Song D.,Seoul National University |
Chae J.,Seoul National University |
Yurchyshyn V.,New Jersey Institute of Technology |
Yurchyshyn V.,Korea Astronomy and Space Science Institute 776 |
And 5 more authors.
Astrophysical Journal | Year: 2017
It is well-known that light bridges (LBs) inside a sunspot produce small-scale plasma ejections and transient brightenings in the chromosphere, but the nature and origin of such phenomena are still unclear. Utilizing the high-spatial and high-temporal resolution spectral data taken with the Fast Imaging Solar Spectrograph and the TiO 7057 Å broadband filter images installed at the 1.6 m New Solar Telescope of Big Bear Solar Observatory, we report arcsecond-scale chromospheric plasma ejections (1.″7) inside a LB. Interestingly, the ejections are found to be a manifestation of upwardly propagating shock waves as evidenced by the sawtooth patterns seen in the temporal-spectral plots of the Ca ii 8542 Å and Hα intensities. We also found a fine-scale photospheric pattern (1″) diverging with a speed of about 2 km s-1 two minutes before the plasma ejections, which seems to be a manifestation of magnetic flux emergence. As a response to the plasma ejections, the corona displayed small-scale transient brightenings. Based on our findings, we suggest that the shock waves can be excited by the local disturbance caused by magnetic reconnection between the emerging flux inside the LB and the adjacent umbral magnetic field. The disturbance generates slow-mode waves, which soon develop into shock waves, and manifest themselves as the arcsecond-scale plasma ejections. It also appears that the dissipation of mechanical energy in the shock waves can heat the local corona. © 2017. The American Astronomical Society. All rights reserved.
Liu T.,Korea Astronomy and Space Science Institute 776 |
Kim K.-T.,Korea Astronomy and Space Science Institute 776 |
Wu Y.,Peking University |
Li D.,CAS National Astronomical Observatories |
And 10 more authors.
Astrophysical Journal | Year: 2015
W49N is a mini-starburst in the Milky Way and is thus an ideal laboratory for high-mass star formation studies. Due to its large distance (11.1-0.7+0.9 kpc), the kinematics inside and between the dense molecular clumps in W49N are far from well-understood. The Submillimeter Array observations resolved the continuum emission into two clumps. The molecular line observation of SO2 (284,24283,25) suggests that the two clumps have a velocity difference of ∼7 km s?1. The eastern clump is very close to two radio sources "G1" and "G2, " and the western clump coincides with a radio source "B." The HCN (32) line reveals an extremely energetic outflow, which is among the most energetic molecular outflows in the Milky Way. This is the first report of high-velocity molecular outflow detection in W49N. The outflow jet might be in precession, which could account for the distribution, velocity, and rotation of water maser spots. Three absorption systems are identified in HCO+ (32) spectra. The absorption features are blueshifted with respect to the emission of SO2 (284,24283,25) lines, indicating that a cold layer is expanding in front of the warm gas. Further analysis indicates that the expansion is decelerated from the geometric expansion centers. © 2015 The American Astronomical Society. All rights reserved.
Liu H.-L.,CAS National Astronomical Observatories |
Liu H.-L.,University of Chinese Academy of Sciences |
Wu Y.,Peking University |
Li J.,CAS National Astronomical Observatories |
And 3 more authors.
Astrophysical Journal | Year: 2015
We present a multi-wavelength study of the IR bubble G24.136+00.436. The J = 1-0 observations of 12CO, 13CO, and C18O were carried out with the PurpleMountain Observatory 13.7mtelescope. Molecular gas with a velocity of 94.8 kms-1 is found prominently in the southeast of the bubble, shaped as a shell with a total mass of ∼2 × 104 M⊙ . It was likely assembled during the expansion of the bubble. The expanding shell consists of six dense cores, whose dense (a few of 103 cm-3 ) and massive (a few of 103 M⊙) characteristics coupled with the broad linewidths (>2.5 kms-1 ) suggest that they are promising sites for forming high-mass stars or clusters. This could be further consolidated by the detection of compact H II regions in Cores A and E. We tentatively identified and classified 63 candidate young stellar objects (YSOs) based on the Spitzer and UKIDSS data. They are found to be dominantly distributed in regions with strong molecular gas emission, indicative of active star formation, especially in the shell. The H II region inside the bubble is mainly ionized by a ∼O8V star(s), of the dynamical age of ∼1.6Myr. The enhanced number of candidate YSOs and secondary star formation in the shell as well as the timescales involved, indicate a possible scenario for triggering star formation, signified by the "collect and collapse" process. © 2015. The American Astronomical Society. All rights reserved.
PubMed | Armagh Observatory, Korea Astronomy and Space Science Institute 776, Institute of Astrophysics of Canarias, National Autonomous University of Mexico and 7 more.
Type: Journal Article | Journal: Nature | Year: 2014
Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.
Inoue S.,Korea Astronomy and Space Science Institute 776 |
Inoue S.,Hebrew University of Jerusalem |
Saitoh T.R.,Tokyo Institute of Technology
Monthly Notices of the Royal Astronomical Society | Year: 2014
We examine a possible formation scenario of galactic thick discs with numerical simulations. Thick discs have previously been argued to form in clumpy disc phase in the high-redshift Universe, which host giant clumps of ≲109 M⊙ in their highly gas-rich discs. We performed N-body/smoothed particle hydrodynamics simulations using isolated galaxy models for the purpose of verifying whether dynamical and chemical properties of the thick discs formed in such clumpy galaxies are compatible with observations. The results of our simulations seem nearly consistent with observations in dynamical properties such as radial and vertical density profiles, significant rotation velocity lag with height and distributions of orbital eccentricities. In addition, the thick discs in our simulations indicate nearly exponential dependence of σθ and σz with radius, nearly isothermal kinematics in vertical direction and negligible metallicity gradients in radial and vertical directions. However, our simulations cannot reproduce altitudinal dependence of eccentricities, metallicity relations with eccentricities or rotation velocities, which shows striking discrepancy from recent observations of the Galactic thick disc. From this result, we infer that the clumpy disc scenario for thick-disc formation would not be suitable at least for the Milky Way. Our study, however, cannot reject this scenario for external galaxies if not all galaxies form their thick discs by the same process. In addition, we found that a significant fraction of thick-disc stars forms in giant clumps. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.