Article
Astronomy & Astrophysics
Hannah C. Gulick, Sarah Sadavoy, Luca Matra, Patrick Sheehan, Nienke van der Marel
Summary: The study of WL 17 reveals unresolved issues in dust self-scattering, with lower polarization fractions observed, while combining high-sensitivity observations shows substructure in the disk for the first time in 233 GHz data.
ASTROPHYSICAL JOURNAL
(2021)
Article
Astronomy & Astrophysics
Enze Chen, Si-Yue Yu, Luis C. Ho
Summary: This study investigates two-armed global spiral density wave modes generated by gravitational instability in self-gravitating protoplanetary disks, revealing the influence of various disk conditions on the spiral arm morphology. The resulting spiral density wave modes closely resemble observations and are found to be insensitive to boundary conditions. Additionally, it is observed that Gaussian disks exhibit more tightly wound spirals compared to power-law disks under similar conditions, and the pitch angle of the spiral arms is influenced by the disk-to-star mass ratio and stability parameters.
ASTROPHYSICAL JOURNAL
(2021)
Article
Astronomy & Astrophysics
Yapeng Zhang, Christian Ginski, Jane Huang, Alice Zurlo, Herve Beust, Jaehan Bae, Myriam Benisty, Antonio Garufi, Michiel R. R. Hogerheijde, Rob G. G. van Holstein, Matthew Kenworthy, Maud Langlois, Carlo F. F. Manara, Paola Pinilla, Christian Rab, Alvaro Ribas, Giovanni P. P. Rosotti, Jonathan Williams
Summary: In this study, we investigated and compared circumstellar disks in three stellar multiple systems using near-infrared scattered-light imaging. Our observations revealed that the presence of companion stars affects the morphology and evolution of the disks, potentially resulting in different planetary populations. The comparison of the three systems showed that the influence of stellar companions on disk structures decreases with increasing separation.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
P. -G. Valegard, C. Ginski, C. Dominik, J. Bae, M. Benisty, T. Birnstiel, S. Facchini, A. Garufi, A. Garufi, M. Hogerheijde, R. G. van Holstein, M. Langlois, C. F. Manara, P. Pinilla, Ch. Rab, A. Ribas, L. B. F. M. Waters, J. Williams
Summary: A dusty disk wind model with micron to sub-micron-sized grains elevated above the disk can explain the optical hiding effect of the disk around young stars at optical wavelengths, while remaining visible in the near-infrared. By creating a geometric model using observational data and conducting synthetic observations, researchers constrained the grain size and dust mass in the wind, and estimated the mass-loss rate.
ASTRONOMY & ASTROPHYSICS
(2022)
Article
Astronomy & Astrophysics
Benedetta Veronesi, Teresa Paneque-Carreno, Giuseppe Lodato, Leonardo Testi, Laura M. Perez, Giuseppe Bertin, Cassandra Hall
Summary: Recent ALMA observations of the protoplanetary disk around Elias 2-27 revealed a two-armed spiral structure, making it an ideal laboratory to investigate the role of self-gravity in early star formation. The dynamical estimate suggests that the disk mass is 17% of the star mass, indicating that the disk could be prone to gravitational instabilities.
ASTROPHYSICAL JOURNAL LETTERS
(2021)
Article
Astronomy & Astrophysics
Dana E. Anderson, L. Ilsedore Cleeves, Geoffrey A. Blake, Edwin A. Bergin, Ke Zhang, John M. Carpenter, Kamber R. Schwarz
Summary: Gas mass is a fundamental quantity for protoplanetary disks' ability to form planets. The current estimates for gas masses in the Lupus star-forming region are based on CO isotopologues, but the uncertainties are large due to the degeneracy between H-2 mass and gas composition. This study explores the gas compositions of seven disks from the Lupus sample and finds tentative correlations among the line fluxes of HCO+, HCN, and N2H+. It highlights the importance of multimolecular studies in determining the physical and chemical properties of gas in protoplanetary disks.
ASTROPHYSICAL JOURNAL
(2022)
Article
Astronomy & Astrophysics
M. Temmink, A. S. Booth, N. van der Marel, E. F. van Dishoeck
Summary: The molecular composition of disks can provide information about the atmospheric composition of forming planets. Observations of the IRS 48 and HD 142527 disks show that dust traps can influence the chemistry through transport and sublimation of ices. This study presents ALMA detections of several molecules in the HD 142527 disk and finds that the dust trap does not significantly change the gas-phase composition.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
Nienke van Der Marel, Alice S. Booth, Margot Leemker, Ewine F. van Dishoeck, Satoshi Ohashi
Summary: This study investigates the chemistry in the asymmetric Oph IRS 48 dust trap, showing the origins of H2CO and CH3OH molecules in warm molecular regions of the disk with temperatures > 100 K and H2CO/CH3OH ~ 0.2, indicative of ice chemistry. The results highlight the importance of dust traps as reservoirs for complex organic molecules in planet-forming disks.
ASTRONOMY & ASTROPHYSICS
(2021)
Article
Astronomy & Astrophysics
Jun Hashimoto, Takayuki Muto, Ruobing Dong, Hauyu Baobab Liu, Nienke van der Marel, Logan Francis, Yasuhiro Hasegawa, Takashi Tsukagoshi
Summary: In this study, the dust disk around DM Tau was analyzed using ALMA observations, revealing two asymmetries that could be related to early evolution. Additionally, millimeter emissions were found inside the ring, suggesting the possibility of forming small icy planets in the region.
ASTROPHYSICAL JOURNAL
(2021)
Article
Astronomy & Astrophysics
L. Bertini, V. Roccatagliata, M. Kim
Summary: This study shows that in the past 5 million years, 90% of the debris disk systems have experienced at least one close flyby event, indicating the significant impact of flybys on the evolution of debris disks. Furthermore, the observed flyby events in resolved systems are consistent with theoretical predictions.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
Satoshi Ohashi, Hiroshi Kobayashi, Jinshi Sai, Nami Sakai
Summary: This study analyzes the growth of dust grains in a protostellar disk and finds that significant growth does not occur in the outer disk. Additionally, a possible ring-like substructure is tentatively identified. These findings suggest that the protostellar disk of L1489 may mark the beginning of planet formation.
ASTROPHYSICAL JOURNAL
(2022)
Article
Astronomy & Astrophysics
I. Mendigutia, E. Solano, M. Vioque, L. Balaguer-Nunez, A. Ribas, N. Huelamo, C. Rodrigo
Summary: This study investigates whether estimates of disk fraction change when considering an extended region around the centers of young stellar clusters. The results show that when the extended region is considered, the disk fraction is comparable to the estimates from the compact region.
ASTRONOMY & ASTROPHYSICS
(2022)
Article
Astronomy & Astrophysics
P. Pinilla, M. Benisty, N. T. Kurtovic, J. Bae, R. Dong, Z. Zhu, S. Andrews, J. Carpenter, C. Ginski, J. Huang, A. Isella, L. Perez, L. Ricci, G. Rosotti, M. Villenave, D. Wilner
Summary: In this paper, new observations of the protoplanetary disk around LkH alpha 330 using scattered light and millimeter techniques are presented. The observations reveal an asymmetric ring, spiral arms, and faint symmetric rings. The comparison with hydrodynamical simulations suggests that a 10 M-Jup mass planet in an eccentric orbit at 60 au can explain most of the observed structures. The results indicate that LkH alpha 330 is an interesting target for studying planet-disk interaction.
ASTRONOMY & ASTROPHYSICS
(2022)
Article
Astronomy & Astrophysics
Luke Keyte, Mihkel Kama, Alice S. Booth, Edwin A. Bergin, L. Ilsedore Cleeves, Ewine F. van Dishoeck, Maria N. Drozdovskaya, Kenji Furuya, Jonathan Rawlings, Oliver Shorttle, Catherine Walsh
Summary: The elemental carbon-to-oxygen ratio (C/O) is a useful diagnostic for understanding the formation history of a giant planet. Using ALMA observations of the protoplanetary disk around HD 100546, researchers have discovered an unexpected azimuthal variation in the C/O ratio, ranging from oxygen-dominated to carbon-dominated. This suggests the need for considering time-dependent azimuthal C/O variations when tracing the formation history of giant exoplanets.
Article
Astronomy & Astrophysics
A. Garufi, L. Podio, C. Codella, D. Segura-Cox, M. Vander Donckt, S. Mercimek, F. Bacciotti, D. Fedele, M. Kasper, J. E. Pineda, E. Humphreys, L. Testi
Summary: Planet-forming disks interact with the surrounding medium, affecting their mass and chemical content. This study reveals that the emission of SO and SO2 originates at the intersection between extended streamers and the planet-forming disk, and late accreting material can induce shocks in the disk that have consequences for its chemical composition and mass content. The observations of SO and SO2 lines play an important role in probing accretion shocks.
ASTRONOMY & ASTROPHYSICS
(2022)
Article
Astronomy & Astrophysics
T. -H. Hsieh, D. M. Segura-Cox, J. E. Pineda, P. Caselli, L. Bouscasse, R. Neri, A. Lopez-Sepulcre, M. T. Valdivia-Mena, M. J. Maureira, Th. Henning, G. V. Smirnov-Pinchukov, D. Semenov, Th. Moeller, N. Cunningham, A. Fuente, S. Marino, A. Dutrey, M. Tafalla, E. Chapillon, C. Ceccarelli, B. Zhao
Summary: This study presents high-sensitivity and high spectral-resolution observations of the binary system SVS13A using NOEMA. Complex line profiles at disk scales suggest the presence of multiple velocity components and significant variations in physical conditions within the circumbinary disk. Furthermore, the observations indicate the possible existence of an infalling streamer feeding material into the central region. Overall, this research provides valuable insights into the chemistry, physics, and accretion processes of interstellar matter.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
S. Hinkley, S. Lacour, G. -D. Marleau, A. -M. Lagrange, J. J. Wang, J. Kammerer, A. Cumming, M. Nowak, L. Rodet, T. Stolker, W. -O. Balmer, S. Ray, M. Bonnefoy, P. Molliere, C. Lazzoni, G. Kennedy, C. Mordasini, R. Abuter, S. Aigrain, A. Amorim, R. Asensio-Torres, C. Babusiaux, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, A. Bohn, H. Bonnet, G. Bourdarot, W. Brandner, F. Cantalloube, P. Caselli, B. Charnay, G. Chauvin, A. Chomez, E. Choquet, V. Christiaens, Y. Clenet, V. Coude du Foresto, A. Cridland, P. Delorme, R. Dembet, A. Drescher, G. Duvert, A. Eckart, F. Eisenhauer, H. Feuchtgruber, F. Galland, P. Garcia, R. Garcia Lopez, T. Gardner, E. Gendron, R. Genzel, S. Gillessen, J. H. Girard, A. Grandjean, X. Haubois, G. Heissel, Th Henning, S. Hippler, M. Horrobin, M. Houlle, Z. Hubert, L. Jocou, M. Keppler, P. Kervella, L. Kreidberg, V. Lapeyrere, J. -B. Le Bouquin, P. Lena, D. Lutz, A-L Maire, F. Mang, A. Merand, N. Meunier, J. D. Monnier, D. Mouillet, E. Nasedkin, T. Ott, G. P. P. L. Otten, C. Paladini, T. Paumard, K. Perraut, G. Perrin, F. Philipot, O. Pfuhl, N. Pourre, L. Pueyo, J. Rameau, E. Rickman, P. Rubini, Z. Rustamkulov, M. Samland, J. Shangguan, T. Shimizu, D. Sing, C. Straubmeier, E. Sturm, L. J. Tacconi, E. F. van Dishoeck, A. Vigan, F. Vincent, K. Ward-Duong, F. Widmann, E. Wieprecht, E. Wiezorrek, J. Woillez, S. Yazici, A. Young, N. Zicher
Summary: Using data from HARPS spectrograph and Gaia host star astrometry, researchers have discovered a companion HD206893c with an orbital separation of 3.53 au. The study also provides a dynamical mass estimation and age for the system. This discovery shows the potential of using Gaia astrometry for direct imaging and characterization of exoplanets.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
S. B. Brown-Sevilla, A. L. Maire, P. Molliere, M. Samland, M. Feldt, W. Brandner, Th. Henning, R. Gratton, M. Janson, T. Stolker, J. Hagelberg, A. Zurlo, F. Cantalloube, A. Boccaletti, M. Bonnefoy, G. Chauvin, S. Desidera, V. D'Orazi, A. -M. Lagrange, M. Langlois, F. Menard, D. Mesa, M. Meyer, A. Pavlov, C. Petit, S. Rochat, D. Rouan, T. Schmidt, A. Vigan, L. Weber
Summary: The aim of this study is to better understand the atmospheric properties of the directly imaged exoplanet 51 Eri b using a retrieval approach with higher signal-to-noise ratio data. The researchers used the radiative transfer code petitRADTRANS to analyze near-infrared SPHERE observations and compare the results with self-consistent models. The findings suggest that atmospheric retrievals should be considered together with self-consistent temperature structures to better characterize and determine the cloudiness of exoplanet atmospheres.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
Jesse Bublitz, Joel H. Kastner, Pierre Hily-Blant, Thierry Forveille, Miguel Santander-Garcia, Javier Alcolea, Valentin Bujarrabal, David J. Wilner, Rodolfo Montez, Isabel Aleman
Summary: Observations of CO+ and HCO+ in the young planetary nebula NGC 7027 reveal that UV radiation from the central star drives CO+ formation, while dust-penetrating soft X-rays enhance the abundance of HCO+ in the surrounding molecular envelope, forming an X-ray dominated region (XDR). These observations clearly demonstrate the influence of stellar irradiation on molecular gas formation.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Carlos E. Munoz-Romero, Karin I. Oberg, Charles J. Law, Richard Teague, Yuri Aikawa, Jennifer B. Bergner, David J. Wilner, Jane Huang, Viviana V. Guzman, L. Ilsedore Cleeves
Summary: Through new observations and archival data, researchers have found similar disk-averaged column densities and kinetic temperatures for DCO+ and DCN in the protoplanetary disk around TW Hya, indicating a common origin near the comet- and planet-forming midplane. The observed DCO+/DCN abundance ratio, combined with modeling results, provide tentative evidence of a gas-phase C/O enhancement within <40 au. Observations of DCO+ and DCN in other disks, as well as HCN and HCO+, will be necessary to fully understand the deuteration mechanisms in disks.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Jane Huang, Edwin A. Bergin, Jaehan Bae, Myriam Benisty, Sean M. Andrews
Summary: DR Tau is known for its high variability compared to other T Tauri stars. Millimeter interferometry observations reveal the distribution of CO and other substances, showing the presence of a protoplanetary disk, gas envelope, asymmetrical outflow, and spiral arm with clumps. The CO arm is much larger than spiral arms seen in scattered light, highlighting the importance of sensitive molecular imaging for understanding the disk environment. Kinematics and compact emission distribution suggest that (CO)-O-18, SO, DCO+, and H2CO primarily originate from the circumstellar disk. The asymmetry in SO emission may be due to interaction with infalling material or unresolved substructure, indicating a complex environment similar to outbursting FUor and EXor sources, suggesting a link between DR Tau's extreme stellar activity and disk instabilities caused by large-scale infall.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
L. Colina, A. Crespo Gomez, J. Alvarez-Marquez, A. Bik, F. Walter, L. Boogaard, A. Labiano, F. Peissker, P. Perez-Gonzalez, G. Ostlin, T. R. Greve, H. U. Norgaard-Nielsen, G. Wright, A. Alonso-Herrero, R. Azollini, K. I. Caputi, D. Dicken, M. Garcia-Marin, J. Hjorth, O. Ilbert, S. Kendrew, J. P. Pye, T. Tikkanen, P. van der Werf, L. Costantin, E. Iani, S. Gillman, I. Jermann, D. Langeroodi, T. Moutard, P. Rinaldi, M. Topinka, E. F. van Dishoeck, M. Guedel, Th. Henning, P. O. Lagage, T. Ray, B. Vandenbussche
Summary: Luminous infrared galaxies at high redshifts (z > 4) have extreme starbursts that form their stellar mass in short periods of time. This study presents the first spatially resolved near-infrared imaging of GN20, a very luminous dusty star-forming galaxy observed at an epoch when the Universe was only 1.5 Gyr old.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
M. Lafarga, I Ribas, M. Zechmeister, A. Reiners, A. Lopez-Gallifa, D. Montes, A. Quirrenbach, P. J. Amado, J. A. Caballero, M. Azzaro, V. J. S. Bejar, A. P. Hatzes, Th Henning, S. Jeffers, A. Kaminski, M. Kuerster, P. Schofer, A. Schweitzer, H. M. Tabernero, M. R. Zapatero Osorio
Summary: This study investigates the impact of stellar activity on the spectral lines of M dwarf stars. It classifies the sensitivity of different lines to activity and uses this information to compute radial velocities. The study finds that a line-by-line approach can deliver precise radial velocities and identifies activity-insensitive lines that mitigate the effects of stellar activity.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
G. Yoffe, R. van Boekel, A. Li, L. B. F. M. Waters, K. Maaskant, R. Siebenmorgen, M. van den Ancker, D. J. M. Petit Dit de la Roche, B. Lopez, A. Matter, J. Varga, M. R. Hogerheijde, G. Weigelt, R. D. Oudmaijer, E. Pantin, M. R. Meyer, J. -C. Augereau, Th. Henning
Summary: Research finds that the emission from polycyclic aromatic hydrocarbons (PAHs) comes from the upper layers of protoplanetary disks, higher than scattered light and similar to millimeter line emission. The intensity profiles of PAHs trace gas distribution and can constrain the penetration depth of UV radiation.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
A. Bello-Garcia, V. M. Passegger, J. Ordieres-Mere, A. Schweitzer, J. A. Caballero, A. Gonzalez-Marcos, I. Ribas, A. Reiners, A. Quirrenbach, P. J. Amado, V. J. S. Bejar, C. Cifuentes, Th. Henning, A. Kaminski, R. Luque, D. Montes, J. C. Morales, S. Pedraz, H. M. Tabernero, M. Zechmeister
Summary: This study demonstrates the feasibility of applying deep transfer learning (DTL) in the framework of determining photospheric stellar parameters. By using internal feature representations and independent parameter measurements as new inputs, DTL provides higher accuracy compared to previous deep learning (DL) methods. The precision of machine learning also improves with DTL. These results indicate that DTL is a robust tool for obtaining M-dwarf stellar parameters comparable to independent estimations.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
Asako Sato, Satoko Takahashi, Shun Ishii, Paul T. P. Ho, Masahiro N. N. Machida, John Carpenter, Luis A. Zapata, Paula Stella Teixeira, Sumeyye Suri
Summary: This study presents high-angular resolution and wide-field observations, revealing that about 80% of protostellar sources and 15% of prestellar sources are gravitationally bound. Twelve molecular outflows, including six newly detected ones, are identified using CO(J = 2-1) emission. Shocked gas structures traced by SiO(J = 5-4) emission are observed for the first time. Interaction between dust condensation and an energetic outflow driven by HOPS-370 within FIR 3 is confirmed. The proposed triggered star formation scenario in FIR 4 is not strongly supported based on a comparison of dynamical timescales and protostellar ages. The spatial distribution of filaments identified in the continuum image is discussed by comparing it with a previously identified hub-fiber system in the N2H+ line.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Charles J. J. Law, Richard Teague, Karin I. I. Oberg, Evan A. A. Rich, Sean M. M. Andrews, Jaehan Bae, Myriam Benisty, Stefano Facchini, Kevin Flaherty, Andrea Isella, Sheng Jin, Jun Hashimoto, Jane Huang, Ryan A. A. Loomis, Feng Long, Carlos E. E. Munoz-Romero, Teresa Paneque-Carreno, Laura M. M. Perez, Chunhua Qi, Kamber R. R. Schwarz, Jochen Stadler, Takashi Tsukagoshi, David J. J. Wilner, Gerrit van der Plas
Summary: High-spatial-resolution observations of CO isotopologue line emission in protoplanetary disks reveal detailed gas structure, including radial and vertical substructures, emission surface heights, and temperature profiles. Observations of CO, (CO)-C-13, and (CO)-O-18 in transition disks provide insights into disk structure and temperature distributions. The derived emission surfaces and temperature models show potential correlations with source characteristics, suggesting the influence of stellar host mass, gas temperature, and disk size on the emitting heights.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Melissa J. Hobson, Andres Jordan, E. M. Bryant, R. Brahm, D. Bayliss, J. D. Hartman, G. A. Bakos, Th. Henning, Jose Manuel Almenara, Khalid Barkaoui, Zouhair Benkhaldoun, Xavier Bonfils, Francois Bouchy, David Charbonneau, Marion Cointepas, Karen A. Collins, Jason D. Eastman, Mourad Ghachoui, Michael Gillon, Robert F. Goeke, Keith Horne, Jonathan M. Irwin, Emmanuel Jehin, Jon M. Jenkins, David W. Latham, Dan Moldovan, Felipe Murgas, Francisco J. Pozuelos, George R. Ricker, Richard P. Schwarz, S. Seager, Gregor Srdoc, Stephanie Striegel, Mathilde Timmermans, Andrew Vanderburg, Roland Vanderspek, Joshua N. Winn
Summary: We have discovered TOI-3235 b, a short-period Jupiter orbiting an M dwarf star close to the transition point from partially to fully convective. The planet was identified through TESS photometry and later confirmed with radial velocities and ground-based photometry. With a mass of 0.665 +/- 0.025 M-J and a radius of 1.017 +/- 0.044 R-J, TOI-3235 b orbits its host star closely but has a lower equilibrium temperature than expected for hot Jupiters. This discovery challenges current planet formation models and makes TOI-3235 b a suitable target for atmospheric characterization.
ASTROPHYSICAL JOURNAL LETTERS
(2023)
Article
Astronomy & Astrophysics
Adrian T. Sutinjo, Danica R. Scott, Clancy W. James, Marcin Glowacki, Keith W. Bannister, Hyerin Cho, Cherie K. Day, Adam T. Deller, Timothy P. Perrett, Ryan M. Shannon
Summary: Studies on the time-domain structure of fast radio bursts (FRBs) require an accurate estimation of the FRB dispersion measure to recover the original burst shape. The dispersion measure itself is also of interest when studying the evolutionary behavior of the medium through which repeating FRBs propagate. However, different authors use varying methods to obtain the structure parameter and do not document the smoothing technique used, and there is a lack of quantitative error estimates in FRB literature. This article introduces a smoothing filter based on the discrete cosine transform and demonstrates how computing the structure parameter allows for the calculation of its uncertainty. The method is illustrated using data from FRB 20181112A and FRB 20210117A detected by the Australian Square Kilometre Array Pathfinder and with available high-time-resolution data.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Pravir Kumar, Ryan M. Shannon, Marcus E. Lower, Adam T. Deller, J. Xavier Prochaska
Summary: The study of impulsive astrophysical radio emission allows us to investigate the plasma between the emission source and the Earth. The circular propagating wave modes in cold electron-ion plasmas affect the linear polarization plane, while in relativistic plasmas, the wave modes can convert linearly polarized emission into circular and vice versa. Fast radio bursts (FRBs) provide an opportunity to study the intergalactic and magneto-ionic medium. This article presents the polarimetric analysis of a repeat burst from the FRB 20201124A source, which displays a unique frequency-dependent circular polarization. The observed circular polarization can be explained by the propagation of a linearly polarized burst signal through a relativistic plasma along the line of sight from the progenitor.