709 research outputs found
Differential Astrometry over 15 degrees
We observed the pair of radio sources 1150+812 and 1803+784 in November 1993
with a VLBI array, simultaneously recording at 8.4 and 2.3 GHz. We determined
the angular separation between the two sources with submilliarcsecond accuracy
by using differential techniques. This result demonstrates the feasibility of
high precision differential astrometry for radio sources separated in the sky
by almost 15 degrees, and opens the avenue to its application to larger samples
of radio sources.Comment: 6 pages, latex2e, 2 figures, To appear in the proceedings of the
EVN/JIVE Symposium No. 4, New Astronomy Reviews (eds. Garret, M. Campbell,
R., and Gurvits, L.
Multi-wavelength differential astrometry of the S5 polar cap sample
We report on the status of our S5 polar cap astrometry program. Since 1997 we
have observed all the 13 radio sources of the complete S5 polar cap sample at
the wavelengths of 3.6 cm, 2 cm and 7 mm. Images of the radio sources at 3.6
and 2 cm have already been published reporting morphological changes.
Preliminary astrometric analyses have been carried out at three frequencies
with precisions in the relative position determination ranging from 80 to 20
microarcseconds. We report also on the combination of our phase-delay global
astrometry results with the microarcsecond-precise optical astrometry that will
be provided by future space-based instruments.Comment: 2 pages. 1 figure. Proceedings of the 7th European VLBI Network
Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R.
Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico
Nacional), p. 323-324. Needs evn2004.cl
Searching for low mass objects around nearby dMe radio stars
Nearby M-dwarfs are best suited for searches of low mass companions. VLBI
phase-referencing observations with sensitive telescopes are able to detect
radio star flux-densities of tenths of mJy as well as to position the star on
the sky with submilliarcsecond precision. We have initiated a long-term
observational program, using EVN telescopes in combination with NASA DSN
dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of
the astrometry allows us to search for possible companions with masses down to
1 Jupiter mass. In this contribution we report preliminary results of the first
observation epochs, in which we could detect some of the radio stars included
in our program.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov A.P., & Zensus J.A. (eds.), MPIfR, Bonn, Germany, p.
255-258 (2002). 4 pages, 3 figures, needs evn2002.cl
VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), sigma Oph, BM Sco, and HD 206859
We add four warmer late-type supergiants to our previous
spectro-interferometric studies of red giants and supergiants.
V766 Cen (=HR 5171 A) is found to be a high-luminosity log(L/L_sun)=5.8+-0.4
source of Teff 4290+-760 K and radius 1490+-540 Rsun located close to both the
Hayashi and Eddington limits; this source is consistent with a 40 Msun
evolutionary track without rotation and current mass 27-36 Msun. It exhibits
NaI in emission arising from a shell of radius 1.5 Rphot and a photocenter
displacement of about 0.1 Rphot. V766 Cen shows strong extended molecular (CO)
layers and a dusty circumstellar background component. This suggest an
optically thick pseudo-photosphere at about 1.5 Rphot at the onset of the wind.
V766 Cen is a red supergiant located close to the Hayashi limit instead of a
yellow hypergiant already evolving back toward warmer Teff as previously
discussed.
The stars sigma Oph, BM Sco, and HD 206859 are found to have lower
luminosities of about log(L/Lsun)=3.4-3.5 and Teff of 3900-5300 K,
corresponding to 5-9 Msun tracks. They do not show extended molecular layers as
observed for higher luminosity red supergiants of our sample. BM Sco shows an
unusually strong contribution by an over-resolved circumstellar dust component.
These stars are more likely high-mass red giants instead of red supergiants.
This leaves us with an unsampled locus in the HR diagram corresponding to
luminosities log(L/Lsun)~3.8-4.8 or masses 10-13 Msun, possibly corresponding
to the mass region where stars explode as type II-P supernovae during the RSG
stage.
Our previously found relation of increasing strength of extended molecular
layers with increasing luminosities is now confirmed to extend to double our
previous luminosities and up to the Eddington limit. This might further point
to steadily increasing radiative winds with increasing luminosity.
[Abridged]Comment: 16 pages, 14 figures, accepted for publication in Astronomy and
Astrophysics (A&A
Assessment of the Accuracy of a Multi-Beam LED Scanner Sensor for Measuring Olive Canopies
MDPI. CC BYCanopy characterization has become important when trying to optimize any kind of agricultural operation in high-growing crops, such as olive. Many sensors and techniques have reported satisfactory results in these approaches and in this work a 2D laser scanner was explored for measuring canopy trees in real-time conditions. The sensor was tested in both laboratory and field conditions to check its accuracy, its cone width, and its ability to characterize olive canopies in situ. The sensor was mounted on a mast and tested in laboratory conditions to check: (i) its accuracy at different measurement distances; (ii) its measurement cone width with different reflectivity targets; and (iii) the influence of the target’s density on its accuracy. The field tests involved both isolated and hedgerow orchards, in which the measurements were taken manually and with the sensor. The canopy volume was estimated with a methodology consisting of revolving or extruding the canopy contour. The sensor showed high accuracy in the laboratory test, except for the measurements performed at 1.0 m distance, with 60 mm error (6%). Otherwise, error remained below 20 mm (1% relative error). The cone width depended on the target reflectivity. The accuracy decreased with the target density
Absolute kinematics of radio source components in the complete S5 polar cap sample. III. First wide-field high-precision astrometry at 15.4 GHz
We report on the first wide-field, high-precision astrometric analysis of the
13 extragalactic radio sources of the complete S5 polar cap sample at 15.4 GHz.
We describe new algorithms developed to enable the use of differenced phase
delays in wide-field astrometric observations and discuss the impact of using
differenced phase delays on the precision of the wide-field astrometric
analysis. From this global fit, we obtained estimates of the relative source
positions with precisions ranging from 14 to 200 as at 15.4 GHz, depending
on the angular separation of the sources (from 1.6 to 20.8
degrees). These precisions are 10 times higher than the achievable
precisions using the phase-reference mapping technique.Comment: 9 pages, 7 figure
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