119 research outputs found
The Extended Methanol Maser Emission in W51
The European VLBI Network (EVN) has been used to make phase referenced,
wide-field (several arcminute) spectral line observations of the 6.7-GHz
methanol maser emission towards W51. In the W51main region, the bulk of the
methanol is offset from an UCHII region. This probably indicates the methanol
emission arises at the interface of the expanding UCHII region and not from an
edge-on circumstellar disc, as previously suggested. Near the W51 IRS2 region,
the methanol emission is associated with a very compact, extremely embedded
source supporting the hypothesis that methanol masers trace the earliest stages
of massive star formation. As well as these two previously well studied sites
of star formation, many previously unknown regions star formation are detected,
demonstrating that methanol masers are powerful means of detection young
massive stars.Comment: 5 pages, 3 figure
Laboratory and tentative interstellar detection of trans-methyl formate using the publicly available Green Bank Telescope PRIMOS survey
The rotational spectrum of the higher-energy trans conformational isomer of
methyl formate has been assigned for the first time using several pulsed-jet
Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This
species has also been sought toward the Sagittarius B2(N) molecular cloud using
the publicly available PRIMOS survey from the Green Bank Telescope. We detect
seven absorption features in the survey that coincide with laboratory
transitions of trans-methyl formate, from which we derive a column density of
3.1 (+2.6, -1.2) \times 10^13 cm-2 and a rotational temperature of 7.6 \pm 1.5
K. This excitation temperature is significantly lower than that of the more
stable cis conformer in the same source but is consistent with that of other
complex molecular species recently detected in Sgr B2(N). The difference in the
rotational temperatures of the two conformers suggests that they have different
spatial distributions in this source. As the abundance of trans-methyl formate
is far higher than would be expected if the cis and trans conformers are in
thermodynamic equilibrium, processes that could preferentially form
trans-methyl formate in this region are discussed. We also discuss measurements
that could be performed to make this detection more certain. This manuscript
demonstrates how publicly available broadband radio astronomical surveys of
chemically rich molecular clouds can be used in conjunction with laboratory
rotational spectroscopy to search for new molecules in the interstellar medium.Comment: 40 pages, 7 figures, 4 tables; accepted for publication in Ap
W51 IRS 2: A Massive Jet Emerging from a Molecular Cloud into an H II Region
We have mapped [Ne II] (12.8um) and [S IV] (10.5um) emission from W51 IRS 2
with TEXES on Gemini North, and we compare these data to VLA free-free
observations and VLT near-infrared images. With 0.5" spatial and 4 km/s
spectral resolution we are able to separate the ionized gas into several
components: an extended H II region on the front surface of the molecular
cloud, several embedded compact H II regions, and a streamer of high velocity
gas. We interpret the high velocity streamer as a precessing or fan-like jet,
which has emerged from the molecular cloud into an OB star cluster where it is
being ionized.Comment: 3 pages, 4 figures, 2 movie
New Molecular Species In Comet C/1995 (Hale-Bopp) Observed with the Caltech Submillimeter Observatory
We present millimeter-wave observations of HNCO, HC3N, SO, NH2CHO, H(13)CN, and H3O(+) in comet C/1995 O1 (Hale-Bopp) obtained in February-April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996 B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H2O is (4-13) x 10(exp -4). HC3N, SO, and NH2CHO are detected for the first time in a comet. The fractional abundance of HC3N based on observations of three rotational lines is (1.9 +/- 0.2) x 10(exp -4). Four transitions of SO are detected and the derived fractional abundance, (2-8) x 10(exp -3), is higher than the upper limits derived from UV observations of previous comets. Observations of NH2CHO imply a fractional abundance of (1-8) x 10(exp -4). H3O(+) is detected for the first time from the ground. The H(13)CN (3-2) transition is also detected and the derived HCN/H(13)CN abundance ratio is 90 +/- 15, consistent with the terrestrial C-13/C-12 ratio. in addition, a number of other molecular species are detected, including HNC, OCS, HCO(+), CO(+), and CN (the last two are first detections in a comet at radio wavelengths)
Ground-state ammonia and water in absorption towards Sgr B2
We have used the Odin submillimetre-wave satellite telescope to observe the
ground state transitions of ortho-ammonia and ortho-water, including their 15N,
18O, and 17O isotopologues, towards Sgr B2. The extensive simultaneous velocity
coverage of the observations, >500 km/s, ensures that we can probe the
conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the
Galactic centre, and the more diffuse gas in the Galactic disk clouds along the
line-of-sight. We present ground-state NH3 absorption in seven distinct
velocity features along the line-of-sight towards Sgr B2. We find a nearly
linear correlation between the column densities of NH3 and CS, and a
square-root relation to N2H+. The ammonia abundance in these diffuse Galactic
disk clouds is estimated to be about (0.5-1)e-8, similar to that observed for
diffuse clouds in the outer Galaxy. On the basis of the detection of H218O
absorption in the 3 kpc arm, and the absence of such a feature in the H217O
spectrum, we conclude that the water abundance is around 1e-7, compared to
~1e-8 for NH3. The Sgr B2 molecular cloud itself is seen in absorption in NH3,
15NH3, H2O, H218O, and H217O, with emission superimposed on the absorption in
the main isotopologues. The non-LTE excitation of NH3 in the environment of Sgr
B2 can be explained without invoking an unusually hot (500 K) molecular layer.
A hot layer is similarly not required to explain the line profiles of the
1_{1,0}-1_{0,1} transition from H2O and its isotopologues. The relatively weak
15NH3 absorption in the Sgr B2 molecular cloud indicates a high [14N/15N]
isotopic ratio >600. The abundance ratio of H218O and H217O is found to be
relatively low, 2.5--3. These results together indicate that the dominant
nucleosynthesis process in the Galactic centre is CNO hydrogen burning.Comment: 10 pages, 5 figure
Perspective from a Younger Generation -- The Astro-Spectroscopy of Gisbert Winnewisser
Gisbert Winnewisser's astronomical career was practically coextensive with
the whole development of molecular radio astronomy. Here I would like to pick
out a few of his many contributions, which I, personally, find particularly
interesting and put them in the context of newer results.Comment: 14 pages. (Co)authored by members of the MPIfR (Sub)millimeter
Astronomy Group. To appear in the Proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium "The Dense Interstellar Medium in Galaxies"
eds. S. Pfalzner, C. Kramer, C. Straubmeier, & A. Heithausen (Springer:
Berlin
The impact and significance of tephra deposition on a Holocene forest environment in the North Cascades, Washington, USA.
© 2016 Elsevier Ltd. High-resolution palaeoecological analyses (stratigraphy, tephra geochemistry, radiocarbon dating, pollen and ordination) were used to reconstruct a Holocene vegetation history of a watershed in the Pacific Northwest of America to evaluate the effects and duration of tephra deposition on a forest environment and the significance of these effects compared to long-term trends. Three tephra deposits were detected and evaluated: MLF-T158 and MLC-T324 from the climactic eruption of Mount Mazama, MLC-T480 from a Late Pleistocene eruption of Mount Mazama and MLC-T485 from a Glacier Peak eruption. Records were examined from both the centre and fringe of the basin to elucidate regional and local effects. The significance of tephra impacts independent of underlying long-term trends was confirmed using partial redundancy analysis. Tephra deposition from the climactic eruption of Mount Mazama approximately 7600 cal. years BP caused a significant local impact, reflected in the fringe location by changes to open habitat vegetation (Cyperaceae and Poaceae) and changes in aquatic macrophytes (Myriophyllum spicatum, Potamogeton, Equisetum and the alga Pediastrum). There was no significant impact of the climactic Mazama tephra or other tephras detected on the pollen record of the central core. Changes in this core are potentially climate driven. Overall, significant tephra fall was demonstrated through high resolution analyses indicating a local effect on the terrestrial and aquatic environment, but there was no significant impact on the regional forest dependent of underlying environmental changes
The polarization of mm methanol masers
We present a survey of the polarization properties of mm-wavelength methanol
masers, comprising both classes, and transitions from 84.5 to 157.0 GHz. Linear
polarization is found in more than half of the observed objects, and circular
polarization is tentatively detected in two sources. Class I and Class II
CH_3OH masers show similar polarization properties.The largest linear
polarization is found in the 133 GHz Class I maser towards L379 (39.5 %), and
in the 157 GHz Class II maser towards G9.62+0.19 (36.7 %). The spectral
profiles of the polarization angle of Class I masers are mostly flat, except
for two sources showing a linear slope. Since the mm-line methanol masers are
expected to be weakly (or not) saturated, we suggest that the stronger
fractional polarizations found by us are enhanced by anisotropic pumping and
radiative losses. In NGC 7538, we find, for both maser classes, a good
agreement between our polarization angles, and those measured for the
submillimeter dust continuum. This can be taken as evidence for magnetic
alignment of dust grains. It is also possible that an unsaturated maser with
equally populated magnetic substates simply amplifies polarized continuum seed
radiation. For Class II masers, the polarization properties of the various
velocity components towards a given source with detectable polarization are
quite homogeneous. A possible explanation is discussed. Since methanol is
non-paramagnetic, the circular polarization of the unsaturated maser emission
can only be due to variations of the angle between the magnetic field and the
line of sight along the maser propagation path.Comment: Astronomy & Astrophysics, in press, received 27 February 2004,
accepted 27 July 200
Determination of the primordial helium abundance from radio recombination line observations: New data. The source W51
Observations of H and He radio recombination lines in the source W51 have been performed with the RT-22 radio telescope (Pushchino) in two transitions: 56α (8 mm) and 65α (13 mm). We have estimated the spectral line parameters and determined the relative abundance of ionized helium, y + = (9.3 ± 0.35)%. We have carried out a model study of the correction (R) for the ionization structure of HII regions (when passing from the observed y + = N(He+)/N(H+) to the actual y = N(He)/N(H)) as a function of the spectral type of the ionizing star. Hence it follows that it is desirable to choose the sources excited by hot stars of spectral types no later than O6 V to estimate the helium abundance. In this case, the correction is expected to be small and essentially constant, R in the range 1.0-1.05. We have analyzed the correction for the ionization structure of W51, obtained an actual abundance of helium in the range y = (8.9-9.7)%, and determined its primordial abundance Y p (produced during primordial nucleosynthesis in the Universe) in this source. We have made a new estimate of the primordial helium abundance from six Galactic HII regions, where we observed H and He radio recombination lines at different times. The weighted mean Y p = 25.64(±0.70)% has been obtained. On the one hand, this value of Y p does not yet disagree strongly with the conclusions of the standard cosmologicalmodel, but, on the other hand, it admits the existence of at least one unknown light particle in the period of primordial nucleosynthesis outside the scope of the standard cosmological model. One should continue to refine Y p for more reliable conclusions to be reached. © 2013 Pleiades Publishing, Inc
Pacific offshore record of plinian arc volcanism in Central America: 1. Along-arc correlations
We collected 56 marine gravity cores from the Pacific seafloor offshore Central America which contain a total of 213 volcanic ash beds. Ash-layer correlations between cores and with their parental tephras on land use stratigraphic, lithologic, and compositional criteria. In particular, we make use of our newly built database of bulk-rock, mineral, and glass major and trace element compositions of plinian and similarly widespread tephras erupted since the Pleistocene along the Central American Volcanic Arc. We thus identify the distal ashes of 11 Nicaraguan, 8 El Salvadorian, 6 Guatemalan, and 1 Costa Rican eruptions. Relatively uniform pelagic sedimentation rates allow us to determine ages of 10 previously undated tephras by their relative position between ash layers of known age. Linking the marine and terrestrial records yields a tephrostratigraphic framework for the Central American volcanic arc from Costa Rica to Guatemala. This is a useful tool and prerequisite to understand the evolution of volcanism at a whole-arc scale
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