77 research outputs found
A 1000 AU Scale Molecular Outflow Driven by a Protostar with an age of <4000 Years
To shed light on the early phase of a low-mass protostar formation process,
we conducted interferometric observations towards a protostar GF9-2 using the
CARMA and SMA. The observations have been carried out in the CO J=3-2 line and
in the continuum emission at the wavelengths of 3 mm, 1 mm and 850 micron. All
the continuum images detected a single point-like source with a radius of
250+/-80 AU at the center of the previously known ~3 Msun molecular cloud core.
A compact emission is detected towards the object at the Spitzer MIPS and IRAC
bands as well as the four bands at the WISE. Our spectroscopic imaging of the
CO line revealed that the continuum source is driving a 1000 AU scale molecular
outflow, including a pair of lobes where a collimated "higher" velocity red
lobe exists inside a poorly collimated "lower" velocity red lobe. These lobes
are rather young and the least powerful ones so far detected. A protostellar
mass of M~<0.06 Msun was estimated using an upper limit of the protostellar age
of (4+/-1)x10^3 yrs and an inferred non-spherical steady mass accretion rate of
~10^{-5} Msun/yr. Together with results from an SED analysis, we discuss that
the outflow system is driven by a protostar whose surface temperature
of~3,000K, and that the natal cloud core is being dispersed by the outflow.Comment: 27 pages, 14 figures, accepted for publication in Astrophysical
Journa
The Initial Conditions for Gravitational Collapse of a Core: An Extremely Young Low-Mass Class 0 Protostar GF9-2
We present a study of the natal core harboring the class 0 protostar GF9-2 in
the filamentary dark cloud GF 9 (d = 200 pc). GF9-2 stands unique in the sense
that it shows H2O maser emission, a clear signpost of protostar formation,
whereas it does not have a high-velocity large-scale molecular outflow
evidenced by our deep search for CO wing emission. These facts indicate that
GF9-2 core is early enough after star formation so that it still retains some
information of initial conditions for collapse. Our 350 um dust continuum
emission image revealed the presence of a protostellar envelope in the center
of a molecular core. The mass of the envelope is ~0.6 Msun from the 350 um flux
density, while LTE mass of the core is ~3 Msun from moleuclar line
observations. Combining visibility data from the OVRO mm-array and the 45m
telescope, we found that the core has a radial density profile of
for 0.003 < r/pc < 0.08 region. Molecular line data
analysis revealed that the velocity width of the core gas increases
inward,while the outermost region maintains a velocity dispersion of a few
times of the ambient sound speed. The broadened velocity width can be
interpreted as infall. Thus, the collapse in GF9-2 is likely to be described by
an extension of the Larson-Penston solution for the period after formation of a
central star. We derived the current mass accretion rate of ~3E-05 Msun/year
from infall velocity of ~ 0.3 km/s at r~ 7000 AU. All results suggest that
GF9-2 core has been undergoing gravitational collapse for ~ 5000 years since
the formation of central protostar(s), and that the unstable state initiated
the collapse ~2E+05 years (the free-fall time) ago.Comment: ApJ Accepted. The preprint including figures with the original
quality is available at http://subarutelescope.org/staff/rsf/publication.htm
High-velocity Molecular Outflow in CO J = 7-6 Emission from the Orion Hot Core
Using the Caltech Submillimeter Observatory 10.4 m telescope, we performed sensitive mapping observations of ^(12)CO J = 7-6 emission at 807 GHz toward Orion IRc2. The image has an angular resolution of 10", which is the highest angular resolution data toward the Orion Hot Core published for this transition. In addition, thanks to the on-the-fly mapping technique, the fidelity of the new image is rather high, particularly in comparison with previous images. We have succeeded in mapping the northwest-southeast high-velocity molecular outflow, whose terminal velocity is shifted by ~70-85 km s^(â1) with respect to the systemic velocity of the cloud. This yields an extremely short dynamical time scale of ~900 years. The estimated outflow mass loss rate shows an extraordinarily high value, on the order of 10^(â3) M_â yr^(â1). Assuming that the outflow is driven by Orion IRc2, our result agrees with the picture so far obtained for a 20 M_â (proto)star in the process of formation
Low-Mass Star Forming Cores in the GF9 Filament
We carried out an unbiased mapping survey of dense molecular cloud cores
traced by the NH3 (1,1) and (2,2) inversion lines in the GF9 filament which
contains an extremely young low-mass protostar GF9-2 (Furuya et al. 2006, ApJ,
653, 1369). The survey was conducted using the Nobeyama 45m telescope over a
region of ~1.5 deg with an angular resolution of 73". The large-scale map
revealed that the filament contains at least 7 dense cores, as well as 3
possible ones, located at regular intervals of ~0.9 pc. Our analysis shows that
these cores have kinetic temperatures of 10 K and LTE-masses of 1.8
-- 8.2 Msun, making them typical sites of low-mass star formation. All the
identified cores are likely to be gravitationally unstable because their
LTE-masses are larger than their virial masses. Since the LTE-masses and
separations of the cores are consistent with the Jeans masses and lengths,
respectively, for the low-density ambient gas, we argue that the identified
cores have formed via the gravitational fragmentation of the natal filamentary
cloud.Comment: accepted by pas
Warm Extended Dense Gas Lurking At The Heart Of A Cold Collapsing Dense Core
In order to investigate when and how the birth of a protostellar core occurs,
we made survey observations of four well-studied dense cores in the Taurus
molecular cloud using CO transitions in submillimeter bands. We report here the
detection of unexpectedly warm (~ 30 - 70 K), extended (radius of ~ 2400 AU),
dense (a few times 10^{5} cm^{-3}) gas at the heart of one of the dense cores,
L1521F (MC27), within the cold dynamically collapsing components. We argue that
the detected warm, extended, dense gas may originate from shock regions caused
by collisions between the dynamically collapsing components and
outflowing/rotating components within the dense core. We propose a new stage of
star formation, "warm-in-cold core stage (WICCS)", i.e., the cold collapsing
envelope encases the warm extended dense gas at the center due to the formation
of a protostellar core. WICCS would constitutes a missing link in evolution
between a cold quiescent starless core and a young protostar in class 0 stage
that has a large-scale bipolar outflow.Comment: Accepted for publication in The Astrophysical Journal Letter
SMA Imaging of the Maser Emission from the H30 Radio Recombination Line in MWC349A
We used the Submillimeter Array to map the angular distribution of the
H30 recombination line (231.9 GHz) in the circumstellar region of the
peculiar star MWC349A. The resolution was 1\farcs2, but because of high
signal-to-noise ratio we measured the positions of all maser components to
accuracies better than 0\farcs01, at a velocity resolution of . The
two strongest maser components (called high velocity components) at velocities
near -14 and are separated by 0\farcs048 \pm 0\farcs001 (60 AU)
along a position angle of 102 \pm 1\arcdeg. The distribution of maser
emission at velocities between and beyond these two strongest components were
also provided. The continuum emission lies at the center of the maser
distribution to within 10 mas. The masers appear to trace a nearly edge-on
rotating disk structure, reminiscent of the water masers in Keplerian rotation
in the nuclear accretion disk of the galaxy NGC4258. However, the maser
components in MWC349A do not follow a simple Keplerian kinematic prescription
with , but have a larger power law index. We explore the
possibility that the high velocity masers trace spiral density or shock waves.
We also emphasize caution in the interpretation of relative centroid maser
positions where the maser is not clearly resolved in position or velocity, and
we present simulations that illustrate the range of applicability of the
centroiding method.Comment: 23 pages with 9 figures (two of these figures are vertically aligned
as Figure 4) submitted to the Astrophysical Journa
High Velocity Outflow in CO J=7-6 from the Orion Hot Core
Using the Caltech Submillimeter Observatory 10.4-meter telescope, we
performed sensitive mapping observations of 12CO J=7-6 emission at 807 GHz
towards Orion IRc2. The image has an angular resolution of 10", which is the
highest angular resolution data toward the Orion Hot Core published for this
transition. In addition, thanks to the on-the-fly mapping technique, the
fidelity of the new image is rather high, particularly in comparison to
previous images. We have succeeded in mapping the northwest-southeast
high-velocity molecular outflow, whose terminal velocity is shifted by ~70-85
km/s with respect to the systemic velocity of the cloud. This yields an
extremely short dynamical time scale of ~900 years. The estimated outflow mass
loss rate shows an extraordinarily high value, on the order of 10^{-3} Msun/yr.
Assuming that the outflow is driven by Orion IRc2, our result agrees with the
picture so far obtained for a 20 Msun (proto)star in the process of formation.Comment: accepted by ApJ main journal, 13 pages 5 color figure
IRC+10216's Innermost Envelope -- The eSMA's View
We used the Extended Submillimeter Array (eSMA) in its most extended
configuration to investigate the innermost (within a radius of 290 R* from the
star) circumstellar envelope (CSE) of IRC+10216. We imaged the CSE using HCN
and other molecular lines with a beam size of 0."22 x 0."46, deeply into the
very inner edge (15 R*) of the envelope where the expansion velocity is only 3
km/s. The excitation mechanism of hot HCN and KCl maser lines is discussed. HCN
maser components are spatially resolved for the first time on an astronomical
object. We identified two discrete regions in the envelope: a region with a
radius of . 15 R*, where molecular species have just formed and the gas has
begun to be accelerated (region I) and a shell region (region II) with a radius
of 23 R* and a thickness of 15 R*, whose expansion velocity has reached up to
13 km/s, nearly the terminal velocity of 15 km/s. The SiS line detected
in region I shows a large expansion velocity of 16 km/s due to strong wing
components, indicating that the emission may arise from a shock region in the
innermost envelope. In region II, the P.A. of the most copious mass loss
direction was found to be 120 +/- 10 degrees, which may correspond to the
equatorial direction of the star. Region II contains a torus-like feature.
These two regions may have emerged due to significant differences in the size
distributions of the dust particles in the two regions.Comment: 26 pages, 8 figures, accepted for publication in The Astrophysical
Journal. Please find the pdf at
http://www.submm.caltech.edu/~hs/astroph/0904.0280.pdf and the ps file at
http://www.submm.caltech.edu/~hs/astroph/0904.0280.p
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