93 research outputs found
Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere
High-resolution spectra of Pluto in the 1.66 um region, recorded with the
VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to
constrain the spatial and vertical distribution of methane in Pluto's
atmosphere and to search for mid-term (4 year) variability. A sensitivity study
to model assumptions (temperature structure, surface pressure, Pluto's radius)
is performed. Results indicate that (i) no variation of the CH4 atmospheric
content (column-density or mixing ratio) with Pluto rotational phase is present
in excess of 20 % (ii) CH4 column densities show at most marginal variations
between 2008 and 2012, with a best guess estimate of a ~20 % decrease over this
time frame. As stellar occultations indicate that Pluto's surface pressure has
continued to increase over this period, this implies a concomitant decrease of
the methane mixing ratio (iii) the data do not show evidence for an
altitude-varying methane distribution; in particular, they imply a roughly
uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high
concentrations of low-temperature methane near the surface can be ruled out.
Our results are also best consistent with a relatively large (> 1180 km) Pluto
radius. Comparison with predictions from a recently developed global climate
model GCM indicates that these features are best explained if the source of
methane occurs in regional-scale CH4 ice deposits, including both low latitudes
and high Northern latitudes, evidence for which is present from the rotational
and secular evolution of the near-IR features due to CH4 ice. Our "best guess"
predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17
ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible
longitudinal variations.Comment: 21 pages, 6 figure
Detection of CO in Triton's atmosphere and the nature of surface-atmosphere interactions
Triton possesses a thin atmosphere, primarily composed of nitrogen, sustained
by the sublimation of surface ices. The goal is to determine the composition of
Triton's atmosphere and to constrain the nature of surface-atmosphere
interactions. We perform high-resolution spectroscopic observations in the
2.32-2.37 m range, using CRIRES at the VLT. From this first spectroscopic
detection of Triton's atmosphere in the infrared, we report (i) the first
observation of gaseous methane since its discovery in the ultraviolet by
Voyager in 1989 and (ii) the first ever detection of gaseous CO in the
satellite. The CO atmospheric abundance is remarkably similar to its surface
abundance, and appears to be controlled by a thin, CO-enriched, surface veneer
resulting from seasonal transport and/or atmospheric escape. The CH partial
pressure is several times larger than inferred from Voyager. This confirms that
Triton's atmosphere is seasonally variable and is best interpreted by the
warming of CH-rich icy grains as Triton passed southern summer solstice in
2000. The presence of CO in Triton's atmosphere also affects its temperature,
photochemistry and ionospheric composition. An improved upper limit on CO in
Pluto's atmosphere is also reported.Comment: 11 pages, including 4 figures and 2 on-line figures Astronomy and
Astrophysics, in press (accepted March 13, 2010
Mid-Infrared Instrumentation for the European Extremely Large Telescope
MIDIR is the proposed thermal/mid-IR imager and spectrograph for the European
Extremely Large Telescope (E-ELT). It will cover the wavelength range of 3 to
at least 20 microns. Designed for diffraction-limited performance over the
entire wavelength range, MIDIR will require an adaptive optics system; a
cryogenically cooled system could offer optimal performance in the IR, and this
is a critical aspect of the instrument design. We present here an overview of
the project, including a discussion of MIDIR's science goals and a comparison
with other infrared (IR) facilities planned in the next decade; top level
requirements derived from these goals are outlined. We describe the optical and
mechanical design work carried out in the context of a conceptual design study,
and discuss some important issues to emerge from this work, related to the
design, operation and calibration of the instrument. The impact of telescope
optical design choices on the requirements for the MIDIR instrument is
demonstrated.Comment: for publication in SPIE Proceedings vol. 6692, Cryogenic Optical
Systems and Instrumentation XII, eds. J.B. Heaney and L.G. Burriesci, San
Diego, Aug 200
Synthesising, using, and correcting for telluric features in high-resolution astronomical spectra
We present a technique to synthesise telluric absorption and emission
features both for in-situ wavelength calibration and for their removal from
astronomical spectra. While the presented technique is applicable for a wide
variety of optical and infrared spectra, we concentrate in this paper on
selected high-resolution near-infrared spectra obtained with the CRIRES
spectrograph to demonstrate its performance and limitation. We find that
synthetic spectra reproduce telluric absorption features to about 2%, even
close to saturated line cores. Thus, synthetic telluric spectra could be used
to replace the observation of telluric standard stars, saving valuable
observing time. This technique also provides a precise in-situ wavelength
calibration, especially useful for high-resolution near-infrared spectra in the
absence of other calibration sources.Comment: 11 pages, 11 figures, accepted for publication in A&A (updated
version
Spectral type dependent rotational braking and strong magnetic flux in three components of the late-M multiple system LHS 1070
We show individual high resolution spectra of components A, B, and C of the
nearby late-M type multiple system LHS 1070. Component A is a mid-M star, B and
C are known to have masses at the threshold to brown dwarfs. From our spectra
we measure rotation velocities and the mean magnetic field for all three
components individually. We find magnetic flux on the order of several
kilo-Gauss in all components. The rotation velocities of the two late-M objects
B and C are similar (vsini = 16km/s), the earlier A component is spinning only
at about half that rate. This suggests weakening of net rotational braking at
late-M spectral type, and that the lack of slowly rotating late-M and L dwarfs
is real. Furthermore, we found that magnetic flux in the B component is about
twice as strong as in component C at similar rotation rate. This indicates that
rotational braking is not proportional to magnetic field strength in fully
convective objects, and that a different field topology is the reason for the
weak braking in low mass objects.Comment: accepted for publication as A&A Lette
Improved orbital solution and masses for the very low-mass multiple system LHS 1070
We present a refined orbital solution for the components A, B, and C of the
nearby late-M type multiple system LHS 1070. By combining astrometric
datapoints from NACO/VLT, CIAO/SUBARU, and PUEO/CFHT, as well as a radial
velocity measurement from the newly commissioned near infrared high-resolution
spectrograph CRIRES/VLT, we achieve a very precise orbital solution for the B
and C components and a first realistic constraint on the much longer orbit of
the A-BC system. Both orbits appear to be co-planar. Masses for the B and C
components calculated from the new orbital solution (M_(B+C) = 0.157 +/- 0.009
M_sun) are in excellent agreement with theoretical models, but do not match
empirical mass-luminosity tracks. The preliminary orbit of the A-BC system
reveals no mass excess for the A component, giving no indication for a
previously proposed fourth (D) component in LHS 1070.Comment: published in A&A, 2008, 484, 429; added CFHT acknowledgemen
Technetium and the third dredge up in AGB stars II. Bulge stars
We searched for Technetium (Tc) in a sample of bright oxygen-rich asymptotic giant branch (AGB) stars located in the outer galactic bulge. Tc is an unstable element synthesised via the s-process in deep layers of AGB stars, thus it is a reliable indicator of both recent s-process activity and third dredge-up. We aim to test theoretical predictions on the luminosity limit for the onset of third dredge-up. Using high resolution optical spectra obtained with the UVES spectrograph at ESO's VLT we search for resonance lines of neutral Tc in the blue spectral region of our sample stars. These measurements allow us to improve the procedure of classification of stars with respect to their Tc content by using flux ratios. Synthetic spectra based on MARCS atmospheric models are presented and compared to the observed spectra around three lines of Tc. Bolometric magnitudes are calculated based on near infrared photometry of the objects. Among the sample of 27 long period bulge variables four were found to definitely contain Tc in their atmospheres. The luminosity of the Tc rich stars is in agreement with predictions from AGB evolutionary models on the minimum luminosity at the time when third dredge-up sets in. However, AGB evolutionary models and a bulge consisting of a single old population cannot be brought into agreement. This probably means that a younger population is present in the bulge, as suggested by various authors, which contains the Tc-rich stars here identified
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