4,011 research outputs found
The diversity of supernovae and impostors shortly after explosion
Observational surveys are now able to detect an increasing number of
transients, such as core-collapse supernovae (SN) and powerful non-terminal
outbursts (SN impostors). Dedicated spectroscopic facilities can follow up
these events shortly after detection. Here we investigate the properties of
these explosions at early times. We use the radiative transfer code CMFGEN to
build an extensive library of spectra simulating the interaction of supernovae
and their progenitor's winds/circumstellar medium (CSM). We consider a range of
progenitor mass-loss rates (Mdot = 5e-4 to 1e-2 Msun/yr), abundances (solar,
CNO-processed, and He-rich), and SN luminosities (L = 1.9e8 to 2.5e10 Lsun).
The models simulate events ~1 day after explosion, and we assume a fixed
location of the shock front as Rin=8.6e13 cm. We show that the large range of
massive star properties at the pre-SN stage causes a diversity of early-time
interacting SN and impostors. We identify three main classes of early-time
spectra consisting of relatively high-ionisation (e.g. Ovi), medium-ionisation
(e.g. Ciii), and low-ionisation lines (e.g. Feii/iii). They are regulated by L
and the CSM density. Given a progenitor wind velocity Vinf, our models also
place a lower limit of Mdot > 5e-4 (Vinf/150 km/s) Msun/yr for detection of CSM
interaction signatures in observed spectra. Early-time SN spectra should
provide clear constraints on progenitors by measuring H, He, and CNO abundances
if the progenitors come from single stars. The connections are less clear
considering the effects of binary evolution. Yet, our models provide a clear
path for linking the final stages of massive stars to their post-explosion
spectra at early times, and guiding future observational follow-up of
transients with facilities such as the Zwicky Transient Facility.Comment: Accepted for publication in A&A. 14 pages, 7 figure
A Lighthouse Effect in Eta Carinae
We present a new model for the behavior of scattered time-dependent,
asymmetric near-UV emission from the nearby ejecta of {\eta} Car. Using a 3-D
hydrodynamical simulation of {\eta} Car's binary colliding winds, we show that
the 3-D binary orientation derived by Madura et al. (2012) is capable of
explaining the asymmetric near-UV variability observed in the Hubble Space
Telescope Advanced Camera for Surveys/High Resolution Camera (HST ACS/HRC)
F220W images of Smith et al. (2004b). Models assuming a binary orientation with
i ~ 130 to 145 degrees, {\omega} ~ 230 to 315 degrees, PAz ~ 302 to 327 degrees
are consistent with the observed F220W near-UV images. We find that the hot
binary companion does not significantly contribute to the near-UV excess
observed in the F220W images. Rather, we suggest that a bore-hole effect and
the reduction of Fe II optical depths inside the wind-wind collision cavity
carved in the extended photosphere of the primary star lead to the
time-dependent directional illumination of circum-binary material as the
companion moves about in its highly elliptical orbit.Comment: 14 pages, 4 figures, 1 table. Accepted for publication in ApJ
Massive star evolution: Luminous Blue Variables as unexpected Supernova progenitors
Stars more massive than about 8 Msun end their lives as a Supernova (SN), an
event of fundamental importance Universe-wide. Theoretically, these stars have
been expected to be either at the red supergiant, blue supergiant, or
Wolf-Rayet stage before the explosion. We performed coupled stellar evolution
and atmospheric modeling of stars with initial masses between 20 Msun and 120
Msun. We found that the 20 Msun and 25 Msun rotating models, before exploding
as SN, have spectra that do not resemble any of the aforementioned classes of
massive stars. Rather, they have remarkable similarities with rare, unstable
massive stars known as Luminous Blue Variables (LBV). While observations show
that some SNe seem to have had LBVs as progenitors, no theoretical model had
yet predicted that a star could explode at this stage. Our models provide
theoretical support for relatively low-luminosity LBVs exploding as SN in the
framework of single stellar evolution. This is a significant shift in paradigm,
meaning that a fraction of LBVs could be the end stage of massive star
evolution, rather than a transitory evolutionary phase. We suggest that type
IIb SN could have LBV as progenitors, and a prime example could be SN 2008ax.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Progenitors of supernova Ibc: a single Wolf-Rayet star as the possible progenitor of the SN Ib iPTF13bvn
Core-collapse supernova (SN) explosions mark the end of the tumultuous life
of massive stars. Determining the nature of their progenitors is a crucial step
towards understanding the properties of SNe. Until recently, no progenitor has
been directly detected for SN of type Ibc, which are believed to come from
massive stars that lose their hydrogen envelope through stellar winds and from
binary systems where the companion has stripped the H envelope from the
primary. Here we analyze recently reported observations of iPTF13bvn, which
could possibly be the first detection of a SN Ib progenitor based on
pre-explosion images. Very interestingly, the recently published Geneva models
of single stars can reproduce the observed photometry of the progenitor
candidate and its mass-loss rate, confirming a recently proposed scenario. We
find that a single WR star with initial mass in the range 31-35 Msun fits the
observed photometry of the progenitor of iPTF13bvn. The progenitor likely has a
luminosity of log (L/Lsun)~5.55, surface temperature ~45000 K, and mass of
~10.9 Msun at the time of explosion. Our non-rotating 32 Msun model
overestimates the derived radius of the progenitor, although this could likely
be reconciled with a fine-tuned model of a more massive (between 40 and 50
Msun), hotter, and luminous progenitor. Our models indicate a very uncertain
ejecta mass of ~8 Msun, which is higher than the average of the SN Ib ejecta
mass that is derived from the lightcurve (2-4 Msun). This possibly high ejecta
mass could produce detectable effects in the iPTF13bvn lightcurve and spectrum.
If the candidate is indeed confirmed to be the progenitor, our results suggest
that stars with relatively high initial masses (>30 Msun) can produce visible
SN explosions at their deaths and do not collapse directly to a black hole.Comment: 4 pages, 2 figures, accepted for publication in A&
Episodic modulations in supernova radio light curves from luminous blue variable supernova progenitor models
Ideally, one would like to know which type of core-collapse SNe is
produced by different progenitors and the channels of stellar evolution leading
to these progenitors. These links have to be very well known to use the
observed frequency of different types of SN events for probing the star
formation rate and massive star evolution in different types of galaxies.
We investigate the link between LBV as SN progenitors and the appearance
of episodic radio light curve modulations of the SN event. We use the
20Msun and 25Msun models with rotation at solar metallicity, part of an
extended grid of stellar models computed by the Geneva team. At their pre-SN
stage, these two models have recently been shown to have spectra similar to
those of LBV stars and possibly explode as Type IIb SNe. Based on the wind
properties before the explosion, we derive the density structure of their
circumstellar medium. This structure is used as input for computing the SN
radio light curve. We find that the 20Msun model shows radio light
curves with episodic luminosity modulations, similar to those observed in some
Type IIb SNe. This occurs because the evolution of the 20Msun model terminates
in a region of the HR diagram where radiative stellar winds present strong
density variations, caused by the bistability limit. The 25Msun model, ending
its evolution in a zone of the HR diagram where no change of the mass-loss
rates is expected, presents no such modulations in its radio SN light curve.
Our results reinforce the link between SN progenitors and LBV
stars. We also confirm the existence of a physical mechanism for a single star
to have episodic radio light curve modulations. In the case of the 25Msun
progenitors, we do not obtain modulations in the radio light curve, but our
models may miss some outbursting behavior in the late stages of massive stars.Comment: 5 pages, 3 figures, accepted by Astronomy & Astrophysics Letter
UV spectroscopy of the blue supergiant SBW1: the remarkably weak wind of a SN 1987A analog
The Galactic blue supergiant SBW1 with its circumstellar ring nebula
represents the best known analog of the progenitor of SN 1987A. High-resolution
imaging has shown H-alpha and IR structures arising in an ionized flow that
partly fills the ring's interior. To constrain the influence of the stellar
wind on this structure, we obtained an ultraviolet (UV) spectrum of the central
star of SBW1 with the HST Cosmic Origins Spectrograph (COS). The UV spectrum
shows none of the typical wind signatures, indicating a very low mass-loss
rate. Radiative transfer models suggest an extremely low rate below 10
Msun/yr, although we find that cooling timescales probably become comparable to
or longer than the flow time below 10 Msun/yr. We therefore adopt this
latter value as a conservative upper limit. For the central star, the model
yields =21,0001000 K, 510 ,
and roughly Solar composition except for enhanced N abundance. SBW1's very low
mass-loss rate may hinder the wind's ability to shape the surrounding nebula.
The very low mass-loss rate also impairs the wind's ability to shed angular
momentum; the spin-down timescale for magnetic breaking is more than 500 times
longer than the age of the ring. This, combined with the star's slow rotation
rate, constrain merger scenarios to form ring nebulae. The mass-loss rate is at
least 10 times lower than expected from mass-loss recipes, without any account
of clumping. The physical explanation for why SBW1's wind is so weak presents
an interesting mystery.Comment: 12 pages, 6 figs. submitted to MNRAS. comments welom
AIDS hospice in Provincetown, Massachusetts.
Thesis (M. Arch.)--Massachusetts Institute of Technology, Dept. of Architecture, 1991.Includes bibliographical references (leave 42).We spend a majority of our lives in and around architecture; it is through architecture that we know our world. Architecture has the ability to orient us in our environment by telling us where we are in place as well as in time. Architecture infonns us about and helps us interpret our natural, social, and technological environments; it is through architecture that these environments can and should be revealed to us. Today, especially in the density of our urban landscape, architecture does little to orient us in the natural environment. The cycles and rhythms in the natural environment such as the movements of the heavens, or the seasonal changes in the wind and rain are rarely experienced through our architecture. Understanding these cycles gives us the ability to gage where we are in both time and place. These rhythms give us an understanding of time not abstracted and dictated by the clock. They 'deal with the reality of things we can observe and understand phenomenally through changes, transformations and the passing of events. They give us an understanding of place by being highly specific to where we live. My thesis is an exploration of how architecture can become a vehicle for understanding and revealing these processes and cycles in the natural environment. In my thesis, I am taking the attitude that architecture should not be merely a window to these natural cycles but rather a filter that selectively reveals them to us. The vehicle for this exploration is an AIDS hospice in Provincetown, Massachusetts. It is in such a context of the chronically ill that I believe an understanding of natural cycles would be especially beneficial to a persons well being.by Paul H. Groh.M.Arch
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