355 research outputs found
A study of the neglected Galactic HII region NGC 2579 and its companion ESO 370-9
The Galactic HII region NGC 2579 has stayed undeservedly unexplored due to
identification problems which persisted until recently. Both NGC 2579 and its
companion ESO 370-9 have been misclassified as planetary or reflection nebula,
confused with each other and with other objects. Due to its high surface
brightness, high excitation, angular size of few arcminutes and relatively low
interstellar extinction, NGC 2579 is an ideal object for investigations in the
optical range. Located in the outer Galaxy, NGC 2579 is an excellent object for
studying the Galactic chemical abundance gradients. In this paper we present
the first comprehensive observational study on the nebular and stellar
properties of NGC 2579 and ESO 370-9, including the determination of electron
temperature, density structure, chemical composition, kinematics, distance, and
the identification and spectral classification of the ionizing stars, and
discuss the nature of ESO 370-9. Long slit spectrophotometric data in the
optical range were used to derive the nebular electron temperature, density and
chemical abundances and for the spectral classification of the ionizing star
candidates. Halpha and UBV CCD photometry was carried out to derive stellar
distances from spectroscopic parallax and to measure the ionizing photon flux.Comment: To be published in Astronomy & Astrophysic
An ASCA Study of the W51 Complex
We present the analysis of ASCA archival data from the Galactic source W51.
The ASCA spectra show that the soft (kT<= 2.5 keV) X-rays are of thermal origin
and are compatible with W51C being a single, isothermal (kT~0.3 keV) supernova
remnant at the far-side of the Sagittarius arm. The ASCA images reveal hard
(kT>=2.5 keV) X-ray sources which were not seen in previous X-ray observations.
Some of these sources are coincident with massive star-forming regions and the
spectra are used to derive X-ray parameters. By comparing the X-ray absorbing
column density with atomic hydrogen column density, we infer the location of
star-forming regions relative to molecular clouds. There are unidentified hard
X-ray sources superposed on the supernova remnant and we discuss the
possibility of their association.Comment: 13 pages, 11 figures, to be published in Astronomical Journa
EXPERIMENTAL ANALYSIS OF PRESSURE DROP IN SINGLE AND TWO PHASE IN MINI CHANNELS
Evaporators with mini and micro channels are one of the main focuses in the design and development of equipment applied to compact refrigeration systems. The objective of this work is to investigate pressure drop of natural refrigerant, isobutane (R-600a), in the single-phase flow through two small tubes, with 1.0 mm and 2.6 mm of internal diameter. Also, the pressure drop was analyzed in the boiling flow in a 2.6 mm internal diameter tube. The experimental tests included mass velocities of 188, 240, 280 and 370 kg/(mÂČs), heat fluxes in the range from 0 to 134 kW/mÂČ and boiling flow the saturation temperature of 22 ÂșC and vapor quality up to 0.8. It was possible to observe the significant influence of the diameter and mass velocity on the total pressure drop and the frictional pressure drop, respectively. The experimental frictional pressure drop in flow boiling in 2.6 mm of internal diameter was compared with four different correlations in literature
Wolf-Rayet Galaxies in the Sloan Digital Sky Survey: the metallicity dependence of the initial mass function
We use a large sample of 174 Wolf-Rayet (WR) galaxies drawn from the Sloan
Digital Sky Survey to study whether and how the slope of the stellar initial
mass function depends on metallicity. We calculate for each object its oxygen
abundance according to which we divide our sample into four metallicity
subsamples. For each subsample, we then measure three quantities: the
equivalent width of \hb emission line, the equivalent width of WR bump around
4650\AA, and the WR bump-to-\hb intensity ratio, and compare to the predictions
of the same quantities by evolutionary synthesis models of Schaerer & Vacca.
Such comparisons lead to a clear dependence of the slope of initial mass
function () on metallicity in that galaxies at higher metallicities
tend to have steeper initial mass functions, with the slope index ranging from
1.00 for the lowest metallicity of to 3.30
for the highest metallicity . We have carefully examined the possible
sources of systematic error either in models or in our observational
measurements and shown that these sources do not change this result.Comment: 12 pages, 6 figures, ApJ accepte
A Survey of Hydroxyl Toward Supernova Remnants: Evidence for Extended 1720 MHz Maser Emission
We present the results of GBT observations of all four ground-state hydroxyl
(OH) transitions toward 15 supernova remnants (SNRs) which show OH(1720 MHz)
maser emission. This species of maser is well established as an excellent
tracer of an ongoing interaction between the SNR and dense molecular material.
For the majority of these objects we detect significantly higher flux densities
with a single dish than has been reported with interferometric observations. We
infer that spatially extended, low level maser emission is a common phenomenon
that traces the large-scale interaction in maser-emitting SNRs. Additionally we
use a collisional pumping model to fit the physical conditions under which OH
is excited behind the SNR shock front. We find the observed OH gas associated
with the SNR interaction having columns less than approximately 10^17 per
square cm, temperatures of 20 to 125 K, and densities 10^5 per cubic cm.Comment: 24 pages, 23 figures, Accepted to ApJ, March 26, 2008; v2 - added
Figure 6, minor clarifications to text in Sections 3 and
Metallicity determination in gas-rich galaxies with semiempirical methods
A study of the precision of the semiempirical methods used in the
determination of the chemical abundances in gas-rich galaxies is carried out.
In order to do this the oxygen abundances of a total of 438 galaxies were
determined using the electronic temperature, the and the P methods.
The new calibration of the P method gives the smaller dispersion for the low
and high metallicity regions, while the best numbers in the turnaround region
are given by the method. We also found that the dispersion correlates
with the metallicity. Finally, it can be said that all the semiempirical
methods studied here are quite insensitive to metallicity with a value of
dex for more than 50% of the total sample.
\keywords{ISM: abundances; (ISM): H {\sc ii} regions}Comment: 26 pages, 9 figures and 2 tables. To appear at AJ, January 200
An environment to share in-service training on the net: An action-research about charge in primary and middle school
The net was one of the determining factors in a training of a Group of 4 primary and middle school teachers (L. M. in Forni Di Sopra and C. D. F. in Villa Santina, P. C. in Tolmezzo and D. S. in Gemona, all places in the mountains
of Friuli, in the Nord-East of Italy) in an action-research on the electric charge. It was developed through meetings with researchers in physics education and Exchange and sharing of experiences and projects on the net, in an environment designed for teaching activities and under the guidance of a project tutor. The environment allowed to establish different kinds of relations: between teachers, between them and researchers, between classes. It offered to teachers (two in middle school and two in primary school) a place for discussions during the design of paths to be presented to their classes, to structure and share the products of their training both during and after it, to reflect on their training path
Benefits from using combined dynamical-statistical downscaling approaches - Lessons from a case study in the Mediterranean region
Abstract. Various downscaling techniques have been developed to bridge the scale gap between global climate models (GCMs) and finer scales required to assess hydrological impacts of climate change. Such techniques may be grouped into two downscaling approaches: the deterministic dynamical downscaling (DD) and the statistical downscaling (SD). Although SD has been traditionally seen as an alternative to DD, recent works on statistical downscaling have aimed to combine the benefits of these two approaches. The overall objective of this study is to assess whether a DD processing performed before the SD permits to obtain more suitable climate scenarios for basin scale hydrological applications starting from GCM simulations. The case study presented here focuses on the Apulia region (South East of Italy, surface area about 20 000 km2), characterised by a typical Mediterranean climate; the monthly cumulated precipitation and monthly mean of daily minimum and maximum temperature distribution were examined for the period 1953â2000. The fifth-generation ECHAM model from the Max-Planck-Institute for Meteorology was adopted as GCM. The DD was carried out with the Protheus system (ENEA), while the SD was performed through a monthly quantile-quantile correction. The SD resulted efficient in reducing the mean bias in the spatial distribution at both annual and seasonal scales, but it was not able to correct the miss-modelled non-stationary components of the GCM dynamics. The DD provided a partial correction by enhancing the spatial heterogeneity of trends and the long-term time evolution predicted by the GCM. The best results were obtained through the combination of both DD and SD approaches
Feedback in the Antennae Galaxies (NGC 4038/9): I. High-Resolution Infrared Spectroscopy of Winds from Super Star Clusters
We present high-resolution (R ~ 24,600) near-IR spectroscopy of the youngest
super star clusters (SSCs) in the prototypical starburst merger, the Antennae
Galaxies. These SSCs are young (3-7 Myr old) and massive (10^5 - 10^7 M_sun for
a Kroupa IMF) and their spectra are characterized by broad, extended Br-gamma
emission, so we refer to them as emission-line clusters (ELCs) to distinguish
them from older SSCs. The Brgamma lines of most ELCs have supersonic widths
(60-110 km/s FWHM) and non-Gaussian wings whose velocities exceed the clusters'
escape velocities. This high-velocity unbound gas is flowing out in winds that
are powered by the clusters' massive O and W-R stars over the course of at
least several crossing times. The large sizes of some ELCs relative to those of
older SSCs may be due to expansion caused by these outflows; many of the ELCs
may not survive as bound stellar systems, but rather dissipate rapidly into the
field population. The observed tendency of older ELCs to be more compact than
young ones is consistent with the preferential survival of the most
concentrated clusters at a given age.Comment: Accepted to Ap
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