2,455 research outputs found
A Survey for Infall Motions toward Starless Cores. II. and Mapping Observations
We present the results of an extensive mapping survey of 53 `starless' cores
in the optically thick line of CS 2-1 and the optically thin lines of N2H+ 1-0
and C18O 1-0. The purpose of this survey was to search for signatures of
extended inward motions.
This study finds 10 `strong' and 9 `probable' infall candidates, based on
analysis and on the spectral shapes of CS lines.
From our analysis of the blue-skewed CS spectra and the
parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple
two layer radiative transfer model to fit the profiles, we derive
one-dimensional infall speeds, half of whose values lie in the range of
0.05-0.09 km s. These values are similar to those found in L1544 by
Tafalla et al., and this result confirms that infall speeds in starless cores
are generally faster than expected from ambipolar diffusion in a strongly
sub-critical core. In addition, the observed infall regions are too extended to
be consistent with the `inside-out' collapse model applied to a very low-mass
star. In the largest cores, the spatial extent of the CS spectra with infall
asymmetry is larger than the extent of the core by a factor of
2-3. All these results suggest that extended inward motions are a common
feature in starless cores, and that they could represent a necessary stage in
the condensation of a star-forming dense core.Comment: Two tex files for manuscript and tables, and 38 figures. To appear in
ApJ
A Survey For Infall Motions Toward Starless Cores. III. CS (3-2) and DCO+(2-1) Observations
We present CS(3-2) and DCO+(2-1) observations of 94 starless cores and
compare the results with previous CS(2-1) and N2H+(1-0) observations to study
inward motions in starless cores. The velocity shifts of the CS(3-2) and (2-1)
lines with respect to N2H+ correlate well with each other and have similar
distributions. This implies that, in many cores, systematic inward motions of
gaseous material may occur over a range of density of at least a factor 4. We
identify 18 infall candidates based on observations of CS(3-2), CS(2-1),
DCO+(2-1) and N2H+(1-0). The eight best candidates, L1355, L1498, L1521F,
L1544, L158, L492, L694-2, and L1155C-1, each show at least four indications of
infall asymmetry and no counter-indications. Fits of the spectra to a 2-layer
radiative transfer model in ten infall candidates suggest that the median
effective line-of-sight speed of the inward-moving gas is ~0.07 km/s for CS
(3-2) and ~0.04 km/s for CS(2-1). Considering that the optical depth obtained
from the fits is usually smaller in CS(3-2) than in (2-1) line, this may imply
that CS(3-2) usually traces inner denser gas in higher inward motions than
CS(2-1). However, it is also possible that this conclusion is not
representative of all starless core infall candidates, due to the statistically
small number analyzed here. Further line observations will be useful to test
this conclusion.Comment: 2 PS files for the manuscript and tables, and 17 gif files for the
figure
The Spitzer c2d Survey of Nearby Dense Cores. V. Discovery of a VeLLO in the "Starless" Dense Core L328
This paper reports the discovery of a Very Low Luminosity Object (VeLLO) in
the "starless" dense core L328, using the Spitzer Space Telescope and ground
based observations from near-infrared to millimeter wavelengths. The Spitzer 8
micron image indicates that L328 consists of three subcores of which the
smallest one may harbor a source, L328-IRS while two other subcores remain
starless. L328-IRS is a Class 0 protostar according to its bolometric
temperature (44 K) and the high fraction ~72 % of its luminosity emitted at
sub-millimeter wavelengths. Its inferred "internal luminosity" (0.04 - 0.06
Lsun) using a radiative transfer model under the most plausible assumption of
its distance as 200 pc is much fainter than for a typical protostar, and even
fainter than other VeLLOs studied previously. Note, however, that its inferred
luminosity may be uncertain by a factor of 2-3 if we consider two extreme
values of the distance of L328-IRS (125 or 310 pc). Low angular resolution
observations of CO do not show any clear evidence of a molecular outflow
activity. But broad line widths toward L328, and Spitzer and near-infrared
images showing nebulosity possibly tracing an outflow cavity, strongly suggest
the existence of outflow activity. Provided that an envelope of at most ~0.1
Msunis the only mass accretion reservoir for L328-IRS, and the star formation
efficiency is close to the canonical value ~30%, L328-IRS has not yet accreted
more than 0.05 Msun. At the assumed distance of 200 pc, L328-IRS is destined to
be a brown dwarf.Comment: 29 pages, 8 figures, 1 table, to be published in Astrophysical
Journa
Target accessibility and signal specificity in live-cell detection of BMP-4 mRNA using molecular beacons
The ability to visualize mRNA in single living cells and monitor in real-time the changes of mRNA level and localization can provide unprecedented opportunities for biological and disease studies. However, the mRNA detection specificity and sensitivity are critically dependent on the selection of target sequences and their accessibility. We carried out an extensive study of the target accessibility of BMP-4 mRNA using 10 different designs of molecular beacons (MBs), and identified the optimal beacon design. Specifically, for MB design 1 and 8 (MB1 and MB8), the fluorescent intensities from BMP-4 mRNA correlated well with the GFP signal after upregulating BMP-4 and co-expressing GFP using adenovirus, and the knockdown of BMP-4 mRNA using siRNA significantly reduced the beacon signals, demonstrating detection specificity. The beacon specificity was further confirmed using blocking RNA and in situ hybridization. We found that fluorescence signal from MBs depends critically on target sequences; the target sequences corresponding to siRNA sites may not be good sites for beacon-based mRNA detection, and vice versa. Possible beacon design rules are identified and approaches for enhancing target accessibility are discussed. This has significant implications to MB design for live cell mRNA detection
Limits on Radio Continuum Emission from a Sample of Candidate Contracting Starless Cores
We used the NRAO Very Large Array to search for 3.6 cm continuum emission
from embedded protostars in a sample of 8 nearby ``starless'' cores that show
spectroscopic evidence for infalling motions in molecular emission lines. We
detect a total of 13 compact sources in the eight observed fields to 5 sigma
limiting flux levels of typically 0.09 mJy. None of these sources lie within 1'
of the central positions of the cores, and they are all likely background
objects. Based on an extrapolation of the empirical correlation between the
bolometric luminosity and 3.6 cm luminosity for the youngest protostars, these
null-detections place upper limits of ~0.1 L_sun (d/140pc)^2 on the
luminosities of protostellar sources embedded within these cores. These limits,
together with the extended nature of the inward motions inferred from molecular
line mapping (Lee et al. 2001), are inconsistent with the inside-out collapse
model of singular isothermal spheres and suggest a less centrally condensed
phase of core evolution during the earliest stages of star formation.Comment: Accepted to the Astronomical Journal; 12 pages, 1 figur
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