51 research outputs found
Atmospheric dispersion correction: model requirements and impact on radial velocity measurements
Observations with ground-based telescopes are affected by differential
atmospheric dispersion when seen at a zenith angle different from zero, a
consequence of the wavelength-dependent index of refraction of the atmosphere.
One of the pioneering technology in detecting exoplanets is the technique of
radial velocity (RV), that can be affected by uncorrected atmospheric
dispersion. The current highest precision spectrographs are expected to deliver
a precision of 10 cm/s (e.g., ESPRESSO). To minimize the atmospheric dispersion
effect, an Atmospheric Dispersion Corrector (ADC) can be employed. ADC designs
are based on sky dispersion models that nonetheless give different results;
these can reach a few tens of milli-arcseconds (mas) in the sky (a difference
up to 40 mas); a value close to the current requirements (20 mas in the case of
ESPRESSO). In this paper we describe tests done with ESPRESSO and HARPS to
understand the influence of atmospheric dispersion and its correction on RV
precision. We also present a comparison of different sky models, using EFOSC2
data (between 600nm and 700nm), that will be used to improve on the design of
ADCs
Integral field spectroscopy of selected areas of the Bright Bar and Orion-S cloud in the Orion Nebula
We present integral field spectroscopy of two selected zones in the Orion
Nebula obtained with the Potsdam Multi-Aperture Spectrophotometer (PMAS),
covering the optical spectral range from 3500 to 7200 A and with a spatial
resolution of 1". The observed zones are located on the prominent Bright Bar
and on the brightest area at the northeast of the Orion South cloud, both
containing remarkable ionization fronts. We obtain maps of emission line fluxes
and ratios, electron density and temperatures, and chemical abundances. We
study the ionization structure and morphology of both fields, which ionization
fronts show different inclination angles with respect to the plane of the sky.
We find that the maps of electron density, O+/H+ and O/H ratios show a rather
similar structure. We interpret this as produced by the strong dependence on
density of the [OII] lines used to derive the O+ abundance, and that our
nominal values of electron density-derived from the [SII] line ratio-may be
slightly higher than the appropriate value for the O+ zone. We measure the
faint recombination lines of OII in the field at the northeast of the Orion
South cloud allowing us to explore the so-called abundance discrepancy problem.
We find a rather constant abundance discrepancy across the field and a mean
value similar to that determined in other areas of the Orion Nebula, indicating
that the particular physical conditions of this ionization front do not
contribute to this discrepancy.Comment: 15 pages, 10 figures. Accepted for publication in MNRA
PEXO : a global modeling framework for nanosecond timing, microsecond astrometry, and μm/s radial velocities
54 pages, 2 tables, 19 figures, accepted for publication in ApJS, PEXO is available at https://github.com/phillippro/pexoThe ability to make independent detections of the signatures of exoplanets with complementary telescopes and instruments brings a new potential for robust identification of exoplanets and precision characterization. We introduce PEXO, a package for Precise EXOplanetology to facilitate the efficient modeling of timing, astrometry, and radial velocity data, which will benefit not only exoplanet science but also various astrophysical studies in general. PEXO is general enough to account for binary motion and stellar reflex motions induced by planetary companions and is precise enough to treat various relativistic effects both in the solar system and in the target system. We also model the post-Newtonian barycentric motion for future tests of general relativity in extrasolar systems. We benchmark PEXO with the pulsar timing package TEMPO2 and find that PEXO produces numerically similar results with timing precision of about 1 ns, space-based astrometry to a precision of 1{\mu}as, and radial velocity of 1 {\mu}m/s and improves on TEMPO2 for decade-long timing data of nearby targets, due to its consideration of third-order terms of Roemer delay. PEXO is able to avoid the bias introduced by decoupling the target system and the solar system and to account for the atmospheric effects which set a practical limit for ground-based radial velocities close to 1 cm/s. Considering the various caveats in barycentric correction and ancillary data required to realize cm/s modeling, we recommend the preservation of original observational data. The PEXO modeling package is available at GitHub (https://github.com/phillippro/pexo).Peer reviewe
Ionized gas diagnostics from protoplanetary discs in the Orion Nebula and the abundance discrepancy problem
We present results from integral field spectroscopy with PMAS. The observed
field contains: five protoplanetary discs (also known as proplyds), the
high-velocity jet HH 514 and a bowshock. Spatial distribution maps are obtained
for different emission line fluxes, the c(H{\beta}) coefficient, electron
densities and temperatures, ionic abundances of different ions from
collisionally excited lines (CELs), C2+ and O2+ abundances from recombination
lines (RLs) and the abundance discrepancy factor of O2+, ADF(O2+). We find that
collisional de-excitation has a major influence on the line fluxes in the
proplyds. If this is not properly accounted for then physical conditions
deduced from commonly used line ratios will be in error, leading to unreliable
chemical abundances for these objects. We obtain the intrinsic emission of the
proplyds 177-341, 170-337 and 170-334 by a direct subtraction of the background
emission, though the last two present some background contamination due to
their small sizes. A detailed analysis of 177-341 spectra reveals the presence
of high-density gas (3.8\times10^5 cm^-3) in contrast to the typical values
observed in the background gas of the nebula (3800 cm^-3). We also explore how
the background subtraction could be affected by the possible opacity of the
proplyd. We construct a physical model for the proplyd 177-341 finding a good
agreement between the predicted and observed line ratios. Finally, we find that
the use of reliable physical conditions returns an ADF(O2+) about zero for the
intrinsic spectra of 177-341, while the background emission presents the
typical ADF(O2+) observed in the Orion Nebula. We conclude that the presence of
high-density ionized gas is severely affecting the abundances determined from
CELs and, therefore, those from RLs should be considered as a better
approximation to the true abundances.Comment: 22 pages, 11 figures. Accepted for publication in MNRA
Rotation matrix from the mean dynamical equator and equinox at J2000.0 to the ICRS
Recommendation VII of Resolution A4 of the XXIst General Assembly of the International Astronomical Union (IAU [CITE]) states, in part, “... that the principal plane of the new conventional celestial reference system be as near as possible to the mean equator at J2000.0 and that the origin in this principal plane be as near as possible to the dynamical equinox of J2000.0, ..." The resulting International Celestial Reference System (ICRS), however, has a small, but significant, offset requiring a rotation matrix. The solutions for the offset between the mean dynamical pole of the Earth at J2000.0 and the pole of the ICRS determined by Lunar Laser Ranging (LLR) and Very Long Baseline Interferometry (VLBI) differ by several σ. Similarly, two different definitions have traditionally been used for the position of the mean equinox. Which of these poles and equinoxes should be used is application dependent. We have shown how the rotation matrix for the rotation from the mean dynamical equator and equinox at J2000.0 to the ICRS changes depending on the various assumptions made in constructing it.
Symposium on the Science of Time
The uses of time in astronomy - from pointing telescopes, coordinating and processing observations, predicting ephemerides, cultures, religious practices, history, businesses, determining Earth orientation, analyzing time-series data and in many other ways - represent a broad sample of how time is used throughout human society and in space. Time and its reciprocal, frequency, is the most accurately measurable quantity and often an important path to the frontiers of science. But the future of timekeeping is changing with the development of optical frequency standards and the resulting challenges of distributing time at ever higher precision, with the possibility of timescales based on pulsars, and with the inclusion of higher-order relativistic effects. The definition of the second will likely be changed before the end of this decade, and its realization will increase in accuracy; the definition of the day is no longer obvious. The variability of the Earth's rotation presents challenges of understanding and prediction. In this symposium speakers took a closer look at time in astronomy, other sciences, cultures, and business as a defining element of modern civilization. The symposium aimed to set the stage for future timekeeping standards, infrastructure, and engineering best practices for astronomers and the broader society. At the same time the program was cognizant of the rich history from Harrison's chronometer to today's atomic clocks and pulsar observations. The theoreticians and engineers of time were brought together with the educators and historians of science, enriching the understanding of time among both experts and the public
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