1,115 research outputs found
Cluster Formation in Protostellar Outflow-Driven Turbulence
Most, perhaps all, stars go through a phase of vigorous outflow during
formation. We examine, through 3D MHD simulation, the effects of protostellar
outflows on cluster formation. We find that the initial turbulence in the
cluster-forming region is quickly replaced by motions generated by outflows.
The protostellar outflow-driven turbulence (``protostellar turbulence'' for
short) can keep the region close to a virial equilibrium long after the initial
turbulence has decayed away. We argue that there exist two types of turbulence
in star-forming clouds: a primordial (or ``interstellar'') turbulence and a
protostellar turbulence, with the former transformed into the latter mostly in
embedded clusters such as NGC 1333. Since the majority of stars are thought to
form in clusters, an implication is that the stellar initial mass function is
determined to a large extent by the stars themselves, through outflows which
individually limit the mass accretion onto forming stars and collectively shape
the environments (density structure and velocity field) in which most cluster
members form. We speculate that massive cluster-forming clumps supported by
protostellar turbulence gradually evolve towards a highly centrally condensed
``pivotal'' state, culminating in rapid formation of massive stars in the
densest part through accretion.Comment: 11 pages (aastex format), 2 figures submitted to ApJ
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An S-shaped outflow from IRAS 03256+3055 in NGC 1333
The IRAS source 03256+3055 in the NGC 1333 star forming region is associated
with extended sub-millimeter emission of complex morphology, showing multiple
clumps. One of these is found to coincide with the driving source of a bipolar
jet of S-shaped morphology seen in the emission lines of H_alpha and [SII] as
well as in the H2 emission lines in the K-band. Detailed images of the driving
source at the wavelengths of H_alpha and [SII] and in the I, J, H, and K bands
as well as a K-band spectrum and polarimetry are discussed. The near-infrared
morphology is characterized by a combination of line emission from the jet and
scattered light from a source with a steep continuum spectrum. The morphology
and proper motion of the jet are discussed in the context of a binary system
with a precessing disk. We conclude that the molecular core associated with
IRAS 03256+3055 consists of several clumps, only one of which shows evidence of
recent star formation at optical and near-infrared wavelengths.We also briefly
discuss a second, newly found near-infrared source associated with a compact
sub-millimeter continuum source near IRAS 03256+3055, and conclude that this
source may be physically unrelated the cluster of molecular clumps.Comment: 25 pages, including 5 figures. Accepted for publication in The
Astronomical Journa
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Large Area Mapping at 850 Microns. V. Analysis of the Clump Distribution in the Orion A South Molecular Cloud
We present results from a 2300 arcmin^2 survey of the Orion A molecular cloud
at 450 and 850 micron using the Submillimetre Common-User Bolometer Array
(SCUBA) on the James Clerk Maxwell Telescope. The region mapped lies directly
south of the OMC1 cloud core and includes OMC4, OMC5, HH1/2, HH34, and L1641N.
We identify 71 independent clumps in the 850 micron map and compute size, flux,
and degree of central concentration in each. Comparison with isothermal,
pressure-confined, self-gravitating Bonnor-Ebert spheres implies that the
clumps have internal temperatures T_d ~ 22 +/- K and surface pressures log
(k^-1 P cm^-3 K) = 6.0 +/- 0.2. The clump masses span the range 0.3 - 22 Msun
assuming a dust temperature T_d ~ 20 K and a dust emissivity kappa_850 = 0.02
cm^2 g^-1. The distribution of clump masses is well characterized by a
power-law N(M) propto M^-alpha with alpha = 2.0 +/- 0.5 for M > 3.0 Msun,
indicating a clump mass function steeper than the stellar Initial Mass
Function. Significant incompleteness makes determination of the slope at lower
masses difficult. A comparison of the submillimeter emission map with an H_2
2.122 micron survey of the same region is performed. Several new Class 0
sources are revealed and a correlation is found between both the column density
and degree of concentration of the submillimeter sources and the likelihood of
coincident H_2 shock emission.Comment: 44 pages, 17 figures, accepted by Ap
Flaring Up All Over -- Radio Activity in Rapidly-Rotating Late-Type M and L Dwarfs
We present Very Large Array observations of twelve late M and L dwarfs in the
Solar neighborhood. The observed sources were chosen to cover a wide range of
physical characteristics - spectral type, rotation, age, binarity, and X-ray
and H\alpha activity - to determine the role of these properties in the
production of radio emission, and hence magnetic fields. Three of the twelve
sources, TVLM513-46546, 2MASS J0036159+182110, and BRI0021-0214, were observed
to flare and also exhibit persistent emission, indicating that magnetic
activity is not quenched at the bottom of the main sequence. The radio emission
extends to spectral type L3.5, and there is no apparent decrease in the ratio
of flaring luminosities to bolometric luminosities between M8-L3.5. Moreover,
contrary to the significant drop in persistent H\alpha activity beyond spectral
type M7, the persistent radio activity appears to steadily increase between
M3-L3.5. Similarly, the radio emission from BRI0021-0214 violates the
phenomenological relations between the radio and X-ray luminosities of
coronally active stars, hinting that radio and X-ray activity are also
uncorrelated at the bottom of the main sequence. The radio active sources that
have measured rotational velocities are rapid rotators, Vsin(i)>30 km/sec,
while the upper limits on radio activity in slowly-rotating late M dwarfs
(Vsin(i)<10 km/sec) are lower than these detections. These observations provide
tantalizing evidence that rapidly-rotating late M and L dwarfs are more likely
to be radio active. This possible correlation is puzzling given that the
observed radio emission requires sustained magnetic fields of 10-1000 G and
densities of 10^12 cm^-3, indicating that the active sources should have slowed
down considerably due to magnetic braking.Comment: Accepted to ApJ; Two new figures; Minor text revision
A statistical study of the mass and density structure of Infrared Dark Clouds
How and when the mass distribution of stars in the Galaxy is set is one of
the main issues of modern astronomy. Here we present a statistical study of
mass and density distributions of infrared dark clouds (IRDCs) and fragments
within them. These regions are pristine molecular gas structures and
progenitors of stars and so provide insights into the initial conditions of
star formation. This study makes use of a IRDC catalogue (Peretto & Fuller
2009), the largest sample of IRDC column density maps to date, containing a
total of ~11,000 IRDCs with column densities exceeding N_{H2} = 1 X10^{22}
cm^{-2} and over 50,000 single peaked IRDC fragments. The large number of
objects constitutes an important strength of this study, allowing detailed
analysis of the completeness of the sample and so statistically robust
conclusions. Using a statistical approach to assigning distances to clouds, the
mass and density distributions of the clouds and the fragments within them are
constructed. The mass distributions show a steepening of the slope when
switching from IRDCs to fragments, in agreement with previous results of
similar structures. IRDCs and fragments are divided into unbound/bound objects
by assuming Larson's relation and calculating their virial parameter. IRDCs are
mostly gravitationally bound, while a significant fraction of the fragments are
not. The density distribution of gravitationally unbound fragments shows a
steep characteristic slope. (see paper for full Abstract).Comment: 15 pages, accepted for publication in Ap
The Environment and Nature of the Class I Protostar Elias 29: Molecular Gas Observations and the Location of Ices
A (sub-)millimeter line and continuum study of the Class I protostar Elias 29 in the Ï Ophiuchi molecular cloud is presented whose goals are to understand the nature of this source and to locate the ices that are abundantly present along this line of sight. Within 15"-60" beams, several different components contribute to the line emission. Two different foreground clouds are detected, an envelope/disk system and a dense ridge of HCO^+-rich material. The latter two components are spatially separated in millimeter interferometer maps. We analyze the envelope/disk system by using inside-out collapse and flared disk models. The disk is in a relatively face-on orientation (<60°), which explains many of the remarkable observational features of Elias 29, such as its flat spectral energy distribution, its brightness in the near-infrared, the extended components found in speckle interferometry observations, and its high-velocity molecular outflow. It cannot account for the ices seen along the line of sight, however. A small fraction of the ices is present in a (remnant) envelope of mass 0.12-0.33 M_â, but most of the ices (~70%) are present in cool (T < 40 K) quiescent foreground clouds. This explains the observed absence of thermally processed ices (crystallized H_2O) toward Elias 29. Nevertheless, the temperatures could be sufficiently high to account for the low abundance of apolar (CO, N_2, O_2) ices. This work shows that it is crucial to obtain spectrally and spatially resolved information from single-dish and interferometric molecular gas observations in order to determine the nature of protostars and to interpret Infrared Space Observatory and future Space Infrared Telescope Facility observations of ices and silicates along a pencil beam
Multiple protostellar systems. I. A deep near infrared survey of Taurus and Ophiuchus protostellar objects
(Abridged) We performed a deep infrared imaging survey of 63 embedded young
stellar objects (YSOs) located in the Taurus and Ophiuchus clouds to search for
companions. The sample includes Class I and flat infrared spectrum protostellar
objects. We find 17 companions physically bound to 15 YSOs with angular
separations in the range 0.8-10" (110-1400 AU) and derive a companion star
fraction of 23+/-9 % and 29+/-7 % for embedded YSOs in Taurus and Ophiuchus,
respectively. In spite of different properties of the clouds and especially of
the prestellar cores, the fraction of wide companions, 27+/-6 % for the
combined sample, is identical in the two star-forming regions. This suggests
that the frequency and properties of wide multiple protostellar systems are not
very sensitive to specific initial conditions. Comparing the companion star
fraction of the youngest YSOs still surrounded by extended envelopes to that of
more evolved YSOs, we find evidence for a possible evolution of the fraction of
wide multiple systems, which seems to decrease by a factor of about 2 on a
timescale of about 10^5 yr. Somewhat contrary to model predictions, we do not
find evidence for a sub-clustering of embedded sources at this stage on a scale
of a few 100 AU that could be related to the formation of small-N protostellar
clusters. Possible interpretations for this discrepancy are discussed.Comment: Accepted for publication in Astronomy & Astrophysics; 16 pages, 3
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