the Science

The amazing spectroscopic precision of this instrument will provide the community with new scientific capabilities which are unique world-wide.


High-resolution spectroscopy has always been at the heart of astrophysics. It provides the physical insights for stars, galaxies, interstellar and intergalactic medium. Correspondingly, high-resolution spectrographs have always been on high demand at major observatories, see e.g. UVES at the VLT or HIRES at Keck. As telescopes get larger, the capabilities of high-resolution spectrographs extend to fainter objects. Besides this increase in photon collecting power another aspect has emerged in recent years: the need for high-precision spectroscopy. In many applications there is the need for highly repeatable observations over long timescales where instrumental effects need to be completely removed or at least minimized. For instance this is the case of radial velocity measurements, or more generally for the determination of the positions and shapes of spectral lines. In this respect, the HARPS spectrograph at the ESO 3.6-m telescope has been a pioneering instrument and ESPRESSO, which keeps its legacy, has a great potential for many new and exciting discoveries.

Hunt for rocky exoplanets

The need for a ground based follow-up facility capable of high RV precision has been recently stressed in the ESO-ESA working group report on extrasolar planets [Perryman, M., Dravins, D., Hainaut, O., Leger, A., Quirrenbach, A. and Rauer, H., in “ESA-ESO Working Group on Extra-Solar Planets” (2005)]. In fact, terrestrial planets in habitable zones are one of the main scientific topics of the next decades in Astronomy, and one of the main science drivers for the new generation of ELT. This ESO-ESA report states p. 63: a) high precision radial velocity instrumentation for the follow-up of astrometric and transit detections, to ensure the detection of a planet by a second independent method, and to determine its true mass. For Jupiter-mass planets, existing instrumentation may be technically adequate but observing time inadequate; for Earth-mass candidates, special purpose instrumentation (like HARPS) on a large telescope would be required. The same concept is reiterated in the first recommendation: b) Support experiments to improve radial velocity mass detection limits, e.g. based on experience from HARPS, down to those imposed by stellar surface phenomena” [Perryman, et al., 2005], p. 72.

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Variations of fundamental constants

Exoplanet research is just one trigger among many others for high-precision spectroscopy. Do the fundamental constants vary? This is one of the six big open questions in cosmology as listed in the ESA-ESO report for fundamental cosmology [Peacock, J. A., Schneider, P., Efstathiou, G., Ellis, J.R., Leibundgut, B., Lilly, S.J., and Mellier, Y., in “ESA-ESO Working Group on Fundamental Cosmology” (2006)]. In the executive summary the document states: “Quasar spectroscopy also offers the possibility of better constraints on any time variation of dimensionless atomic parameters such as the fine-structure constant α and the proton-to-electron mass ratio. Presently there exist controversial claims of evidence for variations in α, which potentially relate to the dynamics of dark energy. It is essential to validate these claims with a wider range of targets and atomic tracers.” This can be done only with improved spectroscopic capabilities.

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A scientific Pandora box

ESPRESSO combines an unprecedented radial-velocity and spectroscopic precision with the largest photon collecting area available today and unique resolving power (R = 225’000). It will certainly provide breakthroughs in many areas of astronomical research, many of which we cannot anticipate. We provide below just few examples.

Chemical composition of stars in local galaxies

One important piece of information in the understanding of the galaxy formation is the chemical composition for local galaxies. In spite of the many successes in this field the majority of local galaxies still lack detailed abundance information. There are about a dozen of nearby galaxies observable from Paranal which, except Sagittarius, have chemical information, albeit generally for a few stars and for a limited set of elements, and for the faintest galaxies based on low to medium resolution spectra. The next nearest galaxy, Leo T, has a distance modulus of 23.1, i.e. more than one magnitude more distant than Leo I (m-M=21.99). The local group galaxies all possess giant stars of magnitude V=20, or fainter. Although some work has been done with UVES at VLT, at this magnitude it is really difficult if not impossible to obtain accurate chemical abundances. For galaxies that possess a young population, like Phoenix or WLM, one can rely on bright O and B supergiants. However, if one considers old metal-poor systems, like Bootes or Hercules, one has to rely on red giants. Although it is clear that most of the chemical information for local galaxies will have to come largely from the ELT, ESPRESSO will give us the chance to have a first but important glimpse of this

Metal poor stars

The most metal poor stars in the Galaxy are probably the most ancient fossil records of the chemical composition and thus can provide clues on the pre-Galactic phases and on the stars that synthesized the first metals. Masses and yields of POP III stars can be inferred from the observed elemental ratios in the most metal poor stars. One crucial question to be answered is the presence of Pop III low-mass stars. For long the Pop III have been thought to be very massive but the recent discovery of a very metal-poor star with [Fe/H] ~ −5.0 and “normal” C and N have shown an entirely new picture.  Several surveys searching for metal poor stars are currently going on or planned, and thousands of EMP stars with [Fe/H] < −3.0, of which maybe several dawn to [Fe/H] ~ −5.0 and hopefully lower, are expected to be found. These will be within the reach of ESPRESSO, which will be able in both the 1-UT and 4-UT mode to provide spectra for an exquisite chemical analysis.

Stellar oscillations, asteroseismology, variability

Stars located in the upper main sequence show non-radial pulsations that cause strong line profile variations. Asteroseismic study (i.e., mode identification) of these pulsating stars (Gamma Dor, Delta Sct, Beta Cep, SPB, etc.) provides constraints on the structure of massive stars, e.g. internal convection, overshooting, core size, extension of acoustic and gravity cavities, mass loss phenomena, interplay between rotation and pulsation.

Galactic winds and tomography of the IGM

Spectroscopy of close, multiple, high-redshift quasars allows in principle to recover the 3-dimensional distribution of matter from the analysis of the HI Lyman-alpha absorption lines. If the multiple lines of sight cross a region where there are known high-redshift galaxies, it is also possible to investigate the properties of outflows and inflows, studying the spectral absorption lines at the redshift of the galaxies and how they evolve moving closer to or far away from the galaxies themselves. The main limitation to the full exploitation of the so-called “tomography” of the IGM is the dearth of quasar pairs at the desired separation, bright enough to be observed with the present high-resolution spectrographs at 10m-class telescopes. ESPRESSO used in the 4UT mode, would result in a gain of ~1.5 magnitude fainter than e.g. UVES, translating into almost a factor 20 of more observable quasar pairs with separation less than 3 arcmin and emission redshift in the range 2 < z < 3.

The expanding Universe

Sandage (1962) first argued that in any cosmological model the redshifts of cosmologically distant objects drift slowly with time. If observed, their redshift drift-rate, dz/dt, would constitute evidence of the Hubble flow’s deceleration or acceleration between redshift z and today. Indeed, this observation would offer a direct, non-geometric, completely model-independent measurement of the Universe’s expansion history. The VLT, even in the 4UT mode, is probably not enough to measure the tiny signal, which is at the level of few cm s-1 yr-1. However, it might provide first accurate historical reference measurements and will in any case represent an important step forward setting the scene for the next generation of high resolution spectrographs at the E-ELTs.

Use of GTO

In particular, the ESPRESSO consortium will invest the Guaranteed-Time Observation (GTO), awarded in return to the major capital and human investment, into two major scientific programmes:

  • 80% of the observing nights will be invested for the search and characterization of rocky planets in the habitable zone of G, K, and M stars in the 1-UT mode.
  • 10% of the time will be dedicated to the determination of possible variability of fundamental constants. Depending on the magnitude of the target, this programme will be carried out partially in the 1-UT, partially in the 4-UT mode.
  • 10% will be reserved for outstanding science cases and allocated as a function of topical questions arising at the moment of the GTO Observations.

ESPRESSO as a CODEX precursor

ESPRESSO have been at first imagined as a prototype for the ultra accurate spectrograph for the European Extremely Large Telescope (E-ELT), CODEX, conceptually designed under ESO leadership by almost the same teams. Later, the first studies have shown that this prototype, installed at the VLT, could have thanks its unique capabilities, a complete line of successful science results. Moreover, the size of the E-ELT is so different from the VLT that it would have been impossible to simply clone ESPRESSO for that telescope. Thus, the two instruments studies have been split in two unique instruments.

ESPRESSO was then  seen as a precursor of CODEX in the sense that both instruments share some concepts and R&D developments, as anamorphic optics, pupil slicing, pupil incoherent recombination, development of large echelle gratings, non-circular fibers for optical scrambling, advanced new wavelength calibrators, as Fabry-Perot cells and laser combs, ultra-stable detector dewars. In the meanwhile, CODEX project has been merged with a near-IR spectrograph project, SIMPLE, to define a more complete instrument for the E-ELT: HIRES. HIRES will naturally inherit some new concepts presently developed for ESPRESSO.

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