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Mallick K.K.,Tata Institute of Fundamental Research | Ojha D.K.,Tata Institute of Fundamental Research | Tamura M.,Japan National Astronomical Observatory | Pandey A.K.,Aryabhatta Research Institute of Observational science | And 7 more authors.
Monthly Notices of the Royal Astronomical Society

We present deep and high-resolution (FWHM ~ 0.4 arcsec) near-infrared (NIR) imaging observations of the NGC 7538 IRS 1-3 region (in JHK bands), and IRS 9 region (in HK bands) using the 8.2m Subaru telescope. The NIR analysis is complemented with Giant Metrewave Radio Telescope (GMRT) low-frequency observations at 325, 610, and 1280 MHz, molecular line observations of H13CO+(J=1-0), and archival Chandra X-ray observations. Using the 'J - H/H - K' diagram, 144 Class II and 24 Class I young stellar object (YSO) candidates are identified in the IRS 1-3 region. Further analysis using 'K/H - K' diagram yields 145 and 96 red sources in the IRS 1-3 and IRS 9 regions, respectively. A total of 27 sources are found to have X-ray counterparts. TheYSOmass function (MF), constructed using a theoretical mass-luminosity relation, shows peaks at substellar (~0.08-0.18M⊙ ) and intermediate (~1- 1.78M⊙ ) mass ranges for the IRS 1-3 region. The MF can be fitted by a power law in the low-mass regime with a slope of Γ ~ 0.54-0.75, which is much shallower than the Salpeter value of 1.35. An upper limit of 10.2 is obtained for the star to brown dwarf ratio in the IRS 1-3 region. GMRT maps show a compact H II region associated with the IRS 1-3 sources, whose spectral index of 0.87 ± 0.11 suggests optical thickness. This compact region is resolved into three separate peaks in higher resolution 1280 MHz map, and the 'east' subpeak coincides with the IRS 2 source. H13CO+ (J=1-0) emission reveals peaks in both IRS 1-3 and IRS 9 regions, none of which are coincident with visible nebular emission, suggesting the presence of dense cloud nearby. The virial masses are approximately of the order of 1000 and 500M⊙ for the clumps in IRS 1-3 and IRS 9 regions, respectively. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Sugiyama K.,Yamaguchi University | Fujisawa K.,Yamaguchi University | Doi A.,Japan Aerospace Exploration Agency | Doi A.,Graduate University for Advanced Studies | And 11 more authors.
Astronomy and Astrophysics

Context. Proper motion observations of masers can provide information on dynamic motions on scales of a few milliarcseconds per year (mas yr -1) at radii of 100-1000 au scales from central young stellar objects (YSOs). Aims. The 6.7 GHz methanol masers are one of the best probes for investigations of the dynamics of high-mass YSOs, and in particular for tracing the rotating disk. We have measured the internal proper motions of the 6.7 GHz methanol masers associated with Cepheus A (Cep A) HW2 using Very Long Baseline Interferometery (VLBI) observations. Methods. We conducted three epochs of VLBI monitoring observations of the 6.7 GHz methanol masers in Cep A-HW2 with the Japanese VLBI Network (JVN) over the period 2006-2008. In 2006, we were able to use phase-referencing to measure the absolute coordinates of the maser emission with an accuracy of a few milliarcseconds. We compared the maser distribution with other molecular line observations that trace the rotating disk. Results. We measured the internal proper motions for 29 methanol maser spots, of which 19 were identified at all three epochs and the remaining ten at only two epochs. The magnitude of proper motions ranged from 0.2 to 7.4 km s-1, with an average of 3.1 km s-1. Although there are large uncertainties in the observed internal proper motions of the methanol maser spots in Cep A, they are well fitted by a disk that includes both rotation and infall velocity components. The derived rotation and infall velocities at the disk radius of 680 au are 0.5 ± 0.7 and 1.8 ± 0.7 km s-1, respectively. Conclusions. Assuming that the modeled disk motion accurately represents the accretion disk around the Cep A-HW2 high-mass YSO, we estimated the mass infall rate to be 3 × 10-4n8M⊙; yr -1 (n8 is the gas volume density in units of 10 8 cm-3). The combination of the estimated mass infall rate and the magnitude of the fitted infall velocity suggests that Cep A-HW2 is at an evolutionary phase of active gas accretion from the disk onto the central high-mass YSO. The infall momentum rate is estimated to be 5 × 10 -4 n8 M⊙; yr-1 km s -1, which is larger than the estimated stellar radiation pressure of the HW2 object, supporting the hypothesis that this object is in an active gas accretion phase. © ESO, 2014. Source

Sawada-Satoh S.,Mizusawa VLBI Observatory | Fujisawa K.,Mizusawa VLBI Observatory | Sugiyama K.,Yamaguchi University | Wajima K.,Yamaguchi University | Honma M.,VERA Project
Proceedings of the International Astronomical Union

We present the internal proper motion of 6.7-GHz methanol masers in S269, an Ultra Compact HII region. The maser distribution in S269 consists of several maser groups, and the spatial structure of the main groups A and B are consistent with the past VLBI image. The remarkable result of comparing the two VLBI maps is that 6.7-GHz methanol maser distribution and velocity range within each group have been kept for eight years. Angular separation between the two groups A and B increases by 3.6 mas, which corresponds to a velocity of 11.5 km s-1. Copyright © International Astronomical Union 2012. Source

Niinuma K.,Yamaguchi University | Lee S.-S.,Korea Astronomy and Space Science Institute | Kino M.,Japan Aerospace Exploration Agency | Sohn B.W.,Yamaguchi University | And 68 more authors.
Publications of the Astronomical Society of Japan

The Korean very-long-baseline interferometry (VLBI) network (KVN) and VLBI Exploration of Radio Astrometry (VERA) Array (KaVA) is the first international VLBI array dedicated to high-frequency (23-43GHz bands) observations in East Asia. Here, we report the first imaging observations of three bright active galactic nuclei (AGNs) known for their complex morphologies: 4C39.25, 3C273, and M87. This is one of the initial results of KaVA's early operation. Our KaVA images reveal extended outflows with complex substructures such as knots and limb brightening, in agreement with previous Very Long Baseline Array (VLBA) observations. Angular resolutions are better than 1.4 and 0.8 mas at 23 and 43GHz, respectively. KaVA achieves a high dynamic range of ∼1000, more than three times the value achieved by VERA. We conclude that KaVA is a powerful array with a great potential for the study of AGN outflows, at least comparable to the best existing radio interferometric arrays. © 2014 © The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. Source

Honma M.,Mizusawa VLBI Observatory | Honma M.,Graduate University for Advanced Studies | Akiyama K.,Mizusawa VLBI Observatory | Akiyama K.,University of Tokyo | And 3 more authors.
Publications of the Astronomical Society of Japan

We propose a new technique to obtain super-resolution images with radio interferometry using sparse modeling. In standard radio interferometry, sampling of (u, v) is quite often incomplete and thus obtaining an image from observed visibilities becomes an underdetermined problem, and a technique of so-called "zero-padding" is often used to fill up unsampled grids in the (u, v) plane, resulting in image degradation by finite beam size as well as numerous side-lobes. In this paper we show that directly solving such an underdetermined problem based on sparse modeling (in this paper, Least Absolute Shrinkage and Selection Operator, known as LASSO) avoids the above problems introduced by zero-padding, leading to super-resolution images in which structure finer than the standard beam size (diffraction limit) can be reproduced. We present results of one-dimensional and two-dimensional simulations of interferometric imaging, and discuss its implications for super-resolution imaging, particularly focusing on imaging of black hole shadows with millimeter VLBI (Very Long Baseline Interferometry). © The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. Source

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