9,485 research outputs found
Photometric studies of some starburst galaxies
We present the results of a detailed morphological analysis of ten starburst
galaxies selected from the Markarian catalogue of uv-excess objects. CCD
surface photometry of these galaxies was carried out based on observations made
in B, V (Johnson) and R, I (Kron-Cousins) band passes. We present the radial
variations of surface brightness, ellipticity, position angle and the colour
indices for each galaxy obtained using ellipse fitting isophotal analysis. The
residual images constructed for extracting the fine structure are also
presented. A variety of morphological types are found to host the starburst
phenomenon. The star formation activity is not confined to the nuclear region
alone, but it also occurs at various locations in the galaxy and is seen as
clumpy regions. The colour index and the residual images are used for deriving
information about the sites of enhanced star formation activity and the
triggers of the starburst. The luminosity profiles show an exponential
behaviour in the outer region. The disk scale lengths and the half-light radii
are derived. The contribution of the burst component has been estimated and the
colours of the burst component are presented. Strong isophotal twisting is
detected in all the S0 and E galaxies: Mrk 1002, Mrk 1308 and Mrk 14, in the
sample. This is accompanied by boxiness in some cases, suggesting that a merger
is responsible for the starburst activity in these galaxies. In case of
isolated spirals, a bar or a central oval distortion appear to be the likely
trigger for the starburst.Comment: 12 pages of text and 28 figures. Uses aastex. To be published in A&A
Intra-night Optical Variability of Luminous Radio Quiet QSOs
In the present paper we report the detection of intra-night variability in
some of the RQQSOs and one LDQ. To study intra-night variability, we carried
out photometric monitoring of seven RQQSOs and one LDQ in Johnson V-passband
using 1.2 meter optical/IR telescope at Gurushikhar, Mount Abu, India.
Observations were made in nine nights during the first half of the year 2000;
seven RQQSOs: 0748+291, 0945+438, 1017+280, 1029+329, 1101+319, 1225+317,
1252+020 and one LDQ: 1103-006 were observed. RQQSOs 0748+291, 1225+317 and LDQ
1103-006 have shown existence of intra-night variations. In the case of
1017+280 (RQQSO) there is indication of intra-night variation in one night
where as the observations in another night do not show convincingly the
existence of intra-night variability. RQQSOs 0945+438, 1029+329, 1101+319 and
1252+020 have not shown any intra-night variations. We compiled intra-night
variability data for radio-loud and radio-quiet AGNs from the literature for
statistical analysis. It is found that a good fraction of radio-quiet AGNs show
intra-night variations with the maximum amplitude of variation being about 10%.
On the other hand blazars show at times intra-night flux variability up to
100%. In case of radio-loud AGNs (excluding blazars), maximum amplitude of
intra-night variation lies between the variability amplitude of radio-quiet
AGNs and blazars i.e. the flux variation is close to 50%. The results indicate
that the energy generation mechanism and the environment around the central
engine in different classes of AGNs may be similar, if not identical. The
standard model for radio-loud AGNs, where shocks are propagating down
relativistic jet or models based on disturbances in accretion disks can also
explain the micro-variability in RQQSOs.Comment: 12 pages, 12 figures, Accepted for publication in A & A, uses aa.cl
Naked Singularities in Higher Dimensional Szekeres Space-time
In this paper we study the quasi-spherical gravitational collapse of (n+2)
dimensional Szekeres space-time. The nature of the central shell focusing
singularity so formed is analyzed by studying both the radial null and
time-like geodesic originated from it. We follow the approach of Barve et al to
analyze the null geodesic and find naked singularity in different situations.Comment: 11 Latex Pages, 4 figures, RevTex styl
Optical polarimetry and photometry of comet 17P/Holmes
Comet 17P/Holmes was observed for linear polarisation using the optical
polarimeter mounted on the 1.2m telescope atop Gurushikhar peak near Mt. Abu
during the period November-December 2007. Observations were conducted through
the IHW narrow band (continuum) filters. During the observing run the phase
angle was near at which the comet showed negative polarisation. On
the basis of the observed polarisation data we find comet 17P/Holmes to be a
typical comet with usual dust characteristics. We note that radial rate of
change of brightness in coma in red band is higher than that in blue band; it
has decreased by a factor of 3.6 and 2.5 respectively in red and blue bands
during the November - December run, indicating relative increase in the
abundance of smaller dust particles out ward. Radial brightness variation seen
near the nucleus on November 6 is indicative of the presence of a blob or
shocked region beyond 10" from the nucleus which has gradually smoothened by
December 13. The brightness distribution is found steeper during November 5-7
as compared to on December 13.Comment: 11pages, 7 figures, accepted for publication in MNRA
Chromospheric plasma ejection above a pore
We present high spatial resolution observations of short lived transients,
ribbon and jets like events above a pore in Ca II H images where fine structure
like umbral dots, lightbridge and penumbral micro filaments are present in the
underlying photosphere. We found that current layers are formed at the edges of
convective fine structure due to the shear between their horizontal field and
the ambient vertical field. High vertical electric current density patches are
observed in the photosphere around these events which indicates the formation
of a current sheet at the reconnection site. In the framework of past studies,
low altitude reconnection could be the mechanism that produces such events. The
reconnection is caused by an opposite polarity field produced by the bending of
field lines by convective downflows at the edge of the pore fine structures.Comment: 10 pages, 5 figures, accepted for publication in MNRA
Impact of vegetation on the simulation of seasonal monsoon rainfall over the Indian subcontinent using a regional model
The change in the type of vegetation fraction can induce major changes in the local effects such as local evaporation, surface radiation, etc., that in turn induces changes in the model simulated outputs. The present study deals with the effects of vegetation in climate modeling over the Indian region using the MM5 mesoscale model. The main objective of the present study is to investigate the impact of vegetation dataset derived from SPOT satellite by ISRO (Indian Space Research Organization) versus that of USGS (United States Geological Survey) vegetation dataset on the simulation of the Indian summer monsoon. The present study has been conducted for five monsoon seasons (1998-2002), giving emphasis over the two contrasting southwest monsoon seasons of 1998 (normal) and 2002 (deficient). The study reveals mixed results on the impact of vegetation datasets generated by ISRO and USGS on the simulations of the monsoon. Results indicate that the ISRO data has a positive impact on the simulations of the monsoon over northeastern India and along the western coast. The MM5-USGS has greater tendency of overestimation of rainfall. It has higher standard deviation indicating that it induces a dispersive effect on the rainfall simulation. Among the five years of study, it is seen that the RMSE of July and JJAS (June-July-August-September) for All India Rainfall is mostly lower for MM5-ISRO. Also, the bias of July and JJAS rainfall is mostly closer to unity for MM5-ISRO. The wind fields at 850 hPa and 200 hPa are also better simulated by MM5 using ISRO vegetation. The synoptic features like Somali jet and Tibetan anticyclone are simulated closer to the verification analysis by ISRO vegetation. The 2 m air temperature is also better simulated by ISRO vegetation over the northeastern India, showing greater spatial variability over the region. However, the JJAS total rainfall over north India and Deccan coast is better simulated using the USGS vegetation. Sensible heat flux over north-west India is also better simulated by MM5-USGS
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