975 research outputs found
Terahertz Spectroscopy in the Lab and at Telescopes
The section of the electromagnetic spectrum extending roughly from wavelengths of 3 mm to 30 μm is commonly known as the far-infrared or TeraHertz (THz) region. It contains the great majority of the photons emitted by the universe, and THz observations of molecules and dust are able penetrate deeply into molecular clouds, thus revealing the full history of star and planet formation. Accordingly, the upcoming deployments of the Herschel, ALMA, and SOFIA observatories promise to revolutionize our understanding of THz astrophysics. To fully realize this promise, however, it is essential that we achieve a quantitative experimental understanding of the dust, ice, and gas which make up the ISM. After outlining the tremendous impact that Tom Phillips has had on astronomical applications of THz radiation, this contribution will describe how emerging technologies in ultrafast lasers are enabling the development of integrated frequency- and time-domain THz facilities that can acquire high dynamic range optical constants of the major components that comprise astrophysical dust, ice and organics across the full wavelength region accessible to Herschel and other THz observatories
Chemistry in Dense Molecular Clouds: Theory and Observational Constraints
For the most part, gas phase models of the chemistry of dense molecular clouds
predict the abundances of simple species rather well. However, for larger molecules and even for
small systems rich in carbon these models often fail spectacularly. We present a brief review of the
basic assumptions and results of large scale modeling of the chemistry in dense molecular clouds.
Particular attention will be paid to the influence of the gas phase ratios of the major elements
in molecular clouds, and the likely role grains play in maintaining these ratios as clouds evolve
from initially diffuse objects to denser cores with associated stellar and planetary formation.
Recent spectral line surveys at centimeter and millimeter wavelengths along with selected
observations in the submillimeter have now produced an accurate "inventory" of the gas phase
elemental budgets in different types of molecular clouds, though gaps in our knowledge clearly
remain. The constraints these observations place on theoretical models of interstellar chemistry
can be used to gain insights into why the models fail, and show also which neglected processes
must be included in more complete analyses. Looking toward the future, truly protostellar regions
are only now becoming available for both experimental and theoretical study, and some of the
expected modifications of molecular cloud chemistry in these sources are therefore outlined
Carbon Chemistry in Dense Molecular Clouds: Theory and Observational Constraints
For the most part, gas phase models of the chemistry of dense molecular clouds
predict the abundances of simple species rather well. However, for larger molecules and even
for small systems rich in carbon these models often fail spectacularly. We present a brief review
of the basic assumptions and results of large scale modeling of the carbon chemistry in dense
molecular clouds. Particular attention will be paid to the influence of the gas phase C/O ratio in
molecular clouds, and the likely role grains play in maintaining this ratio as clouds evolve from
initially diffuse objects to denser cores with associated stellar and planetary formation.
Recent spectral line surveys at centimeter and millimeter wavelengths along with selected
observations in the submillimeter have now produced an accurate "inventory" of the gas phase
carbon budget in several different types of molecular clouds, though gaps in our knowledge clearly
remain. The constraints these observations place on theoretical models of interstellar chemistry
can be used to gain insights into why the models fail, and show also which neglected processes
must be included in more complete analyses. Looking toward the future, larger molecules are
especially difficult to study both experimentally and theoretically in such dense, cold regions, and
some new methods are therefore outlined which may ultimately push the detectability of small
carbon chains and rings to much heavier species
ZEKE-PFI spectroscopy of 1:1 complexes of sodium with water and ammonia
ZEKE-PFI (zero kinetic energy pulsed field ionization) photoelectron spectra of the Na(H_2O), Na(D_2O), Na(NH_3), and Na(ND_3) complexes are reported. Spectra of all four complexes were obtained by single-photon ionization, and, for the Na(NH_3) and Na(ND_3) complexes, by two-color (1 + 1′) photoionization as well, with the Ã^2E state serving as the intermediate resonance. Improved values for the ionization energies (IE) and intermolecular vibrational frequencies of the complexes were determined. The single-photon ZEKE-PFI spectra show transitions only between states of the same vibrational symmetry, in accord with the selection rule for allowed electronic transitions. Some of the two-color ZEKE-PFI spectra, however, show strong transitions between states of different vibrational symmetry which we attribute to vibronic coupling in the intermediate state
Direct measurement of the HCl dimer tunneling rate and Cl isotope dependence by far-infrared laser sideband spectroscopy of planar supersonic jets
The large amplitude tunneling motion of the HCl dimer has been directly studied with a tunable far‐infrared laser sideband/two-dimensional free jet expansion spectrometer at hyperfine resolution. Rotationless tunneling rates for the three common chlorine isotopic forms are v(35–35)=463 979.2(1) MHz, v(35–37)=463 357.7(1) MHz, and v(37–37)=462 733.7(3) MHz. Both the rotational constants and hyperfine parameters indicate that the vibrationally averaged structure shows little variation within a given tunneling state, with both HCl bond angles giving an average projection on the a-axis of 47° in all states with resolved hyperfine patterns
High-Resolution 4.7 Micron Keck/NIRSPEC Spectroscopy of the CO Emission from the Disks Surrounding Herbig Ae Stars
We explore the high-resolution (λ/Δλ = 25,000; Δv = 12 km s^(-1)) M-band (4.7-5.1 μm) spectra of several disk-dominated Herbig Ae (HAe) systems: AB Aur, MWC 758, MWC 480, HD 163296, and VV Ser. All five objects show ^(12)CO v = 1-0 emission lines up to J = 42, but there is little or no evidence of moderate-J, v = 2-1 transitions despite their similar excitation energies. AB Aur shows ^(13)CO emission as well. The line/continuum ratios and intensity profiles are well correlated with inclination, and they trace collisionally driven emission from the inner disk (R_(th) ≾ 0.5-1 AU) as well as resonance fluorescence to much larger radii (R_(hν) ≾ 50-100 AU for J ≾ 10). The temperature, density, and radiation field profiles required to fit the CO emission are in good agreement with models of HAe disks derived from their spectral energy distributions. High-resolution and high dynamic range infrared spectroscopy of CO, and future observations of less abundant species, thus provide direct access to the physicochemical properties and surface structure of disks in regions where planet formation likely occurs
Observations of volatile species in protoplanetary disks
This talk will review recent highlights from early shared risk observations of the rotational emission lines from small molecules in the protoplanetary disks around young Sun-like stars. Particular emphasis will be placed on the synergy of the ALMA observations of the outer disk with high resolution spectroscopy from the ground and space (Herschel, Spitzer) at infrared through THz wavelenghts, and on observational constraints that can be placed on the location of key condensation fronts such as the (water) snow line.Ope
Comparison of submillimeter and ultraviolet observations of neutral carbon toward Zeta Ophiuchi
We have observed the ^3P_1 → ^3P_0 ground state transition of C_I emission toward ζ Oph. We compare this observation with predictions made from Copernicus ultraviolet absorption measurements of the population of the ^3P_1 level and with millimeter wave observations of CO
The circumstellar environment of the FU Orionis pre-outburst candidate V1331 Cygni
High resolution (~4") aperture synthesis maps of the CO (1→ 0), ^(13)CO (1→0), ^(13)CO (2→1), and asociated continuum emission from the FU Orions candidate V1331 Cygni reveal a massive, 0.5 ± 0.15 M_☉, circumstellar disk surrounded by a flattened gaseous envelope, 6000 x 4400 AU in size, mass >0.32 M_☉. These images and lower resolution measurements also trace a bipolar outflow and gaseous ring, 4.1 by 2.8 x 10^4 AU, mass greater than or equal to 0.07 M_☉, radially expanding at 22 ± 4 kms^(-1). We suggest this ring is a swept-up gaseous torus from an energetic mass ejection stage, possibly an FU Orionis outburst or outburts, ~4 x 10^3 yr ago that imparted >10^(45) ergs into the ambient cloud
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