publications

1st author papers

2022

1. Low-mass members of nearby young stellar moving groups from Gaia EDR3

   Jinhee Lee, Inseok Song, and Simon J. Murphy

   MNRAS, April 2022

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   Gaia EDR3 offers greatly improved kinematics for nearby objects, including members of nearby young stellar moving groups (NYMGs). In this study, we aim to identify low-mass NYMG members (spectral types of M0 to mid-L) in Gaia EDR3. We calculated spatio-kinematic membership probabilities by utilizing a Bayesian membership probability calculation scheme developed in our previous study. We evaluated stellar youth primarily based on colour-magnitude diagram positions. Combining spatio-kinematic membership assessment and youth evaluation, we identified ~2900 low-mass NYMG candidate members, including ~700 previously claimed members. In the set of ~2200 new candidate members, ~550 appear to be young based on NUV brightness. Our pilot spectroscopic study with Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope observed 78 candidates, with 79 per cent confirmed as members. Using our new member sample, we estimated an isochronal age of the β Pictoris Moving Group. The mean age (~10 Myr), which is around half the age of recent estimates, suggests either a truly younger age of the β Pictoris Moving Group or inaccuracies in contemporary isochrones. As the main results of this study, we provide lists of newly confirmed and candidate low-mass NYMG members.

2020

1. 2MASS J15460752-6258042: a mid-M dwarf hosting a prolonged accretion disc

   Jinhee Lee, Inseok Song, and Simon J. Murphy

   MNRAS, May 2020

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   We report the discovery of the oldest (∼55 Myr) mid-M type star known to host ongoing accretion. 2MASS J15460752-6258042 (2M1546, spectral type M5, 59.2 pc) shows spectroscopic signs of accretion such as strong H α, He I, and [O I] emission lines, from which we estimate an accretion rate of ∼10-10 M⊙ yr-1. Considering the clearly detected infrared excess in all WISE bands, the shape of its spectral energy distribution (SED) and its age, we believe that the star is surrounded by a transitional disc, clearly with some gas still present at inner radii. The position and kinematics of the star from Gaia DR2 and our own radial-velocity measurements suggest membership in the nearby ∼55 Myr-old Argus moving group. At only 59 pc from Earth, 2M1546 is one of the nearest accreting mid-M dwarfs, making it an ideal target for studying the upper limit on the lifetimes of gas-rich discs around low-mass stars.

2019

1. Evaluation of nearby young moving groups based on unsupervised machine learning

   Jinhee Lee and Inseok Song

   MNRAS, October 2019

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   Nearby young stellar moving groups have been identified by many research groups with different methods and criteria, giving rise to caution regarding the reality of some groups. We aim to utilize moving groups in an unbiased way to create a list of unambiguously recognizable moving groups and their members. For the analysis, two unsupervised machine-learning algorithms (K-means and Agglomerative Clustering) are applied to previously known bona fide members of nine moving groups from our previous study. As a result of this study, we recovered six previously known groups (AB Doradus, Argus, β Pic, Carina, TWA and Volans-Carina). Three other known groups are recognized as well; however, they are combined into two new separate groups (ThOr + Columba and TucHor + Columba).

2019

1. Development of models for nearby young stellar moving groups: creation, revision, and finalization of the models

   Jinhee Lee and Inseok Song

   MNRAS, July 2019

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   In our previous research, we developed a tool for calculating Bayesian membership probability for nearby young stellar moving groups (BAMG: Bayesian Analysis of Moving Groups). The study presented the importance of careful construction of models in moving group membership assessment, using β Pictoris moving group as a test case. In this study, we build models for all major nearby young stellar moving groups (NYMGs hereafter) through four stages. A set of prototype models is created using members listed in the discovery paper of each group. For each group, suggested members after the discovery of the group are used for revising these prototype models. As these additional members being incorporated, group properties of an NYMG changes, thus membership probabilities change as well. A subset of stars shows consistently large membership probabilities regardless of the details of a chosen method for ingesting additional members. Utilizing these members, the NYMG models are finalized. The finalized models are applied for evaluating memberships of all claimed candidate members, resulting in a list of bona fide members. The mass function of bona fide members for the entire set of NYMG members indicates that more late M-type members should be identified. In addition, some objects showing a large difference in membership probability between this study and the other study are presented and discussed. Memberships of some planet host stars are changed, and it can have a significant influence on the estimated planetary masses.

2018

1. Bayesian assessment of moving group membership: importance of models and prior knowledge

   Jinhee Lee and Inseok Song

   MNRAS, April 2018

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   Young nearby moving groups are important and useful in many fields of astronomy such as studying exoplanets, low-mass stars, and the stellar evolution of the early planetary systems over tens of millions of years, which has led to intensive searches for their members. Identification of members depends on the used models sensitively; therefore, careful examination of the models is required. In this study, we investigate the effects of the models used in moving group membership calculations based on a Bayesian framework (e.g. BANYAN II) focusing on the beta-Pictoris moving group (BPMG). Three improvements for building models are suggested: (1) updating a list of accepted members by re-assessing memberships in terms of position, motion, and age, (2) investigating member distribution functions in XYZ, and (3) exploring field star distribution functions in XYZ and UVW. The effect of each change is investigated, and we suggest using all of these improvements simultaneously in future membership probability calculations. Using this improved MG membership calculation and the careful examination of the age, 57 bona fide members of BPMG are confirmed including 12 new members. We additionally suggest 17 highly probable members.

Co-author papers

2023

1. The kinematics of the young stellar population in the W5 region of the Cassiopeia OB6 association: Implication for the formation process of stellar associations

   B. Lim, J. Hong, J. Lee, et al.

   AJ, September 2023

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   The star-forming region W5 is a major part of the Cassiopeia OB6 association. Its internal structure and kinematics may provide hints of the star formation process in this region. Here, we present a kinematic study of young stars in W5 using the Gaia data and our radial velocity data. A total 490 out of 2000 young stars are confirmed as members. Their spatial distribution shows that W5 is highly substructured. We identify a total of eight groups using the k-means clustering algorithm. There are three dense groups in the cavities of H II bubbles, and the other five sparse groups are distributed at the edges of the bubbles. The three dense groups have almost the same age (5 Myr) and show a pattern of expansion. The scale of their expansion is not large enough to account for the overall structure of W5. The three northern groups are, in fact, 3 Myr younger than the dense groups, which indicates independent star formation events. Only one of these groups shows the signature of feedback-driven star formation as its members move away from the eastern dense group. The other two groups might have formed in a spontaneous way. On the other hand, the properties of two southern groups are not understood as those of a coeval population. Their origins can be explained by dynamical ejection of stars and multiple star formation. Our results suggest that the substructures in W5 formed through multiple star-forming events in a giant molecular cloud.

2022

1. A Gaia view on the star formation in the Monoceros OB1 and R1 associations

   B. Lim, Y. Naze, J. Hong, [incl. J. Lee]

   AJ, June 2022

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   Stellar kinematics provides the key to understanding the star formation process. In this respect, we present a kinematic study of the Monoceros OB1 (Mon OB1) and R1 (Mon R1) associations using the recent Gaia data and radial velocities of stars derived from high-resolution spectroscopy and the literature. A total of 728 members are selected using the criteria based on the intrinsic properties of young stars, parallaxes, and proper motions. The spatial distribution and kinematic properties of members show that these associations have distinct substructures. In Mon OB1, we find one northern group and two southern groups. Mon R1 is composed of three small stellar groups that are spatially and kinematically distinct. Some stars are found in a halo around these two associations. We detect patterns of expansion for most stellar groups in the associations. In addition, two stellar groups in Mon OB1 show the signature of rotation, which provides an important constraint on cluster formation. The star formation history of Mon OB1 is slightly revised. Star formation first occurred in the southern region and subsequently in the northern region. Recent star-forming events ignited deeper into the southern region, while some stars are escaping from Mon OB1, forming a halo. Mon R1 might have formed at the same epoch as the formation of the northern group in Mon OB1. Given that star formation is taking place on different scales along a large arc-like structure, Mon OB1 and Mon R1 may be the results of hierarchical star formation.

2021

1. Gemini Planet Imager spectroscopy of the dusty substellar companion HD 206893 B

   K. Ward-Duong, J. Patience, K. Folette, [incl. J. Lee]

   AJ, January 2021

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   We present new near-infrared Gemini Planet Imager (GPI) spectroscopy of HD 206893 B, a substellar companion orbiting within the debris disk of its F5V star. The J, H, K1, and K2 spectra from GPI demonstrate the extraordinarily red colors of the object, confirming it as the reddest substellar object observed to date. The significant flux increase throughout the infrared presents a challenging atmosphere to model with existing grids. Best-fit values vary from 1200 to 1800 K for effective temperature and from 3.0 to 5.0 for log(g), depending on which individual wavelength band is fit and which model suite is applied. The extreme redness of the companion can be partially reconciled by invoking a high-altitude layer of submicron dust particles, similar to dereddening approaches applied to the peculiar red field L dwarf population. However, reconciling the HD 206893 B spectra with even those of the reddest low-gravity L dwarf spectra still requires the contribution of additional atmospheric dust, potentially due to the debris disk environment in which the companion resides. Orbit fitting from 4 yr of astrometric monitoring is consistent with a ∼30 yr period, an orbital inclination of 147°, and a semimajor axis of 10 au, well within the estimated disk inner radius of ∼50 au. As one of a very few substellar companions imaged interior to a circumstellar disk, the properties of this system offer important dynamical constraints on companion-disk interaction and provide a benchmark for substellar and planetary atmospheric study.

2015

1. Discovery and spectroscopy of the young jovian planet 51 Eri b with the Gemini Planet Imager

   B. Macintosh, J. R. Graham, T. Barman, [incl. J. Lee]

   Science, October 2015

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   Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric compositions and luminosities, which are influenced by their formation mechanisms. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20-million-year-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water-vapor absorption. Modeling of the spectra and photometry yields a luminosity (normalized by the luminosity of the Sun) of 1.6 to 4.0 × 10-6 and an effective temperature of 600 to 750 kelvin. For this age and luminosity, “hot-start” formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the “cold-start” core-accretion process that may have formed Jupiter.