183 research outputs found
Non-Equipartition of Energy, Masses of Nova Ejecta, and Type Ia Supernovae
The total masses ejected during classical nova eruptions are needed to answer
two questions with broad astrophysical implications: Can accreting white dwarfs
be pushed towards the Chandrasekhar mass limit to yield type Ia supernovae? Are
Ultra-luminous red variables a new kind of astrophysical phenomenon, or merely
extreme classical novae? We review the methods used to determine nova ejecta
masses. Except for the unique case of BT Mon (nova 1939), all nova ejecta mass
determinations depend on untested assumptions and multi-parameter modeling. The
remarkably simple assumption of equipartition between kinetic and radiated
energy (E_kin and E_rad, respectively) in nova ejecta has been invoked as a way
around this conundrum for the ultra-luminous red variable in M31. The deduced
mass is far larger than that produced by any classical nova model. Our nova
eruption simulations show that radiation and kinetic energy in nova ejecta are
very far from being in energy equipartition, with variations of four orders of
magnitude in the ratio E_kin/E_rad being commonplace. The assumption of
equipartition must not be used to deduce nova ejecta masses; any such
"determinations" can be overestimates by a factor of up to 10,000. We
data-mined our extensive series of nova simulations to search for correlations
that could yield nova ejecta masses. Remarkably, the mass ejected during a nova
eruption is dependent only on (and is directly proportional to) E_rad. If we
measure the distance to an erupting nova and its bolometric light curve then
E_rad and hence the mass ejected can be directly measured.Comment: 9 pages, 4 figures, awaiting publication in ApJ
Open String Moduli in KKLT Compactifications
In the Kachru-Kallosh-Linde-Trivedi (KKLT) de-Sitter construction one
introduces an anti-D3-brane that breaks the supersymmetry and leads to a
positive cosmological constant. In this paper we investigate the open string
moduli associated with this anti-D3-brane, corresponding to its position on the
3-sphere at the tip of the deformed conifold. We show that in the KKLT
construction these moduli are very light, and we suggest a possible way to give
these moduli a large mass by putting orientifold planes in the KKLT "throat".Comment: harvmac, 22 page
WISeREP - An Interactive Supernova Data Repository
We have entered an era of massive data sets in astronomy. In particular, the
number of supernova (SN) discoveries and classifications has substantially
increased over the years from few tens to thousands per year. It is no longer
the case that observations of a few prototypical events encapsulate most
spectroscopic information about SNe, motivating the development of modern tools
to collect, archive, organize and distribute spectra in general, and SN spectra
in particular. For this reason we have developed the Weizmann Interactive
Supernova data REPository - WISeREP - an SQL-based database (DB) with an
interactive web-based graphical interface. The system serves as an archive of
high quality SN spectra, including both historical (legacy) data as well as
data that is accumulated by ongoing modern programs. The archive provides
information about objects, their spectra, and related meta-data. Utilizing
interactive plots, we provide a graphical interface to visualize data, perform
line identification of the major relevant species, determine object redshifts,
classify SNe and measure expansion velocities. Guest users may view and
download spectra or other data that have been placed in the public domain.
Registered users may also view and download data that are proprietary to
specific programs with which they are associated. The DB currently holds >8000
spectra, of which >5000 are public; the latter include published spectra from
the Palomar Transient Factory, all of the SUSPECT archive, the
Caltech-Core-Collapse Program, the CfA SN spectra archive and published spectra
from the UC Berkeley SNDB repository. It offers an efficient and convenient way
to archive data and share it with colleagues, and we expect that data stored in
this way will be easy to access, increasing its visibility, usefulness and
scientific impact.Comment: To be published in PASP. WISeREP:
http://www.weizmann.ac.il/astrophysics/wiserep
The Red Nova-like Variable in M31 - A Blue Candidate in Quiescence
M31-RV was an extraordinarily luminous (~10^6 Lsun) eruptive variable,
displaying very cool temperatures (roughly 1000 Kelvins) as it faded. The
photometric behavior of M31-RV (and several other very red novae, i.e. luminous
eruptive red variables) has led to several models of this apparently new class
of astrophysical object. One of the most detailed models is that of
"mergebursts": hypothetical mergers of close binary stars. These are predicted
to rival or exceed the brightest classical novae in luminosity, but to be much
cooler and redder than classical novae, and to become slowly hotter and bluer
as they age. This prediction suggests two stringent and definitive tests of the
mergeburst hypothesis. First, there should always be a cool red remnant, and
NOT a hot blue remnant at the site of such an outburst. Second, the inflated
envelope of a mergeburst event should be slowly contracting, hence it must
display a slowly rising effective temperature. We have located a luminous,
UV-bright object within 0.4 arcsec (1.5 sigma of the astrometric position) of
M31-RV in archival WFPC2 images taken 10 years after the outburst: it resembles
an old nova. Twenty years after the outburst, the object remains much too hot
to be a mergeburst. Its behavior remains consistent with that of theoretical
nova models which erupt on a low mass white dwarf. Future Hubble UV and visible
images could determine if the M31-RV analogs (in M85 and in M99) are also
behaving like old novae.Comment: Accepted for publication in ApJ, comments welcom
Inhibitory NK Receptor Recognition of HLA-G: Regulation by Contact Residues and by Cell Specific Expression at the Fetal-Maternal Interface
The non-classical HLA-G protein is distinguished from the classical MHC class I molecules by its expression pattern, low polymorphism and its ability to form complexes on the cell surface. The special role of HLA-G in the maternal-fetal interface has been attributed to its ability to interact with specific receptors found on maternal immune cells. However this interaction is restricted to a limited number of receptors. In this study we elucidate the reason for this phenomenon by comparing the specific contact residues responsible for MHC-KIR interactions. This alignment revealed a marked difference between the HLA-G molecule and other MHC class I molecules. By mutating these residues to the equivalent classical MHC residues, the HLA-G molecule regained an ability of interacting with KIR inhibitory receptors found on NK cells derived either from peripheral blood or from the decidua. Functional NK killing assays further substantiated the binding results. Furthermore, double immunofluorescent staining of placental sections revealed that while the conformed form of HLA-G was expressed in all extravillous trophoblasts, the free heavy chain form of HLA-G was expressed in more distal cells of the column, the invasion front. Overall we suggest that HLA-G protein evolved to interact with only some of the NK inhibitory receptors thus allowing a control of inhibition, while permitting appropriate NK cell cytokine and growth factor production necessary for a viable maternal fetal interface
An Extended Grid of Nova Models. III. Very Luminous, Red Novae
Extremely luminous, red eruptive variables like RV in M31 are being suggested
as exemplars of a new class of astrophysical object. Our greatly extended
series of nova simulations shows that classical nova models can produce very
red, luminous eruptions. In a poorly studied corner of 3-D nova parameter space
(very cold, low-mass white dwarfs, accreting at very low rates) we find bona
fide classical novae that are very luminous and red because they eject very
slowly moving, massive envelopes. A crucial prediction of these nova models -
in contrast to the predictions of merging star ("mergeburst") models - is that
a hot remnant, the underlying white dwarf, will emerge after the massive
ejected envelope has expanded enough to become optically thin. This blue
remnant must fade on a timescale of decades - much faster than a "mergeburst",
which must fade on timescales of millennia or longer. Furthermore, the cooling
nova white dwarf and its expanding ejecta must become redder in the years after
eruption, while a contracting mergeburst must become hotter and bluer. We
predict that red novae will always brighten to L~1000 Lsun for about 1 year
before rising to maximum luminosity at L~10^6 - 10^7 Lsun. The maximum
luminosity attainable by a nova is likely to be L~10^7 Lsun, corresponding to
M-12. In an accompanying paper we describe a fading, luminous blue candidate
for the remnant of M31-RV; it is observed with HST to be compatible only with
the nova model.Comment: Accepted for publication in ApJ. Comments welcom
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