1,832 research outputs found
New ammonia masers towards NGC6334I
We report the detection of new ammonia masers in the non-metastable (8,6) and
(11,9) transitions towards the massive star forming region NGC6334I.
Observations were made with the ATCA interferometer and the emitting region
appears unresolved in the 2.7" x 0.8" beam, with deconvolved sizes less than an
arcsecond. We estimate peak brightness temperatures of 7.8 x 10^5 and 1.2 x
10^5 K for the (8,6) and (11,9) transitions, respectively. The masers appear
coincident both spatially and in velocity with a previously detected ammonia
(6,6) maser. We also suggest that emission in the (10,9), (9,9) and (7,6)
transitions may also be masers, based on their narrow line widths and
overlapping velocity ranges with the above masers, as observed with the
single-dish Mopra radiotelescope
The implementation and use of Ada on distributed systems with high reliability requirements
The use and implementation of Ada in distributed environments in which reliability is the primary concern were investigated. In particular, the concept that a distributed system may be programmed entirely in Ada so that the individual tasks of the system are unconcerned with which processors they are executing on, and that failures may occur in the software or underlying hardware was examined. Progress is discussed for the following areas: continued development and testing of the fault-tolerant Ada testbed; development of suggested changes to Ada so that it might more easily cope with the failure of interest; and design of new approaches to fault-tolerant software in real-time systems, and integration of these ideas into Ada
The RMS Survey: 6 cm continuum VLA observations towards candidate massive YSOs in the northern hemisphere
(Abridged) Context: The Red MSX Source (RMS) survey is an ongoing
multi-wavelength observational programme designed to return a large,
well-selected sample of massive young stellar objects (MYSOs). We have
identified 2000 MYSO candidates located throughout the Galaxy by
comparing the colours of MSX and 2MASS point sources to those of known MYSOs.
Aims: To identify the populations of UCHII regions and PNe within the sample
and examine their Galactic distribution. Method: We have conducted high
resolution radio continuum observations at 6 cm towards 659 MYSO candidates in
the northern hemisphere (10\degr< l < 250\degr) using the VLA. In addition to
these targeted observations we present archival data towards a further 315 RMS
sources extracted from a previous VLA survey of the inner Galaxy. Results: We
find radio emission towards 272 (27% of the observed sample). Using
results from other parts of our multi-wavelength survey we separate these
RMS-radio associations into two distinct types of objects, classifying 51 as
PNe and a further 208 as either compact or UC HII regions. Using this well
selected sample of HII regions we estimate their Galactic scale height to be
0.6\degr. Conclusions: Using radio continuum and archival data we have
identified 79 PNe and 391 HII regions within the northern RMS catalogue. We
estimate the total fraction of contamination by PNe in the RMS sample is of
order 10%. The sample of HII regions is probably the best representation to
date of the Galactic population of HII regions as a whole.Comment: Accepted for publication in Astronomy and Astrophysics. 15 pages, 9
figures and 5 tables. Full versions of Tables 3, 4 and 5 and Figs. 2, 4 and 7
will only be available via CDS or the RMS website at
http:/www.ast.leeds.ac.uk/cgi-bin/RMS/RMS_VLA_IMAGES.cg
The RMS Survey: Resolving kinematic distance ambiguities towards a sample of compact HII regions using HI absorption
We present high-resolution HI data obtained using the Australia Telescope
Compact Array to resolve the near/far distance ambiguities towards a sample of
compact HII regions from the Red MSX Source (RMS) survey. The high resolution
data are complemented with lower resolution archival HI data extracted from the
Southern and VLA Galactic Plane surveys. We resolve the distance ambiguity for
nearly all of the 105 sources where the continuum was strong enough to allow
analysis of the HI absorption line structure. This represents another step in
the determination of distances to the total RMS sample, which with over 1,000
massive young stellar objects and compact HII regions, is the largest and most
complete sample of its kind. The full sample will allow the distribution of
massive star formation in the Galaxy to be examined.Comment: Accepted by MNRAS. This paper consists of 15 pages and contains 10
figures and 5 table
Probing discs around massive young stellar objects with CO first overtone emission
We present high resolution (R~50,000) spectroastrometry over the CO 1st
overtone bandhead of a sample of seven intermediate/massive young stellar
objects. These are primarily drawn from the red MSX source (RMS) survey, a
systematic search for young massive stars which has returned a large, well
selected sample of such objects. The mean luminosity of the sample is
approximately 5 times 10^4 L_\odot, indicating the objects typically have a
mass of ~15 solar masses. We fit the observed bandhead profiles with a model of
a circumstellar disc, and find good agreement between the models and
observations for all but one object. We compare the high angular precision
(0.2-0.8 mas) spectroastrometric data to the spatial distribution of the
emitting material in the best-fitting models. No spatial signatures of discs
are detected, which is entirely consistent with the properties of the
best-fitting models. Therefore, the observations suggest that the CO bandhead
emission of massive young stellar objects originates in small-scale disks, in
agreement with previous work. This provides further evidence that massive stars
form via disc accretion, as suggested by recent simulations.Comment: Accepted for publication in MNRA
The RMS Survey: Distribution and properties of a sample of massive young stars
The Red MSX Source (RMS) survey has identified a large sample of massive
young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample
of ~2000 MSX and 2MASS colour selected sources. Using a recent catalogue of
molecular clouds derived from the Boston University-Five College Radio
Astronomy Observatory Galactic Ring Survey (GRS), and by applying a Galactic
scaleheight cut off of 120 pc, we solve the distance ambiguity for RMS sources
located within 18\degr 54\degr. These two steps yield kinematic
distances to 291 sources out of a possible 326 located within the GRS longitude
range. Combining distances and integrated fluxes derived from spectral energy
distributions, we estimate luminosities to these sources and find that > 90%
are indicative of the presence of a massive star. We find the completeness
limit of our sample is ~10^4 Lsun, which corresponds to a zero age main
sequence (ZAMS) star with a mass of ~12 Msun. Selecting only these sources, we
construct a complete sample of 196 sources. Comparing the properties of the
sample of young massive stars with the general population, we find the
RMS-clouds are generally larger, more massive, and more turbulent. We examine
the distribution of this sub-sample with respect to the location of the spiral
arms and the Galactic bar and find them to be spatially correlated. We identify
three significant peaks in the source surface density at Galactocentric radii
of approximately 4, 6 and 8 kpc, which correspond to the proposed positions of
the Scutum, Sagittarius and Perseus spiral arms, respectively. Fitting a scale
height to the data we obtain an average value of ~29+-0.5 pc, which agrees well
with other reported values in the literature, however, we note a dependence of
the scale height on galactocentric radius with it increases from 30 pc to 45 pc
between 2.5 and 8.5 kpc.Comment: Accepted for publication by MNRAS. Paper consists of 15 pages
including 12 figures and four tables. Full versions of Tables 2 and 3 will
only be available online. The resolution of Figure 9 has been reduced - a
full resolution version of the paper can be download from here:
http://www.ast.leeds.ac.uk/cgi-bin/RMS/RMS_PUBLICATIONS.cg
The RMS Survey: Far-Infrared Photometry of Young Massive Stars
Context: The Red MSX Source (RMS) survey is a multi-wavelength campaign of
follow-up observations of a colour-selected sample of candidate massive young
stellar objects (MYSOs) in the galactic plane. This survey is returning the
largest well-selected sample of MYSOs to date, while identifying other dust
contaminant sources with similar mid-infrared colours including a large number
of new ultra-compact (UC)HII regions. Aims:To measure the far-infrared (IR)
flux, which lies near the peak of the spectral energy distribution (SED) of
MYSOs and UCHII regions, so that, together with distance information, the
luminosity of these sources can be obtained. Methods:Less than 50% of RMS
sources are associated with IRAS point sources with detections at 60 micron and
100 micron, though the vast majority are visible in Spitzer MIPSGAL or IRAS
Galaxy Atlas (IGA) images. However, standard aperture photometry is not
appropriate for these data due to crowding of sources and strong spatially
variable far-IR background emission in the galactic plane. A new technique
using a 2-dimensional fit to the background in an annulus around each source is
therefore used to obtain far-IR photometry for young RMS sources.
Results:Far-IR fluxes are obtained for a total of 1113 RMS candidates
identified as young sources. Of these 734 have flux measurements using IGA 60
micron and 100 micron images and 724 using MIPSGAL 70 micron images, with 345
having measurements in both data sets.Comment: 10 pages, 10 figures, 2 Tables, accepted to A&A. A full version of
table 1 is available from the lead author or at the CDS upon publicatio
Discovery of two eclipsing X-ray binaries in M51
We discovered eclipses and dips in two luminous (and highly variable) X-ray sources in M 51. One (CXOM51 J132943.3+471135) is an ultraluminous supersoft source, with a thermal spectrum at a temperature of about 0.1 keV and characteristic blackbody radius of about 104 km. The other (CXOM51 J132946.1+471042) has a two-component spectrum with additional thermal-plasma emission; it approached an X-ray luminosity of 1039 erg s−1 during outbursts in 2005 and 2012. From the timing of three eclipses in a series of Chandra observations, we determine the binary period (52.75 ± 0.63 h) and eclipse fraction (22±0.1 per cent ) of CXOM51 J132946.1+471042. We also identify a blue optical counterpart in archival Hubble Space Telescope images, consistent with a massive donor star (mass of ∼20–35 M⊙). By combining the X-ray light-curve parameters with the optical constraints on the donor star, we show that the mass ratio in the system must be M2 /M1 >~ 18 and therefore the compact object is most likely a neutron star (exceeding its Eddington limit in outburst). The general significance of our result is that we illustrate one method (applicable to high-inclination sources) of identifying luminous neutron star X-ray binaries, in the absence of X-ray pulsations or phase-resolved optical spectroscopy. Finally, we discuss the different X-ray spectral appearance expected from super-Eddington neutron stars and black holes at high viewing angles
The RMS Survey: The Bolometric Fluxes and Luminosity Distributions of Young Massive Stars
Context: The Red MSX Source (RMS) survey is returning a large sample of
massive young stellar objects (MYSOs) and ultra-compact (UC) \HII{} regions
using follow-up observations of colour-selected candidates from the MSX point
source catalogue. Aims: To obtain the bolometric fluxes and, using kinematic
distance information, the luminosities for young RMS sources with far-infrared
fluxes. Methods: We use a model spectral energy distribution (SED) fitter to
obtain the bolometric flux for our sources, given flux data from our work and
the literature. The inputs to the model fitter were optimised by a series of
investigations designed to reveal the effect varying these inputs had on the
resulting bolometric flux. Kinematic distances derived from molecular line
observations were then used to calculate the luminosity of each source.
Results: Bolometric fluxes are obtained for 1173 young RMS sources, of which
1069 have uniquely constrained kinematic distances and good SED fits. A
comparison of the bolometric fluxes obtained using SED fitting with trapezium
rule integration and two component greybody fits was also undertaken, and
showed that both produce considerable scatter compared to the method used here.
Conclusions: The bolometric flux results allowed us to obtain the luminosity
distributions of YSOs and UC\HII{} regions in the RMS sample, which we find to
be different. We also find that there are few MYSOs with L
10\lsol{}, despite finding many MYSOs with 10\lsol{} L
10\lsol{}.Comment: 12 pages, 12 figures, 3 tables, accepted to A&A. The full versions of
tables 1 and 2 will be available via the CDS upon publicatio
- …