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Herczeg, Gregory J.,Johnstone, Doug,Mairs, Steve,Hatchell, Jennifer,Lee, Jeong-Eun,Bower, Geoffrey C.,Chen, Huei-Ru Vivien,Aikawa, Yuri,Yoo, Hyunju,Kang, Sung-Ju,Kang, Miju,Chen, Wen-Ping,Williams, Jo American Astronomical Society 2017 The Astrophysical Journal Vol.849 No.1
<P>Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar 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 amplitude 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 measure the accretion variability of protostars. The total survey area of similar to 1.6 sq.deg. includes similar to 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 similar to 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.</P>
A modest outburst and emission structure of very young protostar HOPS 373
Sung-Yong Yoon,Gregory J. Herczeg,Jeong-Eun Lee,Ho-Gyu Lee,Doug Johnstone,Watson Varricatt,John J. Tobin,Carlos Contreras Peña,Steve Mairs,Klaus Hodapp,P. Manoj,Mayra Osorio,S. Thomas Megeath,the JCM 한국천문학회 2022 天文學會報 Vol.47 No.2
Placing the Spotted T Tauri Star LkCa 4 on an HR Diagram
Gully-Santiago, Michael A.,Herczeg, Gregory J.,Czekala, Ian,Somers, Garrett,Grankin, Konstantin,Covey, Kevin R.,Donati, J. F.,Alencar, Silvia H. P.,Hussain, Gaitee A. J.,Shappee, Benjamin J.,Mace, Gre American Astronomical Society 2017 The Astrophysical journal Vol.836 No.2
<P>Ages and masses of young stars are often estimated by comparing their luminosities and effective temperatures to pre-main-sequence stellar evolution tracks, but magnetic fields and starspots complicate both the observations and evolution. To understand their influence, we study the heavily spotted weak-lined T-Tauri star LkCa 4 by searching for spectral signatures of radiation originating from the starspot or starspot groups. We introduce a new methodology for constraining both the starspot filling factor and the spot temperature by fitting two-temperature stellar atmosphere models constructed from PHOENIX synthetic spectra to a high-resolution near-IR IGRINS spectrum. Clearly discernable spectral features arise from both a hot photospheric component T-hot similar to 4100 K and a cool component T-cool similar to 2700-3000 K, which covers similar to 80% of the visible surface. This mix of hot and cool emission is supported by analyses of the spectral energy distribution, rotational modulation of colors and of TiO band strengths, and features in low-resolution optical/near-IR spectroscopy. Although the revised effective temperature and luminosity make. LkCa 4 appear to be. much younger and of much. lower mass than previous estimates from unspotted stellar evolution models, appropriate estimates will require the production and adoption of spotted evolutionary models. Biases from starspots likely afflict most fully convective young stars and contribute to uncertainties in ages and age spreads of open clusters. In some spectral regions, starspots act as a featureless 'veiling' continuum owing to high rotational broadening and heavy line blanketing in cool star spectra. Some evidence is also found for an anticorrelation between the velocities of the warm and cool components.</P>
New Eruptive YSOs from SPICY and WISE
Carlos Contreras Peña,Mizna Ashraf,이정은,Gregory Herczeg,Phil Lucas,Zhen Guo,Doug Johnstone,이호규,Jessy Jose 한국천문학회 2023 Journal of The Korean Astronomical Society Vol.56 No.2
This work presents four high-amplitude variable YSOs (≃3~mag at near- or mid-IR wavelengths) arising from the SPICY catalog. Three outbursts show a duration that is longer than 1 year, and are still ongoing. And additional YSO brightened over the last two epochs of NEOWISE observations and the duration of the outburst is thus unclear. Analysis of the spectra of the four sources confirms them as new members of the eruptive variable class. We find two YSOs that can be firmly classified as bona fide FUors and one object that falls in the V1647 Ori-like class. Given the uncertainty in the duration of its outburst, an additional YSO can only be classified as a candidate FUor. Continued monitoring and follow-up of these particular sources is important to better understand the accretion process of YSOs.