HIFI – The instrument, its capabilities and the proposed

science

Frank Helmich

SRON – National Institute for Space Research

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Overview

• Introduction to HIFI and its capabilities– Specifications– Facts and figures– Observing modes and AOT’s

• Some images• The proposed science

– General– Spectral surveys– The water Universe– Specific topics (for key programs)

• Preparations for interpretation of the data

• Note that this cannot be in presented in great depth

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Facts & Figures

• HIFI (Heterodyne Instrument for the Far-Infrared) is just an ordinary heterodyne spectrometer with 6 frequency bands over 7 channels!

• But, HIFI is complex and very sensitive!!– Band 1: 480-640 GHz;– Band 2: 640-800 GHz; – Band 3 800-960 GHz; – Band 4 960-1120 GHz; – Band 5: 1120-1250 GHz; – Band 6L+6H: 1410-1910 GHz– 480 GHz is 625 micrometer; 1910 is 158 micrometer– Bands 1-5 SIS technology, Band 6 HEB technology

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Focal Plane Unit

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Facts & Figures II

• At 480 GHz the beam is 42”; at 1910 GHz 13”

• Tsys(SSB) = 200-500K (480-1250 GHz);

• Tsys(SSB) = 1600K in Band 6

• Bandwidth is up to 4 GHz; • Resolution of 135-270-539-1100 kHz

100

1000

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Frequency (GHz)

Baseline Band 1, LERMA Band 2, KOSMA, 4K Band 2, KOSMA, 2K Band 3, SRON Band 4, SRON Band 5, CalTech Band 6, CTH

State-of-the-art mixers

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Facts & Figures III

• HIFI is on-axis in Herschel• HIFI has a cold focal plane (15K) in which the mixers themselves are

cooled to about 4 K• The sky and LO-signals are combined through beam-splitters and

diplexers (Martin-Puplett interferometers)• The LO signal is generated in the service module and send through

waveguides to the focal plane• The sky signal goes from primary to the (stiff) subreflector, and via M3

the 7 (14) sky beams enter HIFI • HIFI has two loads in its Calibration Source Assembly (100 and 15K)• Chopper chooses between loads, sky-on and sky-chop

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Observing Modes & AOTs

• HIFI is thus a sensitive, versatile instrument ideally suited for measurements of high spectral resolution – Very deep for a single frequency

– Relatively shallow over a large (even complete) wavelength range

– Essentially a single-pixel instrument (map by moving the telescope)

H IFI Observing Modes

P os it ion s w itc h F req u en c y sw itch

M ap p in g m od e

D u a l b eam sw itch F req u en c y sw itch

P os it ion s w itc h L oad ch op (w /O F F )

P o in t/sm a ll sou rc em od e

D u a l b eam sw itch

F req u en c y su rveym od e

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Science• HIFI is very well suited for any measurement

requiring very high spectral resolution in the Far-IR/submm region

• Special emphasis on:– The water Universe – water in all kinds of

astrophysical environments– Spectral surveys – unbiased frequency sweeps

The life cycle of gas and dust

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Water

OH & H3O+ are also available in the HIFI freq. range!

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Unbiased frequency scans

IRAS 16293-2422 on JCMT

E. Caux et al.

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Star-formation

Different stages have different characteristics!

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The warm ISM

• Photon-dominated regions and shocks– Determining physical,

chemical and kinematic conditions. Energy balance

– Stringent tests for PDR models

– High spectral resolution needed!

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Late stages of stellar evolution

2mm spectral survey of Pardo & Cernicharo

• Water and high-J lines of CO are major targets

• Spectral surveys for selected sources

Water in W Hya (Barlow et al. ‘96)

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ISM in other galaxies

• Many subtopics, – all specifically aimed at

spectrally resolved lines from HIFI

– And unresolved lines (in maps) from PACS and SPIRE

– Large ground-based component

Fischer 1997

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Solar System

• Water in the giant planets: what is the origin? -> Volume mixing ratios from line profiles

• The chemical composition of the Mars atmosphere

• The chemical composition of cometary material: especially water at different distances from the Sun

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Hydrides & macro molecules

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Preparation

• While HIFI spectra may present you with either complex or straigthforward (Gaussian) profiles, interpretation is always complex– Influence of unknown beam-source coupling;

– different physical (temperature and density) environments in one single beam

– abundances can change over several orders of magnitude on short distances

– Influence of dust or optical depth in excitation of the molecules e.g. mid-IR pumping

– Geometry: e.g. (flared) disks and outflows in spherical (or sheet-like) cores

• Many models will be required

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Preparation II

• Rate coefficients of many molecules with H2 unknown• Rate coefficients of many molecules with He unknown• Frequencies at THz wavelengths often unknown• Chemical reaction rates often unknown or guesses• Radiative transfer often approximated or 1-D

• This also is true for ALMA so we need vast investments in solving all of the above problems– DLR funds spectroscopy in Cologne

– EU funds FP 6 RTN – The Molecular Universe

– Herschel preparatory science groups (own funds)

– More is needed !

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Conclusions

• HIFI is the instrument of choice for any high spectral resolution measurements in the Far-IR/submm– Water

– Spectral surveys

• HIFI data is easily reduced (at least the single spectra)• Interpretation of HIFI data is not straightforward: Prepare well!

– Radiative transfer models

– Chemical models

– Laboratory date and quantum-chemical calculations

– E.g. FP6 RTN “The Molecular Universe” and groups for Herschel Preparatory Research

• HIFI data can be very well combined with PACS, SPIRE and ALMA data