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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>
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>
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
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.
Green, Joel D.,Evans II, Neal J.,Jørgensen, Jes K.,Herczeg, Gregory J.,Kristensen, Lars E.,Lee, Jeong-Eun,Dionatos, Odysseas,Yildiz, Umut A.,Salyk, Colette,Meeus, Gwendolyn,Bouwman, Jeroen,Visser, Ruu IOP Publishing 2013 The Astrophysical journal Vol.770 No.2
<P>We present 50-210 mu m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 mu m spectral energy distributions, as part of the Dust, Ice, and Gas in Time Key Program. Some sources exhibit up to 75 H2O lines ranging in excitation energy from 100 to 2000 K, 12 transitions of OH, and CO rotational lines ranging from J = 14 -> 13 up to J = 40 -> 39. [O I] is detected in all but one source in the entire sample; among the sources with detectable [O I] are two very low luminosity objects. The mean 63/145 mu m [O I] flux ratio is 17.2 +/- 9.2. The [O I] 63 mu m line correlates with L-bol, but not with the time-averaged outflow rate derived from low-J CO maps. [C II] emission is in general not local to the source. The sample L-bol increased by 1.25 (1.06) and T-bol decreased to 0.96 (0.96) of mean (median) values with the inclusion of the Herschel data. Most CO rotational diagrams are characterized by two optically thin components (< N > = ( 0.70 +/- 1.12) x 10(49) total particles). N-CO correlates strongly with L-bol, but neither T-rot nor N-CO(warm)/N-CO(hot) correlates with L-bol, suggesting that the total excited gas is related to the current source luminosity, but that the excitation is primarily determined by the physics of the interaction (e.g., UV-heating/shocks). Rotational temperatures for H2O (< T-rot > = 194 +/- 85 K) and OH (< T-rot > = 183 +/- 117 K) are generally lower than for CO, and much of the scatter in the observations about the best fit is attributed to differences in excitation conditions and optical depths among the detected lines.</P>
Green, Joel D.,Yang, Yao-Lun,II, Neal J. Evans,Karska, Agata,Herczeg, Gregory,Dishoeck, Ewine F. van,Lee, Jeong-Eun,Larson, Rebecca L.,Bouwman, Jeroen American Astronomical Society 2016 The Astronomical journal Vol.151 No.3
<P>We present the COPS-DIGIT-FOOSH (CDF) Herschel spectroscopy data product archive, and related ancillary data products, along with data fidelity assessments, and a user-created archive in collaboration with the Herschel-PACS and SPIRE ICC groups. Our products include datacubes, contour maps, automated line fitting results, and best 1D spectra products for all protostellar and disk sources observed with PACS in RangeScan mode for two observing programs: the DIGIT Open Time Key Program (KPOT_nevans_1 and SDP_nevans_1; PI: N.. Evans), and the FOOSH Open Time Program (OT1_jgreen02_2; PI: J. Green). In addition, we provide our best SPIRE-FTS spectroscopic products for the COPS Open Time Program (OT2_jgreen02_6; PI: J. Green) and FOOSH sources. We include details of data processing, descriptions of output products, and tests of their reliability for user applications. We identify the parts of the data set to be used with caution. The resulting absolute flux calibration has improved in almost all cases. Compared to previous reductions, the resulting rotational temperatures and numbers of CO molecules have changed substantially in some sources. On average, however, the rotational temperatures have not changed substantially (<2%), but the number of warm (T-rot similar to 300 K) CO molecules has increased by about 18%.</P>
Yoo, Hyunju,Lee, Jeong-Eun,Mairs, Steve,Johnstone, Doug,Herczeg, Gregory J.,Kang, Sung-ju,Kang, Miju,Cho, Jungyeon American Astronomical Society 2017 The Astrophysical Journal Vol.849 No.1
<P>During the protostellar phase of stellar evolution, accretion onto the star is expected to be variable, but this suspected variability has been difficult to detect because protostars are deeply embedded. In this paper, we describe a submillimeter luminosity burst of the Class I protostar EC 53 in Serpens Main, the first variable found during our dedicated JCMT/SCUBA-2 monitoring program of eight nearby star-forming regions. EC. 53 remained quiescent for the first six months of our survey, from 2016 February to August. The submillimeter emission began to brighten in 2016 September, reached a peak brightness of 1.5 times the faint state, and has been decaying slowly since 2017 February. The change in submillimeter brightness is interpreted as dust heating in the envelope, generated by a luminosity increase of the protostar of a factor of >= 4 The 850 mu m light curve resembles the historical K-band light curve, which varies by a factor of similar to 6 with a 543 period and is interpreted as accretion variability excited by interactions between the accretion disk and a close binary system. The predictable detections of accretion variability observed at both near-infrared and submillimeter wavelengths make the system a unique test-bed, enabling us to capture the moment of the accretion burst and to study the consequences of the outburst on the protostellar disk and envelope.</P>