102 research outputs found
Ongoing Astrometric Microlensing Events of Two Nearby Stars
Context. Astrometric microlensing is an excellent tool to determine the mass
of a stellar object. By measuring the astrometric shift of a background source
star in combination with precise predictions of its unlensed position and of
the lens position, gravitational lensing allows to determine the mass of the
lensing star with a precision of 1 percent, independent of any prior knowledge.
Aims. Making use of the recently published Gaia Data Release 2 (Gaia DR2) we
predict astrometric microlensing events by foreground stars of high proper
motion passing by a background star in the coming years.
Methods. We compile a list of ~148.000 high-proper-motion stars within Gaia
DR2 with > 150 mas/yr. We then search for background stars close to
their paths and calculate the dates and separations of the closest approaches.
Using color and absolute magnitude, we determine approximate masses of the
lenses. Finally, we calculate the expected astrometric shifts and
magnifications of the predicted events.
Results . We detect two ongoing microlensing events by the high proper motion
stars Luyten 143-23 and Ross 322 and predict closest separations of (108.5
1.4) mas in July 2018 and (125.3 3.4) mas in August 2018,
respectively. The respective expected astrometric shifts are (1.74 0.12)
mas and (0.76 0.06) mas. Furthermore, Luyten 143-23 will pass by another
star in March 2021 with a closest separation of (280.1 1.1) mas, which
results in an expected shift of (0.69 0.05) mas.Comment: Submitted to A&A, accepted June 14, 2018. 4 pages, 3 figures, 2
table
The Effect of Use and Access on Citations
It has been shown (S. Lawrence, 2001, Nature, 411, 521) that journal articles
which have been posted without charge on the internet are more heavily cited
than those which have not been. Using data from the NASA Astrophysics Data
System (ads.harvard.edu) and from the ArXiv e-print archive at Cornell
University (arXiv.org) we examine the causes of this effect.Comment: Accepted for publication in Information Processing & Management,
special issue on scientometric
Prediction of astrometric microlensing events from Gaia DR2 proper motions
Context: Astrometric gravitational microlensing is an excellent tool to
determine the mass of stellar objects. Using precise astrometric measurements
of the lensed position of a background source in combination with accurate
predictions of the positions of the lens and the unlensed source it is possible
to determine the mass of the lens with an accuracy of a few percent. Aims:
Making use of the recently published Gaia Data Release 2 (DR2) catalogue, we
want to predict astrometric microlensing events caused by foreground stars with
high proper motion passing a background source in the coming decades. Methods:
We selected roughly 148 000 high-proper-motion stars from Gaia DR2 with
as potential lenses. We then searched for
background sources close to their paths. Using the astrometric parameters of
Gaia DR2, we calculated the future positions of source and lens. With a
nested-intervals algorithm we determined the date and separation of the closest
approach. Using Gaia DR2 photometry we determined an approximate mass of the
lens, which we used to calculate the expected microlensing effects. Results: We
predict 3914 microlensing events caused by 2875 different lenses between 2010
and 2065, with expected shifts larger than between the
lensed and unlensed positions of the source. Of those, 513 events are expected
to happen between 2014.5 - 2026.5 and might be measured by Gaia. For 127 events
we also expect a magnification between and
.Comment: 11 Pages, 14 figures, 2 tables, Accepted for publication in A&
Stellar laboratories. IX. New Se V, Sr IV - VII, Te VI, and I VI oscillator strengths and the Se, Sr, Te, and I abundances in the hot white dwarfs G191-B2B and RE 0503-289
To analyze spectra of hot stars, advanced non-local thermodynamic equilibrium
(NLTE) model-atmosphere techniques are mandatory. Reliable atomic data is for
the calculation of such model atmospheres.
We aim to calculate new Sr IV - VII oscillator strengths to identify for the
first time Sr spectral lines in hot white dwarf (WD) stars and to determine the
photospheric Sr abundances. o measure the abundances of Se, Te, and I in hot
WDs, we aim to compute new Se V, Te VI, and I VI oscillator strengths.
To consider radiative and collisional bound-bound transitions of Se V, Sr IV
- VII, Te VI, and I VI in our NLTE atmosphere models, we calculated oscillator
strengths for these ions.
We newly identified four Se V, 23 Sr V, 1 Te VI, and three I VI lines in the
ultraviolet (UV) spectrum of RE0503-289. We measured a photospheric Sr
abundance of 6.5 +3.8/-2.4 x 10**-4 (mass fraction, 9500 - 23800 times solar).
We determined the abundances of Se (1.6 +0.9/-0.6 x 10**-3, 8000 - 20000), Te
(2.5 +1.5/-0.9 x 10**-4, 11000 - 28000), and I (1.4 +0.8/-0.5 x 10**-5, 2700 -
6700). No Se, Sr, Te, and I line was found in the UV spectra of G191-B2B and we
could determine only upper abundance limits of approximately 100 times solar.
All identified Se V, Sr V, Te VI, and I VI lines in the UV spectrum of
RE0503-289 were simultaneously well reproduced with our newly calculated
oscillator strengths.Comment: 26 pages, 5 figure
Stellar laboratories. VIII. New Zr IV - VII, Xe IV - V, and Xe VII oscillator strengths and the Al, Zr, and Xe abundances in the hot white dwarfs G191-B2B and RE0503-289
For the spectral analysis of high-resolution and high-signal-to-noise spectra
of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model
atmospheres are mandatory. These are strongly dependent on the reliability of
the atomic data that is used for their calculation.
To search for Zr and Xe lines in the ultraviolet (UV) spectra of G191-B2B and
RE0503-289, new Zr IV-VII, Xe IV-V, and Xe VIII oscillator strengths were
calculated. This allows for the first time, determination of the Zr abundance
in white dwarf (WD) stars and improvement of the Xe abundance determinations.
We calculated Zr IV-VII, Xe IV-V, and Xe VIII oscillator strengths to
consider radiative and collisional bound-bound transitions of Zr and Xe in our
NLTE stellar-atmosphere models for the analysis of their lines exhibited in UV
observations of the hot WDs G191-B2B and RE0503-289.
We identified one new Zr IV, 14 new Zr V, and ten new Zr VI lines in the
spectrum of RE0503-289. Zr was detected for the first time in a WD. We measured
a Zr abundance of -3.5 +/- 0.2 (logarithmic mass fraction, approx. 11 500 times
solar). We dentified five new Xe VI lines and determined a Xe abundance of -3.9
+/- 0.2 (approx. 7500 times solar). We determined a preliminary photospheric Al
abundance of -4.3 +/- 0.2 (solar) in RE0503-289. In the spectra of G191-B2B, no
Zr line was identified. The strongest Zr IV line (1598.948 A) in our model gave
an upper limit of -5.6 +/- 0.3 which is about 100 times solar. No Xe line was
identified in the UV spectrum of G191-B2B and we confirmed the previously
determined upper limit of -6.8 +/- 0.3 (ten times solar).
Precise measurements and calculations of atomic data are a prerequisite for
advanced NLTE stellar-atmosphere modeling. Observed Zr IV - VI and Xe VI - VII
line profiles in the UV spectrum of RE0503-289 were simultaneously well
reproduced.Comment: 137 pages, 11 figure
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