1,175 research outputs found
NLTE spectroscopic analysis of the He anomaly in subluminous B-type stars
Several B-type main-sequence stars show chemical peculiarities. A
particularly striking class are the He stars, which exhibit a remarkable
enrichment of He with respect to He. This isotopic anomaly has also
been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which
are helium-core burning stars of the extreme horizontal branch. Using a hybrid
local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars,
we analyzed high-quality spectra of two known He BHBs and nine known He
sdBs to determine their isotopic helium abundances and He/He abundance
ratios. We redetermined their atmospheric parameters and analyzed selected
neutral helium lines, including 4922 and 6678
, which are very sensitive to He/He. Most of the He
sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are
helium deficient in accordance with previous LTE analyses. BD+48 2721
is reclassified as a BHB star because of its low temperature
( 20700 K). Whereas He is almost absent
(He/He 0.25) in most of the known He stars, other sample stars
show abundance ratios up to He/He2.51. A search for He stars in
the ESO SPY survey led to the discovery of two new He sdB stars (HE
0929-0424 and HE 1047-0436). The observed helium line profiles of all BHBs and
of three sdBs are not matched by chemically homogeneous atmospheres, but hint
at vertical helium stratification. This phenomenon has been seen in other
peculiar B-type stars, but is found for the first time for sdBs. We estimate
helium to increase from the outer to the inner atmosphere by factors ranging
from 1.4 (SB 290) up to 8.0 (BD+48 2721).Comment: 19 pages, 79 figures submitted to Astronomy&Astrophysic
Discovery of Spectroscopic Variations in the DAB White Dwarf GD 323
We report the discovery of spectroscopic variations in GD 323, the
prototypical DAB white dwarf. Simultaneous optical spectroscopic observations
over five consecutive nights of GD 323 and of PG 1234+482, a non-variable
comparison DA white dwarf of similar brightness, are used to reveal
quasi-periodic variations in both the hydrogen and helium absorption lines over
a timescale of hours. The amplitude of the variation of the equivalent width of
Hbeta is ~30 %. Moreover, the strength of the hydrogen lines is shown to vary
in opposite phase from that of He I 4471. These results suggest that the model
currently thought to be the most viable to account for the simultaneous
presence of hydrogen and helium lines in GD 323, namely a static stratified
atmosphere, may need to be reexamined. Instead, a model with an inhomogeneous
surface composition, resulting perhaps from the dilution of a thin hydrogen
atmosphere with the underlying helium convection zone, may be a better
representation of GD 323. The observed variation timescale of ~3.5 hours is
consistent with the slow rotation rate of white dwarf stars.Comment: 23 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Delayed improvement local search
Local search is a fundamental tool in the development of heuristic algorithms. A neighborhood operator takes a current solution and returns a set of similar solutions, denoted as neighbors. In best improvement local search, the best of the neighboring solutions replaces the current solution in each iteration. On the other hand, in first improvement local search, the neighborhood is only explored until any improving solution is found, which then replaces the current solution. In this work we propose a new strategy for local search that attempts to avoid low-quality local optima by selecting in each iteration the improving neighbor that has the fewest possible attributes in common with local optima. To this end, it uses inequalities previously used as optimality cuts in the context of integer linear programming. The novel method, referred to as delayed improvement local search, is implemented and evaluated using the travelling salesman problem with the 2-opt neighborhood and the max-cut problem with the 1-flip neighborhood as test cases. Computational results show that the new strategy, while slower, obtains better local optima compared to the traditional local search strategies. The comparison is favourable to the new strategy in experiments with fixed computation time or with a fixed target.publishedVersio
PG 1610+062: a runaway B star challenging classical ejection mechanisms
Hypervelocity stars are rare objects, mostly main-sequence (MS) B stars, traveling so fast that they will eventually escape from the Milky Way. Recently, it has been shown that the popular Hills mechanism, in which a binary system is disrupted via a close encounter with the supermassive black hole at the Galactic center, may not be their only ejection mechanism. The analyses of Gaia data ruled out a Galactic center origin for some of them, and instead indicated that they are extreme disk runaway stars ejected at velocities exceeding the predicted limits of classical scenarios (dynamical ejection from star clusters or binary supernova ejection). We present the discovery of a new extreme disk runaway star, PG 1610+062, which is a slowly pulsating B star bright enough to be studied in detail. A quantitative analysis of spectra taken with ESI at the Keck Observatory revealed that PG 1610+062 is a late B-type MS star of 4–5 M⊙with low projected rotational velocity. Abundances (C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe) were derived differentially with respect to the normal B star HD 137366 and indicate that PG 1610+062 is somewhat metal rich. A kinematic analysis, based on our spectrophotometric distance (17.3 kpc) and on proper motions from Gaia’s second data release, shows that PG 1610+062 was probably ejected from the Carina-Sagittarius spiral arm at a velocity of 550 ± 40 km s^(−1), which is beyond the classical limits. Accordingly, the star is in the top five of the most extreme MS disk runaway stars and is only the second among the five for which the chemical composition is known
Quantitative spectral analysis of the sdB star HD 188112: a helium-core white dwarf progenitor
HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass
too low to ignite core helium burning and is therefore considered as a
pre-extremely low mass (ELM) white dwarf (WD). ELM WDs (M 0.3 Msun) are
He-core objects produced by the evolution of compact binary systems. We present
in this paper a detailed abundance analysis of HD 188112 based on
high-resolution Hubble Space Telescope (HST) near and far-ultraviolet
spectroscopy. We also constrain the mass of the star's companion. We use hybrid
non-LTE model atmospheres to fit the observed spectral lines and derive the
abundances of more than a dozen elements as well as the rotational broadening
of metallic lines. We confirm the previous binary system parameters by
combining radial velocities measured in our UV spectra with the already
published ones. The system has a period of 0.60658584 days and a WD companion
with M 0.70 Msun. By assuming a tidally locked rotation, combined with
the projected rotational velocity (v sin i = 7.9 0.3 km s) we
constrain the companion mass to be between 0.9 and 1.3 Msun. We further discuss
the future evolution of the system as a potential progenitor of a
(underluminous) type Ia supernova. We measure abundances for Mg, Al, Si, P, S,
Ca, Ti, Cr, Mn, Fe, Ni, and Zn, as well as for the trans-iron elements Ga, Sn,
and Pb. In addition, we derive upper limits for the C, N, O elements and find
HD 188112 to be strongly depleted in carbon. We find evidence of non-LTE
effects on the line strength of some ionic species such as Si II and Ni II. The
metallic abundances indicate that the star is metal-poor, with an abundance
pattern most likely produced by diffusion effects.Comment: Accepted for publication in A&
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Noble gases from the interstellar medium trapped on the MIR space station and analyzed by in vacuo etching
Introduction: The composition of the present interstellar medium (ISM) provides an important benchmark in cosmochemistry. It serves as a reference for galactic chemical evolution (GCE) models, solar mixing predictions and provides information for understanding Big Bang nucleosynthesis. The present-day ISM 3He abundance allows, combined with the protosolar 3He, deduced from the Jovian atmosphere or meteorites [1,2], tracing the GCE over the past 4.56 Ga. 3He/4He = (2.5 0.6) x 10-4 has been determined for the local ISM [3]. However, the uncertainty is too large to better constrain GCE models and - in combination with the present-day solar wind value - the protosolar D/H [4]
The Optical Transient Search in the Bamberg Southern Sky Survey: Preliminary Results
A large fraction of gamma-ray bursts temporarily emit optical light, i.e. optical afterglows and optical transients. So far, optical transients have only been detected after related gamma-ray satellite detection. However, taking into account their optical magnitudes at maximum light, these objects should be detectable in various historical and recent optical surveys, including the photographic sky patrol. Here we report on an extended study based on blink-comparison of 5004 Bamberg Observatory Southern Sky Patrol Plates performed within a student high school project (Jugend Forscht)
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