14 research outputs found
Venus transit 2004: Illustrating the capability of exoplanet transmission spectroscopy
The transit of Venus in 2004 offered the rare possibility to remotely sense a
well-known planetary atmosphere using ground-based observations for absorption
spectroscopy. Transmission spectra of Venus' atmosphere were obtained in the
near infrared using the Vacuum Tower Telescope (VTT) in Tenerife. Since the
instrument was designed to measure the very bright photosphere of the Sun,
extracting Venus' atmosphere was challenging. CO_2 absorption lines could be
identified in the upper Venus atmosphere. Moreover, the relative abundance of
the three most abundant CO_2 isotopologues could be determined. The
observations resolved Venus' limb, showing Doppler-shifted absorption lines
that are probably caused by high-altitude winds.
This paper illustrates the ability of ground-based measurements to examine
atmospheric constituents of a terrestrial planet atmosphere which might be
applied in future to terrestrial extrasolar planets.Comment: 7 pages, 5 figures, 1 tabl
A new concept for the combination of optical interferometers and high-resolution spectrographs
The combination of high spatial and spectral resolution in optical astronomy
enables new observational approaches to many open problems in stellar and
circumstellar astrophysics. However, constructing a high-resolution
spectrograph for an interferometer is a costly and time-intensive undertaking.
Our aim is to show that, by coupling existing high-resolution spectrographs to
existing interferometers, one could observe in the domain of high spectral and
spatial resolution, and avoid the construction of a new complex and expensive
instrument. We investigate in this article the different challenges which arise
from combining an interferometer with a high-resolution spectrograph. The
requirements for the different sub-systems are determined, with special
attention given to the problems of fringe tracking and dispersion. A concept
study for the combination of the VLTI (Very Large Telescope Interferometer)
with UVES (UV-Visual Echelle Spectrograph) is carried out, and several other
specific instrument pairings are discussed. We show that the proposed
combination of an interferometer with a high-resolution spectrograph is indeed
feasible with current technology, for a fraction of the cost of building a
whole new spectrograph. The impact on the existing instruments and their
ongoing programs would be minimal.Comment: 27 pages, 9 figures, Experimental Astronomy; v2: accepted versio
Optical and near-infrared spectrophotometric properties of Long Period Variables and other luminous red stars
Based on a new and large sample of optical and near-infrared spectra obtained
at the Mount Stromlo and Siding Spring Observatories (Lancon & Wood 1998;
Lancon & Wood, in preparation), spectrophotometric properties of cool oxygen-
and carbon-rich Long Period Variables and supergiants are presented.
Temperatures of oxygen-rich stars are assigned by comparison with synthetic
spectra computed from up-to-date oxygen-rich model atmosphere grids. The
existence of reliable optical and near-infrared temperature indicators is
investigated. A narrow relation between the bolometric correction BC(I) and the
broad-band colour I-J is obtained for oxygen-rich cool stars. The ability of
specific near-infrared indices to separate luminosity classes, atmospheric
chemistry or variability subtypes is discussed. Some comments are also given on
extinction effects, water band strengths in Long Period Variables and the
evaluation of 12CO/13CO ratio in red giants.Comment: 14 pages, 21 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/~ralvarez
Laser frequency comb techniques for precise astronomical spectroscopy
Precise astronomical spectroscopic analyses routinely assume that individual
pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons.
Intra-pixel sensitivity (IPS) variations may already cause small systematic
errors in, for example, studies of extra-solar planets via stellar radial
velocities and cosmological variability in fundamental constants via quasar
spectroscopy, but future experiments requiring velocity precisions approaching
~1 cm/s will be more strongly affected. Laser frequency combs have been shown
to provide highly precise wavelength calibration for astronomical
spectrographs, but here we show that they can also be used to measure IPS
variations in astronomical CCDs in situ. We successfully tested a laser
frequency comb system on the Ultra-High Resolution Facility spectrograph at the
Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded
in a single CCD exposure, we find that the average IPS deviates by <8 per cent
if it is assumed to vary symmetrically about the pixel centre. We also
demonstrate that series of comb exposures with absolutely known offsets between
them can yield tighter constraints on symmetric IPS variations from ~100
pixels. We discuss measurement of asymmetric IPS variations and absolute
wavelength calibration of astronomical spectrographs and CCDs using frequency
combs.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
The Elusive Active Nucleus of NGC 4945
We present new HST NICMOS observations of NGC 4945, a starburst galaxy
hosting a highly obscured active nucleus that is one of the brightest
extragalactic sources at 100 keV. The HST data are complemented with ground
based [FeII] line and mid--IR observations. A 100pc-scale starburst ring is
detected in Pa alpha, while H_2 traces the walls of a super bubble opened by
supernova-driven winds. The conically shaped cavity is particularly prominent
in Pa alpha equivalent width and in the Pa alpha/H_2 ratio. Continuum images
are heavily affected by dust extinction and the nucleus of the galaxy is
located in a highly reddened region with an elongated, disk-like morphology. No
manifestation of the active nucleus is found, neither a strong point source nor
dilution in CO stellar features, which are expected tracers of AGN activity.
Even if no AGN traces are detected in the near-IR, with the currently available
data it is still not possible to establish whether the bolometric luminosity of
the object is powered by the AGN or by the starburst: we demonstrate that the
two scenarios constitute equally viable alternatives. However, the absence of
any signature other than in the hard X-rays implies that, in both scenarios,
the AGN is non-standard: if it dominates, it must be obscured in all
directions, conversely, if the starburst dominates, the AGN must lack UV
photons with respect to X-rays. An important conclusion is that powerful AGNs
can be hidden even at mid-infrared wavelengths and, therefore, the nature of
luminous dusty galaxies cannot be always characterized by long-wavelength data
alone but must be complemented with sensitive hard X-ray observations.Comment: Accepted for publication in A&A, high quality color pictures
available at http://www.arcetri.astro.it/~marconi/colpic.htm
Dynamic model atmospheres of cool giants
Cool giant stars are highly dynamical objects, and complex micro-physical
processes play an important role in their extended atmospheres and winds. The
interpretation of observations, and in particular of high-resolution IR
spectra, requires realistic self-consistent model atmospheres. Current
dynamical models include rather detailed micro-physics, and the resulting
synthetic spectra compare reasonably well with observations. A transition from
qualitative to quantitative modelling is taking place at present. We give an
overview of existing dynamical model atmospheres for AGB stars, discussing
recent advances and current trends in modelling, including 3D 'star-in-a-box'
models. When comparing synthetic spectra and other observable properties
resulting from dynamical models with observations we focus on the near- and
mid-IR wavelength range.Comment: 12 pages, 3 figures, to be published in Proc. of ESO Workshop on High
Resolution Infrared Spectroscopy in Astronomy, eds. Kaeufl H.U., Siebenmorgen
R., Moorwood A., ESO Astrophysics Symposia, Springe
VISIR Upgrade Overview and Status
We present an overview of the VISIR upgrade project. VISIR is the mid-infrared imager and spectrograph at ESO’s VLT. The project team is comprised of ESO staff and members of the original VISIR consortium: CEA Saclay and ASTRON. The project plan is based on input from the ESO user community with the goal of enhancing the scientific performance and efficiency of VISIR by a combination of measures: installation of improved hardware, optimization of instrument operations and software support. The cornerstone of the upgrade is the 1k by 1k Si:As Aquarius detector array (Raytheon) which has demonstrated very good performance (sensitivity, stability) in the laboratory IR detector test facility (modified TIMMI 2 instrument). A prism spectroscopic mode will cover the N-band in a single observation. New scientific capabilities for high resolution and high-contrast imaging will be offered by sub-aperture mask (SAM) and phase-mask coronagraphic (4QPM/AGPM) modes. In order to make optimal use of favourable atmospheric conditions a water vapour monitor has been deployed on Paranal, allowing for real-time decisions and the introduction of a userdefined constraint on water vapour. Improved pipelines based on the ESO Reflex concept will provide better support to astronomers. The upgraded VISIR will be a powerful instrument providing background limited performance for diffraction-limited observations at an 8-m telescope. It will offer synergy with facilities such as ALMA, JWST, VLTI and SOFIA, while a wealth of targets is available from survey work (e.g. VISTA, WISE). In addition it will bring confirmation of the technical readiness and scientific value of several aspects of potential mid-IR instrumentation at Extremely Large Telescopes. The intervention on VISIR and installation of hardware has been completed in July and commissioning will take place during July and August. VISIR is scheduled to be available to the users starting Oct 2012
Hydrogen molecular ions: H3+, H5+ and beyond
Three decades after the spectroscopic detection of H3+ in space, the inspiring developments in physics, chemistry and astronomy of Hn+ (n = 3, 5, 7) systems, which led to this Royal Society Discussion Meeting, are reviewed, the present state of the art as represented by the meeting surveyed and future lines of research considered. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'