arXiv subjects

A. Tokunaga

Publications and source records attributed to A. Tokunaga.

3 recordsLinked to original sources

arXiv2006

Collinear-to-Spiral Spin Transformation without Changing Modulation Wavelength upon Ferroelectric Transition in Tb1-xDyxMnO3

Lattice modulation and magnetic structures in magnetoelectric compounds Tb1-xDyxMnO3 have been studied around the ferroelectric (FE) Curie temperature T_C by x-ray and neutron diffraction. Temperature-independent modulation vectors through T_C are observed for the compounds with 0.50< x < 0.68. This indicates that ferroelectricity with a polarization (P) along the c axis in the RMnO3 series cannot be ascribed to such an incommensurate-commensurate transition of an antiferromagnetic order as was previously anticipated. Neutron diffraction study of a single crystal with x=0.59 shows that the FE transition is accompanied by the transformation of the Mn-spin alignment from sinusoidal (collinear) antiferromagnetism into a transverse spiral structure. The observed spiral structure below T_C is expected to produce P along the c axis with the `inverse' Dzialoshinski-Moriya interaction, which is consistent with the observation.

cond-mat.str-el↗

arXiv2002

Near-Infrared Spectroscopy of Very Low-Luminosity Young Stellar Objects in the Taurus Molecular Cloud

We have carried out near-infrared spectroscopic observations of 23 very low-luminosity young stellar object (YSO) candidates and 5 their companions in Heiles Cloud 2, one of the densest parts of the Taurus molecular cloud. Twelve objects were confirmed as YSOs by Br gamma feature. The effective temperatures of the YSOs and of the companions are estimated from the 2.26 micron feature, the 2.21 micron feature, and the H2O band strengths. Detailed comparisons of our photometric and spectroscopic observations with evolutionary tracks on the HR diagram suggest some objects to be very low-mass YSOs.

astro-ph↗

arXiv2000

VLT-detection of two edge-on Circumstellar Disks in the rho Oph dark cloud

Observations of the rho Ophiuchi star forming region with VLT ANTU and ISAAC under 0.35" seeing conditions reveal two bipolar reflection nebulosities intersected by central dust lanes. The sources (OphE-MM3 and CRBR 2422.8-3423) can be identified as spatially resolved circumstellar disks viewed close to edge-on, similar to edge-on disk sources discovered previously in the Taurus and Orion star forming regions. Millimeter continuum fluxes yield disk masses of the order of 0.01 Mo, i.e. about the mass deemed necessary for the minimum solar nebula. Follow-up spectroscopic observations with SUBARU and CISCO show that both disk sources exhibit featureless continua in the K-band. No accretion or outflow signatures were detected. The slightly less edge-on orientation of the disk around CRBR 2422.8-3423 compared to HH 30 leads to a dramatic difference in the flux seen in the ISOCAM 4.5 mu to 12 mu bands. The observations confirm theoretical predictions on the effect of disk geometry and inclination angle on the spectral energy distribution of young stellar objects with circumstellar disks.

astro-ph↗