42 research outputs found
Dust properties from GALEX observations of a UV halo around Spica
GALEX has detected ultraviolet halos extending as far as 5 around
four bright stars (Murthy et al. (2011)). These halos are produced by
scattering of starlight by dust grains in thin foreground clouds that are not
physically associated with the star. Assuming a simple model consisting of a
single layer of dust in front of the star, Murthy et al.(2011) have been able
to model these halo intensities and constrain the value of the phase function
asymmetry factor of the scattering grains in the FUV and NUV. However due
to the uncertainty in the dust geometry they could not constrain the albedo. In
this work we have tried to constrain the optical constants and dust geometry by
modeling the UV halo of Spica. Since the halo emission is not symmetric, we
have modeled the Northern and Southern parts of the halo separately. To the
North of Spica, the best-fit albedo is 0.260.1 and is 0.580.11 in
the FUV at the 90% confidence level. The corresponding limits on the distance
and optical depth () of the dust sheet is 3.651.05 pc and
0.0470.006 respectively. However, owing to a complicated dust distribution
to the South of Spica, we were unable to uniquely constrain the dust parameters
in that region. Nevertheless, by assuming the optical constants of the Northern
region and assuming a denser medium, we were able to constrain the distance of
the dust to 9.51.5 pc and the corresponding to 0.040.01.Comment: 4 pages, accepted for publication in Earth, Planets and Spac
Unravelling the Nuclear Dust Morphology of NGC 1365: A Two Phase Polar - RAT Model for the Ultraviolet to Infrared Spectral Energy Distribution
We present a 3D radiative transfer model for the Spectral Energy Distribution
(SED) of NGC 1365, which is a "changing look" Seyfert 1.8 type AGN. The SED
from the ultraviolet (UV) to the infrared (IR) is constructed using archival
data from the UVIT on-board , along with IR data from the literature.
The SKIRT radiative transfer code is used to model the SED and derive the
geometry and composition of dust in this AGN. Similar to our earlier SED model
of NGC 4151, the nuclear region of NGC 1365 is assumed to contain a ring or
disk-like structure concentric to the accretion disk, composed of large
(0.1m - 1m) graphite grains in addition to the two-phase dusty torus
made up of ISM-type grains (Ring And Torus or RAT model). We also include, for
the first time, an additional component of dusty wind in the form of a bipolar
cone. We carry out a detailed analysis and derive the best-fit parameters from
a test to be = 0.03 pc, = 26 and
= 20 for the assumed ring-torus-polar wind geometry. Our results suggest
the presence of hot dust at a temperature T 1216 K at the location of
the ring which absorbs and scatters the incident UV radiation and emits in the
near-IR (NIR). In the mid-IR (MIR) the major contributors are the polar cone
and warm dust with T 916 K at = 0.1 pc. Not only are our
model radii in agreement with IR interferometric observations, our study also
reiterates the role of high resolution UV observations in constraining the dust
grain size distribution in the nuclear regions of AGN.Comment: 10 pages, 14 Figures, Table 5, Accepted for publication in MNRA