arXiv subjects

M. Saito

Publications and source records attributed to M. Saito.

31 records · Page 2Linked to original sources

arXiv2005

Caught in the act: The Onset of massive star formation

Combining mid-infrared data from the SPITZER Space Telescope with cold gas and dust emission observations from the Plateau de Bure Interferometer, we characterize the Infrared Dark Cloud IRDC18223-3 at high spatial resolution. The millimeter continuum data reveal a massive `184Msun gas core with a projected size of ~28000AU that has no associated protostellar mid-infrared counterpart. However, the detection of 4.5mum emission at the edge of the core indicates early outflow activity, which is supported by broad CO and CS spectral line-wing emission. Moreover, systematically increasing N2H+(1-0) line-width toward the mm core center can be interpreted as additional evidence for early star formation. Furthermore, the N2H+(1-0) line emission reveals a less massive secondary core which could be in an evolutionary stage prior to any star formation activity.

astro-ph↗

arXiv2005

High Mass Starless Cores

We report the identification of a sample of potential High-Mass Starless Cores (HMSCs). The cores were discovered by comparing images of the fields containing candidate High-Mass Protostellar Objects (HMPOs) at 1.2mm and mid-infrared (8.3um; MIR) wavelengths. While the HMPOs are detected at both wavelengths, several cores emitting at 1.2mm in the same fields show absorption or no emission at the MIR wavelength. We argue that the absorption is caused by cold dust. The estimated masses of a few 10^2Msun - 10^3 Msun and the lack of IR emission suggests that they may be massive cold cores in a pre-stellar phase, which could presumably form massive stars eventually. Ammonia (1,1) and (2,2) observations of the cores indicate smaller velocity dispersions and lower rotation temperatures compared to HMPOs and UCHII regions suggesting a quiescent pre-stellar stage. We propose that these newly discovered cores are good candidates for the HMSC stage in high-mass star-formation. This sample of cores will allow us to study the high-mass star and cluster formation processes at the earliest evolutionary stages.

astro-ph↗

arXiv2004

Texture in the Superconducting Order Parameter of CeCoIn_5 Revealed by Nuclear Magnetic Resonance

We present a ^{115}In NMR study of the quasi two-dimensional heavy-fermion superconductor CeCoIn_5 believed to host a Fulde-Ferrel-Larkin-Ovchinnkov (FFLO) state. In the vicinity of the upper critical field and with a magnetic field applied parallel to the ab-plane, the NMR spectrum exhibits a dramatic change below T*(H) which well coincides with the position of reported anomalies in specific heat and ultrasound velocity. We argue that our results provide the first microscopic evidence for the occurrence of a spatially modulated superconducting order parameter expected in a FFLO state. The NMR spectrum also implies an anomalous electronic structure of vortex cores.

cond-mat.supr-con↗

arXiv2004

Imaging the Disk around TW Hya with the Submillimeter Array

We present ~2"-4" aperture synthesis observations of the circumstellar disk surrounding the nearby young star TW Hya in the CO J=2--1 and J=3--2 lines and associated dust continuum obtained with the partially completed Submillimeter Array. The extent and peak flux of the 230 and 345 GHz dust emission follow closely the predictions of the irradiated accretion disk model of Calvet et al. (2002). The resolved molecular line emission extends to a radius of at least 200 AU, the full extent of the disk visible in scattered light, and shows a clear pattern of Keplerian rotation. Comparison of the images with 2D Monte Carlo models constrains the disk inclination angle to 7+/-1 degrees. The CO emission is optically thick in both lines, and the kinetic temperature in the line formation region is ~20K. Substantial CO depletion, by an order of magnitude or more from canonical dark cloud values, is required to explain the characteristics of the line emission.

astro-ph↗

arXiv2004

Muonium as a shallow center in GaN

A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.

cond-mat.mtrl-sci↗

arXiv2004

A High-Resolution Very Large Array Observation of a Protostar in OMC-3: Shock-induced X-ray Emission by a Protostellar Jet

Using the Very Large Array (VLA) in the A-configuration, we have obtained a high-resolution 3.6 cm map of a hard X-ray source detected by the Chandra X-ray Observatory in a protostellar clump in Orion molecular cloud 3. Two radio continuum sources were detected in the vicinity of the X-ray source, both of which have NIR counterparts. We conclude that these VLA sources are free-free emission produced by shocks in protostellar jets from the NIR class I protostars. Using the centimeter data, we determined the power and orientation of the protostellar jets. The center position of the X-ray emission was found to be ~1--2" offset from the exciting sources of the jets, and the displacement is in the direction of the jets and molecular outflows. We discuss the nature of the X-ray emission as the shock-excited plasma at the shock front where the jet propagates through interstellar medium at a speed of ~1000 km/s.

astro-ph↗

arXiv2003

A giant flare on a t tauri star observed at millimeter wavelengths

We have conducted multi-epoch synthesis imaging of 2 and 3 millimeter (mm) continuum emission and near infrared K band 2.2 um imaging of a flare event in January 2003 that occurred on the young stellar object GMR-A which is suggested to be a weak-line T Tauri star in the Orion cluster. Our mm data showed that the flare activity lasted at least over 13 days, whereas the K-band magnitude did not change during this event. In addition, we have succeeded in detecting short time variations of flux on the time scales of 15 minutes. The total energy of the flare is estimated to be 10^{35-36} erg, which makes it one of the most energetic flares reported to date. Comparing the mm continuum luminosities with reported X-ray luminosities, we conclude that the mm flare was similar in nature to solar and other stellar flares. Our results will be a crucial step toward understanding magnetically induced stellar surface activities in T Tauri stars.

astro-ph↗

arXiv2000

Cross sections for nuclide production in 1 GeV proton-irradiated 208-Pb

114 cross sections for nuclide production in a 1.0 GeV proton-irradiated thin 208Pb target have been measured by the direct gamma spectrometry method using a high-resolution Ge detector. The gamma spectra were processed by the GENIE-2000 code. The ITEP-developed SIGMA code was used together with the PCNUDAT nuclear decay database to identify the gamma lines and to determine the cross sections. The 27Al(p,x)22Na reaction was used to monitor the proton flux. Results of a feasibility study of the auxiliary 27Al(p,x)24Na and 27Al(p,x)7Be monitor reactions in the 0.07-2.6 GeV proton-energy range are presented as well. Most of the experimental data have been analyzed by the LAHET (with ISABEL and Bertini options), CEM95, CEM2k, INUCL, CASCADE, CASCADE/INPE, and YIELDX codes that simulate hadron-nucleus interactions.

nucl-th↗

arXiv1999

VLA Observations of H2O Masers in the Class 0 Protostar S106 FIR: Evidence for a 10 AU-Scale Accelerating Jet-like Flow

We conducted VLA observations at 0".06 resolution of the 22 GHz water masers toward the Class 0 source S106 FIR (d=600 pc; 15" west of S106-IRS4) on two epochs separated by ~3 months. Two compact clusters of the maser spots were found in the center of the submillimeter core of S106 FIR. The separation of the clusters was ~80 mas (48 AU) along P. A. = 70 degrees and the size of each cluster was ~20 mas x 10 mas. The western cluster, which had three maser components, was 7.0 km/s redshifted with respect to the ambient cloud velocity. Each component was composed of a few spatially localized maser spots and was aligned on a line connecting the clusters. We found relative proper motions of the components with ~30 mas/yr (18 AU/yr) along the line. In addition, a series of single-dish observations show that the maser components drifted with a radial acceleration of ~1 km/s/yr. These facts indicate that the masers could be excited by a 10 AU-scale jet-like accelerating flow ejected from an assumed protostar located between the two clusters. The outflow size traced by the masers was 50 AU x 5 AU after correction for an inclination angle of 10 degrees which was derived from the relative proper motions and radial velocities of the maser components. The three-dimensional outflow velocity ranged from 40 to 70 km/s assuming symmetric motions for the blue and red components. Since no distinct CO molecular outflows have been detected so far, we suggest that S106 FIR is an extremely young protostar observed just after the onset of outflowing activity.

astro-ph↗

arXiv1999

Dwork Cohomology and Algebraic D-Modules

Using local cohomology and algebraic D-Modules, we generalize a comparison theorem between relative de Rham cohomology and Dwork cohomology due to N. Katz, A. Adolphson and S. Sperber.

math.AG↗

arXiv1998

Photometric Observations of Star Formation Activity in Early Type Spirals

We observationally study the current star formation activities of early type spiral galaxies. We construct a complete sample of 15 early type spirals having far-infrared (FIR) to optical B band luminosity ratios, L(FIR)/L(B), larger than the average of the type, and make their CCD imaging of the R and H-alpha bands. The equivalent widths of H-alpha emission increase with increasing L(FIR)/L(B), indicating that L(FIR)/L(B) can be an indicator of star formation for such early type spirals with star formation activities higher than the average. For all of the observed early type spirals, the extended HII regions exist at the central regions with some asymmetric features. H-alpha emission is more concentrated to the galactic center than the R band light, and the degree of the concentration increases with the star formation activity. We also analyze the relation between the star formation activities and the existence of companion galaxies in the sample galaxies and other bright early type spirals. No correlation is found and this suggests that the interaction is not responsible for all of the star formation activities of early type spirals.

astro-ph↗

arXiv1998

H-Alpha Velocity Fields of H II Regions in Nearby Dwarf Irregular Galaxies

We present H-alpha velocity fields of thirteen giant H II regions in four nearby dwarf irregular galaxies, NGC 2366, Holmberg II, IC 2574, and WLM. We classify the velocity features as well as the morphologies of the H II regions. The H-alpha velocity features are divided into three categories; three H II regions show chaotic feature with a typical scale of variation of a few 100 pc in size and a few 10 km s^{-1} in velocity, one shows expanding-bubble feature, and the remaining nine have calm velocity fields. There is a correlation between the H-alpha velocity feature and the morphology of H II regions. We measured bulk motion of the H II regions relative to the ambient H I velocity for the H II regions with calm velocity fields and found a typical velocity difference of about 5 km s^{-1}. We discuss a model for origin of star-forming regions based on the presence of the velocity difference between H-alpha and H I gas as well as the H I characteristics.

astro-ph↗

arXiv1997

Filtered Perverse Complexes

We introduce the notion of filtered perversity of a filtered differential complex on a complex analytic manifold $X$, without any assumptions of coherence, with the purpose of studying the connection between the pure Hodge modules and the \lt-complexes. We show that if a filtered differential complex $(\cM^\bullet,F_\bullet)$ is filtered perverse then $\aDR(\cM^\bullet,F_\bullet)$ is isomorphic to a filtered $\cD$-module; a coherence assumption on the cohomology of $(\cM^\bullet,F_\bullet)$ implies that, in addition, this $\cD$-module is holonomic. We show the converse: the de Rham complex of a holonomic Cohen-Macaulay filtered $\cD$-module is filtered perverse.

alg-geom↗