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Et Al, ‘How Do Stars Gain Their Mass? a JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star- Forming Regions’, the Astrophysical Journal, Vol

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Et Al, ‘How Do Stars Gain Their Mass? a JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star- Forming Regions’, the Astrophysical Journal, Vol Research Archive Citation for published version: Gregory J. Herczeg, et al, ‘How Do Stars Gain Their Mass? A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star- forming Regions’, The Astrophysical Journal, Vol. 849 (43), October 2017. DOI: https://doi.org/10.3847/1538-4357/aa8b62 Document Version: This is the Accepted Manuscript version. The version in the University of Hertfordshire Research Archive may differ from the final published version. Copyright and Reuse: © 2017 The American Astronomical Society. All rights reserved. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder. Enquiries If you believe this document infringes copyright, please contact the Research & Scholarly Communications Team at [email protected] Draft version September 8, 2017 Preprint typeset using LATEX style emulateapj v. 01/23/15 HOW DO STARS GAIN THEIR MASS? A JCMT/SCUBA-2 TRANSIENT SURVEY OF PROTOSTARS IN NEARBY STAR FORMING REGIONS Gregory J. Herczeg1, Doug Johnstone2,3, Steve Mairs2,3, Jennifer Hatchell4, Jeong-Eun Lee5, Geoffrey C. Bower6, Huei-Ru Vivien Chen7, Yuri Aikawa8, Hyunju Yoo9,5, Sung-Ju Kang10, Miju Kang10, Wen-Ping Chen11, Jonathan P. Williams12, Jaehan Bae13, Michael M. Dunham14,15, Eduard I. Vorobiov16,17,18, Zhaohuan Zhu19, Ramprasad Rao20, Helen Kirk2, Satoko Takahashi21,22, Oscar Morata20, Kevin Lacaille23, James Lane3, Andy Pon24, Aleks Scholz25, Manash R. Samal11, Graham S. Bell26, Sarah Graves26, E'lisa M. Lee26, Harriet Parsons26, Yuxin He27, Jianjun Zhou27, Mi-Ryang Kim28, Scott Chapman23, Emily Drabek-Maunder29, Eun Jung Chung10, Stewart P. S. Eyres30, Jan Forbrich 31,15, Lynne A. Hillenbrand32, Shu-ichiro Inutsuka33, Gwanjeong Kim10, Kyoung Hee Kim34, Yi-Jehng Kuan35,20, Woojin Kwon10,36, Shih-Ping Lai7,20, Bhavana Lalchand11, Chang Won Lee10,36, Chin-Fei Lee20 Feng Long1,37, A-Ran Lyo10, Lei Qian38, Peter Scicluna20, Archana Soam10, Dimitris Stamatellos30, Shigehisa Takakuwa39, Ya-Wen Tang20, Hongchi Wang40, Yiren Wang1,37 Draft version September 8, 2017 ABSTRACT Most protostars have luminosities that are fainter than expected from steady accretion over the pro- tostellar lifetime. The solution to this problem may lie in episodic mass accretion { prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and ampli- tude for variability at the protostellar phase is almost entirely unconstrained. In A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions, we are monitoring monthly with SCUBA-2 the sub-mm emission in eight fields within nearby (< 500 pc) star forming regions to mea- sure the accretion variability of protostars. The total survey area of 1:6 sq.deg. includes 105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded∼ protostars or disks)∼ and 237 peaks of 0.125{0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of sub-mm observations to reach a precision of 2 3% rms, and also provides quantified confidence in any measured variability. The timescales and∼ amplitudes− of any sub- mm variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for sub-mm variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars. Subject headings: stars: protostars | stars: formation | submillimeter: general { circumstellar matter 1 Kavli Institute for Astronomy and Astrophysics, Peking Uni- versity, Yiheyuan 5, Haidian Qu, 100871 Beijing, People's Re- 16 Institute of Fluid Mechanics and Heat Transfer, TU Wien, public of China Vienna, 1060, Austria 2 NRC Herzberg Astronomy and Astrophysics, 5071 West 17 Research Institute of Physics, Southern Federal University, Saanich Rd, Victoria, BC, V9E 2E7, Canada Stachki Ave. 194, Rostov-on-Don, 344090, Russia 3 Department of Physics and Astronomy, University of Victo- 18 University of Vienna, Department of Astrophysics, Vienna, ria, Victoria, BC, V8P 5C2, Canada 1180, Austria 4 Physics and Astronomy, University of Exeter, Stocker Road, 19 Physics and Astronomy Department, University of Nevada, Exeter EX4 4QL, UK Las Vegas 5 School of Space Research, Kyung Hee University, 1732, 20 Academia Sinica Institute of Astronomy and Astrophysics, Deogyeong-Daero, Giheung-gu Yongin-shi, Gyunggi-do 17104, P. O. Box 23-141, Taipei 10617, Taiwan Korea 21 Joint ALMA Observatory, Alonso de C´ordova 3107, Vi- 6 arXiv:1709.02052v1 [astro-ph.SR] 7 Sep 2017 Academia Sinica Institute of Astronomy and Astrophysics, tacura, Santiago, Chile 645 N. A'ohoku Place, Hilo, HI 96720, USA 22 National Astronomical Observatory of Japa, 2-21-1 Osawa, 7 Department of Physics and Institute of Astronomy, National Mitaka, Tokyo 181-8588, Japan Tsing Hua University, Taiwan 23 Department of Physics and Atmospheric Science, Dalhousie 8 Department of Astronomy, University of Tokyo, Tokyo, University, Halifax, NS, B3H 4R2, Canada Japan 24 Department of Physics and Astronomy, The University of 9 Chungnam National University, Korea Western Ontario, 1151 Richmond Street, London, N6A 3K7, 10 Korea Astronomy and Space Science Institute, 776 Canada Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea 25 SUPA, School of Physics & Astronomy, North Haugh, St 11 Graduate Institute of Astronomy, National Central Univer- Andrews, KY16 9SS, United Kingdom sity, 300 Jhongda Road, Zhongli, Taoyuan, Taiwan 26 East Asian Observatory, 660 N. A‘oh¯ok¯uPlace, Hilo, HI 12 Institute for Astronomy, University of Hawaii at Manoa, 96720, USA Honolulu, HI 96822, USA 27 Xinjiang Astronomical Observatory, Chinese Academy of 13 Department of Astronomy, University of Michigan, 1085 S. Sciences, Urumqi, China University Ave., Ann Arbor, MI 48109, USA 28 Department of Physics, Institute for Astrophysics, Chung- 14 Department of Physics, The State University of New York buk National University, Republic of Korea at Fredonia, Fredonia, NY 14063, USA 29 Cardiff University, School of Physics and Astronomy, The 15 Harvard-Smithsonian Center for Astrophysics, 60 Garden Parade, Cardiff, CF24 3AA Street, Cambridge, MA 02138, USA 30 Jeremiah Horrocks Institute for Mathematics, Physics & 2 Herczeg et al. 1. INTRODUCTION tion rate increases of 2-4 orders of magnitude and can Low-mass stars form through the gravitational collapse last for months (called EXors following the prototype of molecular cloud cores. The evolution of mass accretion EX Lup, e.g. Herbig 2008; Aspin et al. 2010) or decades onto a forming protostar depends on the rate at which the (called FUors following the prototype FU Ori, e.g. Her- interior of the core collapses, the role of the circumstel- big 1977; Hartmann & Kenyon 1996). Because most lar disk as a temporary mass reservoir and transporta- transient searches use optical photometry, these accre- tion mechanism, and the physics of how the inner disk tion outbursts are detected only on young stellar objects accretes onto the stellar surface. In dynamical models of that are optically bright and are therefore biased to vari- gravitational collapse of a spherical protostellar core (e.g. ability at or near the end of their main phase of stellar Shu 1977; Shu et al. 1987; Masunaga & Inutsuka 2000), growth. Only a few outbursts have been detected on a the young star grows steadily from the infalling envelope deeply embedded Class 0 star (K´osp´alet al. 2007; Safron −6 −1 et al. 2015; Hunter et al. 2017), the stage when the star at a rate of a few 10 M yr . However, Kenyon et al. (1990) found that luminosities of most protostars fall far should accrete much of its mass { although many FUor below those expected from energy release by steady ac- objects retain some envelopes and are classified as Class I cretion over protostellar lifetimes. This luminosity prob- objects (e.g. Zhu et al. 2008; Caratti o Garatti et al. 2011; lem has since been confirmed with an improved census of Caratti O Garatti et al. 2016; Fischer et al. 2012; Green protostars (e.g., Enoch et al. 2009; Dunham et al. 2010; et al. 2013; K´osp´alet al. 2017). Indirect evidence for out- see also discussions in Dunham et al. 2014 and Fischer bursts includes chemical signatures of past epochs with et al. 2017). high luminosity (e.g. Kim et al. 2012; Vorobyov et al. The resolution of the luminosity problem likely requires 2013; Jørgensen et al. 2015; Frimann et al. 2017) and pe- a either a time-dependent or mass-dependent accretion riodic shocks/bullets along protostellar jets, which may rate (e.g., discussions in Hartmann et al. 2016 and Fis- offer a historical record of accretion events (e.g. Reipurth cher et al. 2017; see also, e.g., Offner & McKee 2011; 1989; Raga et al. 2002; Plunkett et al. 2015). In addition Myers 2012) Observationally, strong but mostly indirect to these large events, instabilities in the inner disk likely evidence suggests that the accretion rate is punctuated lead to more frequent but smaller bursts of accretion, by short bursts of rapid accretion, often termed episodic as seen in more evolved disks (e.g. Costigan et al. 2014; accretion (Kenyon et al. 1990; Dunham et al. 2010; Dun- Venuti et al. 2014; Cody et al. 2017). ham & Vorobyov 2012). The form of this time depen- Directly observing either large outbursts or accretion dence may have a lasting affect on the evolution of stars flickers on protostars is challenging because they are (e.g. Hartmann et al. 1997; Baraffe & Chabrier 2010; deeply embedded in dense envelopes.
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