487 research outputs found
New evidence on the origin of the microquasar GRO J1655-40
Aims. Motivated by the new determination of the distance to the microquasar
GRO J1655-40 by Foellmi et al. (2006), we conduct a detailed study of the
distribution of the atomic and molecular gas, and dust around the open cluster
NGC 6242, the possible birth place of the microquasar. The proximity and
relative height of the cluster on the galactic disk provides a unique
opportunity to study SNR evolution and its possible physical link with
microquasar formation. Methods. We search in the interstellar atomic and
molecular gas around NGC 6242 for traces that may have been left from a
supernova explosion associated to the formation of the black hole in GRO
J1655-40. Furthermore, the 60/100 mu IR color is used as a tracer of
shocked-heated dust. Results. At the kinematical distance of the cluster the
observations have revealed the existence of a HI hole of 1.5*1.5 degrees in
diameter and compressed CO material acumulated along the south-eastern internal
border of the HI cavity. In this same area, we found extended infrared emission
with characteristics of shocked-heated dust. Based on the HI, CO and FIR
emissions, we suggest that the cavity in the ISM was produced by a supernova
explosion occured within NGC 6242. The lower limit to the kinematic energy
transferred by the supernova shock to the surrounding interstellar medium is ~
10^{49} erg and the atomic and molecular mass displaced to form the cavity of ~
16.500 solar masses. The lower limit to the time elapsed since the SN explosion
is ~ 2.2*10^{5} yr, which is consistent with the time required by GRO J1655-40
to move from the cluster up to its present position. The observations suggest
that GRO J1655-40 could have been born inside NGC 6242, being one of the
nearest microquasars known so far.Comment: 6 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
Carbon monoxide in the environs of the star WR 16
Aims. We analyze the carbon monoxide emission around the star WR 16 aiming to study the physical characteristics of the molecular gas linked to the star and to achieve a better understanding of the interaction between massive stars with their surroundings. Methods. We study the molecular gas in a region in size using CO J = 1 → 0 and 13CO J = 1 → 0 line data obtained with the 4-m NANTEN telescope. Radio continuum archival data at 4.85 GHz, obtained from the Parkes-MIT-NRAO Southern Radio Survey, are also analyzed to account for the ionized gas. Available IRAS (HIRES) 60 μm and 100 μm images are used to study the characteristics of the dust around the star. Results. Our new CO and 13CO data allow the low/intermediate density molecular gas surrounding the WR nebula to be completely mapped. We report two molecular features at-5 km s-1 and-8.5 km s-1 (components 1 and 2, respectively) having a good morphological resemblance with the Hα emission of the ring nebula. Component 2 seems to be associated with the external ring, while component 1 is placed at the interface between component 2 and the Hα emission. We also report a third molecular feature ∼10′ in size (component 3) at a velocity of-9.5 km s-1 having a good morphological correspondence with the inner optical and IR emission, although high resolution observations are recommended to confirm its existence. The stratified morphology and kinematics of the molecular gas could be associated to shock fronts and high mass-loss events related to different evolutive phases of the WR star, which have acted upon the surrounding circumstellar molecular gas. An analysis of the mass of component 1 suggests that this feature is composed of swept-up interstellar gas and is probably enriched by molecular ejecta. The direction of the proper motion of WR 16 suggests that the morphology observed at infrared, optical, radio continuum, and probably molecular emission of the inner ring nebula is induced by the stellar motion.Facultad de Ciencias Astronómicas y Geofísica
Characterization of Infrared Dark Clouds -- NH Observations of an Absorption-contrast Selected IRDC Sample
Despite increasing research in massive star formation, little is known about
its earliest stages. Infrared Dark Clouds (IRDCs) are cold, dense and massive
enough to harbour the sites of future high-mass star formation. But up to now,
mainly small samples have been observed and analysed. To understand the
physical conditions during the early stages of high-mass star formation, it is
necessary to learn more about the physical conditions and stability in
relatively unevolved IRDCs. Thus, for characterising IRDCs studies of large
samples are needed. We investigate a complete sample of 218 northern hemisphere
high-contrast IRDCs using the ammonia (1,1)- and (2,2)-inversion transitions.
We detected ammonia (1,1)-inversion transition lines in 109 of our IRDC
candidates. Using the data we were able to study the physical conditions within
the star-forming regions statistically. We compared them with the conditions in
more evolved regions which have been observed in the same fashion as our sample
sources. Our results show that IRDCs have, on average, rotation temperatures of
15 K, are turbulent (with line width FWHMs around 2 km s), have ammonia
column densities on the order of cm and molecular hydrogen
column densities on the order of cm. Their virial masses are
between 100 and a few 1000 M. The comparison of bulk kinetic and
potential energies indicate that the sources are close to virial equilibrium.
IRDCs are on average cooler and less turbulent than a comparison sample of
high-mass protostellar objects, and have lower ammonia column densities. Virial
parameters indicate that the majority of IRDCs are currently stable, but are
expected to collapse in the future.Comment: 21 pages, 11 figures, 7 tables. Paper accepted for publication in
Astronomy & Astrophysic
CBI limits on 31 GHz excess emission in southern HII regions
We have mapped four regions of the southern Galactic plane at 31 GHz with the
Cosmic Background Imager. From the maps, we have extracted the flux densities
for six of the brightest \hii regions in the southern sky and compared them
with multi-frequency data from the literature. The fitted spectral index for
each source was found to be close to the theoretical value expected for
optically thin free-free emission, thus confirming that the majority of flux at
31 GHz is due to free-free emission from ionised gas with an electron
temperature of K.
We also found that, for all six sources, the 31 GHz flux density was slightly
higher than the predicted value from data in the literature. This excess
emission could be due to spinning dust or another emission mechanism.
Comparisons with m data indicate an average dust emissivity of
K (MJy/sr), or a 95 per cent confidence limit of K (MJy/sr). This is lower than that found in diffuse clouds at high
Galactic latitudes by a factor of . The most significant detection
() was found in (RCW49) and may account for up to
per cent of the total flux density observed at 31 GHz. Here, the
dust emissivity of the excess emission is K (MJy/sr) and
is within the range observed at high Galactic latitudes.
Low level polarised emission was observed in all six sources with
polarisation fractions in the range per cent. This is likely to be
mainly due to instrumental leakage and is therefore upper an upper limit to the
free-free polarisation. It corresponds to an upper limit of per cent
for the polarisation of anomalous emission.Comment: Accepted in MNRAS. 12 pages, 10 figures, 5 table
A multiwavelength study of young massive star forming regions: II. The dust environment
We present observations of 1.2-mm dust continuum emission, made with the
Swedish ESO Submillimeter Telescope, towards eighteen luminous IRAS point
sources, all with colors typical of compact HII regions and associated with
CS(2-1) emission, thought to be representative of young massive star forming
regions. Emission was detected toward all the IRAS objects. We find that the
1.2-mm sources associated with them have distinct physical parameters, namely
sizes of 0.4 pc, dust temperatures of 30 K, masses of 2x10^3 Msun, column
densities of 3x10^23 cm^-2, and densities of 4x10^5 cm^-3. We refer to these
dust structures as massive and dense cores. Most of the 1.2-mm sources show
single-peaked structures, several of which exhibit a bright compact peak
surrounded by a weaker extended envelope. The observed radial intensity
profiles of sources with this type of morphology are well fitted with power-law
intensity profiles with power-law indices in the range 1.0-1.7. This result
indicates that massive and dense cores are centrally condensed, having radial
density profiles with power-law indices in the range 1.5-2.2. We also find that
the UC HII regions detected with ATCA towards the IRAS sources investigated
here (Paper I) are usually projected at the peak position of the 1.2-mm dust
continuum emission, suggesting that massive stars are formed at the center of
the centrally condensed massive and dense cores.Comment: 6 figures, accepted by Ap
An Experimental Examination of the Walrasian Tatonnement Mechanism
Joyce (1984) reports results of experiments of a Walrasian tatonnement auction that show that the mechanism is stable, exhibits strong convergence properties and generates efficiency averaging better than 97%. He also found that when subjects could see part of the order flow (excess demand), price tended to be lower (favorable to buyers). His experiments consisted of a stationary environment where subjects were provided with single-unit supply and demand functions. This paper assesses the robustness of his results in a more complex setting and systematically investigates the effect of various order flow information and message restriction rules on the performance of the Walrasian mechanism. In particular, our subjects were provided with multi-unit demands and supplies where equilibrium price and subject values or costs were changed each trading period
ATLASGAL - environments of 6.7 GHz methanol masers
Using the 870 micron APEX Telescope large area survey of the Galaxy, we have identified 577 submillimetre continuum sources with masers from the methanol multibeam survey in the region 280deg lt ell lt 20deg; |b| lt 1deg.5 94 per cent of methanol masers in the region are associated with submillimetre dust emission. We estimate masses for tilde450 maser-associated sources and find that methanol masers are preferentially associated with massive clumps. These clumps are centrally condensed, with envelope structures that appear to be scale-free, the mean maser position being offset from the peak column density by 0 plusmn 4 arcsec. Assuming a Kroupa initial mass function and a star formation efficiency of tilde30 per cent, we find that over two-thirds of the clumps are likely to form clusters with masses gt20 M. Furthermore, almost all clumps satisfy the empirical mass-size criterion for massive star formation. Bolometric luminosities taken from the literature for tilde100 clumps range between tilde100 and 10 L. This confirms the link between methanol masers and massive young stars for 90 per cent of our sample. The Galactic distribution of sources suggests that the star formation efficiency is significantly reduced in the Galactic Centre region, compared to the rest of the survey area, where it is broadly constant, and shows a significant drop in the massive star formation rate density in the outer Galaxy. We find no enhancement in source counts towards the southern Scutum-Centaurus arm tangent at ell tilde 315deg, which suggests that this arm is not actively forming stars.Peer reviewe
Digital compensation of the side-band-rejection ratio in a fully analog 2SB sub-millimeter receiver
In observational radio astronomy, sideband-separating receivers are
preferred, particularly under high atmospheric noise, which is usually the case
in the sub-millimeter range. However, obtaining a good rejection ratio between
the two sidebands is difficult since, unavoidably, imbalances in the different
analog components appear. We describe a method to correct these imbalances
without making any change in the analog part of the sideband-separating
receiver, specifically, keeping the intermediate-frequency hybrid in place.
This opens the possibility of implementing the method in any existing receiver.
We have built hardware to demonstrate the validity of the method and tested it
on a fully analog receiver operating between 600 and 720GHz. We have tested the
stability of calibration and performance vs time and after full resets of the
receiver. We have performed an error analysis to compare the digital
compensation in two configurations of analog receivers, with and without
intermediate frequency (IF) hybrid. An average compensated sideband rejection
ratio of 46dB is obtained. Degradation of the compensated sideband rejection
ratio on time and after several resets of the receiver is minimal. A receiver
with an IF hybrid is more robust to systematic errors. Moreover, we have shown
that the intrinsic random errors in calibration have the same impact for
configuration without IF hybrid and for a configuration with IF hybrid with
analog rejection ratio better than 10dB. Compensated rejection ratios above
40dB are obtained even in the presence of high analog rejection. The method is
robust allowing its use under normal operational conditions at any telescope.
We also demonstrate that a full analog receiver is more robust against
systematic errors. Finally, the error bars associated to the compensated
rejection ratio are almost independent of whether IF hybrid is present or not
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