388 research outputs found
NG7538 IRS1 N: modeling a circumstellar maser disk
We present an edge-on Keplerian disk model to explain the main component of
the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1 N.
The brightness distribution and spectrum of the line of bright masers are
successfully modeled with high amplification of background radio continuum
emission along velocity coherent paths through a maser disk. The bend seen in
the position-velocity diagram is a characteristic signature of differentially
rotating disks. For a central mass of 30 solar masses, suggested by other
observations, our model fixes the masing disk to have inner and outer radii of
about 270 AU and 750 AU.Comment: To appear in The Proceedings of the 2004 European Workshop: "Dense
Molecular Gas around Protostars and in Galatic Nuclei", Eds. Y. Hagiwara,
W.A. Baan, H.J. van Langevelde, 2004, a special issue of ApSS, Kluwe
Physical characterisation of southern massive star-forming regions using Parkes NH observations
We have undertaken a Parkes ammonia spectral line study, in the lowest two
inversion transitions, of southern massive star formation regions, including
young massive candidate protostars, with the aim of characterising the earliest
stages of massive star formation. 138 sources from the submillimetre continuum
emission studies of Hill et al., were found to have robust (1,1) detections,
including two sources with two velocity components, and 102 in the (2,2)
transition.
We determine the ammonia line properties of the sources: linewidth, flux
density, kinetic temperature, NH column density and opacity, and revisit
our SED modelling procedure to derive the mass for 52 of the sources. By
combining the continuum emission information with ammonia observations we
substantially constrain the physical properties of the high-mass clumps. There
is clear complementarity between ammonia and continuum observations for
derivations of physical parameters.
The MM-only class, identified in the continuum studies of Hill et al.,
display smaller sizes, mass and velocity dispersion and/or turbulence than
star-forming clumps, suggesting a quiescent prestellar stage and/or the
formation of less massive stars.Comment: 20 pages, 9 Figures, 1 appendix (to appear in full online only, a
sample appendix in the paper); 7 tables. Accepted by MNRA
Methanol masers probing the ordered magnetic field of W75N
The role of magnetic fields during the protostellar phase of high-mass
star-formation is a debated topic. In particular, it is still unclear how
magnetic fields influence the formation and dynamic of disks and outflows. Most
current information on magnetic fields close to high-mass protostars comes from
H2O and OH maser observations. Recently, the first 6.7 GHz methanol maser
polarization observations were made, and they reveal strong and ordered
magnetic fields. The morphology of the magnetic field during high-mass
star-formation needs to be investigated on small scales, which can only be done
using very long baseline interferometry observations. The massive star-forming
regionW75N contains three radio sources and associated masers, while a
large-scale molecular bipolar outflow is also present. Polarization
observations of the 6.7 GHz methanol masers at high angular resolution probe
the strength and structure of the magnetic field and determine its relation to
the outflow. Eight of the European VLBI network antennas were used to measure
the linear polarization and Zeeman-splitting of the 6.7 GHz methanol masers in
the star-forming region W75N. We detected 10 methanol maser features, 4 of
which were undetected in previous work. All arise near the source VLA1 of W75N.
The linear polarization of the masers reveals a tightly ordered magnetic field
over more than 2000 AU around VLA1 that is exactly aligned with the large-scale
molecular outflow. This is consistent with the twisted magnetic field model
proposed for explaining dust polarization observations. The Zeeman-splitting
measured on 3 of the maser features indicates a dynamically important magnetic
field in the maser region of the order of 50mG. We suggest VLA1 is the powering
sources of the bipolar outflow.Comment: 5 pages, 3 figures, accepted by Astronomy and Astrophysic
The properties and polarization of the H2O and CH3OH maser environment of NGC7538-IRS1
NGC7538 is a complex massive star-forming region. The region is composed of
several radio continuum sources, one of which is IRS1, a high-mass protostar,
from which a 0.3 pc molecular bipolar outflow was detected. Several maser
species have been detected around IRS1. The CH3OH masers have been suggested to
trace a Keplerian-disk, while the H2O masers are almost aligned to the outflow.
More recent results suggested that the region hosts a torus and potentially a
disk, but with a different inclination than the Keplerian-disk that is supposed
to be traced by the CH3OH masers. Tracing the magnetic field close to
protostars is fundamental for determining the orientation of the disk/torus.
Recent studies showed that during the protostellar phase of high-mass star
formation the magnetic field is oriented along the outflows and around or on
the surfaces of the disk/torus. The observations of polarized maser emissions
at milliarcsecond resolution can make a crucial contribution to understanding
the orientation of the magnetic field and, consequently, the orientation of the
disk/torus in NGC7538-IRS1. The NRAO Very Long Baseline Array was used to
measure the linear polarization and the Zeeman-splitting of the 22GHz H2O
masers toward NGC7538-IRS1. The European VLBI Network and the MERLIN telescopes
were used to measure the linear polarization and the Zeeman-splitting of the
6.7GHz CH3OH masers toward the same region. We detected 17 H2O masers and 49
CH3OH masers at high angular resolution. We detected linear polarization
emission toward two H2O masers and toward twenty CH3OH masers. The CH3OH
masers, most of which only show a core structure, seem to trace rotating and
potentially infalling gas in the inner part of a torus. Significant
Zeeman-splitting was measured in three CH3OH masers. [...] We also propose a
new description of the structure of the NGC7538-IRS1 maser region.Comment: 13 pages, 9 figures, 4 Tables, accepted by Astronomy & Astrophysic
High-mass star formation in the Southern Hemisphere sky
We report on a multi-wavelength (IR to cm) and multi-resolution (1 mas to 20
arcsec) exploration of high-mass star formation regions in the Galactic plane,
at longitudes observable from the Southern Hemisphere. Our source sample was
originally identified through methanol masers in the Galactic plane, which
exclusively trace high-mass star-forming regions. (Sub)millimetre continuum and
molecular line observations were carried out with SEST/SIMBA, JCMT/SCUBA and
ATNF/Mopra mm-wave telescopes and have allowed us to identify massive (
M) and luminous ( L) clumps in each star-forming
region. We have also constrained the SED with additional archival IR data, the
physical conditions (, , ) and the chemical composition of each
massive clump. Several types of objects were characterised based on the
ratio, the dust temperature and the molecular line
properties, ranging from class 0-like YSO clusters (,
T=30 K) to hot molecular clumps (, K).
Preliminary high-angular resolution observations for a subset of the sample
with the ATNF/ATCA at 3 mm, the VLA at 15, 22 and 43 GHz and Gemini in MIR have
revealed that several (proto)stellar objects are embedded in the massive
clumps: massive protostars, hot cores and hyper-compact HII regions. We have
thus identified protoclusters of massive YSOs, which are the precursors of the
OB associations. This sample of Southern Hemisphere star-forming regions will
be extremely valuable for the scientific preparation of the ALMA and HSO
observations.Comment: 4 pages, 3 figures, conference proceeding
Outflows and Massive Stars in the protocluster IRAS 05358+3543
We present new near-IR H2, CO J=2-1, and CO J = 3-2 observations to study
outflows in the massive star forming region IRAS 05358+3543. The
Canada-France-Hawaii Telescope H2 images and James Clerk Maxwell Telescope CO
data cubes of the IRAS 05358 region reveal several new outflows, most of which
emerge from the dense cluster of sub-mm cores associated with the Sh 2-233IR NE
cluster to the northeast of IRAS 05358. We used Apache Point Observatory (APO)
JHK spectra to determine line of sight velocities of the outflowing material.
Analysis of archival VLA cm continuum data and previously published VLBI
observations reveal a massive star binary as a probable source of one or two of
the outflows. We have identified probable sources for 6 outflows and candidate
counterflows for 7 out of a total of 11 seen to be originating from the IRAS
05358 clusters. We classify the clumps within Sh 2-233IR NE as an early
protocluster and Sh 2-233IR SW as a young cluster, and conclude that the
outflow energy injection rate approximately matches the turbulent decay rate in
Sh 2-233IR NE.Comment: 15 figures, 42 pages, accepted for publication in the Astrophysical
Journal. Full size figures are included at
http://casa.colorado.edu/~ginsbura/iras05358.htm. Data can be accessed from
figshare: http://figshare.com/articles/IRAS_05358_3543_Data_Cubes/80631
EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions
The role of magnetic fields in the formation of high-mass stars is still
under debate, and recent measurements of their orientation and strength by
using polarized maser emissions are contributing new insights. Masers
polarization, in particular of the 6.7-GHz methanol masers, are one of the best
probes of the magnetic field morphologies around massive protostars.
Determining the magnetic field morphology around an increasing number of
massive protostars at milliarcsecond resolution by observing 6.7-GHz methanol
masers is crucial to better understand the role of magnetic fields in massive
star formation.The First EVN Group consists of 4 massive star-forming
complexes: W51, W48, IRAS18556+0138, and W3(OH). These contain well-studied
\hii ~regions from some of which molecular bipolar outflows were also detected
(W51-e2, G35.20-0.74N). Nine of the European VLBI Network antennas were used to
measure the linear polarization and Zeeman-splitting of the 6.7-GHz methanol
masers in the star-forming regions of the First EVN Group. We detected a total
of 154 CH3OH masers, one third of these towards W3(OH). Fractional linear
polarization (1.2-11.5%) was detected towards 55 masers. The linear
polarization vectors are well-ordered in all the massive star-forming regions.
We measured significant Zeeman-splitting in 3 massive star-forming regions
(W51, W48, and W3(OH)) revealing a range of separations -3.5 m/s<\Delta
V_{z}<3.8 m/s with the smallest |\Delta V_{z}|=0.4m/s. We were also able to
compare our magnetic field results with those obtained from submillimeter
wavelength dust observation in W51 and show that the magnetic field at low and
high resolutions are in perfect agreement.Comment: 15 pages, 11 figures, 5 tables, accepted by Astronomy & Astrophysic
A Survey of OH Masers Towards High Mass Protostellar Objects
We present a survey of OH maser emission towards a sample of high mass
protostellar objects made using the Nancay and GBT telescopes.OH maser emission
was detected towards 63 objects with 36 new detections. There are 56
star-forming regions and 7 OH/IR candidates. There is no evidence that sources
with OH masers have a different range of luminosities from the non-maser
sources. The results of this survey are compared with previous water and class
II methanol maser observations of the same objects. Some of the detected
sources are only associated with OH masers and some sources are only associated
with the 1720 MHz OH maser line. The velocity range of the maser emission
suggests that the water maser sources may be divided into two groups. The
detection rates and velocity range of the OH and Class II methanol masers
support the idea that there is a spatial association of the OH and Class II
methanol masers. The sources span a wide range in R, the ratio of the methanol
maser peak flux to OH 1665 MHz maser peak flux, however there are only a few
sources with intermediate values of R, 8<R<32, which has characterised previous
samples. Sources which have masers of any species, OH, water or methanol, have
redder [100um-12um] IRAS colours than those without masers. However, there is
no evidence for different maser species tracing different stages in the
evolution of these young high mass sources. Previous observations which have
shown that the OH maser emission from similar sources traces the circumstellar
disks around the objects. This combined with the sensitivity of the OH emission
to the magnetic field, make the newly detected sources interesting candidates
for future follow-up at high angular resolution.Comment: Accepted for publication in A&
A Search for Propylene Oxide and Glycine in Sagittarius B2 (LMH) and Orion
We have used the Mopra Telescope to search for glycine and the simple chiral
molecule propylene oxide in the Sgr B2 (LMH) and Orion KL, in the 3-mm band. We
have not detected either species, but have been able to put sensitive upper
limits on the abundances of both molecules. The 3-sigma upper limits derived
for glycine conformer I are 3.7 x 10^{14} cm^{-2} in both Orion-KL and Sgr B2
(LMH), comparable to the reported detections of conformer I by Kuan et al.
However, as our values are 3-sigma upper limits rather than detections we
conclude that this weighs against confirming the detection of Kuan et al. We
find upper limits for the glycine II column density of 7.7 x 10^{12} cm^{-2} in
both Orion-KL and Sgr B2 (LMH), in agreement with the results of Combes et al.
The results presented here show that glycine conformer II is not present in the
extended gas at the levels detected by Kuan et al. for conformer I. Our ATCA
results (Jones et al.) have ruled out the detection of glycine (both conformers
I and II) in the compact hot core of the LMH at the levels reported, so we
conclude that it is unlikely that Kuan et al. have detected glycine in either
Sgr B2 or Orion-KL. We find upper limits for propylene oxide abundance of 3.0 x
10^{14} cm^{-2} in Orion-KL and 6.7 x 10^{14} cm^{-2} in Sgr B2 (LMH). We have
detected fourteen features in Sgr B2 and four features in Orion-KL which have
not previously been reported in the ISM, but have not be able to plausibly
assign these transitions to any carrier.Comment: 12 pages, 3 figures. Accepted by MNRAS 12th January 200
Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry -- Signatures of flow precession
NGC 7538 IRS1 is a high-mass (approx. 30 M_sun) protostar with a CO outflow,
an associated UCHII region, and a linear methanol maser structure, which might
trace a Keplerian-rotating circumstellar disk. The directions of the various
associated axes are misaligned with each other. We investigate the
near-infrared morphology of the source to clarify the relations among the
various axes. K'-band bispectrum speckle interferometry was performed at two
6-meter-class telescopes -- the BTA 6m telescope and the 6.5m MMT.
Complementary IRAC images from the Spitzer Space Telescope Archive were used to
relate the structures detected with the outflow at larger scales. High-dynamic
range images show fan-shaped outflow structure in which we detect 18 stars and
several blobs of diffuse emission. We interpret the misalignment of various
outflow axes in the context of a disk precession model, including numerical
hydrodynamic simulations of the molecular emission. The precession period is
approx. 280 years and its half-opening angle is 40 degrees. A possible
triggering mechanism is non-coplanar tidal interaction of an (undiscovered)
close companion with the circumbinary protostellar disk. Our observations
resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with
separation of 195 mas. We find indications for shock interaction between the
outflow activities in IRS1 and IRS2. Indications of outflow precession have
been discovered to date in a number of massive protostars, all with large
precession angles 20--45 degrees. This might explain the difference between the
outflow widths in low- and high-mass stars and add support to a common
collimation mechanism.Comment: 20 pages; 8 figures; Accepted by A&A on April 10, 2006; Image quality
reduced due to astro-ph file size limitations; Please download a version with
high-quality images from
http://www.mpifr-bonn.mpg.de/staff/tpreibis/ngc7538.pd
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