451 research outputs found
VLT/NACO near-infrared imaging and spectroscopy of N88A in the SMC
We present near-infrared imaging and spectroscopic high spatial resolution
observations of the SMC region N88 containing the bright, excited, extincted
and compact H II region N88A of size ~ 1 pc. To investigate its stellar content
and reddening, N88 was observed using spectroscopy and imagery in the JHKs- and
L'-band at a spatial resolution of ~ 0.1-0.3", using the VLT UT4 equipped with
the NAOS adaptive optics system. In order to attempt to establish if the origin
of the infra-red (IR) excess is due to bright nebulosity, circumstellar
material and/or local dust, we used Ks vs J-K colour-magnitude (CM) and JHK
colour-colour (CC) diagrams, as well as L' imagery.Our IR-data reveal in the
N88 area an IR-excess fraction of geq 30 per cent of the detected stars,as well
as an unprecedently detailed morphology of N88A. It consists of an embedded
cluster of ~3.5" (~ 1 pc) in diameter, of at least thirteen resolved stars
superposed with an unusual bright continuum centered on a very bright star. The
four brightest stars in this cluster lie red-ward of H-K geq 0.45 mag, and
could be classified as young stellar object (YSO) candidates. Four other
probable YSO candidates are also detected in N88 along a south-north bow-shaped
thin H2 filament at ~ 7" east of the young central bright star. At 0.2" east of
this star, a heavily embedded core is detected that could be a massive class I
protostar candidate. The 2.12 mu H2 image of N88A resembles a shell of diameter
~ 3" ~ 0.9 pc) centered on the bright star. The line ratios of H2 2-1 S(1) and
1-0 S(0) relative to 1-0 S(1), as well as the presence of high v lines, are
indicative of photodissociation regions, rather than shocks.Comment: 15 pages, 14 figures, accepted by Astronomy and Astrophysics, uses
pdflatex, aa.cl
Stellar abundances and molecular hydrogen in high-redshift galaxies -the far-ultraviolet view
FUSE spectra of star-forming regions in nearby galaxies are compared to
composite spectra of Lyman-break galaxies (LBGs), binned by strength of Lyman
alpha emission and by mid-UV luminosity. Several far-UV spectral features,
including lines dominated by stellar wind and by photospheric components, are
very sensitive to stellar abundances. Their measurement in Lyman-break galaxies
is compromised by the strong interstellar absorption features, allowing in some
cases only upper limits. The derived C and N abundances in the LBGs are no
higher than half solar (scaled to oxygen abundance for comparison with
emission-line analyses), independent of the strength of Lyman alpha emission. P
V absorption indicates abundances as low as 0.1 solar, with an upper limit near
0.4 solar in the reddest and weakest-emission galaxies. Unresolved interstellar
absorption components would further lower the derived abundances. Trends of
line strength, and derived abundances, are stronger with mid-UV luminosity than
with Lyman-alpha strength. H2 absorption in the Lyman and Werner bands is very
weak in the LBGs. Template H2 absorption spectra convolved to appropriate
resolution show that strict upper limits N(H2)< 10^18 cm^-2 apply in all cases,
with more stringent values appropriate for the stronger-emission composites and
for mixes of H2 level populations like those on Milky Way sight lines. Since
the UV-bright regions are likely to be widespread in these galaxies, these
results rule out massive diffuse reservoirs of H2, and suggest that the
dust/gas ratio is already fairly large at z~3.Comment: Astron J., in press (June 2006
The Massive Wolf-Rayet Binary SMC WR7
We present a study of optical spectra of the Wolf--Rayet star AzV 336a (= SMC
WR7) in the Small Magellanic Cloud. Our study is based on data obtained at
several Observatories between 1988 and 2001. We find SMC WR7 to be a double
lined WN+O6 spectroscopic binary with an orbital period of 19.56 days. The
radial velocities of the He absorption lines of the O6 component and the strong
He{\sc ii} emission at 4686\AA of the WN component describe antiphased
orbital motions. However, they show a small phase shift of 1 day. We
discuss possible explanations for this phase shift. The amplitude of the radial
velocity variations of He {\sc ii} emission is twice that of the absorption
lines. The binary components have fairly high minimum masses, 18
\modot and 34 \modot for the WN and O6 components, respectively.Comment: Accepted by MNRA
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