The Last Glacial Termination Denton, G. H.; Anderson, R. F.; Toggweiler, J. R. ...
Science (American Association for the Advancement of Science),
06/2010, Volume:
328, Issue:
5986
Journal Article
Peer reviewed
A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a ...comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO₂ during southern upwelling pulses augmented warming during the last termination in both polar hemispheres.
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Surface-specific vibrational sum-frequency generation spectroscopy (V-SFG) is frequently used to obtain information about the molecular structure at charged interfaces. Here, we provide experimental ...evidence that not only screening of surface charges but also interference limits the extent to which V-SFG probes interfacial water at sub-mM salt concentrations. As a consequence, V-SFG yields information about the ∼single monolayer interfacial region not only at very high ionic strength, where the surface charge is effectively screened, but also for pure water due to the particularly large screening length at this low ionic strength. At these low ionic strengths, the large screening lengths cause destructive interference between contributions in the surface region. A recently proposed theoretical framework near-quantitatively describes our experimental findings by considering only interference and screening. However, a comparison between NaCl and LiCl reveals ion specific effects in the screening efficiency of different electrolytes. Independent of electrolyte, the hydrogen bonding strength of water right at the interface is enhanced at high electrolyte concentrations.
Vibrational coupling is relevant not only for dissipation of excess energy after chemical reactions but also for elucidating molecular structure and dynamics. It is particularly important for O–H ...stretch vibrational spectra of water, for which it is known that in bulk both intra- and intermolecular coupling alter the intensity and line shape of the spectra. In contrast with bulk, the unified picture of the inter/intra-molecular coupling of O–H groups at the water–air interface has been lacking. Here, combining sum-frequency generation experiments and simulation for isotopically diluted water and alcohols, we unveil effects of inter- and intramolecular coupling on the vibrational spectra of interfacial water. Our results show that both inter- and intramolecular coupling contribute to the O–H stretch vibrational response of the neat H2O surface, with intramolecular coupling generating a double-peak feature, while the intermolecular coupling induces a significant red shift in the O–H stretch response.
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The interaction between minerals and water is manifold and complex: the mineral surface can be (de)protonated by water, thereby changing its charge; mineral ions dissolved into the aqueous phase ...screen the surface charges. Both factors affect the interaction with water. Intrinsically molecular‐level processes and interactions govern macroscopic phenomena, such as flow‐induced dissolution, wetting, and charging. This realization is increasingly prompting molecular‐level studies of mineral–water interfaces. Here, we provide an overview of recent developments in surface‐specific nonlinear spectroscopy techniques such as sum frequency and second harmonic generation (SFG/SHG), which can provide information about the molecular arrangement of the first few layers of water molecules at the mineral surface. The results illustrate the subtleties of both chemical and physical interactions between water and the mineral as well as the critical role of mineral dissolution and other ions in solution for determining those interactions.
The interaction between minerals and water is relevant for a wide range of geological, atmospheric, environmental, and electro(photo‐)chemical systems. This Review highlights the developments made through studying a variety of mineral–water interfaces by nonlinear optical spectroscopy. These methods provide molecular‐level details about the organization of water in the direct vicinity of the interface.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We report new dynamical masses for five pre-main sequence (PMS) stars in the L1495 region of the Taurus star-forming region (SFR) and six in the L1688 region of the Ophiuchus SFR. Since these regions ...have VLBA parallaxes, these are absolute measurements of the stars' masses and are independent of their effective temperatures and luminosities. Seven of the stars have masses , thus providing data in a mass range with little data, and of these, six are measured to precision . We find eight stars with masses in the range 0.09-1.1 that agree well with the current generation of PMS evolutionary models. The ages of the stars we measured in the Taurus SFR are in the range 1-3 Myr, and Myr for those in L1688. We also measured the dynamical masses of 14 stars in the ALMA archival data for Akeson & Jensen's Cycle 0 project on binaries in the Taurus SFR. We find that the masses of seven of the targets are so large that they cannot be reconciled with reported values of their luminosity and effective temperature. We suggest that these targets are themselves binaries or triples.
We present measurements of the orbital positions and flux ratios of 17 binary and triple systems in the Ophiuchus star-forming region and the Upper Centaurus-Lupus cluster based on adaptive optics ...imaging at the Keck Observatory. We report the detection of visual companions in MML 50 and MML 53 for the first time, as well as the possible detection of a third component in WSB 21. For six systems in our sample, our measurements provide a second orbital position following their initial discoveries over a decade ago. For eight systems with sufficient orbital coverage, we analyze the range of orbital solutions that fit the data. Ultimately, these observations will help provide the groundwork toward measuring precise masses for these pre-main-sequence stars and understanding the distribution of orbital parameters in young multiple systems.
The dissolution of minerals in water is typically studied on macroscopic length- and time-scales, by detecting dissolution products in bulk solution and deducing reaction rates from model ...assumptions. Here, we report a direct, real-time measurement of silica dissolution, by monitoring how dissolution changes the first few interfacial layers of water in contact with silica, using surface-specific spectroscopy. We obtain direct information on the dissolution kinetics of this geochemically relevant mineral. The interfacial concentration of dissolution products saturates at the level of the solubility limit of silica (~millimolar) on the surprisingly short timescale of tens of hours. The observed kinetics reveal that the dissolution rate increases substantially with progressing dissolution, suggesting that dissolution is an auto-catalytic process.
Abstract
The stellar companion to the weak-line T Tauri star DI Tau A was first discovered by the lunar occultation technique in 1989 and was subsequently confirmed by a speckle imaging observation ...in 1991. It has not been detected since, despite being targeted by five different studies that used a variety of methods and spanned more than 20 yr. Here, we report the serendipitous rediscovery of DI Tau B during our Young Exoplanets Spectroscopic Survey (YESS). Using radial velocity data from YESS spanning 17 yr, new adaptive optics observations from Keck II, and a variety of other data from the literature, we derive a preliminary orbital solution for the system that effectively explains the detection and (almost all of the) non-detection history of DI Tau B. We estimate the dynamical masses of both components, finding that the large mass difference (
q
∼ 0.17) and long orbital period (≳35 yr) make the DI Tau system a noteworthy and valuable addition to studies of stellar evolution and pre-main-sequence models. With a long orbital period and a small flux ratio (f2/f1) between DI Tau A and B, additional measurements are needed for a better comparison between these observational results and pre-main-sequence models. Finally, we report an average surface magnetic field strength (
B
¯
) for DI Tau A, of ∼0.55 kG, which is unusually low in the context of young active stars.
On the Nature of the T Tauri Triple System Beck, Tracy L.; Schaefer, G. H.; Guilloteau, S. ...
Astrophysical journal/The Astrophysical journal,
10/2020, Volume:
902, Issue:
2
Journal Article
Peer reviewed
Open access
We present a multiwavelength analysis to reveal the nature of the enigmatic T Tauri triple star system. New optical and infrared measurements are coupled with archival X-ray, UV, and millimeter data ...sets to show the morphologies of disk material and outflow kinematics. A dark lane of obscuring material is seen in silhouette in several emission lines and in model-subtracted ALMA millimeter continuum dust residuals near the position of T Tau Sa+Sb, revealing the attenuating circumbinary ring around T Tau S. The flux variability of T Tau S is linked in part to the binary orbit; T Tau Sb brightens near orbital apastron as it emerges from behind circumbinary material. Outflow diagnostics confirm that T Tau N powers the blueshifted western outflow, and the T Tau S binary drives the northwest-southeastern flow. Analysis of the southern outflow shows periodic arcs ejected from the T Tau system. Correlation of these arc locations and tangential kinematics with the orbit timing suggests that launch of the last four southern outflow ejections is contemporaneous with, and perhaps triggered by, the T Tau Sa+Sb binary periastron passage. We present a geometry of the T Tau triple that has the southern components foreground to T Tau N, obscured by a circumbinary ring, with misaligned disks and interacting outflows. Particularly, a wind from T Tauri Sa that is perpendicular to its circumstellar disk might interact with the circumbinary material, which may explain conflicting high-contrast measurements of the system outflows in the literature. T Tauri is an important laboratory for understanding early dynamical processes in young multiple systems. We discuss the historical and future characteristics of the system in this context.
Abstract
Debate over the planet occurrence rates around intermediate-mass stars has hinged on the accurate determination of masses of evolved stars, and has been exacerbated by a paucity of reliable, ...directly measured fundamental properties for these stars. We present long-baseline optical interferometry of five evolved intermediate-mass (∼ 1.5 M⊙) planet-hosting stars using the PAVO beam combiner at the CHARA Array, which we combine with bolometric flux measurements and parallaxes to determine their radii and effective temperatures. We measured the radii and effective temperatures of 6 Lyncis (5.12 ± 0.16 R⊙, 4949 ± 58 K), 24 Sextantis (5.49 ± 0.18 R⊙, 4908 ± 65 K), κ Coronae Borealis (4.77 ± 0.07 R⊙, 4870 ± 47 K), HR 6817 (4.45 ± 0.08 R⊙, 5013 ± 59 K), and HR 8461 (4.91 ± 0.12 R⊙, 4950 ± 68 K). We find disagreements of typically 15 per cent in angular diameter and ∼200 K in temperature compared to interferometric measurements in the literature, yet good agreement with spectroscopic and photometric temperatures, concluding that the previous interferometric measurements may have been affected by systematic errors exceeding their formal uncertainties. Modelling based on BaSTI isochrones using various sets of asteroseismic, spectroscopic, and interferometric constraints tends to favour slightly (∼15 per cent) lower masses than generally reported in the literature.
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