publications

2025

1. The End of the Road for Far-infrared Reddening Maps? Evidence for Reddening Errors Driven by Changes in PAH Abundance

   Dennis Lee, Brandon S. Hensley, Tzu-Ching Chang, and Olivier Doré

   arXiv e-prints, Aug 2025

   Abs NASA/ADS arXiv DOI

   Accurate correction for extinction by Galactic dust is essential for studying the extragalactic sky. In the low-extinction regions of the Ursa Major molecular cloud complex, we demonstrate that Galactic dust reddening maps constructed from observations of far-infrared emission are insensitive to variations in the abundance of polycyclic aromatic hydrocarbons (PAHs), and, as a result, to PAH-induced variations in reddening. Using galaxy counts to validate various reddening maps, we find evidence that maps based on far-infrared emission erroneously under-predict reddening compared to stellar reddening maps. This underestimation by far-infrared emission based reddening maps – representing the largest discrepancy between maps of up to E(B-V)=0.08 mag – is correlated with the relative brightness of PAH emission. Furthermore, we demonstrate theoretically that changes in PAH abundance via accretion from the gas phase is capable of altering extinction significantly with only minor changes to far-infrared emission. We show that modeling the extinction of Ursa Major using both far-infrared and mid- infrared emission more accurately traces dust extinction variations due to changes in PAH abundance. Finally, we discuss how SPHEREx observations of the 3.3 μm PAH feature are a promising way to overcome this limitation of far-infrared emission.

2. SOFIA Polarization Spectrum of Three Star-forming Clouds

   Erin G. Cox, Giles Novak, David T. Chuss, Dennis Lee, Marc Berthoud, Kaitlyn Karpovich, Joseph M. Michail, Zhi-Yun Li, and Peter C. Ashton

   The Astrophysical Journal, Mar 2025

   Abs NASA/ADS arXiv DOI

   The dust emission polarization spectrum—how the polarization percentage changes with wavelength—serves as a probe of dust grain properties in star-forming regions. In this paper, we present 89–214 \ensuremathμm polarization spectrum measurements obtained from SOFIA/HAWC+ for three star- forming clouds: OMC1, M17, and W3. We find that all three clouds have an overall decreasing polarization percentage with increasing wavelength (i.e., a “falling polarization spectrum”). We use SOFIA and Herschel data to create column density and temperature maps for each cloud. We fit for the slope of the polarization spectrum at each sky position in each cloud, and using the Pearson r coefficient, we probe each cloud for possible correlations of slope with column density and slope with temperature. We also create plots of slope versus column density and slope versus temperature for each cloud. For the case of OMC1, our results are consistent with those presented by J. Michail et al., who carried out a similar analysis for that cloud. Our plots of polarization spectrum slope versus column density reveal that for each cloud there exists a critical column density below which a falling polarization spectrum is not observed. For these more diffuse sight lines, the polarization spectrum is instead flat or slightly rising. This finding is consistent with a hypothesis presented 25 yr ago in a paper led by R. Hildebrand based on Kuiper Airborne Observatory data. This hypothesis is that regions shielded from near-IR radiation are required to produce a sharply falling polarization spectrum.

2024

1. Magnetic Field Alignment Relative to Multiple Tracers in the High-mass Star-forming Region RCW 36

   Akanksha Bij, Laura M. Fissel, Lars Bonne, Nicola Schneider, Marc Berthoud, Dennis Lee, Giles A. Novak, Sarah I. Sadavoy, Thushara G. S. Pillai, Maria Cunningham, and 2 more authors

   The Astrophysical Journal, Nov 2024

   Abs NASA/ADS arXiv DOI

   We use polarization data from SOFIA HAWC+ to investigate the interplay between magnetic fields and stellar feedback in altering gas dynamics within the high-mass star-forming region RCW 36, located in Vela C. This region is of particular interest as it has a bipolar H II region powered by a massive star cluster, which may be impacting the surrounding magnetic field. To determine if this is the case, we apply the histogram of relative orientations (HRO) method to quantify the relative alignment between the inferred magnetic field and elongated structures observed in several data sets such as dust emission, column density, temperature, and spectral line intensity maps. The HRO results indicate a bimodal alignment trend, where structures observed with dense gas tracers show a statistically significant preference for perpendicular alignment relative to the magnetic field, while structures probed by the photodissociation region (PDR) tracers tend to align preferentially parallel relative to the magnetic field. Moreover, the dense gas and PDR associated structures are found to be kinematically distinct such that a bimodal alignment trend is also observed as a function of line-of-sight velocity. This suggests that the magnetic field may have been dynamically important and set a preferred direction of gas flow at the time that RCW 36 formed, resulting in a dense ridge developing perpendicular to the magnetic field. However, on filament scales near the PDR region, feedback may be energetically dominating the magnetic field, warping its geometry and the associated flux-frozen gas structures, causing the observed preference for parallel relative alignment.

2. Magnetic Fields Observed along the East–West Outflow of IRAS 16293-2422

   Frankie J. Encalada, Leslie W. Looney, Giles Novak, Sarah Sadavoy, Erin G. Cox, Fabio Pereira-Santos, Dennis Lee, Rachel Harrison, and Kate Pattle

   The Astrophysical Journal, Jun 2024

   Abs NASA/ADS arXiv DOI

   Magnetic fields likely play an important role in the formation of young protostars. Multiscale and multiwavelength dust polarization observations can reveal the inferred magnetic field from scales of the cloud to core to protostar. We present continuum polarization observations of the young protostellar triple system IRAS 16293-2422 at 89 \ensuremathμm using HAWC+ on SOFIA. The inferred magnetic field is very uniform with an average field angle of 89\textdegree \ensuremath\pm 23\textdegree (E of N), which is different from the \ensuremath∼170\textdegree field morphology seen at 850 \ensuremathμm at larger scales (\ensuremath≳2000 au) with JCMT POL-2 and at 1.3 mm on smaller scales (\ensuremath≲300 au) with Atacama Large Millimeter/submillimeter Array. The HAWC+ magnetic field direction is aligned with the known E-W outflow. This alignment difference suggests that the shorter wavelength HAWC+ data is tracing the magnetic field associated with warmer dust likely from the outflow cavity, whereas the longer wavelength data are tracing the bulk magnetic field from cooler dust. Also, we show in this source the dust emission peak is strongly affected by the observing wavelength. The dust continuum peaks closer to source B (northern source) at shorter wavelengths and progressively moves toward the southern A source with increasing wavelength (from 22 to 850 \ensuremathμm).

3. Modeling the Far-infrared Polarization Spectrum of a High-mass Star-forming Cloud

   Dennis Lee, Che-Yu Chen, Giles Novak, David T. Chuss, Erin G. Cox, Kaitlyn Karpovich, Peter Ashton, Marc Berthoud, Zhi-Yun Li, and Joseph M. Michail

   The Astrophysical Journal, Sep 2024

   Abs NASA/ADS arXiv DOI

   The polarization spectrum, or wavelength dependence of the polarization fraction, of interstellar dust emission provides important insights into the grain alignment mechanism of interstellar dust grains. We investigate the far-infrared polarization spectrum of a realistic simulated high-mass star-forming cloud under various models of grain alignment and emission. We find that neither a homogeneous grain alignment model nor a grain alignment model that includes collisional dealignment is able to produce the falling spectrum seen in observations. On the other hand, we find that a grain alignment model with grain alignment efficiency dependent on local temperature is capable of producing a falling spectrum that is in qualitative agreement with observations of OMC-1. For the model most in agreement with OMC-1, we find no correlation between the temperature and the slope of the polarization spectrum. However, we do find a positive correlation between the column density and the slope of the polarization spectrum. We suggest this latter correlation to be the result of wavelength-dependent polarization by absorption.

2022

1. The Twisted Magnetic Field of the Protobinary L483

   Erin G. Cox, Giles Novak, Sarah I. Sadavoy, Leslie W. Looney, Dennis Lee, Marc Berthoud, Tyler L. Bourke, Simon Coudé, Frankie Encalada, Laura M. Fissel, and 9 more authors

   The Astrophysical Journal, Jun 2022

   Abs NASA/ADS arXiv DOI

   We present H-band (1.65 \ensuremathμm) and SOFIA HAWC+ 154 \ensuremathμm polarization observations of the low-mass core L483. Our H-band observations reveal a magnetic field that is overwhelmingly in the E-W direction, which is approximately parallel to the bipolar outflow that is observed in scattered IR light and in single-dish ^12CO observations. From our 154 \ensuremathμm data, we infer a \raisebox-0.5ex\textasciitilde45\textdegree twist in the magnetic field within the inner 5″ (1000 au) of L483. We compare these new observations with published single-dish 350 \ensuremathμm polarimetry and find that the 10,000 au scale H-band data match the smaller-scale 350 \ensuremathμm data, indicating that the collapse of L483 is magnetically regulated on these larger scales. We also present high-resolution 1.3 mm Atacama Large Millimeter/submillimeter Array data of L483 that reveals it is a close binary star with a separation of 34 au. The plane of the binary of L483 is observed to be approximately parallel to the twisted field in the inner 1000 au. Comparing this result to the \raisebox-0.5ex\textasciitilde1000 au protostellar envelope, we find that the envelope is roughly perpendicular to the 1000 au HAWC+ field. Using the data presented, we speculate that L483 initially formed as a wide binary and the companion star migrated to its current position, causing an extreme shift in angular momentum thereby producing the twisted magnetic field morphology observed. More observations are needed to further test this scenario.

2. The TolTEC camera: polarimetric commissioning and performance of the continuously rotating half-wave plate

   Dennis Lee, Giles Novak, Marc Berthoud, John Bussan, Robert Golenia, Eric Van Clepper, Grant Wilson, N. S. DeNigris, Zhiyuan Ma, Michael McCrackan, and 21 more authors

   In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI, Aug 2022

   Abs NASA/ADS DOI

   An ambient-temperature Continuously Rotating Half-Wave Plate (CRHWP) modulates the input polarization signal thereby enabling removal of low-frequency (1/f) noise from polarized flux measurements. This 1/f noise arises from atmospheric turbulence as well as from effects intrinsic to certain detectors. Here, we describe the design and performance of the half wave plate rotator and achromatic half-wave plate for the the new imaging polarimeter, TolTEC. These components are mounted in front of the cryostat window and operate at ambient temperature. The Half-Wave Plate Rotator (HWPR) spins the half-wave plate at 2 revolutions per second. The rotation mechanism consists of nine air bearings to provide low-friction motion and a frameless torque motor to directly drive rotation. The orientation of the rotor and half- wave plate are recorded using a high-precision optical encoder. We review the experimental requirements and technical design of the rotator as well as the associated electronics, pneumatics, and software.

3. The TolTEC camera: the citlali data reduction pipeline engine

   Michael McCrackan, Zhiyuan Ma, Nat S. DeNigris, Caleigh Ryan, Kamal Souccar, Grant W. Wilson, Itziar Aretxaga, Akanksha Bij, Laura Fissel, Joseph E. Golec, and 6 more authors

   In Software and Cyberinfrastructure for Astronomy VII, Aug 2022

   Abs NASA/ADS DOI

   TolTEC is an imaging polarimeter installed on the Large Millimeter Telescope that simultaneously images the sky at 1.1, 1.4, and 2.0 mm. We have developed the open-source, fully parallelized C++ data reduction pipeline, citlali, to process TolTEC’s raw time-ordered data for science and calibration observations into on-sky maps, while also performing map coaddition and post-map- making analyses. Here, we describe citlali’s structure, including its reduction stages, algorithms, and parallelization scheme. We also present the results of the application of citlali to both TolTEC commissioning data and synthetic observations, characterizing the resulting map properties, as well as the software performance and memory usage.

4. The TolTEC camera: optical alignment and characterization

   Emily Lunde, Marc Berthoud, N. S. DeNigris, Simon Doyle, Daniel Ferrusca, Joseph E. Golec, Stephen Kuczarski, Dennis Lee, Zhiyuan Ma, Philip Mauskopf, and 10 more authors

   In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI, Aug 2022

   Abs NASA/ADS DOI

   The TolTEC camera is an imaging polarimeter installed at the Large Millimeter Telescope (LMT) in Mexico during December 2021. This new camera uses polarization-sensitive Kinetic Inductance Detectors (KIDs) coupled to the 50 m LMT to produce high resolution images simultaneously at 1.1, 1.4, and 2.0 mm wavelengths. We present a description of the cryogenic camera optics, the warm coupling optics, and the techniques used for on-site alignment of TolTEC with the telescope and characterization of the optics.

2021

1. HAWC+/SOFIA Polarimetry in L1688: Relative Orientation of Magnetic Field and Elongated Cloud Structure

   Dennis Lee, Marc Berthoud, Che-Yu Chen, Erin G. Cox, Jacqueline A. Davidson, Frankie J. Encalada, Laura M. Fissel, Rachel Harrison, Woojin Kwon, Di Li, and 7 more authors

   The Astrophysical Journal, Sep 2021

   Abs NASA/ADS arXiv DOI

   We present a study of the relative orientation between the magnetic field and elongated cloud structures for the \ensuremathρ Oph A and \ensuremathρ Oph E regions in L1688 in the Ophiuchus molecular cloud. Combining inferred magnetic field orientation from HAWC+ 154 \ensuremathμm observations of polarized thermal emission with column density maps created using Herschel submillimeter observations, we find consistent perpendicular relative alignment at scales of 0.02 pc (33”6 at d \ensuremath≈ 137 pc) using the histogram of relative orientations (HRO) technique. This supports the conclusions of previous work using Planck polarimetry and extends the results to higher column densities. Combining this HAWC+ HRO analysis with a new Planck HRO analysis of L1688, the transition from parallel to perpendicular alignment in L1688 is observed to occur at a molecular hydrogen column density of approximately 10^21.7 cm^-2. This value for the alignment transition column density agrees well with values found for nearby clouds via previous studies using only Planck observations. Using existing turbulent, magnetohydrodynamic simulations of molecular clouds formed by colliding flows as a model for L1688, we conclude that the molecular hydrogen volume density associated with this transition is approximately \raisebox-0.5ex~10^4 cm^-3. We discuss the limitations of our analysis, including incomplete sampling of the dense regions in L1688 by HAWC+.

2020

1. The TolTEC camera: an overview of the instrument and in-lab testing results

   Grant W. Wilson, Sophia Abi-Saad, Peter Ade, Itziar Aretxaga, Jason Austermann, Yvonne Ban, Joseph Bardin, James Beall, Marc Berthoud, Sean Bryan, and 47 more authors

   In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy X, Dec 2020

   Abs NASA/ADS DOI

   TolTEC is a three-band imaging polarimeter for the Large Millimeter Telescope. Simultaneously observing with passbands at 1.1mm, 1.4mm and 2.0mm, TolTEC has diffraction-limited beams with FWHM of 5, 7, and 11 arcsec, respectively. Over the coming decade, TolTEC will perform a combination of PI-led and Open-access Legacy Survey projects. Herein we provide an overview of the instrument and give the first quantitative measures of its performance in the lab prior to shipping to the telescope in 2021.

2. The optical design and performance of TolTEC: a millimeter-wave imaging polarimeter

   Emily Lunde, Peter Ade, Marc Berthoud, Reid Contente, N. S. DeNigris, Simon Doyle, Daniel Ferrusca, Joey Golec, Stephen Kuczarski, Dennis Lee, and 12 more authors

   In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy X, Dec 2020

   Abs NASA/ADS DOI

   TolTEC is an imaging polarimeter that will be mounted on the 50m diameter Large Millimeter Telescope (LMT) in Mexico. This camera simultaneously images the focal plane at three wavebands centered at 1.1, 1.4, and 2.0mm. TolTEC combines polarization- sensitive Kinetic Inductance Detectors (KIDs) with the LMT to produce 5-10 arcmin resolution maps of the sky in both total intensity and polarization. The light from the telescope is coupled to the TolTEC instrument using three room temperature mirrors. Before entering the cryostat, the light passes through a rapid-spinning achromatic half-wave plate, and once inside it passes through a 1 K Lyot stop that controls the telescope illumination. Inside the cryostat, a series of aluminum mirrors, silicon lenses, and dichroic filters split the light into three wavelength bands and direct each band to a different detector array. We will describe the design, and performance of the optics before installation at the telescope.

3. The TolTEC data analysis pipeline and software stack

   Zhiyuan Ma, Michael McCrackan, N. S. DeNigris, Kamal Souccar, Grant W. Wilson, Paul Horton, Dennis Lee, Philip Mauskopf, Giles Novak, Iván. Rodrı́guez-Montoya, and 1 more author

   In Software and Cyberinfrastructure for Astronomy VI, Dec 2020

   Abs NASA/ADS DOI

   TolTEC is a new camera that will shortly be mounted on the Large Millimeter Telescope (LMT). It provides simultaneous, polarization-sensitive imaging at wavelengths of 1.1, 1.4 and 2.0 mm through its 7718 Lumped element Kinetic Inductance Detectors (KIDs). The TolTEC data analysis software stack, TolTECA, has been developed to facilitate the data analysis tasks, producing science-ready data products for both the TolTEC legacy surveys and for future principal investigator projects. The software stack consists of a high performance fully parallelized C++ data reduction pipeline engine citlali, and an infrastructural Python package tolteca, which works at the highest level, with many notable features including data product management, a web-based data visualization framework, timely analysis and quick-look tools for on-site observing, and a TolTEC observation simulator.