2,263 research outputs found
The Variation of Integrated Star IMFs among Galaxies
The integrated galaxial initial mass function (IGIMF) is the relevant
distribution function containing the information on the distribution of stellar
remnants, the number of supernovae and the chemical enrichment history of a
galaxy. Since most stars form in embedded star clusters with different masses
the IGIMF becomes an integral of the assumed (universal or invariant) stellar
IMF over the embedded star-cluster mass function (ECMF). For a range of
reasonable assumptions about the IMF and the ECMF we find the IGIMF to be
steeper (containing fewer massive stars per star) than the stellar IMF, but
below a few Msol it is invariant and identical to the stellar IMF for all
galaxies. However, the steepening sensitively depends on the form of the ECMF
in the low-mass regime. Furthermore, observations indicate a relation between
the star formation rate of a galaxy and the most massive young stellar cluster
in it. The assumption that this cluster mass marks the upper end of a
young-cluster mass function leads to a connection of the star formation rate
and the slope of the IGIMF above a few Msol. The IGIMF varies with the star
formation history of a galaxy. Notably, large variations of the IGIMF are
evident for dE, dIrr and LSB galaxies with a small to modest stellar mass. We
find that for any galaxy the number of supernovae per star (NSNS) is suppressed
relative to that expected for a Salpeter IMF. Dwarf galaxies have a smaller
NSNS compared to massive galaxies. For dwarf galaxies the NSNS varies
substantially depending on the galaxy assembly history and the assumptions made
about the low-mass end of the ECMF. The findings presented here may be of some
consequence for the cosmological evolution of the number of supernovae per
low-mass star and the chemical enrichment of galaxies of different mass.Comment: 27 pages, accepted for publication by Ap
Further evidence for intra-night optical variability of radio-quiet quasars
Although well established for BL Lac objects and radio-loud quasars, the
occurrence of intra-night optical variability (INOV) in radio-quiet quasars is
still debated, primarily since only a handful of INOV events with good
statistical significance, albeit small amplitude, have been reported so far.
This has motivated us to continue intra-night optical monitoring of bona-fide
radio-quiet quasars (RQQs). Here we present the results for a sample of 11 RQQs
monitored by us on 19 nights. On 5 of these nights a given RQQ was monitored
simultaneously from two well separated observatories. In all, two clear cases
and two probable case of INOV were detected. From these data, we estimate an
INOV duty cycle of 8% for RQQs which would increase to 19% if the
`probable variable' cases are also included. Such comparatively small INOV duty
cycles for RQQs, together with the small INOV amplitudes (1%), are in
accord with the previously deduced characteristics of this phenomenon.Comment: 15 Pages, 4 Tables, 24 Figures; Accepted in BAS
Broad Band Optical Polarimetric Study of IC 1805
We present the BVR broad band polarimetric observations of 51 stars belonging
to the young open cluster IC 1805. Along with the photometric data from the
literature we have modeled and subtracted the foreground dust contribution from
the maximum polarization (P_{max}) and colour excess (E_{B-V}). The mean value
of the P_max for intracluster medium and the foreground are found to be 5.008
+/-0.005 % and 4.865 +/-0.022 % respectively. Moreover, the mean value of the
wavelength of maximum polarization (lambda_{max}) for intracluster medium is
0.541 +/- 0.003 micro m, which is quite similar as the general interstellar
medium (ISM). The resulting intracluster dust component is found to have
negligible polarization efficiency as compared to interstellar dust. Some of
the observed stars in IC 1805 have shown the indication of intrinsic
polarization in their measurements.Comment: 8 pages, 7 figures, Accepted for publication in MNRA
Optical observations of the bright long duration peculiar GRB 021004 afterglow
The CCD magnitudes in Johnson and Cousins and photometric
passbands are determined for the bright long duration GRB 021004 afterglow from
2002 October 4 to 16 starting 3 hours after the ray burst.
Light curves of the afterglow emission in ,, and passbands are
obtained by combining these measurements with other published data. The
earliest optical emission appears to originate in a revese shock. Flux decay of
the afterglow shows a very uncommon variation relative to other well-observed
GRBs. Rapid light variations, especially during early times (
days) is superposed on an underlying broken power law decay typical of a jetted
afterglow. The flux decay constants at early and late times derived from least
square fits to the light curve are and respectively,
with a jet break at around 7 day. Comparison with a standard fireball model
indicates a total extinction of mag in the direction of the
burst. Our low-resolution spectra corrected for this extinction provide a
spectral slope . This value and the flux decay constants
agree well with the electron energy index used in the model. The
derived jet opening angle of about implies a total emitted
gamma-ray energy erg at a cosmological distance
of about 20 Gpc. Multiwavelength observations indicate association of this GRB
with a star forming region, supporting the case for collapsar origin of long
duration GRBs.Comment: 19 pages, 3 figures, BASI, 31, 1
Polarization in young open cluster NGC 6823
We present multiwavelength linear polarimetric observations of 104 stars
towards the region of young open cluster NGC 6823. The polarization towards NGC
6823 is dominated by foreground dust grains and we found the evidence for the
presence of several layers of dust towards the line of sight. The first layer
of dust is located approximately within 200 pc towards the cluster, which is
much closer to the Sun than the cluster (~ 2.1 kpc). The radial distribution of
the position angles for the member stars are found to show a systematic change
while the polarization found to reduce towards the outer parts of the cluster
and the average position angle of coronal region of the cluster is very close
to the inclination of the Galactic parallel (~ 32 degree). The size
distribution of the grains within NGC 6823 is similar to those in general
interstellar medium. The patchy distribution of foreground dust grains are
suggested to be mainly responsible for the both differential reddening and
polarization towards NGC 6823. The majority of the observed stars do not show
the evidence of intrinsic polarization in their light.Comment: 16 pages, 6 tables, 11 figures, Accepted for publication in MNRA
Effects of non-denumerable fixed points in finite dynamical systems
The motion of a spinning football brings forth the possible existence of a
whole class of finite dynamical systems where there may be non-denumerably
infinite number of fixed points. They defy the very traditional meaning of the
fixed point that a point on the fixed point in the phase space should remain
there forever, for, a fixed point can evolve as well! Under such considerations
one can argue that a free-kicked football should be non-chaotic.Comment: This paper is a replaced version to modify the not-so-true claim,
made unknowingly in the earlier version, of being first to propose the
peculiar dynamical systems as described in the paper. With respect to the
original workers, we present here our original finding
Signatures of two-dimensionalisation of 3D turbulence in presence of rotation
A reason has been given for the inverse energy cascade in the
two-dimensionalised rapidly rotating 3D incompressible turbulence. For such
system, literature shows a possibility of the exponent of wavenumber in the
energy spectrum's relation to lie between -2 and -3. We argue the existence of
a more strict range of -2 to -7/3 for the exponent in the case of rapidly
rotating turbulence which is in accordance with the recent experiments. Also, a
rigorous derivation for the two point third order structure function has been
provided helping one to argue that even with slow rotation one gets, though
dominated, a spectrum with the exponent -2.87, thereby hinting at the
initiation of the two-dimensionalisation effect with rotation.Comment: An extended and typos-corrected version of the earlier submissio
The star formation history of the Local Group dwarf elliptical galaxy NGC 185: II. Gradients in the stellar population
The star formation history of the dE NGC 185, together with its spatial
variations, has been investigated using new ground-based and
photometry, and synthetic color--magnitude diagrams (CMDs). We find that the
bulk of the stars were formed in NGC 185 at an early epoch of its evolution.
After that, the star formation proceeded at a low rate until the recent past,
the age of the most recent traces of star formation activity detected in the
galaxy being some 100 Myr.
The star formation rate, for old and intermediate ages shows a
gradient in the sense of taking smaller values for higher galactocentric radii.
Moreover, recent star formation is detected in the central
pc only, where the youngest, 100 Myr old population is
found. The luminous blue {\it stars} discovered by Baade (1951) in the center
of NGC 185 are discussed using new CCD images in and Baade's original
photographic plates, reaching the conclusion that most of them are in fact star
clusters. A consistent picture arises in which the gas observed in the central
region of NGC 185 would have an internal origin. The rate at which evolved
stars return gas to the ISM is enough to seed the recent star formation
observed in the center of the galaxy and the SN rate is probably low enough to
allow the galaxy to retain the gas not used in the new stellar generations.Comment: 34 pages, 17 figures, 5 Tables, to be published in AJ October 9
SS 433: Results of a Recent Multi-wavelength Campaign
We conducted a multi-wavelength campaign in September-October, 2002, to
observe SS 433. We used 45 meter sized 30 dishes of Giant Meter Radio Telescope
(GMRT) for radio observation, 1.2 meter Physical Research Laboratory Infra-red
telescope at Mt Abu for IR, 1 meter Telescope at the State Observatory,
Nainital for Optical photometry, 2.3 meter optical telescope at the Vainu Bappu
observatory for spectrum and Rossi X-ray Timing Explorer (RXTE) Target of
Opportunity (TOO) observation for X-ray observations. We find sharp variations
in intensity in time-scales of a few minutes in X-rays, IR and radio
wavelengths. Differential photometry at the IR observation clearly indicated
significant intrinsic variations in short time scales of minutes throughout the
campaign. Combining results of these wavelengths, we find a signature of delay
of about two days between IR and Radio. The X-ray spectrum yielded double Fe
line profiles which corresponded to red and blue components of the relativistic
jet. We also present the broadband spectrum averaged over the campaign
duration.Comment: 17 pages 10 figures MNRAS (submitted
SN 2002ap, the hypernova of class Ic
The supernova SN 2002ap was discovered in the outer regions of the nearby
spiral M74 on January 29.4 UT. Early photometric and spectroscopic observations
indicate the supernova belongs to the class of Ic hypernova. Late time (After
JD 2452500) light curve decay slopes are similar to that of the hypernovae SN
1997ef and SN 1998bw. We present here the photometric light curves and
colour evolutions of SN 2002ap to investigate the late time nature of the light
curve.Comment: 2 pages,1 figure, Poster contribution, Submitted to proceedings of
Astronomical Society of India (ASI) meeting, 200
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