105 research outputs found
Masers and Outflows in the W3(OH)/W3(H2O) region
Methanol masers and molecular shock tracers were observed in the
W3(OH)/W3(HO) region with the BIMA array and the Onsala 20m radiotelescope.
Characteristics of the outflows in the region are discussed. A model of the
W3(OH) methanol maser formation region is constructed.Comment: 4 pages, 2 figures, numerous journal misprints are correcte
Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive YSOs
The central kpc of the Milky Way might be expected to differ significantly
from the rest of the Galaxy with regard to gas dynamics and the formation of
YSOs. We probe this possibility with mid-infrared observations obtained with
IRAC and MIPS on Spitzer and with MSX. We use color-color diagrams and SED fits
to explore the nature of YSO candidates (including objects with 4.5 micron
excesses possibly due to molecular emission). There is an asymmetry in the
distribution of the candidate YSOs, which tend to be found at negative Galactic
longitudes; this behavior contrasts with that of the molecular gas,
approximately 2/3 of which is at positive longitudes. The small scale height of
these objects suggests that they are within the Galactic center region and are
dynamically young. They lie between two layers of infrared dark clouds and may
have originated from these clouds. We identify new sites for this recent star
formation. The methanol masers appear to be associated with young, embedded
YSOs characterized by 4.5 micron excesses. We use the SEDs of these sources to
estimate their physical characteristics. Within the central 400x50 pc
(|l|<1.3\degr and |b|<10') the star formation rate based on the identification
of Stage I evolutionary phase of YSO candidates is about 0.14 solar mass/yr. We
suggest that a recent burst of star formation took place within the last 10^5
years. This suggestion is also consistent with estimates of star formation
rates within the last ~10^7 years showing a peak around 10^5 years ago. Lastly,
we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of
the Galaxy. This implies that star formation does not appear to be dramatically
affected by the extreme physical conditions in the Galactic center region.Comment: 96 pages, ten tables, 35 figures, ApJ (in press), replaced by a
revised versio
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)
Comparative study of complex N- and O-bearing molecules in hot molecular cores
We have observed several emission lines of two Nitrogen-bearing (C2H5CN and
C2H3CN) and two Oxygen-bearing (CH3OCH3 and HCOOCH3) molecules towards a sample
of well-known hot molecular cores (HMCs) in order to check whether the chemical
differentiation seen in the Orion-HMC and W3(H_2O) between O- and N-bearing
molecules is a general property of HMCs. With the IRAM-30m telescope we have
observed 12 HMCs in 21 bands, centered at frequencies from 86250 to 258280 MHz.
The rotational temperatures obtained range from ~100 to ~150 K in these HMCs.
Single Gaussian fits performed to unblended lines show a marginal difference in
the line peak velocities of the C2H5CN and CH3OCH3 lines, indicating a possible
spatial separation between the region traced by the two molecules. On the other
hand, neither the linewidths nor the rotational temperatures and column
densities confirm such a result. By comparing the abundance ratio of the pair
C2H5CN/C2H3CN with the predictions of theoretical models, we derive that the
age of our cores ranges between 3.7 and 5.9x10^{4} yrs. The abundances of
C2H5CN and C2H3CN are strongly correlated, as expected from theory which
predicts that C2H3CN is formed through gas phase reactions involving C2H5CN. A
correlation is also found between the abundances of C2H3CN and CH3OCH3, and
C2H5CN and CH3OCH3. In all tracers the fractional abundances increase with the
H_2 column density while they are not correlated with the gas temperature.Comment: Accepted for publication in A&A, 56 page
Quantum Imaging with Incoherently Scattered Light from a Free-Electron Laser
The advent of accelerator-driven free-electron lasers (FEL) has opened new
avenues for high-resolution structure determination via diffraction methods
that go far beyond conventional x-ray crystallography methods. These techniques
rely on coherent scattering processes that require the maintenance of
first-order coherence of the radiation field throughout the imaging procedure.
Here we show that higher-order degrees of coherence, displayed in the intensity
correlations of incoherently scattered x-rays from an FEL, can be used to image
two-dimensional objects with a spatial resolution close to or even below the
Abbe limit. This constitutes a new approach towards structure determination
based on incoherent processes, including Compton scattering, fluorescence
emission or wavefront distortions, generally considered detrimental for imaging
applications. Our method is an extension of the landmark intensity correlation
measurements of Hanbury Brown and Twiss to higher than second-order paving the
way towards determination of structure and dynamics of matter in regimes where
coherent imaging methods have intrinsic limitations
Embedded Star Clusters in the W51 Giant Molecular Cloud
We present sub-arcsecond (0.35"-0.9"), near-infrared J,H,K band photometric
observations of six fields along the W51 Giant Molecular Cloud (W51 GMC). Our
observations reveal four new, embedded clusters and provide a new
high-resolution (0.35") view of the W51IRS2 (G49.5-0.4) region. The cluster
associated with G48.9-0.3 is found to be a double cluster enclosed in a nest of
near-infrared nebulosity. We construct stellar surface density maps for four
major clusters in the W51 GMC. These unveil the underlying hierarchical
structure. Color-color and color-magnitude diagrams for each of these clusters
show clear differences in the embedded stellar populations and indicate the
relative ages of these clusters. In particular, the clusters associated with
the HII regions G48.9-0.3 and G49.0-0.3 are found to have a high fraction of
YSOs and are therefore considered the youngest of all the near-infrared
clusters in the W51 GMC. The estimated masses of the individual clusters, when
summed, yield a total stellar mass of ~10^4 Msun in the W51 GMC, implying a
star formation efficiency of 5-10%. These results in comparision with the CO
observations of the W51 GMC, suggest for the first time, that star formation in
the W51 GMC is likely triggered by a galactic spiral density wave.Comment: 10 pages and 5 figures. 3 figures in jpeg forma
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 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
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