17,630 research outputs found
Observation of rotation in star forming regions: clouds, cores, disks, and jets
Angular momentum plays a crucial role in the formation of stars and planets.
It has long been noticed that parcels of gas in molecular clouds need to reduce
their specific angular momentum by 6 to 7 orders of magnitude to participate in
the building of a typical star like the Sun. Several physical processes on
different scales and at different stages of evolution can contribute to this
loss of angular momentum. In order to set constraints on these processes and
better understand this transfer of angular momentum, a detailed observational
census and characterization of rotation at all stages of evolution and over all
scales of star forming regions is necessary. This review presents the main
results obtained in low-mass star forming regions over the past four decades in
this field of research. It addresses the search and characterization of
rotation in molecular clouds, prestellar and protostellar cores, circumstellar
disks, and jets. Perspectives offered by ALMA are briefly discussed.Comment: 43 pages, 8 figures. To appear in the Proceedings of the Evry
Schatzman School 2012 of PNPS and CNRS/INSU on the "Role and mechanisms of
angular momentum transport during the formation and early evolution of
stars", Eds. P.Hennebelle and C.Charbonne
On the uniform generation of modular diagrams
In this paper we present an algorithm that generates -noncrossing,
-modular diagrams with uniform probability. A diagram is a labeled
graph of degree over vertices drawn in a horizontal line with arcs
in the upper half-plane. A -crossing in a diagram is a set of
distinct arcs with the property . A diagram without any
-crossings is called a -noncrossing diagram and a stack of length
is a maximal sequence
. A diagram is
-modular if any arc is contained in a stack of length at least
. Our algorithm generates after preprocessing time,
-noncrossing, -modular diagrams in time and space
complexity.Comment: 21 pages, 7 figure
The shape of a random affine Weyl group element and random core partitions
Let be a finite Weyl group and be the corresponding affine
Weyl group. We show that a large element in , randomly generated by
(reduced) multiplication by simple generators, almost surely has one of
-specific shapes. Equivalently, a reduced random walk in the regions of
the affine Coxeter arrangement asymptotically approaches one of -many
directions. The coordinates of this direction, together with the probabilities
of each direction can be calculated via a Markov chain on . Our results,
applied to type , show that a large random -core obtained
from the natural growth process has a limiting shape which is a
piecewise-linear graph. In this case, our random process is a periodic analogue
of TASEP, and our limiting shapes can be compared with Rost's theorem on the
limiting shape of TASEP.Comment: Published at http://dx.doi.org/10.1214/14-AOP915 in the Annals of
Probability (http://www.imstat.org/aop/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Star formation in Perseus. IV. Mass dependent evolution of dense cores
In our SCUBA survey of Perseus, we find that the fraction of protostellar
cores increases towards higher masses and the most massive cores are all
protostellar. In this paper we consider the possible explanations of this
apparent mass dependence in the evolutionary status of these cores, and the
implications for protostellar evolution and the mapping of the embedded core
mass function (CMF) onto the stellar IMF. We consider the following potential
causes: dust temperature; selection effects in the submillimetre and in the
mid-infrared observations used for pre/protostellar classification; confusion
and multiplicity; transient cores; and varying evolutionary timescales. We
develop Core Mass Evolution Diagrams (CMEDs) to investigate how the mass
evolution of individual cores maps onto the observed CMF. Two physical
mechanisms -- short timescales for the evolution of massive cores, and
continuing accumulation of mass onto protostellar cores -- best explain the
relative excess of protostars in high mass cores and the rarity of massive
starless cores. In addition, confusion both increases the likelihood that a
protostar is identified within a core, and increases mass assigned to a core.
The observed pre/protostellar mass distributions are consistent with faster
evolution and a shorter lifetime for higher-mass prestellar cores. We rule out
longer timescales for higher-mass prestellar cores. The differences in the
prestellar and protostellar mass distributions imply that the prestellar CMF
(and possibly the combined pre+protostellar CMF) should be steeper than the
IMF. A steeper prestellar CMF can be reconciled with the observed similarity of
the CMF and the IMF in some regions if a second opposing effect is present,
such as the fragmentation of massive cores into multiple systems.Comment: 11 pages, 5 figures. Accepted by A&
A Combined Spitzer and Chandra Survey of Young Stellar Objects in the Serpens Cloud Core
We present Spitzer and Chandra observations of the nearby (~260 pc) embedded
stellar cluster in the Serpens Cloud Core. We observed, using Spitzer's IRAC
and MIPS instruments, in six wavelength bands from 3 to 70 , to detect
thermal emission from circumstellar disks and protostellar envelopes, and to
classify stars using color-color diagrams and spectral energy distributions
(SEDs). These data are combined with Chandra observations to examine the
effects of circumstellar disks on stellar X-ray properties. Young diskless
stars were also identified from their increased X-ray emission. We have
identified 138 YSOs in Serpens: 22 class 0/I, 16 flat spectrum, 62 class II, 17
transition disk, and 21 class III stars; 60 of which exhibit X-ray emission.
Our primary results are the following: 1.) ten protostars detected previously
in the sub-millimeter are detected at lambda < 24 microns, seven at lambda < 8
microns, 2.) the protostars are more closely grouped than more evolved YSOs
(median separation : ~0.024 pc, and 3.) the luminosity and temperature of the
X-ray emitting plasma around these YSOs does not show any significant
dependence on evolutionary class. We combine the infrared derived values of AK
and X-ray values of NH for 8 class III objects and find that the column density
of hydrogen gas per mag of extinctions is less than half the standard
interstellar value, for AK > 1. This may be the result of grain growth through
coagulation and/or the accretion of volatiles in the Serpens cloud core.Comment: 69 pages, 16 figures, accepted to ApJ. Higher Resolution Figures at:
http://www.cfa.harvard.edu/~ewinston
Young Stellar Population of the Bright-Rimmed Clouds BRC 5, BRC 7 and BRC 39
Bright-rimmed clouds (BRCs), illuminated and shaped by nearby OB stars, are
potential sites of recent/ongoing star formation. Here we present an optical
and infrared photometric study of three BRCs: BRC 5, BRC 7 and BRC 39 to obtain
a census of the young stellar population, thereby inferring the star formation
scenario, in these regions. In each BRC, the Class I sources are found to be
located mostly near the bright rim or inside the cloud, whereas the Class II
sources are preferentially outside, with younger sources closer to the rim.
This provides strong support to sequential star formation triggered by
radiation driven implosion due to the UV radiation. Moreover, each BRC contains
a small group of young stars being revealed at its head, as the next-generation
stars. In particular, the young stars at the heads of BRC 5 and BRC 7 are found
to be intermediate/high mass stars, which, under proper conditions, may
themselves trigger further star birth, thereby propagating star formation out
to long distances.Comment: 30 pages, 7 Figures, 6 Tables, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
- …