# First BISTRO observations of the dark cloud Taurus L1495A-B10: the role of
the magnetic field in the earliest stages of low-mass star formation

Derek Ward-Thompson Jeremiah Horrocks Institute, University of Central
Lancashire, Preston PR1 2HE, UK  Janik Karoly Jeremiah Horrocks Institute,
University of Central Lancashire, Preston PR1 2HE, UK  Kate Pattle Department
of Physics and Astronomy, University College London, WC1E 6BT London, UK
Anthony Whitworth School of Physics and Astronomy, Cardiff University, The
Parade, Cardiff, CF24 3AA, UK  Jason Kirk Jeremiah Horrocks Institute,
University of Central Lancashire, Preston PR1 2HE, UK  David Berry East Asian
Observatory, 660 N. A’ohōkū Place, University Park, Hilo, HI 96720, USA
Pierre Bastien Centre de recherche en astrophysique du Québec & département de
physique, Université de Montréal, C.P. 6128 Succ. Centre-ville, Montréal, QC,
H3C 3J7, Canada  Tao-Chung Ching Zhejiang Lab, Kechuang Avenue, Yuhang
District, Hangzhou 311121, People’s Republic of China  Simon Coudé Department
of Earth, Environment, and Physics, Worcester State University, Worcester, MA
01602, USA  Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden
Street, Cambridge, MA 02138, USA  Jihye Hwang Korea Astronomy and Space
Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of
Korea  University of Science and Technology, Korea, 217 Gajeong-ro, Yuseong-
gu, Daejeon 34113, Republic of Korea  Woojin Kwon Department of Earth Science
Education, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826,
Republic of Korea  SNU Astronomy Research Center, Seoul National University, 1
Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea  Archana Soam Indian
Institute of Astrophysics, II Block, Koramangala, Bengaluru 560034, India
Jia-Wei Wang Academia Sinica Institute of Astronomy and Astrophysics, No.1,
Sec. 4., Roosevelt Road, Taipei 10617, Taiwan  Tetsuo Hasegawa National
Astronomical Observatory of Japan, National Institutes of Natural Sciences,
Osawa, Mitaka, Tokyo 181-8588, Japan  Shih-Ping Lai Institute of Astronomy and
Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
Academia Sinica Institute of Astronomy and Astrophysics, No.1, Sec. 4.,
Roosevelt Road, Taipei 10617, Taiwan  Keping Qiu School of Astronomy and Space
Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, People’s
Republic of China  Key Laboratory of Modern Astronomy and Astrophysics
(Nanjing University), Ministry of Education, Nanjing 210023, People’s Republic
of China  Doris Arzoumanian  Division of Science, National Astronomical
Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan  Tyler L.
Bourke SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT,
UK  Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy,
University of Manchester, Oxford Road, Manchester, UK  Do-Young Byun  Korea
Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon
34055, Republic of Korea  University of Science and Technology, Korea, 217
Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea  Huei-Ru Vivien Chen
Institute of Astronomy and Department of Physics, National Tsing Hua
University, Hsinchu 30013, Taiwan  Academia Sinica Institute of Astronomy and
Astrophysics, No.1, Sec. 4., Roosevelt Road, Taipei 10617, Taiwan  Wen Ping
Chen Institute of Astronomy, National Central University, Zhongli 32001,
Taiwan  Mike Chen  Department of Physics and Astronomy, University of
Victoria, Victoria, BC V8W 2Y2, Canada  Zhiwei Chen  Purple Mountain
Observatory, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing,
People’s Republic of China  Jungyeon Cho  Department of Astronomy and Space
Science, Chungnam National University, Daejeon 34134, Republic of Korea  Minho
Choi  Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-
gu, Daejeon 34055, Republic of Korea  Youngwoo Choi  Department of Physics and
Astronomy, Seoul National University, Seoul 08826, Republic of Korea  Yunhee
Choi  Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-
gu, Daejeon 34055, Republic of Korea  Antonio Chrysostomou  SKA Observatory,
Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK  Eun Jung Chung
Department of Astronomy and Space Science, Chungnam National University,
Daejeon 34134, Republic of Korea  Sophia Dai  National Astronomical
Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District,
Beijing 100012, People’s Republic of China  Victor Debattista Jeremiah
Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
James Di Francesco NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich
Road, Victoria, BC V9E 2E7, Canada  Department of Physics and Astronomy,
University of Victoria, Victoria, BC V8W 2Y2, Canada  Pham Ngoc Diep Vietnam
National Space Center, Vietnam Academy of Science and Technology, Hanoi,
Vietnam  Yasuo Doi Department of Earth Science and Astronomy, Graduate School
of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo
153-8902, Japan  Hao-Yuan Duan  Institute of Astronomy and Department of
Physics, National Tsing Hua University, Hsinchu 30013, Taiwan  Yan Duan
National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun
Road, Chaoyang District, Beijing 100012, People’s Republic of China  Chakali
Eswaraiah Indian Institute of Science Education and Research (IISER) Tirupati,
Rami Reddy Nagar, Karakambadi Road, Mangalam (P.O.), Tirupati 517 507, India
Lapo Fanciullo National Chung Hsing University, 145 Xingda Rd., South Dist.,
Taichung City 402, Taiwan  Jason Fiege  Department of Physics and Astronomy,
The University of Manitoba, Winnipeg, Manitoba R3T2N2, Canada  Laura M. Fissel
Department for Physics, Engineering Physics and Astrophysics, Queen’s
University, Kingston, ON, K7L 3N6, Canada  Erica Franzmann  Department of
Physics and Astronomy, The University of Manitoba, Winnipeg, Manitoba R3T2N2,
Canada  Per Friberg  East Asian Observatory, 660 N. A’ohōkū Place, University
Park, Hilo, HI 96720, USA  Rachel Friesen  National Radio Astronomy
Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA  Gary Fuller
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy,
University of Manchester, Oxford Road, Manchester, UK  Ray Furuya  Institute
of Liberal Arts and Sciences Tokushima University, Minami Jousanajima-machi
1-1, Tokushima 770-8502, Japan  Tim Gledhill School of Physics, Astronomy &
Mathematics, University of Hertfordshire, College Lane, Hatfield,
Hertfordshire AL10 9AB, UK  Sarah Graves  East Asian Observatory, 660 N.
A’ohōkū Place, University Park, Hilo, HI 96720, USA  Jane Greaves  School of
Physics and Astronomy, Cardiff University, The Parade, Cardiff, CF24 3AA, UK
Matt Griffin  School of Physics and Astronomy, Cardiff University, The Parade,
Cardiff, CF24 3AA, UK  Qilao Gu  Shanghai Astronomical Observatory, Chinese
Academy of Sciences, 80 Nandan Road, Shanghai 200030, People’s Republic of
China  Ilseung Han  Korea Astronomy and Space Science Institute, 776
Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea  University of
Science and Technology, Korea, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113,
Republic of Korea  Saeko Hayashi  Subaru Telescope, National Astronomical
Observatory of Japan, 650 N. A’ohōkū Place, Hilo, HI 96720, USA  Thiem Hoang
Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu,
Daejeon 34055, Republic of Korea  University of Science and Technology, Korea,
217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea  Martin Houde
Department of Physics and Astronomy, The University of Western Ontario, 1151
Richmond Street, London N6A 3K7, Canada  Charles L. H. Hull National
Astronomical Observatory of Japan, Alonso de Córdova 3788, Office 61B,
Vitacura, Santiago, Chile  Joint ALMA Observatory, Alonso de Córdova 3107,
Vitacura, Santiago, Chile  NAOJ Fellow  Tsuyoshi Inoue Department of Physics,
Konan University, Okamoto 8-9-1, Higashinada-ku, Kobe 658-8501, Japan  Shu-
ichiro Inutsuka Department of Physics, Graduate School of Science, Nagoya
University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan  Kazunari Iwasaki
Department of Environmental Systems Science, Doshisha University, Tatara,
Miyakodani 1-3, Kyotanabe, Kyoto 610-0394, Japan  Il-Gyo Jeong Department of
Astronomy and Atmospheric Sciences, Kyungpook National University, Republic of
Korea  Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro,
Yuseong-gu, Daejeon 34055, Republic of Korea  Doug Johnstone NRC Herzberg
Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7,
Canada  Department of Physics and Astronomy, University of Victoria, Victoria,
BC V8W 2Y2, Canada  Vera Könyves  Jeremiah Horrocks Institute, University of
Central Lancashire, Preston PR1 2HE, UK  Ji-hyun Kang Korea Astronomy and
Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055,
Republic of Korea  Miju Kang Korea Astronomy and Space Science Institute, 776
Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea  Akimasa Kataoka
Division of Theoretical Astronomy, National Astronomical Observatory of Japan,
Mitaka, Tokyo 181-8588, Japan  Koji Kawabata  Hiroshima Astrophysical Science
Center, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima, Hiroshima
739-8526, Japan  Department of Physics, Hiroshima University, Kagamiyama
1-3-1, Higashi-Hiroshima, Hiroshima 739-8526, Japan  Core Research for
Energetic Universe, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima,
Hiroshima 739-8526, Japan  Francisca Kemper Institute of Space Sciences (ICE),
CSIC, Can Magrans, 08193 Cerdanyola del Vallés, Barcelona, Spain  ICREA, Pg.
Lluís Companys 23, Barcelona, Spain  Institut d’Estudis Espacials de Catalunya
(IEEC), E-08034 Barcelona, Spain  Jongsoo Kim Korea Astronomy and Space
Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of
Korea  University of Science and Technology, Korea, 217 Gajeong-ro, Yuseong-
gu, Daejeon 34113, Republic of Korea  Shinyoung Kim Korea Astronomy and Space
Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of
Korea  Gwanjeong Kim Nobeyama Radio Observatory, National Astronomical
Observatory of Japan, National Institutes of Natural Sciences, Nobeyama,
Minamimaki, Minamisaku, Nagano 384-1305, Japan  Kyoung Hee Kim Korea Astronomy
and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055,
Republic of Korea  Mi-Ryang Kim  School of Space Research, Kyung Hee
University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104,
Republic of Korea  Kee-Tae Kim Korea Astronomy and Space Science Institute,
776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea  University of
Science and Technology, Korea, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113,
Republic of Korea  Hyosung Kim  Department of Earth Science Education, Seoul
National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
Florian Kirchschlager Sterrenkundig Observatorium, Ghent University,
Krijgslaan 281-S9, 9000 Gent, BE  Masato I.N. Kobayashi Division of Science,
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo
181-8588, Japan  Patrick M. Koch Academia Sinica Institute of Astronomy and
Astrophysics, No.1, Sec. 4., Roosevelt Road, Taipei 10617, Taiwan  Takayoshi
Kusune  Astronomical Institute, Graduate School of Science, Tohoku University,
Aoba-ku, Sendai, Miyagi 980-8578, Japan  Jungmi Kwon Department of Astronomy,
Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-0033, Japan  Kevin Lacaille  Department of Physics and Astronomy, McMaster
University, Hamilton, ON L8S 4M1 Canada  Department of Physics and Atmospheric
Science, Dalhousie University, Halifax B3H 4R2, Canada  Chi-Yan Law
Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong
Kong  Department of Space, Earth & Environment, Chalmers University of
Technology, SE-412 96 Gothenburg, Sweden  Chang Won Lee Korea Astronomy and
Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055,
Republic of Korea  University of Science and Technology, Korea, 217 Gajeong-
ro, Yuseong-gu, Daejeon 34113, Republic of Korea  Hyeseung Lee  Department of
Astronomy and Space Science, Chungnam National University, Daejeon 34134,
Republic of Korea  Yong-Hee Lee  School of Space Research, Kyung Hee
University, Gyeonggi-do 17104, Republic of Korea  East Asian Observatory, 660
N. A’ohōkū Place, University Park, Hilo, HI 96720, USA  Chin-Fei Lee  Academia
Sinica Institute of Astronomy and Astrophysics, No.1, Sec. 4., Roosevelt Road,
Taipei 10617, Taiwan  Jeong-Eun Lee  School of Space Research, Kyung Hee
University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104,
Republic of Korea  Sang-Sung Lee  Korea Astronomy and Space Science Institute,
776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea  University of
Science and Technology, Korea, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113,
Republic of Korea  Dalei Li  Xinjiang Astronomical Observatory, Chinese
Academy of Sciences, Urumqi 830011, Xinjiang, People’s Republic of China  Di
Li  CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese
Academy of Sciences, People’s Republic of China  Guangxing Li  Department of
Astronomy, Yunnan University, Kunming, 650091, PR China  Hua-bai Li
Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong
Kong  Sheng-Jun Lin Institute of Astronomy and Department of Physics, National
Tsing Hua University, Hsinchu 30013, Taiwan  Hong-Li Liu Department of
Astronomy, Yunnan University, Kunming, 650091, PR China  Tie Liu Key
Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical
Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030,
People’s Republic of China  Sheng-Yuan Liu Academia Sinica Institute of
Astronomy and Astrophysics, No.1, Sec. 4., Roosevelt Road, Taipei 10617,
Taiwan  Junhao Liu East Asian Observatory, 660 N. A’ohōkū Place, University
Park, Hilo, HI 96720, USA  Steven Longmore Astrophysics Research Institute,
Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
Xing Lu Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80
Nandan Road, Shanghai 200030, People’s Republic of China  A-Ran Lyo  Korea
Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon
34055, Republic of Korea  Steve Mairs East Asian Observatory, 660 N. A’ohōkū
Place, University Park, Hilo, HI 96720, USA  Masafumi Matsumura Faculty of
Education & Center for Educational Development and Support, Kagawa University,
Saiwai-cho 1-1, Takamatsu, Kagawa, 760-8522, Japan  Brenda Matthews  NRC
Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E
2E7, Canada  Department of Physics and Astronomy, University of Victoria,
Victoria, BC V8W 2Y2, Canada  Gerald Moriarty-Schieven NRC Herzberg Astronomy
and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
Tetsuya Nagata  Department of Astronomy, Graduate School of Science, Kyoto
University, Sakyo-ku, Kyoto 606-8502, Japan  Fumitaka Nakamura  Division of
Theoretical Astronomy, National Astronomical Observatory of Japan, Mitaka,
Tokyo 181-8588, Japan  SOKENDAI (The Graduate University for Advanced
Studies), Hayama, Kanagawa 240-0193, Japan  Hiroyuki Nakanishi  Department of
Physics and Astronomy, Graduate School of Science and Engineering, Kagoshima
University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan  Nguyen Bich Ngoc
Vietnam National Space Center, Vietnam Academy of Science and Technology,
Hanoi, Vietnam  Graduate University of Science and Technology, Vietnam Academy
of Science and Technology, Hanoi, Vietnam  Nagayoshi Ohashi Academia Sinica
Institute of Astronomy and Astrophysics, No.1, Sec. 4., Roosevelt Road, Taipei
10617, Taiwan  Takashi Onaka Department of Physics, Faculty of Science and
Engineering, Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan
Department of Astronomy, Graduate School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan  Geumsook Park  Korea Astronomy
and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055,
Republic of Korea  Harriet Parsons  East Asian Observatory, 660 N. A’ohōkū
Place, University Park, Hilo, HI 96720, USA  Nicolas Peretto  School of
Physics and Astronomy, Cardiff University, The Parade, Cardiff, CF24 3AA, UK
Felix Priestley  School of Physics and Astronomy, Cardiff University, The
Parade, Cardiff, CF24 3AA, UK  Tae-Soo Pyo  SOKENDAI (The Graduate University
for Advanced Studies), Hayama, Kanagawa 240-0193, Japan  Subaru Telescope,
National Astronomical Observatory of Japan, 650 N. A’ohōkū Place, Hilo, HI
96720, USA  Lei Qian  CAS Key Laboratory of FAST, National Astronomical
Observatories, Chinese Academy of Sciences, People’s Republic of China
Ramprasad Rao  Academia Sinica Institute of Astronomy and Astrophysics, No.1,
Sec. 4., Roosevelt Road, Taipei 10617, Taiwan  Jonathan Rawlings Department of
Physics and Astronomy, University College London, WC1E 6BT London, UK  Mark
Rawlings Gemini Observatory/NSF’s NOIRLab, 670 N. A’ohōkū Place, Hilo, HI
96720, USA  East Asian Observatory, 660 N. A’ohōkū Place, University Park,
Hilo, HI 96720, USA  Brendan Retter  School of Physics and Astronomy, Cardiff
University, The Parade, Cardiff, CF24 3AA, UK  John Richer  Astrophysics
Group, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, UK
Kavli Institute for Cosmology, Institute of Astronomy, University of
Cambridge, Madingley Road, Cambridge, CB3 0HA, UK  Andrew Rigby  School of
Physics and Astronomy, Cardiff University, The Parade, Cardiff, CF24 3AA, UK
Sarah Sadavoy  Department for Physics, Engineering Physics and Astrophysics,
Queen’s University, Kingston, ON, K7L 3N6, Canada  Hiro Saito  Faculty of Pure
and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki
305-8577, Japan  Giorgio Savini  OSL, Physics & Astronomy Dept., University
College London, WC1E 6BT London, UK  Masumichi Seta  Department of Physics,
School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen,
Sanda, Hyogo 669-1337, Japan  Yoshito Shimajiri Kyushu Kyoritsu University,
1-8, Jiyugaoka, Yahatanishi-ku, Kitakyushu-shi, Fukuoka 807-8585, Japan
Hiroko Shinnaga  Department of Physics and Astronomy, Graduate School of
Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima
890-0065, Japan  Mehrnoosh Tahani Dominion Radio Astrophysical Observatory,
Herzberg Astronomy and Astrophysics Research Centre, National Research Council
Canada, P. O. Box 248, Penticton, BC V2A 6J9 Canada  Motohide Tamura
Department of Astronomy, Graduate School of Science, University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan  Astrobiology Center, National
Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
National Astronomical Observatory of Japan, National Institutes of Natural
Sciences, Osawa, Mitaka, Tokyo 181-8588, Japan  Ya-Wen Tang  Academia Sinica
Institute of Astronomy and Astrophysics, No.1, Sec. 4., Roosevelt Road, Taipei
10617, Taiwan  Xindi Tang Xinjiang Astronomical Observatory, Chinese Academy
of Sciences, 830011 Urumqi, People’s Republic of China  Kohji Tomisaka
Division of Theoretical Astronomy, National Astronomical Observatory of Japan,
Mitaka, Tokyo 181-8588, Japan  Le Ngoc Tram University of Science and
Technology of Hanoi, Vietnam Academy of Science and Technology, Hanoi, Vietnam
Yusuke Tsukamoto  Department of Physics and Astronomy, Graduate School of
Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima
890-0065, Japan  Serena Viti  Physics & Astronomy Dept., University College
London, WC1E 6BT London, UK  Hongchi Wang  Purple Mountain Observatory,
Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, People’s
Republic of China  Jintai Wu  School of Astronomy and Space Science, Nanjing
University, 163 Xianlin Avenue, Nanjing 210023, People’s Republic of China
Jinjin Xie National Astronomical Observatories, Chinese Academy of Sciences,
A20 Datun Road, Chaoyang District, Beijing 100012, People’s Republic of China
Meng-Zhe Yang  Institute of Astronomy and Department of Physics, National
Tsing Hua University, Hsinchu 30013, Taiwan  Hsi-Wei Yen  Academia Sinica
Institute of Astronomy and Astrophysics, No.1, Sec. 4., Roosevelt Road, Taipei
10617, Taiwan  Hyunju Yoo Department of Astronomy and Space Science, Chungnam
National University, Daejeon 34134, Republic of Korea  Jinghua Yuan  National
Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road,
Chaoyang District, Beijing 100012, People’s Republic of China  Hyeong-Sik Yun
Korea Astronomy and Space Science Institute, Yuseong-gu, Daejeon 34055,
Republic of Korea  Tetsuya Zenko  Department of Astronomy, Graduate School of
Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan  Guoyin Zhang  CAS
Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy
of Sciences, People’s Republic of China  Yapeng Zhang Department of Astronomy,
Beijing Normal University, Beijing100875, China  Chuan-Peng Zhang  National
Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road,
Chaoyang District, Beijing 100012, People’s Republic of China  CAS Key
Laboratory of FAST, National Astronomical Observatories, Chinese Academy of
Sciences, People’s Republic of China  Jianjun Zhou Xinjiang Astronomical
Observatory, Chinese Academy of Sciences, Urumqi 830011, Xinjiang, People’s
Republic of China  Lei Zhu  CAS Key Laboratory of FAST, National Astronomical
Observatories, Chinese Academy of Sciences, People’s Republic of China  Ilse
de Looze  Physics & Astronomy Dept., University College London, WC1E 6BT
London, UK  Philippe André  Laboratoire d’Astrophysique (AIM), Université
Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette,
France  C. Darren Dowell  Jet Propulsion Laboratory, M/S 169-506, 4800 Oak
Grove Drive, Pasadena, CA 91109, USA  David Eden  Armagh Observatory and
Planetarium, College Hill, Armagh BT61 9DG, UK  Stewart Eyres  University of
South Wales, Pontypridd, CF37 1DL, UK  Sam Falle Department of Applied
Mathematics, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK  Valentin
J. M. Le Gouellec  SOFIA Science Center, Universities Space Research
Association, NASA Ames Research Center, Moffett Field, California 94035, USA
Laboratoire d’Astrophysique (AIM), Université Paris-Saclay, Université Paris
Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France  Frédérick Poidevin
Instituto de Astrofisíca de Canarias, 38200 La Laguna,Tenerife, Canary
Islands, Spain  Departamento de Astrofísica, Universidad de La Laguna (ULL),
38206 La Laguna, Tenerife, Spain  Jean-François Robitaille Univ. Grenoble
Alpes, CNRS, IPAG, 38000 Grenoble, France  Sven van Loo  School of Physics and
Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

(Received 10 October 2022; Revised 22 February 2023; Accepted 23 February
2023)

###### Abstract

We present BISTRO Survey 850 $\mu$m dust emission polarisation observations of
the L1495A-B10 region of the Taurus molecular cloud, taken at the JCMT. We
observe a roughly triangular network of dense filaments. We detect 9 of the
dense starless cores embedded within these filaments in polarisation, finding
that the plane-of-sky orientation of the core-scale magnetic field lies
roughly perpendicular to the filaments in almost all cases. We also find that
the large-scale magnetic field orientation measured by Planck is not
correlated with any of the core or filament structures, except in the case of
the lowest-density core. We propose a scenario for early prestellar evolution
that is both an extension to, and consistent with, previous models,
introducing an additional evolutionary transitional stage between field-
dominated and matter-dominated evolution, observed here for the first time. In
this scenario, the cloud collapses first to a sheet-like structure. Uniquely,
we appear to be seeing this sheet almost face-on. The sheet fragments into
filaments, which in turn form cores. However, the material must reach a
certain critical density before the evolution changes from being field-
dominated to being matter-dominated. We measure the sheet surface density and
the magnetic field strength at that transition for the first time and show
consistency with an analytical prediction that had previously gone untested
for over 50 years (Mestel, 1965).

ISM: Clouds, Evolution, Magnetic fields

††journal: ApJ††facilities: JCMT (SCUBA2, POL2)††software: Starlink (Currie et
al., 2014), Astropy (Astropy Collaboration et al., 2013, 2018)

##  1 Introduction

Figure 1: Left: False-colour red-green-blue (RGB) image of the L1495A region
taken from the SCUBA2 and Herschel-SPIRE observations of Ward-Thompson et al.
(2016). Red shows the SCUBA2 850 $\mu$m observations and green and blue show
the 500 $\mu$m and 250 $\mu$m Herschel-SPIRE observations respectively. This
region lies at the head of the L1495 filament, which can be seen heading south
from the lower left part of the image. Right: Zoomed in region of the green
square on the left, showing the region mapped in this paper. The RGB channels
are the same as in the left panel.

The L1495 region of the Taurus molecular cloud appears as an obscuring dark
cloud on optical images (Barnard, 1907; Lynds, 1962), with a linear, or
filamentary, structure, coming to a head at a small globule that is referred
to as L1495A (Benson & Myers, 1989; Lee et al., 2001), or sometimes as B10
(e.g. Punanova et al., 2018).

Marsh et al. (2016) studied the L1495 filament and found it to be consistent
with the filamentary star formation model (André et al., 2014) favoured by
Herschel observations of star-forming regions (André et al., 2010). Lee et al.
(2001) found evidence for asymmetric line profiles in the southern part of
L1495A, apparently indicating collapse or contraction. However, this is close
to the Herbig Ae/Be star, V892 Tau, which is clearly affecting at least the
southern part of L1495A, so interpreting asymmetric line profiles is made more
complex in this area.

Ward-Thompson et al. (2016) presented a comparison of SCUBA2 and Herschel
observations of the L1495A cloud – see Figure 1. They found that SCUBA2
detects only the highest-surface-brightness sources, principally detecting
protostellar sources and starless cores embedded in filaments, while Herschel
is sensitive to most of the cloud structure, including extended low-surface-
brightness emission. For similar temperature cores, such as those seen in
L1495A, the surface brightnesses of the cores are determined by their column
densities, with the highest-column-density cores being detected by SCUBA2. For
roughly spherical geometries, column density can be related to volume density,
and so SCUBA2 selects the densest cores from a population at a given
temperature – thus, the cores in Figure 1 appear red. Hence, the polarimeter
POL2 on SCUBA2 is most sensitive to the polarization in the cores of L1495A,
rather than the filaments.

In this paper we present POL2 data of L1495A as part of the BISTRO survey. In
a previous paper from the BISTRO survey (Eswaraiah et al., 2021a) we presented
data from the B213 part of the L1495 filament, which lies to the southeast of
this region. In both cases we concentrate on the polarisation of the cores.
However, whereas the cores in the previous B213 paper mostly contained YSOs,
the cores studied in this paper are all starless.

Table 1: Core characteristics

Core | RA | Dec. |  FWHMb | $\theta_{core}^{b,c}$ | $\theta_{fil}^{c,d}$ |  Temp.b |  N($H_{2}$)e |  n($H_{2}$) |  A${}_{V}^{f}$ | $\theta_{pol}^{c}$
---|---|---|---|---|---|---|---|---|---|---
no.a | (J2000) | (J2000) |  ($\arcsec\times\arcsec$) |  ($\degree$) |  ($\degree$) | (K) |  ($\times 10^{21}cm^{-2}$) |  ($\times 10^{5}cm^{-3}$) | (mag) |  ($\degree$)
1 (2) | 4:17:42.10 | +28:08:44.4 |  54.6 $\times$ 21.4 | 167 | 26 |  10.1 $\pm$ 0.2 |  19.1 $\pm$ 7.6 |  2.0 $\pm$ 0.8 |  17 $\pm$ 7 |  4 $\pm$ 2
2-N (7) | 4:17:43.75 | +28:07:04.6 |  32.0 $\times$ 16.0 | 0 | 0 |  10.6 $\pm$ 0.2 |  14.7 $\pm$ 5.9 |  2.4 $\pm$ 1.0 |  13 $\pm$ 5 |  -3 $\pm$ 4
2-S (7) | 4:17:43.40 | +28:06:04.5 |  32.4 $\times$ 20.7 | 45 | 0 |  10.6 $\pm$ 0.2 |  15.7 $\pm$ 6.3 |  2.1 $\pm$ 0.8 |  14 $\pm$ 6 |  -46 $\pm$ 3
3 (12) | 4:17:34.58 | +28:03:05.0 |  55.2 $\times$ 20.3 | 53 | 37 |  12.7 $\pm$ 0.3 |  15.3 $\pm$ 6.1 |  1.6 $\pm$ 0.6 |  14 $\pm$ 6 |  68 $\pm$ 11
4 ($-$) | 4:17:53.92 | +28:05:28.3 |  31.4 $\times$ 12.1 | 60 | 85 |  12.5 $\pm$ 0.1 |  9.2 $\pm$ 3.7 |  1.7 $\pm$ 0.7 |  8 $\pm$ 3 |  -72 $\pm$ 8
5 (5) | 4:18:08.17 | +28:05:10.3 |  39.6 $\times$ 32.0 | 121 | 150 |  10.1 $\pm$ 0.2 |  17.8 $\pm$ 7.1 |  1.8 $\pm$ 0.7 |  16 $\pm$ 7 |  -4 $\pm$ 12
6 (11) | 4:18:03.08 | +28:07:35.2 |  39.0 $\times$ 20.5 | 126 | 170 |  9.3 $\pm$ 0.2 |  14.0 $\pm$ 5.6 |  1.8 $\pm$ 0.7 |  13 $\pm$ 5 |  18 $\pm$ 5
7 (19) | 4:18:00.55 | +28:11:08.7 |  45.0 $\times$ 22.2 | 165 | 147 |  10.7 $\pm$ 0.2 |  14.4 $\pm$ 5.8 |  1.7 $\pm$ 0.7 |  13 $\pm$ 5 |  9 $\pm$ 6
8 (14) | 4:17:52.08 | +28:12:31.1 |  51.6 $\times$ 48.7 | 93 | 135 (15) |  10.9 $\pm$ 0.2 |  14.1 $\pm$ 5.6 |  1.0 $\pm$ 0.4 |  13 $\pm$ 5 |  -13 $\pm$ 7

a. Core number in parentheses from Ward-Thompson et al. (2016)
b. Values taken from Ward-Thompson et al. (2016)
c. All angle values are measured east of north
d. We adopt $\pm 10\degree$ for the local filament angle except in cores 4 and
8 (see Section 3.2)
e. Column density values from Gould Belt Survey (Palmeirim et al., 2013)
f. AV calculated using NH/A${}_{V}\sim 2.2\times\,10^{21}$ cm-2 mag-1 (Güver &
Özel, 2009) and N($H_{2}$)$\sim$0.5 NH

##  2 Observations and Data Reduction

###  2.1 SCUBA2-POL2 Observations

L1495 was observed at 450 and 850 $\mu$m, using SCUBA2-POL2 on the JCMT.
SCUBA2 only detects Stokes I, whereas POL2 also detects Q and U. Observations
were carried out as part of the BISTRO large program between January 2020 and
January 2021 (Project ID: M17BL011). Additional observations as part of BISTRO
were further carried out between March and November 2021 (Project ID:
M20AL018). There were 20 observations completed, each of $\approx$41 minutes
for a total on source time of $\approx$14 hours. The observations were carried
out in weather bands 1 and 2 which is equivalent to atmospheric opacity at 225
GHz ($\tau_{225}$) $\leq$0.05 for the first sets of observations and in the
range 0.05–0.08 for the later observations respectively.

The JCMT has a primary dish diameter of 15 m and a beamsize of 14$\farcs$6 at
850 $\mu$m when approximated with a two-component Gaussian (Dempsey et al.,
2013). This corresponds to roughly 0.01 pc at a nominal distance to Taurus of
140 pc (Torres et al., 2009; Schlafly et al., 2014; Roccatagliata et al.,
2020). The observations were carried out using a SCUBA2 DAISY mode that is
optimized for POL2 observations. This produces a central 3$\arcmin$ diameter
region with uniform coverage and exposure time. Coverage decreases and noise
increases towards the edge of the field of view (Holland et al., 2013). This
mode has a scan speed of 8 arcsec/sec and a half-wave plate with a rotation
frequency of 2 Hz (Friberg et al., 2016).

###  2.2 Data Reduction

The data were reduced using the Submillimetre User Reduction Facility (SMURF)
package (Chapin et al., 2013) from the Starlink software (Currie et al.,
2014). The SMURF package contains the data reduction routine for SCUBA2-POL2
observations named pol2map which is used for both 450 and 850 $\mu$m. In this
paper we only consider the 850-$\mu$m data.

Initially, the raw bolometer timestreams are separated into separate Stokes I,
Q and U timestreams. The command makemap (Chapin et al., 2013) is then called
to create an initial Stokes I map from the Stokes I timestreams. The second
step of the reduction creates the final Stokes I, Q and U maps and a
polarization half-vector catalog. Note that we refer to the polarization
measurements as half-vectors because they have a 180∘ ambiguity.

We included the parameters skyloop and mapvar in our reduction (Chapin et al.,
2013). Skyloop allows for each observation to be compared to each other at
each iteration, as opposed to the previous map-making method where each
individual set of observations would iterate individually and then be coadded
at the end. Using skyloop allows us to pick up fainter, more extended
emission.

Normally, variances in the map are calculated as linear combinations of the
variances of each observation (where variances are calculated by the spread of
bolomoter signals which fall within the pixel). However when using mapvar,
variances are calculated by the spread across the individual observations.

The reduction was carried out using a pixel size of 8$\arcsec$. This is a
larger pixel size than previous SCUBA2-POL2 reductions which tend to use a
pixel size of 4$\arcsec$. However, with lower flux density starless sources
such as those in L1495A, a larger pixel size increases the signal-to-noise
ratio of the Stokes I, Q and U maps.

The 850-$\mu$m Stokes I, Q and U maps were also multiplied by a Flux
Conversion Factor (FCF) of 748 Jy beam-1 pW-1 to convert from pW to Jy beam-1
and account for loss of flux from POL2 inserted into the telescope. This value
is the standard 495 Jy beam-1 pW-1 for reductions using 4$\arcsec$ pixels,
multiplied by the standard 1.35 factor from POL2 (Mairs et al., 2021) and then
multiplied by a factor of 1.12 to account for the 8$\arcsec$ pixels. This
factor was determined from SCUBA2 calibration
plots.111https://www.eaobservatory.org/jcmt/instrumentation/continuum/SCUBA2/calibration

To further increase the signal-to-noise ratio (SNR) of our polarization half-
vectors, we binned them to a resolution of 12$\arcsec$ as is standard in
previous BISTRO work and to roughly match the primary beam size of the JCMT at
850 $\mu$m (Dempsey et al., 2013). The polarization half-vectors are also
debiased as described by Wardle & Kronberg (1974) to remove statistical bias
in regions of low SNR (see Equation 1).

The values for the debiased degree of polarization $P$ were calculated from

$P=\frac{1}{I}\sqrt{Q^{2}+U^{2}-\frac{1}{2}(\delta Q^{2}+\delta U^{2})}\,\,,$
(1)

where I, Q, and U are the Stokes parameters, and $\delta Q$, and $\delta U$
are the uncertainties in Stokes Q and U. The uncertainty $\delta P$ of the
degree of polarization was obtained using

$\delta P=\sqrt{\frac{(Q^{2}\delta Q^{2}+U^{2}\delta
U^{2})}{I^{2}(Q^{2}+U^{2})}+\frac{\delta I^{2}(Q^{2}+U^{2})}{I^{4}}}\,\,,$ (2)

with $\delta I$ being the uncertainty in Stokes I (total intensity).

The polarization position angles $\theta$, measured from north through east on
the plane of the sky, were calculated using the relation

${\theta=\frac{1}{2}{\rm tan}^{-1}\frac{U}{Q}}\,.$ (3)

The corresponding uncertainties in $\theta$ were calculated using

$\delta\theta=\frac{1}{2}\frac{\sqrt{Q^{2}\delta U^{2}+U^{2}\delta
Q^{2}}}{(Q^{2}+U^{2})}\times\frac{180\degree}{\pi}\,\,.$ (4)

We chose SNR cuts of I/$\delta$I $>$ 10 and P/$\delta$P $>$ 2, as is standard
for making SCUBA2-POL2 polarisation maps. This corresponded to a minimum flux
density of around 10 mJy/beam. The number of individual positions at which we
detect polarisation at this level varies in each core from a couple up to 20
or more. Typically, all of the half-vectors in each core were seen to be lying
roughly parallel to each other, so we subsequently averaged the measured
polarisation half-vectors as a weighted mean (weighted according to their
SNR), averaged over the FWHM size of each core (see Table 1), to make a single
polarisation measurement on each core.

The plane-of-sky orientation of the magnetic field is inferred by rotating the
polarization angles by 90$\degree$, assuming that the polarization is caused
by elongated dust grains aligned perpendicular to the magnetic field (see
Andersson et al., 2015, and references therein). This is true for dust grains
in the Rayleigh regime (Kirchschlager et al., 2019; Guillet et al., 2020), and
for observations at 850 $\mu$m this is fulfilled for dust grains with sizes up
to roughly 100 $\mu$m, which form the majority of those we observe. This
single measurement of the B-field is what is plotted at the position of each
core in Figure 2 and listed in Table 1 below.

##  3 Results

Figure 2: Same background image as Figure 1. The filaments seen in the
original SCUBA2 and Herschel images are clearly seen and are labelled ‘A’, ‘B’
and ‘C’, as described in the text. The cores that we identify in this paper
are numbered 1 to 8 as described in the text. Superposed on this image are
rotated polarization half-vectors to demonstrate the orientation of the
magnetic field. The red vectors show the mean magnetic field orientation in
each core from the POL2 observations. The yellow vectors show the orientation
of the large-scale magnetic field (over-sampled) from Planck observations
(there are only about 4 independent Planck beams in this whole field of view).
The blue half-vectors show the local filament major axis orientation (for core
8 we only show the axis of filament B). Each set of vectors in this image has
a constant length for clarity.

###  3.1 Cores

Figure 1 (left) shows a false-colour image (adapted from Ward-Thompson et al.
(2016)), in which green and blue are taken from the Herschel-SPIRE images of
the region at 500 and 250 $\mu$m respectively, and red is the SCUBA2
850-$\mu$m Stokes I emission. The much larger-scale B211-213 L1495 filament
(e.g. Marsh et al., 2016) can be seen beginning at the southern edge of this
field and extending southwards out of the field of view. Figure 1 (right)
shows a close-up of the triangle of filamentary structures observed by Ward-
Thompson et al. (2016). This is the region observed by BISTRO. The dense cores
show up clearly in the 850-$\mu$m data as red sources, as mentioned above.
These are all starless cores and candidate pre-stellar cores (Ward-Thompson et
al., 2016; Howard et al., 2019).

We here assign the cores numbers as shown in Figure 2, which has the same
background as Figure 1 (right). One core where we have detected polarisation
was not identified as a core by Ward-Thompson et al. (2016), as it was only
detected very faintly in the earlier work. This is labelled on Figure 2 as
core 4. Hence, if we include core 4 amongst the list of cores detected
earlier, we find that we only detect polarisation at this level on the cores
themselves. We also note that core 2 appears to be split into two cores in the
Stokes I image, which we here label 2N and 2S. Core 2 was not identified as a
double core by Ward-Thompson et al. (2016). The two cores 2N and 2S have very
different polarisation orientations. We therefore have a sample of 9 cores.

Table 1 lists the cores detected in L1495A here in our BISTRO POL2 survey. We
also state the core numbers assigned to them by Ward-Thompson et al. (2016),
who fitted elliptical gaussians to each of the cores and we list the
properties of those ellipses also in Table 1 (recall that 30′′ is roughly 0.02
pc at a nominal distance of 140 pc to Taurus). We note that core 21 from Ward-
Thompson et al. (2016) is in our field of view, but no detectable polarised
emission was observed at this position. For cores 2N, 2S and 4 we calculated
the parameters of the elliptical gaussians here.

Table 1 also lists the orientation of the local filament major axis at the
position of each core, which we measure here from Figure 2, and the core major
axis orientation. Core 8 has two filament angles listed, because it sits at
the junction of two filaments. The value that is in brackets is for the
western filament (see also below).

Table 1 further lists the chief core parameters of temperature as measured by
Ward-Thompson et al. (2016) and column density which was calculated by
Palmeirim et al. (2013) using the Herschel bands at the resolution of SPIRE
250 $\mu$m. We calculate the volume number density in Table 1 using the column
density values and the 850 $\mu$m core sizes from Ward-Thompson et al. (2016).
The optical extinction values from Table 1 are calculated using the column
density values. We also list the weighted mean of the polarisation position
angle in each core from this work measured north through east.

Figure 3: A plot of core magnetic field orientation for each core on the
y-axis, versus 90o minus its local filament major axis angle on the x-axis.
The cores are numbered as in Figure 2. The solid line indicates a one-to-one
correlation, which is where the points would be located if the B-field lay
exactly orthogonal to the local filament orientation in every case. The two
dashed lines represent $\pm$20o, roughly matching our predicted systematic
angle error at our chosen signal-to-noise ratio cut-off. The shaded area is
$\pm$45∘. See text for details.

Figure 2 shows the SCUBA2-POL2 polarisation half-vectors overlaid in red,
rotated by 90o to indicate the orientation of the plane-of-sky magnetic field,
within each dense core. As explained above, we refer to them as half-vectors
because they have a 180o ambiguity (i.e. we don’t know which end of the
B-field half-vectors to put the arrow on). Each half-vector represents the
weighted mean polarisation angle measured for each core respectively, rotated
by 90o.

All half-vectors are shown at the same length for clarity. Also shown on
Figure 2 in yellow are the half-vectors of the Planck measurements (over-
sampled), also rotated by 90o to indicate the large-scale plane-of-sky
magnetic field orientation. Additionally, the blue vectors in each core
indicate the local filament major axis as listed in Table 1.

###  3.2 Fields and filaments

Examination of Figure 2 shows a number of interesting features. The filaments
form a roughly triangular pattern on the sky. We here label the western
filament ‘Filament A’, the eastern filament ‘Filament B’ and the southern
filament that runs roughly east-west ‘Filament C’. Cores 1, 2 and 3 lie in
Filament A, with cores 5, 6 and 7 in Filament B. Core 4 lies in Filament C.
Core 8 lies at the northern apex of the triangle where Filaments A and B meet.

Figure 2 shows that the magnetic field in cores 1, 2N and 3 lies roughly
orthogonal to the local filament (Filament A) long axis. In core 2S the
magnetic field is not orthogonal, but at this point the filament turns through
90o, so it is difficult to uniquely define a filament orientation. Core 4 has
a magnetic field orientation roughly orthogonal to the western half of
Filament C, which turns somewhat at the position of core 4. The magnetic field
orientations in cores 5, 6 and 7 also lie roughly orthogonal to their local
filament (Filament B) major axis orientation. Core 7 is slightly further from
orthogonal than the other two, although we note that here also the local
filament orientation turns slightly. The magnetic field of core 8 is roughly
orthogonal to filament B.

Figure 3 shows a plot of core magnetic field orientation for each core on the
y-axis, versus 90o minus its local filament major axis angle on the x-axis, in
order to quantify the above discussion. Any angle that lay between 180o and
360o has had 180o subtracted from it and any angle that lay between 0o and
$-$180o has had 180o added to it due to the fact that both the magnetic field
orientation and the filament orientation are half-vectors, as discussed above.
The exception to this was core 4, whose error-bar overlaps the origin, so we
extend the plot to slightly negative numbers to accommodate core 4.

Figure 4: Artist’s impression of the three stages of evolution of the
filaments and cores in L1495A-B10. The grey-scale represents the observer’s
view of column density, as seen in Figure 2. The dashed lines with arrows
represent the magnetic field and circles represent the positions of cores. The
magnetic field lies initially mostly along the line of sight, with only a
small component in the plane of the sky: (a) The magnetically-dominated phase:
the initial cloud is threaded by a large-scale, uniform magnetic field,
largely along the line of sight, as the matter forms a sheet in the plane of
the sky; (b) The intermediate phase: the sheet starts to fragment and form
filaments, in which low-mass core formation starts; (c) The matter-dominated
phase: once the matter of the fragmenting structures crosses a critical
surface density, the evolution becomes matter dominated and the subsequent
evolution of the matter affects the magnetic field to the extent that the
local field is turned to lie perpendicular to the long axis of the local
filament orientation at the position of each core.

We took the error-bar in filament orientation to be $\pm$10o as an indication
of how accurately we could measure this orientation. For core 4 where the
filament is curving we took the filament orientation to be the tangent to the
curve and the error-bar to be the amount of curvature. For core 8, which lies
at the junction of two filaments we took the local filament orientation to be
that of filament B, which is denser. The error-bar used in the B-field
orientation in each case is the standard deviation of the angles of the
weighted mean half-vectors shown in Figure 2 and listed in Table 1.

If all B-field orientations lay exactly orthogonal to their respective
filaments, then this plot would show a one-to-one correlation. This is shown
by the solid line on Figure 3. We also plot two dashed lines, at $+$20o and
$-$20o from orthogonal. This is the typical systematic error that we would
expect for a 2-$\sigma$ detection of polarization (Naghizadeh-Khouei & Clarke,
1993), the value we chose for our cut, as described above. The shaded area
represents $\pm$45∘.

It can be seen that there is a good degree of correlation in this plot, with
most of the points consistent (including error bars) with lying between, or
very close to, the dashed lines, and all lying within the shaded area. Hence
we conclude that the magnetic fields that we have measured in these cores
generally tend to lie closer to orthogonal than parallel to the local filament
orientation in which the core is embedded. The core that lies furthest from
the correlation line is 2S. This is one of the most dense cores and may have
been affected by the proximity of core 2N or the effect of the filament
changing orientation from the south to the north of core 2S.

A similar plot to Figure 3 looking for a correlation between B-field
orientation and core major axis shows no correlation whatsoever. Likewise a
plot of our measured small-scale B-field position angles shows no correlation
with the large-scale B-field orientation measured by Planck, as shown in
Figure 2. It can be seen that the local field that we have measured in the
starless cores within the filaments has totally dissociated from the large-
scale field orientation seen by Planck, and there is no correlation between
them.

The exception to this is core 4, whose small-scale B-field that we have
measured lies almost exactly parallel to the large-scale B-field orientation
measured by Planck – see Figure 2. This may be a coincidence, because the
B-field in core 4 also lies roughly perpendicular to its host filament.
However, we note that core 4 and the filament in which it sits have the lowest
column density of any of our cores by a factor of 1.5–2 (see Table 1),
calculated using standard conversion factors (Güver & Özel, 2009; Marsh et
al., 2016). Thus it may be the youngest core.

##  4 Discussion: An evolutionary scenario

All of the foregoing leads us to propose an evolutionary scenario for this
cloud that is illustrated graphically in Figure 4. This is an artist’s
impression of the three stages of evolution of the filaments and cores in
L1495A-B10. The grey-scale represents the observer’s view of column density,
as seen in Figure 2. The dashed lines with arrows represent the magnetic field
and circles represent the positions of cores.

We split this evolution into three distinct phases, illustrated by the three
panels in Figure 4. We label these phases the magnetically-dominated phase
(4a), the intermediate phase (4b) and the matter-dominated phase (4c). Figure
4 is an attempt to reproduce the image seen in Figure 2 at different times
during this cloud’s evolution.

###  4.1 The magnetically-dominated phase

This phase is illustrated in Figure 4(a) and is predicted by theory to be the
earliest phase of evolution from an interstellar cloud to the formation of a
star. Flux-freezing in this phase predicts that the magnetic field should have
a significant effect on any structure that is formed. This phase is also
commonly referred to as being magnetically sub-critical (Crutcher, 2004).

Hence we hypothesise a scenario to explain why, in our observations herein of
L1495A-B10 on the plane of the sky, the filament orientations appear totally
unrelated to one another and to the orientation of the large-scale Planck
field. We hypothesise that the B-field lies mostly along the line-of-sight
orientation, with a small component in the plane of sky. Note that this can be
explained, because, for a small range of viewing orientations around the
B-field axis, there will be both a clearly visible filament, and a significant
component of the field apparently approximately perpendicular to the filament.

If the large-scale field does indeed lie close to the line of sight to the
cloud, then the percentage polarisation seen in the Planck data would be lower
at this point than elsewhere in Taurus. We looked at the Planck data (Planck
XIX) and found that this is in fact is the case. L1495A-B10 does indeed lie in
a ‘polarisation hole’ (a minimum of percentage polarisation) relative to other
nearby regions of Taurus. Hence, our hypothesis to explain our data is also
consistent with the Planck data, although this does not constitute proof,
since polarisation holes can also be caused by other mechanisms.

In this magnetically-dominated phase, the material moves mostly along the line
of sight, parallel to the B-field, both towards and away from the observer
(see Figure 4a). These counter-flows towards and away from us, that feed
matter into a denser layer, are assumed to be oblique, and therefore produce a
mildly sheared layer, as one would expect for the general case. Since we are
assuming ideal MHD the flow is mostly aligned with the magnetic field (the
path of least resistance). Any motion of the matter perpendicular to the field
lines has to drag the field with it.

Consequently, the material that we see lies mostly in a sheet-like structure
close to the plane of the sky – i.e. we are seeing the sheet almost face-on.

We illustrate the evolution occurring during this phase in Figure 5. This
cartoon illustrates the view roughly orthogonal to the plane-of-sky views in
Figures 2 and 4. Here the magnetic field (in red) is orthogonal to the viewing
angle. The observer is either looking upwards from the bottom of this image or
downwards from the top.

On the left of Figure 5 we see the initial cloud threaded by a magnetic field.
The blue arrows on the cloud edges indicate the principal directions of
collapse along the magnetic field orientation. On the right we see the cloud
at the end of the magnetically-dominated phase. The cloud has now collapsed to
a sheet-like structure, here seen edge-on, orthogonal to the orientation of
the magnetic field.

Figure 5: Cartoon illustrating the view roughly orthogonal to the plane-of-sky
views in Figures 2 and 4. Here the magnetic field (red arrows) is orthogonal
to the viewing angle. The blue arrows are indicating the direction the matter
is flowing. The observer is either looking upwards from the bottom of this
image or downwards from the top. The transition from left to right is
indicating the evolution occurring during the phase illustrated in Figure
4(a), but here seen edge-on. See text for discussion.

###  4.2 The intermediate phase

Once the structures are sufficiently dense to cross a certain critical surface
density threshold, the gas motions of the cloud start to transition from
magnetically-dominated to matter-dominated. The sheet fragments under self-
gravity and filaments form in a fairly complex web-like manner, all orthogonal
to the large-scale magnetic field (see Figure 4b). The convergent velocity
field producing each filament (i.e. an approximately linear over-density
within the layer) requires a kink in the magnetic field, so that material can
slide down the field lines into the filament.

Simulations suggest that the transition to matter-dominated dynamics is
associated with a transition from the magnetic field being preferentially
parallel to density structures at low column densities to preferentially
perpendicular at high column densities (Soler et al., 2013). This transition
appears to require converging, likely super-Alfvénic, gas flows (Soler &
Hennebelle, 2017), and is likely associated with, and potentially a signature
of, the transition of the region to gravitational instability (Chen et al.,
2016). Although the precise mechanism by which (and gas density at which) this
transition occurs varies between models (see, e.g. Pattle et al. 2022a for a
recent review), a large-scale magnetic field strong enough to impose a
preferred gas flow direction parallel to the field at low densities is
typically required (Soler & Hennebelle, 2017).

Figure 4(b) is intended to show the cloud in the process of crossing from the
magnetically-dominated phase to the matter-dominated phase. Clearly, this
change is not instantaneous and takes a finite length of time. The period of
the transition is what we here refer to as the intermediate phase seen in
Figure 4(b). We here hypothesise that core 4 may still be in this phase, with
its magnetic field orientation still parallel to that of the initial large-
scale field.

###  4.3 The matter-dominated phase

We would expect the filaments starting to form low density cores would cause
the realignment of the magnetic field orientations within them (see change
from Figure 4b to c). This phase is sometimes referred to as being
magnetically super-critical (Crutcher, 2004), or matter-dominated (our chosen
term).

This is now the scenario depicted in Figure 4(c), which is intended to show
the picture largely seen in the observations across the majority of Figure 2.

Thus we see that the filaments influence the local small-scale magnetic field
orientation, turning it to tend to lie apparently roughly orthogonal to the
long axis of the local filament (see Figure 4c). Thereafter, the filaments
start to form low density cores. All cores in L1495A-B10 other than core 4 are
here hypothesised to be in this phase.

The cores and filaments then start to increase in density due to accretion of
surrounding material. However, the cores are still at an early stage in the
star-forming process, since none of them contains a compact protostellar
object yet (Ward-Thompson et al., 2016). Similarly, the filaments are probably
also in a fairly early stage of development. This is a slight variation on a
picture that has previously been proposed – e.g. André et al. (2014); Seo et
al. (2019) – in that it adds an extra transitional evolutionary phase to the
picture that we have here observed for the first time.

Simulations suggest that a transition in magnetic field orientation within
molecular clouds from lying parallel to perpendicular to density structure
occurs only in the case where magnetic fields are dynamically important on
large scales (e.g. Soler et al., 2013; Seifried et al., 2020). Planck
Collaboration et al. (2016) find a significantly sub-critical mass-to-flux
ratio of $\sim 0.2-0.4$ on large scales in Taurus, while Soler (2019) find
from Planck observations that in the L1495/B213 filament (just to the south of
the region studied here), a transition from preferentially parallel to
preferentially perpendicular occurs at $N_{\rm H}\sim 10^{21.5}\,$cm-2. The
balance of evidence suggests that the cloud is still magnetically-dominated on
large scales.

Note that our scenario for the evolution of L1495A-B10 is consistent with
BISTRO observations of other nearby filaments with embedded starless cores
(except that the field is not along the line of sight), including Perseus NGC
1333 (Doi et al., 2020), Ophiuchus L1689 (Pattle et al., 2021) and Serpens
Main (Kwon et al., 2022). All of these clouds have magnetic field orientations
consistent with being perpendicular to the local filament orientation, and
apparently agnostic to the large-scale magnetic field orientation in the
region.

###  4.4 Comparison with theory

The magnetically- to matter-dominated transition critical volume density is
not well-defined. Different theoretical simulations produce transitions at
volume number densities in the range $10^{2}-10^{6}\,$cm-3, with no apparent
dependence on Alfvén Mach number (Pattle et al., 2022a). We thus need
observational constraints on the transition critical surface density in a
range of environments.

However, calculations of the critical column density at which this transition
occurs date back over 50 years (e.g. Mestel, 1965, – see his equation 85), in
which a relatively idealised case of a uniform magnetic field threads a
uniform density cloud (see also Ward-Thompson & Whitworth, 2011, equation
4.82). Figure 5 shows a simplified cartoon of the picture we are hypothesising
of a cloud collapsing to a sheet, followed by the fragmentation of the sheet
to form filaments and cores (Figure 4) above a certain sheet critical surface
density, which we are referring to as the surface density of the transition
from the magnetically- to the matter-dominated phase.

The above-cited equations lead to a prediction of the relation between the
critical surface density, $\Sigma_{c}$, and the magnetic field strength, $B$,
at which the transition occurs, of the form

$\Sigma_{c}\,=\,(5/G)^{1/2}\,(B/3\pi)\,$ (5)

which yields

$[N({\rm H_{2}})/{\rm cm}^{-2}]\,\simeq\,2\times 10^{20}\,\times\,[B/\mu
G]\,.$ (6)

This equation contains uncertainties of a small numerical factor of order
unity (e.g. Nakano & Nakamura, 1978) but is nevertheless a useful theoretical
order-of-magnitude prediction.

Our hypothesised scenario outlined in Sections 4.1-4.3 above, together with
our interpretation of the data in Section 3.2 that core 4 may be the youngest
core and still undergoing the change from the magnetically- to the matter-
dominated phase, would lead us to set the critical column density N(H2) at
around 9.2 $\times$ 1021 cm-2. Inserting this value into equation (6) above
predicts an order-of-magnitude magnetic field strength in L1495A-B10 of
$\sim$46$\mu$G. If core 4 has passed the critical point of the intermediate
phase, then this B-field value is an upper limit.

This compares to a literature value of 25–77 $\mu$G from optical and NIR
observations (Chapman et al., 2011) for L1495A-B10. The equivalent Planck
value is 13–32 $\mu$G (Planck Collaboration et al., 2016) for the large-scale
field around the Taurus region as a whole.

We performed an approximate Davis-Chandrasekhar-Fermi (Davis, 1951;
Chandrasekhar & Fermi, 1953, DCF) analysis in core 1, where we have sufficient
half-vectors to make this statistical analysis. For this DCF analysis (e.g.
Pattle et al., 2021), we used $\Delta V_{NT}$=0.206 km s-1 which is found from
NH3 velocity dispersion observations (Seo et al., 2015) and then removed the
thermal component assuming a temperature of $\sim$10 K. We calculated a
dispersion in the magnetic field position angle of 11.8o and used the n(H2)
value of core 1 from Table 1. We obtained an upper limit of $\approx$70
$\mu$G.

To summarise all of the above: we have hypothesised that L1495A-B10 is
evolving from the magnetically-dominated phase to the matter-dominated phase,
as the face-on sheet that we are observing starts to fragment into filaments
and cores. We have further hypothesised that our observed core 4 may be
exactly in that transition phase. We have used theoretical arguments to
predict what field strength would correspond to this transition at the column
densities we observe. Our theoretical prediction for the field strength
required is $\sim$46$\mu$G. We have estimated the field strength and, along
with literature values, found a range of observed field strengths of 13–32,
25–77 and $\leq$70 $\mu$G.

Hence we see that the whole picture is mutually self-consistent. The theory
predicts a B-field strength that is right in the range of measured values.
Hence the theoretical prediction of Mestel (1965) is consistent with our
observations, and we have managed to fully test this prediction for the first
time in over 50 years.

###  4.5 Subsequent evolution

Using the ratio NH/A${}_{V}\sim 2.2\times\,10^{21}$ cm-2 mag-1 (Güver & Özel,
2009), the corresponding value of AV for core 4 at the column density
discussed above is A${}_{V}\sim 8$ (assuming N($H_{2}$)$\sim$0.5 NH). This is
consistent with the star formation threshold at which molecular clouds can
begin to form stars at A${}_{V}\sim$7–8, discussed by, e.g. Johnstone et al.
(2004); Könyves et al. (2015); Marsh et al. (2016), among others, and with the
observations collated by Soler (2019), given that it has not yet begun to form
stars.

The subsequent behaviour of magnetic fields in regions with embedded young
stellar objects (YSOs) appears to be more complex. Notably, in Taurus B213, a
more evolved region located elsewhere in the filament of which L1495A-B10
forms the head, the average magnetic field orientation within the embedded
dense cores varies significantly, with no clear correlation with either
filament or outflow orientation (Eswaraiah et al., 2021b). More generally, YSO
outflows appear to have the potential to reshape magnetic fields in their
vicinities (e.g. Yen et al., 2021; Lyo et al., 2021; Pattle et al., 2022b),
suggesting that we cannot infer the magnetic initial conditions for star
formation in environments with embedded YSOs.

Therefore, as a region without embedded YSOs, L1495A-B10 represents a more
pristine environment in which to study the behaviour of magnetic fields in the
early stages of filament fragmentation and core formation.

##  5 Summary

In this paper we have presented the first polarization data from BISTRO of the
dark cloud L1495A-B10 in the Taurus molecular cloud, at a wavelength of 850
$\mu$m. The cloud contains a complex pattern of small-scale filaments, in a
roughly triangular morphology, surrounded by a web of lower density filaments.
The main filaments contain cores of varying densities. We detect polarization
from 9 of the cores, all of which have previously been shown to be starless.

We calculate the mean plane-of-sky orientation of the small-scale magnetic
field in each of the cores and compare this to the large-scale field
orientation measured by Planck and to the orientation of the filaments and
long axes of the cores. There is no correlation between our measurements and
those of Planck other than in the lowest density case, core 4, as expected for
the ‘youngest’ core. There is also no correlation at all with the core
orientation.

However, we find a correlation between our small-scale field measurements and
filament orientation, in which the field tends to lie orthogonal to the long
axes of the filaments in all cases except for a core that lies near to a bend
in its local filament.

We hypothesise a scenario to explain our data, which is an extension of the
previous magnetic filamentary model, adding an additional transitional
evolutionary phase to the model, that we here observe for the first time. We
suggest we are seeing a cloud which was originally magnetically-dominated in
the process of transition to being matter-dominated in its denser regions,
with core 4 defining the transition point, since the local field in core 4 is
still aligned with the large-scale field, whereas in the others it tends more
towards lying orthogonal to the local filament.

We measure the sheet surface density and the magnetic field strength of that
transitionary phase for the first time and show consistency with the original
analytical prediction that has gone untested for over 50 years (Mestel, 1965).

D.W.T. acknowledges funding support from the UK STFC through grant number
ST/R000786/1. J.K. acknowledges funding from the Moses Holden Scholarship for
his PhD. K.P is a Royal Society University Research Fellow, supported by grant
number URF\R1\211322 The JCMT is operated by the East Asian Observatory on
behalf of National Astronomical Observatory of Japan; the UK STFC under the
auspices of grant number ST/N005856/1; Academia Sinica Institute of Astronomy
and Astrophysics; the Korea Astronomy and Space Science Institute; the
Operation, Maintenance and Upgrading Fund for Astronomical Telescopes and
Facility Instruments, budgeted from the Ministry of Finance of China. SCUBA2
and POL2 were built through grants from the Canada Foundation for Innovation.
This research used the facilities of the Canadian Astronomy Data Centre
operated by the National Research Council of Canada with the support of the
Canadian Space Agency. The data taken in this paper were observed under the
project code M20AL018. The Starlink software (Currie et al., 2014) is
currently supported by the East Asian Observatory. The authors wish to
recognize and acknowledge the very significant cultural role and reverence
that the summit of Maunakea has always had within the indigenous Hawaiian
community. We are most fortunate to have the opportunity to conduct
observations from this mountain. M.T. is supported by JSPS KAKENHI grant
Nos.18H05442, 15H02063, and 22000005. J.K.is supported by JSPS KAKENHI grant
No.19K14775. F.P. acknowledges support from the Spanish State Research Agency
(AEI) under grant number PID2019-105552RB-C43. F.K. is supported by the
Spanish program Unidad de Excelencia María de Maeztu CEX2020-001058-M,
financed by MCIN/AEI/10.13039/501100011033. CE acknowledges the financial
support from grant RJF/2020/000071 as a part of Ramanujan Fellowship awarded
by the Science and Engineering Research Board (SERB), Department of Science
and Technology (DST), Government of India. W.K. was supported by the National
Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT)
(NRF-2021R1F1A1061794). The work of M.G.R. is supported by NOIRLab, which is
managed by the Association of Universities for Research in Astronomy (AURA)
under a cooperative agreement with the National Science Foundation.

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