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A Way to Other Worlds |
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A Way to Other Worlds |
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Resp. Alessandro Sozzetti
INAF - Osservatorio Astrofisico di Torino
Since the first discovery of a Jupiter-mass companion to a nearby
solar-type star in 1995, high-precision radial-velocity (RV)
measurements have played a pivotal role in gathering the body of
observational evidence that is now ushering us into the era of
''comparative planetology'', in which our Solar System can finally
be placed in the broader context of the astrophysics of planetary
systems. Indeed, ground-based and space-borne photometric survey
programs are today providing crucial data for characterizing the
exciting class of transiting planets. However, the RV technique is
still not only of essential value (e.g., to confirm transiting
planet candidates and determine their actual masses), but is beginning
to explore new, uncharted territories. This is happening thanks to
a) improved sensitivity and stability of the new generation of
high-resolution spectrographs, and b) new experiments which are
being devised to probe more and more in depth our theoretical
understanding of planet formation and evolution.
The discovery and characterization of habitable rocky planets is,
arguably, one of the most exciting scientific endeavours of the
coming years. However, the way forward into the realm of potentially
habitable exoplanets is a challenging one, particularly when seen
in connection to the physical properties of their parent stars. To
this end, ultra-high precision (<1 m/s) RV measurements from the
ground will play once more a fundamental role, as the pioneering
experience of the HARPS instrument on the 3.6m ESO telescope is
there testify. At optical wavelengths, the HARPS-N spectrograph on
the Telescopio Nazionale Galileo (TNG) is bound to surpass the RV
performance of its southern twin HARPS, allowing for deeper
understanding of the architectural properties of low-mass planetary
systems (containing Super-Earths with Mp<10MEARTH) orbiting solar-type
stars. The upcoming ESPRESSO instrument on the VLT will exceed
HARPS-N's RV precision by over an order of magnitude, aiming at
detecting habitable Earth-mass planets around low-mass stars. In
the near-infrared, new instruments such as GIANO@TNG will deliver
m/s-level precision, opening the doors to the detection of Earth-mass
planets in the Habitable Zone of red M dwarfs at the bottom of the
main sequence.
The Italian community is leading relevant efforts in the arena of
exoplanet detection and characterization with high-precision RVs.
In the following section we discuss the status of our ongoing
projects and the related development plans.
HARPS-N is an echelle spectrograph covering the visible wavelength
range between 383 and 693 nm. It is a near-twin of the HARPS
instrument mounted at the ESO 3.6-m telescope in La Silla. It was
installed at the TNG in spring 2012. After instrument commissioning
in late spring and summer 2012, it was offered for open time programs
starting in August 2012. The instrument is located in a
thermally-controlled room, within a vacuum-controlled enclosure to
ensure the required stability, and is fed by two fibres at the
Nasmyth B focus of the TNG. The second fibre can be used for
simultaneous calibration (currently with a Th-Ar hollow-cathode
lamp) or for monitoring of the sky depending on the science goal
and target brightness. Both fibres have an aperture on the sky of
1 arcsec. The spectra are recorded on an E2V 4k4 CCD 231 with a 15
micron pixel size. The resulting sampling is about 3.3 pixels (FWHM)
and the spectral resolution is about 115,000.
INAF astronomers lead a large community effort for the exploitation
of HARPS-N's surgical RV precision through a long-term observational
program dubbed GAPS (Global Architecture of Planetary Systems).
INAF is also directly involved in the scientific exploitation of
the HARPS-N GTO Program both in terms of the follow-up of candidate
transiting planets in the Kepler field and of a rocky planets planet
search around nearby, quiet solar-type stars. Currently, HARPS-N
delivers high-precision RVs, typically achieving ~1 m/s precision
on a V=12 mag solar-type star in 1-hr integration time. However,
the goal of accurately modelling orbits of planets with masses
similar to Earth's implies the detection of RV amplitude close or
even below the single-measurement precision of HARPS-N. Simultaneous
calibration with a reference lamp is required in order to follow
small changes in radial velocity induced by the instrument and local
environment, but currently used Th-Ar sources have intrinsic, single
line accuracy of tens of m/s and the use of ~10000 line averages
increases substantially but not sufficiently the precision of a
spectrograph. In turn, hundreds, thousands of measurements over
months and years are then necessary to tackle planets orbiting stars
which by reflex move at ~1 m/s around the common centre of mass.
Astronomical use of laser comb (astrocombs) as simultaneous calibration
sources will improve the precision of single-line position measurement,
as well as greatly increase the number and homogeneity of lines in
the various orders, allowing overall precision of few cm/s.
In collaboration with the Center for Astrophysics (CfA), we are
conducting an experiment (partially funded by NSF, USA) at TNG with
the HARPS-N spectrograph in order to a) evaluate a new fiber-based
astrocomb at the CfA laboratory and then b) test it as a new upgrade
of the HARPS-N instrument. The new astrocomb will be constructed
from the combination of three elements: (i) a laser frequency comb
producing a dense comb of delta function lines evenly spaced in
optical frequency and referenced to an atomic frequency standard;
(ii) a highly nonlinear optical fiber used to coherently shift near
infrared light generated by the laser frequency comb into the
wavelength range desired for spectrograph calibration; and (iii) a
Fabry-Perot filter cavity designed to match the resolution of the
astrocomb to that of the spectrograph.
The timeline envisioned for
experiment completion is of three years.
GIANO is an high resolution cross-dispersed spectrograph covering,
in a single exposure, the complete near infrared wavelengths range,
from 0.95 to 2.43 microns. With a resolving power of R=50,000 it
is the first and only instrument in the world which can extend to
near infrared wavelengths the type of study currently performed
with HARPS-N at visual wavelengths. GIANO can effectively study
extrasolar planets around low-mass stars (late M-dwarfs) which are
too red for HARPS-N. GIANO was recently (July 2012) commissioned
at the Telescopio Nazionale Galileo.
The first results indicate that a radial velocity accuracy of about
15 m/s can be achieved using the telluric absorption lines as
reference source for wavelength calibration. This limit is set by
the systematic variations which are intrinsically related to the
telluric lines. However, the accuracy can be increased by an order
of magnitude (i.e. to 1.5 m/s) by 1) upgrading the fore-optics
system which takes the light from the telescope to the two fibres
of GIANO. This would allow us to take simultaneous spectra of the
star and of the wavelength calibration source through the two fibres,
and 2) substituting the current wavelengths-calibration source (U-Ne
lamp) with a stabilized Fabry-Perot or a laser-comb (taking advantage
of the experience that will be acquired during the development of
the astrocomb for HARPS-N), which produce a regular pattern of lines
with similar intensities over the whole wavelengths range. This
would provide a proper lambda-coverage of the K-band (where there
are just a few U-Ne lines) and avoid spurious effects (saturation,
ghosts etc.) produced by the bright (and least useful for calibration)
Ne lines, whose intensities are up to 3 orders of magnitude higher
than the U-lines. In particular, the first element of activity,
including then opto-mechanical design of the new fore-optics, bidding
and procurement of optics and mechanics, assembling and verification
in the INAF-Arcetri labs, shipping to the TNG and commissioning at
the telescope, is expected to last 1 year, while the second activity
will require 2 years.
Providing a wide wavelength coverage from the visible to the near-IR
is extremely desirable for the optimization of the scientific output
from key program elements carried out with both HARPS-N and GIANO,
such as the search and characterization of low-mass planetary systems
around M stars. For example, simultaneous visible through near-IR
observations can yield the highest possible precision for targeted
M-type stars while permitting at the same time the discrimination
of false positive RV signals caused by stellar activity. Activity-induced
RV variations are expected to be wavelength-dependent, which is
strictly not the case for orbital variations. The wavelength
dependence of activity-induced RV signals will result in at least
a factor of 2 to 3 different amplitude in the range 500-2000 nm,
and thus provide an efficient and safe way to discard spurious
signals. The wavelength dependence will also yield valuable information
on the spot temperature and distribution. Modelling of light and
colour curves of cool spotted stars show that near-IR colors
(comparing V with I or K fluxes) are more sensitive than visible
colours to differences in the spot distribution or structure
(two-component spots vs. solid spots, distribution over the surface,
etc.). The presence of bright facular components can even produce
blue colour curves in anti-phase with respect to the near-IR ones.
The effect of these spot structures on RVs is not yet known but
similar differences might be expected, hence the importance of a
large wavelength coverage reaching out to the near-IR bands. As a
bonus, the high-resolution near-IR data will allow us to study the
target stars with unprecedented detail, including their full
characterization regarding atmospheric parameters and activity, and
possibly carry out powerful diagnostics via astroseismic analyses.
We plan to carry out an upgrade at the TNG in order to perform
simultaneous spectroscopic observations from the visible to the
near-infrared wavelength range with both HARPS-N and GIANO. We have
envisaged two possibilities for such an upgrade at the TNG, taking
into account the actual operations of HARPS-N and the still tuneable
integration of the infrared arm GIANO. Basically the dichroic optical
element will share the incoming light at around 800-900 nm, redward
of HARPS-N wavelength range, but the actual value will be traded
off with the foreseen GIANO performances and leaving open the
possibility to add other instruments or simply technological
demonstrators at the same Nasmyth focus. In practice, option 1)
will entail the transformation of the M4 mirror into a dichroic,
while option 2) encompasses the possibility of inserting a dichroic
in the HARPS-N Front End Unit after the Tip/Tilt mirror. The foreseen
activities will be divided in two phases, lasting a total of two
years. Initially, a feasibility study will assess the merit of
choosing option 1) or 2), and the chosen option will then be
implemented.
ESPRESSO, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, is a super-stable Optical High Resolution Spectrograph built by an European Consortium including INAF-OATs, INAF-OABr, ESO, and Portuguese, Spanish and Swiss institutes. The instrument will be mounted on the combined Coude' focus of the VLT. It can be operated by either one of the UTs (with the primary goal of obtaining unprecedented radial velocity precision) or collecting the light from up to 4 UTs simultaneously in a configuration equivalent to a 16m telescope (to measure precisely faint or high redshift objects). The main scientific driver for ESPRESSO is the measurement of high precision radial velocities of solar type stars for search for rocky planets in habitable zones around dwarfs G-M. This science case requires an efficient, high-resolution, extremely stable and accurate spectrograph. In particular, with an expected single-measurement RV precision better than 10cm/s, Earth-mass planets in the Habitable Zone of stars not much different from our Sun can be detected. INAF has the responsibility of the front-end, of the optical design, of the electronics and control software and data analysis, in addition to the role of Project Scientist and System Engineer. In particular, this is the first ESO instrument for which the consortium will provide a Data Analysis (DA) package tailored on the science cases of the instrument itself. We foresee two DA branches, one for quasar spectra and one for star spectra. The input will be the products of the Data reduction pipeline and the output are meant to be accurate scientific quantities and not just quick look results. For star spectra, we will provide tools to determine the radial velocity, the bisector, activity indexes, stellar parameters (for FGK dwarfs). ESPRESSO is the successor of HARPS and the predecessor of HIRES for ELT. ESPRESSO will start the operations in 2016.