552 research outputs found
Period derivative of the M15 X-ray Binary AC211/X2127+119
We have combined Rossi X-ray Timing Explorer observations of X2127+119, the
low-mass X-ray binary in the globular cluster M15, with archival X-ray
lightcurves to study the stability of the 17.1 hr orbital period. We find that
the data cannot be fit by the Ilovaisky (1993) ephemeris, and requires either a
7sigma change to the period or a period derivative Pdot/P~9x10e-7 per year.
Given its remarkably low L_X/L_opt such a Pdot lends support to models that
require super-Eddington mass transfer in a q~1 binary.Comment: 11 pages, 3 figures, to be published in New Astronom
The Phonon Drag Effect in Single-Walled Carbon Nanotubes
A variational solution of the coupled electron-phonon Boltzmann equations is
used to calculate the phonon drag contribution to the thermopower in a 1-D
system. A simple formula is derived for the temperature dependence of the
phonon drag in metallic, single-walled carbon nanotubes. Scattering between
different electronic bands yields nonzero values for the phonon drag as the
Fermi level varies.Comment: 8 pages, 4 figure
Multiwavelength analysis of the young open cluster NGC 2362
We present a multiwavelength analysis of the young open cluster NGC 2362.
UBVRcIc CCD photometric observations, together with available data in the
Chandra data base, near infrared data from the Two Micron All Sky Survey
(2MASS), and recently published Halpha spectroscopy were used to get
information about the evolutionary stage of the cluster and the main physical
properties of its stellar content. Cluster membership is estimated for every
individual star by means of ZAMS and isochrone fitting. The cluster is
confirmed to host a richly populated pre-main sequence (PMS), and to contain a
large amount of X-ray emitting stars, which reach from the PMS members of GK
spectral type, up to the most luminous OB type main sequence (MS) members. The
PMS cluster members show no significant age spread, and the comparison to both
PMS and post-MS isochrones suggests a younger age for the more massive MS than
for lower mass PMS members. The analysis allows to asses the validity of
currently used pre-main sequence evolutionary models, and supports the
suggestion of a well defined positive correlation of the X-ray emission from
PMS stars with their bolometric luminosity. Clear differences are found on the
other hand, between the X-ray activity properties of MS and PMS cluster
members, both in the relation between X-ray luminosity and bolometric
luminosity, and in spectral properties as well.Comment: 1 gzipped file: 1 tex file with 9 pages text. 5 ps files with
figures. Submitted to Astrophysical Journa
The Formation Rate of Blue Stragglers in 47 Tucanae
We investigate the effects of changes in the blue straggler formation rate in
globular clusters on the blue straggler distribution in the color-magnitude
diagram. We find that the blue straggler distribution is highly sensitive to
the past formation rate. Comparing our models to new UBV observations of a
region close to the core of 47 Tucanae suggests that this cluster may have
stopped forming blue straggler formation several Gyr ago. This cessation of
formation can be associated with an epoch of primordial binary burning which
has been invoked in other clusters to infer the imminence of core collapse.Comment: 17 pages, 9 figures, submitted to the Astrophysical Journa
A new mass-ratio for the X-ray Binary X2127+119 in M15?
The luminous low-mass X-ray binary X2127+119 in the core of the globular
cluster M15 (NGC 7078), which has an orbital period of 17 hours, has long been
assumed to contain a donor star evolving off the main sequence, with a mass of
0.8 solar masses (the main-sequence turn-off mass for M15). We present
orbital-phase-resolved spectroscopy of X2127+119 in the H-alpha and He I 6678
spectral region, obtained with the Hubble Space Telescope. We show that these
data are incompatible with the assumed masses of X2127+119's component stars.
The continuum eclipse is too shallow, indicating that much of the accretion
disc remains visible during eclipse, and therefore that the size of the donor
star relative to the disc is much smaller in this high-inclination system than
the assumed mass-ratio allows. Furthermore, the flux of X2127+119's He I 6678
emission, which has a velocity that implies an association with the stream-disc
impact region, remains unchanged through eclipse, implying that material from
the impact region is always visible. This should not be possible if the
previously-assumed mass ratio is correct. In addition, we do not detect any
spectral features from the donor star, which is unexpected for a 0.8 solar-mass
sub-giant in a system with a 17-hour period.Comment: 6 pages, 4 figures, accepted by A&
Gravitational Radiation From Globular Clusters
Space-based gravitational wave detectors will have the ability to observe
continuous low frequency gravitational radiation from binary star systems. They
can determine the direction to continuous sources with an angular resolution
approaching tens of arcminutes. This resolution should be sufficient to
identify binary sources as members of some nearby globular clusters. Thus,
gravitational radiation can be used to determine the population of hard
binaries in globular clusters. For particularly hard binaries, the orbital
period may change as a result of gravitational wave emission. If one of these
binaries can be identified with a globular cluster, then the distance to that
cluster can be determined. Thus, gravitational radiation may provide
reddening-independent distance measurements to globular clusters.Comment: 26 pages, 1 figure, LaTeX, uses aasms4.sty, submitted to Ap.
A Geometric, Dynamical Approach to Thermodynamics
We present a geometric and dynamical approach to the micro-canonical ensemble
of classical Hamiltonian systems. We generalize the arguments in \cite{Rugh}
and show that the energy-derivative of a micro-canonical average is itself
micro-canonically observable. In particular, temperature, specific heat and
higher order derivatives of the entropy can be observed dynamically. We give
perturbative, asymptotic formulas by which the canonical ensemble itself can be
reconstructed from micro-canonical measurements only. In a purely
micro-canonical approach we rederive formulas by Lebowitz et al \cite{LPV},
relating e.g. specific heat to fluctuations in the kinetic energy. We show that
under natural assumptions on the fluctuations in the kinetic energy the
micro-canonical temperature is asymptotically equivalent to the standard
canonical definition using the kinetic energy.Comment: 7 pages, LaTeX, uses RevTex. New sections and examples using
fluctuations in the kinetic energy adde
What grounded theory is ... a critically reflective conversation among scholars
Grounded theory (GT) is taught in many doctoral schools across the world and exemplified in most methodological books and publications in top-tier journals as a qualitative research method. This limited view of GT does not allow full use of possible resources and restrains researchers’ creativity and capabilities. Thus, it blocks some innovative possibilities and the emergence of valuable theories, which are badly needed. Therefore, understanding the full reach and scope of GT is becoming urgent, and we brought together a panel of established grounded theory scholars to help us in this endeavor through a reflective conversation
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