1,532 research outputs found
The Progenitors of Recent Core-Collapse Supernovae
We present the results of our analysis of Hubble Space Telescope
(HST) and deep ground-based images to isolate the massive progenitor stars
of the two recent core-collapse supernovae 2008bk and 2008cn. The identification of the progenitors is facilitated in one of these two cases by high-precision
astrometry based on our HST imaging of SNe at late times
On the progenitor and early evolution of the type II supernova 2009kr
We report the identification of a source coincident with SN 2009kr in HST
pre-explosion images. The object appears to be a single point source with an
intrinsic colour V-I = 1.1 and M_V = -7.6. If this is a single star it would be
a yellow supergiant of log L/L_{sol} \sim 5.1 and a mass of 15 (+5/-4) M_{sol}.
The spatial resolution does not allow us yet to definitively determine if the
progenitor object is a single star, a binary system, or a compact cluster. We
show that the early lightcurve is flat, similar to IIP SNe, but that the the
spectra are somewhat peculiar, displaying unusual P-Cygni profiles. The
evolution of the expanding ejecta will play an important role in understanding
the progenitor object.Comment: Submitted to ApJ Letter
The Progenitors of Recent Core-Collapse Supernovae
We present the results of our analysis of Hubble Space Telescope (HST) and deep ground-based images to isolate the massive progenitor stars of the two recent core-collapse supernovae 2008 bk and 2008 cn. The identification of the progenitors is facilitated in one of these two cases by high-precision astrometry based on our HST imaging of SNe at late times
High luminosity, slow ejecta and persistent carbon lines: SN 2009dc challenges thermonuclear explosion scenarios
Extended optical and near-IR observations reveal that SN 2009dc shares a number of similarities with normal Type Ia supernovae (SNe Ia), but is clearly overluminous, with a (pseudo-bolometric) peak luminosity of log (L) = 43.47 (erg s^(−1)). Its light curves decline slowly over half a year after maximum light [Δm_(15)(B)_true= 0.71], and the early-time near-IR light curves show secondary maxima, although the minima between the first and the second peaks are not very pronounced. The bluer bands exhibit an enhanced fading after ~200 d, which might be caused by dust formation or an unexpectedly early IR catastrophe. The spectra of SN 2009dc are dominated by intermediate-mass elements and unburned material at early times, and by iron-group elements at late phases. Strong C ii lines are present until ~2 weeks past maximum, which is unprecedented in thermonuclear SNe. The ejecta velocities are significantly lower than in normal and even subluminous SNe Ia. No signatures of interaction with a circumstellar medium (CSM) are found in the spectra. Assuming that the light curves are powered by radioactive decay, analytic modelling suggests that SN 2009dc produced ~1.8 M_⊙ of ^(56)Ni assuming the smallest possible rise time of 22 d. Together with a derived total ejecta mass of ~2.8 M_⊙, this confirms that SN 2009dc is a member of the class of possible super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg, 2006gz and 2007if. A study of the hosts of SN 2009dc and other superluminous SNe Ia reveals a tendency of these SNe to explode in low-mass galaxies. A low metallicity of the progenitor may therefore be an important prerequisite for producing superluminous SNe Ia. We discuss a number of possible explosion scenarios, ranging from super-Chandrasekhar-mass white-dwarf progenitors over dynamical white-dwarf mergers and Type I(1/2) SNe to a core-collapse origin of the explosion. None of the models seems capable of explaining all properties of SN 2009dc, so that the true nature of this SN and its peers remains nebulous
A Binary Progenitor for the Type IIb Supernova 2011dh in M51
We perform binary stellar evolutionary calculations following the
simultaneous evolution of both stars in the system to study a potential
progenitor system for the Type IIb supernova 2011dh. Pre-explosion photometry
as well as light-curve modeling have provided constraints on the physical
properties of the progenitor system. Here we present a close binary system that
is compatible with such constraints. The system is formed by stars of solar
composition with 16 Msun + 10 Msun on a circular orbit with an initial period
of 125 days. The primary star ends its evolution as a yellow supergiant with a
mass of ~4 Msun, a final hydrogen content of ~3-5E-03 Msun and with an
effective temperature and luminosity in agreement with the HST pre-explosion
observations of SN 2011dh. These results are nearly insensitive to the adopted
accretion efficiency factor beta. At the time of explosion, the companion star
has an effective temperature of 22 to 40 thousand Kelvin, depending on the
value of beta, and lies near the zero age main sequence. Considering the
uncertainties in the HST pre-SN photometry the secondary star is only
marginally detectable in the bluest observed band. Close binary systems, as
opposed to single stars, provide a natural frame to explain the properties of
SN 2011dh.Comment: 12 pages, 15 figures, 3 tables. Accepted for publication in The
Astrophysical Journal on October June 27, 201
The Massive Progenitor of the Possible Type II-Linear Supernova 2009hd in Messier 66
We present observations of SN2009hd in the nearby galaxy M66. This SN is one
of the closest to us in recent years but heavily obscured by dust, rendering it
unusually faint in the optical, given its proximity. We find that the observed
properties of SN2009hd support its classification as a possible Type II-L SN, a
relatively rare subclass of CC-SNe. High-precision relative astrometry has been
employed to attempt to identify a SN progenitor candidate, based on a
pixel-by-pixel comparison between HST F555W and F814W images of the SN site
prior to explosion and at late times. A progenitor candidate is identified in
the F814W images only; this object is undetected in F555W. Significant
uncertainty exists in the astrometry, such that we cannot definitively identify
this object as the SN progenitor. Via insertion of artificial stars into the
pre-SN HST images, we are able to constrain the progenitor's properties to
those of a possible supergiant, with M(F555W)0>-7.6 mag and (V-I) 0>0.99 mag.
The magnitude and color limits are consistent with a luminous RSG; however,
they also allow for the possibility that the star could have been more yellow
than red. From a comparison with theoretical massive-star evolutionary tracks,
which include rotation and pulsationally enhanced mass loss, we can place a
conservative upper limit on the initial mass for the progenitor of <20 M_sun.
If the actual mass of the progenitor is near the upper range allowed by our
derived mass limit, then it would be consistent with that for the identified
progenitors of the SNII-L 2009kr and the high-luminosity SNII-P 2008cn. The
progenitors of these three SNe may possibly bridge the gap between lower-mass
RSG that explode as SNeII-P and LBV, or more extreme RSG, from which the more
exotic SNeII-n may arise. Very late-time imaging of the SN2009hd site may
provide us with more clues regarding the true nature of its progenitor.Comment: 19 pages, 9 figures, 3 tables, accepted for publication in Ap
EVLA Observations of the Radio Evolution of SN 2011dh
We report on Expanded Very Large Array (EVLA) observations of the Type IIb
supernova 2011dh, performed over the first 100 days of its evolution and
spanning 1-40 GHz in frequency. The radio emission is well-described by the
self-similar propagation of a spherical shockwave, generated as the supernova
ejecta interact with the local circumstellar environment. Modeling this
emission with a standard synchrotron self-absorption (SSA) model gives an
average expansion velocity of v \approx 0.1c, supporting the classification of
the progenitor as a compact star (R_* \approx 10^11 cm). We find that the
circumstellar density is consistent with a {\rho} \propto r^-2 profile. We
determine that the progenitor shed mass at a constant rate of \approx 3 \times
10^-5 M_\odot / yr, assuming a wind velocity of 1000 km / s (values appropriate
for a Wolf-Rayet star), or \approx 7 \times 10^-7 M_\odot / yr assuming 20 km /
s (appropriate for a yellow supergiant [YSG] star). Both values of the
mass-loss rate assume a converted fraction of kinetic to magnetic energy
density of {\epsilon}_B = 0.1. Although optical imaging shows the presence of a
YSG, the rapid optical evolution and fast expansion argue that the progenitor
is a more compact star - perhaps a companion to the YSG. Furthermore, the
excellent agreement of the radio properties of SN 2011dh with the SSA model
implies that any YSG companion is likely in a wide, non-interacting orbit.Comment: 6 pages, 2 figures; accepted to ApJ
Essays on arts entrepreneurship : exploring creative entrepreneurial processes
Entrepreneurship is fundamental to both the success of artistic careers and economic development and growth. However, scholars have paid little attention, thus far, to the entrepreneurial efforts involved in the creation of artistic work and aesthetic value. In an attempt to call scholarly attention to arts entrepreneurship, this three-paper dissertation (1) challenges the dominant perspective in organizational entrepreneurship that entrepreneurs' primary goal is to maximize economic profit, (2) suggests that three major dimensions -- embodied imagination, contemplation, and consensus -- are involved in the process by which entrepreneurs and customers collaborate in the co-creation of unique ideas, and (3) proposes that entrepreneurs may go through five elements -- experiencing, early creating, reaching an impasse and gestating, (re)creating and evaluating imagined futures, and choosing and enterprising -- as they imagine, create, and act on unique ideas. Overall, this dissertation provides insight into processes that are essential to entrepreneurship while calling scholars' attention to the notion that individuals may become entrepreneurial in the pursuit of entrepreneurial dreams, creative freedom, artistic passion, and social change
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