1,719 research outputs found
Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud
Using photometry from the Ultraviolet Imaging Telescope (UIT) and photometry
and spectroscopy from three ground-based optical datasets we have analyzed the
stellar content of OB associations and field areas in and around the regions N
79, N 81, N 83, and N 94 in the LMC. We compare data for the OB association
Lucke-Hodge 2 (LH 2) to determine how strongly the initial mass function (IMF)
may depend on different photometric reductions and calibrations. We also
correct for the background contribution of field stars, showing the importance
of correcting for field star contamination in determinations of the IMF of star
formation regions. It is possible that even in the case of an universal IMF,
the variability of the density of background stars could be the dominant factor
creating the differences between calculated IMFs for OB associations.
We have also combined the UIT data with the Magellanic Cloud Photometric
Survey to study the distribution of the candidate O-type stars in the field. We
find a significant fraction, roughly half, of the candidate O-type stars are
found in field regions, far from any obvious OB associations. These stars are
greater than 2 arcmin (30 pc) from the boundaries of existing OB associations
in the region, which is a distance greater than most O-type stars with typical
dispersion velocities will travel in their lifetimes. The origin of these
massive field stars (either as runaways, members of low-density star-forming
regions, or examples of isolated massive star formation) will have to be
determined by further observations and analysis.Comment: 16 pages, 10 figures (19 PostScript files), tabular data + header
file for Table 1 (2 ASCII files). File format is LaTeX/AASTeX v.502 using the
emulateapj5 preprint style (included). Also available at
http://www.boulder.swri.edu/~joel/papers.html . To appear in the February
2001 issue of the Astronomical Journa
The Discovery of a Twelfth Wolf-Rayet Star in the Small Magellanic Cloud
We report the discovery of a relatively faint (V=15.5) early-type WN star in
the SMC. The line strength and width of He II lambda 4686 emission is similar
to that of the other SMC WNs, and the presense of N V lambda 4603,19 emission
(coupled with the lack of N III) suggests this star is of spectral type
WN3-4.5, and thus is similar in type to the other SMC WRs. Also like the other
SMC WN stars, an early-type absorption spectrum is weakly present. The absolute
magnitude is comparable to that of other (single) Galactic early-type WNs. The
star is located in the Hodge 53 OB association, which is also the home of two
other SMC WNs. This star, which we designate SMC-WR12, was actually detected at
a high significance level in an earlier interference-filter survey, but the
wrong star was observed as part of a spectroscopic followup, and this case of
mistaken identity resulted in its Wolf-Rayet nature not being recognized until
now.Comment: Accepted by PASP (November 2003 issue
The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov-Gerasimenko observed by the Alice Far-ultraviolet Spectrograph on Rosetta
Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst
multiple objectives, is designed to observe emissions from various atomic and
molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The
initial observations, made following orbit insertion in August 2014, showed
emissions of atomic hydrogen and oxygen spatially localized close to the
nucleus and attributed to photoelectron impact dissociation of H2O vapor.
Weaker emissions from atomic carbon were subsequently detected and also
attributed to electron impact dissociation, of CO2, the relative H I and C I
line intensities reflecting the variation of CO2 to H2O column abundance along
the line-of-sight through the coma. Beginning in mid-April 2015, Alice
sporadically observed a number of outbursts above the sunward limb
characterized by sudden increases in the atomic emissions, particularly the
semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a
corresponding enhancement in long wavelength solar reflected light
characteristic of dust production. A large increase in the brightness ratio O I
1356/O I 1304 suggests O2 as the principal source of the additional gas. These
outbursts do not correlate with any of the visible images of outbursts taken
with either OSIRIS or the navigation camera. Beginning in June 2015 the nature
of the Alice spectrum changed considerably with CO Fourth Positive band
emission observed continuously, varying with pointing but otherwise fairly
constant in time. However, CO does not appear to be a major driver of any of
the observed outbursts.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
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