52 research outputs found
Laser frequency combs for astronomical observations
A direct measurement of the universe's expansion history could be made by
observing in real time the evolution of the cosmological redshift of distant
objects. However, this would require measurements of Doppler velocity drifts of
about 1 centimeter per second per year, and astronomical spectrographs have not
yet been calibrated to this tolerance. We demonstrate the first use of a laser
frequency comb for wavelength calibration of an astronomical telescope. Even
with a simple analysis, absolute calibration is achieved with an equivalent
Doppler precision of approximately 9 meters per second at about 1.5 micrometers
- beyond state-of-the-art accuracy. We show that tracking complex, time-varying
systematic effects in the spectrograph and detector system is a particular
advantage of laser frequency comb calibration. This technique promises an
effective means for modeling and removal of such systematic effects to the
accuracy required by future experiments to see direct evidence of the
universe's putative acceleration.Comment: Science, 5th September 2008. 18 pages, 7 figures (7 JPG files),
including Supporting Online Material. Version with higher resolution figures
available at http://astronomy.swin.edu.au/~mmurphy/pub.htm
A Frequency Comb calibrated Solar Atlas
The solar spectrum is a primary reference for the study of physical processes
in stars and their variation during activity cycles. In Nov 2010 an experiment
with a prototype of a Laser Frequency Comb (LFC) calibration system was
performed with the HARPS spectrograph of the 3.6m ESO telescope at La Silla
during which high signal-to-noise spectra of the Moon were obtained. We exploit
those Echelle spectra to study the optical integrated solar spectrum . The
DAOSPEC program is used to measure solar line positions through gaussian
fitting in an automatic way. We first apply the LFC solar spectrum to
characterize the CCDs of the HARPS spectrograph. The comparison of the LFC and
Th-Ar calibrated spectra reveals S-type distortions on each order along the
whole spectral range with an amplitude of +/-40 m/s. This confirms the pattern
found by Wilken et al. (2010) on a single order and extends the detection of
the distortions to the whole analyzed region revealing that the precise shape
varies with wavelength. A new data reduction is implemented to deal with CCD
pixel inequalities to obtain a wavelength corrected solar spectrum. By using
this spectrum we provide a new LFC calibrated solar atlas with 400 line
positions in the range of 476-530, and 175 lines in the 534-585 nm range. The
new LFC atlas improves the accuracy of individual lines by a significant factor
reaching a mean value of about 10 m/s. The LFC--based solar line wavelengths
are essentially free of major instrumental effects and provide a reference for
absolute solar line positions. We suggest that future LFC observations could be
used to trace small radial velocity changes of the whole solar photospheric
spectrum in connection with the solar cycle and for direct comparison with the
predicted line positions of 3D radiative hydrodynamical models of the solar
photosphere.Comment: Accept on the 15th of October 2013. 9 pages, 10 figures. ON-lINE data
A&A 201
High-precision wavelength calibration of astronomical spectrographs with laser frequency combs
We describe a possible new technique for precise wavelength calibration of
high-resolution astronomical spectrographs using femtosecond-pulsed mode-locked
lasers controlled by stable oscillators such as atomic clocks. Such `frequency
combs' provide a series of narrow modes which are uniformly spaced according to
the laser's pulse repetition rate and whose absolute frequencies are known a
priori with relative precision better than 10^{-12}. Simulations of frequency
comb spectra show that the photon-limited wavelength calibration precision
achievable with existing echelle spectrographs should be ~1 cm/s when
integrated over a 4000A range. Moreover, comb spectra may be used to accurately
characterise distortions of the wavelength scale introduced by the spectrograph
and detector system. The simulations show that frequency combs with pulse
repetition rates of 5-30GHz are required, given the typical resolving power of
existing and possible future echelle spectrographs. Achieving such high
repetition rates, together with the desire to produce all comb modes with
uniform intensity over the entire optical range, represent the only significant
challenges in the design of a practical system. Frequency comb systems may
remove wavelength calibration uncertainties from all practical spectroscopic
experiments, even those combining data from different telescopes over many
decades.Comment: 10 pages, 6 figures, 1 table. Accepted by MNRAS. v2: Fig. 3 augmented
and minor changes to text (including extended title
Elliptic and hyperelliptic magnetohydrodynamic equilibria
The present study is a continuation of a previous one on "hyperelliptic"
axisymmetric equilibria started in [Tasso and Throumoulopoulos, Phys. Plasmas
5, 2378 (1998)].
Specifically, some equilibria with incompressible flow nonaligned with the
magnetic field and restricted by appropriate side conditions like "isothermal"
magnetic surfaces, "isodynamicity" or P + B^2/2 constant on magnetic surfaces
are found to be reducible to elliptic integrals. The third class recovers
recent equilibria found in [Schief, Phys. Plasmas 10, 2677 (2003)]. In contrast
to field aligned flows, all solutions found here have nonzero toroidal magnetic
field on and elliptic surfaces near the magnetic axis.Comment: 9 page
Atmospheric Rossiter-McLaughlin effect and transmission spectroscopy of WASP-121b with ESPRESSO
WASP-121b is one of the most studied Ultra-hot Jupiters: many recent analyses
of its atmosphere report interesting features at different wavelength ranges.
In this paper we analyze one transit of WASP-121b acquired with the
high-resolution spectrograph ESPRESSO at VLT in 1-telescope mode, and one
partial transit taken during the commissioning of the instrument in 4-telescope
mode. We investigate the anomalous in-transit radial velocity curve and study
the transmission spectrum of the planet. By analysing the in-transit radial
velocities we were able to infer the presence of the atmospheric
Rossiter-McLaughlin effect. We measured the height of the planetary atmospheric
layer that correlates with the stellar mask (mainly Fe) to be 1.0520.015
Rp and we also confirmed the blueshift of the planetary atmosphere. By
examining the planetary absorption signal on the stellar cross-correlation
functions we confirmed the presence of a temporal variation of its blueshift
during transit, which could be investigated spectrum-by-spectrum. We detected
significant absorption in the transmission spectrum for Na, H, K, Li, CaII, Mg,
and we certified their planetary nature by using the 2D tomographic technique.
Particularly remarkable is the detection of Li, with a line contrast of
0.2% detected at the 6 level. With the cross-correlation
technique we confirmed the presence of FeI, FeII, CrI and VI. H and
CaII are present up to very high altitudes in the atmosphere (1.44 Rp and
2 Rp, respectively), and also extend beyond the transit-equivalent Roche
lobe radius of the planet. These layers of the atmosphere have a large line
broadening that is not compatible with being caused by the tidally-locked
rotation of the planet alone, and could arise from vertical winds or
high-altitude jets in the evaporating atmosphere.Comment: 15 pages, 19 figures. Accepted for publication in A&
The CODEX-ESPRESSO experiment: cosmic dynamics, fundamental physics, planets and much more..
CODEX, a high resolution, super-stable spectrograph to be fed by the E-ELT,
the most powerful telescope ever conceived, will for the first time provide the
possibility of directly measuring the change of the expansion rate of the
Universe with time and much more, from the variability of fundamental constants
to the search for other earths. A study for the implementation at the VLT of a
precursor of CODEX, dubbed ESPRESSO, is presently carried out by a
collaboration including ESO, IAC, INAF, IoA Cambridge and Observatoire de
Geneve. The present talk is focused on the cosmological aspects of the
experiment.Comment: 6 pages Latex, to appear in the proceedings of `A Century of
Cosmology', S. Servolo, August 2007, to be published in Il Nuovo Ciment
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