The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs

We analyze radial velocity observations of the 12 extremely low-mass <0.25 Msol white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the 12 WDs are binaries with orbital periods shorter than 14 hours; the one non-variable WD is possibly a pole-on system among our non-kinematically selected targets. Our sample is unique: it is complete in a well-defined range of apparent magnitude and color. The orbital mass functions imply that the unseen companions are most likely other WDs, although neutron star companions cannot be excluded. Six of the 11 systems with orbital solutions will merge within a Hubble time due to the loss of angular momentum through gravitational wave radiation. The quickest merger is J0923+3028, a g=15.7 ELM WD binary with a 1.08 hr orbital period and a <130 Myr merger time. The chance of a supernova Ia event among our ELM WDs is only 1%-7%, however. Three binary systems (J0755+4906, J1233+1602, and J2119-0018) have extreme mass ratios and will most likely form stable mass-transfer AM CVn systems. Two of these objects, SDSS J1233+1602 and J2119-0018, are the lowest surface gravity WDs ever found; both show Ca II absorption likely from accretion of circumbinary material. We predict that at least one of our WDs is an eclipsing detached double WD system, important for constraining helium core WD models.Comment: ApJ, in pres

Direct Distance Measurement to the Dusty White Dwarf GD 362

We present trigonometric parallax observations of GD 362 obtained over seven epochs using the MDM 2.4m Hiltner Telescope. The existence of a dust disk around this possibly massive white dwarf makes it an interesting target for parallax observations. The measured parallax for GD 362 places it at a distance of 50.6 pc, which implies that its radius and mass are ~ 0.0106 Rsun and 0.71 Msun, respectively. GD 362 is not as massive as initially thought (1.2Msun). Our results are entirely consistent with the distance and mass estimates (52.2 pc and 0.73 Msun) by Zuckerman et al., who demonstrated that GD 362 has a helium dominated atmosphere. Dropping GD 362 from the list of massive white dwarfs, there are no white dwarfs with M > 0.9 Msun that are known to host circumstellar dust disks.Comment: ApJ Letters, in pres

The ELM Survey. III. A Successful Targeted Survey for Extremely Low Mass White Dwarfs

Extremely low mass (ELM) white dwarfs (WDs) with masses <0.25 Msun are rare objects that result from compact binary evolution. Here, we present a targeted spectroscopic survey of ELM WD candidates selected by color. The survey is 71% complete and has uncovered 18 new ELM WDs. Of the 7 ELM WDs with follow-up observations, 6 are short-period binaries and 4 have merger times less than 5 Gyr. The most intriguing object, J1741+6526, likely has either a pulsar companion or a massive WD companion making the system a possible supernova Type Ia or .Ia progenitor. The overall ELM Survey has now identified 19 double degenerate binaries with <10 Gyr merger times. The significant absence of short orbital period ELM WDs at cool temperatures suggests that common envelope evolution creates ELM WDs directly in short period systems. At least one-third of the merging systems are halo objects, thus ELM WD binaries continue to form and merge in both the disk and the halo.Comment: 10 pages, accepted to Ap

Forecasting Volatility of Turkish Markets: A Comparison of Thin and Thick Models

Volatility of financial markets is an important topic for academics, policy makers and market participants. In this study first I summarized several specifications for the conditional variance and also define some methods for combination of these specifications. Then assuming that the squared returns are the benchmark estimate for actual volatility of the day, I compare all of the models with respect to how much efficient they are to mimic the realized volatility. At the same time I used a VaR approach to compare these forecasts. With the help of these analyses I examine if combination of the forecast could outperform the single models.volatility, arch, garch, combination, VaR

Violation duration as a better way of VaR model evaluation : evidence from Turkish market portfolio

Financial crisis those we have been experienced during last two decades encouraged the efforts of both academicians and the market participants to develop clear representations of the risk exposure of a �nancial institute. As a useful tool for measuring market risk of a portfolio, Value-at-Risk has emerged as the standard. However, there are several alternative Value-at-Risk implementations which may pro- duce signi�cantly di¤erent Value-at-Risk forecasts. Thus, evaluation of Value-at-Risk forecasts is as crucial as VaR itself. In this paper I will use the methodology which has described by Christoffersen and Pelletier[6] and I extended the methodology to create duration based analogous of unconditional coverage, conditional coverage and inde- pendence tests. I evaluated 14 Value-at-Risk implementation by using a Turkish Market portfolio which contain foreing currency, stock and bonds.Value-at-Risk; model evaluation; conditional cover- age; duration based coverage testing

Broadband Technologies on Residential Acces

The diffusion of broadband technologies is a hot topic for developed and many developing countries. Although the provision of service has many similar aspects, the overall and specific penetration of broadband technologies varies significantly in these countries. This study aims to examine the place of users' perceptions in the broadband issue by studying the development of the selected technologies and national policies in the light of the general information technology diffusion aspects.Diffusion of Technology, Broadband Technologies, Development of National Policies and Pricing Issues

Can A Pseudo-Nambu-Goldstone Higgs Lead To Symmetry Non-Restoration?

The calculation of finite temperature contributions to the scalar potential in a quantum field theory is similar to the calculation of loop corrections at zero temperature. In natural extensions of the Standard Model where loop corrections to the Higgs potential cancel between Standard Model degrees of freedom and their symmetry partners, it is interesting to contemplate whether finite temperature corrections also cancel, raising the question of whether a broken phase of electroweak symmetry may persist at high temperature. It is well known that this does not happen in supersymmetric theories because the thermal contributions of bosons and fermions do not cancel each other. However, for theories with same spin partners, the answer is less obvious. Using the Twin Higgs model as a benchmark, we show that although thermal corrections do cancel at the level of quadratic divergences, subleading corrections still drive the system to a restored phase. We further argue that our conclusions generalize to other well-known extensions of the Standard Model where the Higgs is rendered natural by being the pseudo-Nambu-Goldstone mode of an approximate global symmetry.Comment: 21 pages, 5 figures. v2: fixed problem related to references with 9-digit arXiv identifiers. v3: references added v4: Some clarifications and more references added; matches published versio

Integrability of a Non-autonomous Coupled KdV System

The Painlev\'{e} property of coupled, non-autonomous Korteweg-de Vries (KdV) type of systems is studied. The conditions under which the systems pass the Painlev\'{e} test for integrability are obtained. For some of the integrable cases, exact solutions are given.Comment: 7 pages, 2 figures, to be published in IJMPC. For related figures, see http://www.metu.edu.tr/~akarasu/figures.htm

A Nonparametric Way of Distribution Testing

Testing the distribution of a random sample can be considered ,indeed, as a goodness-of-fit problem. If we use the nonparametric density estimation of the sample as a consistent estimate of exact distribution, the problem reduces, more specifically, to the distance of two functions. This paper examines the distribution testing from this point of view and suggests a nonparametric procedure. Although the procedure is applicable for all distributions, paper emphasizes on normality test.The critical values for this normality test generated by using Monte Carlo techniques.distribution testing, normality, monte carlo simulation