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The HERMES Project: Science Case & Requirements

Galactic Archaeology

The main scientific goal for HERMES is to unravel for the first time the 12-billion yearlong stellar built-up history of our Galaxy. Of particular importance is to identify the early accretion events of primordial stellar fragments that are believed to have been the first founding blocks of the disk of our Galaxy.

Image of our galaxy from 2MASS

2MASS all-sky near-infrared picture of our Galaxy showing its three main stellar components (bulge, halo, thin & thick disk) and its two companions (the large & small Magellanic Clouds).

(Atlas Image obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.)

To identify these ancient events, now dissolved throughout our Galaxy, HERMES will measure accurate (roughly at the 0.5% level) chemical abundances of elements involved in the main nucleogenesis processes for a million stars up to a limiting magnitude V=14 and distributed over the galactic disk.

Sun spectra showing lines from many elements

404.7-406.4 nm spectral window on the Sun revealing the chemical signature of many elements at once (Fe, Cr, Ti, V, Co, Mg, Mn, Nd, Cu, Ce, Sc, Gd, Zr, Dy). Spectral resolution is ~ 80,000.

There is a huge synergy between this program and the forthcoming (2012-2017) GAIA Survey mission by the European Space Agency: from GAIA's exquisite astrometric measurements and HERMES' accurate abundances and line of sight velocities, the three-dimensional orbits of the one-million chemically-tagged stars will be precisely derived, a big bonus to unravel our Galaxy highly complex alchemic history.

Detailed simulations have shown that this ambitious scientific goal translates into a set of challenging technical requirements, namely: 

Ancillary Science Cases

HERMES will be able to make significant inroads in many other scientific fields. One broad area is stellar astrophysics, in particular globular cluster formation and evolution as well as the study of stellar magnetic dynamos through Doppler imaging. Another is the study of chemical abundances and the energy balance of the interstellar medium in our Galaxy and in the Magellanic clouds. Repeated precise (~10-20 m/s) line of sight (radial) velocity measurements will open up the study of multiple stellar systems. An optional (very large) iodine cell would boost this precision by an order of magnitude, opening the exciting field of exo-solar planet detection around a huge sample of faint stars, up to V ~ 14.

Some fine-tuning of the HERMES instrument will be needed to open this large scientific palette. That includes shifting the spectral windows on the detectors and/or getting a higher spectral resolution ~ 50,000. This will be achievable through exchanges of optical components (slits, gratings and/or beam splitters) inside the HERMES spectrograph. For operational reasons, this must be done during the day and must require only one trained technician during a few hours. The instrument's detailed design incorporates this somewhat challenging requirement.

One important requirement for any astronomical instrument is its stability. The current constraint for the HERMES spectrograph is an end-to-end mechanical and thermal stability of ± 1/10th of a detector pixel during a one-hour exposure. This rather tough limit comes from the precise radial velocity science case. It is also seen as necessary to avoid line depth uncertainties due to detector fringing, especially in the infrared channel; this will be reassessed when the actual detectors are in hand.

A summary of the main science requirements is given in the Table below.

Parameters Galactic Archaeology Stellar Astrophysics Interstellar Medium Radial Velocity Variations
Target V Mag 10-14 for main survey; down to 16.5 for targeted observations Typically brighter than 14 Wide range; strictly not relevant for emission line objects. Typically brighter than 14
λ range (Å) 4708 - 4893
5649 - 5873
6481 - 6739
7590 - 7890
Wide range; in some cases GA windows appropriate. Wide range across optical spectrum Wide range. If using iodine cell, 4800-6000.
R = λ/δλ 28,000 28,000 ~50,000 25,000 - 28,000
SNR 100 at V ~ 14 in 1 hour
(GA Survey)
Wide range Wide range Wide range

A detailed study of the full HERMES science case and the corresponding technical requirements has been developed by the HERMES Science Team and can be accessed here.