107 research outputs found
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 Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. II. Fluid-Dynamical Models
We report a dynamical determination of the separate contributions of disk and
dark halo masses to the rotation curve of a spiral galaxy. We use
fluid-dynamical models of gas flow in the barred galaxy NGC 4123 to constrain
the dynamical properties of the galaxy: disk M/L, bar pattern speed, and the
central density and scale radius of the dark halo. We derive a realistic barred
potential directly from the light distribution. For each model we assume a
value of the stellar M/L and a bar pattern speed Omega_p and add a dark halo to
fit the rotation curve. We then compute the gas flow velocities with a 2-D gas
dynamical code, and compare the model flow patterns to a 2-D velocity field
derived from Fabry-Perot observations. The strong shocks and non-circular
motions in the observed gas flow require a high stellar M/L and a fast-rotating
bar. Models with I-band disk M/L of 2.0 -- 2.5 h_75, or 80 -- 100% of the
maximum disk value, are highly favored. The corotation radius of the bar must
be <= 1.5 times the bar semi-major axis. These results contradict some recent
claimed ``universal'' galaxy disk/halo relations, since NGC 4123 is of modest
size (rotation curve maximum 145 km/sec, and V_flat = 130 km/sec) yet is quite
disk-dominated. The dark halo of NGC 4123 is less concentrated than favored by
current models of dark halos based on cosmological simulations. Since some 30%
of bright disk galaxies are strongly barred and have dust lanes indicating
shock morphology similar to that of NGC 4123, it is likely that they also have
high stellar M/L and low density halos. We suggest that luminous matter
dominates inside the optical radius R_25 of high surface brightness disk
galaxies.Comment: accepted by ApJ, 20 pages, 11 figures (2 color), uses emulateapj.sty,
onecolfloat.st
The Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. I. Observations
The non-circular streaming motions in barred galaxies are sensitive to the
mass of the bar and can be used to lift the degeneracy between disk and dark
matter halo encountered when fitting axisymmetric rotation curves of disk
galaxies. In this paper, we present photometric and kinematic observations of
NGC 4123, a barred galaxy of modest size (V_rot = 130 km/sec, L = 0.7 L_*),
which reveal strong non-circular motions. The bar has straight dust lanes and
an inner Lindblad resonance. The disk of NGC 4123 has no sign of truncation out
to 10 scale lengths, and star-forming regions are found well outside R_25. A
Fabry-Perot H-alpha velocity field shows velocity jumps of >100 km/sec at the
location of the dust lanes within the bar, indicating shocks in the gas flow.
VLA observations yield the velocity field of the H I disk. Axisymmetric mass
models yield good fits to the rotation curve outside the bar regionfor disk
I-band M/L of 2.25 or less, and dark halos with either isothermal or power-law
profiles can fit the data well. In a companion paper, we model the full 2-D
velocity field, including non-circular motions, to determine the stellar M/L
and the mass of the dark halo.Comment: accepted by ApJ, 16 pages, 9 figures (1 color), uses emulateapj.sty,
onecolfloat.st
The almost ubiquitous association of 6.7 GHz methanol masers with dust
We report the results of 870-m continuum observations, using the Large
APEX Bolometer Camera (LABOCA), towards 77 class-II, 6.7-GHz methanol masers
identified by the Methanol Multibeam (MMB) survey to map the thermal emission
from cool dust towards these objects. These data complement a study of 630
methanol masers associated with compact dense clumps identified from the
ATLASGAL survey. Compact dust emission is detected towards 70 sources, which
implies a dust-association rate of 99% for the full MMB catalogue. Evaluation
of the derived dust and maser properties leads us to conclude that the combined
sample represents a single population tracing the same phenomenon. We find
median clump masses of a few 10 M and that all but a handful of
sources satisfy the mass-size criterion required for massive star formation.
This study provides the strongest evidence of the almost ubiquitous association
of methanol masers with massive, star-forming clumps. The fraction of
methanol-maser associated clumps is a factor of ~2 lower in the outer Galaxy
than the inner Galaxy, possibly a result of the lower metallicity environment
of the former. We find no difference in the clump-mass and maser-luminosity
distributions of the inner and outer Galaxy. The maser-pumping and
clump-formation mechanisms are therefore likely to be relatively invariant to
Galactic location. Finally, we use the ratio of maser luminosity and clump mass
to investigate the hypothesis that the maser luminosity is a good indicator of
the evolutionary stage of the embedded source, however, we find no evidence to
support this.Comment: Accepted by MNRAS. 17 pages, 17 figures and 5 tables. The full
version of Figs. 3 and 5 are only available in electronic form of the journal
while the full versions of Tables 1, 2 and 4 will only be available through
CDS. A complete version of the paper is available on reques
The RMS survey: galactic distribution of massive star formation
We have used the well-selected sample of~1750 embedded, young, massive stars identified by the Red MSX Source (RMS) survey to investigate the Galactic distribution of recent massive star formation. We present molecular line observations for ~800 sources without existing radial velocities. We describe the various methods used to assign distances extracted from the literature and solve the distance ambiguities towards approximately 200 sources located within the solar circle using archival HI data. These distances are used to calculate bolometric luminosities and estimate the survey completeness (~2 × 10 4 L⊙). In total, we calculate the distance and luminosity of ~1650 sources, one third of which are above the survey's completeness threshold. Examination of the sample's longitude, latitude, radial velocities and mid-infrared images has identified~120 small groups of sources, many of which are associated with well-known star formation complexes, such as G305, G333, W31, W43, W49 and W51. We compare the positional distribution of the sample with the expected locations of the spiral arms, assuming a model of the Galaxy consisting of four gaseous arms. The distribution of young massive stars in the Milky Way is spatially correlated with the spiral arms, with strong peaks in the source position and luminosity distributions at the arms' Galactocentric radii. The overall source and luminosity surface densities are both well correlated with the surface density of the molecular gas, which suggests that the massive star formation rate per unit molecular mass is approximately constant across the Galaxy. A comparison of the distribution of molecular gas and the young massive stars to that in other nearby spiral galaxies shows similar radial dependences. We estimate the total luminosity of the embedded massive star population to be ~0.76 × 10 8 L⊙, 30 per cent of which is associated with the 10 most active star-forming complexes.We measure the scaleheight as a function of the Galactocentric distance and find that it increases only modestly from ~20-30 pc between 4 and 8 kpc, but much more rapidly at larger distances. © 2013 The Authors
MALT90 Kinematic Distances to Dense Molecular Clumps
Using molecular-line data from the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90), we have estimated kinematic distances to 1905 molecular clumps identified in the ATLASGAL 870 μm continuum survey over the longitude range 295° < l < 350°. The clump velocities were determined using a flux-weighted average of the velocities obtained from Gaussian fits to the HCO+, HNC, and N2H+ (1–0) transitions. The near/far kinematic distance ambiguity was addressed by searching for the presence or absence of absorption or self-absorption features in 21 cm atomic hydrogen spectra from the Southern Galactic Plane Survey. Our algorithm provides an estimation of the reliability of the ambiguity resolution. The Galactic distribution of the clumps indicates positions where the clumps are bunched together, and these locations probably trace the locations of spiral arms. Several clumps fall at the predicted location of the far side of the Scutum–Centaurus arm. Moreover, a number of clumps with positive radial velocities are unambiguously located on the far side of the Milky Way at galactocentric radii beyond the solar circle. The measurement of these kinematic distances, in combination with continuum or molecular-line data, now enables the determination of fundamental parameters such as mass, size, and luminosity for each clump
Bodily tides near spin-orbit resonances
Spin-orbit coupling can be described in two approaches. The method known as
"the MacDonald torque" is often combined with an assumption that the quality
factor Q is frequency-independent. This makes the method inconsistent, because
the MacDonald theory tacitly fixes the rheology by making Q scale as the
inverse tidal frequency.
Spin-orbit coupling can be treated also in an approach called "the Darwin
torque". While this theory is general enough to accommodate an arbitrary
frequency-dependence of Q, this advantage has not yet been exploited in the
literature, where Q is assumed constant or is set to scale as inverse tidal
frequency, the latter assertion making the Darwin torque equivalent to a
corrected version of the MacDonald torque.
However neither a constant nor an inverse-frequency Q reflect the properties
of realistic mantles and crusts, because the actual frequency-dependence is
more complex. Hence the necessity to enrich the theory of spin-orbit
interaction with the right frequency-dependence. We accomplish this programme
for the Darwin-torque-based model near resonances. We derive the
frequency-dependence of the tidal torque from the first principles, i.e., from
the expression for the mantle's compliance in the time domain. We also explain
that the tidal torque includes not only the secular part, but also an
oscillating part.
We demonstrate that the lmpq term of the Darwin-Kaula expansion for the tidal
torque smoothly goes through zero, when the secondary traverses the lmpq
resonance (e.g., the principal tidal torque smoothly goes through nil as the
secondary crosses the synchronous orbit).
We also offer a possible explanation for the unexpected frequency-dependence
of the tidal dissipation rate in the Moon, discovered by LLR
The H2O southern Galactic Plane Survey(HOPS): NH3 (1,1) and (2,2) catalogues
The H2O Southern Galactic Plane Survey (HOPS) has mapped a 100 degree strip
of the Galactic plane (-70deg > l > 30deg, |b| < 0.5deg) using the 22-m Mopra
antenna at 12-mm wavelengths. Observations were conducted in on-the-fly mode
using the Mopra spectrometer (MOPS), targeting water masers, thermal molecular
emission and radio-recombination lines. Foremost among the thermal lines are
the 23 GHz transitions of NH3 J,K = (1,1) and (2,2), which trace the densest
parts of molecular clouds (n > 10^4 cm^{-3}). In this paper we present the NH3
(1,1) and (2,2) data, which have a resolution of 2 arcmin and cover a velocity
range of +/-200 km/s. The median sensitivity of the NH3 data-cubes is sigma_Tmb
= 0.20 +/1 0.06 K. For the (1,1) transition this sensitivity equates to a 3.2
kpc distance limit for detecting a 20 K, 400 Msun cloud at the 5-sigma level.
Similar clouds of mass 5,000 Msun would be detected as far as the Galactic
centre, while 30,000 Msun clouds would be seen across the Galaxy. We have
developed an automatic emission finding procedure based on the ATNF DUCHAMP
software and have used it to create a new catalogue of 669 dense molecular
clouds. The catalogue is 100 percent complete at the 5-sigma detection limit
(Tmb = 1.0 K). A preliminary analysis of the ensemble cloud properties suggest
that the near kinematic distances are favoured. The cloud positions are
consistent with current models of the Galaxy containing a long bar. Combined
with other Galactic plane surveys this new molecular-line dataset constitutes a
key tool for examining Galactic structure and evolution. Data-cubes, spectra
and catalogues are available to the community via the HOPS website.Comment: 22 pages, 19 figures. Accepted for publication in MNRAS
(25-July-2012
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