arXiv subjects

Beomdu Lim

Publications and source records attributed to Beomdu Lim.

At least 19 recordsLinked to original sources

arXiv2026

Kinematics and Dynamics of the Open Cluster NGC 2302

Open clusters are ideal observational testbeds to understand the dynamics of stellar systems. We present a dynamical study of the young open cluster NGC 2302. The latest Gaia data and $UBVIJHK_s$ photometric data are used in this study. A total of 117 stars are selected as the genuine members using the Gaia data. This cluster is, on average, reddened by $ = 0.24 \pm 0.06$ (s.d.). The ratio of total-to-selective extinction ($R_V$) in the direction of NGC 2302 is $2.8 \pm 0.1$. The cluster distance is determined to be $1.16 \pm 0.08$ kpc using Gaia parallaxes. Theoretical isochrone fitting for $Z = 0.008$ on color-magnitude diagrams yields an age of $80 \pm 20$ Myr. The relative proper motions of individual members show no significant radial expansion or contraction. NGC 2302 contains a total stellar mass of $333 \pm 48 M_{\odot}$. The one-dimensional velocity dispersion is approximately 0.26 km s$^{-1}$, which is comparable to the viral velocity dispersion of 0.27 km s$^{-1}$ derived from its total mass. Its relaxation time is estimated to be approximately 90 Myr, which is similar to the age of the cluster within the uncertainty in age estimation. Finally, we report a pattern of mass segregation in the radial distribution of stellar masses. Our results suggest that NGC 2302 is virialized and currently approaching a state of dynamical relaxation. However, because no definitive evidence of kinetic energy equipartition is found, the possibility of the in-situ formation of high-mass stars within the central region should be carefully considered.

astro-ph.GA↗

arXiv2026

A Search for Exoplanets around Northern Circumpolar Stars X. The origin of radial velocity variations in the evolved star HD 216595

Detecting planetary companions around evolved stars, particularly asymptotic giant branch (AGB) stars, is challenging due to intrinsic stellar variability such as surface convection, pulsations, and mass loss, which can produce radial velocity (RV) signals that mimic Keplerian motion. We investigate the origin of long-period, low-amplitude RV variations observed in the AGB star HD 216595 based on high-resolution spectroscopic data spanning approximately 16 years obtained with the Bohyunsan Optical Astronomy Observatory Echelle Spectrograph (BOES) and the Las Cumbres Observatory Network of Robotic Echelle Spectrographs (NRES) instruments. The RV measurements reveal a statistically significant periodic signal at 567 days that can be described by a Keplerian model consistent with a substellar companion. However, no strong correlations are found between the RV variations and stellar activity indicators, including line bisectors, chromospheric activity, and photometric variability, although weak signals at similar timescales are present in some diagnostics. Given the stellar properties of HD 216595 and similarities to previously reported cases, the observed RV variations are likely related to intrinsic stellar processes, although a companion-induced origin cannot be definitively ruled out. Further progress will require improved diagnostics and more sophisticated modeling to disentangle stellar variability from genuine orbital signals.

astro-ph.SR↗

arXiv2026

Initial Mass Functions of Young Stellar Clusters from the Gemini Spectroscopic Survey of Nearby Galaxies. II. Young Clusters in NGC 1313

We present a spectroscopic study of young stellar clusters in the barred spiral galaxy NGC 1313. Integrated light spectra of 11 clusters, obtained using the GMOS-S instrument on the 8.1 m Gemini South telescope, are analyzed using a simple stellar population model. A subsolar metallicity (Z = 0.008) is adopted, consistent with previous studies. Cluster ages are constrained primarily through absorption lines and prominent emission bands of Wolf-Rayet stars. Utilizing these constraints, we match the observed spectra with synthetic counterparts generated from the simple stellar population model, determining key physical parameters including age, cluster mass, and the underlying initial mass function (IMF). Furthermore, the impact of stochastic sampling on the derived parameters of several low-mass clusters is rigorously evaluated using Monte Carlo simulations. The sampled clusters exhibit ages ranging from 2.5 to 300 Myr and stellar masses between 2.8 x 10^3 Msun and 2.6 x 10^5 Msun. Notably, for stellar masses exceeding 0.8 Msun, the power-law index (Gamma) of the underlying IMFs is found to be smaller than the standard Salpeter/Kroupa IMF. Furthermore, a correlation is observed where more massive clusters tend to possess top-light IMFs. This finding aligns with trends observed in the young clusters of the Antennae Galaxies, despite the differing mass scales between the two systems. Our results suggest that applying a universal standard IMF to spatially unresolved systems warrants caution, given the inherent complexities revealed in this study.

astro-ph.GA↗

arXiv2025

Formation process of young stellar population in Messier 16 from a kinematic perspective

We present a kinematic study of young stars in Messier 16 (M16) using the Gaia Data Release 3 and high-resolution spectra. A total of 345 stars are selected as genuine members using the published lists of X-ray, infrared sources, and early-type stars as well as the Gaia data. There is severe differential reddening across this region and the reddening law of the intracluster medium appears abnormal. The distance to M16, derived from the parallaxes of the members, is about 1.7 kpc. The ages of members, estimated by comparing their color-magnitude diagram with theoretical isochrones, range from 1 Myr to 4 Myr. This star-forming region is composed of an open cluster (NGC 6611) and a distributed population. This cluster shows a clear pattern of expansion and rotation. Some of the distributed population are spatially associated with the gas pillars located at the ridge of H II bubble. In particular, several stars moving away from the cluster are physically associated with the northeastern pillar. In addition, their younger ages support the idea that the formation of these stars was triggered by the feedback from massive stars in NGC 6611. On the other hand, the other stars do not show systematic radial or stream motions; therefore, they likely formed through spontaneous star formation events. We discuss the formation of young stars in the context of cluster expansion, spontaneous star formation, and feedback-driven star formation, and suggest that all of these mechanisms possibly contributed to their formation.

astro-ph.SR↗

arXiv2024

Initial Mass Functions of Young Stellar Clusters from the Gemini Spectroscopic Survey of Nearby Galaxies I. Young Massive Clusters in the Antennae galaxies

The stellar initial mass function (IMF) is a key parameter to understand the star formation process and the integrated properties of stellar populations in remote galaxies. We present a spectroscopic study of young massive clusters (YMCs) in the starburst galaxies NGC 4038/39. The integrated spectra of seven YMCs obtained with GMOS-S attached to the 8.2-m Gemini South telescope reveal the spectral features associated with stellar ages and the underlying IMFs. We constrain the ages of the YMCs using the absorption lines and strong emission bands from Wolf-Rayet stars. The internal reddening is also estimated from the strength of the Na I D absorption lines. Based on these constraints, the observed spectra are matched with the synthetic spectra generated from a simple stellar population model. Several parameters of the clusters including age, reddening, cluster mass, and the underlying IMF are derived from the spectral matching. The ages of the YMCs range from 2.5 to 6.5 Myr, and these clusters contain stellar masses ranging from 1.6 X 10^5 M_sun to 7.9 X 10^7 M_sun. The underlying IMFs appear to differ from the universal form of the Salpeter/Kroupa IMF. Interestingly, massive clusters tend to have the bottom-heavy IMFs, although the masses of some clusters are overestimated due to the crowding effect. Based on this, our results suggest that the universal form of the IMF is not always valid when analyzing integrated light from unresolved stellar systems. However, further study with a larger sample size is required to reach a definite conclusion.

astro-ph.GA↗

arXiv2023

A Morpho-Kinematic Study of the Enigmatic Emission Nebula NGC 6164/5 Surrounding the Magnetic O-type Star HD 148937

HD 148937 is a peculiar massive star (Of?p) with a strong magnetic field (1kG). The hourglass-shaped emission nebula NGC 6164/5 surrounds this star. This nebula is presumed to originate from episodic mass-loss events of the central O-type star, but the detailed formation mechanism is not yet well understood. Grasping its three-dimensional structure is essential to uncover the origin of this nebula. Here we report the high-resolution multi-object spectroscopic observation of NGC 6164/5 using the GIRAFFE on the 8.2m Very Large Telescope. Integrated intensity maps constructed from several spectral lines delineate well the overall shape of this nebula, such as the two bright lobes and the inner gas region. The position-velocity diagrams show that the two bright lobes are found to be redshifted and blueshifted, respectively, while the inner region has multiple layers. We consider a geometric model composed of a bilateral outflow harboring nitrogen-enriched knots and expanding inner shells. Its spectral features are then simulated by using a Monte-Carlo radiative transfer technique for different sets of velocities. Some position-velocity diagrams from simulations are very similar to the observed ones. According to the model that best reproduces the observational data, the two bright lobes and the nitrogen-enriched knots are moving away from HD 148937 at about 120 km s$^{-1}$. Their minimum kinematic age is estimated to be about 7,500 years. We discuss possible formation mechanisms of this nebula in the context of binary interaction.

astro-ph.SR↗

arXiv2023

A Search for exoplanets around northern circumpolar stars VIII. filter out a planet cycle from the multi-period radial velocity variations in M giant HD 3638

This paper is written as a follow-up observations to reinterpret the radial velocity (RV) of HD 36384, where the existence of planetary systems is known to be ambiguous. In giants, it is, in general, difficult to distinguish the signals of planetary companions from those of stellar activities. Thus, known exoplanetary giant hosts are relatively rare. We, for many years, have obtained RV data in evolved stars using the high-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO). Here, we report the results of RV variations in the M giant HD 36384. We have found two significant periods of 586d and 490d. Considering the orbital stability, it is impossible to have two planets at so close orbits. To determine the nature of the RV variability variations, we analyze the HIPPARCOS photometric data, some indicators of stellar activities, and line profiles. A significant period of 580d was revealed in the HIPPARCOS photometry. H{\alpha} EW variations also show a meaningful period of 582d. Thus, the period of 586d may be closely related to the rotational modulations and/or stellar pulsations. On the other hand, the other significant period of 490d is interpreted as the result of the orbiting companion. Our orbital fit suggests that the companion was a planetary mass of 6.6 MJ and is located at 1.3 AU from the host.

astro-ph.EP↗

arXiv2023

Long-period radial velocity variations of nine M red giants: The detection of sub-stellar companions around HD 6860 and HD 112300

Context. Certain periodic variations of radial velocities (RV) of wobbling giants originate from exoplanets. Indeed, a number of exoplanets have been discovered around giant stars. Aims. The purpose of our study is to find low-amplitude and long-period RV variations around bright M (super) giants in the RGB (or AGB) stage, which are long-period variables (LPVs) or high-proper-motion (HPM) stars. Methods. High-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO) was used to record numerous spectra of nine giants. The observation period for the targets spans 16 years, from 2005 to 2022. Results. We found from the precise RV observations of nine M giants two sub-stellar companions, one with a 28.26$^{+2.05}_{-2.17}$ $M_{J}$ orbiting period of 663.87$^{+4.61}_{-4.31}$ days at a distance of 2.03$^{+0.01}_{-0.01}$ AU (HD 6860) and the other, with a 15.83$^{+2.33}_{-2.74}$ $M_{J}$ orbiting period of 466.63 $^{+1.47}_{-1.28}$ days at a distance of 1.33 $^{+0.08}_{-0.11}$ AU (HD 112300). Our estimate of the stellar parameters for HD 6860 makes it currently the largest star with a sub-stellar companion. We also found RV variations mimicking a planetary companion in HD 18884 and confirmed LPVs in two stars, HD 39801 and HD 42995. The RV variations of some stars seem to be associated with stellar activities rather than reflex orbital motion due to their companions. Such variations are also detected even for HD 6860 and HD 112300, hosting sub-stellar companions.

astro-ph.SR↗

arXiv2023

The kinematics of young stellar population in the W5 region of the Cassiopeia OB6 association: implication on the formation process of stellar associations

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 2,000 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 ridge of the bubbles. The three dense groups have almost the same ages (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 the independent star formation events. Only one group of them 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.

astro-ph.GA↗

arXiv2023

A deep optical survey of young stars in the Carina Nebula. I. -- UBVRI photometric data and fundamental parameters

We present the deep homogeneous $UBVRI$ photometric data of 135,071 stars down to $V\sim23$ mag and I ~ 22 mag toward the Carina Nebula. These stars are cross-matched with those from the previous surveys in the X-ray, near-infrared, and mid-infrared wavelengths as well as the Gaia Early Data Release 3 (EDR3). This master catalog allows us to select reliable members and determine the fundamental parameters distance, size, stellar density of stellar clusters in this star-forming region. We revisit the reddening toward the nebula using the optical and the near-infrared colors of early-type stars. The foreground reddening [E(B-V)_fg] is determined to be 0.35+/-0.02, and it seems to follow the standard reddening law. On the other hand, the total-to-selective extinction ratio of the intracluster medium (R_V,cl) decreases from the central region (Trumpler 14 and 16, R_V,cl ~ 4.5) to the northern region (Trumpler 15, R_V,cl ~ 3.4). It implies that the central region is more dusty than the northern region. We find that the distance modulus of the Carina Nebula to be 11.9+/-0.3 mag (d = 2.4+/-0.35 kpc) using a zero-age main-sequence fitting method, which is in good agreement with that derived from the Gaia EDR3 parallaxes. We also present the catalog of 3,331 pre-main-sequence (PMS) members and 14,974 PMS candidates down to V ~ 22 mag based on spectrophotometric properties of young stars at infrared, optical, and X-ray wavelengths. From the spatial distribution of PMS members and PMS candidates, we confirm that the member selection is very reliable down to faint stars. Our data will have a legacy value for follow-up studies with different scientific purposes.

astro-ph.SR↗

arXiv2022

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

Stellar kinematics provides the key to understanding 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.

astro-ph.SR↗

arXiv2021

The Sejong Open Cluster Survey (SOS). VII. A Photometric Study of the Young Open Cluster IC 1590

Young open clusters are ideal laboratories to understand star formation process. We present deep UBV I and Halpha photometry for the young open cluster IC 1590 in the center of the H II region NGC 281. Early-type members are selected from UBV photometric diagrams, and low-mass pre-main sequence (PMS) members are identified by using Halpha photometry. In addition, the published X-ray source list and Gaia astrometric data are also used to isolate probable members. A total of 408 stars are selected as members. The mean reddening obtained from early-type members is 1 M_sun). The signature of mass segregation is detected from the difference in the slopes of the initial mass functions for the inner (r < 2.'5) and outer region of this cluster. We finally discuss the star formation history in NGC 281.

astro-ph.SR↗

arXiv2021

A kinematic perspective on the formation process of the stellar groups in the Rosette Nebula

Stellar kinematics is a powerful tool for understanding the formation process of stellar associations. Here, we present a kinematic study of the young stellar population in the Rosette nebula using the recent Gaia data and high-resolution spectra. We first isolate member candidates using the published mid-infrared photometric data and the list of X-ray sources. A total of 403 stars with similar parallaxes and proper motions are finally selected as members. The spatial distribution of the members shows that this star-forming region is highly substructured. The young open cluster NGC 2244 in the center of the nebula has a pattern of radial expansion and rotation. We discuss its implication on the cluster formation, e.g., monolithic cold collapse or hierarchical assembly. On the other hand, we also investigate three groups located around the border of the H II bubble. The western group seems to be spatially correlated with the adjacent gas structure, but their kinematics is not associated with that of the gas. The southern group does not show any systematic motion relative to NGC 2244. These two groups might be spontaneously formed in filaments of a turbulent cloud. The eastern group is spatially and kinematically associated with the gas pillar receding away from NGC 2244. This group might be formed by feedback from massive stars in NGC 2244. Our results suggest that the stellar population in the Rosette Nebula may form through three different processes: the expansion of stellar clusters, hierarchical star formation in turbulent clouds, and feedback-driven star formation.

astro-ph.SR↗

arXiv2021

Planck Cold Clumps in the lambda Orionis Complex. III. A chemical probe of stellar feedback on cores in the lambda Orionis cloud

Massive stars have a strong impact on their local environments. However, how stellar feedback regulates star formation is still under debate. In this context, we studied the chemical properties of 80 dense cores in the Orion molecular cloud complex composed of the Orion A (39 cores), B (26 cores), and lambda Orionis (15 cores) clouds using multiple molecular line data taken with the Korean Very Long Baseline Interferometry Network (KVN) 21-m telescopes. The lambda Orionis cloud has an H ii bubble surrounding the O-type star lambda Ori, and hence it is exposed to the ultraviolet (UV) radiation field of the massive star. The abundances of C2H and HCN, which are sensitive to UV radiation, appear to be higher in the cores in the lambda Orionis cloud than those in the Orion A and B clouds, while the HDCO to H2CO abundance ratios show an opposite trend, indicating a warmer condition in the lambda Orionis cloud. The detection rates of dense gas tracers such as the N2H+, HCO+, and H13CO+ lines are also lower in the lambda Orionis cloud. These chemical properties imply that the cores in the lambda Orionis cloud are heated by UV photons from lambda Ori. Furthermore, the cores in the lambda Orionis cloud do not show any statistically significant excess in the infall signature of HCO+ (1 - 0), unlike the Orion A and B clouds. Our results support the idea that feedback from massive stars impacts star formation in a negative way by heating and evaporating dense materials, as in the lambda Orionis cloud.

astro-ph.GA↗

arXiv2020

The origin of a distributed stellar population in the star-forming region W4

Stellar kinematics provides the key to understanding the formation process and dynamical evolution of stellar systems. Here, we present a kinematic study of the massive star-forming region W4 in the Cassiopeia OB6 association using the Gaia Data Release 2 and high-resolution optical spectra. This star-forming region is composed of a core cluster (IC 1805) and a stellar population distributed over 20 pc, which is a typical structural feature found in many OB associations. According to a classical model, this structural feature can be understood in the context of the dynamical evolution of a star cluster. The core-extended structure exhibits internally different kinematic properties. Stars in the core have an almost isotropic motion, and they appear to reach virial equilibrium given their velocity dispersion (0.9 +/- 0.3 km/s) comparable to that in a virial state (~0.8 km/s). On the other hand, the distributed population shows a clear pattern of radial expansion. From the N-body simulation for the dynamical evolution of a model cluster in subvirial state, we reproduce the observed structure and kinematics of stars. This model cluster experiences collapse for the first 2 Myr. Some members begin to radially escape from the cluster after the initial collapse, eventually forming a distributed population. The internal structure and kinematics of the model cluster appear similar to those of W4. Our results support the idea that the stellar population distributed over 20 pc in W4 originate from the dynamical evolution of IC 1805.

astro-ph.SR↗

arXiv2019

A Gaia view of the two OB associations Cygnus OB2 and Carina OB1: The signature of their formation process

OB associations are the prime star forming sites in galaxies. However the detailed formation process of such stellar systems still remains a mystery. In this context, identifying the presence of substructures may help tracing the footprints of their formation process. Here, we present a kinematic study of the two massive OB associations Cygnus OB2 and Carina OB1 using the precise astrometry from the Gaia Data Release 2 and radial velocities. From the parallaxes of stars, these OB associations are confirmed to be genuine stellar systems. Both Cygnus OB2 and Carina OB1 are composed of a few dense clusters and a halo which have different kinematic properties: the clusters occupy regions of 5-8 parsecs in diameter and display small dispersions in proper motion, while the halos spread over tens of parsecs with a 2-3 times larger dispersions in proper motion. This is reminiscent of the so-called "line width-size" relation of molecular clouds related to turbulence. Considering that the kinematics and structural features were inherited from those of their natal clouds would then imply that the formation of OB associations may result from structure formation driven by supersonic turbulence, rather than from the dynamical evolution of individual embedded clusters.

astro-ph.SR↗

arXiv2018

Extended main sequence turn-off originated from a broad range of stellar rotational velocities

Star clusters have long been considered to comprise a simple stellar population, but this paradigm is being challenged, since apart from multiple populations in Galactic globular clusters^{1,2}, a number of intermediate-age star clusters exhibit a significant colour spread at the main sequence turn-off (MSTO)^{3,4,5,6,7,8,9,10,11}. A sequential evolution of multiple generations of stars formed over 100-200 million years is a natural explanation of this colour spread^{12}. Another approach to explain this feature is to introduce the effect of stellar rotation^{13}. However, its effectiveness has not yet been proven due to the lack of direct measurements of rotational velocities. Here we report the distribution of projected rotational velocities (V sin i) of stars in the Galactic open cluster M11, measured through a Fourier transform analysis. Cluster members display a broad V sin i distribution, and fast rotators including Be stars have redder colours than slow rotators. Monte Carlo simulations infer that cluster members have highly aligned spin axes and a broad distribution of equatorial velocities biased towards high velocities. Our findings demonstrate how stellar rotation affects the colours of cluster members, suggesting that the colour spread observed in populous clusters can be understood in the context of stellar evolution even without introducing multiple stellar populations.

astro-ph.SR↗

arXiv2018

Kinematic evidence for feedback-driven star formation in NGC 1893

OB associations are the prevailing star forming sites in the Galaxy. Up to now, the process of how OB associations were formed remained a mystery. A possible process is self-regulating star formation driven by feedback from massive stars. However, although a number of observational studies uncovered various signposts of feedback-driven star formation, the effectiveness of such feedback has been questioned. Stellar and gas kinematics is a promising tool to capture the relative motion of newborn stars and gas away from ionizing sources. We present high-resolution spectroscopy of stars and gas in the young open cluster NGC 1893. Our findings show that newborn stars and the tadpole nebula Sim 130 are moving away from the central cluster containing two O-type stars, and that the timescale of sequential star formation is about 1 Myr within a 9 parsec distance. The newborn stars formed by feedback from massive stars account for at least 18 per cent of the total stellar population in the cluster, suggesting that this process can play an important role in the formation of OB associations. These results support the self-regulating star formation model.

astro-ph.SR↗