1,619 research outputs found
A Systematic Search for Molecular Outflows Toward Candidate Low-Luminosity Protostars and Very Low Luminosity Objects
We present a systematic single-dish search for molecular outflows toward a
sample of 9 candidate low-luminosity protostars and 30 candidate Very Low
Luminosity Objects (VeLLOs; L_int < 0.1 L_sun). The sources are identified
using data from the Spitzer Space Telescope catalogued by Dunham et al. toward
nearby (D < 400 pc) star forming regions. Each object was observed in 12CO and
13CO J = 2-1 simultaneously using the sideband separating ALMA Band-6 prototype
receiver on the Heinrich Hertz Telescope at 30 arcsecond resolution. Using
5-point grid maps we identify five new potential outflow candidates and make
on-the-fly maps of the regions surrounding sources in the dense cores B59,
L1148, L1228, and L1165. Of these new outflow candidates, only the map of B59
shows a candidate blue outflow lobe associated with a source in our survey. We
also present larger and more sensitive maps of the previously detected L673-7
and the L1251-A IRS4 outflows and analyze their properties in comparison to
other outflows from VeLLOs. The accretion luminosities derived from the outflow
properties of the VeLLOs with detected CO outflows are higher than the observed
internal luminosity of the protostars, indicating that these sources likely had
higher accretion rates in the past. The known L1251-A IRS3 outflow is detected
but not remapped. We do not detect clear, unconfused signatures of red and blue
molecular wings toward the other 31 sources in the survey indicating that
large-scale, distinct outflows are rare toward this sample of candidate
protostars. Several potential outflows are confused with kinematic structure in
the surrounding core and cloud. Interferometric imaging is needed to
disentangle large-scale molecular cloud kinematics from these potentially weak
protostellar outflows.Comment: 42 pages, 19 figures, Accepted for publication in the Astronomical
Journa
Internal Motions in Starless Dense Cores
This paper discusses the statistics of internal motions in starless dense
cores and the relation of these motions to core density and evolution. Four
spectral lines from three molecular species are analyzed from single-pointing
and mapped observations of several tens of starless cores. Blue asymmetric
profiles are dominant, indicating that inward motions are prevalent in
sufficiently dense starless cores. These blue profiles are found to be more
abundant, and their asymmetry is bluer, at core positions with stronger line emission or higher column density. Thirty three starless cores are
classified into four types according to the blue and red shifts of the lines in
their molecular line maps. Among these cores, contracting motions dominate: 19
are classified as contracting, 3 as oscillating, 3 as expanding, and 8 as
static. Contracting cores have inward motions all over the core with
predominance of those motions near the region of peak density. Cores with the
bluest asymmetry tend to have greater column density than other cores and all
five cores with peak column density are found
to be contracting. This suggests that starless cores are likely to have
contracting motions if they are sufficiently condensed. Our classification of
the starless cores may indicate a sequence of core evolution in the sense that
column density increases from static to contracting cores: the static cores in
the earliest stage, the expanding and/or the oscillating cores in the next, and
the contracting cores in the latest stage.Comment: Accepted for publication in The Astrophysical Journal, 34 pages, and
14 figure
CO2 Ice toward Low-luminosity, Embedded Protostars: Evidence for Episodic Mass Accretion via Chemical History
We present Spitzer IRS spectroscopy of CO2 ice bending mode spectra at 15.2
micrometer toward 19 young stellar objects with luminosity lower than 1 Lsun (3
with luminosity lower than 0.1 Lsun). Ice on dust grain surfaces can encode the
history of heating because pure CO2 ice forms only at elevated temperature, T >
20 K, and thus around protostars of higher luminosity. Current internal
luminosities of YSOs with L < 1 Lsun do not provide the conditions needed to
produce pure CO2 ice at radii where typical envelopes begin. The presence of
detectable amounts of pure CO2 ice would signify a higher past luminosity. Many
of the spectra require a contribution from a pure, crystalline CO2 component,
traced by the presence of a characteristic band splitting in the 15.2
micrometer bending mode. About half of the sources (9 out of 19) in the low
luminosity sample have evidence for pure CO2 ice, and six of these have
significant double-peaked features, which are very strong evidence of pure CO2
ice. The presence of the pure CO2 ice component indicates that the dust
temperature, and hence luminosity of the central star/accretion disk system,
must have been higher in the past. An episodic accretion scenario, in which
mixed CO-CO2 ice is converted to pure CO2 ice during each high luminosity
phase, explains the presence of pure CO2 ice, the total amount of CO2 ice, and
the observed residual C18O gas.Comment: Accepted for publication in ApJ, total 24 pages, 14 figure
Discovery of A Binary System in IRAM 04191+1522
We present high angular resolution observations of the Class 0 protostar
IRAM04191+1522, using the Submillimeter Array (SMA). The SMA 1.3 mm continuum
images reveal within IRAM04191+1522 two distinct sources with an angular
separation of 7.8\,\,0.2. The two continuum sources are located in the
southeast-northwest direction, with total gas masses of about 0.011 M_sun and
about 0.005 M_sun, respectively. The southeastern source, associated with an
infrared source seen in the Spitzer images, is the well-known Class 0 protostar
with a bolometric luminosity of about 0.08 L_sun. The newly-discovered
northwestern continuum source is not visible in the Spitzer images at
wavelengths from 3.6 to 70 micron, and has an extremely low bolometric
luminosity (< 0.03 L_sun). Complementary IRAM N2H+(1-0) data that probe the
dense gas in the common envelope suggest that the two sources were formed
through the rotational fragmentation of an elongated dense core. Furthermore,
comparisons between IRAM04191+1522 and other protostars suggest that most cores
with binary systems formed therein have ratios of rotational energy to
gravitational energy > 1%. This is consistent with
theoretical simulations and indicates that the level of rotational energy in a
dense core plays an important role in the fragmentation process.Comment: 15 pages, 4 figures, to be published by ApJ Letter
Cold electron beams from cryo-cooled, alkali antimonide photocathodes
In this letter we report on the generation of cold electron beams using a
Cs3Sb photocathode grown by co-deposition of Sb and Cs. By cooling the
photocathode to 90 K we demonstrate a significant reduction in the mean
transverse energy validating the long standing speculation that the lattice
temperature contribution limits the mean transverse energy or thermal emittance
near the photoemission threshold, opening new frontiers in generating
ultra-bright beams. At 90 K, we achieve a record low thermal emittance of 0.2
m (rms) per mm of laser spot diameter from an ultrafast (sub-picosecond)
photocathode with quantum efficiency greater than using a
visible laser wavelength of 690 nm
Comparison of wind velocity in thunderstorms determined from measurements by a ground-based Doppler radar and an F-106B airplane
As a part of the NASA Storm Hazards Program, the wind velocity in several thunderstorms was measured by an F-106B instrumented airplane and a ground-based Doppler radar. The results of five airplane penetrations of two storms in 1980 and six penetrations of one storm in 1981 are given. Comparisons were made between the radial wind velocity components measured by the radar and the airplane. The correlation coefficients for the 1980 data and part of the 1981 data were 0.88 and 0.78, respectively. It is suggested that larger values for these coefficients may be obtained by improving the experimental technique and in particular by slaving the radar to track the airplane during such tests
Building a Better Mousetrap: Enhanced Dollar Cost Averaging
This paper presents a simple, intuitive investment strategy that improves upon the popular dollarcost- averaging (DCA) approach. The investment strategy, which we call enhanced dollar-costaveraging (EDCA), is a simple, rule-based strategy that retains most of the attributes of traditional DCA that are appealing to most investors but yet adjusts to new information, which traditional DCA does not. Simulation results show that the EDCA strategy reliably outperforms the DCA strategy in terms of higher dollar-weighted returns about 90% of the time and nearly always delivers greater terminal wealth for reasonable values of the risk premium. EDCA is most effective when applied to high volatility assets, when cash flows are highly sensitive to past returns, and during secular bear markets. Historical back-testing on equity indexes and mutual funds indicates that investor dollar-weighted returns can be enhanced by between 30 and 70 basis points per year simply by switching from DCA to EDCA
The Spitzer c2d Survey of Nearby Dense Cores: Jet and Molecular Outflow Associated with a YSO in core A of L1251
A long infrared jet has been discovered by the Spitzer c2d legacy program in
core A of L1251. It is associated with a very embedded Class 0 object with an
accretion luminosity of about 0.9 Lsun derived by radiative transfer model
fitting to the observed SED. Comparing the observed IRAC colors along the
infrared jet with those calculated from a model of an admixture of gas with a
power-law temperature distribution indicates that the jet is possibly created
by a paraboloidal bow shock propagating into the ambient medium of n(H_2)=10^5
cm^{-3}. In addition, the variation of the power-law index along the jet
suggests that the portion of hot gas decreases with distance from the jet
engine. The molecular outflow in this region has been mapped for the first time
using CO data. From the calculated outflow momentum flux, a very strong lower
limit to the average accretion luminosity is 3.6 sin i/cos^3 i Lsun, indicative
of a decrease in the accretion rate with time.Comment: To appear in ApJ Letter
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