233 research outputs found
Detection of CO in the inner part of M31's bulge
We report the first detection of CO in M31's bulge. The 12CO (1-0) and (2-1)
lines are both detected in the dust complex D395A/393/384, at 1.3" (~0.35 kpc)
from the centre. From these data and from visual extinction data, we derive a
CO-luminosity to reddening ratio (and a CO-luminosity to H_2 column density
ratio) quite similar to that observed in the local Galactic clouds. The (2-1)
to (1-0) line intensity ratio points to a CO rotational temperature and a gas
kinetic temperature > 10 K. The molecular mass of the complex, inside a 25'
(100 pc) region, is 1.5 10^4 Mo.Comment: 5 pages including 4 figures (2 in colour
The effect of violent star formation on the state of the molecular gas in M82
We present the results of a high angular resolution, multi-transition analysis of the molecular gas in M82. The analysis is based on the two lowest transitions of 12CO and the ground transition of the rare isotopes 13CO and C18O measured with the PdBI, the BIMA array and the IRAM 30m telescope. In order to address the question of how the intrinsic molecular cloud properties are influenced by massive star formation we have carried out radiative transfer calculations based on the observed CO line ratios. The calculations suggest that the kinetic temperature of the molecular gas is high in regions with strong star formation and drops towards the outer molecular lobes with less ongoing star formation. The location of the highest kinetic temperature is coincident with that of the mid infrared peaks which trace emission from hot dust. The hot gas is associated with low H2 densities while the cold gas in the outer molecular lobes has high H2 densities. We find that CO intensities do not trace H2 column densities well. Most of the molecular gas is distributed in a double-lobed distribution which surrounds the starburst. A detailed analysis of the conversion factor from CO intensity to H2 column density shows that X_CO depends on the excitation conditions. We find X_CO ~ Sqrt(n_H2)/T_kin, as expected for virialized clouds
First detection of ammonia in M82
We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion
lines of ammonia (NH3) towards the south--western molecular lobe in M82. The
relative intensities of the ammonia lines are characterized by a rotational
temperature of T_rot=29+/-5 K which implies an average kinetic temperature of
T_kin~60 K. A Gaussian decomposition of the observed spectra indicates
increasing kinetic temperatures towards the nucleus of M82, consistent with
recent findings based on CO observations. The observations imply a very low NH3
abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a
decreasing NH3 abundance towards the central active regions in M82 and
interpret this abundance gradient in terms of photodissociation of NH3 in PDRs.
The low temperature derived here from NH3 also explains the apparent
underabundance of complex molecules like CH_3OH and HNCO, which has previously
been reported.Comment: 4 pages, 4 figures, accepted by ApJ
Radio supernovae, supernova remnants and HII regions in NGC 2146 observed with MERLIN and the VLA
We present a high-resolution 5-GHz radio continuum map of the starburst
galaxy NGC 2146 made with MERLIN and the VLA, in a search of radio supernovae
and supernova remnants expected to be already produced by the most massive
stars in the starburst. At 5 GHz, about 20 point sources were detected earlier
by Glendenning & Kronberg (1986) in the central 800 pc of NGC 2146. Our
observations with higher sensitivity and resolution made with MERLIN and the
VLA confirms the detection of 18 sources, and resolves 7 of them. Additional
1.6-GHz MERLIN observations disclose 9 sources coincident in position with
those detected at 5 GHz, which allows us to derive their spectral indices. Only
3 sources have indices consistent with synchrotron emission from supernova
remnants or radio supernovae, while the others have very steep inverted
spectra. We suggest that the sources with positive spectral index are optically
thick ultra-compact and/or ultra-dense HII regions with high electron densities
and high emission measures (EM > 10^(7) cm^(-6) pc). Minimum energy
requirements indicate that these regions may contain up to 1000 equivalent
stars of type O6. When compared with M 82, the galaxy NGC 2146 lacks however a
large number of supernova remnants. We suggest that NGC 2146 is experiencing a
burst of star formation stronger than that in M 82, but being in a younger
phase. We may, however, not exclude an alternative scenario in which strong
free-free absorption at 1.6 GHz in foreground ionized gas with very high
emission measures (EM > 10^(8) cm^(-6) pc) hides a certain number of supernova
remnants, thus rendering for some sources the observed inverted spectra.Comment: 10 pages, including 2 figures. Accepted for publication in Astronomy
and Astrophysic
Broadband 300-GHz Power Amplifier MMICs in InGaAs mHEMT Technology
In this article, we report on compact solid-state power amplifier (SSPA) millimeter-wave monolithic integrated circuits (MMICs) covering the 280–330-GHz frequency range. The technology used is a 35-nm gate-length InGaAs metamorphic highelectron- mobility transistor (mHEMT) technology. Two power amplifier MMICs are reported, based on a compact unit amplifier cell, which is parallelized two times using two different Wilkinson power combiners. The Wilkinson combiners are designed using elevated coplanar waveguide and air-bridge thin-film transmission lines in order to implement low-loss 70-Ω lines in the back-endof-line of this InGaAs mHEMT technology. The five-stage SSPA MMICs achieve a measured small-signal gain around 20 dB over the 280–335-GHz frequency band. State-of-the-art output power performance is reported, achieving at least 13 dBm over the 286–310-GHz frequency band, with a peak output power of 13.7 dBm (23.4 mW) at 300 GHz. The PA MMICs are designed for a reduced chip width while maximizing the total gate width of 512 μm in the output stage, using a compact topology based on cascode and common-source devices, improving the output power per required chip width significantly
Atmospheric observation-based global SF6 emissions - comparison of top-down and bottom-up estimates
Emissions of sulphur hexafluoride (SF6), one of the strongest greenhouse gases on a per molecule basis, are targeted to be collectively reduced under the Kyoto Protocol. Because of its long atmospheric lifetime (≈3000 years), the accumulation of SF6 in the atmosphere is a direct measure of its global emissions. Examination of our extended data set of globally distributed high-precision SF6 observations shows an increase in SF6 abundance from near zero in the 1970s to a global mean of 6.7 ppt by the end of 2008. In-depth evaluation of our long-term data records shows that the global source of SF6 decreased after 1995, most likely due to SF6 emission reductions in industrialised countries, but increased again after 1998. By subtracting those emissions reported by Annex I countries to the United Nations Framework Convention of Climatic Change (UNFCCC) from our observation-inferred SF6 source leaves a surprisingly large gap of more than 70–80% of non-reported SF6 emissions in the last decade
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