3,028 research outputs found
A Very Large Telescope imaging and spectroscopic survey of the Wolf-Rayet population in NGC 7793
We present a VLT/FORS1 imaging and spectroscopic survey of the Wolf-Rayet
(WR) population in the Sculptor group spiral galaxy NGC 7793. We identify 74
emission line candidates from archival narrow-band imaging, from which 39 were
observed with the Multi Object Spectroscopy (MOS) mode of FORS1. 85% of these
sources displayed WR features. Additional slits were used to observe HII
regions, enabling an estimate of the metallicity gradient of NGC 7793 using
strong line calibrations, from which a central oxygen content of log (O/H) + 12
= 8.6 was obtained, falling to 8.25 at R_25. We have estimated WR populations
using a calibration of line luminosities of Large Magellanic Cloud stars,
revealing ~27 WN and ~25 WC stars from 29 sources spectroscopically observed.
Photometric properties of the remaining candidates suggest an additional ~27 WN
and ~8 WC stars. A comparison with the WR census of the LMC suggests that our
imaging survey has identified 80% of WN stars and 90% for the WC subclass.
Allowing for incompleteness, NGC 7793 hosts ~105 WR stars for which
N(WC)/N(WN)~0.5. From our spectroscopy of HII regions in NGC 7793, we revise
the global Halpha star formation rate of Kennicutt et al. upward by 50% to 0.45
M_sun/yr. This allows us to obtain N(WR)/N(O)~0.018, which is somewhat lower
than that resulting from the WR census by Schild et al. of another Sculptor
group spiral NGC 300, whose global physical properties are similar to NGC 7793.
Finally, we also report the fortuitous detection of a bright (m_V = 20.8 mag)
background quasar Q2358-32 at z~2.02 resulting from CIV 1548-51 redshifted to
the 4684 passband.Comment: 17 pages, 13 figures, accepted for MNRAS (detailed finding charts
omitted)
On two Tasmanian Crania (Immature)
In describing these two crania I am infiuenced by the
comparative rarity of such specimens, and the opportunity,
now offered, of comparison with the adult Crania in the
large collection of the Tasmanian Museum.
There remain for description of this extinct race the
Crania of only eight children (of which one is of doubtful
authcnticity), (1899, p. 191).
The Crania under consideration are as follows:
A). Cranium of a child of about 6 years or a little more,
with the Mandible in situ. This was found by Dr. Inglis
Clark and myself about Christmas, 1908, at Oyster Cove.
B). The Cranium of a child of 6-7 years discovered by
Mrs. R. W. Legge, on the West Coast of Tasmania, in the
summer of 1927.
It will be of interest to see to what extent the features,
that are regarded as typical of the Adult Tasmanian
Cranium, are present in these immature types.
In Conclusion, generally, it may be said that in their
Physical Characters both these immature Crania conform
to the Adult types of their race, and could be distinguished
by their Tasmanian characteristics. Neither, however,
shows the very typical carination of the Vault of the
Cranium. The Facial Features are worthy of detailed attention,
in a larger series of skullls; as it seems that in early
life a marked width of the face at the expense of its Height
is not pronounced
Study of drift-field solar cells damaged by low-energy protons Progress report, Sep. 10 - Oct. 15, 1965
Irradiation damage of drift-field solar cells by low energy protons - computer analysis of current-voltage and spectral response dat
Cluster and nebular properties of the central star-forming region of NGC 1140
We present new high spatial resolution HST/ACS imaging of NGC 1140 and high
spectral resolution VLT/UVES spectroscopy of its central star-forming region.
The central region contains several clusters, the two brightest of which are
clusters 1 and 6 from Hunter, O'Connell & Gallagher, located within
star-forming knots A and B, respectively. Nebular analysis indicates that the
knots have an LMC-like metallicity of 12 + log(O/H) = 8.29 +/- 0.09. According
to continuum subtracted H alpha ACS imaging, cluster 1 dominates the nebular
emission of the brighter knot A. Conversely, negligible nebular emission in
knot B originates from cluster 6. Evolutionary synthesis modelling implies an
age of 5 +/- 1 Myr for cluster 1, from which a photometric mass of (1.1 +/-
0.3) x 10^6 Msun is obtained. For this age and photometric mass, the modelling
predicts the presence of ~5900 late O stars within cluster 1. Wolf-Rayet
features are observed in knot A, suggesting 550 late-type WN and 200 early-type
WC stars. Therefore, N(WR)/N(O) ~ 0.1, assuming that all the WR stars are
located within cluster 1. The velocity dispersions of the clusters were
measured from constituent red supergiants as sigma ~ 23 +/- 1 km/s for cluster
1 and sigma ~ 26 +/- 1 km/s for cluster 6. Combining sigma with half-light
radii of 8 +/- 2 pc and 6.0 +/- 0.2 pc measured from the F625W ACS image
implies virial masses of (10 +/- 3) x 10^6 Msun and (9.1 +/- 0.8) x 10^6 Msun
for clusters 1 and 6, respectively. The most likely reason for the difference
between the dynamical and photometric masses of cluster 1 is that the velocity
dispersion of knot A is not due solely to cluster 1, as assumed, but has an
additional component associated with cluster 2.Comment: 13 pages, 7 figure
Reduced Wolf-Rayet Line Luminosities at Low Metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Magellanic Clouds
are presented, from which HeII 4686 line luminosities have been derived, and
compared with observations of other Magellanic Cloud WR stars. SMC WN3-4 stars
possess line luminosities which are a factor of 4 times lower than LMC
counterparts, incorporating several binary SMC WN3-4 stars. Similar results are
found for WN5-6 stars, despite reduced statistics, incorporating observations
of single LMC WN5-9 stars. CIV 5808 line luminosities of carbon sequence WR
stars in the SMC and IC1613 (both WO subtypes) are a factor of 3 lower than LMC
WC stars from Mt Stromlo/DBS spectrophotometry, although similar results are
also obtained for the sole LMC WO star. We demonstrate how reduced line
luminosities at low metallicity follow naturally if WR winds are Z-dependent,
as recent results suggest. We apply mass loss-Z scalings to atmospheric non-LTE
models of Milky Way and LMC WR stars to predict the wind signatures of WR stars
in the metal-poor star forming WR galaxy IZw18. WN HeII 4686 line luminosities
are 7-20 times lower than in Z-rich counterparts of identical bolometric
luminosity, whilst WC CIV 5808 line luminosities are 3-6 times lower.
Significant He^+ Lyman continuum fluxes are predicted for Z-poor early-type WR
stars. Consequently, our results suggest the need for larger population of WR
stars in IZw18 than is presently assumed, particularly for WN stars,
potentially posing a severe challenge to evolutionary models at very low Z.
Finally, reduced wind strengths from WR stars at low Z impacts upon the
immediate circumstellar environment of long duration GRB afterglows,
particularly since the host galaxies of high-redshift GRBs tend to be Z-poor.Comment: 14 pages, 12 figures, accepted for A&A, revision fixes error with Eqn
A Spectroscopic Search for the non-nuclear Wolf-Rayet Population of the metal-rich spiral galaxy M83
We present a catalogue of non-nuclear regions containing Wolf-Rayet stars in
the metal-rich spiral galaxy M83 (NGC5236). From a total of 283 candidate
regions identified using HeII 4686 imaging with VLT-FORS2, Multi Object
Spectroscopy of 198 regions was carried out, confirming 132 WR sources. From
this sub-sample, an exceptional content of 1035 +/- 300 WR stars is inferred,
with N(WC)/N(WN) approx 1.2, continuing the trend to larger values at higher
metallicity amongst Local Group galaxies, and greatly exceeding current
evolutionary predictions at high metallicity. Late-type stars dominate the WC
population of M83, with N(WC8-9)/N(WC4-7)=9 and WO subtypes absent, consistent
with metallicity dependent WC winds. Equal numbers of late to early WN stars
are observed, again in contrast to current evolutionary predictions. Several
sources contain large numbers of WR stars. In particular, #74 (alias region 35
from De Vaucouleurs et al. contains 230 WR stars, and is identified as a Super
Star Cluster from inspection of archival HST/ACS images. Omitting this
starburst cluster would result in revised statistics of N(WC)/N(WN) approx 1
and N(WC8-9)/N(WC4-7) approx 6 for the `quiescent' disk population. Including
recent results for the nucleus and accounting for incompleteness in our
spectroscopic sample, we suspect the total WR population of M83 may exceed 3000
stars.Comment: 39 pages, 13 figures, 17 finding charts, accepted for Astronomy &
Astrophysics. Version will full resolution images available at
ftp://astro1.shef.ac.uk/pub/pac/m83.ps.g
A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini near-Infrared spectroscopy
We present H- and K-band spectroscopy of OB and Wolf-Rayet (WR) members of
the Milky Way cluster 1806-20 (G10.0-0.3), to obtain a revised cluster distance
of relevance to the 2004 giant flare from the SGR 1806-20 magnetar. From GNIRS
spectroscopy obtained with Gemini South, four candidate OB stars are confirmed
as late O/early B supergiants, while we support previous mid WN and late WC
classifications for two WR stars. Based upon an absolute Ks-band magnitude
calibration for B supergiants and WR stars, and near-IR photometry from NIRI at
Gemini North plus archival VLT/ISAAC datasets, we obtain a cluster distance
modulus of 14.7+/-0.35 mag. The known stellar content of the 1806-20 cluster
suggests an age of 3-5 Myr, from which theoretical isochrone fits infer a
distance modulus of 14.7+/-0.7 mag. Together, our results favour a distance
modulus of 14.7+/-0.4 mag (8.7^+1.8_-1.5 kpc) to the 1806-20 cluster, which is
significantly lower than the nominal 15 kpc distance to the magnetar. For our
preferred distance, the peak luminosity of the December 2004 giant flare is
reduced by a factor of three to 7 X 10^46 erg/s, such that the contamination of
BATSE short gamma ray bursts (GRB's) from giant flares of extragalactic
magnetars is reduced to a few percent. We infer a magnetar progenitor mass of
~48^+20_-8 Msun, in close agreement with that obtained recently for the
magnetar in Westerlund 1.Comment: 6 pages, 4 figures, accepted for MNRAS Letter
Recommended from our members
The Wolf-Rayet population of Westerlund 1
New NTT/SOFI near-IR narrow-band imaging and spectroscopy reveals an additional four Wolf-Rayet (WR) stars in the massive cluster Westerlund 1, bringing the total WR population to 24. Sixteen of the WR stars in Wd1 have been classified WN5–11, while eight are WC8–9. An observed WR to RSG/YHG ratio of ∼3 suggests an age of 4.5–5.0 Myr, with WR stars descended from 40–55MSolar progenitors. On the basis of dust and hard X-ray emission, we estimate that 40–65% are probable members of massive star binary systems
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