283 research outputs found
The Rotating Molecular Structures and the Ionized Outflow Associated with IRAS 16547-4247
We present VLA 1.3 cm radio continuum and water maser observations as well as
SMA SO (226.300 GHz) and 1.3 mm dust continuum observations toward the
massive star formation region IRAS 16547-4247. We find evidence of multiple
sources in the central part of the region. There is evidence of a rotating
structure associated with the most massive of these sources, traced at small
scales (~50 AU) by the water masers. At large scales (~1000 AU) we find a
velocity gradient in the SO2 molecular emission with a barely resolved
structure that can be modeled as a rotating ring or two separate objects. The
velocity gradients of the masers and of the molecular emission have the same
sense and may trace the same structure at different size scales. The position
angles of the structures associated with the velocity gradients are roughly
perpendicular to the outflow axis observed in radio continuum and several
molecular tracers. We estimate the mass of the most massive central source to
be around 30 solar masses from the velocity gradient in the water maser
emission. The main source of error in this estimate is the radius of the
rotating structure. We also find water masers that are associated with the
large scale molecular outflow of the system, as well as water masers that are
associated with other sources in the region. Our results suggest that the
formation of this source, one of the most luminous protostars or protostellar
clusters known, is taking place with the presence of ionized jets and disk-like
structures.Comment: 26 pages, 7 figure
Dissecting a hot molecular core: The case of G31.41+0.31
We made a detailed observational analysis of a well known hot molecular core
lying in the high-mass star-forming region G31.41+0.31. This core is believed
to contain deeply embedded massive stars and presents a velocity gradient that
has been interpreted either as rotation or as expansion, depending on the
authors. Our aim was to shed light on this question and possibly prepare the
ground for higher resolution ALMA observations which could directly detect
circumstellar disks around the embedded massive stars. Observations at
sub-arcsecond resolution were performed with the Submillimeter Array in methyl
cyanide, a typical hot molecular core tracer, and 12CO and 13CO, well known
outflow tracers. We also obtained sensitive continuum maps at 1.3 mm. Our
findings confirm the existence of a sharp velocity gradient across the core,
but cannot confirm the existence of a bipolar outflow perpendicular to it. The
improved angular resolution and sampling of the uv plane allow us to attain
higher quality channel maps of the CH3CN lines with respect to previous studies
and thus significantly improve our knowledge of the structure and kinematics of
the hot molecular core. While no conclusive argument can rule out any of the
two interpretations (rotation or expansion) proposed to explain the velocity
gradient observed in the core, in our opinion the observational evidence
collected so far indicates the rotating toroid as the most likely scenario. The
outflow hypothesis appears less plausible, because the dynamical time scale is
too short compared to that needed to form species such as CH3CN, and the mass
loss and momentum rates estimated from our measurements appear too high.Comment: Astronomy and Astrophysics, in pres
Spherical Infall in G10.6-0.4: Accretion Through an Ultracompact HII Region
We present high resolution (0.''12 x 0.''079) observations of the
ultracompact HII region G10.6-0.4 in 23 GHz radio continuum and the NH3(3,3)
line. Our data show that the infall in the molecular material is largely
spherical, and does not flatten into a molecular disk at radii as small as 0.03
pc. The spherical infall in the molecular gas matches in location and velocity
the infall seen in the ionized gas. We use a non-detection to place a stringent
upper limit on the mass of an expanding molecular shell associated with
pressure driven expansion of the HII region. These data support a scenario in
which the molecular accretion flow passes through an ionization front and
becomes an ionized accretion flow onto one or more main sequence stars, not the
classical pressure-driven expansion scenario. In the continuum emission we see
evidence for externally ionized clumps of molecular gas, and cavities evacuated
by an outflow from the central source.Comment: Accepted for publication in Astrophysical Journal Letter
Mopra line survey mapping of NGC6334I and I(N) at 3mm
A 5'x5' region encompassing NGC6334I and I(N) has been mapped at a wavelength
of 3mm (from 83.5 to 115.5GHz) with the Mopra telescope at an angular
resolution between 33 arcsec and 36 arcsec. This investigation has made use of
the recently installed 3mm MMIC receiver and the Mopra Spectrometer (MOPS) with
broadband capabilities permitting total coverage of the entire frequency range
with just five different observations. In total, the spatial distribution of
nineteen different molecules, ions and radicals, along with additional selected
isotopologues have been studied. Whilst most species trace the sites of star
formation, CH_3CN appears to be most closely associated with NGC6334I and I(N).
Both CN and C_2H appear to be widespread, tracing gas that is not associated
with active star formation. Both N_2H^+ and HC_3N closely resemble dust
continuum emission, showing they are reliable tracers of dense material, as
well as the youngest stages of high mass star formation. Hot (E_u/k>100K)
thermal CH_3OH emission is preferentially found towards NGC6334I, contrasting
with I(N), where only cold (E_u/k<22K) thermal CH_3OH emission is found.Comment: Accepted by MNRA
Methanol and excited OH masers towards W51: I - Main and South
MERLIN phase-referenced polarimetric observations towards the W51 complex
were carried out in March 2006 in the Class II methanol maser transition at
6.668 GHz and three of the four excited OH maser hyperfine transitions at 6
GHz. Methanol maser emission is found towards both W51 Main and South. We did
not detect any emission in the excited OH maser lines at 6.030 and 6.049 GHz
down to a 3 sigma limit of ~20 mJy per beam. Excited OH maser emission at 6.035
GHz is only found towards W51 Main. This emission is highly circularly
polarised (typically 45% and up to 87%). Seven Zeeman pairs were identified in
this transition, one of which contains detectable linear polarisation. The
magnetic field strength derived from these Zeeman pairs ranges from +1.6 to
+6.8 mG, consistent with the previously published magnetic field strengths
inferred from the OH ground-state lines. The bulk of the methanol maser
emission is associated with W51 Main, sampling a total area of ~3"x2.2" (i.e.,
~16200x11900 AU), while only two maser components, separated by ~2.5", are
found in the W51 South region. The astrometric distributions of both 6.668-GHz
methanol and 6.035-GHz excited-OH maser emission in the W51 Main/South region
are presented here. The methanol masers in W51 Main show a clear coherent
velocity and spatial structure with the bulk of the maser components
distributed into 2 regions showing a similar conical opening angle with of a
central velocity of ~+55.5 km/s and an expansion velocity of =<5 km/s. The mass
contained in this structure is estimated to be at least 22 solar masses. The
location of maser emission in the two afore-mentioned lines is compared with
that of previously published OH ground-state emission. Association with the
UCHII regions in the W51 Main/South complex and relationship of the masers to
infall or outflow in the region are discussed.Comment: 19 pages, 16 figures and 4 tables, accepted for publication in MNRA
A VLA Study of Ultracompact and Hypercompact H II Regions from 0.7 to 3.6 cm
We report multi-frequency Very Large Array observations of three massive star
formation regions (MSFRs) containing radio continuum components that were
identified as broad radio recombination line (RRL) sources and hypercompact
(HC) H II region candidates in our previous H92alpha and H76alpha study:
G10.96+0.01 (component W), G28.20-0.04 (N), and G34.26+0.15 (B). An additional
HC H II region candidate, G45.07+0.13, known to have broad H66alpha and
H76alpha lines, small size, high electron density and emission measure, was
also included. We observed with high spatial resolution (0.9" to 2.3") the
H53alpha, H66alpha, H76alpha, and H92alpha RRLs and the radio continuum at the
corresponding wavelengths (0.7 to 3.6 cm). The motivation for these
observations was to obtain RRLs over a range of principal quantum states to
look for signatures of pressure broadening and macroscopic velocity structure.
We find that pressure broadening contributes significantly to the line widths,
but it is not the sole cause of the broad lines. We compare radio continuum and
dust emission distributions and find a good correspondence. We also discuss
maser emission and multi-wavelength observations reported in the literature for
these MSFRs.Comment: Accepted for publication in ApJ; 55 pages, 10 tables, 12 figure
Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun
Theory predicts and observations confirm that low-mass stars (like the Sun)
in their early life grow by accreting gas from the surrounding material. But
for stars ~ 10 times more massive than the Sun (~10 M_sun), the powerful
stellar radiation is expected to inhibit accretion and thus limit the growth of
their mass. Clearly, stars with masses >10 M_sun exist, so there must be a way
for them to form. The problem may be solved by non-spherical accretion, which
allows some of the stellar photons to escape along the symmetry axis where the
density is lower. The recent detection of rotating disks and toroids around
very young massive stars has lent support to the idea that high-mass (> 8
M_sun) stars could form in this way. Here we report observations of an ammonia
line towards a high-mass star forming region. We conclude from the data that
the gas is falling inwards towards a very young star of ~20 M_sun, in line with
theoretical predictions of non-spherical accretion.Comment: 11 pages, 2 figure
Molecular outflows towards O-type young stellar objects
We have searched for massive molecular outflows in a sample of high-mass star
forming regions, and we have characterised both the outflow properties and
those of their associated molecular clumps. With a sample composed largely of
more luminous objects than previous ones, this work complements analogous
surveys performed by other authors by adding the missing highest luminosity
sources. The sample under study has been selected so as to favour the earliest
evolutionary phases of star formation, and is composed of very luminous objects
(L_bol > 2x10^4 L_sun and up to ~10^6 L_sun), possibly containing O-type stars.
Each source has been mapped in 13CO(2-1) and C18O(2-1) with the IRAM-30m
telescope on Pico Veleta (Spain). The whole sample shows high-velocity wings in
the 13CO(2-1) spectra, indicative of outflowing motions. In addition, we have
obtained outflow maps in 9 of our 11 sources, which display well-defined blue
and/or red lobes. For these sources, the outflow parameters have been derived
from the line wing 13CO(2-1) emission. An estimate of the clump masses from the
C18O(2-1) emission is also provided and found to be comparable to the virial
masses. From a comparison between our results and those found by other authors
at lower masses, it is clear that the outflow mechanical force increases with
the bolometric luminosity of the clump and with the ionising photon rate of the
associated HII regions, indicating that high-mass stars drive more powerful
outflows. A tight correlation between outflow mass and clump mass is also
found. Molecular outflows are found to be as common in massive star forming
regions as in low-mass star forming regions. This, added to the detection of a
few tentative large-scale rotating structures suggests that high-mass stars may
generally form via accretion, as low-mass stars.Comment: 16 pages, 10 figures, accepted by Astronomy and Astrophysic
ATCA 3mm observations of NGC6334I and I(N): dense cores, outflows and an UCHII region
Aims: Investigation of the dense gas, the outflows and the continuum emission
from the massive twin cores NGC6334I and I(N) at high spatial resolution.
Methods: We imaged the region with the Australia Telescope Compact Array (ATCA)
at 3.4mm wavelength in continuum as well as CH3CN(5_K-4_K) and HCN(1-0)
spectral line emission. Results: While the continuum emission in NGC6334I
mainly traces the UCHII region, toward NGC6334I(N) we detect line emission from
four of the previously identified dust continuum condensations that are of
protostellar or pre-stellar nature. The CH3CN(5_K-4_K) lines are detected in
all K-components up to energies of 128K above ground toward two protostellar
condensations in both regions. We find line-width increasing with increasing K
for all sources, which indicates a higher degree of internal motions closer to
the central protostars. Toward the main mm and CH3CN source in NGC6334I we
identify a velocity gradient approximately perpendicular to the large-scale
molecular outflow. This may be interpreted as a signature of an accretion disk,
although other scenarios, e.g., an unresolved double source, could produce a
similar signature as well. No comparable signature is found toward any of the
other sources. HCN does not trace the dense gas well but it is dominated by the
molecular outflows. While the outflow in NGC6334I exhibits a normal Hubble-law
like velocity structure, the data indicate a precessing outflow close to the
plane of the sky for NGC6334I(N). Furthermore, we observe a wide (~15.4km/s)
HCN absorption line, much broader than the previously observed CH3OH and NH3
absorption lines. Several explanations for the difference are discussed.Comment: 14 pages, 14 figures, accepted for A&
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Organic C and N stabilization in a forest soil: evidence from sequential density fractionation
In mineral soil, organic matter (OM) accumulates mainly on and around surfaces of silt- and clay-size particles. When fractionated according to particle density, C and N concentration (per g fraction) and C/N of these soil organo-mineral particles decrease with increasing particle density across soils of widely divergent texture, mineralogy, location, and management. The variation in particle density is explained potentially by two factors: (1) a decrease in the mass ratio of organic to mineral phase of these particles, and (2) variations in density of the mineral phase. The first explanation implies that the thickness of the organic accumulations decreases with increasing particle density. The decrease in C/N can be explained at least partially by especially stable sorption of cationic peptidic compounds (amine, amide, and pyrrole) directly to mineral surfaces, a phenomenon well documented both empirically and theoretically. These peptidic compounds, along with ligand-exchanged carboxylic compounds, could then form a stable inner organic layer onto which less polar organics could sorb more readily than onto the highly charged mineral surfaces (''onion'' layering model). To explore mechanisms underlying this trend in C concentration and C/N with particle density, we sequentially density fractionated an Oregon andic soil at 1.65, 1.85, 2.00, 2.28, and 2.55 g cm{sup -3} and analyzed the six fractions for measures of organic matter and mineral phase properties. All measures of OM composition showed either: (1) a monotonic change with density, or (2) a monotonic change across the lightest fractions, then little change over the heaviest fractions. Total C, N, and lignin phenol concentration all decreased monotonically with increasing density, and {sup 14}C mean residence time (MRT) increased with particle density from ca. 150 y to >980 y in the four organo-mineral fractions. In contrast, C/N, {sup 13}C and {sup 15}N concentration all showed the second pattern. All these data are consistent with a general pattern of an increase in extent of microbial processing with increasing organo-mineral particle density, and also with an ''onion'' layering model. X-ray diffraction before and after separation of magnetic materials showed that the sequential density fractionation isolated pools of differing mineralogy, with layer-silicate clays dominating in two of the intermediate fractions and primary minerals in the heaviest two fractions. There was no indication that these differences in mineralogy controlled the differences in density of the organo-mineral particles in this soil. Thus, our data are consistent with the hypothesis that variation in particle density reflects variation in thickness of the organic accumulations and with an ''onion'' layering model for organic matter accumulation on mineral surfaces. However, the mineralogy differences among fractions made it difficult to test either the layer-thickness or ''onion'' layering models with this soil. Although sequential density fractionation isolated pools of distinct mineralogy and organic-matter composition, more work will be needed to understand mechanisms relating the two factors
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