25 research outputs found
HOCO+ toward the Galactic Center
We have identified a weak thermal line U42.767, which has been detected only
in the directions toward Sgr A and Sgr B2, as the HOCO+ 2_{02}--1_{01}
transition. Because of the proximity of this line to the SiO maser line at
42.821 GHz (J=1-0 v=2),it was observable simultaneously in the ~43 GHz SiO
maser searches at Nobeyama. From the past data of SiO maser surveys of infrared
objects in the Galactic center, we created a map of emission distribution of
HOCO+ in the Sgr A molecular cloud as well as maps of the 29SiO J=1-0 v=0
thermal emission and H53alpha emission. The emission distribution of HOCO+ was
quite similar to the distribution of 29SiO emission. It suggests that the
enhancement of the HOCO+ abundance in the galactic center is induced by shock
activities which release the CO2 molecules frozen on grains into gases.Comment: PASJ Dec. 25, 2006 issue in pres
Milky Way Supermassive Black Hole: Dynamical Feeding from the Circumnuclear Environment
The supermassive black hole (SMBH), Sgr A*, at the Galactic Center is
surrounded by a molecular circumnuclear disk (CND) lying between 1.5-4 pc
radii. The irregular and clumpy structures of the CND, suggest dynamical
evolution and episodic feeding of gas towards the central SMBH. New sensitive
data from the SMA and GBT, reveal several >5-10 pc scale molecular arms, which
either directly connect to the CND, or may penetrate inside the CND. The CND
appears to be the convergence of the innermost parts of largescale gas
streamers, which are responding to the central gravitational potential well.
Rather than being a quasi-stationary structure, the CND may be dynamically
evolving, incorporating inflow via streamers, and feeding gas towards the
center.Comment: 9 pages, 7 figures, accepted to Ap
The First Very Long Baseline Interferometry Image of 44 GHz Methanol Maser with the KVN and VERA Array (KaVA)
We have carried out the first very long baseline interferometry (VLBI)
imaging of 44 GHz class I methanol maser (7_{0}-6_{1}A^{+}) associated with a
millimeter core MM2 in a massive star-forming region IRAS 18151-1208 with KaVA
(KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI
Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in
imaging compact maser features with a synthesized beam size of 2.7
milliarcseconds x 1.5 milliarcseconds (mas). These features are detected at a
limited number of baselines within the length of shorter than approximately 650
km corresponding to 100 Mlambda in the uv-coverage. The central velocity and
the velocity width of the 44 GHz methanol maser are consistent with those of
the quiescent gas rather than the outflow traced by the SiO thermal line. The
minimum component size among the maser features is ~ 5 mas x 2 mas, which
corresponds to the linear size of ~ 15 AU x 6 AU assuming a distance of 3 kpc.
The brightness temperatures of these features range from ~ 3.5 x 10^{8} to 1.0
x 10^{10} K, which are higher than estimated lower limit from a previous Very
Large Array observation with the highest spatial resolution of ~ 50 mas. The 44
GHz class I methanol maser in IRAS 18151-1208 is found to be associated with
the MM2 core, which is thought to be less evolved than another millimeter core
MM1 associated with the 6.7 GHz class II methanol maser.Comment: 19 pages, 3 figure
Emission Mechanism of "Green Fuzzies" in High-mass Star Forming Regions
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope has revealed
that a number of high-mass protostars are associated with extended mid-infrared
emission, particularly prominent at 4.5-micron. These are called "Green Fuzzy"
emission or "Extended Green Objects". We present color analysis of this
emission toward six nearby (d=2-3 kpc) well-studied high-mass protostars and
three candidate high-mass protostars identified with the Spitzer GLIMPSE
survey. In our color-color diagrams most of the sources show a positive
correlation between the [3.6]-[4.5] and [3.5]-[5.8] colors along the extinction
vector in all or part of the region. We compare the colors with those of
scattered continuum associated with the low-mass protostar L 1527, modeled
scattered continuum in cavities, shocked emission associated with low-mass
protostars, modeled H2 emission for thermal and fluorescent cases, and modeled
PAH emission. Of the emission mechanisms discussed above, scattered continuum
provides the simplest explanation for the observed linear correlation. In this
case, the color variation within each object is attributed to different
foreground extinctions at different positions. Alternative possible emission
mechanisms to explain this correlation may be a combination of thermal and
fluorescent H2 emission in shocks, and a combination of scattered continuum and
thermal H2 emission, but detailed models or spectroscopic follow-up are
required to further investigate this possibility. Our color-color diagrams also
show possible contributions from PAHs in two objects. However, none of our
sample show clear evidence for PAH emission directly associated with the
high-mass protostars, several of which should be associated with ionizing
radiation. This suggests that those protostars are heavily embedded even at
mid-infrared wavelengths.Comment: 32 pages, 14 figures, 3 tables, accepted for publication in
Astrophysical Journa