We report on the design and performance of an echelon-based single shot visible/near-infrared spe... more We report on the design and performance of an echelon-based single shot visible/near-infrared spectrometer with adequate sensitivity to measure the nonlinear optical and terahertz Kerr effects in neat molecular liquids at room temperature. Useful molecular information spanning tens of picoseconds can be measured in just a few milliseconds, and the signal-to-noise performance scales favorably with respect to the standard stage scan technique. These results demonstrate the viability of stage-free nonlinear Kerr effect measurements and provide a route for improvements to the speed of future multidimensional Kerr effect studies.
Proceedings of the 70th International Symposium on Molecular Spectroscopy, 2015
We report the generation and detection of a decade-spanning TeraHertz (THz) frequency comb (0.15-... more We report the generation and detection of a decade-spanning TeraHertz (THz) frequency comb (0.15-2.4 THz) using two Ti:Sapphire femtosecond laser oscillators and ASynchronous OPtical Sampling THz Time-Domain Spectroscopy (ASOPS-THz-TDS). The measured linewidth of the comb at 1.5 THz is 3 kHz over a 60 second acquisition. With timedomain detection of the comb, we measure three transitions of water vapor at 10 mTorr between 1-2 THz with an average Doppler-limited fractional uncertainty of 5.9 × 10 −8. Significant improvements in bandwidth, resolution, and sensitivity are possible with existing technologies and will enable future studies of jet-cooled hydrogen-bonded clusters.
Proceedings of the 70th International Symposium on Molecular Spectroscopy, 2015
The ultrafast TeraHertz Kerr effect (TKE) has recently been demonstrated as a nonlinear spectrosc... more The ultrafast TeraHertz Kerr effect (TKE) has recently been demonstrated as a nonlinear spectroscopic technique capable of measuring the dielectric relaxation of liquids. The true power of this technique lies in its ability to provide complementary information to measurements taken using heterodyne-detected optical Kerr effect (OKE) spectroscopy. The optical pulses in OKE measurements interact with the sample via the molecular polarizability, a rank-two tensor, in contrast with THz pulses that interact with the molecules via the dipole moment, a rank-one tensor. Given the different light-matter interactions in the two techniques, TKE measurements help complete the physical picture of intermolecular interactions at short timescales. We report here our implementation of heterodyne-detected TKE spectroscopy, along with measurements of pure liquids, and binary mixtures. Some of the liquids presented here were previously believed to be TKE inactive, thus showing that we have achieved a greater sensitivity than the previous implementation in the literature. In addition, we will discuss a variety of binary mixtures and show how the TKE data can be compared with OKE data to deepen our physical understanding of intermolecular interactions in liquids.
The composition of fragments C and B of the Jupiter-family comet 73P/Schwassmann-Wachmann 3 (SW3)... more The composition of fragments C and B of the Jupiter-family comet 73P/Schwassmann-Wachmann 3 (SW3) was investigated in early April of 2006 at IR wavelengths using high-dispersion echelle spectroscopy. Both fragments were depleted in ethane, and C was depleted in most forms of volatile carbon. In particular, fragment C shows a severe depletion of CH 3 OH but a "normal" abundance of HCN (which has a similar volatility). Thermal processing is a possible explanation, but since fragment B is perhaps sublimating fresher material because of the frequent outbursts and fragmentation, the observed depletions might have cosmogonic implications. The chemistry of the volatile ices in SW3, like in the Oort Cloud comet C/1999 S4 (LINEAR), may be associated with sublimation of icy mantles from precometary grains followed by subsequent gas-phase chemistry and recondensation.
Aims. We derive the dense core structure and the water abundance in four massive star-forming reg... more Aims. We derive the dense core structure and the water abundance in four massive star-forming regions in the hope of understanding the earliest stages of massive star formation. Methods. We present Herschel/HIFI observations of the para-H 2 O 1 11 −0 00 and 2 02 −1 11 and the para-H 18 2 O 1 11 −0 00 transitions. The envelope contribution to the line profiles is separated from contributions by outflows and foreground clouds. The envelope contribution is modeled with Monte-Carlo radiative transfer codes for dust and molecular lines (MC3D and RATRAN), and the water abundance and the turbulent velocity width as free parameters. Results. While the outflows are mostly seen in emission in high-J lines, envelopes are seen in absorption in ground-state lines, which are almost saturated. The derived water abundances range from 5 ×10 −10 to 4 ×10 −8 in the outer envelopes. We detect cold clouds surrounding the protostar envelope, thanks to the very high quality of the Herschel/HIFI data and the unique ability of water to probe them. Several foreground clouds are also detected along the line of sight. Conclusions. The low H 2 O abundances in massive dense cores are in accordance with the expectation that high densities and low temperatures lead to freeze-out of water on dust grains. The spread in abundance values is not clearly linked to physical properties of the sources.
Early results from the Herschel Space Observatory revealed the water cation H 2 O + to be an abun... more Early results from the Herschel Space Observatory revealed the water cation H 2 O + to be an abundant ingredient of the interstellar medium. Here we present new observations of the H 2 O and H 2 O + lines at 1113.3 and 1115.2 GHz using the Herschel Space Observatory toward a sample of high-mass star-forming regions to observationally study the relation between H 2 O and H 2 O +. Nine out of ten sources show absorption from H 2 O + in a range of environments: the molecular clumps surrounding the forming and newly formed massive stars, bright high-velocity outflows associated with the massive protostars, and unrelated low-density clouds along the line of sight. Column densities per velocity component of H 2 O + are found in the range of 10 12 to a few 10 13 cm −2. The highest N(H 2 O +) column densities are found in the outflows of the sources. The ratios of H 2 O + /H 2 O are determined in a range from 0.01 to a few and are found to differ strongly between the observed environments with much lower ratios in the massive (proto)cluster envelopes (0.01−0.1) than in outflows and diffuse clouds. Remarkably, even for source components detected in H 2 O in emission, H 2 O + is still seen in absorption.
Aims. We aim to study the composition and energetics of the circumstellar material of DK Cha, an ... more Aims. We aim to study the composition and energetics of the circumstellar material of DK Cha, an intermediate-mass star in transition from an embedded configuration to a star plus disk stage, during this pivotal stage of its evolution. Methods. Using the range scan mode of PACS on the Herschel Space Observatory, we obtained a spectrum of DK Cha from 55 to 210 μm as part of the DIGIT key program. Results. Almost 50 molecular and atomic lines were detected, many more than the 7 lines detected in ISO-LWS. Nearly the entire ladder of CO from J = 14-13 to 38-37 (E u /k = 4080 K), water from levels as excited as J K −1 K +1 = 7 07 (E u /k = 843 K), and OH lines up to E u /k = 290 K were detected. Conclusions. The continuum emission in our PACS SED scan matches the flux expected by a model consisting of a star, a surrounding disk of 0.03 M , and an envelope of a similar mass, supporting the suggestion that the object is emerging from its main accretion stage. Molecular, atomic, and ionic emission lines in the far-infrared reveal the outflow's influence on the envelope. The inferred hot gas may be photon-heated, but some emission may be caused by C-shocks in the walls of the outflow cavity.
Context. We present far-infrared spectroscopic observations, taken with the Photodetector Array C... more Context. We present far-infrared spectroscopic observations, taken with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory, of the protoplanetary disk around the pre-main-sequence star HD 100546. These observations are the first within the DIGIT Herschel key program, which aims to follow the evolution of dust, ice, and gas from young stellar objects still embedded in their parental molecular cloud core, through the final pre-main-sequence phases when the circumstellar disks are dissipated. Aims. Our aim is to improve the constraints on temperature and chemical composition of the crystalline olivines in the disk of HD 100546 and to give an inventory of the gas lines present in its far-infrared spectrum. Methods. The 69 μm feature is analyzed in terms of position and shape to derive the dust temperature and composition. Furthermore, we detected 32 emission lines from five gaseous species and measured their line fluxes. Results. The 69 μm emission comes either from dust grains with ∼70 K at radii larger than 50 AU, as suggested by blackbody fitting, or it arises from ∼200 K dust at ∼13 AU, close to the midplane, as supported by radiative transfer models. We also conclude that the forsterite crystals have few defects and contain at most a few percent iron by mass. Forbidden line emission from [C ii] at 157 μm and [O i] at 63 and 145 μm, most likely due to photodissociation by stellar photons, is detected. Furthermore, five H 2 O and several OH lines are detected. We also found high-J rotational transition lines of CO, with rotational temperatures of ∼300 K for the transitions up to J = 22−21 and T ∼ 800 K for higher transitions.
Two-dimensional terahertz-terahertz-Raman spectroscopy can provide insight into the anharmoniciti... more Two-dimensional terahertz-terahertz-Raman spectroscopy can provide insight into the anharmonicities of low-energy phonon modes-knowledge of which can help develop strategies for coherent control of material properties. Measurements on LiNbO 3 reveal THz and Raman nonlinear transitions between the EðTO 1) and EðTO 3) phonon polaritons. Distinct coherence pathways are observed with different THz polarizations. The observed pathways suggest that the origin of the third-order nonlinear responses is due to mechanical anharmonicities, as opposed to electronic anharmonicities. Further, we confirm that the EðTO 1) and EðTO 3) phonon polaritons are excited through resonant one-photon THz excitation.
We present maps of over 1.5 square degrees in Chamaeleon (Cha) II at 24, 70, and 160 µm observed ... more We present maps of over 1.5 square degrees in Chamaeleon (Cha) II at 24, 70, and 160 µm observed with the Spitzer Space Telescope Multiband Imaging
For several hours on 2008 April 19 the T Tauri spectroscopic binary DQ Tau was observed to bright... more For several hours on 2008 April 19 the T Tauri spectroscopic binary DQ Tau was observed to brighten, reaching a maximum detected flux of 468 mJy and likely making it (briefly) the brightest object at 3 mm in the Taurus star-forming region. We present the light curve of a rarely before observed millimeter flare originating in the region around a pre-main-sequence star, and the first from a classical T Tauri star. We discuss the properties and nature of the flaring behavior in the context of pulsed accretion flows (the current picture based largely on studies of this object's optically variable spectrum), as well as magnetospheric re-connection models (a separate theory that predicts millimeter flares for close binaries of high orbital eccentricity). We believe that the flare mechanism is linked to the binary orbit, and therefore periodic. DQ Tau makes a strong case for multi-wavelength follow-up studies, performed in parallel, of future flares to help determine whether magnetosph...
We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the Chamaeleon II molecular cloud... more We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the Chamaeleon II molecular cloud. The observed area covers about 1 square degree defined by A_V >2. Analysis of the data in the 2005 c2d catalogs reveals a small number of sources (40) with properties similar to those of young stellaror substellar objects (YSOs). The surface density of these YSO candidates is low, and contamination by background galaxies appears to be substantial, especially for sources classified as Class I or flat SED. We discuss this problem in some detail and conclude that very few of the candidate YSOs in early evolutionary stages are actually in the Cha II cloud. Using a refined set of criteria, we define a smaller, but more reliable, set of 24 YSO candidates.
We present the first images of the 691.473 GHz CO J=6–5 line in a protoplanetary disk, obtained a... more We present the first images of the 691.473 GHz CO J=6–5 line in a protoplanetary disk, obtained along with the 690 GHz dust continuum, toward the classical T Tauri star TW Hya using the Submillimeter Array. Imaging in the CO J=6–5 line reveals a rotating disk, consistent with previous observations of CO J=3–2 and 2–1 lines. Using an irradiated accretion disk model and 2D Monte Carlo radiative transfer, we find that additional surface heating is needed to fit simultaneously the absolute and relative intensities of the CO J=6–5, 3–2 and 2–1 lines. In particular, the vertical gas temperature gradient in the disk must be steeper than that of the dust, mostly likely because the CO emission lines probe nearer to the surface of the disk. We have used an idealized X-ray heating model to fit the line profiles of CO J=2–1 and 3–2 with χ 2 analysis, and the prediction of this model yields CO J=6–5 emission consistent with the observations.
Abstract. Motivated by Yung and Miller’s [1997] sugges-tion that N2O is isotopically fractionated... more Abstract. Motivated by Yung and Miller’s [1997] sugges-tion that N2O is isotopically fractionated during UV pho-tolysis in the stratosphere, we have studied the photoly-sis rates of the 14N15N16O and 15N14N16O structural iso-topomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photol-ysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single-stage enrichment fac-tors of (14N15N16O / 14N14N16O) =-73 ± 5 per mil and (15N14N16O / 14N14N16O) =-41 ± 10 per mil. As predicted by Yung and Miller [1997], the photolysis rate of 15N14N16O is faster than 14N15N16O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
Our study began with the goal of developing new methods to test the radically new understanding o... more Our study began with the goal of developing new methods to test the radically new understanding of solar system formation that has recently emerged, and to identify innovative instrumentation targeted to this purpose. In particular, we were seeking to test predictions of dynamical models such as the Nice model, and to do so through interdisciplinary collaboration between the planetary dynamics communities that have formulated (and largely dominated discussion of) these new ideas, and the meteoritics and cosmochemistry communities who will be most involved in any in-situ mission to an outer solar system body. Our study was principally focused on coming up with explicit tests of the predictions of these new dynamical models of solar system evolution.
Motivated by Yung and Miller's [1997] suggestion that N2O is isotopically fractionated during... more Motivated by Yung and Miller's [1997] suggestion that N2O is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the ^(14)N^(15)N^(16)O and ^(15)N^(14)N^(16)O structural isotopomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photolysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single‐stage enrichment factors of ϵ(^(14)N^(15)N^(16)O / ^(14)N^(14)N^(16)O) = −73 ± 5 per mil and ϵ(^(15)N^(14)N^(16)O / ^(14)N^(14)N^(16)O) = −41 ± 10 per mil. As predicted by Yung and Miller [1997], the photolysis rate of ^(15)N^(14)N^(16)O is faster than ^(14)N^(15)N^(16)O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
Motivated by Yung and Miller's [199• suggestion that N20 is isotopically fractionated during ... more Motivated by Yung and Miller's [199• suggestion that N20 is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the •4N•SN•60 and •SN•4N•60 structural isotopomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photolysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single-stage enrichment factors of e(•4N•SN•O / •4N•4N•O) = -73 q5 per mil and e(•SN•4N•O / •4N•4N•O) -41 q10 per mil. As predicted by Yung and Miller [199•, the photolysis rate of •SN•4N•60 is faster than •4N•SN•O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
We demonstrate that halogenated methane 2D-Terahertz Terahertz Raman (2D-TTR) spectra are determi... more We demonstrate that halogenated methane 2D-Terahertz Terahertz Raman (2D-TTR) spectra are determined by the complicated structure of the instrument response function (IRF) along f 1 and by the molecular coherences along f 2. Experimental improvements have helped increase the resolution and dynamic range of the measurements, including accurate THz pulse shape characterization. Sum-frequency excitations convolved with the IRF are found to quantitatively reproduce the 2D-TTR signal. A new Reduced Density Matrix model which incorporates sum-frequency pathways, with linear and harmonic operators fully supports this (re)interpretation of the 2D-TTR spectra.
We report on the design and performance of an echelon-based single shot visible/near-infrared spe... more We report on the design and performance of an echelon-based single shot visible/near-infrared spectrometer with adequate sensitivity to measure the nonlinear optical and terahertz Kerr effects in neat molecular liquids at room temperature. Useful molecular information spanning tens of picoseconds can be measured in just a few milliseconds, and the signal-to-noise performance scales favorably with respect to the standard stage scan technique. These results demonstrate the viability of stage-free nonlinear Kerr effect measurements and provide a route for improvements to the speed of future multidimensional Kerr effect studies.
Proceedings of the 70th International Symposium on Molecular Spectroscopy, 2015
We report the generation and detection of a decade-spanning TeraHertz (THz) frequency comb (0.15-... more We report the generation and detection of a decade-spanning TeraHertz (THz) frequency comb (0.15-2.4 THz) using two Ti:Sapphire femtosecond laser oscillators and ASynchronous OPtical Sampling THz Time-Domain Spectroscopy (ASOPS-THz-TDS). The measured linewidth of the comb at 1.5 THz is 3 kHz over a 60 second acquisition. With timedomain detection of the comb, we measure three transitions of water vapor at 10 mTorr between 1-2 THz with an average Doppler-limited fractional uncertainty of 5.9 × 10 −8. Significant improvements in bandwidth, resolution, and sensitivity are possible with existing technologies and will enable future studies of jet-cooled hydrogen-bonded clusters.
Proceedings of the 70th International Symposium on Molecular Spectroscopy, 2015
The ultrafast TeraHertz Kerr effect (TKE) has recently been demonstrated as a nonlinear spectrosc... more The ultrafast TeraHertz Kerr effect (TKE) has recently been demonstrated as a nonlinear spectroscopic technique capable of measuring the dielectric relaxation of liquids. The true power of this technique lies in its ability to provide complementary information to measurements taken using heterodyne-detected optical Kerr effect (OKE) spectroscopy. The optical pulses in OKE measurements interact with the sample via the molecular polarizability, a rank-two tensor, in contrast with THz pulses that interact with the molecules via the dipole moment, a rank-one tensor. Given the different light-matter interactions in the two techniques, TKE measurements help complete the physical picture of intermolecular interactions at short timescales. We report here our implementation of heterodyne-detected TKE spectroscopy, along with measurements of pure liquids, and binary mixtures. Some of the liquids presented here were previously believed to be TKE inactive, thus showing that we have achieved a greater sensitivity than the previous implementation in the literature. In addition, we will discuss a variety of binary mixtures and show how the TKE data can be compared with OKE data to deepen our physical understanding of intermolecular interactions in liquids.
The composition of fragments C and B of the Jupiter-family comet 73P/Schwassmann-Wachmann 3 (SW3)... more The composition of fragments C and B of the Jupiter-family comet 73P/Schwassmann-Wachmann 3 (SW3) was investigated in early April of 2006 at IR wavelengths using high-dispersion echelle spectroscopy. Both fragments were depleted in ethane, and C was depleted in most forms of volatile carbon. In particular, fragment C shows a severe depletion of CH 3 OH but a "normal" abundance of HCN (which has a similar volatility). Thermal processing is a possible explanation, but since fragment B is perhaps sublimating fresher material because of the frequent outbursts and fragmentation, the observed depletions might have cosmogonic implications. The chemistry of the volatile ices in SW3, like in the Oort Cloud comet C/1999 S4 (LINEAR), may be associated with sublimation of icy mantles from precometary grains followed by subsequent gas-phase chemistry and recondensation.
Aims. We derive the dense core structure and the water abundance in four massive star-forming reg... more Aims. We derive the dense core structure and the water abundance in four massive star-forming regions in the hope of understanding the earliest stages of massive star formation. Methods. We present Herschel/HIFI observations of the para-H 2 O 1 11 −0 00 and 2 02 −1 11 and the para-H 18 2 O 1 11 −0 00 transitions. The envelope contribution to the line profiles is separated from contributions by outflows and foreground clouds. The envelope contribution is modeled with Monte-Carlo radiative transfer codes for dust and molecular lines (MC3D and RATRAN), and the water abundance and the turbulent velocity width as free parameters. Results. While the outflows are mostly seen in emission in high-J lines, envelopes are seen in absorption in ground-state lines, which are almost saturated. The derived water abundances range from 5 ×10 −10 to 4 ×10 −8 in the outer envelopes. We detect cold clouds surrounding the protostar envelope, thanks to the very high quality of the Herschel/HIFI data and the unique ability of water to probe them. Several foreground clouds are also detected along the line of sight. Conclusions. The low H 2 O abundances in massive dense cores are in accordance with the expectation that high densities and low temperatures lead to freeze-out of water on dust grains. The spread in abundance values is not clearly linked to physical properties of the sources.
Early results from the Herschel Space Observatory revealed the water cation H 2 O + to be an abun... more Early results from the Herschel Space Observatory revealed the water cation H 2 O + to be an abundant ingredient of the interstellar medium. Here we present new observations of the H 2 O and H 2 O + lines at 1113.3 and 1115.2 GHz using the Herschel Space Observatory toward a sample of high-mass star-forming regions to observationally study the relation between H 2 O and H 2 O +. Nine out of ten sources show absorption from H 2 O + in a range of environments: the molecular clumps surrounding the forming and newly formed massive stars, bright high-velocity outflows associated with the massive protostars, and unrelated low-density clouds along the line of sight. Column densities per velocity component of H 2 O + are found in the range of 10 12 to a few 10 13 cm −2. The highest N(H 2 O +) column densities are found in the outflows of the sources. The ratios of H 2 O + /H 2 O are determined in a range from 0.01 to a few and are found to differ strongly between the observed environments with much lower ratios in the massive (proto)cluster envelopes (0.01−0.1) than in outflows and diffuse clouds. Remarkably, even for source components detected in H 2 O in emission, H 2 O + is still seen in absorption.
Aims. We aim to study the composition and energetics of the circumstellar material of DK Cha, an ... more Aims. We aim to study the composition and energetics of the circumstellar material of DK Cha, an intermediate-mass star in transition from an embedded configuration to a star plus disk stage, during this pivotal stage of its evolution. Methods. Using the range scan mode of PACS on the Herschel Space Observatory, we obtained a spectrum of DK Cha from 55 to 210 μm as part of the DIGIT key program. Results. Almost 50 molecular and atomic lines were detected, many more than the 7 lines detected in ISO-LWS. Nearly the entire ladder of CO from J = 14-13 to 38-37 (E u /k = 4080 K), water from levels as excited as J K −1 K +1 = 7 07 (E u /k = 843 K), and OH lines up to E u /k = 290 K were detected. Conclusions. The continuum emission in our PACS SED scan matches the flux expected by a model consisting of a star, a surrounding disk of 0.03 M , and an envelope of a similar mass, supporting the suggestion that the object is emerging from its main accretion stage. Molecular, atomic, and ionic emission lines in the far-infrared reveal the outflow's influence on the envelope. The inferred hot gas may be photon-heated, but some emission may be caused by C-shocks in the walls of the outflow cavity.
Context. We present far-infrared spectroscopic observations, taken with the Photodetector Array C... more Context. We present far-infrared spectroscopic observations, taken with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory, of the protoplanetary disk around the pre-main-sequence star HD 100546. These observations are the first within the DIGIT Herschel key program, which aims to follow the evolution of dust, ice, and gas from young stellar objects still embedded in their parental molecular cloud core, through the final pre-main-sequence phases when the circumstellar disks are dissipated. Aims. Our aim is to improve the constraints on temperature and chemical composition of the crystalline olivines in the disk of HD 100546 and to give an inventory of the gas lines present in its far-infrared spectrum. Methods. The 69 μm feature is analyzed in terms of position and shape to derive the dust temperature and composition. Furthermore, we detected 32 emission lines from five gaseous species and measured their line fluxes. Results. The 69 μm emission comes either from dust grains with ∼70 K at radii larger than 50 AU, as suggested by blackbody fitting, or it arises from ∼200 K dust at ∼13 AU, close to the midplane, as supported by radiative transfer models. We also conclude that the forsterite crystals have few defects and contain at most a few percent iron by mass. Forbidden line emission from [C ii] at 157 μm and [O i] at 63 and 145 μm, most likely due to photodissociation by stellar photons, is detected. Furthermore, five H 2 O and several OH lines are detected. We also found high-J rotational transition lines of CO, with rotational temperatures of ∼300 K for the transitions up to J = 22−21 and T ∼ 800 K for higher transitions.
Two-dimensional terahertz-terahertz-Raman spectroscopy can provide insight into the anharmoniciti... more Two-dimensional terahertz-terahertz-Raman spectroscopy can provide insight into the anharmonicities of low-energy phonon modes-knowledge of which can help develop strategies for coherent control of material properties. Measurements on LiNbO 3 reveal THz and Raman nonlinear transitions between the EðTO 1) and EðTO 3) phonon polaritons. Distinct coherence pathways are observed with different THz polarizations. The observed pathways suggest that the origin of the third-order nonlinear responses is due to mechanical anharmonicities, as opposed to electronic anharmonicities. Further, we confirm that the EðTO 1) and EðTO 3) phonon polaritons are excited through resonant one-photon THz excitation.
We present maps of over 1.5 square degrees in Chamaeleon (Cha) II at 24, 70, and 160 µm observed ... more We present maps of over 1.5 square degrees in Chamaeleon (Cha) II at 24, 70, and 160 µm observed with the Spitzer Space Telescope Multiband Imaging
For several hours on 2008 April 19 the T Tauri spectroscopic binary DQ Tau was observed to bright... more For several hours on 2008 April 19 the T Tauri spectroscopic binary DQ Tau was observed to brighten, reaching a maximum detected flux of 468 mJy and likely making it (briefly) the brightest object at 3 mm in the Taurus star-forming region. We present the light curve of a rarely before observed millimeter flare originating in the region around a pre-main-sequence star, and the first from a classical T Tauri star. We discuss the properties and nature of the flaring behavior in the context of pulsed accretion flows (the current picture based largely on studies of this object's optically variable spectrum), as well as magnetospheric re-connection models (a separate theory that predicts millimeter flares for close binaries of high orbital eccentricity). We believe that the flare mechanism is linked to the binary orbit, and therefore periodic. DQ Tau makes a strong case for multi-wavelength follow-up studies, performed in parallel, of future flares to help determine whether magnetosph...
We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the Chamaeleon II molecular cloud... more We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the Chamaeleon II molecular cloud. The observed area covers about 1 square degree defined by A_V >2. Analysis of the data in the 2005 c2d catalogs reveals a small number of sources (40) with properties similar to those of young stellaror substellar objects (YSOs). The surface density of these YSO candidates is low, and contamination by background galaxies appears to be substantial, especially for sources classified as Class I or flat SED. We discuss this problem in some detail and conclude that very few of the candidate YSOs in early evolutionary stages are actually in the Cha II cloud. Using a refined set of criteria, we define a smaller, but more reliable, set of 24 YSO candidates.
We present the first images of the 691.473 GHz CO J=6–5 line in a protoplanetary disk, obtained a... more We present the first images of the 691.473 GHz CO J=6–5 line in a protoplanetary disk, obtained along with the 690 GHz dust continuum, toward the classical T Tauri star TW Hya using the Submillimeter Array. Imaging in the CO J=6–5 line reveals a rotating disk, consistent with previous observations of CO J=3–2 and 2–1 lines. Using an irradiated accretion disk model and 2D Monte Carlo radiative transfer, we find that additional surface heating is needed to fit simultaneously the absolute and relative intensities of the CO J=6–5, 3–2 and 2–1 lines. In particular, the vertical gas temperature gradient in the disk must be steeper than that of the dust, mostly likely because the CO emission lines probe nearer to the surface of the disk. We have used an idealized X-ray heating model to fit the line profiles of CO J=2–1 and 3–2 with χ 2 analysis, and the prediction of this model yields CO J=6–5 emission consistent with the observations.
Abstract. Motivated by Yung and Miller’s [1997] sugges-tion that N2O is isotopically fractionated... more Abstract. Motivated by Yung and Miller’s [1997] sugges-tion that N2O is isotopically fractionated during UV pho-tolysis in the stratosphere, we have studied the photoly-sis rates of the 14N15N16O and 15N14N16O structural iso-topomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photol-ysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single-stage enrichment fac-tors of (14N15N16O / 14N14N16O) =-73 ± 5 per mil and (15N14N16O / 14N14N16O) =-41 ± 10 per mil. As predicted by Yung and Miller [1997], the photolysis rate of 15N14N16O is faster than 14N15N16O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
Our study began with the goal of developing new methods to test the radically new understanding o... more Our study began with the goal of developing new methods to test the radically new understanding of solar system formation that has recently emerged, and to identify innovative instrumentation targeted to this purpose. In particular, we were seeking to test predictions of dynamical models such as the Nice model, and to do so through interdisciplinary collaboration between the planetary dynamics communities that have formulated (and largely dominated discussion of) these new ideas, and the meteoritics and cosmochemistry communities who will be most involved in any in-situ mission to an outer solar system body. Our study was principally focused on coming up with explicit tests of the predictions of these new dynamical models of solar system evolution.
Motivated by Yung and Miller's [1997] suggestion that N2O is isotopically fractionated during... more Motivated by Yung and Miller's [1997] suggestion that N2O is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the ^(14)N^(15)N^(16)O and ^(15)N^(14)N^(16)O structural isotopomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photolysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single‐stage enrichment factors of ϵ(^(14)N^(15)N^(16)O / ^(14)N^(14)N^(16)O) = −73 ± 5 per mil and ϵ(^(15)N^(14)N^(16)O / ^(14)N^(14)N^(16)O) = −41 ± 10 per mil. As predicted by Yung and Miller [1997], the photolysis rate of ^(15)N^(14)N^(16)O is faster than ^(14)N^(15)N^(16)O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
Motivated by Yung and Miller's [199• suggestion that N20 is isotopically fractionated during ... more Motivated by Yung and Miller's [199• suggestion that N20 is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the •4N•SN•60 and •SN•4N•60 structural isotopomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photolysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single-stage enrichment factors of e(•4N•SN•O / •4N•4N•O) = -73 q5 per mil and e(•SN•4N•O / •4N•4N•O) -41 q10 per mil. As predicted by Yung and Miller [199•, the photolysis rate of •SN•4N•60 is faster than •4N•SN•O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
We demonstrate that halogenated methane 2D-Terahertz Terahertz Raman (2D-TTR) spectra are determi... more We demonstrate that halogenated methane 2D-Terahertz Terahertz Raman (2D-TTR) spectra are determined by the complicated structure of the instrument response function (IRF) along f 1 and by the molecular coherences along f 2. Experimental improvements have helped increase the resolution and dynamic range of the measurements, including accurate THz pulse shape characterization. Sum-frequency excitations convolved with the IRF are found to quantitatively reproduce the 2D-TTR signal. A new Reduced Density Matrix model which incorporates sum-frequency pathways, with linear and harmonic operators fully supports this (re)interpretation of the 2D-TTR spectra.
Uploads
Papers by Geoffrey Blake