Person: Wilner, David
Loading...
Email Address
AA Acceptance Date
Birth Date
Research Projects
Organizational Units
Job Title
Last Name
Wilner
First Name
David
Name
Wilner, David
17 results
Search Results
Now showing 1 - 10 of 17
Publication DCO+, DCN, and N2D+ Reveal Three Different Deuteration Regimes in the Disk Around the Herbig Ae Star HD 163296(EDP Sciences, 2017-10-24) Salinas, Victoria; Hogerheijde, M; Mathews, G; Oberg, Karin; Qi, C; Williams, J; Wilner, DavidContext: Deuterium fractionation has been used to study the thermal history of pre-stellar environments. Their formation pathways trace different regions of the disk and may shed light into the disk’s physical structure, including locations of important features for planetary formation. Aims: We aim to constrain the radial extent of main deuterated species; we are particularly interested in spatially characterizing the high and low temperature pathways for enhancing deuteration of these species. Methods: We observed the disk surrounding the Herbig Ae star HD 163296 using ALMA in Band 6 and obtained resolved spectral imaging data of DCO + + ′′ ′′ ′′ ′′ (J=3–2), DCN (J=3–2) and N2D (J=3–2) with synthesized beam sizes of 0. 53× 0. 42, 0. 53× 0. 42 and ′′ ′′ 0. 50× 0. 39 respectively. We adopt a physical model of the disk from the literature and use the 3D radiative transfer code LIME to estimate an excitation temperature profile for our detected lines. We model the radial emission profiles of DCO+, DCN and N2D+, assuming their emission is optically thin, using a parametric model of their abundances and our excitation temperature estimates. Results. DCO+ can be described by a three-region model, with constant-abundance rings centered at 70 AU, 150 AU and 260 AU. The DCN radial profile peaks at about 60 AU and N2D+ is seen in a ring at 160 AU. Simple models of both molecules using constant abundances reproduce the data. Assuming reasonable average excitation temperatures for the whole disk, their disk-averaged column densities (and deuterium fractionation ratios) are 1.6–2.6×1012 cm−2 (0.04–0.07), 2.9–5.2×1012 cm−2 (∼0.02) and 1.6–2.5 ×1011 cm−2 (0.34–0.45) for DCO+, DCN and N2D+, respectively. Conclusions: Our simple best-fit models show a correlation between the radial location of the first two rings in DCO+ and the DCN and N2D+ abundance distributions that can be interpreted as the high and low temperature deuteration pathways regimes. The origin of the third DCO+ ring at 260 AU is unknown but may be due to a local decrease of ultraviolet opacity allowing the photodesorption of CO or due to thermal desorption of CO as a consequence of radial drift and settlement of dust grains. The derived deuterium fractionation values agree with previous estimates of 0.05 for DCO+/HCO+ and 0.02 for DCN/HCN in HD163296, and 0.3-0.5 for N2D+/N2H+ in AS 209, a T Tauri disk. The high N2D+/N2H+ confirms N2D+ as a good candidate for tracing ionization in the cold outer disk.Publication Submillimeter Array Imaging of the Maser Emission from the \(H30\alpha\) Radio Recombination Line in MWC 349A(American Astronomical Society, 2008) Weintroub, Jonathan; Moran, James; Wilner, David; Young, Ken; Rao, Ramprasad; Shinnaga, HirokoWe used the Submillimeter Array to map the angular distribution of the H30α recombination line (231.9 GHz) in the circumstellar region of the peculiar star MWC 349A. The resolution was 1.2'', but because of high signal-to-noise ratio we measured the positions of all maser components to accuracies better than 0.01'', at a velocity resolution of \(1 km s^{−1}\). The two strongest maser components (called high-velocity components) at velocities near –14 and \(32 km s^{−1}\) are separated by 0.048'' ± 0.001'' (60 AU) along a position angle of 102° ± 1°. The distribution of maser emission at velocities between and beyond these two strongest components were also provided. The continuum emission lies at the center of the maser distribution to within 10 mas. The masers appear to trace a nearly edge-on rotating disk structure, reminiscent of the water masers in Keplerian rotation in the nuclear accretion disk of the galaxy NGC 4258. However, the maser components in MWC 349A do not follow a simple Keplerian kinematic prescription with v ~ \(r^{−1/2}\), but have a larger power-law index. We explore the possibility that the high-velocity masers trace spiral density or shock waves. We also emphasize caution in the interpretation of relative centroid maser positions where the maser is not clearly resolved in position or velocity, and we present simulations that illustrate the range of applicability of the centroiding method.Publication H2CO and N2H+ in Protoplanetary Disks: Evidence for a CO-Ice Regulated Chemistry(IOP Publishing, 2013) Qi, Chunhua; Oberg, Karin; Wilner, DavidWe present Submillimeter Array observations of H2CO and N2H+ emission in the disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296 at 2"-6" resolution and discuss the distribution of these species with respect to CO freeze-out. The H2CO and N2H+ emission toward HD 163296 does not peak at the continuum emission center that marks the stellar position but is instead significantly offset. Using a previously developed model for the physical structure of this disk, we show that the H2CO observations are reproduced if H2CO is present predominantly in the cold outer disk regions. A model where H2CO is present only beyond the CO snow line (estimated at a radius of 160 AU) matches the observations well. We also show that the average H2CO excitation temperature, calculated from two transitions of H2CO observed in these two disks and a larger sample of disks around T Tauri stars in the DISCS (the Disk Imaging Survey of Chemistry with SMA) program, is consistent with the CO freeze-out temperature of 20 K. In addition, we show that N2H+ and H2CO line fluxes in disks are strongly correlated, indicative of co-formation of these species across the sample. Taken together, these results imply that H2CO and N2H+ are generally present in disks only at low temperatures where CO depletes onto grains, consistent with fast destruction of N2H+ by gas-phase CO, and in situ formation of H2CO through hydrogenation of CO ice. In this scenario H2CO, CH3OH and N2H+ emission in disks should appear as rings with the inner edge at the CO midplane snow line. This prediction can be tested directly using observations from ALMA with higher resolution and better sensitivity.Publication First Detection of c-C3H2 in a Circumstellar Disk(IOP Publishing, 2013) Qi, Chunhua; Oberg, Karin; Wilner, David; Rosenfeld, KatherineWe report the first detection of c-C3H2 in a circumstellar disk. The c-C3H2 J = 6-5 line (217.882 GHz) is detected and imaged through Atacama Large Millimeter Array (ALMA) Science Verification observations toward the disk around the Herbig Ae star HD 163296 at 0.''8 resolution. The emission is consistent with that arising from a Keplerian rotating disk. Two additional c-C3H2 transitions are also tentatively detected, bolstering the identification of this species, but with insufficient signal-to-noise ratio to constrain the spatial distribution. Using a previously developed model for the physical structure of this disk, we fit a radial power-law distribution model to the c-C3H2 6-5 emission and find that c-C3H2 is present in a ring structure from an inner radius of about 30 AU to an outer radius of about 165 AU. The column density is estimated to be 1012-1013 cm–2. The clear detection and intriguing ring structure suggest that c-C3H2 has the potential to become a useful probe of radiation penetration in disks.Publication Imaging of the CO Snow Line in a Solar Nebula Analog(American Association for the Advancement of Science (AAAS), 2013) Qi, Chunhua; Oberg, Karin; Wilner, David; D'Alessio, Paola; Bergin, E.; Andrews, Sean; Blake, G. A.; Hogerheijde, M. R.; van Dishoeck, E. F.Planets form in the disks around young stars. Their formation efficiency and composition are intimately linked to the protoplanetary disk locations of “snow lines” of abundant volatiles. We present chemical imaging of the carbon monoxide (CO) snow line in the disk around TW Hya, an analog of the solar nebula, using high spatial and spectral resolution Atacama Large Millimeter/Submillimeter Array observations of diazenylium (N2H+), a reactive ion present in large abundance only where CO is frozen out. The N2H+ emission is distributed in a large ring, with an inner radius that matches CO snow line model predictions. The extracted CO snow line radius of ∼30 astronomical units helps to assess models of the formation dynamics of the solar system, when combined with measurements of the bulk composition of planets and comets.Publication First Detection of Millimeter/Submillimeter Extragalactic H2O Maser Emission(IOP Publishing, 2005) Humphreys, E. M. L.; Greenhill, L. J.; Reid, M. J.; Beuther, H.; Moran, James; Gurwell, Mark; Wilner, David; Kondratko, P. T.We report the first detection of an extragalactic millimeter wavelength H2O maser at 183 GHz towards NGC 3079 using the Submillimeter Array (SMA), and a tentative submillimeter wave detection of the 439 GHz maser towards the same source using the James Clerk Maxwell Telescope (JCMT). These H2O transitions are known to exhibit maser emission in star-forming regions and evolved stars. NGC 3079 is a well-studied nuclear H2O maser source at 22 GHz with a timevariable peak flux density in the range 3 – 12 Jy. The 183 GHz H2O maser emission, with peak flux density ∼0.5 Jy (7σ detection), also originates from the nuclear region of NGC 3079 and is spatially coincident with the dust continuum peak at 193 GHz (53 mJy integrated). Peak emission at both 183 and 439 GHz occurs in the same range of velocity as that covered by the 22 GHz spectrum. We estimate the gas to dust ratio of the nucleus of NGC 3079 to be ≈150, comparable to the Galactic value of 160. Discovery of maser emission in an active galactic nucleus beyond the long-known 22 GHz transition opens the possibility of future position-resolved radiative transfer modeling of accretion disks and outflows < 1 pc from massive black holes.Publication Submillimeter Array 440 μm/690 GHz Line and Continuum Observations of Orion KL(IOP Publishing, 2006) Beuther, H.; Zhang, Qizhou; Reid, Mark; Hunter, T. R.; Gurwell, Mark; Wilner, David; Zhao, Jun-Hui; Shinnaga, H.; Keto, Eric; Ho, P. T. P.; Moran, James; Liu, S.‐Y.Submillimeter Array observations of Orion-KL at ∼ 1′′ resolution in the 440 µm/690 GHz band reveal new insights about the continuum and line emission of the region. The 440 µm continuum flux density measurement from source I allows us to differentiate among the various proposed physical models: Source I can be well modeled by a “normal” protostellar SED consisting of a proton-electron free-free emission component at low frequencies and a strong dust component in the submillimeter bands. Furthermore, we find that the protostellar object SMA1 is clearly distinct from the hot core. The non-detection of SMA1 at cm and infrared wavelengths suggests that it may be one of the youngest sources in the entire Orion-KL region. The molecular line maps show emission mainly from the sources I, SMA1 and the hot core peak position. An analysis of the CH 3CN(37 K − 36 K) K-ladder ( K = 0...3) indicates a warm gas component of the order 600 ± 200K. In addition, we detect a large fraction ( ∼ 58%) of unidentified lines and discuss the difficulties of line identifications at these frequencies.Publication Line Imaging of Orion KL at 865 μm with the Submillimeter Array(IOP Publishing, 2005) Beuther, H.; Zhang, Qizhou; Greenhill, Lincoln; Reid, Mark; Wilner, David; Keto, Eric; Shinnaga, H.; Ho, P. T. P.; Moran, James; Liu, S.‐Y.; Chang, Chih-WeiWe present the first submm (865 µm) imaging spectral line survey at one arcsecond resolution conducted with the Submillimeter Array toward Orion-KL. Within the two × two GHz bandpasses (lower and upper sidebands, 337.2–339.2GHz and 347.2–349.2GHz), we find about 145 spectral lines from 13 species, 6 isotopologues, and 5 vibrational excited states. Most nitrogen-bearing molecules are strong toward the hot core, whereas the oxygen-bearing molecules peak toward the south-west in the so-called compact ridge. Imaging of spectral lines is shown to be an additional tool to improve the identifications of molecular lines. Arcsecond spatial resolution allows us to distinguish the molecular line emission of the sources I and n from that of the hot core. The only molecular species detected strongly toward source I is SiO, delineating mainly the collimated north-east south-west low-velocity outflow. The two positions close to source I, which have previously been reported to show maser emission in the v=0 28SiO(1–0) and (2–1) lines, show no detectable maser emission in the v=0 28SiO(8–7) line at our spatial resolution. SiO is weak toward source n, and thus source n may not currently be driving a molecular outflow. CH 3OH is the molecule with the highest number of identified lines (46) in this spectral window. This “line forest” allows us to estimate temperatures in the region, and we find temperatures between 50 and 350 K, with the peak temperatures occurring toward the hot core. The detection of strong vibrational excited line emission from the submm continuum peak SMA1 supports the interpretation that the source SMA1 is likely of protostellar nature.Publication Subarcsecond Submillimeter Continuum Observations of Orion KL(IOP Publishing, 2004) Beuther, H.; Zhang, Qizhou; Greenhill, Lincoln; Reid, Mark; Wilner, David; Keto, Eric; Marrone, D.; Ho, P. T. P.; Moran, James; Rao, R.; Shinnaga, H.; Liu, S.-Y.We present the first 865 µm continuum image with sub-arcsecond resolution obtained with the Submillimeter Array. These data resolve the Orion-KL region into the hot core, the nearby radio source I, the sub-mm counterpart to the infrared source n (radio source L), and new sub-mm continuum sources. The radio to submillimeter emission from source I may be modeled as either the result of proton-electron free-free emission that is optically thick to ∼ 100 GHz plus dust emission that accounts for the majority of the submillimeter flux, or H− free-free emission that gives rise to a power-law spectrum with power-law index of ∼ 1.6. The latter model would indicate similar physical conditions as found in the inner circumstellar environment of Mira variable stars. Future sub-arcsecond observations at shorter sub-mm wavelengths should easily discriminate between these two possibilities. The sub-mm continuum emission toward source n can be interpreted in the framework of emission from an accretion disk.Publication The comet-like composition of a protoplanetary disk as revealed by complex cyanides(Nature Publishing Group, 2015) Oberg, Karin; Guzmán, Viviana V.; Furuya, Kenji; Qi, Chunhua; Aikawa, Yuri; Andrews, Sean; Loomis, Ryan; Wilner, DavidObservations of comets and asteroids show that the Solar Nebula that spawned our planetary system was rich in water and organic molecules. Bombardment brought these organics to the young Earth's surface, seeding its early chemistry. Unlike asteroids, comets preserve a nearly pristine record of the Solar Nebula composition. The presence of cyanides in comets, including 0.01% of methyl cyanide (CH3CN) with respect to water, is of special interest because of the importance of C-N bonds for abiotic amino acid synthesis. Comet-like compositions of simple and complex volatiles are found in protostars, and can be readily explained by a combination of gas-phase chemistry to form e.g. HCN and an active ice-phase chemistry on grain surfaces that advances complexity[3]. Simple volatiles, including water and HCN, have been detected previously in Solar Nebula analogues - protoplanetary disks around young stars - indicating that they survive disk formation or are reformed in situ. It has been hitherto unclear whether the same holds for more complex organic molecules outside of the Solar Nebula, since recent observations show a dramatic change in the chemistry at the boundary between nascent envelopes and young disks due to accretion shocks[8]. Here we report the detection of CH3CN (and HCN and HC3N) in the protoplanetary disk around the young star MWC 480. We find abundance ratios of these N-bearing organics in the gas-phase similar to comets, which suggests an even higher relative abundance of complex cyanides in the disk ice. This implies that complex organics accompany simpler volatiles in protoplanetary disks, and that the rich organic chemistry of the Solar Nebula was not unique.