VLT Telescopes Research Articles

Testing Newtonian gravity with distant globular clusters: NGC 1851 and NGC 1904

Globular clusters are useful to test the validity of Newtonian dynamics in the low acceleration regime typical of galaxies, without the complications of non-baryonic dark matter. Specifically, in absence of disturbing effects, e.g. tidal heating, their velocity dispersion is expected to vanish at large radii. If such behaviour is not observed, and in particular if, as observed in elliptical galaxies, the dispersion is found constant at large radii below a certain threshold acceleration, this might indicate a break down of Newtonian dynamics. To minimise the effects of tidal heating in this paper we study the velocity dispersion profile of two distant globular clusters, NGC 1851 and NGC 1904. The velocity dispersion profile is derived from accurate radial velocities measurements, obtained at the ESO 8m VLT telescope. Reliable data for 184 and 146 bona fide cluster star members, respectively for NGC 1851 and NGC 1904, were obtained. These data allow to trace the velocity dispersion profile up to ~2r0, where r0 is the radius at which the cluster internal acceleration of gravity is a0 = 10e-8 cm/s/s. It is found that in both clusters the velocity dispersion becomes constant beyond ~r0. These new results are fully in agreement with those found for other five globular clusters previously investigated as part of this project. Taken all together, these 7 clusters support the claim that the velocity dispersion is constant beyond r0, irrespectively of the specific physical properties of the clusters: mass, size, dynamical history, and distance from the Milky Way. The strong similarly with the constant velocity dispersion observed in elliptical galaxies beyond r0 is suggestive of a common origin for this phenomenon in the two class of objects, and might indicate a breakdown of Newtonian dynamics below a0.

The Supernova Legacy Survey 3-year sample: Type Ia supernovae photometric distances and cosmological constraints

We present photometric properties and distance measurements of 252 high redshift Type Ia supernovae (0.15 < z < 1.1) discovered during the first three years of the Supernova Legacy Survey (SNLS). These events were detected and their multi-colour light curves measured using the MegaPrime/MegaCam instrument at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in four bands. Follow-up spectroscopy was performed at the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and to measure their redshifts. Systematic uncertainties arising from light curve modeling are studied, making use of two techniques to derive the peak magnitude, shape and colour of the supernovae, and taking advantage of a precise calibration of the SNLS fields. A flat LambdaCDM cosmological fit to 231 SNLS high redshift Type Ia supernovae alone gives Omega_M = 0.211 +/- 0.034(stat) +/- 0.069(sys). The dominant systematic uncertainty comes from uncertainties in the photometric calibration. Systematic uncertainties from light curve fitters come next with a total contribution of +/- 0.026 on Omega_M. No clear evidence is found for a possible evolution of the slope (beta) of the colour-luminosity relation with redshift.

Statistical equilibrium of silicon in the atmospheres of metal-poor stars

<i>Aims. <i/>The statistical equilibrium of neutral and ionized silicon in the atmospheres of metal-poor stars is discussed. Non-local thermodynamic equilibrium effects (NLTE) are investigated and the silicon abundances in metal-poor stars determined.<i>Methods. <i/>We have used high resolution, high signal to noise ratio spectra from the UVES spectragraph at the ESO VLT telescope. Line formation calculations of Si i and Si ii in the atmospheres of metal-poor stars are presented for atomic models of silicon including 174 terms and 1132 line transitions. Recent improved calculations of Si i and Si ii photoionization cross-sections are taken into account, and the influence of the free-free quasi-molecular absorption in the Ly<i>α<i/> wing is investigated by comparing theoretical and observed fluxes of metal-poor stars. All abundance results are derived from LTE and NLTE statistical equilibrium calculations and spectrum synthesis methods.<i>Results. <i/>It is found that the extreme ultraviolet radiation is very important for metal-poor stars, especially for the high temperature, very metal-poor stars. The radiative bound-free cross-sections also play a very important role for these stars.<i>Conclusions. <i/>NLTE effects for Si are found to be important for metal-poor stars, in particular for warm metal-poor stars. It is found that these effects depend on the temperature. For warm metal-poor stars, the NLTE abundance correction reaches ~0.2 dex relative to standard LTE calculations. Our results indicate that Si is overabundant for metal-poor stars.

New insights on the binary Asteroid 121 Hermione

We report on the results of a six-month photometric study of the main-belt binary C-type asteroid 121 Hermione, performed during its 2007 opposition. We took advantage of the rare observational opportunity afforded by one of the annual equinoxes of Hermione occurring close to its opposition in June 2007. The equinox provides an edge-on aspect for an Earth-based observer, which is well suited to a thorough study of Hermione's physical characteristics. The catalog of observations carried out with small telescopes is presented in this work, together with new adaptive optics (AO) imaging obtained between 2005 and 2008 with the Yepun 8-m VLT telescope and the 10-m Keck telescope. The most striking result is confirmation that Hermione is a bifurcated and elongated body, as suggested by Marchis et al., (2005). A new effective diameter of 187 +/- 6 km was calculated from the combination of AO, photometric and thermal observations. The new diameter is some 10% smaller than the hitherto accepted radiometric diameter based on IRAS data. The reason for the discrepancy is that IRAS viewed the system almost pole-on. New thermal observations with the Spitzer Space Telescope agree with the diameter derived from AO and lightcurve observations. On the basis of the new AO astrometric observations of the small 32-km diameter satellite we have refined the orbit solution and derived a new value of the bulk density of Hermione of 1.4 +0.5/-0.2 g cm-3. We infer a macroscopic porosity of ~33 +5/-20%.

FLUORINE IN ASYMPTOTIC GIANT BRANCH CARBON STARS REVISITED

A reanalysis of the fluorine abundance in three Galactic AGB carbon stars (TX Psc, AQ Sgr and R Scl) has been performed from the molecular HF (1-0) R9 line at 2.3358 $\mu$m. High-resolution (R$\sim 50000$) and high signal to noise spectra obtained with the CRIRES spectrograph and the VLT telescope or from the NOAO archive (for TX Psc) have been used. Our abundance analysis uses the latest generation of MARCS model atmospheres for cool carbon rich stars. Using spectral synthesis in LTE we derive for these stars fluorine abundances that are systematically lower by $\sim 0.8$ dex in average with respect to the sole previous estimates by Jorissen, Smith & Lambert (1992). The possible reasons of this discrepancy are explored. We conclude that the difference may rely on the blending with C-bearing molecules (CN and C$_2$) that were not properly taken into account in the former study. The new F abundances are in better agreement with the prediction of full network stellar models of low mass AGB stars. These models also reproduce the $s$-process elements distribution in the sampled stars. This result, if confirmed in a larger sample of AGB stars, might alleviate the current difficulty to explain the largest [F/O] ratios found by Jorissen et al. In particular, it may not be necessary to search for alternative nuclear chains affecting the production of F in AGB stars.

Ground-based secondary eclipse detection of the very-hot Jupiter OGLE-TR-56b

We report on the detection of the secondary eclipse of the very-hot Jupiter OGLE-TR-56b from combined z-band time series photometry obtained with the VLT and Magellan telescopes. We measure a flux decrement of 0.0363+/-0.0091 percent from the combined Magellan and VLT datasets, which indicates a blackbody brightness temperature of 2718 (+127/-107) K, a very low albedo, and a small incident radiation redistribution factor, indicating a lack of strong winds in the planet's atmosphere. The measured secondary depth is consistent with thermal emission, but our precision is not sufficient to distinguish between a black-body emitting planet, or emission as predicted by models with strong optical absorbers such as TiO/VO. This is the first time that thermal emission from an extrasolar planet is detected at optical wavelengths and with ground-based telescopes.

Visible spectroscopy of the new ESO large program on trans-Neptunian objects and Centaurs

Aims. A second large observational program was started to improve our knowledge of the physical properties of Centaurs and transNeptunian objects, TNOs. The program is being executed in the facilities at the Cerro Paranal of the European Southern Observatory, ESO, in Chile. Methods. Visible spectroscopy was carried out using FORS1 at the UT2 of the VLT telescope. We computed the spectral slope, S , for each object, and we searched for the possible existence of weak absorption features. We compared our data with those from the literature.

C, N, O abundances in the most metal-poor damped Lyman alpha systems

This study focuses on some of the most metal-poor damped Lyα (DLA) absorbers known in the spectra of high-redshift QSOs, using new and archival observations obtained with ultraviolet-sensitive echelle spectrographs on the Keck and VLT telescopes. The weakness and simple velocity structure of the absorption lines in these systems allow us to measure the abundances of several elements, and in particular those of C, N and O, a group that is difficult to study in DLAs of more typical metallicities. We find that when the oxygen abundance is less than ∼1/100 of solar, the C/O ratio in high-redshift DLAs and sub-DLAs matches that of halo stars of similar metallicity and shows higher values than expected from galactic chemical evolution models based on conventional stellar yields. Furthermore, there are indications that at these low metallicities the N/O ratio may also be above simple expectations and may exhibit a minimum value, as proposed by Centurion and her collaborators in 2003. Both results can be interpreted as evidence for enhanced production of C and N by massive stars in the first few episodes of star formation, in our Galaxy and in the distant protogalaxies seen as QSO absorbers. The higher stellar yields implied may have an origin in stellar rotation which promotes mixing in the stars' interiors, as considered in some recent model calculations. We briefly discuss the relevance of these results to current ideas on the origin of metals in the intergalactic medium and the universality of the stellar initial mass function.

Surface properties of Rosetta's targets (21) Lutetia and (2867) Steins from ESO observations

Aims. The aim of this work is to constrain the size, composition and surface properties of asteroids (2867) Steins and (21) Lutetia, targets of the Rosetta mission. Rosetta is en route to rendezvous with comet 67P/Churyumov-Gerasimenko. Methods. Thermal-Infrared N-band observations for Lutetia and Steins were obtained using, respectively, TIMMI2 on the ESO 3.6-m telescope at La Silla and VISIR at the UT3 VLT telescope on Cerro Paranal; visible light curves for Steins were obtained using NTT+SUSI2, while R-band photometry for Lutetia was obtained with the 2.0-m Faulkes Telescope North on Haleakala. For Steins, the NEATM model was used to constrain its visible geometric albedo and beaming parameter. A detailed thermophysical model was implemented and used to analyze our set of observations of Lutetia as well as previous reported measurements. Results. The visible photometry of Steins was used along with data from the literature to yield a slope parameter of G = 0.32 +0.14 −0.11 . Problems during the observations led to the loss of measurements on two of the three N-band filters requested for Steins. Using the remaining data and the polarimetric albedo recently published, we were able to constrain the thermal beaming parameter as η> 1.2, which is more similar to near-Earth asteroids and suggests either high thermal inertia or a very rough surface. For Lutetia, the best fit visible geometric albedo obtained with our model and the reported observation is pv = 0.129, significantly lower than that obtained if one applies the same model to previously reported measurements. The discrepancy cannot be explained solely by assuming inhomogeneities in the surface properties and we suggest that the most plausible explanation is the presence of one or more large craters on the northern hemisphere. For both sets of measurements, the implied single scattering albedo of Lutetia is compatible with laboratory measurements of carbonaceous chondrite meteorites.

Testing turbulence model at metric scales with mid-infrared VISIR images at the VLT

We probe turbulence structure from centimetric to metric scales by simultaneous imagery at mid-infrared and visible wavelengths at the VLT telescope and show that it departs significantly from the commonly used Kolmogorov model. The data can be fitted by the von Karman model with an outer scale of the order of 30 m and we see clear signs of the phase structure function saturation across the 8-m VLT aperture. The image quality improves in the infrared faster than the standard lambda^{-1/5} scaling and may be diffraction-limited at 30-m apertures even without adaptive optics at wavelengths longer than 8 micron.

A Statistical Study of Multiply Imaged Systems in the Lensing Cluster Abell 68

We have carried out an extensive spectroscopic survey with the Keck and VLT telescopes, targeting lensed galaxies in the background of the massive cluster Abell 68. Spectroscopic measurements are obtained for 26 lensed images, including a distant galaxy at z=5.4 . Redshifts have been determined for 5 out of 7 multiply-image systems. Through a careful modeling of the mass distribution in the strongly-lensed regime, we derive a mass estimate of 5.3 x 10^14 Msun within 500 kpc. Our mass model is then used to constrain the redshift distribution of the remaining multiply-imaged and singly-imaged sources. This enables us to examine the physical properties for a subsample of 7 Lyman-alpha emitters at 1.7 < z < 5.5, whose unlensed luminosities of ~ 10^41 ergs/s are fainter than similar objects found in blank fields. Of particular interest is an extended Lyman-alpha emission region surrounding a highly magnified source at z=2.6, detected in VIMOS Integral Field Spectroscopy data. The physical scale of the most distant lensed source at z=5.4 is very small (<300 pc), similar to the lensed z ~ 5.6 emitter reported by Ellis et al. (2001) in Abell 2218. New photometric data available for Abell 2218 allow for a direct comparison between these two unique objects. Our survey illustrates the practicality of using lensing clusters to probe the faint end of the z ~ 2-5 Lyman-alpha luminosity function in a manner that is complementary to blank field narrow-band surveys.

The Infrared Extinction Law at Extreme Depth in a Dark Cloud Core

We combined sensitive near-infrared data obtained with ground-based imagers on the ESO NTT and VLT telescopes with space mid-infrared data acquired with the IRAC imager on the Spitzer Space Telescope to calculate the extinction law A_\lambda/A_K as a function of \lambda between 1.25 and 7.76 micron to an unprecedented depth in Barnard 59, a star forming, dense core located in the Pipe Nebula. The ratios A_\lambda/A_K were calculated from the slopes of the distributions of sources in color-color diagrams \lambda-K vs. H-K. The distributions in the color-color diagrams are fit well with single slopes to extinction levels of A_K ~ 7 (A_V ~ 59 mag). Consequently, there appears to be no significant variation of the extinction law with depth through the B59 line of sight. However, when slopes are translated into the relative extinction coefficients A_\lambda/A_K, we find an extinction law which departs from the simple extrapolation of the near-infrared power law extinction curve, and agrees more closely with a dust extinction model for a cloud with a total to selective absorption R_V=5.5 and a grain size distribution favoring larger grains than those in the diffuse ISM. Thus, the difference we observe could be possibly due to the effect of grain growth in denser regions. Finally, the slopes in our diagrams are somewhat less steep than those from the study of Indebetouw et al. (2005) for clouds with lower column densities, and this indicates that the extinction law between 3 and 8 micron might vary slightly as a function of environment.

Line profile variations in rapidly oscillating Ap stars: resolution of the enigma*

We have carried out the first survey of the pulsational line profile variability in rapidly oscillating Ap (roAp) stars. We analysed high signal-to-noise time-series observations of ten sharp-lined roAp stars obtained with the high-resolution spectrographs attached to the VLT and CFHT telescopes. We investigated in detail the variations of Pr III, Nd II, Nd III and Tb III lines and discovered a prominent change of the profile variability pattern with height in the atmospheres of all studied roAp stars. In every investigated star profile variability of at least one rare-earth ion is characterized by unusual blue-to-red moving features - a behaviour inexplicable in the framework of the standard oblique pulsator model of slowly rotating roAp stars. Using analysis of the line profile moments and spectrum synthesis calculations, we demonstrate that unusual oscillations in spectral lines of roAp stars arise from the pulsational modulation of line widths. This variation occurs approximately in quadrature with the radial velocity changes, and its amplitude rapidly increases with height in stellar atmosphere. We propose that the line width modulation is a consequence of the periodic expansion and compression of turbulent layers in the upper atmospheres of roAp stars. Thus, the line profile changes observed in slowly rotating magnetic pulsators should be interpreted as a superposition of two types of variability: the usual time-dependent velocity field due to an oblique low-order pulsation mode and an additional line width modulation, synchronized with the changes of stellar radius. Our explanation of the line profile variations of roAp stars solves the long-standing observational puzzle and opens new possibilities for constraining geometric and physical properties of the stellar magnetoacoustic pulsations.

Spectroscopic monitoring of the BL Lac object AO 0235+164

Aims. Spectroscopic monitoring of BL Lac objects is a difficult taskthat nonetheless can provide important information on the different components of the active galactic nucleus. Methods. We performed optical spectroscopic monitoring of the BL Lac object AO 0235+164 (z = 0.94) with the VLT and TNG telescopes from Aug. 2003 to Dec. 2004, during an extended WEBT campaign. The flux of this source is both contaminated and absorbed by a foreground galactic system at z = 0.524, the stars of which can act as gravitational micro-lenses. Results. In this period the object was in an optically faint, though variable state, and a broad Mg ii emission line was visible at all epochs. The spectroscopic analysis reveals an overall variation in the Mg ii line flux of a factor 1.9, while the corresponding continuum flux density changed by a factor 4.3. Most likely, the photoionising radiation can be identified with the emission component that was earlier recognised to be present as a UV–soft-X-ray bump in the source spectral energy distribution and that is visible in the optical domain only in very faint optical states. We estimate an upper limit to the broad line region (BLR) size of a few light months from the historical minimum brightness level; from this we infer the maximum amplification of the Mg ii line predicted by the microlensing scenario. Conclusions. Unless we have strongly overestimated the size of the BLR, only very massive stars could significantly magnify the broad Mg ii emission line, but the time scale of variations due to these (rare) events would be of several years. In contrast, the continuum flux, coming from much smaller emission regions in the jet, could be affected by microlensing from the more plausible MACHO deflectors, with variability time scales of the order of some months.

Rest‐Frame Optical Spectroscopic Classifications for Submillimeter Galaxies

We report the results of a systematic near-IR spectroscopic survey using the Subaru, VLT, and Keck Telescopes of a sample of high-redshift ULIRGs mainly composed of submillimeter-selected galaxies. Our observations span the rest-frame optical range containing nebular emission lines such as Hβ, [O III] λλ4959, 5007, and [O II] λ3727, which are essential for making robust diagnostics of the physical properties. Using the Hα/Hβ emission line ratios, we derive internal extinction estimates for these galaxies similar to those of local ULIRGs: A_V ~ 2.9 ± 0.5. Correcting the Hα estimates of the star formation rate for dust extinction results in rates that are consistent with those estimated from the far-IR luminosity. The majority (>60%) of our sample show spectral features characteristic of AGNs (although this partially reflects an observational bias), with ~65% exhibiting broad Balmer emission lines. A proportion of these sources show relatively low [O III] λ5007/Hβ line ratios, which are similar to those of narrow-line Seyfert 1 galaxies, suggesting low-mass black holes that are rapidly growing. In the subsample of our survey with both [O III] λ5007 and hard X-ray coverage, at least ~60% show an excess of [O III] λ5007 emission, by a factor of 5-10, relative to the hard X-ray luminosity compared to the correlation between these two properties seen in Seyfert galaxies and QSOs locally. From our spectral diagnostics, we propose that the strong [O III] λ5007 emission in these galaxies arises from shocks in dense gaseous regions. Due to sensitivity and resolution limits, our sample is biased to strong-line emitters and hence our results do not yet provide a complete view of the physical properties of the whole high-redshift ULIRG population.

Hypernova Signatures in the Late Rebrightening of GRB 050525A

We report observations of GRB 050525A, for which a Gemini North spectrum shows its redshift to be z = 0.606. This is the third closest long GRB discovered by Swift. We observed its afterglow using the VLT, Gemini and TNG telescopes to search for an associated SN. We find that the early-time light curve is described by a broken power law with a break at t ~ 0.3 d after the burst. About 5 d after the burst, a flattening is apparent, followed by a further dimming. Both the magnitude and the shape of the light curve suggest that a supernova was emerging during the late decay of the afterglow. This supernova, dubbed SN 2005nc, had a rise time faster than SN 1998bw and a long-lasting maximum. A spectrum obtained about 20 d (rest-frame) after the GRB resembles the spectrum of SN 1998bw obtained close to maximum light.

The Supernova Legacy Survey: measurement of $\Omega_{\mathsf{M}}$, $\Omega_\mathsf{\Lambda}$ andwfrom the first year data set

We present distance measurements to 71 high redshift type Ia supernovae discovered during the first year of the 5-year Supernova Legacy Survey (SNLS). These events were detected and their multi-color light-curves measured using the MegaPrime/MegaCam instrument at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in four bands. Follow-up spectroscopy was performed at the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and to measure their redshift. With this data set, we have built a Hubble diagram extending to z=1, with all distance measurements involving at least two bands. Systematic uncertainties are evaluated making use of the multi-band photometry obtained at CFHT. Cosmological fits to this first year SNLS Hubble diagram give the following results : Omega_M = 0.263 +/- 0.042(stat) +/- 0.032(sys) for a flat LambdaCDM model; and w = -1.023 +/- 0.090(stat) +/- 0.054(sys) for a flat cosmology with constant equation of state w when combined with the constraint from the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations.

Sulphur abundance in Galactic stars

We investigate sulphur abundance in 74 Galactic stars by using high resolution spectra obtained at ESO VLT and NTT telescopes. For the first time the abundances are derived, where possible, from three optical multiplets: Mult. 1, 6, and 8. By combining our own measurements with data in the literature we assemble a sample of 253 stars in the metallicity range -3.2 ≤ [Fe/H] ≤ +0.5. Two important features, which could hardly be detected in smaller samples, are obvious from this large sample: I) a sizeable scatter in [S/Fe] ratios around (Fe/H] ∼ -1;2) at low metallicities we observe stars with [S/Fe] ∼0.4, as well as stars with higher [S/Fe] ratios. The latter do not seem to be kinematically different from the former ones. Whether the latter finding stems from a distinct population of metal-poor stars or simply from an increased scatter in sulphur abundances remains an open question.

LP 349-25: A new tight M8V binary

We present the discovery of a tight M8V binary, with a separation of only 1.2 astronomical units, obtained with the PUEO and NACO adaptive optics systems, respectively at the CFHT and VLT telescopes. The estimated period of LP 349-25 is approximately 5 years, and this makes it an excellent candidate for a precise mass measurement.

A search for interstellar absorptions towards Sgr dSph

We searched for Na I D1,D2 interstellar absorption lines towards 12 giant stars of the Sgr dwarf spheroidal and 3 stars of the associated globular cluster Terzan 7, observed at high resolution with the UVES spectrograph at the 8.2 m Kueyen VLT telescope. These stars are not an ideal background source for the analysis of interstellar (IS) absorption lines since they have been observed in the framework of a stellar abundances study. However they are sufficiently luminous to allow a decent S/N ratio at a resolution of about 43000, i.e. ~7 km/s, which allows an exploratory study. We detected two distinct groups of IS absorptions, a ``local'' group, with radial velocities ranging from -43 km/s to 50 km/s, and a high velocity group with radial velocities ranging from 150 km/s to 165 km/s. Likely the high velocity components are due to gas falling on the Sgr dSph since have been observed only in 2 stars which are separated by only 1.5' in the sky. We argue that the observations suggest a stripping of gas due to the passage of Sgr through the Galactic disc.