407 research outputs found
Photoluminescence spectroscopy of bandgap reduction in dilute InNAs alloys
Photoluminescence (PL) has been observed from dilute InNxAs1âx epilayers grown by molecular-beam epitaxy. The PL spectra unambiguously show band gap reduction with increasing N content. The variation of the PL spectra with temperature is indicative of carrier detrapping from localized to extended states as the temperature is increased. The redshift of the free exciton PL peak with increasing N content and temperature is reproduced by the band anticrossing model, implemented via a (5Ă5) k·p Hamiltonian
Interwell relaxation times in p-Si/SiGe asymmetric quantum well structures: the role of interface roughness
We report the direct determination of nonradiative lifetimes in SiâSiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band kâp model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height â©Ÿ1 Ă
), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAsâAlxGa1âxAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV material
The substellar population of the young cluster lambda Orionis
By collecting optical and infrared photometry and low resolution
spectroscopy, we have identified a large number of low mass stars and brown
dwarf candidates belonging to the young cluster (~5 Myr) associated with the
binary star lambda Orionis. The lowest mass object found is a M8.5 with an
estimated mass of 0.02 Msun (~0.01 Msun for objects without spectroscopic
confirmation).
For those objects with spectroscopy, the measured strength of the Halpha
emission line follows a distribution similar to other clusters with the same
age range, with larger equivalent widths for cooler spectral types. Three of
the brown dwarfs have Halpha emission equivalent widths of order 100 \AA,
suggestive that they may have accretion disks and thus are the substellar
equivalent of Classical T Tauri stars. We have derived the Initial Mass
Function for the cluster. For the substellar regime, the index of the mass
spectrum is alpha=0.60$+-0.06, very similar to other young associations.Comment: Astrophysica Journal, accepted April 2, 200
An Investigation of Be/X-ray Pulsars with OGLE-III Data
We have studied five seasons of OGLE-III data for eight SMC Be/X-ray pulsars
for which no other survey data were available. We have determined orbital
periods for four of these binary systems, one of which also shows nonradial
pulsations. Optical identification of SMC X-2 is reconsidered, but no periods
were found for either of the two possible candidates
Band anticrossing in GaNxSb1âx
Fourier transform infrared absorption measurements are presented from the dilute nitride semiconductor GaNSb with nitrogen incorporations between 0.2% and 1.0%. The divergence of transitions from the valence band to Eâ and E+ can be seen with increasing nitrogen incorporation, consistent with theoretical predictions. The GaNSb band structure has been modeled using a five-band k·p Hamiltonian and a band anticrossing fitting has been obtained using a nitrogen level of 0.78 eV above the valence band maximum and a coupling parameter of 2.6 eV
Young core collapse supernova remnants and their supernovae
Massive star supernovae can be divided into four categories depending on the
amount of mass loss from the progenitor star and the star's radius: red
supergiant stars with most of the H envelope intact (SN IIP), stars with some H
but most lost (IIL, IIb), stars with all H lost (Ib, Ic), and blue supergiant
stars with a massive H envelope (SN 1987A-like). Various aspects of the
immediate aftermath of the supernova are expected to develop in different ways
depending on the supernova category: mixing in the supernova, fallback on the
central compact object, expansion of any pulsar wind nebula, interaction with
circumstellar matter, and photoionization by shock breakout radiation. The
observed properties of young supernova remnants allow many of them to be placed
in one of the supernova categories; all the categories are represented except
for the SN 1987A-like type. Of the remnants with central pulsars, the pulsar
properties do not appear to be related to the supernova category. There is no
evidence that the supernova categories form a mass sequence, as would be
expected in a single star scenario for the evolution. Models for young pulsar
wind nebulae expanding into supernova ejecta indicate initial pulsar periods of
10-100 ms and approximate equipartition between particle and magnetic energies.
Ages are obtained for pulsar nebulae, including an age of 2400 pm 500 yr for
3C58, which is not consistent with an origin in SN 1181. There is no evidence
that mass fallback plays a role in neutron star properties.Comment: 43 pages, ApJ, revised, discussion of 3C58 changed, in press for Feb.
1, 200
Study of the Cir X--1 Broad Band Spectrum at Orbital Phases Close to the Apoastron
We report on the results of a BeppoSAX (1.8--200 keV) observation of the
peculiar X-ray binary source Circinus X--1 (Cir X--1) at the orbital phases
between 0.61 and 0.63. We find that three components are needed to fit the
broad band spectrum: a blackbody component, at a temperature of keV,
a Comptonized component, with a seed-photon temperature of keV,
electron temperature of keV and optical depth of , and a
power-law component dominating the spectrum at energies higher than 20 keV. We
interpret the blackbody as the emission from the accretion disk, while the
Comptonized component probably comes from a corona surrounding the inner part
of the system. This spectrum is different from that observed at the periastron
(Iaria et al. 2001a) because of the presence of the blackbody component. We
discuss the implications of this difference and the presence of the power-law
component.Comment: 15 pages, 6 figures, accepted by Ap
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Si:P as a laboratory analogue for hydrogen on high magnetic field white dwarf stars
Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10 5 T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8 T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H 2 analogues, and for investigation of He 2, a bound molecule predicted under extreme field conditions
Observations of the Crab Nebula and its pulsar in the far-ultraviolet and in the optical
We present HST/STIS far-UV observations of the Crab nebula and its pulsar.
Broad, blueshifted absorption arising in the nebula is seen in C IV 1550,
reaching about 2500 km/s. This can be interpreted as evidence for a fast outer
shell, and we adopt a spherically symmetric model to constrain the properties
of this. We find that the density appears to decrease outward in the shell. A
lower limit to the mass is 0.3 solar masses with an accompanying kinetic energy
of 1.5EE{49} ergs. A massive 10^{51} erg shell cannot be excluded, but is less
likely if the density profile is much steeper than R^{-4} and the velocity is
<6000 km/s. The observations cover the region 1140-1720 A. With the time-tag
mode of the spectrograph we obtain the pulse profile. It is similar to that in
the near-UV, although the primary peak is marginally narrower. Together with
the near-UV data, and new optical data from NOT, our spectrum of the pulsar
covers the entire region from 1140-9250 A. Dereddening the spectrum gives a
flat spectrum for E(B-V)=0.52, R=3.1. This dereddened spectrum of the Crab
pulsar can be fitted by a power law with spectral index alpha_{\nu} = 0.11 +/-
0.04. The main uncertainty is the amount and characteristics of the interstel-
lar reddening, and we have investigated the dependence of \alpha_{\nu} on
E(B-V) and R. In the extended emission covered by our 25" x 0.5" slit in the
far-UV, we detect C IV 1550 and He II 1640 emission lines from the Crab nebula.
Several interstellar absorption lines are detected toward the pulsar. The Ly
alpha absorption indicates a column density of 3.0+/-0.5\EE{21} cm^{-2} of
neutral hydrogen, which agrees well with our estimate of E(B-V)=0.52 mag. Other
lines show no evidence of severe depletion of metals in atomic gas.Comment: 18 pages emulateapj style, including 10 figures. ApJ, accepte
A Multi-Wavelength, Multi-Epoch Study of the Soft X-Ray Transient Prototype, V616 Mon (A0620-00)
We have obtained optical and infrared photometry of the soft x-ray transient
prototype V616 Mon (A0620-00). From this photometry, we find a spectral type of
K4 for the secondary star in the system, which is consistent with spectroscopic
observations. We present J-, H-, and K-band light curves modeled with WD98 and
ELC. Combining detailed, independently run models for ellipsoidal variations
due to a spotted, non-spherical secondary star, and the observed ultraviolet to
infrared spectral energy distribution of the system, we show that the most
likely value for the orbital inclination is 40.75 +/- 3 deg. This inclination
angle implies a primary black hole mass of 11.0 +/- 1.9 solar masses.Comment: 29 pages (preprint format), including 7 figures and 4 tables,
accepted for publication in the Nov 2001 issue of A
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