298 research outputs found
Discovery of optical candidate supernova remnants in Sagittarius
During an [O III] survey for planetary nebulae, we identified a region in
Sagittarius containing several candidate Supernova Remnants and obtained deep
optical narrow-band images and spectra to explore their nature. The images of
the unstudied area have been obtained in the light of Halpha+[N II], [S II] and
[O III]. The resulting mosaic covers an area of 1.4x1.0 deg^2 where filamentary
and diffuse emission was discovered, suggesting the existence of more than one
supernova remnants (SNRs) in the area. Deep long slit spectra were also taken
of eight different regions. Both the flux calibrated images and the spectra
show that the emission from the filamentary structures originates from
shock-heated gas, while the photo-ionization mechanism is responsible for the
diffuse emission. Part of the optical emission is found to be correlated with
the radio at 4850 MHz suggesting their association, while the WISE infrared
emission found in the area at 12 and 22 micron marginally correlates with the
optical. The presence of the [O III] emission line in one of the candidate SNRs
suggests shock velocities into the interstellar "clouds" between 120 and 200
km/s, while the absence in the other indicates slower shock velocities. For all
candidate remnants the [S II] 6716/6731 ratio indicates electron densities
below 240 cm^{-3}, while the Halpha emission has been measured to be between
0.6 to 41x10^{-17} erg/s/cm^2/arcsec^2. The existence of eight pulsars within
1.5deg away from the center of the candidate SNRs also supports the scenario of
many SNRs in the area as well as that the detected optical emission could be
part of a number of supernovae explosions.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Oscillations and running waves observed in sunspots. III. Multilayer study
We continue our study of waves and oscillations observed in sunspots using an improved method for enhancing the waves, giving the opportunity to identify them and determine their properties in far Hα wings. We found that the running penumbral waves are observable at least up to the formation height of the Hα ± 0.5 Å line, but not in the H Hα ± 0.75 Å or the FeI ± 0.12 Å. We found a time lag between the waves in the blue and the red wing of the Hα line corresponding to a phase shift of 180°, that indicates a pure Doppler shift of the line. There is a lag in the propagation of the waves seen at Hα center and at Hα wings. Also there is a lag in the variation of the umbral oscillations as they are observed from lower to higher atmospheric layers. The correlation between umbral oscillations at various atmospheric heights and running penumbral waves strongly indicates that the latter are excited by photospheric umbral oscillations and not the chromospheric ones. We found a new category of photospheric waves that originate at approximately 0.7 of the distance between the umbra and the penumbra boundary and propagate beyond the outer penumbra boundary with a velocity of the order of 3-4 km s^(-1). Further, we found 3 min penumbral oscillations apparent in the inner penumbra at lower chromospheric layers (far Hα wings)
Chromospheric Evershed flow
We studied the chromospheric Evershed flow from filtergrams obtained at nine wavelengths along the Hα profile. We computed line-of-sight velocities based on Becker's cloud model and we determined the components of the flow velocity vector as a function of distance from the center of the sunspot, assuming an axial symmetry of both the spot and the flow. We found that the flow velocity decreases with decreasing height and that the maximum of the velocity shifts towards the inner penumbral boundary. The flow related to some fibrils deviates significantly from the average Evershed flow. The profile of the magnitude of the flow velocity as a function of distance from the spot center, indicates that the velocity attains its maximum value in the downstream part of the flow channels (assumed to have the form of a loop). This behavior can be understood in terms of a critical flow that pass from subsonic to supersonic near the apex of the loop, attains its higher velocity at the downstream part of the loop and finally relaxes to subsonic through a tube shock. We computed the average flow vector from segmented line-of-sight velocity maps, excluding bright or dark fibrils alternatively. We found that the radial component of the velocity does not show a significant difference, but the magnitude of the vertical component of the velocity related to dark fibrils is higher than that related to bright fibrils.
Deep Halpha imagery of the Eridanus shells
A deep \ha image of interlocking filamentary arcs of nebulosity has been
obtained with a wide-field ( 30\degree diameter) narrow-band filter
camera combined with a CCD as a detector. The resultant mosaic of images,
extending to a galactic latitude of 65, has been corrected for field
distortions and had galactic coordinates superimposed on it to permit accurate
correlations with the most recent H{\sc i} (21 cm), X-ray (0.75 kev) and FIR
(IRAS 100 m) maps.
Furthermore, an upper limit of 0.13 arcsec/yr to the expansion proper motion
of the primary 25\degree long nebulous arc has been obtained by comparing a
recent \ha image obtained with the San Pedro Martir telescope of its
filamentary edge with that on a POSS E plate obtained in 1951.
It is concluded that these filamentary arcs are the superimposed images of
separate shells (driven by supernova explosions and/or stellar winds) rather
than the edges of a single `superbubble' stretching from Barnard's Arc (and the
Orion Nebula) to these high galactic latitudes. The proper motion measurement
argues against the primary \ha emitting arc being associated with the giant
radio loop (Loop 2) except in extraordinary circumstances.Comment: 9 pages, 5 figures, accepted for MNRAS publicatio
An assessment to evaluate potential passive cooling patterns for climate change adaptation in a residential neighbourhood of a Mediterranean coastal city (Athens, Greece)
© 2018 Inderscience Enterprises Ltd. This study investigates the potential for passive cooling patterns inside the urban fabric in the Mediterranean climate city of Athens (Greece), especially with regard to quantify air temperature reduction and thermal comfort amelioration at the neighbourhood scale. Using both field measurements and an urban microclimate simulation model, we assessed cooling and warming patterns in various sites of an Athens residential neighbourhood. Results show that, under Mediterranean climate conditions, urban design elements such as wooded courtyards and appropriately oriented urban design elements such as galleries have a considerable cooling effect and can be used as cool places inside the neighbourhood for occupants\u27 comfort amelioration and also as passive cooling tools for buildings to reduce summer energy consumption. They may then function as passive design strategies to adapt the urban site form to different climate change scenarios
An Assessment To Evaluate Potential Passive Cooling Patterns For Climate Change Adaptation In A Residential Neighbourhood Of A Mediterranean Coastal City (Athens, Greece)
This study investigates the potential for passive cooling patterns inside the urban fabric in the Mediterranean climate city of Athens (Greece), especially with regard to quantify air temperature reduction and thermal comfort amelioration at the neighbourhood scale. Using both field measurements and an urban microclimate simulation model, we assessed cooling and warming patterns in various sites of an Athens residential neighbourhood. Results show that, under Mediterranean climate conditions, urban design elements such as wooded courtyards and appropriately oriented urban design elements such as galleries have a considerable cooling effect and can be used as cool places inside the neighbourhood for occupants\u27 comfort amelioration and also as passive cooling tools for buildings to reduce summer energy consumption. They may then function as passive design strategies to adapt the urban site form to different climate change scenarios
An Updated Catalog of 4680 Northern Eclipsing Binaries with Algol-Type light curve morphology in the Catalina Sky Surveys
We present an updated catalog of 4680 northern eclipsing binaries (EBs) with
Algol-type light curve morphology (i.e., with well-defined beginning and end of
primary and secondary eclipses), using data from the Catalina Sky Surveys. Our
work includes revised period determinations, phenomenological parameters of the
light curves, and system morphology classification based on machine learning
techniques. While most of the new periods are in excellent agreement with those
provided in the original Catalina catalogs, improved values are now available
for ~10% of the stars. A total of 3456 EBs were classified as detached and 449
as semi-detached, while 145 cannot be classified unambiguously into either
subtype. The majority of the SD systems seems to be comprised of short-period
Algols. By applying color criteria, we searched for K- and M-type dwarfs in
these data, and present a subsample of 609 EB candidates for further
investigation. We report 119 EBs (2.5% of the total sample) that show maximum
quadrature light variations over long timescales, with periods bracketing the
range 4.5-18 yrs and fractional luminosity variance of 0.04-0.13. We discuss
possible causes for this, making use of models of variable starspot activity in
our interpretation of the results
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