The nature of the KFR08 stellar stream

The origin of a new kinematically identified metal-poor stellar stream, the KFR08 stream, has not been established. We present stellar parameters, stellar ages, and detailed elemental abundances for Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Ni, Zn, Sr, Y, Zr, Ba, La, and Eu for 16 KFR08 stream members based on analysis of high resolution spectra. Based on the abundance ratios of 14 elements, we use the chemical tagging method to identify the stars which have the same chemical composition, and thus, might have a common birthplace, such as a cluster. Although three stars were tagged with similar elemental abundances ratios, we find that, statistically, it is not certain that they originate from a dissolved star cluster. This conclusion is consistent with the large dispersion of [Fe/H] ($\sigma_{\rm{[Fe/H]}} = 0.29$) among the 16 stream members. We find that our stars are $\alpha$ enhanced and that the abundance patterns of the stream members are well matched to the thick disk. In addition, most of the stream stars have estimated stellar ages larger than 11 Gyr. These results, together with the hot kinematics of the stream stars, suggest that the KFR08 stream is originated from the thick disk population which was perturbed by a massive merger in the early universe.Comment: 9 pages, 6 figures, accepted for publication in A&

NLTE analysis of Sr lines in spectra of late-type stars with new R-matrix atomic data

We investigate statistical equilibrium of neutral and singly-ionized strontium in late-type stellar atmospheres. Particular attention is given to the completeness of the model atom, which includes new energy levels, transition probabilities, photoionization and electron-impact excitation cross-sections computed with the R-matrix method. The NLTE model is applied to the analysis of Sr I and Sr II lines in the spectra of the Sun, Procyon, Arcturus, and HD 122563, showing a significant improvement in the ionization balance compared to LTE line formation calculations, which predict abundance discrepancies of up to 0.5 dex. The solar Sr abundance is log A = 2.93 \pm 0.04 dex, in agreement with the meteorites. A grid of NLTE abundance corrections for Sr I and Sr II lines covering a large range of stellar parameters is presented.Comment: 11 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

A test of speculative arbitrage : is the cross-section of volatility invariant?

We derive testable implications of Kyle and Obizhaeva’s (2016) notion of “bet invariance” for the cross-section of trade-time volatilities. We jointly develop theoretical foundations of “no speculative arbitrage” whose implications incorporate those of bet invariance. Our proposed test circumvents the unobservable nature of “bets.” Utilizing a large sample of U.S. stocks post decimilization, we show that using realized volatilities rather than expected volatilities introduces noise that substantially biases the tests. This leads us to use estimates of normalized volatilities based on running 24 month windows. We find strong support for no speculative arbitrage at a moment in time, but not across time

The night and day of Amihud’s (2002) liquidity measure

Amihud’s (2002) stock (il)liquidity measure averages the daily ratio of absolute closeto-close return to dollar volume, including overnight returns, while trading volumes come from regular trading hours. Our modified measure addresses this mis-match by using open-to-close returns. It better explains cross-sections of returns, doubling estimated liquidity premia over 1964–2017. Using non-synchronous trading near close as an instrument reveals that overnight returns are primarily information-driven and orthogonal to price impacts of trading. Thus, including them in liquidity proxies magnifies measurement error, understating liquidity premia. Our modification especially matters when applications in finance and accounting render use of intraday data infeasible/undesirable

Lender forbearance

We use a threshold-based design to study ex post discretion in lenders’ contractual enforcement of covenant violations. At preset thresholds, lenders enforce contractual breaches only infrequently, but this enforcement is associated with material consequences, e.g., fees and renegotiations. Enforcement varies significantly over time and peaks when credit conditions are tightest, indicating that enforcement is procyclical. Costly coordination reduces enforcement: syndicates with ex ante restrictive voting requirements enforce at lower rates. Consistent with theories of lender competition and implicit contracting, enforcement rates are lower for borrowers with access to alternative sources of financing and well-reputed lead arrangers

Old puzzle, new insights: a lithium rich giant quietly burning helium in its core

About 1% of giant stars have been shown to have large surface Li abundances, which is unexpected according to standard stellar evolution models. Several scenarios for lithium production have been proposed, but it is still unclear why these Li-rich giants exist. A missing piece in this puzzle is the knowledge of the exact stage of evolution of these stars. Using low-and-high-resolution spectroscopic observations, we have undertaken a survey of lithium-rich giants in the Kepler field. In this letter, we report the finding of the first confirmed Li-rich core-helium-burning giant, as revealed by asteroseismic analysis. The evolutionary timescales constrained by its mass suggest that Li-production most likely took place through non-canonical mixing at the RGB-tip, possibly during the helium flash.Comment: 16 pages, 4 figures, 1 table, accepted in ApJ Letter

Stellar science from a blue wavelength range - A possible design for the blue arm of 4MOST

From stellar spectra, a variety of physical properties of stars can be derived. In particular, the chemical composition of stellar atmospheres can be inferred from absorption line analyses. These provide key information on large scales, such as the formation of our Galaxy, down to the small-scale nucleosynthesis processes that take place in stars and supernovae. By extending the observed wavelength range toward bluer wavelengths, we optimize such studies to also include critical absorption lines in metal-poor stars, and allow for studies of heavy elements (Z>38) whose formation processes remain poorly constrained. In this context, spectrographs optimized for observing blue wavelength ranges are essential, since many absorption lines at redder wavelengths are too weak to be detected in metal-poor stars. This means that some elements cannot be studied in the visual-redder regions, and important scientific tracers and science cases are lost. The present era of large public surveys will target millions of stars. Here we describe the requirements driving the design of the forthcoming survey instrument 4MOST, a multi-object spectrograph commissioned for the ESO VISTA 4m-telescope. We focus here on high-density, wide-area survey of stars and the science that can be achieved with high-resolution stellar spectroscopy. Scientific and technical requirements that governed the design are described along with a thorough line blending analysis. For the high-resolution spectrograph, we find that a sampling of >2.5 (pixels per resolving element), spectral resolution of 18000 or higher, and a wavelength range covering 393-436 nm, is the most well-balanced solution for the instrument. A spectrograph with these characteristics will enable accurate abundance analysis (+/-0.1 dex) in the blue and allow us to confront the outlined scientific questions. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A

The RAVE Survey: Constraining the Local Galactic Escape Speed

We report new constraints on the local escape speed of our Galaxy. Our analysis is based on a sample of high velocity stars from the RAVE survey and two previously published datasets. We use cosmological simulations of disk galaxy formation to motivate our assumptions on the shape of the velocity distribution, allowing for a significantly more precise measurement of the escape velocity compared to previous studies. We find that the escape velocity lies within the range 498\kms < \ve < 608 \kms (90 per cent confidence), with a median likelihood of 544\kms. The fact that \ve^2 is significantly greater than 2\vc^2 (where \vc=220\kms is the local circular velocity) implies that there must be a significant amount of mass exterior to the Solar circle, i.e. this convincingly demonstrates the presence of a dark halo in the Galaxy. For a simple isothermal halo, one can calculate that the minimum radial extent is $\sim58$ kpc. We use our constraints on \ve to determine the mass of the Milky Way halo for three halo profiles. For example, an adiabatically contracted NFW halo model results in a virial mass of $1.42^{+1.14}_{-0.54}\times10^{12}M_\odot$ and virial radius of $305^{+66}_{-45}$ kpc (90 per cent confidence). For this model the circular velocity at the virial radius is 142^{+31}_{-21}\kms. Although our halo masses are model dependent, we find that they are in good agreement with each other.Comment: 19 pages, 9 figures, MNRAS (accepted). v2 incorporates minor cosmetic revisions which have no effect on the results or conclusion

The Gaia-ESO Survey: the selection function of the Milky Way field stars

The Gaia-ESO Survey was designed to target all major Galactic components (i.e., bulge, thin and thick discs, halo and clusters), with the goal of constraining the chemical and dynamical evolution of the Milky Way. This paper presents the methodology and considerations that drive the selection of the targeted, allocated and successfully observed Milky Way field stars. The detailed understanding of the survey construction, specifically the influence of target selection criteria on observed Milky Way field stars is required in order to analyse and interpret the survey data correctly. We present the target selection process for the Milky Way field stars observed with VLT/FLAMES and provide the weights that characterise the survey target selection. The weights can be used to account for the selection effects in the Gaia-ESO Survey data for scientific studies. We provide a couple of simple examples to highlight the necessity of including such information in studies of the stellar populations in the Milky Way.Comment: 18 pages, 19 figures, Accepted for publication in MNRAS (April 25, 2016

A High-resolution Scintillating Fiber Tracker With Silicon Photomultiplier Array Readout

We present prototype modules for a tracking detector consisting of multiple layers of 0.25 mm diameter scintillating fibers that are read out by linear arrays of silicon photomultipliers. The module production process is described and measurements of the key properties for both the fibers and the readout devices are shown. Five modules have been subjected to a 12 GeV/c proton/pion testbeam at CERN. A spatial resolution of 0.05 mm and light yields exceeding 20 detected photons per minimum ionizing particle have been achieved, at a tracking efficiency of more than 98.5%. Possible techniques for further improvement of the spatial resolution are discussed.Comment: 31 pages, 27 figures, pre-print version of an article published in Nuclear Instruments and Methods in Physics Research Section A, Vol. 62