484 research outputs found
The Lesser Role of Shear in Star Formation: Insight from the Galactic Ring Survey
We analyse the role played by shear in regulating star formation in the
Galaxy on the scale of individual molecular clouds. The clouds are selected
from the 13^CO J=1-0 line of the Galactic Ring Survey. For each cloud, we
estimate the shear parameter which describes the ability of density
perturbations to grow within the cloud. We find that for almost all molecular
clouds considered, there is no evidence that shear is playing a significant
role in opposing the effects of self-gravity. We also find that the shear
parameter of the clouds does not depend on their position in the Galaxy.
Furthermore, we find no correlations between the shear parameter of the clouds
with several indicators of their star formation activity. No significant
correlation is found between the shear parameter and the star formation
efficiency of the clouds which is measured using the ratio of the massive young
stellar objects luminosities, measured in the Red MSX survey, to the cloud
mass. There are also no significant correlations between the shear parameter
and the fraction of their mass that is found in denser clumps which is a proxy
for their clump formation efficiency, nor with their level of fragmentation
expressed in the number of clumps per unit mass. Our results strongly suggest
that shear is playing only a minor role in affecting the rates and efficiencies
at which molecular clouds convert their gas into dense cores and thereafter
into stars.Comment: Accepted to ApJ. 30 pages, 11 figures. Content substantially enlarged
and includes quantitative correlations between the SFE of molecular clouds
and their shear parameters. Some references correcte
The spontaneous formation of stereotypes via cumulative cultural evolution
All people share knowledge of cultural stereotypes of social groupsâbut what are the origins of these stereotypes? We examined whether stereotypes form spontaneously as information is repeatedly passed from person to person. As information about novel social targets was passed down a chain of individuals, what initially began as a set of random associations evolved into a system that was simplified and categorically structured. Over time, novel stereotypes emerged that not only were increasingly learnable but also allowed generalizations to be made about previously unseen social targets. By illuminating how cognitive and social factors influence how stereotypes form and change, these findings show how stereotypes might naturally evolve or be manipulated
Towards Anytime Classification in Early-Exit Architectures by Enforcing Conditional Monotonicity
Modern predictive models are often deployed to environments in which
computational budgets are dynamic. Anytime algorithms are well-suited to such
environments as, at any point during computation, they can output a prediction
whose quality is a function of computation time. Early-exit neural networks
have garnered attention in the context of anytime computation due to their
capability to provide intermediate predictions at various stages throughout the
network. However, we demonstrate that current early-exit networks are not
directly applicable to anytime settings, as the quality of predictions for
individual data points is not guaranteed to improve with longer computation. To
address this shortcoming, we propose an elegant post-hoc modification, based on
the Product-of-Experts, that encourages an early-exit network to become
gradually confident. This gives our deep models the property of conditional
monotonicity in the prediction quality -- an essential stepping stone towards
truly anytime predictive modeling using early-exit architectures. Our empirical
results on standard image-classification tasks demonstrate that such behaviors
can be achieved while preserving competitive accuracy on average.Comment: NeurIPS 202
A Mid-Infrared Census of Star Formation Activity in Bolocam Galactic Plane Survey Sources
We present the results of a search for mid-infrared signs of star formation
activity in the 1.1 mm sources in the Bolocam Galactic Plane Survey (BGPS). We
have correlated the BGPS catalog with available mid-IR Galactic plane catalogs
based on the Spitzer Space Telescope GLIMPSE legacy survey and the Midcourse
Space Experiment (MSX) Galactic plane survey. We find that 44% (3,712 of 8,358)
of the BGPS sources contain at least one mid-IR source, including 2,457 of
5,067 (49%) within the area where all surveys overlap (10 deg < l < 65 deg).
Accounting for chance alignments between the BGPS and mid-IR sources, we
conservatively estimate that 20% of the BPGS sources within the area where all
surveys overlap show signs of active star formation. We separate the BGPS
sources into four groups based on their probability of star formation activity.
Extended Green Objects (EGOs) and Red MSX Sources (RMS) make up the highest
probability group, while the lowest probability group is comprised of
"starless" BGPS sources which were not matched to any mid-IR sources. The mean
1.1 mm flux of each group increases with increasing probability of active star
formation. We also find that the "starless" BGPS sources are the most compact,
while the sources with the highest probability of star formation activity are
on average more extended with large skirts of emission. A subsample of 280 BGPS
sources with known distances demonstrates that mass and mean H_2 column density
also increase with probability of star formation activity.Comment: 20 pages, 12 figures, 3 tables. Accepted for publication in ApJ. Full
Table 2 will be available online through Ap
The RMS Survey: Critical Tests of Accretion Models for the Formation of Massive Stars
There is currently no accepted theoretical framework for the formation of the
most massive stars, and the manner in which protostars continue to accrete and
grow in mass beyond \sim10Msun is still a controversial topic. In this study we
use several prescriptions of stellar accretion and a description of the
Galactic gas distribution to simulate the luminosities and spatial distribution
of massive protostellar population of the Galaxy. We then compare the
observables of each simulation to the results of the Red MSX Source (RMS)
survey, a recently compiled database of massive young stellar objects. We find
that the observations are best matched by accretion rates which increase as the
protostar grows in mass, such as those predicted by the turbulent core and
competitive accretion (i.e. Bondi-Hoyle) models. These 'accelerating accretion'
models provide very good qualitative and quantitative fits to the data, though
we are unable to distinguish between these two models on our simulations alone.
We rule out models with accretion rates which are constant with time, and those
which are initially very high and which fall away with time, as these produce
results which are quantitatively and/or qualitatively incompatible with the
observations. To simultaneously match the low- and high-luminosity YSO
distribution we require the inclusion of a 'swollen-star' pre-main-sequence
phase, the length of which is well-described by the Kelvin-Helmholz timescale.
Our results suggest that the lifetime of the YSO phase is \sim 10^5yrs, whereas
the compact Hii-region phase lasts between \sim 2 - 4 \times 10^5yrs depending
on the final mass of the star. Finally, the absolute numbers of YSOs are best
matched by a globally averaged star-formation rate for the Galaxy of
1.5-2Msun/yr.Comment: 22 pages, 24 figures. Accepted for publication in MNRA
A Possible Third Body in the X-Ray System GRS 1747-312 and Models with Higher-Order Multiplicity
GRS 1747-312 is a bright Low-Mass X-ray Binary in the globular cluster Terzan
6, located at a distance of 9.5 kpc from the Earth. It exhibits regular
outbursts approximately every 4.5 months, during which periodic eclipses are
known to occur. These eclipses have only been observed in the outburst phase,
and are not clearly seen when the source is quiescent. Recent Chandra
observations of the source were performed in June 2019 and April, June, and
August of 2021. Two of these observations captured the source during its
outburst, and showed clear flux decreases at the expected time of eclipse. The
other two observations occurred when the source was quiescent. We present the
discovery of a dip that occurred during the quiescent state. The dip is of
longer duration and its time of occurrence does not fit the ephemeris of the
shorter eclipses. We study the physical characteristics of the dip and
determine that it has all the properties of an eclipse by an object with a well
defined surface. We find that there are several possibilities for the nature of
the object causing the 5.3 ks eclipse. First, GRS 1747-312 may be an X-ray
triple, with an LMXB orbited by an outer third object, which could be an
M-dwarf, brown dwarf, or planet. Second, there could be two LMXBs in close
proximity to each other, likely bound together. Whatever the true nature of the
eclipser, its presence suggests that the GRS 1747-312 system is unique.Comment: 38 pages, 30 figures, submitted to MNRA
- âŠ