1,369 research outputs found
Performance Evaluation with Stochastic Discount Factors
We study the use of stochastic discount factor (SDF) models in evaluating the investment performance of portfolio managers. By constructing artificial mutual funds with known levels of investment ability, we evaluate a large set of SDF models. We find that the measures of performance are not highly sensitive to the SDF model, and that most of the models have a mild negative bias when performance is neutral. We use the models to evaluate a sample of U.S. equity mutual funds. Adjusting for the observed bias, we find that the average mutual fund has enough ability to cover its transactions costs. Extreme funds are more likely to have good rather than poor risk adjusted performance. Our analysis also reveals a number of implementation issues relevant to other applications of SDF models.
Hot subdwarf stars and their connection to thermonuclear supernovae
Hot subdwarf stars (sdO/Bs) are evolved core helium-burning stars with very
thin hydrogen envelopes, which can be formed by common envelope ejection. Close
sdB binaries with massive white dwarf (WD) companions are potential progenitors
of thermonuclear supernovae type Ia (SN Ia). We discovered such a progenitor
candidate as well as a candidate for a surviving companion star, which escapes
from the Galaxy. More candidates for both types of objects have been found by
crossmatching known sdB stars with proper motion and light curve catalogues.
The Gaia mission will provide accurate astrometry and light curves of all the
stars in our hot subdwarf sample and will allow us to compile a much larger
all-sky catalogue of those stars. In this way we expect to find hundreds of
progenitor binaries and ejected companions.Comment: Proceedings of the 11th Pacific Rim Conference on Stellar
Astrophysics, Hong Kong 2015, Journal of Physics: Conference Series, in pres
The orbits of subdwarf-B + main-sequence binaries. II. Three eccentric systems; BD+29 3070, BD +34 1543 and Feige 87
The predicted orbital-period distribution of the subdwarf-B (sdB) population
is bi-modal with a peak at short ( 250 days) periods.
Observationally, many short-period sdB systems are known, but the predicted
long period peak is missing as orbits have only been determined for a few
long-period systems. As these predictions are based on poorly understood
binary-interaction processes, it is of prime importance to confront the
predictions with reliable observational data. We therefore initiated a
monitoring program to find and characterize long-period sdB stars. In this
paper we aim to determine the orbital parameters of the three long-period
sdB+MS binaries BD+29 3070, BD+34 1543 and Feige 87, to constrain their
absolute dimensions and the physical parameters of the components.
High-resolution spectroscopic time series were obtained with HERMES at the
Mercator telescope on La Palma, and analyzed to determine the radial velocities
of both the sdB and MS components. Photometry from the literature was used to
construct the spectral-energy distribution (SED) of the binaries. Atmosphere
models were used to fit these SEDs and to determine the surface gravities and
temperatures of both components of all systems. Spectral analysis was used to
check the results of the SEDs. An orbital period of 1283 +- 63 d, a mass ratio
of q = 0.39 +- 0.04 and a significant non-zero eccentricity of e = 0.15 +- 0.01
were found for BD+29 3070. For BD+34 1543 we determined P = 972 +- 2 d, q =
0.57 +- 0.01 and again a clear non-zero eccentricity of e = 0.16 +- 0.01. Last,
for Feige 87 we found P = 936 +- 2 d, q = 0.55 +- 0.01 and e = 0.11 +- 0.01.
BD+29 3070, BD+34 1543 and Feige 87 are long period sdB + MS binaries on
clearly eccentric orbits. These results are in conflict with the predictions of
stable Roche-lobe overflow models.Comment: 15 pages, 6 figures, Accepted by A&
The MUCHFUSS photometric campaign
Hot subdwarfs (sdO/Bs) are the helium-burning cores of red giants, which lost
almost all of their hydrogen envelopes. This mass loss is often triggered by
common envelope interactions with close stellar or even substellar companions.
Cool companions like late-type stars or brown dwarfs are detectable via
characteristic light curve variations like reflection effects and often also
eclipses. To search for such objects we obtained multi-band light curves of 26
close sdO/B binary candidates from the MUCHFUSS project with the BUSCA
instrument. We discovered a new eclipsing reflection effect system
(~d) with a low-mass M dwarf companion ().
Three more reflection effect binaries found in the course of the campaign were
already published, two of them are eclipsing systems, in one system only
showing the reflection effect but no eclipses the sdB primary is found to be
pulsating. Amongst the targets without reflection effect a new long-period sdB
pulsator was discovered and irregular light variations were found in two sdO
stars. The found light variations allowed us to constrain the fraction of
reflection effect binaries and the substellar companion fraction around sdB
stars. The minimum fraction of reflection effect systems amongst the close sdB
binaries might be greater than 15\% and the fraction of close substellar
companions in sdB binaries might be as high as . This would result in a
close substellar companion fraction to sdB stars of about 3\%. This fraction is
much higher than the fraction of brown dwarfs around possible progenitor
systems, which are solar-type stars with substellar companions around 1 AU, as
well as close binary white dwarfs with brown dwarf companions. This might be a
hint that common envelope interactions with substellar objects are
preferentially followed by a hot subdwarf phase.Comment: accepted for A&
The Deuterium, Oxygen, and Nitrogen Abundance Toward LSE 44
We present measurements of the column densities of interstellar DI, OI, NI,
and H2 made with FUSE, and of HI made with IUE toward the sdO star LSE 44, at a
distance of 554+/-66 pc. This target is among the seven most distant Galactic
sight lines for which these abundance ratios have been measured. The column
densities were estimated by profile fitting and curve of growth analyses. We
find D/H = (2.24 +1.39 -1.32)E-5, D/O = (1.99 +1.30 -0.67)E-2, D/N = (2.75
+1.19 -0.89)E-1, and O/H = (1.13 +0.96 -0.71)E-3 (2 sigma). Of the most distant
Galactic sight lines for which the deuterium abundance has been measured LSE 44
is one of the few with D/H higher than the Local Bubble value, but D/O toward
all these targets is below the Local Bubble value and more uniform than the D/H
distribution. (Abstract abridged.)Comment: 20 pages, including 9 figures. Accepted for publication in Ap
The MultiSite Spectroscopic Telescope campaign: 2m spectroscopy of the V361 Hya variable PG1605+072
We present results and analysis for the 2m spectroscopic part of the
MultiSite Spectroscopic Telescope (MSST) campaign undertaken in May/June 2002.
The goal of the project was to observe the pulsating subdwarf B star PG1605+072
simultaneously in velocity and photometry and to resolve as many of the >50
known modes as possible, which will allow a detailed asteroseismological
analysis. We have obtained over 150 hours of spectroscopy, leading to an
unprecedented noise level of only 207m/s. We report here the detection of 20
frequencies in velocity, with two more likely just below our detection
threshold. In particular, we detect 6 linear combinations, making PG1605+072
only the second star known to show such frequencies in velocity. We investigate
the phases of these combinations and their parent modes and find relationships
between them that cannot be easily understood based on current theory. These
observations, when combined with our simultaneous photometry, should allow
asteroseismology of this most complicated of sdB pulsators.Comment: 9 pages, 5 figures, accepted for publication in A&A; Figure 1 at
lower resolution than accepted versio
The Unseen Population of F to K-type Companions to Hot Subdwarf Stars
We present a method to select hot subdwarf stars with A to M-type companions
using photometric selection criteria. We cover a wide range in wavelength by
combining GALEX ultraviolet data, optical photometry from the SDSS and the
Carlsberg Meridian telescope, near-infrared data from 2MASS and UKIDSS. We
construct two complimentary samples, one by matching GALEX, CMC and 2MASS, as
well as a smaller, but deeper, sample using GALEX, SDSS and UKIDSS. In both
cases, a large number of composite subdwarf plus main-sequence star candidates
were found. We fit their spectral energy distributions with a composite model
in order to estimate the subdwarf and companion star effective temperatures
along with the distance to each system. The distribution of subdwarf effective
temperature was found to primarily lie in the 20,000 - 30,000 K regime, but we
also find cooler subdwarf candidates, making up ~5-10 per cent. The most
prevalent companion spectral types were seen to be main-sequence stars between
F0 and K0, while subdwarfs with M-type companions appear much rarer. This is
clear observational confirmation that a very efficient first stable Roche-lobe
overflow channel appears to produce a large number of subdwarfs with F to
K-type companions. Our samples thus support the importance of binary evolution
for subdwarf formation.Comment: 30 pages, 10 figures, 11 tables. Accepted for publication in MNRA
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