arXiv subjects

Yoon Kyung Choi

Publications and source records attributed to Yoon Kyung Choi.

15 recordsLinked to original sources

arXiv2025

On the Nature of the Protoplanetary Nebula OH\,231.8+4.2

Aims. OH 231.8+4.2 , also known as the Rotten Egg or Calabash nebula is a protoplanetary nebula which is seen in the direction of the open cluster M 46. While an association of the nebula with the cluster has been suggested in the past, this has been never confirmed. Here, we present accurate trigonometric parallax and proper motion measurements using VLBI observations of masers in the nebula and Gaia DR3 data for the cluster. Methods. We observed 22 GHz H2O and 43 GHz SiO masers around OH 231.8+4.2 using the Very Long Baseline Array at 4 epochs over 1 year. We also calculated the parallax and proper motion of the open star cluster M 46 using Gaia DR3 data. Results. Based on astrometric monitoring for 1 year, we measured an annual parallax of OH 231.8+4.2 to be 0.65 +\- 0.01 mas (stat.) +\- 0.02 mas (syst.), corresponding to a distance of 1.54 +\- 0.05 kpc. This agrees well with the parallax of M 46 from the Gaia DR3 data, which is 0.639 +\- 0.001 mas (stat.) +\- 0.010 mas (syst.). We re-estimated the luminosity of OH 231.8+4.2 to be 1.4 x 10^4 L_sol. However, there is a 15 km/s velocity difference between OH 231.8+4.2 and M 46 which could be caused by a past merger event.

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arXiv2020

44 GHz methanol masers: Observations toward 95 GHz methanol masers

We report a simultaneous 44 and 95 GHz class I methanol maser survey toward 144 sources from the 95 GHz class I methanol maser catalog. The observations were made with the three telescopes of the Korean very long baseline interferometry network operating in single-dish mode. The detection rates are 89% at 44 GHz and 77% at 95 GHz. There are 106 new discoveries at 44 GHz. Comparing the previous 95 GHz detections with new observations of the same transitions made using the Purple Mountain Observatory 13.7 m radio telescope shows no clear evidence of variability on a timescale of six years. Emission from the 44 and 95 GHz transitions shows strong correlations in peak velocity, peak flux density, and integrated flux density, indicating that they are likely cospatial. We found that the peak flux density ratio Spk,95/Spk,44 decreases as the 44 GHz peak flux density increases. We found that some class I methanol masers in our sample could be associated with infrared dark clouds, while others are associated with H II regions, indicating that some sources occur at an early stage of high-mass star formation, while others are located toward more evolved sources.

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arXiv2020

The First VERA Astrometry Catalog

We present the first astrometry catalog from the Japanese VLBI (very long baseline interferometer) project VERA (VLBI Exploration of Radio Astrometry). We have compiled all the astrometry results from VERA, providing accurate trigonometric annual parallax and proper motion measurements. In total, 99 maser sources are listed in the VERA catalog. Among them, 21 maser sources are newly reported while the rest of 78 sources are referred to previously published results or those in preparation for forthcoming papers. The accuracy in the VERA astrometry are revisited and compared with those from the other VLBI astrometry projects such as BeSSeL (The Bar and Spiral Structure Legacy) Survey and GOBELINS (the Gould's Belt Distances Survey) with the VLBA (Very Long Baseline Array). We have confirmed that most of the astrometry results are consistent with each other, and the largest error sources are due to source structure of the maser features and their rapid variation, along with the systematic calibration errors and different analysis methods. Combined with the BeSSeL results, we estimate the up-to-date fundamental Galactic parameter of $R_{0}=7.92\pm0.16_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~kpc and $Ω_{\odot}=30.17\pm0.27_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~km~s$^{-1}$~kpc$^{-1}$, where $R_{0}$ and $Ω_{\odot}$ are the distance from the Sun to the Galactic center and the Sun's angular velocity of the Galactic circular rotation, respectively.

astro-ph.GA↗

arXiv2018

Simultaneous VLBI Astrometry of H2O and SiO Masers toward the Semiregular Variable R Crateris

We obtained, for the first time, astrometrically registered maps of the 22.2 GHz H2O and 42.8, 43.1, and 86.2 GHz SiO maser emission toward the semiregular b-type variable (SRb) R Crateris, at three epochs (2015 May 21, and 2016 January 7 and 26) using the Korean Very-long-baseline Interferometry Network. The SiO masers show a ring-like spatial structure, while the H2O maser shows a very asymmetric one-side outflow structure, which is located at the southern part of the ring-like SiO maser feature. We also found that the 86.2 GHz SiO maser spots are distributed in an inner region, compared to those of the 43.1 GHz SiO maser, which is different from all previously known distributions of the 86.2 GHz SiO masers in variable stars. The different distribution of the 86.2 GHz SiO maser seems to be related to the complex dynamics caused by the overtone pulsation mode of the SRb R Crateris. Furthermore, we estimated the position of the central star based on the ring fitting of the SiO masers, which is essential for interpreting the morphology and kinematics of a circumstellar envelope. The estimated stellar coordinate corresponds well to the position measured by Gaia.

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arXiv2018

Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii

The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers. However, previous VLBI observations have been performed separately, which makes it difficult to spatially trace the outward transfer of the material consecutively. Here we present the astrometrically registered, simultaneous maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward VX Sagittarii. The H2O masers detected above the dust-forming layers have an asymmetric distribution. The multi-transition SiO masers are nearly circular ring, suggesting spherically symmetric wind within a few stellar radii. These results provide the clear evidence that the asymmetry in the outflow is enhanced after the smaller molecular gas clump transform into the inhomogeneous dust layers. The 129.3 GHz maser arises from the outermost region compared to that of 43.1/42.8/86.2 GHz SiO masers. The ring size of the 129.3 GHz maser is maximized around the optical maximum, suggesting that radiative pumping is dominant.

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arXiv2014

VLBI observations of bright AGN jets with KVN and VERA Array (KaVA): Evaluation of Imaging Capability

The Korean very-long-baseline interferometry (VLBI) network (KVN) and VLBI Exploration of Radio Astrometry (VERA) Array (KaVA) is the first international VLBI array dedicated to high-frequency (23 and 43 GHz bands) observations in East Asia. Here, we report the first imaging observations of three bright active galactic nuclei (AGNs) known for their complex morphologies: 4C 39.25, 3C 273, and M 87. This is one of the initial result of KaVA early science. Our KaVA images reveal extended outflows with complex substructure such as knots and limb brightening, in agreement with previous Very Long Baseline Array (VLBA) observations. Angular resolutions are better than 1.4 and 0.8 milliarcsecond at 23 GHz and 43 GHz, respectively. KaVA achieves a high dynamic range of ~1000, more than three times the value achieved by VERA. We conclude that KaVA is a powerful array with a great potential for the study of AGN outflows, at least comparable to the best existing radio interferometric arrays.

astro-ph.IM↗

arXiv2012

Fundamental Parameters of the Milky Way Galaxy Based on VLBI astrometry

We present analyses to determine the fundamental parameters of the Galaxy based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA and EVN. We model the Galaxy's structure with a set of parameters including the Galaxy center distance R_0, the angular rotation velocity at the LSR Omega_0, mean peculiar motion of the sources with respect to Galactic rotation (U_src, V_src, W_src), rotation-curve shape index, and the V component of the Solar peculiar motions V_sun. Based on a Markov chain Monte Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be R_0 = 8.05 +/- 0.45 kpc, where the error bar includes both statistical and systematic errors. We also find that the two components of the source peculiar motion U_src and W_src are fairly small compared to the Galactic rotation velocity, being U_src = 1.0 +/- 1.5 km/s and W_src = -1.4 +/- 1.2 km/s. Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand, we find a linear relation between V_src and V_sun as V_src = V_sun -19 (+/- 2) km/s, suggesting that the value of V_src is fully dependent on the adopted value of V_sun. Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between Omega_0 and V_sun. We find that the angular velocity of the Sun, Omega_sun, which is defined as Omega_sun = Omega_0 + V_sun/R_0, can be well constrained with the best estimate of Omega_sun = 31.09 +/- 0.78 km/s/kpc. This corresponds to Theta_0 = 238 +/- 14 km/s if one adopts the above value of R_0 and recent determination of V_sun ~ 12 km/s.

astro-ph.GA↗

arXiv2008

VLBI Astrometry of AGB Variables with VERA -- A Semiregular Variable S Crateris --

We present a distance measurement for the semiregular variable S Crateris (S Crt) based on its annual parallax. With the unique dual beam system of the VLBI Exploration for Radio Astrometry (VERA) telescopes, we measured the absolute proper motion of a water maser spot associated with S Crt, referred to the quasar J1147-0724 located at an angular separation of 1.23$^{\circ}$. In observations spanning nearly two years, we have detected the maser spot at the LSR velocity of 34.7 km s$^{-1}$, for which we measured the annual parallax of 2.33$\pm$0.13 mas corresponding to a distance of 430$^{+25}_{-23}$ pc. This measurement has an accuracy one order of magnitude better than the parallax measurements of HIPPARCOS. The angular distribution and three-dimensional velocity field of maser spots indicate a bipolar outflow with the flow axis along northeast-southwest direction. Using the distance and photospheric temperature, we estimate the stellar radius of S Crt and compare it with those of Mira variables.

astro-ph↗

arXiv2008

Distance to VY Canis Majoris with VERA

We report astrometric observations of H2O masers around the red supergiant VY Canis Majoris (VY CMa) carried out with VLBI Exploration of Radio Astrometry (VERA). Based on astrometric monitoring for 13 months, we successfully measured a trigonometric parallax of 0.88 +/- 0.08 mas, corresponding to a distance of 1.14 +0.11/-0.09 kpc. This is the most accurate distance to VY CMa and the first one based on an annual parallax measurement. The luminosity of VY CMa has been overestimated due to a previously accepted distance. With our result, we re-estimate the luminosity of VY CMa to be (3 +/- 0.5) x 10^5 L_sun using the bolometric flux integrated over optical and IR wavelengths. This improved luminosity value makes location of VY CMa on the Hertzsprung-Russel (HR) diagram much closer to the theoretically allowable zone (i.e. the left side of the Hayashi track) than previous ones, though uncertainty in the effective temperature of the stellar surface still does not permit us to make a final conclusion.

astro-ph↗

arXiv2008

Astrometry of H$_{2}$O Masers in Nearby Star-Forming Regions with VERA. III. IRAS 22198+6336 in L1204G

We present results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H$_{2}$O masers associated with a young stellar object (YSO) IRAS 22198+6336 in a dark cloud L1204G. Based on the phase-referencing VLBI astrometry, we derive an annual parallax of IRAS 22198+6336 to be 1.309$\pm$0.047 mas, corresponding to the distance of 764$\pm$27 pc from the Sun. Although the most principal error source of our astrometry is attributed to the internal structure of the maser spots, we successfully reduce the errors in the derived annual parallax by employing the position measurements for all of the 26 detected maser spots. Based on this result, we reanalyze the spectral energy distribution (SED) of IRAS 22198+6336 and find that the bolometric luminosity and total mass of IRAS 22198+6336 are 450$L_{\odot}$ and 7$M_{\odot}$, respectively. These values are consistent with an intermediate-mass YSO deeply embedded in the dense dust core, which has been proposed to be an intermediate-mass counterpart of a low-mass Class 0 source. In addition, we obtain absolute proper motions of the H$_{2}$O masers for the most blue-shifted components. We propose that the collimated jets aligned along the east-west direction are the most plausible explanation for the origin of the detected maser features.

astro-ph↗

arXiv2008

Distance to NGC 281 in a Galactic Fragmenting Superbubble: Parallax Measurements with VERA

We have used the Japanese VLBI array VERA to perform high-precision astrometry of an H2O maser source in the Galactic star-forming region NGC 281 West, which has been considered to be part of a 300-pc superbubble. We successfully detected a trigonometric parallax of 0.355+/-0.030 mas, corresponding to a source distance of 2.82+/-0.24 kpc. Our direct distance determination of NGC 281 has resolved the large distance discrepancy between previous photometric and kinematic studies; likely NGC 281 is in the far side of the Perseus spiral arm. The source distance as well as the absolute proper motions were used to demonstrate the 3D structure and expansion of the NGC 281 superbubble, ~650 pc in size parallel to the Galactic disk and with a shape slightly elongated along the disk or spherical, but not vertically elongated, indicating the superbubble expansion may be confined to the disk. We estimate the expansion velocity of the superbubble as ~20 km/s both perpendicular to and parallel to the Galactic disk with a consistent timescale of ~20 Myr.

astro-ph↗

arXiv2007

Astrometry of Water Maser Sources in Nearby Molecular Clouds with VERA - II. SVS 13 in NGC 1333

We report on the results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We have carried out phase-referencing VLBI astrometry and measured an annual parallax of the maser features in SVS 13 of 4.25+/-0.32 mas, corresponding to the distance of 235+/-18 pc from the Sun. Our result is consistent with a photometric distance of 220 pc previously reported. Even though the maser features were detectable only for 6 months, the present results provide the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H2O masers with the LSR (local standard of rest) velocities of 7-8 km s-1 are most likely associated with VLA4A, which is a radio counterpart of SVS 13. The origin of the observed proper motions of the maser features are currently difficult to attribute to either the jet or the rotating circumstellar disk associated with VLA4A, which should be investigated through future high-resolution astrometric observations of VLA4A and other radio sources in NGC 1333.

astro-ph↗

arXiv2007

Astrometry of Galactic Star Forming Region Sharpless 269 with VERA : Parallax Measurements and Constraint on Outer Rotation Curve

We have performed high-precision astrometry of H2O maser sources in Galactic star forming region Sharpless 269 (S269) with VERA. We have successfully detected a trigonometric parallax of 189+/-8 micro-arcsec, corresponding to the source distance of 5.28 +0.24/-0.22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc. The source distance as well as proper motions are used to constrain the outer rotation curve of the Galaxy, demonstrating that the difference of rotation velocities at the Sun and at S269 (which is 13.1 kpc away from the Galaxy's center) is less than 3%. This gives the strongest constraint on the flatness of the outer rotation curve and provides a direct confirmation on the existence of large amount of dark matter in the Galaxy's outer disk.

astro-ph↗

arXiv2007

Distance to Orion KL Measured with VERA

We present the initial results of multi-epoch VLBI observations of the 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we have carried out phase-referencing VLBI astrometry and successfully detected an annual parallax of Orion KL to be 2.29+/-0.10 mas, corresponding to the distance of 437+/-19 pc from the Sun. The distance to Orion KL is determined for the first time with the annual parallax method in these observations. Although this value is consistent with that of the previously reported, 480+/-80 pc, which is estimated from the statistical parallax method using proper motions and radial velocities of the H2O maser features, our new results provide the much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we have detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.

astro-ph↗

arXiv2007

Absolute Proper Motions of H2O Masers Away from the Galactic Plane Measured with VERA in the "Superbubble" Region NGC 281

We report on absolute proper-motion measurements of an H2O maser source in the NGC 281 West molecular cloud, which is located ~320 pc above the Galactic plane and is associated with an HI loop extending from the Galactic plane. We have conducted multi-epoch phase-referencing observations of the maser source with VERA (VLBI Exploration of Radio Astrometry) over a monitoring period of 6 months since May 2006. We find that the H2O maser features in NGC 281 West are systematically moving toward the southwest and further away from the Galactic plane with a vertical velocity of ~20-30 km/s at its estimated distance of 2.2-3.5 kpc. Our new results provide the most direct evidence that the gas in the NGC 281 region on the HI loop was blown out from the Galactic plane, most likely in a superbubble driven by multiple or sequential supernova explosions in the Galactic plane.

astro-ph↗