1,192 research outputs found
Gas dynamics in Massive Dense Cores in Cygnus-X
We study the kinematic properties of dense gas surrounding massive protostars
recognized by Bontemps et a. (2010) in a sample of five Massive Dense Cores in
Cygnus-X. We investigate whether turbulent support plays a major role in
stabilizing the core against fragmentation into Jeans-mass objects or
alternatively, the observed kinematics could indicate a high level of dynamics.
We present IRAM 30m single-dish (HCO+ and H13CO+) and IRAM PdBI high
angular-resolution observations of dense gas tracers (H13CO+ and H13CN) to
reveal the kinematics of molecular gas at scales from 0.03 to 0.1 pc. Radiative
transfer modeling shows that H13CO+ is depleted within the envelopes of massive
protostars and traces the bulk of material surrounding the protostars rather
than their inner envelopes. H13CN shows a better correspondence with the peak
of the continuum emission, possibly due to abundance anomalies and specific
chemistry in the close vicinity of massive protostars. Analyzing the
line-widths we show that the observed line-dispersion of H13CO+ at the scale of
MDCs is smaller than expected from the quasi-static, turbulent-core model. At
large-scales, global organized bulk motions are identified for 3 of the MDCs.
At small-scales, several spectral components are identified in all MDCs showing
filamentary structures and intrinsic velocity gradients towards the continuum
peaks. The dynamics of these flows show diversity among the sample and we link
this to the specific fragmentation properties of the MDCs. No clear evidence is
found for a turbulence regulated, equilibrium scenario within the sample of
MDCs. We propose a picture in which MDCs are not in equilibrium and their
dynamics is governed by small-scale converging flows, which may initiate
star-formation via their shears
Nonparametric vs parametric binary choice models: An empirical investigation
Research Methods/ Statistical Methods,
Millimeter- and Submillimeter-Wave Observations of the OMC-2/3 Region; I. Dispersing and Rotating Core around an Intermediate-mass Protostar MMS 7
We report the results of H13CO+(1-0), CO(1-0), and 3.3 mm dust continuum
observations toward one of the strongest mm-wave sources in OMC-3, MMS 7, with
the Nobeyama Millimeter Array (NMA) and the Nobeyama 45 m telescope. With the
NMA, we detected centrally-condensed 3.3 mm dust-continuum emission which
coincides with the MIR source and the free-free jet. Our combined H13CO+
observations have revealed a disk-like envelope. The size and the mass of the
disk-like envelope are 0.15 times 0.11 pc and 5.1 - 9.1 M_sun, respectively.
The combined map also shows that the outer portion of the disk-like envelope
has a fan-shaped structure which delineates the rim of the CO(1-0) outflow
observed with the NMA. The position-velocity (P-V) diagrams in the H13CO+ (1-0)
emission show that the velocity field in the disk-like envelope is composed of
a dispersing gas motion and a possible rigid-like rotation. The mass dispersing
rate is estimated to be (3.4 - 6.0) times 10^-5 M_sun/yr, which implies that
MMS 7 has an ability to disperse ~10 M_sun during the protostellar evolutional
time of a few times 10^5 yr. The specific angular momentum of the possible
rotation in the disk-like envelope is nearly two orders of magnitude larger
than that in low-mass cores. The turn-over point of the power law of the
angular momentum distribution in the disk-like envelope (< 0.007 pc), which is
likely to be related to the outer radius of the central mass accretion, is
similar to the size of the 3.3 mm dust condensation. The intermediate-mass
protostar MMS 7 is in the last stage of the main accretion phase and that the
substantial portion of the outer gas has already been dispersed, while the mass
accretion may still be on-going at the innermost region traced by the dusty
condensation.Comment: 19 pages, 9 figures, ApJ accepted pape
Massive Infrared-Quiet Dense Cores: Unveiling the Initial Conditions of High-Mass Star Formation
As Pr. Th. Henning said at the conference, cold precursors of high-mass stars
are now "hot topics". We here propose some observational criteria to identify
massive infrared-quiet dense cores which can host the high-mass analogs of
Class 0 protostars and pre-stellar condensations. We also show how far-infrared
to millimeter imaging surveys of entire complexes forming OB stars are starting
to unveil the initial conditions of high-mass star formation
First detection of CF+ towards a high-mass protostar
We report the first detection of the J = 1 - 0 (102.6 GHz) rotational lines
of CF+ (fluoromethylidynium ion) towards CygX-N63, a young and massive
protostar of the Cygnus X region. This detection occurred as part of an
unbiased spectral survey of this object in the 0.8-3 mm range, performed with
the IRAM 30m telescope. The data were analyzed using a local thermodynamical
equilibrium model (LTE model) and a population diagram in order to derive the
column density. The line velocity (-4 km s-1) and line width (1.6 km s-1)
indicate an origin from the collapsing envelope of the protostar.
We obtain a CF+ column density of 4.10e11 cm-2. The CF+ ion is thought to be
a good tracer for C+ and assuming a ratio of 10e-6 for CF+/C+, we derive a
total number of C+ of 1.2x10e53 within the beam. There is no evidence of carbon
ionization caused by an exterior source of UV photons suggesting that the
protostar itself is the source of ionization. Ionization from the protostellar
photosphere is not efficient enough. In contrast, X-ray ionization from the
accretion shock(s) and UV ionization from outflow shocks could provide a large
enough ionizing power to explain our CF+ detection.
Surprisingly, CF+ has been detected towards a cold, massive protostar with no
sign of an external photon dissociation region (PDR), which means that the only
possibility is the existence of a significant inner source of C+. This is an
important result that opens interesting perspectives to study the early
development of ionized regions and to approach the issue of the evolution of
the inner regions of collapsing envelopes of massive protostars. The existence
of high energy radiations early in the evolution of massive protostars also has
important implications for chemical evolution of dense collapsing gas and could
trigger peculiar chemistry and early formation of a hot core.Comment: 6 page
First results from the CALYPSO IRAM-PdBI survey. I. Kinematics of the inner envelope of NGC1333-IRAS2A
The structure and kinematics of Class 0 protostars on scales of a few hundred
AU is poorly known. Recent observations have revealed the presence of Keplerian
disks with a diameter of 150-180 AU in L1527-IRS and VLA1623A, but it is not
clear if such disks are common in Class 0 protostars. Here we present
high-angular-resolution observations of two methanol lines in NGC1333-IRAS2A.
We argue that these lines probe the inner envelope, and we use them to study
the kinematics of this region. Our observations suggest the presence of a
marginal velocity gradient normal to the direction of the outflow. However, the
position velocity diagrams along the gradient direction appear inconsistent
with a Keplerian disk. Instead, we suggest that the emission originates from
the infalling and perhaps slowly rotating envelope, around a central protostar
of 0.1-0.2 M. If a disk is present, it is smaller than the disk of
L1527-IRS, perhaps suggesting that NGC1333-IRAS2A is younger.Comment: Accepted for publication in A&A letter
SCUBA observations of the Horsehead Nebula - what did the horse swallow?
We present observations taken with SCUBA on the JCMT of the Horsehead Nebula
in Orion (B33), at wavelengths of 450 and 850 \mum. We see bright emission from
that part of the cloud associated with the photon-dominated region (PDR) at the
`top' of the horse's head, which we label B33-SMM1. We characterise the
physical parameters of the extended dust responsible for this emission, and
find that B33-SMM1 contains a more dense core than was previously suspected. We
compare the SCUBA data with data from the Infrared Space Observatory (ISO) and
find that the emission at 6.75-\mum is offset towards the west, indicating that
the mid-infrared emission is tracing the PDR while the submillimetre emission
comes from the molecular cloud core behind the PDR. We calculate the virial
balance of this core and find that it is not gravitationally bound but is being
confined by the external pressure from the HII region IC434, and that it will
either be destroyed by the ionising radiation, or else may undergo triggered
star formation. Furthermore we find evidence for a lozenge-shaped clump in the
`throat' of the horse, which is not seen in emission at shorter wavelengths. We
label this source B33-SMM2 and find that it is brighter at submillimetre
wavelengths than B33-SMM1. SMM2 is seen in absorption in the 6.75-\mum ISO
data, from which we obtain an independent estimate of the column density in
excellent agreement with that calculated from the submillimetre emission. We
calculate the stability of this core against collapse and find that it is in
approximate gravitational virial equilibrium. This is consistent with it being
a pre-existing core in B33, possibly pre-stellar in nature, but that it may
also eventually undergo collapse under the effects of the HII region.Comment: 11 pages, 6 figures, accepted by MNRA
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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