HAWC High-resolution Airborne Wideband Camera

The Facility Far-Infrared Imager for SOFIA

How Are We Coming along?

June 2010 SIDAG Review: HAWC Science1

HAWCHigh-resolution Airborne Wideband Camera

• Facility far infrared camera for SOFIA: spectral range 40-300 µm• Built by University of Chicago in collaboration with Goddard Space Flight Center and

Rochester Institute of Technology• Four filters at 53, 88, 155, and 215 µm, 0.1<<0.2• 12 × 32 Pop-up Detectors (PUD) array cooled to 0.2˚K with adiabatic demagnetization

refrigerator (ADR)• Design optimized to give the highest possible angular resolution at each wavelength• Science: formation and evolution of stars, galaxies, and planetary systems; physics of dust and

the interstellar medium• Status: hardware complete, except for fabrication of handling cart and fore-optics assembly.

Tests, software development, and documentation ongoing at University of Chicago.• Future development potential: polarimetry, larger detectors, additional passbands

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2.3 HAWC Science

Many infrared sources are dusty. Absorption of starlight typically heats the dust grains to temperatures of tens to hundreds of degrees Kelvin. The dust grains radiate most of their energy in the far-infrared at wavelengths of 40-300 µm . This spectral region is inaccessible from the ground. Far-infrared imagery is the natural starting point from which to develop an understanding of source energetics and morphology. It also plays a central role in studies of the physics and chemistry of the interstellar medium.

SOFIA's combination of sensitivity and angular resolution will make it possible to study the evolution of stars, planetary systems, and galaxies in unprecedented detail. SOFIA will provide the best far-infrared images for comparison with the wealth of arcsecond-scale data now available at other wavelengths. Some of the scientific problems which will be addressed include the following (for additional details, see the HAWC proposal and SOF 1009):

• The formation of stars and stellar clusters within our galaxy• Star formation in external galaxies• Structure and evolution of protoplanetary and remnant disks• The return of gas and dust to the interstellar medium from stars• The composition and life-cycle of interstellar dust• The interactions between young stars and their environments• Conditions in regions surrounding active galactic nuclei.

2.1 HAWC and Its Mission

When SOFIA enters operation, it will be the largest far-infrared telescope available, so it will have the best intrinsic angular resolution. HAWC is a first-generation facility instrument for SOFIA. It is a far-infrared camera designed to cover the 40-300 µm spectral region at the highest possible angular resolution. HAWC's goal is to provide a sensitive, versatile, and reliable far-infrared imaging capability for the astronomical community during SOFIA's first years of operation.

HAWC ScienceFrom HAWC Project Implementation Plan (Dec. 1999)

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HAWC Science- 2012• HAWC’s primary mission and high-level science requirements remain essentially the

same as those outlined in the PIP– When the Herschel mission ends in 2012, HAWC/SOFIA will provide the astronomical

community’s principal access to the far infrared spectral region– We expect the interest and the science opportunities in this spectral region will only increase

because of the legacy of Spitzer and Herschel– For many Spitzer and Herschel sources, effective follow-up will depend not primarily on

photometric sensitivity but on the ability to provide more detailed spectral and spatial information, particularly on spatially complex sources

– HAWC will be able to monitor time-variable phenomena over intervals up to 20 years– Far infrared polarimetry with HAWC will be an entirely new science opportunity

• The HAWC instrument provides a versatile and robust platform that can be commissioned soon and progressively enhanced with additional capabilities such as:

– Additional filter passbands– Polarimetry– Larger detector arrays

• In the second half of our presentation (Status and Plans), we will present a plan for commissioning and developing HAWC that we believe will maximize HAWC’s scientific capabilities and its usefulness to the astronomical community while working within the budgetary constraints expected in FY 11 and FY 12

• In this part of our presentation, we discuss some of the types of science that HAWC will be able to address and, in particular, the unique new opportunities enabled by polarimetry

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HAWC Spectral & Imaging Specifications

HAWC currently has four spectral passbands with spectral resolution of R~5. There is a separate lens for each band that re-images the telescope focal plane such that there are two pixels per diffraction-limited beam for each band. The lens/filter carousel has six positions, only four of which are currently filled, so two additional bands can be easily added. There is also an eight-position pupil carousel which can carry different pupil masks and/or auxiliary narrow-band filters. The design goals were to non-redundantly sample the continuum emission from large dust grains in the 40-300 µm spectral region inaccessible to ground-based telescopes while preserving the maximum possible angular resolution and minimizing the residual atmospheric absorption in each band.

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HAWC Photometric & Polarimetric Specifications

Quantity Units Band1 Band2 Band3 Band4

Central Wavelength μm 53 89 155 216

Bandwidth Δλ/λ 0.16 0.19 0.22 0.21

Pixel Size arcsec 2.25 3.50 6.00 8.00

Image Diameter (Diffraction-limit FWHM) arcsec 5.4 9 16 22

Pixels per Airy FWHM 2.4 2.6 2.6 2.7

Detector array field of view arcsec 27×72 42×112 72×192 96×256

Detector array areal filling factor % >95 >95 >95 >95

Mean transmission (cold optics & detector QE) % 15 21 29 25

Mean transmission (warm optics & vacuum window) % 34 43 42 53

Mean transmission (atmosphere, 10μm p.w.v., 40° elevation) % 73 74 65 81

Background Power, per pixel pW 52 51 54 24

NEP (thermal background), per pixel fW Hz-1/2 63 50 42 24

NEFD (chopped, background limited, AΩ=λ2) Jy s1/2 beam-1 0.94 0.61 0.61 0.49

NEFD (chopped, background limited, AΩ=λ2) mJy (4σ, 900s) pixel-1 57 35 34 26

Polarization uncertainty (single-beam, 5 Jy source, 1 hour) %P 0.63 0.41 0.41 0.33

Position Angle uncertainty (single-beam, P=3%,5 Jy source, 1 hr) degrees 6.0 3.9 3.9 3.1

Polarization uncertainty (dual-beam, 5 Jy source, 1 hour) %P 0.44 0.29 0.29 0.23

Position Angle uncertainty (dual-beam, P=3%,5 Jy source, 1 hr) degrees 4.2 2.8 2.8 2.2

(dual-beam = simultaneous measurement of 2 orthogonal polarizations; single-beam = one polarization mode only)

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HAWC

Sensitivity and Angular Resolution

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HAWCFORCAST

Emission Spectrum of Interstellar Dust

•example of Diffuse ISM (e.g., Compiegne et al. 2010)•dense ISM has similar components, different ratios

HAWC covers the emission peak of large interstellar dust grains emitting at 50 – 250 μm with temperatures in the range 12 – 120 Kelvin.•Fits to SED yield information on column density or optical depth τ(λ), temperature T, and emissivity spectral index β.•Estimates of total mass & total luminosity are not possible without data spanning this peak•HAWC will be particularly well suited to constrain temperatures in the range 12-50 K 8

• T-β anti-correlation may be intrinsic property of grains• Requires SED fits at multiple points in cloud• Would like to do so in different physical

environments to understand how grain composition is affected

• There exists degeneracy between T and β in fits which is often not accounted for. At best, Paladini et al. (2010) sees 2σ detection of real anti-correlation.

• Improving this requires more wavelengths & narrower passbands. These are provided by HAWC.

• T-β anti-correlation may be intrinsic property of grains• Requires SED fits at multiple points in cloud• Would like to do so in different physical

environments to understand how grain composition is affected

• There exists degeneracy between T and β in fits which is often not accounted for. At best, Paladini et al. (2010) sees 2σ detection of real anti-correlation.

• Improving this requires more wavelengths & narrower passbands. These are provided by HAWC.

HAWC

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• Wish to determine temperature, column density, and spectral index of multiple-T components.

Cannot do with only 4 λ's. • HAWC can add more λ's, lowering uncertainties

• also (λ/Δλ)HAWC ≈ 2.5(λ/Δλ)Herschel; helps separate background from clump

• HAWC can be enhanced with new filter centers and bandwidths, tuned to specific science goals

HAWCband centers

SOFIA Science Case by Staguhn et al. propose observing 28 IRDCs with HAWC(http://www.sofia.usra.edu/Science/science_cases/Staguhn2008.pdf)

Infrared Dark Clouds (IRDCs)Dense molecular clouds seen in extinction against NIR background

(possible sites of pre-stellar cores)

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HAWC follow-up of Herschel discoveries, one example

•new protostars seen with Herschel, but not Spitzer. (number "4" in 70 μm image below)•Short-λ follow-up is required to classify age (Class I, II, etc.) and constrain mass.•(also, too dim for FORCAST at 24 – 38 μm)

Sensitivity limit (4σ) for 8 minute HAWC integration

Bontemps et al. (2010)

HAWC beams: 53 and 89 μm

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60 μmKAO

100 μmKAO

350 μmCSO

All data smoothed to ~ 30''the KAO resolution at 100 μm

38 μmKAO

True-color FIR images

arrayfootprint

10'' beam

Polarimetry with SOFIA

thanks to D. Chuss, J. Dotson, D. Dowell,R. Hildebrand, G. Novak, M. Houde, J. Vaillancourt, et al.

Broadest Goals:•constrain the effect of magnetic fields on ISM evolution (and vice versa), from the scales of proto-stars to galaxies•study the physics of interstellar grains, their properties, and their interaction with B-fields

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Why do polarimetry from SOFIA?

• covers a unique niche: No other observatory in active development (orbital, sub-orbital, or ground) will have polarization capabilities at λ = 50−200 m

• signal-to-noise: observe at peak of GMC spectrum; e.g., SOFIA Design Reference Mission Case Study by Novak et al. 2005 estimates 100,000 vectors in nine fields during 113 hours of observation

• angular resolution: 5 arcsec at 53 m; 3 mpc at Oph

• magnetic fields are ubiquitous in astrophysics having an influence on cloud, star, and galaxy formation

• polarization spectrum: Grain alignment prescription is a key input for any cloud/core polarization model. Many experiments provide access to the “submm rise”, but only SOFIA will provide access to the “far-IR drop”

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Example SOFIA/HAWC Coverage &Polarization sensitivity

contour: pointings where (P) < 0.3% in 4 minutes with SOFIA

KAO polarization map: Schleuning (1998)

polarimeter will be able to measure polarizations with un- certainties as low as 0.3 % for source intensities of a few Jy/beam. (1% polarization is a typical value for ISM clouds at 100 μm.) This is comparable to the sensitivity of other instruments expected to come online in the next decade. A SOFIA polar- imeter will provide important complementary short-wavelength coverage.

With only modest integration times (≈ 1 hr.) a SOFIA

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Multi-wavelength polarimetry & the pol.-spectrum

BLASTpol SCUBA-Pol2

HAWCpolW3

60 m100 m350 m850 m

W3

4 %

arcm

inut

es

arcminutes

• Different wavelengths trace different grain temperature and emission components along the line of sight.

• Therefore, it also traces different magnetic field geometries at different cloud depths• Grain alignment, temperature, and emission properties depend on the radiation field, so mapping SEDs

across a cloud tests theory in a diverse range of environments.• SOFIA/HAWC can extend current measurements to new frontiers including: the diffuse ISM, infrared

cirrus clouds, starless environments, infrared dark clouds (IRDCs)

FIR-drop(from embedded stars?) Sub-mm rise

(equilibrium w/ ISRF?)16

New Frontiers − Diffuse Clouds

SOFIA/HAWC polarimeter can:•investigate the role of magnetic fields in filamentary and other diffuse clouds

• measure the polarization spectrum in diffuse regions to inform upcoming CMB polarization experiments

• compare with grain alignment and theoretical predictions

Where:• ≈ 50 filamentary structures identified by

Jackson, Werner & Gautier (2003) are observable with a SOFIA/HAWC polarimeter

• IRAS (and now Herschel) discovered the highly filamentary structure of the diffuse

Galactic ISM, as well as a range of other structures shown in the picture. Are these

structures defined by the local magnetic field?

3°

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Turbulent Power Spectra & Polarization Angles

Pol'n-Angle Power Spectrum

Numerical Simulations with MHD andself-gravity (Lazarian et al.)

slope changes with Mach number

Measurements (Vaillancourt,

Houde, Hildebrand,

et al., 2008 − 2010)

turbulent part of angular dispersion functionturbulent correlation

length, ≈7.3'' ≈ 16 mpc

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The Galactic Center Magnetic Field

Outstanding Questions1. What is the geometry of the magnetic field?2. What is the strength of the magnetic field?3. How do the radio filaments form?

What can a SOFIA polarimeter do?• Study B-fields threading warm dust at high

spatial resolution (5 – 20 arcseconds)• Multi-frequency observations sample different

dust populations along the line of sight• High sensitivity will probe the interaction sites

between filaments and associated molecular clouds

• Bright large-scale non-thermal filaments indicate B-fields perpendicular to the Galactic plane• Large-scale submillimeter polarimetry implies a magnetic field parallel to the Galactic plane• The small-scale field observed by submillimeter polarimetry is quite complex

100 μm (KAO) and 350 μm (CSO) polarimetry

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External Galaxies

•Nuclei of Starburst Galaxies• Where does the vertical field

transition to planar?• What is the strength of the

magnetic field? The turbulent component?

•IR Bright Galaxies• Are there blowouts?• What is the strength of the

magnetic field? The turbulent component?

There exist many outstanding questions about the global role of magnetic fields in galaxies. Addressing these questions requires multi-wavelength polarimetry. Some of the issues a SOFIA

polarimeter may address include:

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From large-scales to small-scales with SOFIA/HAWC

Red = 350 μm (Stephens, Dowell, et al.) Black = optical (Heiles 2000)

inferred B-field direction

SHARP

SMA

B-fields affect star/gas dynamics on a large range of scales(kpc to fractions of an AU). Theoretical tests need observations whichfollow these connections across size-scales. SOFIA/HAWC provides info. on intermediate scales (few arcsec – few arcmin), in the gap between interferometers like SMA and ALMA (0.1's – few arcsec) and the largest scales (degrees) covered by CMB experiments like Planck.

2'

10''4'

1°

NGC 6334 (Novak et al. 2009)

(Attard et al. 2009)

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Summary

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HAWC Spectral & Imaging Specifications

HAWC currently has four spectral passbands with spectral resolution of R~5. There is a separate lens for each band that re-images the telescope focal plane such that there are two pixels per diffraction-limited beam for each band. The lens/filter carousel has six positions, only four of which are currently filled, so two additional bands can be easily added. There is also an eight-position pupil carousel which can carry different pupil masks and/or auxiliary narrow-band filters. The design goals were to non-redundantly sample the continuum emission from large dust grains in the 40-300 µm spectral region inaccessible to ground-based telescopes while preserving the maximum possible angular resolution and minimizing the residual atmospheric absorption in each band.

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HAWC’s Unique Contributions to SOFIA

• Far infrared 12×32 pixel camera operating in four bands between 50 and 250 μm- Provides highest possible (diffraction limited) spatial resolution for each band (5 − 22 arcsec)- HAWC will be the ONLY instrument capable of collecting data in this region when Herschel retires in 2012High-resolution spatial mapping of physical properties such as luminosity, total dust mass and mass density,

optical depth, dust temperature, dust emissivity of: star forming clouds, infrared dark clouds, photodissociation regions, planetary debris disks, dusty envelopes around evolved stars, external galaxies

Constrains the peak of the dust spectral-energy-distribution (SED) emitting at temperatures between 15K and 150K (e.g. in dense cloud cores)

• Offers narrower filter bandpass than PACS on Herschel, (λ/Δλ)HAWC ≈ 5.3 ≈ 2.5 (λ/Δλ)Herschel

- Minimizes ambiguities caused by spectral/spatial convolutionUnderstand the stellar content of cold opaque high-mass star forming coresProvide accurate and more dense SED sampling of envelope/accretion disk dynamics, linking formation processes

between low mass and high mass starsConduct broadband photometry to constrain planetary atmosphere temperature profiles

• Offers direct path for enhancement of spatial mapping in the Far-IR- Potential for larger detector array could access the SOFIA telescope’s full 8-arcminute field of view Map the large-scale spatial distribution of photodissociation regions (PDRs) and cooling lines

• Offers direct path for additional combinations of filters, grisms, focal scales, and pupil and coronographic masks - Potential for unique add-on imaging polarimetry capability with minimum impact to instrumentEnable magnetic field mapping in the Galactic Center, protostars and disks, turbulent clouds, and nearby external

galaxies Understand the role of magnetic fields in star-forming clouds and enable statistical tests of turbulent theories of

cloud and star-formationMeasure spatial fluctuations in the magnetic field in the Galactic Center and field geometry of the CMZ Constrain grain size growth in proto-planetary disks & probe the physics of dust-grain alignment in ISM

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Why a facility instrument?

• Overall science quality and quantity need to be greater to address far infrared science of the SOFIA era. Need more resources for instrument and pipeline support than afforded by PI instrument model

• Calibration quality would be improved by flying more often than a PI instrument

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Presentation contributors

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