Statistical Study of the Reconnection Rate in Solar Flares Observed with YOHKOH/SXT

We report a statistical study of flares observed with the Soft X-ray Telescope (SXT) onboard Yohkoh in the year of 2000. We measure physical parameters of 77 flares, such as the temporal scale, the size, and the magnetic flux density and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationship between these parameters and find weak correlation between temporal and spatial scales of flares. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using above observed values. The inflow velocities are distributed from a few km/s to several tens km/s and the Alfven velocities in the corona are in the range from 10^3 to 10^4 km/s. Hence the reconnection rate is 10^-3 - 10^-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within one order of magnitude from the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.Comment: 21 pages, 8 figures, accepted for publication in Ap

A Catalogue of Solar X-ray Plasma Ejections observed by the Soft X-ray Telescope onboard YOHKOH

A catalogue of X-ray Plasma Ejections (XPEs) observed by the Soft X-ray Telescope onboard the YOHKOH satellite has been recently developed in the Astronomical Institute of the University of Wroc{\l}aw. The catalogue contains records of 368 events observed in years 1991-2001 including movies and crossreferences to associated events like flares and Coronal Mass Ejections (CMEs). 163 XPEs from 368 in the catalogue were not reported until now. A new classification scheme of XPEs is proposed in which morphology, kinematics, and recurrence are considered. The relation between individual subclasses of XPEs and the associated events was investigated. The results confirm that XPEs are strongly inhomogeneous, responding to different processes that occur in the solar corona. A subclass of erupting loop-like XPEs is a promising candidate to be a high-temperature precursor of CMEs.Comment: 70 pages, 25 figures, 7 tables, accepted by The Astrophysical Journal Supplement Serie

What Controls the Star Formation in Luminous Starburst Mergers ?

In order to understand what controls the star formation process in luminous starburst mergers (e.g., NGC 6240, Arp 220, and so on), we investigate observational properties of two samples of high-luminosity starburst galaxies mapped in CO($J$=1--0) independently using both the Owens Valley Radio Observatory (Scoville et al. 1991) and the IRAM interferometer (Downes & Solomon 1998). We find that the surface density of far-infrared luminosity, $\Sigma$(FIR), is proportional linearly to the H$_2$ surface mass density, $\Sigma$(H$_2$), for the two samples; $\Sigma$(FIR) $\propto \Sigma$(H$_2$)$^{1.01\pm0.06}$ with a correlation coefficient of 0.96. It is often considered that $\Sigma$(FIR) provides a good measure of the star formation rate per unit area, $\Sigma$(SFR). It is also known that molecular gas is dominated in circumnuclear regions in the luminous starburst mergers; i.e., $\Sigma$(gas) $\simeq \Sigma$(H$_2$). Therefore, the above relationship suggests a star formation law; $\Sigma$(SFR) $\propto \Sigma$(gas). We suggest that this star formation law favors the gravitational instability scenario rather than the cloud-cloud collision one.Comment: 14 pages, 2 figures. The Astrophysical Journal (Letters), in pres

Sub-parsec-scale Accleration of the Radio Jet in the Powerful Radio Galaxy NGC 6251

In order to investigate the genesis of powerful radio jet, we have mapped the central 10 pc region of the nearby radio galaxy NGC 6251 with a 0.2 pc resolution using Very Long Baseline Interferometer (VLBI) at two radio frequencies, 5 GHz and 15 GHz, we have found the sub-parsec-scale counterjet for the first time in this radio galaxy. This discovery allows us to investigate the jet acceleration based on the relativistic beaming model.Comment: 7 pages with 7 figures. To appear in PASJ, 52, No. 5, Oct. 25, 200

Large-scale distributions of mid- and far-infrared emission from the center to the halo of M82 revealed with AKARI

The edge-on starburst galaxy M82 exhibits complicated distributions of gaseous materials in its halo, which include ionized superwinds driven by nuclear starbursts, neutral materials entrained by the superwinds, and large-scale neutral streamers probably caused by a past tidal interaction with M81. We investigate detailed distributions of dust grains and polycyclic aromatic hydrocarbons (PAHs) around M82 to understand their interplay with the gaseous components. We performed mid- (MIR) and far-infrared (FIR) observations of M82 with the Infrared Camera and Far-Infrared Surveyor on board AKARI. We obtain new MIR and FIR images of M82, which reveal both faint extended emission in the halo and very bright emission in the center with signal dynamic ranges as large as five and three orders of magnitude for the MIR and FIR, respectively. We detect MIR and FIR emission in the regions far away from the disk of the galaxy, reflecting the presence of dust and PAHs in the halo of M82. We find that the dust and PAHs are contained in both ionized and neutral gas components, implying that they have been expelled into the halo of M82 by both starbursts and galaxy interaction. In particular, we obtain a tight correlation between the PAH and H$\alpha$ emission, which provides evidence that the PAHs are well mixed in the ionized superwind gas and outflowing from the disk.Comment: 12 pages, 8 figures, accepted for publication in A&

VV 655 and NGC 4418: Implications of an interaction for the evolution of a LIRG

VV 655, a dwarf irregular galaxy with HI tidal debris, is a companion to the lenticular luminous infrared galaxy (LIRG) NGC 4418. NGC 4418 stands out among nearby LIRGs due to its dense central concentration of molecular gas and the dusty, bi-polar structures along its minor axis suggestive of a wind driven by a central starburst and possible nuclear activity. We seek to understand the consequences of the ongoing minor interaction between VV 655 and NGC 4418 for the evolution of the LIRG, including the origin of the gas supply responsible for its unusual nuclear properties. We investigate the structural, kinematic, and chemical properties of VV 655 and NGC 4418 by analyzing archival imaging data and optical spectroscopic observations from the SDSS-III and new spectra from SALT-RSS. We characterize their gas-phase metal abundances and spatially resolved, ionized gas kinematics, and demonstrate that the gas-phase metallicity in NGC 4418 significantly exceeds that in VV 655. No kinematic disturbances in the ionized gas are observed along the minor axis of NGC 4418, but we see evidence for ionized gas outflows from VV 655 that may increase the cross-section for gas stripping in grazing collisions. A faint, asymmetric outer arm is detected in NGC 4418 of the type normally associated with galaxy-galaxy interactions. The simplest model suggests that the minor interaction between VV 655 and NGC 4418 produced the unusual nuclear properties of the LIRG via tidal torquing of the interstellar medium of NGC 4418 rather than through a significant gas transfer event. In addition to inducing a central concentration of gas in NGC 4418, this interaction also produced an enhanced star formation rate and an outer tidal arm in the LIRG. The VV 655-NGC 4418 system offers an example of the potential for minor collisions to alter the evolutionary pathways of giant galaxies.Comment: 9 pages, 4 figures, accepted for publication in Astronomy & Astrophysic