13 research outputs found
The first spectral line surveys searching for signals from the Dark Ages
Our aim is to observationally investigate the cosmic Dark Ages in order to
constrain star and structure formation models, as well as the chemical
evolution in the early Universe. Spectral lines from atoms and molecules in
primordial perturbations at high redshifts can give information about the
conditions in the early universe before and during the formation of the first
stars in addition to the epoch of reionisation. The lines may arise from moving
primordial perturbations before the formation of the first stars (resonant
scattering lines), or could be thermal absorption or emission lines at lower
redshifts. The difficulties in these searches are that the source redshift and
evolutionary state, as well as molecular species and transition are unknown,
which implies that an observed line can fall within a wide range of
frequencies. The lines are also expected to be very weak. Observations from
space have the advantages of stability and the lack of atmospheric features
which is important in such observations. We have therefore, as a first step in
our searches, used the Odin satellite to perform two sets of spectral line
surveys towards several positions. The first survey covered the band 547-578
GHz towards two positions, and the second one covered the bands 542.0-547.5 GHz
and 486.5-492.0 GHz towards six positions selected to test different sizes of
the primordial clouds. Two deep searches centred at 543.250 and 543.100 GHz
with 1 GHz bandwidth were also performed towards one position. The two lowest
rotational transitions of H2 will be redshifted to these frequencies from
z~20-30, which is the predicted epoch of the first star formation. No lines are
detected at an rms level of 14-90 and 5-35 mK for the two surveys,
respectively, and 2-7 mK in the deep searches with a channel spacing of 1-16
MHz. The broad bandwidth covered allows a wide range of redshifts to be
explored for a number of atomic and molecular species and transitions. From the
theoretical side, our sensitivity analysis show that the largest possible
amplitudes of the resonant lines are about 1 mK at frequencies <200 GHz, and a
few micro K around 500-600 GHz, assuming optically thick lines and no
beam-dilution. However, if existing, thermal absorption lines have the
potential to be orders of magnitude stronger than the resonant lines. We make a
simple estimation of the sizes and masses of the primordial perturbations at
their turn-around epochs, which previously has been identified as the most
favourable epoch for a detection. This work may be considered as an important
pilot study for our forthcoming observations with the Herschel Space
Observatory.Comment: 15 pages, 9 figures, 3 on-line pages. Accepted for publication in
Astronomy & Astrophysics 8 March 2010
A search for Ejecta Nebulae around Wolf-Rayet Stars using the SHS H survey
Recent large scale Galactic Plane H surveys allow a re-examination of
the environs of Wolf-Rayet (WR) stars for the presence of a circumstellar
nebula. Using the morphologies of WR nebulae known to be composed of stellar
ejecta as a guide, we constructed ejecta nebula criteria similar to those of
Chu (1991) and searched for likely WR ejecta nebula in the SHS H
survey. A new Wolf-Rayet ejecta nebula around WR 8 is found and its morphology
discussed. The fraction of WR stars with ejecta type nebulae is roughly
consistent between the MilkyWay (MW) and LMC at around 5-6%, with the MW sample
dominated by nitrogen rich WR central stars (WN type) and the LMC stars having
a higher proportion of carbon rich WR central stars (WC type). We compare our
results with those of previous surveys, including those of Marston (1997) and
Miller & Chu (1993), and find broad consistency. We investigate several trends
in the sample: most of the clear examples of ejecta nebulae have WNh central
stars; and very few ejecta nebulae have binary central stars. Finally, the
possibly unique evolutionary status of the nebula around the binary star WR 71
is explored.Comment: 13 pages, 18 figures; Accepted for publication in MNRA
Synchrotron superbubble in the galaxy IC10: the ionized gas structure, kinematics, and emission spectrum
We have investigated the structure, kinematics, and emission spectrum of the
ionized gas in the synchrotron superbubble in the irregular galaxy IC 10 based
on observations with the 6-m Special Astrophysical Observatory telescope with
the SCORPIO focal reducer in three modes: direct imaging in the
[SII](6717+6731)A lines, long-slit spectroscopy, and spectroscopy with a
scanning Fabry--Perot interferometer. We have identified a bright (in the [SII]
lines) filamentary optical shell and determined its expansion velocity, mass,
and kinetic energy. The nature of the object is discussed.Comment: 12 pages, 9 EPS figure
CLOUD STRUCTURE OF INTERSTELLAR MATTER. OBSERVATIONAL PARAMETERS
Parameters of 7600 HI clouds were determined on the base of HI RATAN-600 Survey in the second and third quadrants of galactic longitudes. The spectra of cloud linear diameters, HI densities and masses are obtained first for such huge population of clouds. Mass spectrum of HI clouds shows that, in middle mass range, the process of coalescence in cloud{cloud collisions predominates, but the clouds with low masses are probably evaporated due to the very hot ISM component. It is found that mean clouds linear diameters along Galactic plane are 2.5 times greater than in transverse direction. The relation between HI concentrations and cloud diameters is obtained in the form of nH ∞d 1.25±0.01 probably regardless of selection effects. It is shown that velocity dispersion does not depend on cloud diameters, as distinct from molecular clouds. It is found that 16% of HI clouds have systematic velocity gradients across cloud disks that is may be due to clouds rotation. Mean clouds angular rotational velocity is about 5x10 -14s -1 and observable quantities of clouds with opposite directions of rotation are equal within 5% in both galactic quadrants investigated