A high-dispersion spectrograph on GTC will enable the study of chemical compositions of celestial objects (stars, stellar clusters, comets, nebulae, …), opening up to GTC many research fields such as stellar nucleosynthesis, stellar structure and evolution,  galactic chemical evolution or galaxy assembly (chemical tagging). By ensuring extreme wavelength stability, HORUS on GTC will also give the IAC the ability to characterize stars hosting planets, and to discover and follow-up planetary systems from radial velocity variations. 


The origin of globular cluster systems in other galaxies

Globular clusters provide a record of the early history of a galaxy. The Milky Way has a system of  about 150 globular clusters with a bimodal distribution of metallicity and age (see, e.g., Aparicio et al. 2009). A more massive galaxy such as M31 has a census of over 300 globular clusters, which separate into three distinct components in age and metallicity, including a group of intermediate age and metallicity with no counterpart in the Milky Way  (Beasley et al. 2005).

It is possible to determine precise chemical compositions from integrated light of globular clusters. This requires combining an initial mass function, stellar evolution models (which connect  the masses and ages of stars to their surface temperatures and gravities), and model atmospheres (which can be used to predict stellar spectra). The technique has already been proven for unresolved observations of clusters in the Milky Way, where it is possible to compare with abundances for individual stars (McWilliam & Bernstein 2008). Globulars are bright enough to be observed in other galaxies of the local group, fully resolving their modest velocity dispersion (see Fig. 1), and sampling the chemical evolution of these galaxies (Colucci et al. 2009).   

The fact that HORUS will share the focal plane with OSIRIS opens the possibility to carry out parallel observations in the outer parts of nearby galaxies while their central parts are being studied with OSIRIS – a high-efficiency tandem.


Stellar nucleosynthesis in the early Galaxy

HORUS will also enable follow-up observations of sources with unusual chemical compositions (e.g. neutron-capture enhanced stars) that will be identified by on-going and planned massive spectroscopic surveys such as HERMES, APOGEE, RAVE, or Gaia. These surveys will collect large numbers of moderate-to-high resolution spectra, but all have limited spectral windows,  and therefore each program necessarily misses some elements, which can be easily obtained with the large spectral  coverage provided  by HORUS.

Only two stars with iron abundances below [Fe/H]=-5 are known, and both have been discovered recently,  after years of dedicated efforts starting with wide-angle low-resolution surveys. Both of these stars are extremely enhanced in carbon and present a challenge for Type-II supernova models (Beers & Christlieb 2005).  Among the halo stellar population, several families of “rare” stars have been identified with common abundance ratios. The nature of these families, most with only a few dozen members known, is still far from clear, but can shed light on the important puzzle of nucleosynthesis and the formation of the early Galaxy.       

Teams at the IAC are actively working on the problem of spectral classification and the identification of chemically interesting stars. HORUS on GTC will allow the IAC to competitively follow-up and decipher the messages coded in the chemical abundances of the most interesting stars born in the early Milky Way.


Spectroscopy of exoplanets

Some 15 years after the discovery of the first planet orbiting a main-sequence star beyond our Solar System, exoplanetary science has become an established, cutting-edge branch of astrophysics. As technology improves and more is learnt about exoplanets and their host stars, the community’s attention is progressively turning towards the characterization of the exoplanets’ atmospheres. A planet’s atmosphere contains valuable information on the formation and evolution of the planet.

When the exoplanet transits its host star,  in-transit transmission spectroscopy has beeen demonstrated to be a powerful technique for the characterization of exoplanetary atmospheres. The wavelength-dependent absorption during the transit is sensitive to the composition of the planet’s terminator,  and therefore indicative of the bulk composition of its atmosphere (including gases, hazes, clouds, etc).

To overcome the difficulty inherent to the many orders of magnitude that separate the brightness of a planet and that of its stellar host, ultra-high signal-to-noise observations are necessary. This requires a high-dispersion spectrograph and a large-diameter telescope, in order to get the necessary ultra-high signal-to-noise ratio observations.

HORUS on GTC  will make it possible to follow-up other transiting systems,  extracting invaluable information on the structure and composition of planetary atmospheres. In 2009, Charbonneau et al announced the discovery of a transiting super-earth, GJ 1214b, around a bright, nearby, M-type star. In the near future more and more of these low mass rocky planets will be discovered, opening the door to characterizing their atmospheres. In some cases, such a GJ1214b is, these planets will be within the technical reach of GTC. The high spectral resolution of HORUS will allow the separation of the atomic and molecular lines in their atmospheres from the local telluric lines.


Spectroscopy of individual stars in Local-Group galaxies

Spectroscopy of individual giant stars in Local Group galaxies provides detailed chemical abundances and kinematics which allow an in depth characterization of different aspects of the galaxy. With a 10-m class telescope, abundance determinations are possible for the nearest galaxies only, i.e. the Milky Way satellites, while radial velocity measurement are possible, and have been performed for objects over the whole Local Group (e.g. Fraternali et al. 2009). The VLT, with the high multiplexing capability of FLAMES, has meant a revolution in this field, but there are a number of very interesting Northern Hemisphere objects which are out of its reach.  HORUS and OSIRIS could observe the same galaxy for complementary purposes (e.g. search for, and determination of periods of the ancient RR Lyrae variable stars), thus teaming up efficiently.

Stellar kinematics allows us to constrain the total mass of a stellar system. In the case of dwarf galaxies, total mass determinations have shown that these systems, and particularly dwarf spheroidal galaxies, are highly dark matter dominated (e.g.  Mateo 1998). The measurements of stellar kinematics in galaxies beyond the Milky Way satellite system are still scarce due to the faintness of the stars to be observed (V~ 20), but they are particularly important because i) they will allow us to study isolated objects which internal properties are more likely not to have been affected by the influence of a the giant galaxy host; and ii) gas-rich dwarf irregular galaxies will be among the targets, which will allow us to study the complex relationship between the kinematics of stars and gas (Cook et al. 1999).


Precision Asteroseismology

Asteroseismology is currently living a renaissance thanks on one hand to the extraordinary quality of uninterrupted observations from space. The Kepler mission, launched in 2009, observing a single field, providing continuous high-precision photometry of thousands of stars (Gilliland et al. 2010). These measurements need to be complemented with  ground-based high-resolution spectroscopy, which can be provided by HORUS on GTC, in order to constrain fundamental stellar parameters that cannot be derived from the the photometry time series (Uytterhoeven et al. 2010). 

At the same time, stable high-resolution spectrographs have recently started to be used for asteroseismology with great success (Bedding & Kjeldsen 2008). The contrast in the power spectrum of solar-like oscillations, relative to the background associated with granulation, is significantly higher for velocity than for photometry, allowing the detection of many more modes.  The IAC has expressed interest in participating in the SONG network for asteroseismology.  SONG  is deploying a series of stations around the globe, each  consisting of a robotic 1-m telescope coupled to an echelle spectrograph, devoted to asteroseismology.

By using a iodine cell to imprint a reference spectrum on the stellar spectra,  the SONG instruments will routinely derive a velocity accuracy of the order of 2 m/s at V~4 mag, and 10 m/s at V~7 mag. HORUS on GTC would provide an excellent instrument able to  follow up targets that are fainter, or for which a higher accuracy is sought.