19 research outputs found
Planetary Systems Around Neutron Stars
This project was initiated in 1993, about one year after the announcement of two planets around PSR B1257+12. Its goal was to investigate planetary systems around neutron stars using high precision timing of radio pulsars as a tool. A microsecond precision of the pulse timing analysis, which is equivalent to a millimeter-per-second radial velocity resolution, makes it possible to detect asteroid-mass bodies in orbit around pulsars and to study the dynamics of pulsar planetary systems. The project originally consisted of two longterm efforts: (i) routine observations and timing analysis of the millisecond pulsar PSR B1257+12 which was found to be orbited by at least two earth-mass bodies (Wolszczan and Frail, Nature, 355, 145) and (ii) a sensitive all-sky search for millisecond pulsars to detect further examples of neutron stars with planetary systems. In the third year of the project, it was expanded to include long-term timing observations of slow pulsars in search for planetary systems around these younger neutron stars. The instrumentation used to conduct these investigations included the 305-m Arecibo antenna with the Penn State Pulsar Machine (PSPM-1), the 100-m Effelsberg telescope with the local pulse timing hardware, and the 32-m paraboloid of the Torun Centre for Astronomy in Torun, Poland (TCFA) with the PSPM-2, the second pulsar machine built at Penn State. The PI's collaborators included pulsar groups led by D. Backer (Berkeley), R. Foster (NRL), S. Kulkarni (Caltech), J. Taylor (Princeton) and R. Wielebinski (Bonn). One postdoc (Stuart Anderson), one graduate student (Brian Cadwell) and several undergraduates have been engaged in various aspects of research related to this project
Survivability of radio-loud planetary cores orbiting white dwarfs
The discovery of the intact metallic planetary core fragment orbiting the white dwarf SDSS J1228+1040 within one Solar radius highlights the possibility of detecting larger, unfragmented conducting cores around magnetic white dwarfs through radio emission. Previous models of this decades-old idea focussed on determining survivability of the cores based on their inward Lorentz drift towards the star. However, gravitational tides may represent an equal or dominant force. Here, we couple both effects by assuming a Maxwell rheological model and performing simulations over the entire range of observable white dwarf magnetic field strengths (103 – 109 G) and their potential atmospheric electrical conductivities (10−1 – 104 S/m) in order to more accurately constrain survivability lifetimes. This force coupling allows us to better pinpoint the physical and orbital parameters which allow planetary cores to survive for over a Gyr, maximizing the possibility that they can be detected. The most robust survivors showcase high dynamic viscosities (≳ 1024 Pa·s) and orbit within kG-level magnetic fields
Extrasolar Planet Inferometric Survey (EPIcS)
The discovery of the nature of the solar system was a crowning achievement of Renaissance science. The quest to evaluate the properties of extrasolar planetary systems is central to both the intellectual understanding of our origins and the cultural understanding of humanity's place in the Universe; thus it is appropriate that the goals and objectives of NASA's breakthrough Origins program emphasize the study of planetary systems, with a focus on the search for habitable planets. We propose an ambitious research program that will use SIM - the first major mission of the Origins program - to explore planetary systems in our Galactic neighborhood. Our program is a novel two-tiered SIM survey of nearby stars that exploits the capabilities of SIM to achieve two scientific objectives: (i) to identify Earth-like planets in habitable regions around nearby Sunlike stars: and (ii) to explore the nature and evolution of planetary systems in their full variety. The first of these objectives was recently recommended by the Astronomy and Astrophysics Survey Committee (the McKee-Taylor Committee) as a prerequisite for the development of the Terrestrial Planet Finder mission later in the decade. Our program combines this two-part survey with preparatory and contemporaneous research designed to maximize the scientific return from the limited and thus precious observing resources of SIM
The Outcome of the Protoplanetary Disk of Very Massive Stars
We suggest that planets, brown dwarfs, and even low mass stars can be formed
by fragmentation of protoplanetary disks around very massive stars M>~100 solar
masses. We discuss how fragmentation conditions make the formation of very
massive planetary systems around very massive stars favorable. Such planetary
systems are likely to be composed of brown dwarfs and low mass stars of
~0.1-0.3 solar masses, at orbital separations of ~ few x 100 - 10^4 AU. In
particular, scaling from solar-like stars suggests that hundreds of
Mercury-like planets might orbit very massive stars at ~1000 AU, where
conditions might favor liquid water. Such fragmentation objects can be
excellent targets for the James Webb Space Telescope and other large telescopes
working in the IR bands. We predict that deep observations of very massive
stars would reveal these fragmentation objects, orbiting in the same orbital
plane in cases where there are more than one object.Comment: Accepted to New Astronom
On the progenitors of millisecond pulsars by the recycling evolutionary channel
The recycling model suggested that low-mass X-ray binaries (LMXBs) could
evolve into binary millisecond pulsars (BMSPs). In this work, we attempt to
investigate the progenitor properties of BMSPs formed by the recycling
evolutionary channel, and if sub-millisecond pulsars can be produced by this
channel. Using Eggleton's stellar evolution code, considering that the dead
pulsars can be spun up to a short spin period by the accreting material and
angular momentum from the donor star, we have calculated the evolution of close
binaries consisting of a neutron star and a low-mass main-sequence donor star,
and the spin evolution of NSs. In calculation, some physical process such as
the thermal and viscous instability of a accretion disk, propeller effect, and
magnetic braking are included. Our calculated results indicate that, all LMXBs
with a low-mass donor star of 1.0 - 2.0 and a short orbital period
(\la 3-4 \rm d) can form millisecond pulsars with a spin period less than 10
ms. However, it is difficult to produce sub-millisecond pulsars by this
evolutionary channel. In addition, our evolutionary scenario cannot account for
the existence of BMSPs with a long orbital period (P_{\rm orb}\ga 70-80\rm
d).Comment: 7 pages,5 figures, MNRAS in pres
The characteristics of millisecond pulsar emission: I. Spectra, pulse shapes and the beaming fraction
We have monitored a large sample of millisecond pulsars using the 100-m
Effelsberg radio telescope in order to compare their radio emission properties
to the slowly rotating population. With some notable exceptions, our findings
suggest that the two groups of objects share many common properties. A
comparison of the spectral indices between samples of normal and millisecond
pulsars demonstrates that millisecond pulsar spectra are not significantly
different from those of normal pulsars. There is evidence, however, that
millisecond pulsars are slightly less luminous and less efficient radio
emitters compared to normal pulsars. We confirm recent suggestions that a
diversity exists among the luminosities of millisecond pulsars with the
isolated millisecond pulsars being less luminous than the binary millisecond
pulsars. There are indications that old millisecond pulsars exhibit somewhat
flatter spectra than the presumably younger ones. We present evidence that
millisecond pulsar profiles are only marginally more complex than those found
among the normal pulsar population. Moreover, the development of the profiles
with frequency is rather slow, suggesting very compact magnetospheres. The
profile development seems to anti-correlate with the companion mass and the
spin period, again suggesting that the amount of mass transfer in a binary
system might directly influence the emission properties. The angular radius of
radio beams of millisecond pulsars does not follow the scaling predicted from a
canonical pulsar model which is applicable for normal pulsars. Instead they are
systematically smaller. The smaller inferred luminosity and narrower emission
beams will need to be considered in future calculations of the birth-rate of
the Galactic population.Comment: 40 pages, 14 figures, accepted for publication in Ap
The characteristics of millisecond pulsar emission: II. Polarimetry
We have made polarimetric monitoring observations of millisecond pulsars
visible from the northern hemisphere at 1410 MHz. Their emission properties are
compared with those of normal pulsars. Although we demonstrated in paper I that
millisecond pulsars exhibit the same flux density spectra and similar profile
complexity, our results presented here suggest that millisecond pulsar profiles
do not comply with the predictions of classification schemes based on
``normal'' pulsars. The frequency development of a large number of millisecond
pulsar profiles is abnormal when compared with the development seen for normal
pulsars. Moreover, the polarization characteristics suggest that
millisecond-pulsar magnetospheres might not simply represent scaled versions of
the magnetospheres of normal pulsars, supporting results of paper I. However,
phenomena such as mode-changing activity in both intensity and polarization are
recognized here for the first time (e.g., J1730--2304). This suggests that
while the basic emission mechanism remains insensitive to rotational period,
the conditions that, according to the canonical pulsar model, regulate the
radio emission, might be satisfied at different regions in millisecond pulsar
magnetospheres.
At least three types of model have been proposed to describe the millisecond
pulsar magnetospheres. A comparison of the predictions of these models with the
observations suggests that individual cases are better explained by different
processes. However, we show that millisecond pulsars can be grouped according
to common emission properties, a grouping that awaits verification from future
multifrequency observations.Comment: 38 pages, 8 figures, accepted for publication in ApJ, (see
astro-ph/9801177 for paper I