HERMES SCIENCE OVERVIEW

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HERMES SCIENCE OVERVIEW Ken Freeman RSAA, ANU
AAO February 26, 2008
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HERMES spectral resolution 30,000 400 fibres
over ? square degrees four non-contiguous echelle
orders
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Galactic Archaeology Other HERMES science
Stellar physics Galactic bulge
LMC Globular clusters
Interstellar medium
HERMES science will benefit greatly from GAIA
which will start generating data around 2015
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The goals of galactic archaeology
We seek signatures or fossils from the epoch of
Galaxy formation, to give us insight about the
processes that took place as the Galaxy
formed. Aim to reconstruct the star-forming
aggregates that built up the disk, bulge and halo
of the Galaxy
Some of these dispersed aggregates can be still
recognised kinematically as stellar moving
groups. For others, the dynamical information
was lost through disk heating processes, but
they are still recognizable by their chemical
signatures (chemical tagging).
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A major goal is to identify how important
mergers and accretion events were in building up
the Galactic disk and the bulge. CDM predicts a
high level of merger activity which
conflicts with many observed properties of disk
galaxies.
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The galactic disk shows kinematical substructure
usually called moving stellar groups. The stars
of the moving groups are all around us
Some are associated with dynamical resonances
(bar) or spiral structure (eg Hercules moving
group) Some are debris of star-forming
aggregates in the disk (eg the HR1614 moving
group), dispersed into extended regions of
the Galaxy Others may be debris of
infalling objects, as seen in ?CDM
simulations (eg Arcturus moving group)
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Now look at the HR1614 group (age 2 Gyr,
Fe/H 0.2) which appears to be a relic of a
dispersed star forming event. Its stars are
scattered all around us. This group has not
lost its dynamical identity despite its age. De
Silva et al (2007) measured accurate differential
chemical abundances for many elements in HR1614
stars, and finds a very small spread in
abundances.
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HR1614 moving group stars the (U,V) plane
The small tilt is expected because epicyclic
theory is not valid for these larger V-values.
De Silva et al 2007
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HR 1614 o field stars
The HR 1614 stars (age 2 Gyr) are chemically
homogeneous. They are probably the dispersed
relic of an old star forming event.
De Silva et al 2007
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Expect that most debris from star forming events
in the thin disk and thick disk will have phase
mixed and become unrecognizable kinematically.
If the original star forming aggregate was
chemically homogeneous, its stellar debris will
have retained its chemical abundance
distribution over many elements Even if we
cannot recognize its stars from their kinematics,
we can identify them from their common chemical
signature (chemical tagging)
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Galactic Archaeology with HERMES
Imagine a large complete stellar survey down to
V 14 (matches the fiber density) Cover about
half the southern sky (b 30) 10,000 square
degrees 3000 pointings gives 1.2 x 106
stars At V 14, R 30,000, expect SNR 100
per resolution element in 60 minutes Do 8
fields per night for 400 clear nights (bright
time program)
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Fractional contribution from galactic
components Dwarf Giant Thin
disk 0.58 0.20 Thick disk 0.10 0.07 Halo 0.0
2 0.03
Old disk dwarfs are seen out to distances of
about 1 kpc Disk giants ________________________
______ 5 Halo giants __________________________
____ 15
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About 9 of the thick disk stars and about 14
of the thin disk stars pass through our 1 kpc
dwarf horizon Assume that all of their formation
aggregates are azimuthally mixed right around
the Galaxy, so all of their formation sites are
represented within our horizon
For the halo, theWFMOS halo giants are visible
out to 15 kpc, so we sample a large fraction of
the galactic halo
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A complete random sample of 1.2 x 106 stars
with V thick disk dwarfs from each of about 4,500 star
formation sites 10 thin disk dwarfs from each
of about 35,000 star formation sites
A smaller survey means less stars from a
similar number of sites
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Can we detect 4500 different thick disk
sites using chemical tagging techniques ? Yes
we would need 7 independent chemical
elements each with 4 measurable abundance levels
to get enough independent cells (47) in
chemical abundance space. Are there 7
independent elements or element groups ? Yes
light elements (Na,Al) Mg
other alpha-elements (Ca, Si, Ti) Fe
and Fe-peak elements light s-process
elements (Sr,Zr) heavy s-process
elements (Ba) r-process (Eu)
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GAIA is a major element of a WFMOS-A survey
WFMOS and GAIA

• GAIA ( 2015) will provide precision astrometry
  for about 109 stars
• For V 14, ?? 10 ?as, ?? 10 ?as yr -1
  this is GAIA at its best
• (1 distance errors at 1 kpc, 0.7 km s -1
  velocity errors at 15 kpc)
• accurate transverse velocities for all stars in
  the WFMOS-A
• sample, and
• accurate distances for all of the survey stars
• therefore accurate color-(absolute magnitude)
  diagram for all
• of the survey stars independent check
  that chemically tagged
• groups have common age.

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HERMES GAIA will give accurate abundances and 3D
motions, and isochrone ages for subgiants in the
thin disk, thick disk and halo
subgiants are numerous (about 10 of the
sample) and are observable out to about 1 kpc
Bertelli et al 1994
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The ultimate goal of the archaeology program is
unravelling the star formation history of the
thick disk and halo via chemical tagging.
In the shorter term data products include
distribution of stars in position, velocity,
chemical space for a million stars
(isochrone ages for about 200,000)
distribution of Fe/H, ?/Fe and X/Fe for vast
samples of stars from each component
thin and thick disks, halo. detailed
abundance gradients in each component
chemical and kinematical correlations in inner
and outer thick and thin disks
Pre-GAIA, we will have photometric parallaxes (
10) and groundbased proper motions ( 2 mas/yr)
for most stars.
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Looking at a 7-10 year program, including
construction The timing meshes very nicely with
the GAIA schedule. GAIA will detect a lot of
phase space substructure. From the HERMES
survey, we will be able to determine which
phase-space substructures are real and which are
dynamical artifacts (resonances). AAO will have
this field to ourselves no obvious
competition on the horizon (Hectochelle,
MIKE-fibers, FLAMES do not compete with HERMES
in overall scope fiber numbers, field,
wavelength coverage, resolution)
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Great opportunities for PI science by AAO
community from such a very large uniform data
set. Input catalog would be planned in
consultation with community to meet scientific
goals of teams.
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We asked Martin Asplund (MPA) what could such
a survey do for stellar astrophysics ?
Imagine you were able to get R 30,000 spectra
over 400A for a million stars, most having 1
distance errors. Can you conceive of important
stellar physics programs with these data ?
His response this is a goldmine for studies
of stellar evolution, nucleosynthesis and
internal mixing excellent for identifying
rare kinds of stars derive accurate ages for
subgiants of different populations test mixing
length theory across the HR diagram ideal
database for deriving the primordial helium
abundance
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Other HERMES PI science some with longer
exposures, down to V 15 some of this
science involves GAIA also
Abundances of large samples of stars in the
outer disk of the Galaxy the galactic
abundance gradient in young and old stars
Chemical evolution in the galactic bulge,
complemented by astrometry from JASMINE
(similar quality and schedule to GAIA) Globular
clusters, open star clusters and
superclusters membership, dynamics and
detailed abundance distributions AGB stars in
the Magellanic Clouds s-process element
evolution ISM in Galaxy, Magellanic Clouds and
other nearby galaxies
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So far, considered only high SNR ( 100) science
with HERMES reaching to V 14-15. Interesting
opportunities for lower SNR ( 10) observations
which would go much fainter (V 18-19)
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Carney et al (1987) R 30,000 spectra with
SNR 10 per resolution element give M/H
estimates with errors of 0.12. Radial velocity
errors are 24
This capability would allow work on the outer
disk and halo of the LMC. e.g. are there very
metal-poor stars present in the LMC - Fe/H -3. Such stars are seen in our Galaxy but not
(yet ?) in the nearby dwarf spheroidal galaxies.
Helmi et al 2006
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The next two talks are on important technical
issues Liz Wylie on the feasibility of a
chemical pipeline Simon Ellis on the choice
of wavelength windows
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WFMOS-A GAIA will give accurate abundances and
3D motions, plus direct isochrone ages for most
of the subgiants in the old thin disk, thick disk
and halo
for the subgiants, abundances and GAIA
luminosities give their ages directly
subgiants are numerous (about 10 of the
sample) and are observable out to about 1 kpc