57 research outputs found
The Electron Temperature Gradient in the Galactic Disk
We derive the electron temperature gradient in the Galactic disk using a
sample of HII regions that spans Galactocentric distances 0--17 kpc. The
electron temperature was calculated using high precision radio recombination
line and continuum observations for more than 100 HII regions. Nebular
Galactocentric distances were calculated in a consistent manner using the
radial velocities measured by our radio recombination line survey. The large
number of nebulae widely distributed over the Galactic disk together with the
uniformity of our data provide a secure estimate of the present electron
temperature gradient in the Milky Way. Because metals are the main coolants in
the photoionized gas, the electron temperature along the Galactic disk should
be directly related to the distribution of heavy elements in the Milky Way. Our
best estimate of the electron temperature gradient is derived from a sample of
76 sources for which we have the highest quality data. The present gradient in
electron temperature has a minimum at the Galactic Center and rises at a rate
of 287 +/- 46 K/kpc. There are no significant variations in the value of the
gradient as a function of Galactocentric radius or azimuth. The scatter we find
in the HII region electron temperatures at a given Galactocentric radius is not
due to observational error, but rather to intrinsic fluctuations in these
temperatures which are almost certainly due to fluctuations in the nebular
heavy element abundances. Comparing the HII region gradient with the much
steeper gradient found for planetary nebulae suggests that the electron
temperature gradient evolves with time, becoming flatter as a consequence of
the chemical evolution of the Milky Way's disk.Comment: 43 pages, 9 figures (accepted for publication in the ApJ
Ionized gas, molecules, and dust in Sh2-132
We analyze the various interstellar components of the HII region Sh2-132. The
main stellar source is the double binary system that includes the Wolf-Rayet
star WR153ab. We use radio continuum images at 408 and 1420 MHz, and HI 21cm
line data taken from the Canadian Galactic Plane Survey, molecular observations
of the 12CO(1-0) line at 115 GHz from the Five College Radio Astronomy
Observatory, and available mid and far IR observations obtained with the MSX
and IRAS satellites, respectively.
Sh2-132 is composed of two shells showing radio continuum counterparts at
both frequencies. The emission is thermal in nature. The estimated rms electron
density and ionized mass of the nebula are n_e = 20 cm^{-3} and M_HII = 1500
Mo. The distribution of the CO emission shows molecular gas bordering the
ionized nebula and interacting with it. The velocities of the molecular gas is
in the range --38 to --53 km/s, similar to the velocity of the ionized gas. The
emission at 8.3 mic. reveals a ring like feature of about 15' that encircles
the bright optical regions. This emission is due to the PAHs and marks the
location of photodissociation regions.
The gas distribution in the environs of Sh2-132 can be explained in a
scenario where the massive stars in the region photodissociated, ionized, and
swept-up the dense molecular material from the parental cloud through their
strong stellar winds and intense UV photon flux.Comment: 11 figures and 5 tables, accepted in MNRA
Galaxy rotation curves: the effect of j x B force
Using the Galaxy as an example, we study the effect of j x B force on the
rotational curves of gas and plasma in galaxies. Acceptable model for the
galactic magnetic field and plausible physical parameters are used to fit the
flat rotational curve for gas and plasma based on the observed baryonic
(visible) matter distribution and j x B force term in the static MHD equation
of motion. We also study the effects of varied strength of the magnetic field,
its pitch angle and length scale on the rotational curves. We show that j x B
force does not play an important role on the plasma dynamics in the
intermediate range of distances 6-12 kpc from the centre, whilst the effect is
sizable for larger r (r > 15 kpc), where it is the most crucial.Comment: Accepted for publication in Astrophysics & Space Science (final
printed version, typos in proofs corrected
A multifrequency study of the active star forming complex NGC6357. I. Interstellar structures linked to the open cluster Pis24
We investigate the distribution of the gas (ionized, neutral atomic and
molecular), and interstellar dust in the complex star forming region NGC6357
with the goal of studying the interplay between the massive stars in the open
cluster Pis24 and the surrounding interstellar matter. Our study of the
distribution of the ionized gas is based on narrow-band Hhalfa, [SII], and
[OIII] images obtained with the Curtis-Schmidt Camera at CTIO, Chile, and on
radio continuum observations at 1465 MHz taken with the VLA with a synthesized
beam of 40 arcsec. The distribution of the molecular gas is analyzed using
12CO(1-0) data obtained with the Nanten radiotelescope, Chile (angular
resolution = 2.7 arcmin). The interstellar dust distribution was studied using
mid-infrared data from the GLIMPSE survey and far-infrared observations from
IRAS. NGC6357 consists of a large ionized shell and a number of smaller optical
nebulosities. The optical, radio continuum, and near- and mid-IR images
delineate the distributions of the ionized gas and interstellar dust in the HII
regions and in previously unknown wind blown bubbles linked to the massive
stars in Pis24 revealing surrounding photodissociation regions. The CO line
observations allowed us to identify the molecular counterparts of the ionized
structures in the complex and to confirm the presence of photodissociation
regions. The action of the WR star HD157504 on the surrounding gas was also
investigated. The molecular mass in the complex is estimated to be (4+/-2)X10^5
Mo. Mean electron densities derived from the radio data suggest electron
densities > 200 cm^-3, indicating that NGC6357 is a complex formed in a region
of high ambient density. The known massive stars in Pis24 and a number of newly
inferred massive stars are mainly responsible for the excitation and
photodissociation of the parental molecular cloud.Comment: 16 pages, 9 figures. Accepted for publication in MNRA
The ionization mechanism of NGC 185: how to fake a Seyfert galaxy?
NGC 185 is a dwarf spheroidal satellite of the Andromeda galaxy. From
mid-1990s onwards it was revealed that dwarf spheroidals often display a varied
and in some cases complex star formation history. In an optical survey of
bright nearby galaxies, NGC 185 was classified as a Seyfert galaxy based on its
emission line ratios. However, although the emission lines in this object
formally place it in the category of Seyferts, it is probable that this galaxy
does not contain a genuine active nucleus. NGC 185 was not detected in radio
surveys either in 6 or 20 cm, or X-ray observations, which means that the
Seyfert-like line ratios may be produced by stellar processes. In this work, we
try to identify the possible ionization mechanisms for this galaxy. We
discussed the possibility of the line emissions being produced by planetary
nebulae (PNe), using deep spectroscopy observations obtained with GMOS-N, at
Gemini. Although the fluxes of the PNe are high enough to explain the
integrated spectrum, the line ratios are very far from the values for the
Seyfert classification. We then proposed that a mixture of supernova remnants
and PNe could be the source of the ionization, and we show that a composition
of these two objects do mimic Seyfert-like line ratios. We used chemical
evolution models to predict the supernova rates and to support the idea that
these supernova remnants should be present in the galaxy.Comment: 9 pages, 7 figures, accepted for publication in MNRA
GS100-02-41: a new large HI shell in the outer part of the Galaxy
Massive stars have a profound effect on the surrounding interstellar medium.
They ionize and heat the neutral gas, and due to their strong winds, they swept
the gas up forming large HI shells. In this way, they generate a dense shell
where the physical conditions for the formation of new stars are given. The aim
of this study is to analyze the origin and evolution of the large HI shell
GS100-02-41 and its role in triggering star forming processes.To characterize
the shell and its environs, we carry out a multi-wavelength study. We analyze
he HI 21 cm line, the radio continuum, and infrared emission distributions. The
analysis of the HI data shows an expanding shell structure centred at (l, b) =
(100.6 deg, -2.04 deg) in the velocity range from -29 to -51.7 km/s.
We infer for GS100-02-41, a kinematical distance of 2.8 +/- 0.6 kpc. Several
massive stars belonging to Cep OB1 are located in projection within the large
HI, shell boundaries. The analysis of the radio continuum and infrared data
reveal that there is no continuum counterpart of the HI shell. On the other
hand, three slightly extended radio continuum sources are observed in
projection onto the dense HI shell. From their flux density determinations we
infer that they are thermal in nature. An analysis of the HI emission
distribution in the environs of these sources shows, for each of them, a region
of low emissivity having a good morphological correlation with the ionized gas
in a velocity range similar to the one where GS100-02-41 is detected. The
origin of GS100-02-41 could have been mainly due to the action of the Cep OB1
massive stars located inside the HI shell. The obtained age difference between
the HI shell and the HII regions, together with their relative location, led us
to conclude that the ionizing stars could have been created as a consequence of
the shell evolution.Comment: Accepted for publication in A&
Extreme infrared variables from UKIDSS-I. A concentration in star-forming regions
We present initial results of the first panoramic search for high-amplitude near-infrared variability in theGalactic plane.We analyse the widely separated two-epoch K-band photometry in the fifth and seventh data releases of the UKIDSS Galactic plane survey.We find 45 stars with δK > 1 mag, including two previously known OH/IR stars and a Nova. Even though the midplane is not yet included in the data set, we find the majority (66 per cent) of our sample to be within known star-forming regions (SFRs), with two large concentrations in the Serpens OB2 association (11 stars) and the Cygnus-X complex (12 stars). Sources in SFRs show spectral energy distributions that support classification as young stellar objects (YSOs). This indicates that YSOs dominate the Galactic population of high-amplitude infrared variable stars at low luminosities and therefore likely dominate the total high-amplitude population. Spectroscopic follow up of the DR5 sample shows at least four stars with clear characteristics of eruptive premain- sequence variables, two of which are deeply embedded. Our results support the recent concept of eruptive variability comprising a continuum of outburst events with different timescales and luminosities, but triggered by a similar physical mechanism involving unsteady accretion. Also, we find what appears to be one of the most variable classical Be stars. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Low-ionization pairs of knots in planetary nebulae: physical properties and excitation
We obtained optical long-slit spectra of four planetary nebulae (PNe) with
low-ionization pair of knots, namely He 1-1, IC 2149, KjPn 8 and NGC 7662.
These data allow us to derive the physical parameters and excitation of the
pairs of knots, and those of higher ionization inner components of the nebulae,
separately.
Our results are as follows. 1) The electron temperatures of the knots are
within the range 9500 to 14500 K, similar to the temperatures of the higher
ionization rims/shells. 2) Typical knots' densities are 500 to 2000 cm^{-3}. 3)
Empirical densities of the inner rims/shells are higher than those of the pairs
of knots, by up to a factor of 10. Theoretical predictions, at variance with
the empirical results, suggest that knots should be denser than the inner
regions, by at least a factor of 10. 4) Empirical and theoretical density
contrasts can be reconciled if we assume that at least 90% of the knots' gas is
neutral (likely composed of dust and molecules). 5) By using Raga et al. (2008)
shock modeling and diagnostic diagrams appropriated for spatially resolved PNe,
we suggest that high-velocity shocked knots traveling in the photoionized outer
regions of PNe can explain the emission of the pairs of knots analysed in this
paper.Comment: The paper contains 11 pages (6 figures and 5 tables). Accepted for
publication in MNRAS; added last paragraph for subsection 7.
NGC 3503 and its molecular environment
We present a study of the molecular gas and interstellar dust distribution in
the environs of the HII region NGC 3503 associated with the open cluster Pis 17
with the aim of investigating the spatial distribution of the molecular gas
linked to the nebula and achieving a better understanding of the interaction of
the nebula and Pis 17 with their molecular environment.
We based our study in ^{12}CO(1-0) observations of a region of ~0.6 deg. in
size obtained with the 4-m NANTEN telescope, unpublished radio continuum data
at 4800 and 8640 MHz obtained with the ATCA telescope, radio continuum data at
843 MHz obtained from SUMSS, and available IRAS, MSX, IRAC-GLIMPSE, and MIPSGAL
images.
We found a molecular cloud (Component 1) having a mean velocity of -24.7 km
s^{-1}, compatible with the velocity of the ionized gas, which is associated
with the nebula and its surroundings. Adopting a distance of 2.9 +/- 0.4 kpc
the total molecular mass and density yield (7.6 +/- 2.1) x 10^3 Msun and 400
+/- 240 cm^{-3}, respectively.
The radio continuum data confirm the existence of an electron density
gradient in NGC 3503. The IR emission shows the presence of a PDR bordering the
higher density regions of the nebula. The spatial distribution of the CO
emission shows that the nebula coincides with a molecular clump, with the
strongest CO emission peak located close to the higher electron density region.
The more negative velocities of the molecular gas (about -27 km s^{-1}), is
coincident with NGC 3503. Candidate YSOs were detected towards the HII region,
suggesting that embedded star formation may be occurring in the neighbourhood
of the nebula. The presence of a clear electron density gradient, along with
the spatial distribution of the molecular gas and PAHs in the region indicates
that NGC 3503 is a blister-type HII region that probably has undergone a
champagne phase
Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission
We present a new estimate of foreground emission in the WMAP data, using a
Markov chain Monte Carlo (MCMC) method. The new technique delivers maps of each
foreground component for a variety of foreground models, error estimates of the
uncertainty of each foreground component, and provides an overall
goodness-of-fit measurement. The resulting foreground maps are in broad
agreement with those from previous techniques used both within the
collaboration and by other authors.
We find that for WMAP data, a simple model with power-law synchrotron,
free-free, and thermal dust components fits 90% of the sky with a reduced
chi-squared of 1.14. However, the model does not work well inside the Galactic
plane. The addition of either synchrotron steepening or a modified spinning
dust model improves the fit. This component may account for up to 14% of the
total flux at Ka-band (33 GHz). We find no evidence for foreground
contamination of the CMB temperature map in the 85% of the sky used for
cosmological analysis.Comment: accepted by ApJS, 49 pages, 4 tables, 21 figures. PS and PDF versions
with high-resolution figures available at
http://lambda.gsfc.nasa.gov/product/map/dr3/map_bibliography.cf
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