Jorge Lima

Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation

Atmospheric aerosols exert an important influence on climate through their effects on stratiform cloud albedo and lifetime and the invigoration of convective storms. Model calculations suggest that almost half of the global cloud condensation nuclei in the atmospheric boundary layer may originate from the nucleation of aerosols from trace condensable vapours, although the sensitivity of the number of cloud condensation nuclei to changes of nucleation rate may be small. Despite extensive research, fundamental questions remain about the nucleation rate of sulphuric acid particles and the mechanisms responsible, including the roles of galactic cosmic rays and other chemical species such as ammonia. Here we present the first results from the CLOUD experiment at CERN. We find that atmospherically relevant ammonia mixing ratios of 100 parts per trillion by volume, or less, increase the nucleation rate of sulphuric acid particles more than 100–1,000-fold. Time-resolved molecular measurements reveal that nucleation proceeds by a base-stabilization mechanism involving the stepwise accretion of ammonia molecules. Ions increase the nucleation rate by an additional factor of between two and more than ten at ground-level galactic-cosmic-ray intensities, provided that the nucleation rate lies below the limiting ion-pair production rate. We find that ion-induced binary nucleation of H2SO4–H2O can occur in the mid-troposphere but is negligible in the boundary layer. However, even with the large enhancements in rate due to ammonia and ions, atmospheric concentrations of ammonia and sulphuric acid are insufficient to account for observed boundary-layer nucleation.

ESPRESSO: the Echelle spectrograph for rocky exoplanets and stable spectroscopic observations

ESPRESSO, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, will combine the efficiency of modern echelle spectrograph design with extreme radial-velocity precision. It will be installed on ESOs VLT in order to achieve a gain of two magnitudes with respect to its predecessor HARPS, and the instrumental radialvelocity precision will be improved to reach cm/s level. Thanks to its characteristics and the ability of combining incoherently the light of 4 large telescopes, ESPRESSO will offer new possibilities in various fields of astronomy. The main scientific objectives will be the search and characterization of rocky exoplanets in the habitable zone of quiet, nearby G to M-dwarfs, and the analysis of the variability of fundamental physical constants. We will present the ambitious scientific objectives, the capabilities of ESPRESSO, and the technical solutions of this challenging project.

CHEOPS/ZIMPOL: a VLT instrument study for the polarimetric search of scattered light from extrasolar planets

We present results from a phase A study supported by ESO for a VLT instrument for the search and investigation of extrasolar planets. The envisaged CHEOPS (CHaracterizing Extrasolar planets by Opto-infrared Polarization and Spectroscopy) instrument consists of an extreme AO system, a spectroscopic integral field unit and an imaging polarimeter. This paper describes the conceptual design of the imaging polarimeter which is based on the ZIMPOL (Zurich IMaging POLarimeter) technique using a fast polarization modulator combined with a demodulating CCD camera. ZIMPOL is capable of detecting polarization signals on the order of p=0.001% as demonstrated in solar applications. We discuss the planned implementation of ZIMPOL within the CHEOPS instrument, in particular the design of the polarization modulator. Further we describe strategies to minimize the instrumental effects and to enhance the overall measuring efficiency in order to achieve the very demanding science goals.

MAD star oriented: laboratory results for ground layer and multi-conjugate adaptive optics

The Multi-Conjugate Adaptive Optics Demonstrator (MAD) built by ESO with the contribution of two external consortia is a powerful test bench for proving the feasibility of Ground Layer (GLAO) and Multi-Conjugate Adaptive Optics (MCAO) techniques both in the laboratory and on the sky. The MAD module will be installed at one of the VLT unit telescope in Paranal observatory to perform on-sky observations. MAD is based on a two deformable mirrors correction system and on two multi-reference wavefront sensors (Star Oriented and Layer Oriented) capable to observe simultaneously some pre-selected configurations of Natural Guide Stars. MAD is expected to correct up to 2 arcmin field of view in K band. MAD is completing the test phase in the Star Oriented mode based on Shack-Hartmann wavefront sensing. The GLAO and MCAO loops have been successfully closed on simulated atmosphere after a long phase of careful system characterization and calibration. In this paper we present the results obtained in laboratory for GLAO and MCAO corrections testing with bright guide star flux in Star Oriented mode paying also attention to the aspects involving the calibration of such a system. A short overview of the MAD system is also given.

Uncovering the kiloparsec-scale stellar ring of NGC 5128 ,

We reveal the stellar light emerging from the kiloparsec-scale, ring-like structure of the NGC 5128 (Centaurus A) galaxy in unprecedented detail. We use arcsecond-scale resolution near infrared images to create a “dust-free” view of the central region of the galaxy, which we then use to quantify the shape of the revealed structure. At the resolution of the data, the structure contains several hundreds of discreet, point-like or slightly elongated sources. The typical extinction-corrected surface brightness of the structure is KS ≈ 16. 5m ag/arcsec 2 , and we estimate the total near infrared luminosity of the structure to be M ≈− 21 mag. We use diffraction limited (FWHM resolution of ≈0.1 �� , or 1.6 pc) near infrared data taken with the NACO instrument on the VLT to show that the structure decomposes into thousands of separate, mostly point-like sources. According to the tentative photometry, the most luminous sources have MK ≈− 12 mag, making them red supergiants or relatively low-mass star clusters. We also discuss the large-scale geometry implied by the reddening signatures of dust in our near infrared images.

MAD on sky results in star oriented mode

The Multi-Conjugate Adaptive Optics Demonstrator (MAD) built by ESO with the contribution of two external consortia is a powerful test bench for proving the feasibility of Multi-Conjugate (MCAO) and Ground Layer Adaptive Optics (GLAO) techniques both in the laboratory and on the sky. MAD is based on a two deformable mirrors correction system and on two multi-reference wavefront sensors (Star Oriented and Layer Oriented) capable to observe simultaneously some pre-selected configurations of Natural Guide Stars. MAD corrects up to 2 arcmin field of view in K band. After a long laboratory test phase, it has been installed at the VLT and it successfully performed on-sky demonstration runs on several astronomical targets for evaluating the correction performance under different atmospheric turbulence conditions. In this paper we present the results obtained on the sky in Star Oriented mode for MCAO and GLAO configurations and we correlate them with different atmospheric turbulence parameters. Finally we compare some of the on-sky results with numerical simulations including real turbulence profile measured at the moment of the observations.

The final design of the GRAVITY acquisition camera and associated VLTI beam monitoring strategy

The GRAVITY acquisition camera measurements are part of the overall beam stabilization by measuring each second the tip-tilt and the telescope pupil lateral and longitudinal positions, while monitoring at longer intervals the full telescope pupil, and the VLTI beam higher order aberrations. The infrared acquisition camera implements a mosaic of field, pupil, and Shack Hartman type images for each telescope. Star light is used to correct the tip-tilt while laser beacons placed at the telescope spiders are used to measure the pupil lateral positions. Dedicated optimized algorithms are applied to each image, extracting the beam parameters and storing them on the instrument database. The final design is built into the GRAVITY beam combiner, around a structural plane where the 4 telescope folding optics and field imaging lenses are attached. A fused silica prism assembly, kept around detector temperature, is placed near to the detector implementing the different image modes.

The GRAVITY acquisition and guiding system

GRAVITY is a VLTI second generation instrument designed to deliver astrometry at the level of 10 μas. The beam transport to the beam combiner is stabilized by means of a dedicated guiding system whose specifications are mainly driven by the GRAVITY astrometric error budget. In the present design, the beam is monitored using an infrared acquisition camera that implements a mosaic of field, pupil and Shack-Hartmann images for each of the telescopes. Star and background H-band light from the sky can be used to correct the tip-tilt and pupil lateral position, within the GRAVITY specifications, each 10 s. To correct the beam at higher frequencies laser guiding beams are launched in the beam path, on field and pupil planes, and are monitored using position sensor detectors. The detection, in the acquisition camera, of metrology laser light back reflected from the telescopes, is also being investigated as an alternative for the pupil motion control.

CHEOPS NIR IFS: exploring stars neighborhood spectroscopically

CHEOPS is a 2nd generation VLT instrument for the direct detection of extrasolar planets. The project is currently in its Phase A. It consists of an high order adaptive optics system which provides the necessary Strehl ratio for the differential polarimetric imager (ZIMPOL) and an Integral Field Spectrograph (IFS). The IFS is a very low resolution spectrograph (R~15) which works in the near IR (0.95-1.7 μm), an ideal wavelength range for the ground based detection of planetary features. In our baseline design, the Integral Field Unit (IFU) is a microlens array of about 250x250 elements which will cover a field of view of about 3.5x3.5 arcsecs2 in proximity of the target star. In this paper we describe the instrument, its preliminary optical design and the basic requirements about detectors. In a separate contribution to this conference, we present the very low resolution disperser.

ESPRESSO: design and analysis of a Coudé-train for a stable and efficient simultaneous optical feeding from the four VLT unit telescopes

ESPRESSO is a fiber-fed, cross-dispersed, high-resolution, echelle spectrograph. Being the first purpose of ESPRESSO to develop a competitive and innovative high-resolution spectrograph to fully exploit the VLT (Very Large Telescope), and allow new science, it is important to develop the VLT array concept bearing in mind the need to obtain the highest stability, while preserving its best efficiency. This high-resolution ultra-stable spectrograph will be installed in the VLT at the Combined Coudé Laboratory (CCL), fed by four Coudé Trains, which brings the light from the Nasmyth platforms of the four VLT Unit Telescopes to the CCL. A previous trade-off analysis, considering the use of mirrors, prisms, lenses or fibers and several possible combinations of them, pointed towards a Full Optics solution, using only conventional optics to launch the light from the telescope into the front-end unit. In this case, the system is composed of a set of prisms and lenses to deliver a pupil and an image in the CCL, including an Atmospheric Dispersion Compensator. In this paper, we present the optical design of the Coudé Trains, the opto-mechanical concept, the main characteristics and expected performances.