5,328 research outputs found
A Physical Limit to the Magnetic Fields of T Tauri Stars
Recent estimates of magnetic field strengths in T Tauri stars yield values
--. In this paper, I present an upper limit to the
photospheric values of by computing the equipartition values for different
surface gravities and effective temperatures. The values of derived from
the observations exceed this limit, and I examine the possible causes for this
discrepancy
Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275
Far ultraviolet emission has been detected from a knot of Halpha emission in
the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected
relative to the brightness of the Halpha line in the same spatial region is
very close to that expected from Hydrogen two-photon continuum emission in the
particle heating model of Ferland et al. (2009) if reddening internal to the
filaments is taken into account. We find no need to invoke other sources of far
ultraviolet emission such as hot stars or emission lines from CIV in
intermediate temperature gas to explain these data.Comment: 9 pages, 8 figures. Accepted for publication in MNRA
Cyanobacteria blooms cannot be controlled by effective microorganisms (EM) from mud- or Bokashi-balls
In controlled experiments, the ability of ‘‘Effective Microorganisms (EM, in the form of mudballs or Bokashi-balls)’’ was tested for clearing waters from cyanobacteria. We found suspensions of EM-mudballs up to 1 g l-1 to be ineffective in reducing cyanobacterial growth. In all controls and EM-mudball treatments up to 1 g l-1 the cyanobacterial chlorophyll-a (Chl-a) concentrations increased within 4 weeks from&120 to 325–435 lg l-1. When pieces of EM-mudballs (42.5 g) were added to 25-l lake water with cyanobacteria, no decrease of cyanobacteria as compared to untreated controls was observed. In contrast, after 4 weeks cyanobacterial Chl-a concentrations were significantly higher in EM-mudball treatments (52 lg l-1) than in controls (20 lg l-1). Only when suspensions with extremely high EM-mudball concentrations were applied (i.e., 5 and 10 g l-1), exceeding the recommended concentrations by orders of magnitude, cyanobacterial growth was inhibited and a bloom forming concentration was reduced strongly. In these high dosing treatments, the oxygen concentration dropped initially to very low levels of 1.8 g l-1. This was most probably through forcing strong light limitation on the cyanobacteria caused by the high amount of clay and subsequent high turbidity of the water. Hence, this study yields no support for the hypothesis that EM is effective in preventing cyanobacterial proliferation or in terminating blooms. We consider EM products to be ineffective because they neither permanently bind nor remove phosphorus from eutroficated systems, they have no inhibiting effect on cyanobacteria, and they could even be an extra source of nutrients
ASCA and ROSAT observations of nearby cluster cooling flows
We present a detailed analysis of the X-ray properties of the cooling flows
in a sample of nearby, X-ray bright clusters of galaxies using high-quality
ASCA spectra and ROSAT X-ray images. We demonstrate the need for multiphase
models to consistently explain the spectral and imaging X-ray data for the
clusters. The mass deposition rates of the cooling flows, independently
determined from the ASCA spectra and ROSAT images, exhibit reasonable
agreement. We confirm the presence of intrinsic X-ray absorption in the
clusters using a variety of spectral models. We also report detections of
extended m infrared emission, spatially coincident with the cooling
flows, in several of the systems studied. The observed infrared fluxes and flux
limits are in good agreement with the predicted values due to reprocessed X-ray
emission from the cooling flows. We present precise measurements of the
abundances of iron, magnesium, silicon and sulphur in the central regions of
the Virgo and Centaurus clusters. Our results firmly favour models in which a
high mass fraction (70-80 per cent) of the iron in the X-ray gas in these
regions is due to Type Ia supernovae. Finally, we present a series of methods
which may be used to measure the ages of cooling flows from the X-ray data. The
results for the present sample of clusters indicate ages of between 2.5 and 7
Gyr. If the ages of cooling flows are primarily set by subcluster merger
events, then our results suggest that in the largest clusters, mergers with
subclusters with masses of approximately 30 per cent of the final cluster mass
are likely to disrupt cooling flows.Comment: Final version. MNRAS, in press. 36 pages, 9 figs, 14 tables in MNRAS
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Collisional excitation of [C II], [O I] and CO in Massive Galaxies
Many massive galaxies at the centres of relaxed galaxy clusters and groups
have vast reservoirs of cool (~10,000 K) and cold (~100 K) gas. In many low
redshift brightest group and cluster galaxies this gas is lifted into the hot
ISM in filamentary structures, which are long lived and are typically not
forming stars. Two important questions are how far do these reservoirs cool and
if cold gas is abundant what is the cause of the low star formation efficiency?
Heating and excitation of the filaments from collisions and mixing of hot
particles in the surrounding X-ray gas describes well the optical and near
infra-red line ratios observed in the filaments. In this paper we examine the
theoretical properties of dense, cold clouds emitting in the far infra-red and
submillimeter through the bright lines of [C II]157 \mu m , [O I]63 \mu m and
CO, exposed to these energetic ionising particles. While some emission lines
may be optically thick we find this is not sufficient to model the emission
line ratios. Models where the filaments are supported by thermal pressure
support alone also cannot account for the cold gas line ratios but a very
modest additional pressure support, either from turbulence or magnetic fields
can fit the observed [O I]/[C II] line ratios by decreasing the density of the
gas. This may also help stabilise the filaments against collapse leading to the
low rates of star formation. Finally we make predictions for the line ratios
expected from cold gas under these conditions and present diagnostic diagrams
for comparison with further observations. We provide our code as an Appendix.Comment: 17 pages, submitted to MNRA
New bird records for the island of Lombok
Seventeen species are recorded for the first time from the island of Lombok. Nine of these are maritime birds on passage and four are widely occurring migrant shore and land birds. Gallirallus striatus may be a local migrant, Chrysococcyx basalis is a visitor from Australia, and Anthreples malaccensis is presumed to be either a previously overlooked resident or a recent colonist. Hemiprocne longipenis, previously unrecorded from the Lesser Sundas, appears to be a recent colonist
Filamentary Star Formation in NGC 1275
We examine the star formation in the outer halo of NGC~1275, the central
galaxy in the Perseus cluster (Abell 426), using far ultraviolet and optical
images obtained with the Hubble Space Telescope. We have identified a
population of very young, compact star clusters with typical ages of a few Myr.
The star clusters are organised on multiple-kiloparsec scales. Many of these
star clusters are associated with "streaks" of young stars, the combination of
which has a cometary appearance. We perform photometry on the star clusters and
diffuse stellar streaks, and fit their spectral energy distributions to obtain
ages and masses. These young stellar populations appear to be normal in terms
of their masses, luminosities and cluster formation efficiency; <10% of the
young stellar mass is located in star clusters. Our data suggest star formation
is associated with the evolution of some of the giant gas filaments in NGC~1275
that become gravitationally unstable on reaching and possibly stalling in the
outer galaxy. The stellar streaks then could represent stars moving on
ballistic orbits in the potential well of the galaxy cluster. We propose a
model where star-forming filaments, switched on ~50~Myr ago and are currently
feeding the growth of the NGC~1275 stellar halo at a rate of ~2-3 solar masses
per year. This type of process may also build stellar halos and form isolated
star clusters in the outskirts of youthful galaxies.Comment: 15 pages, 10 figures, accepted for publication in MNRA
A Deep Look at the Emission-Line Nebula in Abell 2597
The close correlation between cooling flows and emission-line nebulae in
clusters of galaxies has been recognized for over a decade and a half, but the
physical reason for this connection remains unclear. Here we present deep
optical spectra of the nebula in Abell 2597, one of the nearest strong
cooling-flow clusters. These spectra reveal the density, temperature, and metal
abundances of the line-emitting gas. The abundances are roughly half-solar, and
dust produces an extinction of at least a magnitude in V. The absence of [O
III] 4363 emission rules out shocks as a major ionizing mechanism, and the
weakness of He II 4686 rules out a hard ionizing source, such as an active
galactic nucleus or cooling intracluster gas. Hot stars are therefore the best
candidate for producing the ionization. However, even the hottest O stars
cannot power a nebula as hot as the one we see. Some other nonionizing source
of heat appears to contribute a comparable amount of power. We show that the
energy flux from a confining medium can become important when the ionization
level of a nebula drops to the low levels seen in cooling-flow nebulae. We
suggest that this kind of phenomenon, in which energy fluxes from the
surrounding medium augment photoelectric heating, might be the common feature
underlying the diverse group of objects classified as LINERS.Comment: 33 Latex pages, including 16 Postscript figures, to appear in 1997
September 1 Astrophysical Journa
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