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2. physics

Stars, The Galaxy and its Neighbours - III
19th Feb 2015
14:30 - 16:00
ASI-2015: Parallel Sessions
Stars, The Galaxy and its Neighbours - III [Chairperson: Ranjan Gupta]
Time: 14:30 - 16:00
ASI2015_633
Thursday, 19 February 2015
Kanak Saha
14:30 - 15:00
Is there a classical bulge in our Milky Way?
Kanak Saha
COBE/DIRBE satellite revealed that the central region of our Milky Way hosts a boxy shaped bulge
rather than a spheroid. A number of evidences have already established that the Milky Way also hosts a
stellar bar in the disk mid-plane and the boxy bulge is thought to have formed as a result of buckling
instability of the bar. N-body modelling of the kinematic data from recently completed surveys such as
BRAVA, VVV, etc. indicates that our Milky Way is a pure disk galaxy without a classical bulge.
However, the stellar population and metallicity gradients in the bulge region do not rule out the existence
of a classical bulge. After giving a brief overview of the current status of the Milky Way's bulge, I will
discuss some new results that might shed some new light on this ongoing debate.
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Stars, The Galaxy and its Neighbours - III
ASI2015_473
19th Feb 2015
Arabindo Roy
14:30 - 16:00
15:00 - 15:15
The Line-mass Power Spectrum of Interstellar Filaments - A Possible Link to the IMF ?
Roy, A., Andr'e, Ph., CEA, Saclay, France Arzoumanian, D., et al., IAS, Orsay, France
Herschel observations (Konyves et al. 2010 & Andre' et al. 2013) have revealed that a majority of
prestellar cores (>70%) are formed along thermally supercritical filaments due to gravitational instability.
The virtue of being supercritical, however, is not a sufficient condition for the formation of prestellar
cores because in theory, a supercritical filament (M_line > 2Cs^2/G) collapses radially into a line without
any fragmentation. The presence of longitudinal line-mass perturbation modes along the z axis of the
filaments combined with self-gravity assists in the formation of cores. Interestingly, the statistical
properties of the perturbations are directly linked to the statistics of prestellar core masses. Analytically,
Inutsuka (2001) showed that a line mass fluctuating field along a filament characterized by a power
spectrum slope of -1.5 generates a mass spectrum of collapsed cores i.e., dN/dM proportional to M^-2.5,
similar to the Salpeter mass function. Our analysis on the Herschel Gould Belt Survey dust emission data
has revealed that the line mass fluctuations along the long axes of filaments (Roy et al. 2014) have a
characteristic 1-D power spectrum slope of -1.7+/-0.3. The power spectrum slope bears a striking
similarity with 1-D Kolomogorov turbulence spectrum of -1.67. The observational derivation of the
characteristic power spectrum slope may have implications on our understanding of the IMF. The
observationally derived power spectrum slope (-1.7) in the present study suggests that thermally
supercritical filaments will fragment into a core population whose mass function approaches dN/dM
proportional to M^-2.3. This is very close to the Salpeter slope pointing to the possibility that the density
perturbations due to turbulence is prerequisite for generating a Salpeter-like mass function toward the high
mass end. References Andre', P., Di Francesco, J., Ward-Thompson, D., et al. 2013, PPVI Inutsuka, S.-i.
2001, ApJ, 559, L149 K¨onyves, V., Andr´e, P., Men’shchikov, A., et al. 2010, A&A, 518, L106 Roy, A.,
Andre', P., Arzoumanian, D., et al. 2014, to be submitted
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Stars, The Galaxy and its Neighbours - III
ASI2015_661
19th Feb 2015
Lab Saha
14:30 - 16:00
15:15 - 15:30
Radiative recombination structures in galactic supernova remnants
T. Ergin (1,2), A. Sezer (1,2), P. Majumdar (3), A. Chatterjee (3), A. Bayirli (2), E. N. Ercan (2) (1)
TUBITAK Space Technologies Research Institute,Ankara, Turkey (2) Physics Department, Bogazici
University, Istanbul, Turkey (3) Saha Institute of Nuclear Physics, Kolkata, India
The recent X-ray observations have revealed that some of the galactic gamma-ray-emitting supernova
remnants (SNRs) have overionized plasmas. G31.9+0.0 (3C391) is one of the such galactic mixedmorphology SNRs observed in GeV gamma-rays by the Fermi Gamma Ray Space Telescope LAT
(Fermi-LAT), as well as in the 0.3- 10 keV X-ray band by Suzaku. We analyzed the Suzaku data of 3C391
and we first discovered radiative recombination structures of silicon and sulfur from 3C391. We have also
analysed gamma-ray data of 3C391 taken with Fermi-LAT and it was detected in GeV gamma rays with a
significance of ∼18σ and we showed that the GeV emission is point-like in nature. The GeV gamma-ray
spectrum was shown to be best explained by the decay of neutral pions assuming a broken power-law
proton distribution. Here, we will discuss the possible origin of the of radiative recombination structures
of two such SNRs including 3C391 and origin of gamma rays.
ASI2015_585
Pragati Pradhan
15:30 - 15:45
Broadband properties of accretion powered X-ray pulsars: a study with Suzaku
Biswajit Paul Raman Research Institute, Sadashivnagar, Bangalore 560080, India B.C Paul North Bengal
University, Raja Rammohanpur, District Darjeeling, 734013, West Bengal, India
We will present a comprehensive study of the hard X-ray timing and spectral properties of accreting X-ray
pulsars, most of which are high mass X-ray binaries (HMXBs). Using Suzaku observations we have
examined the energy dependence of the pulsations upto highest detectable energies in these sources and
compare the energy dependence with the X-ray luminosity and the magnetic field strength of these
sources, if known. Similarly, the hard X-ray spectral properties of the sources are examined against the Xray luminosity and the magnetic field strength. These measurements give valuable insight into physical
properties of the emission region in accreting pulsars. In the lower energy band, we measured the emission
lines, the absorbing column density, and the reprocessed soft X-ray emission. All of these together show
the variety of the physical environment in the X-ray pulsars.
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Stars, The Galaxy and its Neighbours - III
ASI2015_640
19th Feb 2015
Prasanta Bera
14:30 - 16:00
15:45 - 16:00
Mass–radius relation of strongly magnetized white dwarfs: nearly independent of Landau quantization
Prof. Dipankar Bhattacharya, IUCAA, PUNE
Super-Chandrasekhar mass white dwarfs are believed to be the progenitors of over-luminous type-Ia
supernovae. Modification of equation of state in presence of strong magnetic field has been considered to
be a probable explanation of single degenerate system. We study the strongly magnetized white dwarf
configurations in a self consistent manner. We compute static equilibria of white dwarf stars containing
strong poloidal and toroidal magnetic field, and present the modification of white dwarf mass–radius
relation caused by the magnetic field. We find that a maximum white dwarf mass of about 1.9M_Sun may
be supported if the interior field is as strong as approximately 10^10 T. This mass is over 30 per cent
larger than the traditional Chandrasekhar Limit. The equation of state of electron degenerate matter can be
strongly modified due to Landau quantization at such high magnetic fields.We find, however, that this
does not significantly affect the structure of the white dwarf.
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