solvation by a polar aprotic solvent

A series of strong H-bonded complexes of trimethylglycine, also known as betaine, with acetic, chloroacetic, dichloroacetic, trifluoroacetic and hydrofluoric acids as well as the homo-conjugated cation of betaine with trifluoroacetate as the counteranion were investigated by low-temperature (120–160 K) liquid-state NMR spectroscopy using CDF3/CDF2Cl mixture as the solvent. The temperature dependencies of 1H NMR chemical shifts are analyzed in terms of the solvent–solute interactions. The experimental data are explained assuming the combined action of two main effects. Firstly, the solvent ordering around the negatively charged OHX region of the complex (X = O, F) at low temperatures, which leads to a contraction and symmetrisation of the H-bond; this effect dominates for the homo-conjugated cation of betaine. Secondly, at low temperatures structures with a larger dipole moment are preferentially stabilized, an effect which dominates for the neutral betaine–acid complexes. The way this second contribution affects the H-bond geometry seems to depend on the proton position. For the Be+COO−⋯HOOCCH3 complex (Be = (CH3)3NCH2–) the proton displaces towards the hydrogen bond center (H-bond symmetrisation, O⋯O contraction). In contrast, for the Be+COOH⋯−OOCCF3 complex the proton shifts further away from the center, closer to the betaine moiety (H-bond asymmetrisation, O⋯O elongation). Hydrogen bond geometries and their changes upon lowering the temperature were estimated using previously published H-bond correlations

Stellar photometry with Multi Conjugate Adaptive Optics

We overview the current status of photometric analyses of images collected with Multi Conjugate Adaptive Optics (MCAO) at 8-10m class telescopes that operated, or are operating, on sky. Particular attention will be payed to resolved stellar population studies. Stars in crowded stellar systems, such as globular clusters or in nearby galaxies, are ideal test particles to test AO performance. We will focus the discussion on photometric precision and accuracy reached nowadays. We briefly describe our project on stellar photometry and astrometry of Galactic globular clusters using images taken with GeMS at the Gemini South telescope. We also present the photometry performed with DAOPHOT suite of programs into the crowded regions of these globulars reaching very faint limiting magnitudes Ks ~21.5 mag on moderately large fields of view (~1.5 arcmin squared). We highlight the need for new algorithms to improve the modeling of the complex variation of the Point Spread Function across the field of view. Finally, we outline the role that large samples of stellar standards plays in providing a detailed description of the MCAO performance and in precise and accurate colour{magnitude diagrams.Comment: 17 pages, 12 figures, SPIE 201

Weak Galactic halo--dwarf spheroidal connection from RR Lyrae stars

We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in six dwarf spheroidals and eleven ultra faint dwarf galaxies (1,300) show a Gaussian period distribution well peaked around a mean period of =0.610+-0.001 days (sigma=0.03). The Halo RRL (15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dwarf spheroidals apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P 0.75mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered eighteen globulars covering a broad range in metallicity (-2.3< [Fe/H]< -1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal--rich than [Fe/H] -1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of 50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.Comment: accepted for publication in ApJ Lette

The ACS LCID project. X. The Star Formation History of IC 1613: Revisiting the Over-Cooling Problem

We present an analysis of the star formation history (SFH) of a field near the half light radius in the Local Group dwarf irregular galaxy IC 1613 based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our observations reach the oldest main sequence turn-off, allowing a time resolution at the oldest ages of ~1 Gyr. Our analysis shows that the SFH of the observed field in IC 1613 is consistent with being constant over the entire lifetime of the galaxy. These observations rule out an early dominant episode of star formation in IC 1613. We compare the SFH of IC 1613 with expectations from cosmological models. Since most of the mass is in place at early times for low mass halos, a naive expectation is that most of the star formation should have taken place at early times. Models in which star formation follows mass accretion result in too many stars formed early and gas mass fractions which are too low today (the "over-cooling problem"). The depth of the present photometry of IC 1613 shows that, at a resolution of ~1 Gyr, the star formation rate is consistent with being constant, at even the earliest times, which is difficult to achieve in models where star formation follows mass assembly.Comment: 13 pages, 12 figures, accepted for publication in the Ap

First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A

We present the abundance analyses of three isolated A-type supergiant stars in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES spectrograph at the VLT. Detailed model atmosphere analyses have been used to determine the stellar atmospheric parameters and the elemental abundances of the stars. The mean iron group abundance was determined from these three stars to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of the present-day iron group abundances in Sextans A. These three stars now represent the most metal-poor massive stars for which detailed abundance analyses have been carried out. The mean stellar alpha element abundance was determined from the alpha element magnesium as [alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the nebular alpha element abundances as determined from oxygen in the H II regions. These results are consistent from star-to-star with no significant spatial variations over a length of 0.8 kpc in Sextans A. This supports the nebular abundance studies of dwarf irregular galaxies, where homogeneous oxygen abundances are found throughout, and argues against in situ enrichment. The alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which is consistent with the solar ratio. This is consistent with the results from A-supergiant analyses in other Local Group dwarf irregular galaxies but in stark contrast with the high [alpha/Fe] results from metal-poor stars in the Galaxy, and is most clearly seen from these three stars in Sextans A because of their lower metallicities. The low [alpha/Fe] ratios are consistent with the slow chemical evolution expected for dwarf galaxies from analyses of their stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A

The ACS LCID Project: RR Lyrae stars as tracers of old population gradients in the isolated dwarf spheroidal galaxy Tucana

We present a study of the radial distribution of RR Lyrae variables, which present a range of photometric and pulsational properties, in the dwarf spheroidal galaxy Tucana. We find that the fainter RR Lyrae stars, having a shorter period, are more centrally concentrated than the more luminous, longer period RR Lyrae variables. Through comparison with the predictions of theoretical models of stellar evolution and stellar pulsation, we interpret the fainter RR Lyrae stars as a more metal-rich subsample. In addition, we show that they must be older than about 10 Gyr. Therefore, the metallicity gradient must have appeared very early on in the history of this galaxy.Comment: 5 pages, 5 figures in emulateapj style. Submitted to ApJ Letter

The ACS LCID Project. I. Short-Period Variables in the Isolated Dwarf Spheroidal Galaxies Cetus & Tucana

(abridged) We present the first study of the variable star populations in the isolated dwarf spheroidal galaxies (dSph) Cetus and Tucana. Based on Hubble Space Telescope images obtained with the Advanced Camera for Surveys in the F475W and F814W bands, we identified 180 and 371 variables in Cetus and Tucana, respectively. The vast majority are RR Lyrae stars. In Cetus we also found three anomalous Cepheids, four candidate binaries and one candidate long-period variable (LPV), while six anomalous Cepheids and seven LPV candidates were found in Tucana. Of the RR Lyrae stars, 147 were identified as fundamental mode (RRab) and only eight as first-overtone mode (RRc) in Cetus, with mean periods of 0.614 and 0.363 day, respectively. In Tucana we found 216 RRab and 82 RRc giving mean periods of 0.604 and 0.353 day. These values place both galaxies in the so-called Oosterhoff Gap, as is generally the case for dSph. We calculated the distance modulus to both galaxies using different approaches based on the properties of RRab and RRc, namely the luminosity-metallicity and period-luminosity-metallicity relations, and found values in excellent agreement with previous estimates using independent methods: (m-M)_{0,Cet}=24.46+-0.12 and (m-M)_{0,Tuc}=24.74+-0.12, corresponding to 780+-40 kpc and 890+-50 kpc. We also found numerous RR Lyrae variables pulsating in both modes simultaneously (RRd): 17 in Cetus and 60 in Tucana. Tucana is, after Fornax, the second dSph in which such a large fraction of RRd (~17%) has been observed. We provide the photometry and pulsation parameters for all the variables, and compare the latter with values from the literature for well-studied dSph of the Local Group and Galactic globular clusters.Comment: 26 pages, 24 figures, in emulateapj format. To be published in ApJ. Some figures heavily degraded; See http://www.iac.es/project/LCID/?p=publications for a version with full resolution figure

Leo A: A Late-Blooming Survivor of the Epoch of Reionization in the Local Group

As part of a major program to use isolated Local Group dwarf galaxies as near-field probes of cosmology, we have obtained deep images of the dwarf irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble Space Telescope. From these images we have constructed a color-magnitude diagram (CMD) reaching apparent [absolute] magnitudes of (M475, M814) > (29.0 [+4.4], 27.9 [+3.4]), the deepest ever achieved for any irregular galaxy beyond the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of all the star formation that ever occurred in Leo A happened more recently than 8 Gyr ago. The CMD shows only a very small amount of star formation in the first few billion years after the Big Bang; a possible burst at the oldest ages cannot be claimed with high confidence. The peak SFR occurred ~1.5-4 Gyr ago, at a level 5-10 times the current value. Our modelling indicates that Leo A has experienced very little metallicity evolution; the mean inferred metallicity is consistent with measurements of the present-day gas-phase oxygen abundance. We cannot exclude a scenario in which ALL of the ancient star formation occurred prior to the end of the era of reionization, but it seems unlikely that the lack of star formation prior to ~8 Gyr ago was due to early loss or exhaustion of the in situ gas reservoir.Comment: Accepted for publication in the Astrophysical Journal Letters; 5 pages, 3 figures (2 in color); uses emulateapj.st

The Cascadia Initiative : a sea change In seismological studies of subduction zones

Author Posting. © The Oceanography Society, 2014. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 27, no. 2 (2014): 138-150, doi:10.5670/oceanog.2014.49.Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology—shielded ocean bottom seismometers—is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data.The Cascadia Initiative is supported by the National Science Foundation; the CIET is supported under grants OCE- 1139701, OCE-1238023, OCE‐1342503, OCE-1407821, and OCE-1427663 to the University of Oregon