arXiv subjects

Y. Kobayashi

Publications and source records attributed to Y. Kobayashi.

94 records · Page 6Linked to original sources

arXiv2002

The Type Ic Hypernova SN 2002ap

Photometric and spectroscopic data of the energetic Type Ic supernova (SN) 2002ap are presented, and the properties of the SN are investigated through models of its spectral evolution and its light curve. The SN is spectroscopically similar to the "hypernova" SN 1997ef. However, its kinetic energy [$\sim (4-10) \times 10^{51}$ erg] and the mass ejected (2.5-5 $M_{\odot}$) are smaller, resulting in a faster-evolving light curve. The SN synthesized $\sim 0.07 M_{\odot}$ of $^{56}$Ni, and its peak luminosity was similar to that of normal SNe. Brightness alone should not be used to define a hypernova, whose defining character, namely very broad spectral features, is the result of a high kinetic energy. The likely main-sequence mass of the progenitor star was 20-25 $M_{\odot}$, which is also lower than that of both hypernovae SNe 1997ef and 1998bw. SN 2002ap appears to lie at the low-energy and low-mass end of the hypernova sequence as it is known so far. Observations of the nebular spectrum, which is expected to dominate by summer 2002, are necessary to confirm these values.

astro-ph↗

arXiv1997

The Interpretation of Near-Infrared Star Counts at the South Galactic Pole

We present new deep $K'$ counts of stars at the South Galactic Pole (SGP) taken with the NAOJ PICNIC camera to $K'=17.25$. Star-galaxy separation to $K'=17.5$ was accomplished effectively using image profiles because the pixel size we used is 0.509 arcsec. We interpret these counts using the SKY (Cohen 1994) model of the Galactic point source sky and determine the relative normalization of halo-to-disk populations, and the location of the Sun relative to the Galactic plane, within the context of this model. The observed star counts constrain these parameters to be: halo/disk $\sim$ 1/900 and z$_\odot$=16.5$\pm$2.5 pc. These values have been used to correct our SGP galaxy counts for contamination by the point source Galactic foreground.

astro-ph↗

arXiv1997

K-Band Galaxy Counts in the South Galactic Pole Region

We present new K-band galaxy number counts from K=13 to 20.5 obtained from $K'$-band surveys in the south galactic pole region, which cover 180.8 arcmin$^2$ to a limiting magnitude of K=19, and 2.21 arcmin$^2$ to K=21. These are currently the most precise K-band galaxy counts at $17.5<K<19.0$ because the area of coverage is largest among the existing surveys for this magnitude range. The completeness and photometry corrections are estimated from the recovery of simulated galaxy and stellar profiles added to the obtained field image. Many simulations were carried out to construct a probability matrix which corrects the galaxy counts at the faint-end magnitudes of the surveys so the corrected counts can be compared with other observations. The K-band star counts in the south galactic pole region to $K=17.25$ are also presented for use to constrain the vertical structure of the Galaxy.

astro-ph↗

arXiv1996

Equivalence principle in the new general relativity

We study the problem of whether the active gravitational mass of an isolated system is equal to the total energy in the tetrad theory of gravitation. The superpotential is derived using the gravitational Lagrangian which is invariant under parity operation, and applied to an exact spherically symmetric solution. Its associated energy is found equal to the gravitational mass. The field equation in vacuum is also solved at far distances under the assumption of spherical symmetry. Using the most general expression for parallel vector fields with spherical symmetry, we find that the equality between the gravitational mass and the energy is always true if the parameters of the theory $a_1$, $a_2$ and $a_3$ satisfy the condition, $(a_1+ a_2) (a_1-4a_3/9)\neq0$. In the two special cases where either $(a_1+a_2)$ or $(a_1-4a_3/9)$ is vanishing, however, this equality is not satisfied for the solutions when some components of the parallel vector fields tend to zero as $1/\sqrt{r}$ for large $r$.

gr-qc↗