2,265 research outputs found
The young stellar population of Lynds 1340. An infrared view
We present results of an infrared study of the molecular cloud Lynds 1340,
forming three groups of low and intermediate-mass stars. Our goals are to
identify and characterise the young stellar population of the cloud, study the
relationships between the properties of the cloud and the emergent stellar
groups, and integrate L1340 into the picture of the star-forming activity of
our Galactic environment. We selected candidate young stellar objects from the
Spitzer and WISE data bases using various published color criteria, and
classified them based on the slope of the spectral energy distribution. We
identified 170 Class II, 27 Flat SED, and Class 0/I sources. High angular
resolution near-infrared observations of the RNO 7 cluster, embedded in L1340,
revealed eight new young stars of near-infrared excess. The surface density
distribution of young stellar objects shows three groups, associated with the
three major molecular clumps of L1340, each consisting of less than 100
members, including both pre-main sequence stars and embedded protostars. New
Herbig--Haro objects were identified in the Spitzer images. Our results
demonstrate that L1340 is a prolific star-forming region of our Galactic
environment in which several specific properties of the intermediate-mass mode
of star formation can be studied in detail.Comment: 73 pages, 33 figures, 15 tables. Accepted for publication in ApJ
Transition-metal interactions in aluminum-rich intermetallics
The extension of the first-principles generalized pseudopotential theory
(GPT) to transition-metal (TM) aluminides produces pair and many-body
interactions that allow efficient calculations of total energies. In
aluminum-rich systems treated at the pair-potential level, one practical
limitation is a transition-metal over-binding that creates an unrealistic TM-TM
attraction at short separations in the absence of balancing many-body
contributions. Even with this limitation, the GPT pair potentials have been
used effectively in total-energy calculations for Al-TM systems with TM atoms
at separations greater than 4 AA. An additional potential term may be added for
systems with shorter TM atom separations, formally folding repulsive
contributions of the three- and higher-body interactions into the pair
potentials, resulting in structure-dependent TM-TM potentials. Towards this
end, we have performed numerical ab-initio total-energy calculations using VASP
(Vienna Ab Initio Simulation Package) for an Al-Co-Ni compound in a particular
quasicrystalline approximant structure. The results allow us to fit a
short-ranged, many-body correction of the form a(r_0/r)^{b} to the GPT pair
potentials for Co-Co, Co-Ni, and Ni-Ni interactions.Comment: 18 pages, 5 figures, submitted to PR
The Enigmatic Radio Afterglow of GRB 991216
We present wide-band radio observations spanning from 1.4 GHz to 350 GHz of
the afterglow of GRB 991216, taken from 1 to 80 days after the burst. The
optical and X-ray afterglow of this burst were fairly typical and are explained
by a jet fireball. In contrast, the radio light curve is unusual in two
respects: (a) the radio light curve does not show the usual rise to maximum
flux on timescales of weeks and instead appears to be declining already on day
1 and (b) the power law indices show significant steepening from the radio
through the X-ray bands. We show that the standard fireball model, in which the
afterglow is from a forward shock, is unable to account for (b) and we conclude
that the bulk of the radio emission must arise from a different source. We
consider two models, neither of which can be ruled out with the existing data.
In the first (conventional) model, the early radio emission is attributed to
emission from the reverse shock as in the case of GRB 990123. We predict that
the prompt optical emission would have been as bright (or brighter) than 8th
magnitude. In the second (exotic) model, the radio emission originates from the
forward shock of an isotropically energetic fireball (10^54 erg) expanding into
a tenuous medium (10^-4 cm^-3). The resulting fireball would remain
relativistic for months and is potentially resolvable with VLBI techniques.
Finally, we note that the near-IR bump of the afterglow is similar to that seen
in GRB 971214 and no fireball model can explain this bump.Comment: ApJ, submitte
A Coordinated Radio Afterglow Program
We describe a ground-based effort to find and study afterglows at centimeter
and millimeter wavelengths. We have observed all well-localized gamma-ray
bursts in the Northern and Southern sky since BeppoSAX first started providing
rapid positions in early 1997. Of the 23 GRBs for which X-ray afterglows have
been detected, 10 have optical afterglows and 9 have radio afterglows. A
growing number of GRBs have both X-ray and radio afterglows but lack a
corresponding optical afterglow.Comment: To appear in Proc. of the 5th Huntsville Gamma-Ray Burst Symposium, 5
pages, LaTe
Test of Quantum Action for Inverse Square Potential
We present a numerical study of the quantum action previously introduced as a
parametrisation of Q.M. transition amplitudes. We address the questions: Is the
quantum action possibly an exact parametrisation in the whole range of
transition times ()? Is the presence of potential terms beyond
those occuring in the classical potential required? What is the error of the
parametrisation estimated from the numerical fit? How about convergence and
stability of the fitting method (dependence on grid points, resolution, initial
conditions, internal precision etc.)? Further we compare two methods of
numerical determination of the quantum action: (i) global fit of the Q.M.
transition amplitudes and (ii) flow equation. As model we consider the inverse
square potential, for which the Q.M. transition amplitudes are analytically
known. We find that the relative error of the parametrisation starts from zero
at T=0 increases to about at and then decreases to zero
when . Second, we observe stability of the quantum action under
variation of the control parameters. Finally, the flow equation method works
well in the regime of large giving stable results under variation of
initial data and consistent with the global fit method.Comment: Text (LaTeX), Figures(ps
The Broadband Afterglow of GRB980329
We present radio observations of the afterglow of the bright gamma-ray burst
GRB980329 made between one month and several years after the burst, a
re-analysis of previously published submillimeter data, and late-time optical
and near-infrared (NIR) observations of the host galaxy. From the absence of a
spectral break in the optical/NIR colors of the host galaxy, we exclude the
earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our
data with the numerous multi-wavelength observations of the early afterglow,
fit a comprehensive afterglow model to the entire broadband dataset, and derive
fundamental physical parameters of the blast-wave and its host environment.
Models for which the ejecta expand isotropically require both a high
circumburst density and extreme radiative losses from the shock. No low density
model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51}
erg, with the ejecta narrowly collimated to an opening angle of a few degrees,
driven into a surrounding medium with density ~ 20 cm^{-3}, provides a
satisfactory fit to the lightcurves over a range of redshifts.Comment: 27 pages, incl. 6 figures, minor revisions (e.g. added/updated
references) Accepted by Ap
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