25,703 research outputs found
Short-duration lensing events: II. Expectations and Protocols
Ongoing microlensing observations by OGLE and MOA regularly identify and
conduct high-cadence sampling of lensing events with Einstein diameter crossing
time, tau_E, of 16 or fewer days. Events with estimated values of tau_E of one
to two days have been detected. Short duration events tend to be generated by
low-mass lenses or by lenses with high transverse velocities. We compute the
expected rates, demonstrate the expected ranges of parameters for lenses of
different mass, and develop a protocol for observing and modeling
short-duration events. Relatively minor additions to the procedures presently
used will increase the rate of planet discovery, and also discover or place
limits on the population of high-speed dim stars and stellar remnants in the
vicinity of the Sun.Comment: 17 pages; 3 figures; submitted to ApJ 3 July 200
Spin-Up/Spin-Down models for Type Ia Supernovae
In the single degenerate scenario for Type Ia supernova (SNeIa), a white
dwarf (WD) must gain a significant amount of matter from a companion star.
Because the accreted mass carries angular momentum, the WD is likely to achieve
fast spin periods, which can increase the critical mass, , needed for
explosion. When is higher than the maximum mass achieved by the WD,
the WD must spin down before it can explode. This introduces a delay between
the time at which the WD has completed its epoch of mass gain and the time of
the explosion. Matter ejected from the binary during mass transfer therefore
has a chance to become diffuse, and the explosion occurs in a medium with a
density similar to that of typical regions of the interstellar medium. Also,
either by the end of the WD's mass increase or else by the time of explosion,
the donor may exhaust its stellar envelope and become a WD. This alters,
generally diminishing, explosion signatures related to the donor star.
Nevertheless, the spin-up/spin-down model is highly predictive. Prior to
explosion, progenitors can be super- WDs in either wide binaries with
WD companions, or else in cataclysmic variables. These systems can be
discovered and studied through wide-field surveys. Post explosion, the
spin-up/spin-down model predicts a population of fast-moving WDs, low-mass
stars, and even brown dwarfs. In addition, the spin-up/spin-down model provides
a paradigm which may be able to explain both the similarities and the diversity
observed among SNeIa.Comment: Submitted to ApJ Letter
The Progenitors of Type Ia Supernovae: II. Are they Double-Degenerate Binaries? The Symbiotic Channel
In order for a white dwarf (WD) to achieve the Chandrasekhar mass, M_C, and
explode as a Type Ia supernova (SNIa), it must interact with another star,
either accreting matter from or merging with it. The failure to identify the
types of binaries which produce SNeIa is the "progenitor problem". Its solution
is required if we are to utilize the full potential of SNeIa to elucidate basic
cosmological and physical principles. In single-degenerate models, a WD
accretes and burns matter at high rates. Nuclear-burning WDs (NBWDs) with mass
close to M_C are hot and luminous, potentially detectable as supersoft x-ray
sources (SSSs). In previous work we showed that > 90-99% of the required number
of progenitors do not appear as SSSs during most of the crucial phase of mass
increase. The obvious implication is that double-degenerate (DD) binaries form
the main class of progenitors. We show in this paper, however, that many
binaries that later become DDs must pass through a long-lived NBWD phase during
which they are potentially detectable as SSSs. The paucity of SSSs is therefore
not a strong argument in favor of DD models. Those NBWDs that are the
progenitors of DD binaries are likely to appear as symbiotic binaries for
intervals > 10^6 years. In fact, symbiotic pre-DDs should be common, whether or
not the WDs eventually produce SNeIa. The key to solving the progenitor problem
lies in understanding the appearance of NBWDs. Most do not appear as SSSs most
of the time. We therefore consider the evolution of NBWDs to address the
question of what their appearance may be and how we can hope to detect them.Comment: 24 pages; 5 figures; submitted to Ap
The Progenitors of Type Ia Supernovae: Are They Supersoft Sources?
In a canonical model, the progenitors of Type Ia supernovae (SNe Ia) are
accreting, nuclear-burning white dwarfs (NBWDs), which explode when the white
dwarf reaches the Chandrasekhar mass, M_C. Such massive NBWDs are hot (kT ~100
eV), luminous (L ~ 10^{38} erg/s), and are potentially observable as luminous
supersoft X-ray sources (SSSs). During the past several years, surveys for soft
X-ray sources in external galaxies have been conducted. This paper shows that
the results falsify the hypothesis that a large fraction of progenitors are
NBWDs which are presently observable as SSSs. The data also place limits on
sub-M_C models. While Type Ia supernova progenitors may pass through one or
more phases of SSS activity, these phases are far shorter than the time needed
to accrete most of the matter that brings them close to M_C.Comment: submitted to ApJ 18 November 2009; 17 pages, 2 figure
Populations of Supersoft X-ray Sources: Novae, tidal disruption, Type Ia supernovae, accretion-induced collapse, ionization, and intermediate-mass black holes?
Observations of hundreds of supersoft x-ray sources (SSSs) in external
galaxies have shed light on the diversity of the class and on the natures of
the sources. SSSs are linked to the physics of Type Ia supernovae and
accretion-induced collapse, ultraluminous x-ray sources and black holes, the
ionization of the interstellar medium, and tidal disruption by supermassive
black holes. The class of SSSs has an extension to higher luminosities:
ultraluminous SSSs have luminosities above 10^39 erg/s. There is also an
extension to higher energies: quasisoft x-ray sources (QSSs) emit photons with
energies above 1 eV, but few or none with energies above 2 keV. Finally, a
significant fraction of the SSSs found in external galaxies switch states
between observations, becoming either quasisoft or hard. For many systems
``supersoft'' refers to a temporary state; SSSs are sources, possibly including
a variety of fundamentally different system types, that pass through such a
state. We review those results derived from extragalactic data and related
theoretical work that are most surprising and that suggest directions for
future research.Comment: submitted to Astron.Nachr.; latex, 6 figure
Discovering habitable Earths, hot Jupiters and other close planets with microlensing
Searches for planets via gravitational lensing have focused on cases in which
the projected separation, a, between planet and star is comparable to the
Einstein radius, R_E. This paper considers smaller orbital separations and
demonstrates that evidence of close-orbit planets can be found in the
low-magnification portion of the light curves generated by the central star. We
develop a protocol to discover hot Jupiters as well as Neptune-mass and
Earth-mass planets in the stellar habitable zone. When planets are not
discovered, our method can be used to quantify the probability that the lens
star does not have planets within specified ranges of the orbital separation
and mass ratio. Nearby close-orbit planets discovered by lensing can be subject
to follow-up observations to study the newly-discovered planets or to discover
other planets orbiting the same star. Careful study of the low-magnification
portions of lensing light curves should produce, in addition to the discoveries
of close-orbit planets, definite detections of wide-orbit planets through the
discovery of "repeating" lensing events. We show that events exhibiting
extremely high magnification can effectively be probed for planets in close,
intermediate, and wide distance regimes simply by adding several-time-per-night
monitoring in the low-magnification wings, possibly leading to gravitational
lensing discoveries of multiple planets occupying a broad range of orbits, from
close to wide, in a single planetary system.Comment: 21 pages, 5 figures, submitted to the Astrophysical Journa
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