1,585 research outputs found
Probing the centre of the large circumstellar disc in M17
We investigated the nature of the hitherto unresolved elliptical infrared
emission in the centre of the ~20000 AU disc silhouette in M 17. We combined
high-resolution JHKsL'M' band imaging carried out with NAOS/CONICA at the VLT
with [Fe II] narrow band imaging using SOFI at the NTT. The analysis is
supported by Spitzer/GLIMPSE archival data and by already published SINFONI/VLT
Integral Field Spectroscopy data. For the first time, we resolve the elongated
central infrared emission into a point-source and a jet-like feature that
extends to the northeast in the opposite direction of the recently discovered
collimated H2 jet. They are both orientated almost perpendicular to the disc
plane. In addition, our images reveal a curved southwestern emission nebula
whose morphology resembles that of the previously detected northeastern one.
Both nebulae are located at a distance of 1500 AU from the disc centre. We
describe the infrared point-source in terms of a protostar that is embedded in
circumstellar material producing a visual extinction of 60 <= Av <= 82. The
observed Ks band magnitude is equivalent to a stellar mass range of 2.8 Msun <=
Mstar <= 8 Msun adopting conversions for a main-sequence star. Altogether, we
suggest that the large M 17 accretion disc is forming an intermediate to
high-mass protostar. Part of the accreted material is expelled through a
symmetric bipolar jet/outflow.Comment: 6 pages, 3 figures, accepted by MNRAS (16 May 2008
The Masses Of The B-Stars In The High Galactic Latitude Eclipsing Binary IT Lib
A number of blue stars which appear to be similar to Population I B-stars in
the star forming regions of the galactic disk are found more than 1 kpc from
the galactic plane. Uncertainties about the true distances and masses of these
high latitude B-stars has fueled a debate as to their origin and evolutionary
status. The eclipsing binary IT Lib is composed of two B-stars, is
approximately one kiloparsec above the galactic plane, and is moving back
toward the plane. Observations of the light and velocity curves presented here
lead to the conclusion that the B-stars in this system are massive young
main-sequence stars. While there are several possible explanations, it appears
most plausible that the IT Lib system formed in the disk about 30 million years
ago and was ejected on a trajectory taking it to its present position.Comment: 26 pages, 3 figures, accepted for publication in the PASP (January
2003
Perceptions about alcohol use during pregnancy in France, Portugal and Spain: a cross-cultural qualitative study
info:eu-repo/semantics/publishedVersio
Prospectus, July 7, 1993
https://spark.parkland.edu/prospectus_1993/1010/thumbnail.jp
Radial Velocity Studies of Close Binary Stars.IV
Radial-velocity measurements and sine-curve fits to the orbital velocity
variations are presented for the fourth set of ten close binary systems: 44
Boo, FI Boo, V2150 Cyg, V899 Her, EX Leo, VZ Lib, SW Lyn, V2377 Oph, Anon Psc
(GSC 8-324), HT Vir. All systems are double-lined spectroscopic binaries with
only two of them not being contact systems (SW Lyn and GSC 8-324) and with five
(FI Boo, V2150 Cyg, V899 Her, EX Leo, V2377 Oph) being the recent photometric
discoveries of the Hipparcos satellite project. Five of the binaries are
triple-lined systems (44 Boo, V899 Her, VZ Lib, SW Lyn, HT Vir). Three (or
possibly four) companions in the triple-lined systems show radial-velocity
changes during the span of our observations suggesting that these are in fact
quadruple systems. Several of the studied systems are prime candidates for
combined light and radial-velocity synthesis solutions.Comment: aastex5.0, 5 figures in PS; submitted to Astron.
Evaluating the accuracy of two: In situ optical sensors to estimate DOC concentrations for drinking water production
Two in situ optical sensors, a single-excitation fluorescence-based sensor (fDOM) mounted on a multi-parameter EXO2 sonde (YSI), and a stand-alone, multispectral absorbance-based instrument (spectro::lyser, scan Messtechnik GmbH), were evaluated for their capability to (i) estimate river dissolved organic carbon (DOC) concentrations and (ii) provide oversight of drinking water production. The sensors were deployed between March and November 2017 in the river Fyris, which drains a mixed forested and agricultural 2003 km catchment and serves as a drinking water source by managed aquifer recharge. Grab samples were collected every 2 to 3 weeks and compared with logged sensor data collected at 15 minute intervals. The fDOM probe signal was used to estimate DOC concentrations in the range of 10.4 to 24.4 mg L using linear regression (R = 0.71, RMSE = 2.5 mg L), after correction for temperature, turbidity and inner-filter effects. Temporal changes in DOC character associated with the mixed land use landscape, as indicated by optical indices, reduced this sensor accuracy for estimating DOC concentration. Nevertheless, humic substance concentrations, the fraction of DOC that is preferentially removed during artificial infiltration, were well captured. The spectrolyser signal was used to establish a 2-component partial least square model that captured DOC fluctuations from 10.2 to 29.4 mg L (R = 0.92; RMSE = 1.3 mg L). This multiple-wavelength model (220 to 720 nm) effectively handled the changes in DOC composition while accurately estimating DOC concentrations. This study explores the advantages and limitations of optical sensors for their use in managed aquifer recharge and drinking water production in relation to DOC levels
Mid - infrared interferometry of massive young stellar objects II Evidence for a circumstellar disk surrounding the Kleinmann - Wright object
The formation scenario for massive stars is still under discussion. To
further constrain current theories, it is vital to spatially resolve the
structures from which material accretes onto massive young stellar objects
(MYSOs). Due to the small angular extent of MYSOs, one needs to overcome the
limitations of conventional thermal infrared imaging, regarding spatial
resolution, in order to get observational access to the inner structure of
these objects.We employed mid - infrared interferometry, using the MIDI
instrument on the ESO /VLTI, to investigate the Kleinmann - Wright Object, a
massive young stellar object previously identified as a Herbig Be star
precursor. Dispersed visibility curves in the N- band (8 - 13 {\mu}m) have been
obtained at 5 interferometric baselines. We show that the mid - infrared
emission region is resolved. A qualitative analysis of the data indicates a non
- rotationally symmetric structure, e.g. the projection of an inclined disk. We
employed extensive radiative transfer simulations based on spectral energy
distribution fitting. Since SED - only fitting usually yields degenerate
results, we first employed a statistical analysis of the parameters provided by
the radiative transfer models. In addition, we compared the ten best - fitting
self - consistent models to the interferometric observations. Our analysis of
the Kleinmann - Wright Object suggests the existence of a circumstellar disk of
0.1M\odot at an intermediate inclination of 76\circ, while an additional dusty
envelope is not necessary for fitting the data. Furthermore, we demonstrate
that the combination of IR interferometry with radiative transfer simulations
has the potential to resolve ambiguities arising from the analysis of spectral
energy distributions alone.Comment: 12 pages, 22 figures accepted for publication in A&
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