168 research outputs found
A Stellar Rotation Census of B Stars: from ZAMS to TAMS
Two recent observing campaigns provide us with moderate dispersion spectra of
more than 230 cluster and 370 field B stars. Combining them and the spectra of
the B stars from our previous investigations (430 cluster and 100
field B stars) yields a large, homogeneous sample for studying the rotational
properties of B stars. We derive the projected rotational velocity ,
effective temperature, gravity, mass, and critical rotation speed for each star. We find that the average is significantly lower
among field stars because they are systematically more evolved and spun down
than their cluster counterparts. The rotational distribution functions of
for the least evolved B stars show that lower mass B
stars are born with a larger proportion of rapid rotators than higher mass B
stars. However, the upper limit of that may separate
normal B stars from emission line Be stars (where rotation promotes mass loss
into a circumstellar disk) is smaller among the higher mass B stars. We compare
the evolutionary trends of rotation (measured according to the polar gravity of
the star) with recent models that treat internal mixing. The spin-down rates
observed in the high mass subset () agree with predictions, but
the rates are larger for the low mass group (). The faster spin
down in the low mass B stars matches well with the predictions based on
conservation of angular momentum in individual spherical shells. Our results
suggest the fastest rotators (that probably correspond to the emission line Be
stars) are probably formed by evolutionary spin up (for the more massive stars)
and by mass transfer in binaries (for the full range of B star masses).Comment: 44 pages, 10 figures, accepted for publication in Ap
Orbital and physical parameters of the spectroscopic binary HD37737
We report the physical and orbital parameters of the visible component of the spectroscopic binary HD37737 (m V = 8.03). The observations were performed with the 1.2-m telescope of the Kourovka Astronomical Observatory of the Ural Federal University in 2012 and the 6-m BTA telescope of the SAO RAS in 2007 and 2009. Radial velocities were measured separately from each spectral line of the list by the cross-correlation method with a synthetic spectrum. The latter was calculated using the grids of non-LTE model atmospheres with solar chemical compositions. A significant difference in the epochs of observations (2005-2012) allowed to refine the orbital period of the star (7· d84705) and the orbital elements of the binary system. We obtained an estimate of the mass function f(m) = 0.23 ± 0.02M ⊙. The best agreement between the synthetic and observed spectra is achieved at T eff = 30 000 K and log g = 3.50 according to the observations on both instruments. The obtained parameters correspond to a star of spectral type O9.5 III, with mass estimated at 26 ± 2M ⊙. The minimum mass estimate of the secondary component of the binary is 6.2 ± 0.5M ⊙. We have discovered a fact that the velocities, obtained from different spectral lines, differ, which is typical for giant stars. Engaging additional spectra, obtained in 2005 with the 2.1-m KPNO telescope, we investigated the effect of this fact on the estimate of the speed of the system's center of mass. The difference in the velocities of various lines is approximately the same in the spectra, obtained at all the three instruments. The obtained ratios suggest that the deeper layers of the atmosphere of the star are moving with a greater velocity than the outer layers. Depending on the line, the estimate of the heliocentric velocity of the binary's center of mass varies in the range from -11 to 1 km/s. © 2013 Pleiades Publishing, Ltd
Optical Geolocation for Small Unmanned Aerial Systems
This paper presents an airborne optical geolocation system using four optical targets to provide position and attitude estimation for a sUAS supporting the NASA Acoustic Research Mission (ARM), where the goal is to reduce nuisance airframe noise during approach and landing. A large precision positioned microphone array captures the airframe noise for multiple passes of a Gulfstream III aircraft. For health monitoring of the microphone array, the Acoustic Calibration Vehicle (ACV) sUAS completes daily flights with an onboard speaker emitting tones at frequencies optimized for determining microphone functionality. An accurate position estimate of the ACV relative to the array is needed for microphone health monitoring. To this end, an optical geolocation system using a downward facing camera mounted to the ACV was developed. The 3D positioning of the ACV is computed using the pinhole camera model. A novel optical geolocation algorithm first detects the targets, then a recursive algorithm tightens the localization of the targets. Finally, the position of the sUAS is computed using the image coordinates of the targets, the 3D world coordinates of the targets, and the camera matrix. A Real-Time Kinematic GPS system is used to compare the optical geolocation system
HD 69686: A Mysterious High Velocity B Star
We report on the discovery of a high velocity B star, HD 69686. We estimate
its space velocity, distance, surface temperature, gravity, and age. With these
data, we are able to reconstruct the trajectory of the star and to trace it
back to its birthplace. We use evolutionary tracks for single stars to estimate
that HD 69686 was born 73 Myr ago in the outer part of our Galaxy (
kpc) at a position well below the Galactic plane ( kpc), a very
unusual birthplace for a B star. Along the star's projected path in the sky, we
also find about 12 other stars having similar proper motions, and their
photometry data suggest that they are located at the same distance as HD 69686
and probably have the same age. We speculate on the origin of this group by
star formation in a high velocity cloud or as a Galactic merger fragment.Comment: 28 pages, 6 figures, accepted for publication in Ap
Multi-wavelength observations of the binary system PSR B1259-63/LS 2883 around the 2010-2011 periastron passage
We report on broad multi-wavelength observations of the 2010-2011 periastron
passage of the gamma-ray loud binary system PSR B1259-63. High resolution
interferometric radio observations establish extended radio emission trailing
the position of the pulsar. Observations with the Fermi Gamma-ray Space
Telescope reveal GeV gamma-ray flaring activity of the system, reaching the
spin-down luminosity of the pulsar, around 30 days after periastron. There are
no clear signatures of variability at radio, X-ray and TeV energies at the time
of the GeV flare. Variability around periastron in the H emission line,
can be interpreted as the gravitational interaction between the pulsar and the
circumstellar disk. The equivalent width of the H grows from a few days
before periastron until a few days later, and decreases again between 18 and 46
days after periastron. In near infrared we observe the similar decrease of the
equivalent width of Br line between the 40th and 117th day after the
periastron. For the idealized disk, the variability of the H line
represents the variability of the mass and size of the disk. We discuss
possible physical relations between the state of the disk and GeV emission
under assumption that GeV flare is directly related to the decrease of the disk
size.Comment: accepted to MNRA
Spectral energy distribution of the gamma-ray microquasar LS 5039
The microquasar LS 5039 has recently been detected as a source of very high
energy (VHE) -rays. This detection, that confirms the previously
proposed association of LS 5039 with the EGRET source 3EG~J18241514, makes
of LS 5039 a special system with observational data covering nearly all the
electromagnetic spectrum. In order to reproduce the observed spectrum of LS
5039, from radio to VHE -rays, we have applied a cold matter dominated
jet model that takes into account accretion variability, the jet magnetic
field, particle acceleration, adiabatic and radiative losses, microscopic
energy conservation in the jet, and pair creation and absorption due to the
external photon fields, as well as the emission from the first generation of
secondaries. The radiative processes taken into account are synchrotron,
relativistic Bremsstrahlung and inverse Compton (IC). The model is based on a
scenario that has been characterized with recent observational results,
concerning the orbital parameters, the orbital variability at X-rays and the
nature of the compact object. The computed spectral energy distribution (SED)
shows a good agreement with the available observational data.Comment: 8 pages, 3 figures, submitted to A&A, revised vesion accounting for
referee comments, small improvements of the results due to better
calculation
A numerical model for the gamma-ray emission of the microquasar LS 5039
The possible association between the microquasar LS 5039 and the EGRET source
3EG J1824-1514 suggests that microquasars could also be sources of high energy
gamma-rays. In this paper, we explore, with a detailed numerical model, if this
system can produce the emission detected by EGRET (>100 MeV) through inverse
Compton (IC) scattering. Our numerical approach considers a population of
relativistic electrons entrained in a cylindrical inhomogeneous jet,
interacting with both the radiation and the magnetic fields, taking into
account the Thomson and Klein-Nishina regimes of interaction. The computed
spectrum reproduces the observed spectral characteristics at very high energy.Comment: 14 pages, 4 figures, 3 tables, LaTeX, uses aa.cls. published in A&A,
final published version (referee format)mnkdi
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