9,733 research outputs found
Magnetically Regulated Star Formation in Turbulent Clouds
We investigate numerically the combined effects of supersonic turbulence,
strong magnetic fields and ambipolar diffusion on cloud evolution leading to
star formation. We find that, in clouds that are initially magnetically
subcritical, supersonic turbulence can speed up star formation, through
enhanced ambipolar diffusion in shocks. The speedup overcomes a major objection
to the standard scenario of low-mass star formation involving ambipolar
diffusion, since the diffusion time scale at the average density of a molecular
cloud is typically longer than the cloud life time. At the same time, the
strong magnetic field can prevent the large-scale supersonic turbulence from
converting most of the cloud mass into stars in one (short) turbulence crossing
time, and thus alleviate the high efficiency problem associated with the
turbulence-controlled picture for low-mass star formation. We propose that
relatively rapid but inefficient star formation results from supersonic
collisions of somewhat subcritical gas in strongly magnetized, turbulent
clouds. The salient features of this shock-accelerated, ambipolar
diffusion-regulated scenario are demonstrated with numerical experiments.Comment: 10 pages, 3 figures, accepted for publication in ApJ
Disintegrating Asteroid P/2013 R3
Splitting of the nuclei of comets into multiple components has been
frequently observed but, to date, no main-belt asteroid has been observed to
break-up. Using the Hubble Space Telescope, we find that main-belt asteroid
P/2013 R3 consists of 10 or more distinct components, the largest up to 200 m
in radius (assumed geometric albedo of 0.05) each of which produces a coma and
comet-like dust tail. A diffuse debris cloud with total mass roughly 2x10^8 kg
further envelopes the entire system. The velocity dispersion among the
components is about V = 0.2 to 0.5 m/s, is comparable to the gravitational
escape speeds of the largest members, while their extrapolated plane-of-sky
motions suggest break-up between February and September 2013. The broadband
optical colors are those of a C-type asteroid. We find no spectral evidence for
gaseous emission, placing model-dependent upper limits to the water production
rate near 1 kg/s. Breakup may be due to a rotationally induced structural
failure of the precursor body.Comment: 16 pages, 3 figures; accepted by ApJ
Nucleus and Mass Loss from Active Asteroid 313P/Gibbs
We present Hubble Space Telescope observations of active asteroid 313P/Gibbs
(formerly P/2014 S4) taken over the five month interval from 2014 October to
2015 March. This object has been recurrently active near perihelion (at 2.4 AU)
in two different orbits, a property that is naturally explained by the
sublimation of near surface ice but which is difficult to reconcile with other
activity mechanisms. We find that the mass loss peaks near 1 kg s in
October and then declines over the subsequent months by about a factor of five,
at nearly constant heliocentric distance. This decrease is too large to be
caused by the change in heliocentric distance during the period of observation.
However, it is consistent with sublimation from an ice patch shadowed by local
topography, for example in a pit like those observed on the nuclei of
short-period comet 67P/Churyumov-Gerasimenko. While no unique interpretation is
possible, a simple self shadowing model shows that sublimation from a pit with
depth to diameter ratio near 1/2 matches the observed rate of decline of the
activity, while deeper and shallower pits do not. We estimate the nucleus
radius to be 700100 m (geometric albedo 0.05 assumed). Measurements of the
spatial distribution of the dust were obtained from different viewing
geometries. They show that dust was ejected continuously not impulsively, that
the effective particle size is large, 50 , and that the ejection
speed is 2.5 m s. The total dust mass ejected is 10 kg,
corresponding to 10 of the nucleus mass. The observations are
consistent with partially shadowed sublimation from 10 m of ice,
corresponding to 0.2\% of the nucleus surface. For ice to survive in 313P
for billion-year timescales requires that the duty cycle for sublimation be
10.Comment: 34 pages, 11 figures, 4 tables; Astronomical Journal: in pres
The Nucleus of Active Asteroid 311P/(2013 P5) PANSTARRS
The unique inner-belt asteroid 311P/PANSTARRS (formerly P/2013 P5) is notable
for its sporadic, comet-like ejection of dust in nine distinct epochs spread
over 250 days in 2013. This curious behavior has been interpreted as the
product of localized, equator-ward landsliding from the surface of an asteroid
rotating at the brink of instability. We obtained new Hubble Space Telescope
observations to directly measure the nucleus and to search for evidence of its
rapid rotation. However, instead of providing photometric evidence for rapid
nucleus rotation, our data set a lower limit to the lightcurve period,
5.4 hour. The dominant feature of the lightcurve is a V-shaped minimum,
0.3 magnitudes deep, that is suggestive of an eclipsing binary. Under
this interpretation, the time-series data are consistent with a
secondary/primary mass ratio, 1:6, a ratio of separation/primary
radius, 4 and an orbit period 0.8 days. These properties lie
within the range of other asteroid binaries that are thought to be formed by
rotational breakup. While the lightcurve period is long, centripetal dust
ejection is still possible if one or both components rotates rapidly
( 2 hour) and has a small lightcurve variation because of azimuthal
symmetry. Indeed, radar observations of asteroids in critical rotation reveal
"muffin-shaped" morphologies which are closely azimuthally symmetric and which
show minimal lightcurves. Our data are consistent with 311P being a close
binary in which one or both components rotates near the centripetal limit. The
mass loss in 2013 suggests that breakup occurred recently and could even be
on-going. A search for fragments that might have been recently ejected beyond
the Hill sphere reveals none larger than effective radius 10 m.Comment: 37 pages, 9 figures, Astronomical Journal, in pres
Anatomy of an Asteroid Break-Up: The Case of P/2013 R3
We present an analysis of new and published data on P/2013 R3, the first
asteroid detected while disintegrating. Thirteen discrete components are
measured in the interval between UT 2013 October 01 and 2014 February 13. We
determine a mean, pair-wise velocity dispersion amongst these components of
m s and find that their separation times are
staggered over an interval of 5 months. Dust enveloping the system has,
in the first observations, a cross-section 30 km but fades
monotonically at a rate consistent with the action of radiation pressure
sweeping. The individual components exhibit comet-like morphologies and also
fade except where secondary fragmentation is accompanied by the release of
additional dust. We find only upper limits to the radii of any embedded solid
nuclei, typically 100 to 200 m (geometric albedo 0.05 assumed). Combined,
the components of P/2013 R3 would form a single spherical body with radius
400 m, which is our best estimate of the size of the precursor
object. The observations are consistent with rotational disruption of a weak
(cohesive strength 50 to 100 N m) parent body, 400 m in
radius. Estimated radiation (YORP) spin-up times of this parent are 1
Myr, shorter than the collisional lifetime. If present, water ice sublimating
at as little as 10 kg s could generate a torque on the parent
body rivaling the YORP torque. Under conservative assumptions about the
frequency of similar disruptions, the inferred asteroid debris production rate
is 10 kg s, which is at least 4% of the rate needed to
maintain the Zodiacal Cloud.Comment: 44 pages, 13 figures, accepted by Astronomical Journa
Quiescent Cores and the Efficiency of Turbulence-Accelerated, Magnetically Regulated Star Formation
The efficiency of star formation, defined as the ratio of the stellar to
total (gas and stellar) mass, is observed to vary from a few percent in regions
of dispersed star formation to about a third in cluster-forming cores. This
difference may reflect the relative importance of magnetic fields and
turbulence in controlling star formation. We investigate the interplay between
supersonic turbulence and magnetic fields using numerical simulations, in a
sheet-like geometry. We demonstrate that star formation with an efficiency of a
few percent can occur over several gravitational collapse times in moderately
magnetically subcritical clouds that are supersonically turbulent. The
turbulence accelerates star formation by reducing the time for dense core
formation. The dense cores produced are predominantly quiescent, with subsonic
internal motions. These cores tend to be moderately supercritical. They have
lifetimes long compared with their local gravitational collapse time. Some of
the cores collapse to form stars, while others disperse away without star
formation. In turbulent clouds that are marginally magnetically supercritical,
the star formation efficiency is higher, but can still be consistent with the
values inferred for nearby embedded clusters. If not regulated by magnetic
fields at all, star formation in a multi-Jeans mass cloud endowed with a strong
initial turbulence proceeds rapidly, with the majority of cloud mass converted
into stars in a gravitational collapse time. The efficiency is formally higher
than the values inferred for nearby cluster-forming cores, indicating that
magnetic fields are dynamically important even for cluster formation.Comment: submitted to Ap
Nonaxisymmetric Evolution of Magnetically Subcritical Clouds: Bar Growth, Core Elongation, and Binary Formation
We have begun a systematic numerical study of the nonlinear growth of
nonaxisymmetric perturbations during the ambipolar diffusion-driven evolution
of initially magnetically subcritical molecular clouds, with an eye on the
formation of binaries, multiple stellar systems and small clusters. In this
initial study, we focus on the (or bar) mode, which is shown to be
unstable during the dynamic collapse phase of cloud evolution after the central
region has become magnetically supercritical. We find that, despite the
presence of a strong magnetic field, the bar can grow fast enough that for a
modest initial perturbation (at 5% level) a large aspect ratio is obtained
during the isothermal phase of cloud collapse. The highly elongated bar is
expected to fragment into small pieces during the subsequent adiabatic phase.
Our calculations suggest that the strong magnetic fields observed in some
star-forming clouds and envisioned in the standard picture of single star
formation do not necessarily suppress bar growth and fragmentation; on the
contrary, they may actually promote these processes, by allowing the clouds to
have more than one (thermal) Jeans mass to begin with without collapsing
promptly. Nonlinear growth of the bar mode in a direction perpendicular to the
magnetic field, coupled with flattening along field lines, leads to the
formation of supercritical cores that are triaxial in general. It removes a
longstanding objection to the standard scenario of isolated star formation
involving subcritical magnetic field and ambipolar diffusion based on the
likely prolate shape inferred for dense cores. Continuted growth of the bar
mode in already elongated starless cores, such as L1544, may lead to future
binary and multiple star formation.Comment: 5 pages, 2 figures, accepted by ApJ
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