55 research outputs found
H2O maser emission from bright rimmed clouds in the southern hemisphere
Water maser emission is a powerful tracer of the presence of embedded sources in dense clouds since it requires elevated
temperatures (>100 K) and densities (>107 cmâ3) that can be found in circumstellar disks and/or jets/outflows associated with Young
Stellar Objects. Bright rimmed clouds compressed by ionization fronts from nearby massive stars are considered good examples of
externally triggered star formation, possibly resulting in the formation of massive stars.
Aims. We aim to determine the water maser emission frequency and characteristics of 45 bright rimmed clouds in the southern
hemisphere identified by Sugitani & Ogura (1994, ApJS, 92, 163).
Methods. We have used the Tidbinbilla 70-m radiotelescope to perform a high sensitivity survey at 22.2 GHz of the maser emission
from the 616â523 rotational transition of H2O molecules.
Results. We found 7 water maser sources out of 44 (16% detection rate), 5 being new detections. With the exception of the maser
associated with BRC 68, all the other maser are characterized by low integrated fluxes and luminosities.
Conclusions. Most maser sources fall below the correlation between the H2O and far-infrared luminosity found in other studies
towards a variety of star forming regions. These results are similar to those found in the companion survey of BRCs in the northern
hemisphere by Valdettaro et al. (2005, A&A, 443, 535). The low detection frequency and the properties of water maser emission from
BRCs indicate that low-mass star formation is the most natural outcome of the external compression induced by the ionization front
from nearby massive stars
Revisiting the relativistic ejection event in XTE J1550-564 during the 1998 outburst
We revisit the discovery outburst of the X-ray transient XTE J1550â564 during which relativistic jets were observed in 1998 September, and review the radio images obtained with the Australian Long Baseline Array, and light curves obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array. Based on Hi spectra, we constrain the source distance to between 3.3 and 4.9 kpc. The radio images, taken some 2 d apart, show the evolution of an ejection event. The apparent separation velocity of the two outermost ejecta is at least 1.3c and may be as large as 1.9c; when relativistic effects are taken into account, the inferred true velocity is â„ 0.8c. The flux densities appear to peak simultaneously during the outburst, with a rather flat (although still optically thin) spectral index of â0.2
Radio variability of 1st 3-months Fermi blazars at 5 GHz: affected by interstellar scintillation?
Blazars from the first-three-months Fermi-AGN list were observed with the
Urumqi 25m radio telescope at 5GHz in IDV (Intra-Day Variability) mode and
inter-month observation mode. A significant correlation between the flux
density at 5GHz and the gamma-ray intensity for the Fermi-LAT detected blazars
is seen. There is a higher IDV detection rate in Fermi detected blazars than
those reported for other samples. Stronger variability appears at lower
Galactic latitudes; IDV appears to be stronger in weaker sources, indicating
that the variability is affected by interstellar scintillation.Comment: 4 pages, 4 figures, in proceedings of 'Multiwavelength Variability of
Blazars' in Guangzhou Uni. of China, 22-24, Sep. 2010, to appear in JA
The Hubble Constant
I review the current state of determinations of the Hubble constant, which
gives the length scale of the Universe by relating the expansion velocity of
objects to their distance. There are two broad categories of measurements. The
first uses individual astrophysical objects which have some property that
allows their intrinsic luminosity or size to be determined, or allows the
determination of their distance by geometric means. The second category
comprises the use of all-sky cosmic microwave background, or correlations
between large samples of galaxies, to determine information about the geometry
of the Universe and hence the Hubble constant, typically in a combination with
other cosmological parameters. Many, but not all, object-based measurements
give values of around 72-74km/s/Mpc , with typical errors of 2-3km/s/Mpc.
This is in mild discrepancy with CMB-based measurements, in particular those
from the Planck satellite, which give values of 67-68km/s/Mpc and typical
errors of 1-2km/s/Mpc. The size of the remaining systematics indicate that
accuracy rather than precision is the remaining problem in a good determination
of the Hubble constant. Whether a discrepancy exists, and whether new physics
is needed to resolve it, depends on details of the systematics of the
object-based methods, and also on the assumptions about other cosmological
parameters and which datasets are combined in the case of the all-sky methods.Comment: Extensively revised and updated since the 2007 version: accepted by
Living Reviews in Relativity as a major (2014) update of LRR 10, 4, 200
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