We measured maps of atmospheric water (H2O) and its deuterated form (HDO) across the martian globe, showing strong isotopic anomalies and a significant high deuterium/hydrogen (D/H) enrichment ...indicative of great water loss. The maps sample the evolution of sublimation from the north polar cap, revealing that the released water has a representative D/H value enriched by a factor of about 7 relative to Earth's ocean Vienna standard mean ocean water (VSMOW). Certain basins and orographic depressions show even higher enrichment, whereas high-altitude regions show much lower values (1 to 3 VSMOW). Our atmospheric maps indicate that water ice in the polar reservoirs is enriched in deuterium to at least 8 VSMOW, which would mean that early Mars (4.5 billion years ago) had a global equivalent water layer at least 137 meters deep.
Context. Pluto possesses a thin atmosphere, primarily composed of nitrogen, in which the detection of methane has been reported. Aims. The goal is to constrain essential but so far unknown parameters ...of Pluto's atmosphere, such as the surface pressure, lower atmosphere thermal stucture, and methane mixing ratio. Methods. We use high-resolution spectroscopic observations of gaseous methane and a novel analysis of occultation lightcurves. Results. We show that (i) Pluto's surface pressure is currently in the 6.5–24 μbar range, (ii) the methane mixing ratio is 0.5±0.1%, adequate to explain Pluto's inverted thermal structure and ~100 K upper atmosphere temperature, and (iii) a troposphere is not required by our data, but if present, it has a depth of at most 17 km, i.e. less than one pressure scale height; in this case methane is supersaturated in most of it. The atmospheric and bulk surface abundances of methane are strikingly similar, a possible consequence of a CH4-rich top surface layer.
Context.
The direct imaging of rocky exoplanets is one of the major science goals of upcoming large telescopes. The contrast requirement for imaging such planets is challenging. However, the mid-IR ...(infrared) regime provides the optimum contrast to directly detect the thermal signatures of exoplanets in our solar neighbourhood.
Aims.
We aim to exploit novel fast-chopping techniques newly developed for astronomy with the aid of adaptive optics to look for thermal signatures of exoplanets around bright stars in the solar neighbourhood.
Methods.
We used the upgraded Very Large Telescope Imager and Spectrometer for the mid-InfraRed (VISIR) instrument with high-contrast imaging capability optimised for observations at 10 μm to look for exoplanets around five nearby (
d
< 4 pc) stars. The instrument provides an improved signal-to-noise ratio (S/N) by a factor of ~4 in the
N
-band compared to standard VISIR for a given S/N and time.
Results.
In this work, we achieve a detection sensitivity of sub-mJy, which is sufficient to detect a few Jupiter mass planets in nearby systems. Although no detections are made, we achieve most sensitive limits within <2″ for all the observed targets compared to previous campaigns. For
ϵ
Indi A and
ϵ
Eri, we achieve detection limits very close to the giant planets discovered by RV, with the limits on
ϵ
Indi A being the most sensitive to date. Our non-detection therefore supports an older age for
ϵ
Indi A. The results presented here are promising for high-contrast imaging and exoplanet detections in the mid-IR regime.
Context. Multiple populations have been detected in several globular clusters (GC) that do not display a spread in metallicity. Unusual features of their observed colour–magnitude diagrams (CMD) can ...be interpreted in terms of differences in the helium content of the stars belonging to the sub-populations. Aims. Even if evidence gathered so far is compelling, differences in He abundance have never been directly observed. We attempt to measure these differences in two giant stars of NGC 2808 with very similar astrophysical parameters but different Na and O abundances, hence that presumably belong to different sub-populations, by directly comparing their He I 10 830 Å lines. Methods. The He 10 830 Å line forms in the upper chromosphere. Our detailed models derive the chromospheric structure using the Ca II and Hα chromospheric lines, and simulate the corresponding He I 10 830 line profiles as a function of the helium abundance. We show that, at a given value of He abundance, the He I 10 830 equivalent width cannot significantly change without a corresponding much larger change in the Ca II chromospheric lines. We have used the VLT-CRIRES to obtain high-resolution spectra in the 10 830 Å region, and the VLT-UVES to obtain spectra of the Ca II and Hα lines of our target stars. Results. The two target stars have very similar Ca II and Hα chromospheric lines, but different appearances in the He region. One line, blueshifted by 17 km s-1 with respect to the He 10 830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas the Na-poor star spectrum is consistent with a non-detection. From a detailed chromospheric modeling, we show that the difference in the spectra is consistent and most closely explained by an He abundance difference between the two stars of ΔY ≥ 0.17. Our optical observations bracket the infrared ones over a range of about 50 days and we do not observe any substantial variability in the Ca II and Hα lines. Conclusions. We provide direct evidence of a significant He line strength difference in giant stars of NGC 2808 belonging to different sub-populations, which had been previously detected by other photometric and spectroscopic means. The use of appropriate model chromospheres allows us for the first time to provide an approximate quantitative estimate of this difference, which is clearly consistent with the expected difference in abundance required by the stellar evolution theory to account for the observed peculiarities of this cluster’s cmD.
Context. Technetium (Tc) is an unstable element synthesised via the s-process in deep layers of asymptotic giant branch (AGB) stars, making it a reliable indicator of both recent s-process activity ...and third dredge-up. Aims. We searched for Tc in a sample of bright oxygen-rich asymptotic giant branch (AGB) stars located in the outer galactic bulge. We test theoretical predictions of the luminosity limit for the onset of third dredge-up. Methods. Using high-resolution optical spectra obtained with the UVES spectrograph at ESO's VLT, we searched 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 the near-infrared photometry of the objects. Results. 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 agrees 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, that contains the Tc-rich stars identified here.
We measured organic volatiles (CH sub(4), CH sub(3)OH, C sub(2)H sub(6), H sub(2)CO), CO, and water in comet 8P/Tuttle, a comet from the Oort Cloud reservoir now in a short-period Halley-type orbit. ...We compare its composition with two other comets in Halley-type orbits, and with comets of the "organics-normal" and "organlcs-depleted" classes. Chemical gradients are expected in the comet-forming region of the protoplanetary disk, and an individual comet should reflect its specific heritage. If Halley-type comets came from the inner Oort Cloud as proposed, we see no common characteristics that could distinguish such comets from those that were stored in the outer Oort Cloud.
Context. Triton possesses a thin atmosphere, primarily composed of nitrogen, sustained by the sublimation of surface ices. Aims. We aim at determining the composition of Triton's atmosphere to ...constrain the nature of surface-atmosphere interactions. Methods. We perform high-resolution spectroscopic observations in the 2.32–2.37 μm range, using CRIRES at the VLT. Results. 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 CH4 partial pressure is several times higher than inferred by Voyager. This confirms that Triton's atmosphere is seasonally variable and is best interpreted by the warming of CH4-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.
ABSTRACT We have carried out two sets of observations to quantify the properties of SiO gas in the unusual HD 172555 debris disk: (1) a search for the J = 8-7 rotational transition from the ...vibrational ground state, carried out with the Atacama Pathfinder EXperiment (APEX) submillimeter telescope and heterodyne receiver at 863 m and (2) a search at 8.3 m for the P(17) ro-vibrational transition of gas phase SiO, carried out with the Very Large Telescope (VLT)/VISIR with a resolution, λ/Δλ, of 30,000. The APEX measurement resulted in a 3.3 detection of an interstellar feature, but only an upper limit to emission at the radial velocity and line width expected from HD 172555. The VLT/VISIR result was also an upper limit. These were used to provide limits for the abundance of gas phase SiO for a range of temperatures. The upper limit from our APEX detection, assuming an 8000 K primary star photospheric excitation, falls more than an order of magnitude below the self-shielding stability threshold derived by Johnson et al. (2012). Our results thus favor a solid-state origin for the 8.3 m feature seen in the Spitzer IRS spectrum of the circumstellar excess emission and the production of circumstellar O i and Si i by SiO UV photolysis. The implications of these estimates are explored in the framework of models of the HD 172555 circumstellar disk.
Context.
T Tauri has long been the prototypical young pre-main-sequence star. However, with increasing resolution and sensitivity, T Tauri has now been decomposed into a triple system with a complex ...disk and outflow geometry.
Aims.
We aim to measure the brightness of all three components of the T Tauri system (T Tau N, T Tau Sa, and T Tau Sb) in the mid-infrared in order to obtain photometry around the ∼9.7
μ
m silicate feature. This allows us to study their variability and to investigate the distribution of dust and the geometry of circumstellar and circumbinary disks in this complex system.
Methods.
We observe the T Tauri system with the Very Large Telescope (VLT)/VISIR-NEAR instrument, performing diffraction-limited imaging in the mid-infrared. With kernel phase interferometry post-processing of the data, and using the astrometric positions of all three components from VLT/SPHERE, we measure the three components’ individual brightnesses (including the southern binary at an angular separation down to ∼0.2
λ
/
D
) and obtain their photometry. In order to validate our methods, we simulate and recover mock data of the T Tauri system using the observed reference point-spread function of HD 27639.
Results.
We find that T Tau N is rather stable and shows weak silicate emission, while T Tau Sa is highly variable and shows prominent silicate absorption. T Tau Sb became significantly fainter compared to data from 2004 and 2006, suggesting increased extinction by dust. The precision of our photometry is limited by systematic errors in kernel phase interferometry, which is consistent with previous studies using this technique.
Conclusions.
Our results confirm the complex scenario of misaligned disks in the T Tauri system that had been observed previously, and they are in agreement with the recently observed dimming of T Tau Sb in the near-infrared. Our mid-infrared photometry supports the interpretation that T Tau Sb has moved behind the dense region of the Sa-Sb circumbinary disk on its tight orbit around Sa, therefore suffering increased extinction.
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