791 research outputs found
The Progenitor of SN 2005cs in the Whirlpool Galaxy
The progenitor of SN 2005cs, in the galaxy M51, is identified in
pre-explosion HST ACS WFC imaging. Differential astrometry, with post-explosion
ACS HRC F555W images, permitted the identification of the progenitor with an
accuracy of 0.006". The progenitor was detected in the F814W pre-explosion
image with I=23.3+/-0.2, but was below the detection thresholds of the F435W
and F555W images, with B<24.8 and V<25 at 5-sigma. Limits were also placed on
the U and R band fluxes of the progenitor from pre-explosion HST WFPC2 F336W
and F675W images. Deep images in the infra-red from NIRI on the Gemini-North
telescope were taken 2 months prior to explosion, but the progenitor is not
clearly detected on these. The upper limits for the JHK magnitudes of the
progenitor were J<21.9,H<21.1 and K<20.7. Despite having a detection in only
one band, a restrictive spectral energy distribution of the progenitor star can
be constructed and a robust case is made that the progenitor was a red
supergiant with spectral type between mid-K to late-M. The spectral energy
distribution allows a region in the theoretical HR diagram to be determined
which must contain the progenitor star. The initial mass of the star is
constrained to be M(ZAMS)=9+3/-2 M_solar, which is very similar to the
identified progenitor of the type II-P SN 2003gd, and also consistent with
upper mass limits placed on five other similar SNe. The upper limit in the deep
K-band image is significant in that it allows us to rule out the possibility
that the progenitor was a significantly higher mass object enshrouded in a dust
cocoon before core-collapse. This is further evidence that the trend for type
II-P SNe to arise in low to moderate mass red supergiants is real.Comment: Accepted (31/08/05) for publication in MNRAS Letter
Assortativity and leadership emergence from anti-preferential attachment in heterogeneous networks
Many real-world networks exhibit degree-assortativity, with nodes of similar
degree more likely to link to one another. Particularly in social networks, the
contribution to the total assortativity varies with degree, featuring a
distinctive peak slightly past the average degree. The way traditional models
imprint assortativity on top of pre-defined topologies is via degree-preserving
link permutations, which however destroy the particular graph's hierarchical
traits of clustering. Here, we propose the first generative model which creates
heterogeneous networks with scale-free-like properties and tunable realistic
assortativity. In our approach, two distinct populations of nodes are added to
an initial network seed: one (the followers) that abides by usual preferential
rules, and one (the potential leaders) connecting via anti-preferential
attachments, i.e. selecting lower degree nodes for their initial links. The
latter nodes come to develop a higher average degree, and convert eventually
into the final hubs. Examining the evolution of links in Facebook, we present
empirical validation for the connection between the initial anti-preferential
attachment and long term high degree. Thus, our work sheds new light on the
structure and evolution of social networks
Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy
Recent weak emission-line long-slit surveys and modelling studies of PNe have
convincingly argued in favour of the existence of an unknown component in the
planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an
explanation for the long-standing recombination-line versus forbidden-line
temperature and abundance discrepancy problems. Here we describe the rationale
and initial results from a detailed spectroscopic study of three Galactic PNe
undertaken with the VLT FLAMES integral-field unit spectrograph, which advances
our knowledge about the small-scale physical properties, chemical abundances
and velocity structure of these objects across a two-dimensional field of view,
and opens up for exploration an uncharted territory in the study and modelling
of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU
Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in
Hawaii, April 2006
High Resolution mid-Infrared Imaging of SN 1987A
Using the Thermal-Region Camera and Spectrograph (T-ReCS) attached to the
Gemini South 8m telescope, we have detected and resolved 10 micron emission at
the position of the inner equatorial ring (ER) of supernova SN 1987A at day
6067. ``Hot spots'' similar to those found in the optical and near-IR are
clearly present. The morphology of the 10 micron emission is globally similar
to the morphology at other wavelengths from X-rays to radio. The observed
mid-IR flux in the region of SN1987A is probably dominated by emission from
dust in the ER. We have also detected the ER at 20 micron at a 4 sigma level.
Assuming that thermal dust radiation is the origin of the mid-IR emission, we
derive a dust temperature of 180^{+20}_{-10} K, and a dust mass of 1.- 8.
10^{-5} Mo for the ER. Our observations also show a weak detection of the
central ejecta at 10 micron. We show that previous bolometric flux estimates
(through day 2100) were not significantly contaminated by this newly discovered
emission from the ER. If we assume that the energy input comes from radioactive
decays only, our measurements together with the current theoretical models set
a temperature of 90 leq T leq 100 K and a mass range of 10^{-4} - 2. 10^{-3} Mo
for the dust in the ejecta. With such dust temperatures the estimated thermal
emission is 9(+/-3) 10^{35} erg s^{-1} from the inner ring, and 1.5 (+/-0.5)
10^{36} erg s^{-1} from the ejecta. Finally, using SN 1987A as a template, we
discuss the possible role of supernovae as major sources of dust in the
Universe.Comment: aastex502, 14 pages, 4 figures; Accepted for publication in ApJ
Content changed: new observations, Referee's comments and suggestion
Hydrogen issue in Core Collapse Supernovae
We discuss results of analyzing a time series of selected
photospheric-optical spectra of core collapse supernovae (CCSNe). This is
accomplished by means of the parameterized supernovae synthetic spectrum (SSp)
code
``SYNOW''.
Special attention is addressed to traces of hydrogen at early phases,
especially for the stripped-envelope SNe (i.e. SNe Ib-c). A thin low mass
hydrogen layer extending to very high ejection velocities above the helium
shell, is found to be the most likely scenario for Type Ib SNe.Comment: 8 pages. Proceedings of the conference ``The Multicoloured Landscape
of Compact Objects and their Explosive Origins'', 2006 June 11--24, Cefalu,
Sicily, to be published by AI
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