UVES Spectrograph Research Articles

Toward detection of terrestrial planets in the habitable zone of our closest neighbor: proxima Centauri

Context. The precision of radial velocity (RV) measurements to detect indirectly planetary companions of nearby stars has improved to enable the discovery of extrasolar planets in the Neptune and Super-Earth mass range. Detections of extremely low mass planets, even as small as 1 Earth mass or below, in short-period orbits now appears conceivable in ongoing RV planet searches. Discoveries of these Earth-like planets by means of ground-based RV programs will help to determine the parameter η⊕, the frequency of potentially habitable planets around other stars. Aims. In search of low-mass planetary companions we monitored Proxima Centauri (M5V) as part of our M dwarf program. In the absence of a significant detection, we use these data to demonstrate the general capability of the RV method in finding terrestrial planets. For late M dwarfs the classic liquid surface water habitable zone (HZ) is located close to the star, in which circumstances the RV method is most effective. We want to demonstrate that late M dwarfs are ideal targets for the search of terrestrial planets with the RV technique. Methods. Using the iodine cell technique we obtained differential RV measurements of Proxima Cen over a time span of 7 years with the UVES spectrograph at the ESO VLT. We determine upper limits to the masses of companions in circular orbits by means of numerical simulations. Results. The RV data of Proxima Cen have a total rms scatter of 3. 1ms −1 and a period search does not reveal any significant signals. In contrast to our earlier results for Barnard’s star, the RV results for the active M dwarf Proxima Cen are only weakly correlated with Hα line index measurements. As a result of our companion limit calculations, we find that we successfully recover all test signals with RV amplitudes corresponding to planets with m sini ≥ 2–3 M⊕ residing inside the HZ of Proxima Cen with a statistical significance of >99%. Over the same period range, we can recover 50% of the test planets with masses of m sini ≥ 1.5–2.5 M⊕. Based on our simulations, we exclude the presence of any planet in a circular orbit with m sini ≥ 1 MNeptune at separations of a ≤ 1A U.

Nitrogen abundance in turn-off stars of NGC 6397 and NGC 6752

Aims. Our goal is to measure the nitrogen abundance in 5 turn off (TO) stars of the two globular clusters NGC 6397 and NGC 6752, and to compare the cluster abundances with those of field stars of comparable metallicity. In addition, we investigate the variations in nitrogen abundance and its connections to variations in the abundances of other light elements. Methods. We determine the nitrogen abundance from the compact band-head of the (0-0) vibrational band of the NH A 3 Π − X 3 Σ − system at 3360 A, using spectra of resolution R ∼ 45 000 acquired using the UVES spectrograph on the ESO Kueyen-VLT 8.2 m telescope, analysed with spectrum synthesis based on plane parallel LTE model atmospheres. We apply a method previously applied to field stars, to enable a direct comparison to be made between the results. Results. The nitrogen abundances of two stars in NGC 6397 are found to be almost identical, even though there is a difference of one order of magnitude in their oxygen abundances. In a third star of the same cluster, the nitrogen abundance is slighty lower, although consistent, within the calculated margins of error, with the other two measurements. All stars in NGC 6397 are N-rich with respect to field objects of similar metallicity. The two stars in NGC 6752 show a difference in nitrogen abundance of over one order of magnitude. The same stars differ in the abundances of other elements such as Na, O, and Li, although only by a factor ∼3–4. Conclusions. NGC 6397 and NGC 6752 are, at present, the only two globular clusters in which N has been measured in TO stars in a way consistent with similar measurements for field stars. The behaviour of N is different in the two clusters: no variation is observed between stars in NGC 6397, while a large variation is observed in NGC 6752. This is consistent with a picture in which the stars in NGC 6752 have been formed by a mixture of “pristine” material and material which has been processed by an early generation of stars, referred to as “polluters”. The N abundances reported will help to constrain the properties of the polluters. In the case of NGC 6397, a simple pollution history is probably not viable, since the observed variations in O abundances are not accompanied by corresponding variations in N or Li.

A transiting planet among 23 new near-threshold candidates from the OGLE survey – OGLE-TR-182

By re-processing the data of the second season of the OGLE survey for planetary transits and adding new mesurements on the same fields gathered in subsequent years with the OGLE telescope, we have identified 23 new transit candidates, recorded as OGLE-TR-178 to OGLE-TR-200. We studied the nature of these objects with the FLAMES/UVES multi-fiber spectrograph on the VLT. One of the candidates, OGLE-TR-182, was confirmed as a transiting gas giant planet on a 4-day orbit. We characterised it with further observations using the FORS1 camera and UVES spectrograph on the VLT. OGLE-TR-182b is a typical “hot Jupiter” with an orbital period of 3.98 days, a mass of and a radius of 1.13. Confirming this transiting planet required a large investment in telescope time with the best instruments available, and we comment on the difficulty of the confirmation process for transiting planets in the OGLE survey. We delineate the zone were confirmation is difficult or impossible, and discuss the implications for the CoRoT space mission in its quest for transiting telluric planets.

Studying hot gas using the ultraviolet: High ions in galactic halos

Among the many spectral lines falling in the ultraviolet are the resonance doublets of the highly ionized species O VI, N V, C IV, and Si IV. By using both space-based UV observations and ground-based optical observations, these lines can be used to trace diffuse astrophysical plasma over a large fraction of the age of the Universe. In this paper we discuss observations of these absorption lines in damped Lyman-α systems at z≈2–3, using the UVES spectrograph on the VLT in Chile.

Isotopic anomaly and stratification of Ca in magnetic Ap stars

We have carried out an accurate investigation of the Ca isotopic composition and stratification in the atmospheres of 23 magnetic chemically peculiar (Ap) stars of different temperature and magnetic field strength. With the UVES spectrograph at the 8m ESO VLT, we have obtained high-resolution spectra of Ap stars in the wavelength range 3000-10000 A. Using a detailed spectrum synthesis calculations, we have reproduced a variety of Ca lines in the optical and ultraviolet spectral regions, inferring the overall vertical distribution of Ca abundance, then we have deduced the relative isotopic composition and its dependence on height using the profile of the the IR-triplet Ca II line at lambda 8498 A. In 22 out of 23 studied stars, we found that Ca is strongly stratified, being usually overabundant by 1.0-1.5 dex below log tau_5000 ~ -1, and strongly depleted above log tau_5000 = -1.5. The IR-triplet Ca II line at lambda 8498 A reveals a significant contribution of the heavy isotopes 46Ca and 48Ca, which represent less than 1% of the terrestrial Ca isotopic mixture. We confirm our previous finding that the presence of heavy Ca isotopes is generally anticorrelated with the magnetic field strength. Moreover, we discover that in Ap stars with relatively small surface magnetic fields (<=4-5 kG), the light isotope 40Ca is concentrated close to the photosphere, while the heavy isotopes are dominant in the outer atmospheric layers. This vertical isotopic separation, observed for the first time for any metal in a stellar atmosphere, disappears in stars with magnetic field strength above 6-7 kG. We suggest that the overall Ca stratification and depth-dependent isotopic anomaly observed in Ap stars may be attributed to a combined action of the radiatively-driven diffusion and light-induced drift.

First Stellar Abundances in the Dwarf Irregular Galaxy IC 1613

Chemical abundances in three M supergiants in the Local Group dwarf irregular galaxy IC 1613 have been determined using high-resolution spectra obtained with the UVES spectrograph on the ESO 8.2 m Kueyen telescope. A detailed synthetic-spectrum analysis has been used to determine the atmospheric parameters and abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Fe, Co, Ni, La, and Eu. We find the overall metallicity of the stars to be [Fe/H] = -0.67 ± 0.09 and the age 9-13 Myr, which is in excellent agreement with the present-day values in the age-metallicity relationship model of IC 1613 by Skillman et al. We have found that the three supergiants investigated have a mean [α/Fe] equal to about -0.1, which is lower than seen in Galactic stars at the same metallicity and is in agreement with the results obtained in other dwarf irregular galaxies. The oxygen abundances are in agreement with the upper values of the nebular oxygen determinations in IC 1613. The abundance ratios of s- and r-process elements to iron are enhanced relative to solar by about 0.3 dex. The abundance pattern of the elements studied is similar to that of the Small Magellanic Cloud, except for Co and Ni, which are underabundant in the SMC. The observed elemental abundances are generally in very good agreement with the recent chemical evolution model of Yuk and Lee.

Variations in the lithium abundances of turn off stars in the globular cluster 47 Tucanae

Aims. Our aim is to determine Li abundances in turn off stars of the globular cluster 47 Tuc and test theories about Li variations among turn off stars. Methods. We make use of high resolution (R ∼ 43 000), high signal-to-noise ratio (S /N = 50–70) spectra of 4 turn off (TO) stars obtained with the UVES spectrograph at the 8.2 m VLT Kueyen telescope. Results. The four stars observed span the range 1.6 < A(Li) < 2.14, providing a mean A(Li) = 1.84 with a standard deviation of 0.25 dex. When coupled with data of other two TO stars of the cluster, available in the literature, the full range in Li abundances observed in this cluster is 1.6 < A(Li) < 2.3. The variation in A(Li) is at least 0.6 dex (0.7 dex considering also the data available in the literature) and the scatter is six times larger than what is expected from the observational error. We claim that these variations are real. A(Li) seems to be anti-correlated with A(Na) exactly as observed in NGC 6752. No systematic error in our analysis could produce such an anti-correlation. Conclusions. Na production through p captures on 22 Ne at temperatures in excess of 3 × 10 7 K and the contemporary Li destruction could result in this anti-correlation. However, such nuclear processing cannot have taken place in the stars themselves, which do not reach such high temperatures, even at their centre. This points towards the processing in a previous generation of stars. The low N/O ratios in the observed stars and the apparent lack of correlation between N an Li abundances, place a strong constraint on the properties of this previous generation. Our results indicate a different behaviour among the globular clusters so far studied, as far as the abundance patterns are concerned.

Synthesis of nanocrystalline CdS thin films in PVA matrix

Nanocrystalline thin films of CdS are deposited on glass substrates by chemical bath deposition technique using polyvinyl alcohol (PVA) matrix solution. Crystallite sizes of the nanocrystalline films are determined from broadening of X-ray diffraction lines and are found to vary from 5·4–10·2 nm. The band gap of the nanocrystalline material is determined from the UV spectrograph. The absorption edge is shifted towards the lower wave length side (i.e. blue shift) and are found to be within the range from 2·48–2·8 eV as grain sizes decrease from 10·2–5·4 nm. This is also supported by the spectral response curves. An increase of molarity decreases the grain size which in turn increases the band gap.

Effects of Low Energetic Neutral Atoms on Martian and Venusian Dayside Exospheric Temperature Estimations

The heating of the Upper atmospheres and the formation of the ionospheres on Venus and Mars are mainly controlled by the solar X-ray and extreme ultraviolet (EUV) radiation (λ = 0.1 − 102.7 nm and can be characterized by the 10.7 cm solar radio flux). Previous estimations of the average Martian dayside exospheric temperature inferred from topside plasma scale heights, UV airglow and Lyman-a dayglow observations of up to ∼500 K imply a stronger dependence on solar activity than that found on Venus by the Pioneer Venus Orbiter (PVO) and Magellan spacecraft. However, this dependence appears to be inconsistent with exospheric temperatures (<250 K) inferred from aerobraking maneuvers of recent spacecraft like Mars Pathfinder, Mars Global Surveyor and Mars Odyssey during different solar activity periods and at different orbital locations of the planet. In a similar way, early Lyman-α dayglow and UV airglow observations by Venera 4, Mariner 5 and 10, and Venera 9–12 at Venus also suggested much higher exospheric temperatures of up to 1000 K as compared with the average dayside exospheric temperature of about 270 K inferred from neutral gas mass spectrometry data obtained by PVO. In order to compare Venus and Mars, we estimated the dayside exobase temperature of Venus by using electron density profiles obtained from the PVO radio science experiment during the solar cycle and found the Venusian temperature to vary between 250–300 K, being in reasonable agreement with the exospheric temperatures inferred from Magellan aerobraking data and PVO mass spectrometer measurements. The same method has been applied to Mars by studying the solar cycle Variation of the ionospheric peak plasma density observed by Mars Global Surveyor during both solar minimum and maximum conditions, yielding a temperature range between 190–220 K. This result clearly indicates that the average Martian dayside temperature at the exobase does not exceed a value of about 240 K during high solar activity conditions and that the response of the Upper atmosphere temperature on Mars to solar activity near the ionization maximum is essentially the same as on Venus. The reason for this discrepancy between exospheric temperature determinations from topside plasma scale heights and electron distributions near the ionospheric maximum seems to lie in the fact that thermal and photochemical equilibrium applies only at altitudes below 170 km, whereas topside scale heights are derived for much higher altitudes where they are modified by transport processes and where local thermodynamic equilibrium (LTE) conditions are violated. Moreover, from simulating the energy density distribution of photochemically produced moderately energetic H, C and O atoms, as well as CO molecules, we argue that exospheric temperatures inferred from Lyman-α dayglow and UV airglow observations result in too high values, because these particles, as well as energetic neutral atoms, transformed from solar wind protons into hydrogen atoms via Charge exchange, may contribute to the observed planetary hot neutral gas coronae. Because the low exospheric temperatures inferred from neutral gas mass spectrometer and aerobraking data, as well as from CO 2 + UV doublet emissions near 180–260 nm obtained from the Mars Express SPICAM UV spectrograph suggest rather low heating efficiencies, some hitherto unidentified additional IR-cooling mechanism in the thermospheres of both Venus and Mars is likely to exist.

MCP-based dual band far UV spectrograph with single channel readouts for space use

A compact far ultraviolet (FUV) spectrograph has been developed and applied to space observation on a micro-satellite. The dual channel imaging spectrograph utilized two micro-channel plate (MCP) detectors with a single crossed delay line (XDL) anode to record photon arrival events. The unconventional anode design allows for the use of a single set of position encoding electronics for both detector fields, thereby reducing the size, weight, and power of the associated electronics. The ground and on-orbit performance tests verified the successful application of the system for astrophysical observations. In this note, we report the design, the development, and the test results of the system, focusing on the XDL anode system.

Na-O anticorrelation and horizontal branches

We present the LTE abundance analysis of high resolution spectra for red giant stars in the peculiar bulge globular cluster NGC 6388. Spectra of seven members were taken using the UVES spectrograph at the ESO VLT2 and the multiobject FLAMES facility. We exclude any intrinsic metallicity spread in this cluster: on average, [Fe/H] = -0.44 ±0.01 ± 0.03 dex on the scale of the present series of papers, where the first error bar refers to individual star-to-star errors and the second is systematic, relative to the cluster. Elements involved in H-buming at high temperatures show large spreads, exceeding the estimated errors in the analysis. In particular, the pairs Na and O. Al and Mg are anticorrelated and Na and Al are correlated among the giants in NGC 6388, the typical pattern observed in all galactic globular clusters studied so far. Stars in NGC 6388 shows an excess of α-process elements, similar to the one found in the twin bulge cluster NGC 6441. Mn is found underabundant in NGC 6388, in agreement with the average abundance ratio shown by clusters of any metallicity. Abundances of neutron-capture elements are homogeneously distributed within NGC 6388; the [Eu/Fe] ratio stands above the value found in field stars of similar metallicity.

Beryllium abundance in turn-off stars of NGC 6752

Aims: To measure the beryllium abundance in two TO stars of the Globular Cluster NGC 6752, one oxygen rich and sodium poor, the other presumably oxygen poor and sodium rich. Be abundances in these stars are used to put on firmer grounds the hypothesis of Be as cosmochronometer and to investigate the formation of Globular Clusters. Method:We present near UV spectra with resolution R$\sim 45000$ obtained with the UVES spectrograph on the 8.2m VLT Kueyen telescope, analysed with spectrum synthesis based on plane parallel LTE model atmospheres. Results:Be is detected in the O rich star with log(Be/H)=-12.04 $\pm$0.15, while Be is not detected in the other star for which we obtain the upper limit log(Be/H)$<$-12.2. A large difference in nitrogen abundance (1.6 dex) is found between the two stars. Conclusions:The Be measurement is compatible with what found in field stars with the same [Fe/H] and [O/H]. The 'Be age' of the cluster is found to be 13.3 Gyrs, in excellent agreement with the results from main sequence fitting and stellar evolution. The presence of Be confirms the results previously obtained for the cluster NGC 6397 and supports the hypothesis that Be can be used as a clock for the early formation of the Galaxy. Since only an upper limit is found for the star with low oxygen abundance, we cannot decide between competing scenarios of Globular Cluster formation, but we can exclude that 'polluted' stars are substantially younger than 'unpolluted' ones. We stress that the Be test might be the only measurement capable of distinguishing between these scenarios.

On Martian nitrogen dayglow emission observed by SPICAM UV spectrograph/Mars Express

SPICAM UV spectrograph on board Mars Express has reported, for the first time, the identification of the N2 VK (0,5) and (0,6) band emissions in the Martian dayglow. The derived exospheric temperature of the N2 species in the upper Martian atmosphere is equal to 190 ± 51 K for areocentric longitudes between 100° and 171° and equal to 257 ± 71 K for areocentric longitudes between 287° and 321°. These temperatures are in agreement with the ones deduced from the analysis of the 289 nm emission associated with the CO2 major Martian atmospheric species. The intensity of the N2 VK (0,6) band is found to be ∼3 times smaller than the calculated intensity of Fox and Dalgarno (1979) which is reasonable when considering the uncertainties in the parameters used by Fox and Dalgarno (1979) and the uncertainties on SPICAM UVS observations.

Mn, Cu, and Zn abundances in metal-rich globular clusters

Metal-rich globular clusters are important to study the chemical evolution of the Galaxy and one of the best approaches to better understand the physical processes occurring in stellar populations as building blocks of galaxies. We report on Mn, Cu, and Zn abundances of giant stars in metal-rich Galactic globular clusters (−0.70 < [Fe/H] < −0.11), measured on spectra obtained with the UVES spectrograph on the 8-m VLT UT2-Kueyen telescope.

Alice: The rosetta Ultraviolet Imaging Spectrograph

We describe the design, performance and scientific objectives of the NASA-funded ALICE instrument aboard the ESA Rosetta asteroid flyby/comet rendezvous mission. ALICE is a lightweight, low-power, and low-cost imaging spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy. It will be the first UV spectrograph to study a comet at close range. It is designed to obtain spatially-resolved spectra of Rosetta mission targets in the 700–2050 A spectral band with a spectral resolution between 8 A and 12 A for extended sources that fill its ∼0.05^ × 6.0^ field-of-view. ALICE employs an off-axis telescope feeding a 0.15-m normal incidence Rowland circle spectrograph with a toroidal concave holographic reflection grating. The microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr and CsI) and employs a two-dimensional delay-line readout array. The instrument is controlled by an internal microprocessor. During the prime Rosetta mission, ALICE will characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus, and nucleus/coma coupling; during cruise to the comet, ALICE will make observations of the mission's two asteroid flyby targets and of Mars, its moons, and of Earth's moon. ALICE has already successfully completed the in-flight commissioning phase and is operating well in flight. It has been characterized in flight with stellar flux calibrations, observations of the Moon during the first Earth fly-by, and observations of comet C/2002 T7 (LINEAR) in 2004 and comet 9P/Tempel 1 during the 2005 Deep Impact comet-collision observing campaign.

Dynamical masses of two young globular clusters in the blue compact galaxy ESO 338–IG04

We present high-resolution echelle spectroscopy, obtained with the UVES spectrograph on ESO/VLT, of two luminous star clusters in the metal-poor blue compact galaxy ESO 338-IG04 at a distance of 37 ...

On the age of stars harboring transiting planets

Results of photometric surveys have brought to light the existence of a population of giant planets orbiting their host stars even closer than the hot Jupiters (HJ), with orbital periods below 3 days. The reason why radial velocity surveys were not able to detect these very-hot Jupiters (VHJ) is under discussion. A possible explanation is that these close-in planets are short-lived, being evaporated on short time-scales due to UV flux of their host stars. In this case, stars hosting transiting VHJ planets would be systematically younger than those in the radial velocity sample. We have used the UVES spectrograph (VLT-UT2 telescope) to obtain high resolution spectra of 5 faint stars hosting transiting planets, namely, OGLE-TR-10, 56, 111, 113 and TrES-1. Previously obtained CORALIE spectra of HD189733, and published data on the other transiting planet-hosts were also used. The immediate objective is to estimate ages via Li abundances, using the Ca II activity-age relation, and from the analysis of the stellar rotational velocity. For the stars for which we have spectra, Li abundances were computed as in Israelian et al. (2004) using the stellar parameters derived in Santos et al. (2006). The chromospheric activity index $S_{US}$ was built as the ratio of the flux within the core of the Ca II H & K lines and the flux in two nearby continuum regions. The index $S_{US}$ was calibrated to Mount Wilson index $S_{MW}$ allowing the computation of the Ca II H & K corrected for the photospheric contribution. These values were then used to derive the ages by means of the Henry et al. (1996) activity-age relation. Bearing in mind the limitations of the ages derived by Li abundances, chromospheric activity, and stellar rotational velocities, none of the stars studied in this paper seem to be younger than 0.5 Gyr.

Origins of the Martian aurora observed by Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) on board Mars Express

On the 11 August 2004, the UV spectrograph Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) on board Mars Express made the first observation of auroral‐type emission on the Martian nightside. In this paper, we describe the results of a new analysis of the observed emission owing to a better calibration of SPICAM UV channel and the use of all spectral information obtained during this observation. Several possibilities for the origin of this emission are discussed. We discussed, in particular, the possible exact geometry of the observation and the possible origins of the Martian aurorae. The emissions measured by SPICAM ultraviolet spectrometer have most probably been produced by electrons with an energy distribution peaking at few tens of eV rather than by electron distributions peaking above 100 eV.

Martian dayglow as seen by the SPICAM UV spectrograph on Mars Express

In this paper we describe measurements of the Martian dayglow obtained by SPICAM UV spectrograph on board Mars Express between October 2004 and March 2005. Typical spectra (of more than 24,000 individual spectra) display the main features of the dayglow already reported more than 30 years ago (Barth et al., 1971, 1972). The variations with altitude, solar zenith angle, and aerocentric longitude of the main emissions, namely, the CO Cameron band emissions (a3Π – X1Σ+) from 180–260 nm, the CO2+ ultraviolet doublet (B2Σ+ ‐ X2Π) emission near 289 nm, the O resonance line at 130.4 nm, and the H Lyman alpha line at 121.6 nm, are detailed. The average temperature deduced from the CO2+ (B2Σ+ ‐ X2Π) emission scale height between 150 and 190 km is T∞ = 201 ± 10 K. The altitude of the peak of the dayglow emission varies from 120 km at low solar zenith angle (∼30°) up to 132 km at high solar zenith angle (∼75°). We also find 24 orbits in which the individual derived exospheric temperatures do not show significant variations with respect to solar zenith angle, aerocentric longitude, or longitude. The presence of crustal magnetic fields appears to correlate with a significant (∼100 K) increase in exospheric temperature. We report also the first observations of emissions associated with the N2 Vegard Kaplan band system in the Martian upper atmosphere.

High resolution spectroscopy of stars with transiting planets

Context: During the past years photometric surveys, later complemented by follow-up radial-velocity measurements, have revealed the presence of several new extra-solar transiting planets, in very short period orbits. Many of the host stars are extremely faint (V$\sim$16), making high-precision spectroscopic measurements challenging. Aims: We have used the UVES spectrograph (VLT-UT2 telescope) to obtain high resolution spectra of 5 stars hosting transiting planets, namely for OGLE-TR-10, 56, 111, 113 and TrES-1. The immediate objective is to derive accurate stellar parameters and chemical abundances. Methods: The stellar parameters were derived from an LTE analysis of a set of FeI and FeII lines. Results: Complementing the spectroscopic information with photometric transit curves and radial-velocity data from the literature, we have then refined the stellar and planetary radii and masses. The obtained data were also used to study and discuss the relation between the stellar metallicity and orbital period of the planets.