29 research outputs found
Infrared emission from interstellar dust cloud with two embedded sources: IRAS 19181+1349
Mid and far infrared maps of many Galactic star forming regions show multiple
peaks in close proximity, implying more than one embedded energy sources. With
the aim of understanding such interstellar clouds better, the present study
models the case of two embedded sources. A radiative transfer scheme has been
developed to deal with an uniform density dust cloud in a cylindrical geometry,
which includes isotropic scattering in addition to the emission and absorption
processes. This scheme has been applied to the Galactic star forming region
associated with IRAS 19181+1349, which shows observational evidence for two
embedded energy sources. Two independent modelling approaches have been
adopted, viz., to fit the observed spectral energy distribution (SED) best; or
to fit the various radial profiles best, as a function of wavelength. Both the
models imply remarkably similar physical parameters.Comment: 17 pages, 6 Figures, uses epsf.sty. To appear in Journal of
Astronophysics & Astronom
GMRT observations of the field of INTEGRAL X-ray sources- II (newly discovered hard X-ray sources)
We have conducted low-frequency radio observations with the Giant Metrewave
Radio Telescope (GMRT) of 40 new hard X-ray sources discovered by the INTEGRAL
satellite. This survey was conducted in order, to study radio emissions from
these sources, to provide precise position and to identify new microquasar
candidates. From our observations we find that 24 of the X-ray sources have
radio candidates within the INTEGRAL error circle. Based on the radio
morphology, variability and information available from different wavelengths,
we categorize them as seventeen Galactic sources (4 unresolved, 7 extended, 6
extended sources in diffuse region) and seven extragalactic sources (2
unresolved, 5 extended). Detailed account for seventeen of these sources was
presented in earlier paper. Based on the radio data for the remaining sources
at 0.61 GHz, and the available information from NVSS, DSS, 2MASS and NED, we
have identified possible radio counterparts for the hard X-ray sources. The
three unresolved sources, viz IGR J173030601, IGR J174643213, and IGR
J184060539 are discussed in detail. These sources have been identified as
X-ray binaries with compact central engine and variable in X-ray and in the
radio, and are most likely microquasar candidates. The remaining fourteen
sources have extended radio morphology and are either diffuse Galactic regions
or extragalactic in origin.Comment: 9 pages, 7 figures, submitted to A&A. submitted to A&
Radio observations of the planetary nebula around the OH/IR Star OH354.88-0.54 (V1018 Sco)
We present radio observations of the unique, recently formed, planetary
nebula (PN) associated with a very long-period OH/IR variable star V1018 Sco
that is unequivocally still in its asymptoticgiant branch phase. Two regions
within the optical nebula are clearly detected in nonthermal radio continuum
emission, with radio spectral indices comparable to those seen in
colliding-wind Wolf-Rayet binaries. We suggest that these represent shocked
interactions between the hot, fast stellar wind and the cold nebular shell that
represents the PN's slow wind moving away from the central star. This same
interface produces both synchrotron radio continuum and the optical PN
emission. The fast wind is neither spherical in geometry nor aligned withany
obvious optical or radio axis. We also report the detection of transient H2O
maser emission in this nebula.Comment: 11 pages, LaTeX (mn2e.cls), incl. 9 PostScript (ps or eps) figures
and 2 tables. Accepted by MNRA
Far and mid infrared observations of two ultracompact H II regions and one compact CO clump
Two ultracompact H II regions (IRAS 19181+1349 and 20178+4046) and one
compact molecular clump (20286+4105) have been observed at far infrared
wavelengths using the TIFR 1 m balloon-borne telescope and at mid infrared
wavelengths using ISO. Far infrared observations have been made simultaneously
in two bands with effective wavelengths of ~ 150 and ~ 210 micron, using liquid
3He cooled bolometer arrays. ISO observations have been made in seven spectral
bands using the ISOCAM instrument; four of these bands cover the emission from
Polycyclic Aromatic Hydrocarbon (PAH) molecules. In addition, IRAS survey data
for these sources in the four IRAS bands have been processed using the HIRES
routine. In the high resolution mid infrared maps as well as far infrared maps
multiple embedded energy sources have been resolved. There are structural
similarities between the images in the mid infrared and the large scale maps in
the far infrared bands, despite very different angular resolutions of the two.
Dust temperature and optical depth (tau_150 um) maps have also been generated
using the data from balloon-borne observations. Spectral energy distributions
(SEDs) for these sources have been constructed by combining the data from all
these observations. Radiation transfer calculations have been made to
understand these SEDs. Parameters for the dust envelopes in these sources have
been derived by fitting the observed SEDs. In particular, it has been found
that radial density distribution for three sources is diffrent. Whereas in the
case of IRAS 20178+4046, a steep distribution of the form r^-2 is favoured, for
IRAS 20286+4105 it is r^-1 and for IRAS 19181+1349 it the uniform distribution
(r^0). Line ratios for PAH bands have generally been found to be similar to
those for other compact H II regions but different from general H II regions.Comment: To appear in Astronomy & Astrophysics; (19 pages including 14 Figures
and 6 Tables
Recent star formation in the inner Galactic Bulge seen by ISOGAL. I - Classification of bright mid-IR sources in a test field
Context: The stellar populations in the central region of the Galaxy are
poorly known because of the high visual extinction and very great source
density in this direction.
Aims: To use recent infrared surveys for studying the dusty stellar objects
in this region.
Methods: We analyse the content of a 20x20 arcmin^2 field centred at
(l,b)=(-0.27,-0.06) observed at 7 and 15 microns as part of the ISOGAL survey.
These ISO observations are more than an order of magnitude better in
sensitivity and spatial resolution than the IRAS observations. The sources are
cross-associated with other catalogues to identify various types of objects. We
then derive criteria to distinguish young objects from post-main sequence
stars.
Results: We find that a sample of about 50 young stellar objects and
ultra-compact HII regions emerges, out of a population of evolved AGB stars. We
demonstrate that the sources colours and spatial extents, as they appear in the
ISOGAL catalogue, possibly complemented with MSX photometry at 21 microns, can
be used to determine whether the ISOGAL sources brighter than 300 mJy at 15
microns (or [15] < 4.5 mag) are young objects or late-type evolved stars.Comment: 15 pages, 12 figures. Accepted for publication in Astronomy and
Astrophysic
Radio sources at low Galactic latitudes
We present high-resolution radio observations of a sample of 65 radio sources
at low Galactic latitudes. The sources were all observed at 5 GHz with the Very
Large Array A-array. MERLIN observations at 5 GHz of the ultracompact HII
region G34.26+0.15 and one of the extragalactic sources, B1857-000, are also
presented, as are GMRT observations of HI in the direction of three sources,
B1801-203, B1802-196 and B1938+229. These observations were made with the
objectives of (i) finding compact components suitable for studying the effects
of interstellar scattering at lower frequencies, (ii) identifying high
surface-brightness lobes of background radio sources to probe the Galactic
magnetic field on different scales via polarization observations, and (iii)
searching for young supernova remnants. We discuss the nature of the sources
found to have shell or shell-like structure and exhibiting both thermal and
non-thermal spectra. Of the remaining sources, B1749-281 is coincident within
the positional errors of a known pulsar, not detected earlier at 5 GHz. The
rest are likely to be background extragalactic objects.Comment: 12 pages, 6 figures (most with multiple images), 1 table. Accepted
for publicaton in MNRA
GMRT observations of four suspected supernova remnants near the Galactic Centre
We have observed two fields - Field-I (l=3.2 degrees, b=-1.0 degree) and
Field-II (l=356.8 degrees, b=-0.1 degree) with the Giant Metrewave Radio
Telescope (GMRT) at 330 MHz. In the first field, we have studied the candidate
supernova remnant (SNR) G3.1-0.6 and based on its observed morphology, spectral
index and polarisation confirmed it to be an SNR. We find this supernova to
have a double ring appearance with a strip of emission on it's western side
passing through it's centre.
We have discovered two extended curved objects in the second field, which
appears to be part of a large shell like structure. It is possibly the remains
of an old supernova in the region. Three suspected supernova remnants,
G356.3-0.3, G356.6+0.1 and G357.1-0.2 detected in the MOST 843 MHz survey of
the Galactic Centre region appears to be located on this shell like structure.
While both G356.3-0.3 and G356.6+0.1 seem to be parts of this shell, G357.1-0.2
which has a steeper spectrum above 1 GHz, could be a background SNR seen
through the region. Our HI absorption observation towards the candidate SNR
G357.1-0.2 indicates that it is at a distance of more than 6 kpc from us.Comment: 13 pages, 13 figures, accepted for publication in MNRA
The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X
We have made an extensive 1.2mm continuum mosaicing study of the Cygnus X
molecular cloud complex using the MAMBO cameras at the IRAM 30 m telescope. We
then compared our mm maps with mid-IR images, and have made SiO(2-1) follow-up
observations of the best candidate progenitors of high-mass stars. Our complete
study of Cygnus X provides, for the first time, an unbiased census of massive
young stellar objects. We discover 129 massive dense cores, among which 42 are
probable precursors of high-mass stars. Our study qualifies 17 cores as good
candidates for hosting massive IR-quiet protostars, while up to 25 cores
potentially host high-luminosity IR protostars. We fail to discover the
high-mass analogs of pre-stellar dense cores in CygnusX, but find several
massive starless clumps that might be gravitationally bound. Since our sample
is derived from a single molecular complex and covers every embedded phase of
high-mass star formation, it gives the first statistical estimates of their
lifetime. In contrast to what is found for low-mass class 0 and class I phases,
the IR-quiet protostellar phase of high-mass stars may last as long as their
better-known high-luminosity IR phase. The statistical lifetimes of high-mass
protostars and pre-stellar cores (~ 3 x 10^4 yr and < 10^3 yr) in Cygnus X are
one and two order(s) of magnitude smaller, respectively, than what is found in
nearby, low-mass star-forming regions. We therefore propose that high-mass
pre-stellar and protostellar cores are in a highly dynamic state, as expected
in a molecular cloud where turbulent processes dominate.Comment: 32 pages, 62 figures to be published in Astronomy & Astrophysics
journa
Extragalactic sources towards the central region of the Galaxy
We have observed a sample of 64 small diameter sources towards the central -6
degree < l< 6 degree, -2 degree < b < 2 degree of the Galaxy with the aim of
studying the Faraday rotation measure near the Galactic Centre (GC) region. All
the sources were observed at 6 and 3.6 cm wavelengths using the ATCA and the
VLA. Fifty nine of these sources are inferred to be extragalactic. The
observations presented here constitute the first systematic study of the radio
polarisation properties of the background sources towards this direction and
increases the number of known extragalactic radio sources in this part of the
sky by almost an order of magnitude. Based on the morphology, spectral indices
and lack of polarised emission, we identify four Galactic HII regions in the
sample.Comment: 24 pages, 67 figures, published earlier in MNRA
Understanding the Spectral Energy Distributions of the Galactic Star Forming Regions IRAS 18314-0720, 18355-0532 & 18316-0602
Embedded Young Stellar Objects (YSO) in dense interstellar clouds is treated
self-consistently to understand their spectral energy distributions (SED).
Radiative transfer calculations in spherical geometry involving the dust as
well as the gas component, have been carried out to explain observations
covering a wide spectral range encompassing near-infrared to radio continuum
wavelengths. Various geometric and physical details of the YSOs are determined
from this modelling scheme. In order to assess the effectiveness of this
self-consistent scheme, three young Galactic star forming regions associated
with IRAS 18314-0720, 18355-0532 and 18316-0602 have been modelled as test
cases. They cover a large range of luminosity ( 40). The modelling of
their SEDs has led to information about various details of these sources, e.g.
embedded energy source, cloud structure & size, density distribution,
composition & abundance of dust grains etc. In all three cases, the best fit
model corresponds to the uniform density distribution.Comment: AAMS style manuscript with 3 tables (in a separate file) and 4
figures. To appear in Journal of Astronophysics & Astronom