28 research outputs found
The massive binary companion star to the progenitor of supernova 1993J
The massive star which underwent core-collapse to produce SN1993J was
identified as a non-variable red supergiant star in images of the galaxy M81
taken before explosion. However the stellar source showed an excess in UV and
B-band colours that suggested it had either a hot, massive companion star or
was embedded in an unresolved young stellar association. The spectra of SN1993J
underwent a remarkable transformation between a hydrogen-rich Type II supernova
and a helium-rich (hydrogen-deficient) Type Ib. The spectral and photometric
peculiarities were explained by models in which the 13-20 solar mass supergiant
had lost almost its entire hydrogen envelope to a close binary companion. The
binary scenario is currently the best fitting model for the production of such
type IIb supernovae, however the hypothetical massive companion stars have so
far eluded discovery. Here we report the results of new photometric and
spectroscopic observations of SN1993J, 10 years after explosion. At the
position of the fading SN we detect the unambiguous signature of a massive
star, the binary companion to the progenitor. This is evidence that this type
of SN originate in interacting binary systems.Comment: 18 pages (3 figures
A binary model for the UV-upturn of elliptical galaxies (MNRAS version)
The discovery of a flux excess in the far-ultraviolet (UV) spectrum of
elliptical galaxies was a major surprise in 1969. While it is now clear that
this UV excess is caused by an old population of hot helium-burning stars
without large hydrogen-rich envelopes, rather than young stars, their origin
has remained a mystery. Here we show that these stars most likely lost their
envelopes because of binary interactions, similar to the hot subdwarf
population in our own Galaxy. We have developed an evolutionary population
synthesis model for the far-UV excess of elliptical galaxies based on the
binary model developed by Han et al (2002, 2003) for the formation of hot
subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces
most of the properties of elliptical galaxies with a UV excess: the range of
observed UV excesses, both in and , and their evolution
with redshift. We also present colour-colour diagrams for use as diagnostic
tools in the study of elliptical galaxies. The model has major implications for
understanding the evolution of the UV excess and of elliptical galaxies in
general. In particular, it implies that the UV excess is not a sign of age, as
had been postulated previously, and predicts that it should not be strongly
dependent on the metallicity of the population, but exists universally from
dwarf ellipticals to giant ellipticals.Comment: accepted for publication in MNRAS, 24 pages, 15 figures, 2 table
Observational and Physical Classification of Supernovae
This chapter describes the current classification scheme of supernovae (SNe).
This scheme has evolved over many decades and now includes numerous SN Types
and sub-types. Many of these are universally recognized, while there are
controversies regarding the definitions, membership and even the names of some
sub-classes; we will try to review here the commonly-used nomenclature, noting
the main variants when possible. SN Types are defined according to
observational properties; mostly visible-light spectra near maximum light, as
well as according to their photometric properties. However, a long-term goal of
SN classification is to associate observationally-defined classes with specific
physical explosive phenomena. We show here that this aspiration is now finally
coming to fruition, and we establish the SN classification scheme upon direct
observational evidence connecting SN groups with specific progenitor stars.
Observationally, the broad class of Type II SNe contains objects showing strong
spectroscopic signatures of hydrogen, while objects lacking such signatures are
of Type I, which is further divided to numerous subclasses. Recently a class of
super-luminous SNe (SLSNe, typically 10 times more luminous than standard
events) has been identified, and it is discussed. We end this chapter by
briefly describing a proposed alternative classification scheme that is
inspired by the stellar classification system. This system presents our
emerging physical understanding of SN explosions, while clearly separating
robust observational properties from physical inferences that can be debated.
This new system is quantitative, and naturally deals with events distributed
along a continuum, rather than being strictly divided into discrete classes.
Thus, it may be more suitable to the coming era where SN numbers will quickly
expand from a few thousands to millions of events.Comment: Extended final draft of a chapter in the "SN Handbook". Comments most
welcom
Study of the HII regions in the spiral galaxy NGC6384
The galaxy NGC6384 has been observed with an IPCS through H alpha and [NII]
narrow-band interference filters for direct imagery with the 2.6 m Byurakan
telescope. We studied main physical parameters of identified 98 HII regions,
their diameter and luminosity functions, as well [NII]/H alpha ratio
distribution. The integrated distribution function of the HII region diameters
can be well fitted by the exponential function. The characteristic diameter has
the value (Do = 217 pc) predicted for a galaxy of its measured absolute
luminosity. The luminosity function of HII regions has double power laws
profile with relatively shallow slope at low luminosities (a = - 0.4), an
abrupt turnover at log L(H alpha) = 38.75, and sharper slope at higher
luminosities (a = - 2.3). Correlation between luminosity and diameter of HII
regions confirms that in general they are constant density, radiation-bound
systems. [NII]/H alpha ratio data for HII regions show that there is a negative
radial gradient of [NII]/H alpha. In the central region of the galaxy, nitrogen
abundance is higher than in the periphery. The properties of the HII regions
population of this AGN galaxy does not differs significantly from the
properties of the HII regions population of the "normal" galaxies. Reexamining
the location of the type Ia SN 1971L in the galaxy, we confirm that it lies on
the spiral arm at about 8.6" far from the closest HII region #53 (F81). Such a
location can be taken as prove that the progenitor of this SN do not belong to
an old, evolved stellar population.Comment: 17 pages, 7 figure
A bridge between worlds: understanding network structure to understand change strategy
A number of scholars are exploring district and site relations in organizational change efforts in the larger policy context of No Child Left Behind. These studies suggest the importance of the central office as a support to the work of reform and offer strategies for building relations between district offices and sites in order to implement and sustain change efforts. What is frequently overlooked in these studies is that organizational change efforts are often socially constructed. Therefore, examining the underlying social networks may provide insight into structures that support or constrain efforts at change. This exploratory case study uses social network analysis and interviews to examine the communication and knowledge network structures of central office and site leaders in an ‘in need of improvement’ district facing sanctions under No Child Left Behind. Findings indicate sparse ties among and between school site and central office administrators, as well as a centralized network structure that may constrain the exchange of complex information and ultimately inhibit efforts at change