structure and evolution will require more sensitive observations, which may be best carried out from above Earth's atmosphere.

Large-scale structure of molecular gas in Heiles Cloud 2: a remarkable rotating ring.

We have mapped the J = 1 --> 0 transition of 13CO over a 3 deg2 region in Heiles Cloud 2 using the Five College Radio Astronomy Observatory 14 m telescope. The complete map contains 3600 individual spectra of which 2400 were sampled with 1' spacing. The map suggest that the structure of Heiles Cloud 2 is dominated by a quasi equilibrium rotating ring similar to those found in numerical calculations of the gravitational collapse of a rotating cloud. Within this ring, several dense condensations have been identified and partially mapped in C18O. These subcondensations, among which is the dense filament TMC-1, probably result from the instability of the ring to fragmentation and have masses on the order of the Jeans mass. Thus, they are marginally unstable to further collapse and may be the precursors of fragments that will eventually form stars.

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.