27,999 research outputs found
On the Star Formation Rates in Molecular Clouds
In this paper we investigate the level of star formation activity within
nearby molecular clouds. We employ a uniform set of infrared extinction maps to
provide accurate assessments of cloud mass and structure and compare these with
inventories of young stellar objects within the clouds. We present evidence
indicating that both the yield and rate of star formation can vary considerably
in local clouds, independent of their mass and size. We find that the surface
density structure of such clouds appears to be important in controlling both
these factors. In particular, we find that the star formation rate (SFR) in
molecular clouds is linearly proportional to the cloud mass (M_{0.8}) above an
extinction threshold of A_K approximately equal to 0.8 magnitudes,
corresponding to a gas surface density threshold of approximaely 116 solar
masses per square pc. We argue that this surface density threshold corresponds
to a gas volume density threshold which we estimate to be n(H_2) approximately
equal to 10^4\cc. Specifically we find SFR (solar masses per yr) = 4.6 +/- 2.6
x 10^{-8} M_{0.8} (solar masses) for the clouds in our sample. This relation
between the rate of star formation and the amount of dense gas in molecular
clouds appears to be in excellent agreement with previous observations of both
galactic and extragalactic star forming activity. It is likely the underlying
physical relationship or empirical law that most directly connects star
formation activity with interstellar gas over many spatial scales within and
between individual galaxies. These results suggest that the key to obtaining a
predictive understanding of the star formation rates in molecular clouds and
galaxies is to understand those physical factors which give rise to the dense
components of these clouds.Comment: accepted for publicaton in the Astrophysical Journal; 22 pages, 4
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Statistical stability and limit laws for Rovella maps
We consider the family of one-dimensional maps arising from the contracting
Lorenz attractors studied by Rovella. Benedicks-Carleson techniques were used
by Rovella to prove that there is a one-parameter family of maps whose
derivatives along their critical orbits increase exponentially fast and the
critical orbits have slow recurrent to the critical point. Metzger proved that
these maps have a unique absolutely continuous ergodic invariant probability
measure (SRB measure).
Here we use the technique developed by Freitas and show that the tail set
(the set of points which at a given time have not achieved either the
exponential growth of derivative or the slow recurrence) decays exponentially
fast as time passes. As a consequence, we obtain the continuous variation of
the densities of the SRB measures and associated metric entropies with the
parameter. Our main result also implies some statistical properties for these
maps.Comment: 1 figur
Performances of a Newly High Sensitive Trilayer F/Cu/F GMI Sensor
We have selected stress-annealed nanocrystalline Fe-based ribbons for
ferromagnetic/copper/ferromagnetic sensors exhibiting high magneto-impedance
ratio. Longitudinal magneto-impedance reaches 400% at 60 kHz and longitudinal
magneto-resistance increases up to 1300% around 200 kHz.Comment: 4 pages, 6 figures, Sensors and Actuators A (in review
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