2,049 research outputs found
Proper Motion of H2O Masers in IRAS 20050+2720 MMS1: An AU Scale Jet Associated with An Intermediate-Mass Class 0 Source
We conducted a 4 epoch 3 month VLBA proper motion study of HO masers
toward an intermediate-mass class 0 source IRAS 20050+2720 MMS1 (d=700 pc).
From milli-arcsecond (mas) resolution VLBA images, we found two groups of H2O
maser spots at the center of the submillimeter core of MMS1. One group consists
of more than intense maser spots; the other group consisting of
several weaker maser spots is located at 18 AU south-west of the intense group.
Distribution of the maser spots in the intense group shows an arc-shaped
structure which includes the maser spots that showed a clear velocity gradient.
The spatial and velocity structures of the maser spots in the arc-shape did not
significantly change through the 4 epochs. Furthermore, we found a relative
proper motion between the two groups. Their projected separation increased by
1.13+/-0.11 mas over the 4 epochs along a line connecting them. The spatial and
velocity structures of the intense group and the relative proper motions
strongly suggest that the maser emission is associated with a protostellar jet.
Comparing the observed LSR velocities with calculated radial velocities from a
simple biconical jet model, we conclude that the most of the maser emission are
likely to be associated with an accelerating biconical jet which has large
opening angle. The large opening angle of the jet traced by the masers would
support the hypothesis that poor jet collimation is an inherent property of
luminous (proto)stars.Comment: 14 pages, 10 figures, Fig.3 was downsized significantly. accepted for
publication in A&
First results from a VLBA proper motion survey of H2O masers in low-mass YSOs: the Serpens core and RNO15-FIR
This article reports first results of a long-term observational program aimed
to study the earliest evolution of jet/disk systems in low-mass YSOs by means
of VLBI observations of the 22.2 GHz water masers. We report here data for the
cluster of low-mass YSOs in the Serpens molecular core and for the single
object RNO~15-FIR. Towards Serpens SMM1, the most luminous sub-mm source of the
Serpens cluster, the water maser emission comes from two small (< 5 AU in size)
clusters of features separated by ~25 AU, having line of sight velocities
strongly red-shifted (by more than 10 km/s) with respect to the LSR velocity of
the molecular cloud. The two maser clusters are oriented on the sky along a
direction that is approximately perpendicular to the axis of the radio
continuum jet observed with the VLA towards SMM1. The spatial and velocity
distribution of the maser features lead us to favor the interpretation that the
maser emission is excited by interaction of the receding lobe of the jet with
dense gas in the accretion disk surrounding the YSO in SMM1. Towards
RNO~15-FIR, the few detected maser features have both positions and (absolute)
velocities aligned along a direction that is parallel to the axis of the
molecular outflow observed on much larger angular scales. In this case the
maser emission likely emerges from dense, shocked molecular clumps displaced
along the axis of the jet emerging from the YSO. The protostar in Serpens SMM1
is more massive than the one in RNO~15-FIR. We discuss the case where a high
mass ejection rate can generate jets sufficiently powerful to sweep away from
their course the densest portions of circumstellar gas. In this case, the
excitation conditions for water masers might preferably occur at the interface
between the jet and the accretion disk, rather than along the jet axis.Comment: 18 pages (postscript format); 9 figures; to be published into
Astronomy & Astrophysics, Main Journa
The extremely collimated bipolar H_2O jet from the NGC 1333-IRAS 4B protostar
We have performed observations of water maser emission towards a sample of
low-mass protostars, in order to investigate the properties of jets associated
with the earliest stages of star formation and their interaction with the
surrounding medium. The main aim is to measure the absolute positions and
proper motions of the H_2O spots in order to investigate the kinematics of the
region from where the jet is launched. We imaged the protostars in the nearby
region NGC 1333-IRAS 4 in the water maser line at 22.2 GHz by using the VLBA in
phase-reference mode at the milliarcsecond scale over four epochs, spaced by
one month to measure proper motions. Two protostars (A2 and B) were detected in
a highly variable H_2O maser emission, with an active phase shorter than four
weeks. The H_2O maps allow us to trace the fast jet driven by the B protostar:
we observed both the red- and blue-shifted lobes very close to the protostar,
=< 35 AU, moving away with projected velocities of ~10-50 km/s. The comparison
with the molecular outflow observed at larger scale suggests a jet precession
with a 18'/yr rate. By measuring the positional spread of the H_2O spots we
estimate a jet width of ~2 AU at a distance of ~12 AU from the driving
protostar.Comment: 9 pages, 8 figures, A&A accepte
Entanglement in the dispersive interaction of trapped ions with a quantized field
The mode-mode entanglement between trapped ions and cavity fields is
investigated in the dispersive regime. We show how a simple initial preparation
of Gaussian coherent states and a postselection may be used to generate
motional non-local mesoscopic states (NLMS) involving ions in different traps.
We also present a study of the entanglement induced by dynamical Stark-shifts
considering a cluster of N-trapped ions. In this case, all entanglement is due
to the dependence of the Stark-shifts on the ions' state of motion manifested
as a cross-Kerr interaction between each ion and the field.Comment: 10 pages, 5 figures, corrected typo
Relative Evolutionary Time Scale of Hot Molecular Cores with Respect to Ultra Compact HII Regions
Using the Owens Valley and Nobeyama Radio Observatory interferometers, we
carried out an unbiased search for hot molecular cores and ultracompact UC HII
regions toward the high-mass star forming region G19.61--0.23. In addition, we
performed 1.2 mm imaging with SIMBA, and retrieved 3.5 and 2 cm images from the
VLA archive data base. The newly obtained 3 mm image brings information on a
cluster of high-mass (proto)stars located in the innermost and densest part of
the parsec scale clump detected in the 1.2 mm continuum. We identify a total of
10 high-mass young stellar objects: one hot core (HC) and 9 UC HII regions,
whose physical parameters are obtained from model fits to their continuum
spectra. The ratio between the current and expected final radii of the UC \HII
regions ranges from 0.3 to 0.9, which leaves the possibility that all O-B stars
formed simultaneously. Under the opposite assumption -- namely that star
formation occurred randomly -- we estimate that HC lifetime is less than
1/3 of that of UCHII regions on the basis of the source number ratio
between them.Comment: 13 pages, 2 figs, including a color fi
The impact of the Herbig Haro object HH2 on local dust and gas
We present results from a study of molecular gas and dust in the vicinity of
the Herbig Haro object HH2. Emission from the sub-mm continuum, 12CO and HCO+
was mapped with angular resolutions ranging from 14 arcsec to 5 arcsec (or
0.01pc at the distance of HH2). The continuum shows an extended dust clump of
mass 3.8Msun and temperature 22K, located downstream of the bright optical HH
knots. However, a compact emission peak lies within ~0.01pc of the
low-excitation H2-prominent shocks, with a luminosity consistent with local
heating by the outflow.
The HCO+ emission shows two velocity components: firstly, ambient-velocity
gas lying in a region roughly corresponding to the dust clump, with abundance
enhanced by a factor of a few close to the H2-prominent knots. Secondly a
component of high-velocity emission (20 km/s linewidth), found mainly in a
collimated jet linking the low-excitation HH objects. In this high-velocity
jet, the line wings show an abundance ratio HCO+/CO proportional to v^2, with
an HCO+ enhancement compared with ambient gas of up to 10^3 at the most extreme
velocities. Such high abundances are consistent with models of shock chemistry
in turbulent mixing layers at the interaction boundaries of jets. Extrapolating
this effect to low velocities suggests that the more modest HCO+ enhancement in
the clump gas could be caused by low velocity shocks. A UV precursor may not,
therefore be necessary to explain the elevated HCO+ abundance in this gas.Comment: 8 pages. 9 figures. To be published in MNRA
Dressed-State Approach to Population Trapping in the Jaynes-Cummings Model
The phenomenon of atomic population trapping in the Jaynes-Cummings Model is
analysed from a dressed-state point of view. A general condition for the
occurrence of partial or total trapping from an arbitrary, pure initial
atom-field state is obtained in the form of a bound to the variation of the
atomic inversion. More generally, it is found that in the presence of initial
atomic or atom-field coherence the population dynamics is governed not by the
field's initial photon distribution, but by a `weighted dressedness'
distribution characterising the joint atom-field state. In particular,
individual revivals in the inversion can be analytically described to good
approximation in terms of that distribution, even in the limit of large
population trapping. This result is obtained through a generalisation of the
Poisson Summation Formula method for analytical description of revivals
developed by Fleischhauer and Schleich [Phys. Rev. A {\bf 47}, 4258 (1993)].Comment: 24 pages, 5 figures, to appear in J. Mod. Op
The L723 low mass star forming protostellar system: resolving a double core
We present 1.35 mm SMA observations around the low-mass Class 0 source IRAS
19156+1906, at the the center of the L723 dark cloud. We detected emission from
dust as well as emission from H2CO, DCN and CN, which arise from two cores, SMA
1 and SMA 2, separated by 2.9" (880 AU). SMA 2 is associated with VLA 2. SiO
5-4 emission is detected, possibly tracing a region of interaction between the
dense envelope and the outflow. We modeled the dust and the H2CO emission from
the two cores: they have similar physical properties but SMA 2 has a larger
p-H2CO abundance than SMA 1. The p-H2CO abundances found are compatible with
the value of the outer part of the circumstellar envelopes associated with
Class 0 sources. SMA 2 is likely more evolved than SMA 1. The kinematics of the
two sources show marginal evidence of infall and rotation motions. The mass
detected by the SMA observation, which trace scales of ~1000 AU, is only a
small fraction of the mass contained in the large scale molecular envelope,
which suggests that L723 is still in a very early phase of star formation.
Despite the apparent quiescent nature of the L723, fragmentation is occurring
at the center of the cloud at different scales. Thus, at 1000 AU the cloud has
fragmented in two cores, SMA 1 and SMA 2. At the same time, at least one of
these cores, SMA 2, has undergone additional fragmentation at scales of 150 AU,
forming a multiple stellar system.Comment: 35 pages, 15 figures. Accepted to the Astrophysical Journa
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