173 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
Towards an explanation for the 30 Dor (LMC) Honeycomb nebula - the impact of recent observations and spectral analysis
The unique Honeycomb nebula, most likely a peculiar supernova remnant, lies
in 30 Doradus in the Large Magellanic Cloud. Due to its proximity to SN1987A,
it has been serendipitously and intentionally observed at many wavelengths.
Here, an optical spectral analysis of forbidden line ratios is performed in
order to compare the Honeycomb high-speed gas with supernova remnants in the
Galaxy and the LMC, with galactic Wolf-Rayet nebulae and with the optical line
emission from the interaction zone of the SS433 microquasar and W50 supernova
remnant system. An empirical spatiokinematic model of the images and spectra
for the Honeycomb reveals that its striking appearance is most likely due to a
fortuitous viewing angle. The Honeycomb nebula is more extended in soft X-ray
emission and could in fact be a small part of the edge of a giant LMC shell
revealed for the first time in this short wavelength domain. It is also
suggested that a previously unnoticed region of optical emission may in fact be
an extension of the Honeycomb around the edge of this giant shell. A secondary
supernova explosion in the edge of a giant shell is considered for the creation
of the Honeycomb nebula. A microquasar origin of the Honeycomb nebula as
opposed to a simple supernova origin is also evaluated.Comment: 12 pages, 9 figures, accepted for publication in MNRA
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
Multi-band optical micro-variability observations of the BL Lac object S4 0954+658
We have observed S4 0954+658 in the BVRI bands in 2001, and in the BI bands
in 2002. The observations resulted in almost evenly sampled light curves, 3-8
hours long, with an average sampling interval of ~5-15 min. Because of the
dense sampling and the availability of light curves in more than one optical
bands we are able to study the intra-night flux and spectral variability of the
source in detail. Significant observations were observed in all but one cases.
On average, the flux variability amplitude, on time scales of minutes/hours,
increases from ~2-3% in the I, to ~3.5-5.5% in the B band light curves. We do
not detect any flares within the individual light curves. However, there is a
possibility that the April 2001 and late May 2002 observations sample two
flares which lasted longer than ~1-3 days. The evidence is only suggestive
though, due to the limited length of the present light curves with regard to
the duration of the assumed flares. No spectral variations are detected during
the April 2001 observations. The source flux rises and decays with the same
rate, in all bands. This variability behaviour is typical of S4 0954+658, and
is attributed to geometrical effects. However, significant spectral variations
are observed in May 2002. We find that the spectrum hardens/softens as the flux
increases/decreases, respectively. Furthermore, the "hardening" rate of the
energy spectrum is faster than the rate with which the spectrum becomes
"redder" as the flux decays. We also find evidence (although of low statistical
significance) that the I band variations are delayed with respect to the B band
variations. If the May 2002 observations sample a flaring event, these results
suggest that the variations are caused by energetic processes which are
associated with the particle cooling and the source light travel time scales.Comment: 7 pages, 7 figures, accepted for publication in A&
First optical detection from the supernova remnant G 15.1-1.6
Deep optical CCD images of the supernova remnant G 15.1-1.6 were obtained and
filamentary and diffuse emission has been discovered. The images, taken in the
emission lines of Halpha+[N II], [S II] and [O III], reveal filamentary and
diffuse structures all around the remnant. The radio emission at 4850 MHz in
the same area is found to be well correlated with the brightest optical
filaments. The IRAS 60 micron emission may also be correlated with the optical
emission but to a lesser extent. The flux calibrated images suggest that the
optical emission originates from shock-heated gas ([S II]/Halpha > 0.4), while
there is a possible HII region ([S II]/Halpha ~0.3) contaminating the supernova
remnant's emission to the east. Furthermore, deep long-slit spectra were taken
at two bright filaments and also show that the emission originates from shock
heated gas. An [O III] filamentary structure has also been detected further to
the west but it lies outside the remnant's boundaries and possibly is not
associated to it. The [O III] flux suggests shock velocities into the
interstellar "clouds" ~100 km/s, while the [S II] 6716/6731 ratio indicates
electron densities up to ~250 cm^{-3}. Finally, the Halpha emission has been
measured to be between 2 to 7 x 10^{-16} erg/s/cm^2/arcsec^2, while the lower
limit to the distance is estimated at 2.2 kpc.Comment: 12 pages, 6 figures, 3 tables. Accepted for pubication in A&
The distribution of the ISM in the Milky Way A three-dimensional large-scale model
We use the COBE/DIRBE (1.2, 2.2, 60, 100, 140, and 240 m) maps and the
COBE/FIRAS spectra (for the wavelength range 100 - 1000 m) to constrain a
model for the spatial distribution of the dust, the stars, and the gas in the
Milky Way. By assuming exponential axisymmetric distributions for the dust and
the stars and by performing the corresponding radiative transfer calculations
we closely (given the simple geometry of the model) reproduce the FIR and NIR
maps of the Milky Way. Similar distributions for the atomic and molecular
hydrogen in the disk are used (with an inner cut-off radius for the atomic
hydrogen) to fit the gas data. The star formation rate as a function of the
Galactic radius is derived from the FIR emission and is well in agreement with
existing estimates from various star formation tracers. The gas surface density
is plotted against the star formation rate density and an ``intrinsic''
Galactic Schmidt law is derived with excellent agreement with the ``external''
Schmidt law found for spiral galaxies. The Milky Way is found to consume and of its gas in the outer and inner regions respectively (for
a period of 0.1 Gyr) to make stars. The dust-induced B-V color excess observed
in various directions and distances (up to kpc) with well-studied
Cepheid stars is compared with the model predictions showing a good agreement.
The simple assumption of exponential distributions of stars and dust in the
Galaxy is found to be quite instructive and adequate in modeling all the
available data sets from 0.45 m (B-band) to 1000 m.Comment: 14 Pages, 10 figure
Abundant dust found in intergalactic space
Galactic dust constitutes approximately half of the elements more massive
than helium produced in stellar nucleosynthesis. Notwithstanding the formation
of dust grains in the dense, cool atmospheres of late-type stars, there still
remain huge uncertainties concerning the origin and fate of galactic stardust.
In this paper, we identify the intergalactic medium (i.e. the region between
gravitationally-bound galaxies) as a major sink for galactic dust. We discover
a systematic shift in the colour of background galaxies viewed through the
intergalactic medium of the nearby M81 group. This reddening coincides with
atomic, neutral gas previously detected between the group members. The
dust-to-HI mass ratio is high (1/20) compared to that of the solar neighborhood
(1/120) suggesting that the dust originates from the centre of one or more of
the galaxies in the group. Indeed, M82, which is known to be ejecting dust and
gas in a starburst-driven superwind, is cited as the probable main source.Comment: 5 pages, 3 figures, 1 table. ApJ Letters in pres
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