The Dynamical State of Barnard 68: A Thermally Supported, Pulsating Dark Cloud

We report sensitive, high resolution molecular-line observations of the dark cloud Barnard 68 obtained with the IRAM 30-m telescope. We analyze spectral-line observations of C18O, CS(2--1), C34S(2--1), and N2H+(1--0) in order to investigate the kinematics and dynamical state of the cloud. We find extremely narrow linewidths in the central regions of the cloud. These narrow lines are consistent with thermally broadened profiles for the measured gas temperature of 10.5 K. We determine the thermal pressure to be a factor 4 -- 5 times greater than the non-thermal (turbulent) pressure in the central regions of the cloud, indicating that thermal pressure is the primary source of support against gravity in this cloud. This confirms the inference of a thermally supported cloud drawn previously from deep infrared extinction measurements. The rotational kinetic energy is found to be only a few percent of the gravitational potential energy, indicating that the contribution of rotation to the overall stability of the cloud is insignificant. Finally, our observations show that CS line is optically thick and self-reversed across nearly the entire projected surface of the cloud. The shapes of the self-reversed profiles are asymmetric and are found to vary across the cloud in such a manner that the presence of both inward and outward motions are observed within the cloud. Moreover, these motions appear to be globally organized in a clear and systematic alternating spatial pattern which is suggestive of a small amplitude, non-radial oscillation or pulsation of the outer layers of the cloud about an equilibrium configuration.Comment: To appear in the Astrophysical Journal; 23 pages, 8 figures; Manuscript and higher resolution images can be obtained at http://cfa-www.harvard.edu/~ebergin/pubs_html/b68_vel.htm

Herschel observations of EXtra-Ordinary Sources (HEXOS): The present and future of spectral surveys with Herschel/HIFI

We present initial results from the Herschel GT key program: Herschel observations of EXtra-Ordinary Sources (HEXOS) and outline the promise and potential of spectral surveys with Herschel/HIFI. The HIFI instrument offers unprecedented sensitivity, as well as continuous spectral coverage across the gaps imposed by the atmosphere, opening up a largely unexplored wavelength regime to high-resolution spectroscopy. We show the spectrum of Orion KL between 480 and 560 GHz and from 1.06 to 1.115 THz. From these data, we confirm that HIFI separately measures the dust continuum and spectrally resolves emission lines in Orion KL. Based on this capability we demonstrate that the line contribution to the broad-band continuum in this molecule-rich source is ~20−40% below 1 THz and declines to a few percent at higher frequencies. We also tentatively identify multiple transitions of HD^(18)O in the spectra. The first detection of this rare isotopologue in the interstellar medium suggests that HDO emission is optically thick in the Orion hot core with HDO/H_2O ~ 0.02. We discuss the implications of this detection for the water D/H ratio in hot cores

The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks

We show in this Letter that the spectral details of the FUV radiation fields have a large impact on the chemistry of protoplanetary disks surrounding T Tauri stars. We show that the strength of a realistic stellar FUV field is significantly lower than typically assumed in chemical calculations and that the radiation field is dominated by strong line emission, most notably Lyman alpha radiation. The effects of the strong Lyman alpha emission on the chemical equilibrium in protoplanetary disks has previously been unrecognized. We discuss the impact of this radiation on molecular observations in the context of a radiative transfer model that includes both direct attenuation and scattering. In particular, Lyman alpha radiation will directly dissociate water vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter

A combined IRAM and Herschel/HIFI study of cyano(di)acetylene in Orion KL: tentative detection of DC3N

We present a study of cyanoacetylene (HC3N) and cyanodiacetylene (HC5N) in Orion KL, through observations from two line surveys performed with the IRAM 30m telescope and the HIFI instrument on board the Herschel telescope. The frequency ranges covered are 80-280 GHz and 480-1906 GHz. We model the observed lines of HC3N, HC5N, their isotopologues (including DC3N), and vibrational modes, using a non-LTE radiative transfer code. To investigate the chemical origin of HC3N and DC3N in Orion KL, we use a time-dependent chemical model. We detect 40 lines of the ground state of HC3N and 68 lines of its 13C isotopologues. We also detect 297 lines of six vibrational modes of this molecule (nu_7, 2nu_7, 3nu_7, nu_6, nu_5, and nu_6+nu_7) and 35 rotational lines of the ground state of HC5N. We report the first tentative detection of DC3N in a giant molecular cloud with a DC3N/HC3N abundance ratio of 0.015. We provide column densities and isotopic and molecular abundances. We also perform a 2x2" map around Orion IRc2 and we present maps of HC3N lines and maps of lines of the HC3N vibrational modes nu_6 and nu_7. In addition, a comparison of our results for HC3N with those in other clouds allows us to derive correlations between the column density, the FWHM, the mass, and the luminosity of the clouds. The high column densities of HC3N obtained in the hot core, make this molecule an excellent tracer of hot and dense gas. In addition, the large frequency range covered reveals the need to consider a temperature and density gradient in the hot core in order to obtain better line fits. The high D/H ratio (comparable to that obtained in cold clouds) that we derive suggests a deuterium enrichment. Our chemical models indicate that the possible deuterated HC3N present in Orion KL is formed during the gas-phase. This fact provides new hints concerning the processes leading to deuteration.Comment: 50 pages, 33 figures, 13 tables. Accepted for publication in A&

A Direct Measurement of the Total Gas Column Density in Orion KL

The large number of high-J lines of C^(18)O available via the Herschel Space Observatory provide an unprecedented ability to model the total CO column density in hot cores. Using the emission from all the observed lines (up to J = 15-14), we sum the column densities in each individual level to obtain the total column after correcting for the population in the unobserved states. With additional knowledge of source size, V_(LSR), and line width, and both local thermodynamic equilibrium (LTE) and non-LTE modeling, we have determined the total C^(18)O column densities in the Extended Ridge, Outflow/Plateau, Compact Ridge, and Hot Core components of Orion KL to be 1.4 × 10^(16) cm^(–2), 3.5 × 10^(16) cm^(–2), 2.2 × 10^(16) cm^(–2), and 6.2 × 10^(16) cm^(–2), respectively. We also find that the C^(18)O/C^(17)O abundance ratio varies from 1.7 in the Outflow/Plateau, 2.3 in the Extended Ridge, 3.0 in the Hot Core, and to 4.1 in the Compact Ridge. This is in agreement with models in which regions with higher ultraviolet radiation fields selectively dissociate C^(17)O, although care must be taken when interpreting these numbers due to the size of the uncertainties in the C^(18)O/C^(17)O abundance ratio

Chemical Differentiation toward the Pipe Nebula Starless Cores

We used the new IRAM 30-m FTS backend to perform an unbiased ~15 GHz wide survey at 3 mm toward the Pipe Nebula young diffuse starless cores. We found an unexpectedly rich chemistry. We propose a new observational classification based on the 3 mm molecular line emission normalized by the core visual extinction (Av). Based on this classification, we report a clear differentiation in terms of chemical composition and of line emission properties, which served to define three molecular core groups. The "diffuse" cores, Av<~15, show poor chemistry with mainly simple species (e.g. CS and CCH). The "oxo-sulfurated" cores, Av~15--22, appear to be abundant in species like SO and SO2, but also in HCO, which seem to disappear at higher densities. Finally, the "deuterated" cores, Av>~22, show typical evolved chemistry prior to the onset of the star formation process, with nitrogenated and deuterated species, as well as carbon chain molecules. Based on these categories, one of the "diffuse" cores (Core 47) has the spectral line properties of the "oxo-sulfurated" ones, which suggests that it is a possible failed core.Comment: Accepted for publication in A&A. 5 pages, 2 figure

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Experiments were performed during the period May–July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42− were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 μm and greater than 0.01 μm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42− is the dominant aerosol anion species, with Na+, NH4+, and Ca2+being the dominant cations. Impactor results indicate that MSA and SO42− have similar mass size distributions. Furthermore, MSA and SO42− have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/− 0.011 cm/s for NH4+ to 0.110 cm/s +/− 0.021 cm/s for Ca. SO42− dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor-estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles \u3e 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles \u3c 0.5 μm are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42−serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3−, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO42− and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals

The Dynamical State fo the Starless Dense Core FeSt 1-457: A Pulsating Globule?

High resolution molecular line observations of CS, HCO+, C18O and N2H+ were obtained toward the starless globule FeSt 1-457 in order to investigate its kinematics and chemistry. The HCO+ and CS spectra show clear self-reversed and asymmetric profiles across the face of the globule. The sense of the observed asymmetry is indicative of the global presence of expansion motions in the outer layers of the globule. These motions appear to be subsonic and significantly below the escape velocity of the globule. Comparison of our observations with near-infrared extinction data indicate that the globule is gravitationally bound. Taken together these considerations lead us to suggest that the observed expansion has its origin in an oscillatory motion of the outer layers of the globule which itself is likely in a quasi-stable state near hydrostatic equilibrium. Analysis of the observed linewidths of CO and N2H+ confirm that thermal pressure is the dominant component of the cloud's internal support. A simple calculation suggests that the dominant mode of pulsation would be an l = 2 mode with a period of 0.3 Myr. Deformation of the globule due to the large amplitude l = 2 oscillation may be responsible for the double-peaked structure of the core detected in high resolution extinction maps. Detailed comparison of the molecular-line observations and extinction data provides evidence for significant depletion of C18O and perhaps HCO+ while N2H+ may be undepleted to a cloud depth of about 40 magnitudes of visual extinction.Comment: to appear in ApJ vol 665 20 August 2007