2,877 research outputs found
How extreme are the Wolf-Rayet clusters in NGC3125?
We reinvestigate the massive stellar content of NGC3125 (Tol3) using
VLT/FORS1 imaging & spectroscopy, plus archival VLT/ISAAC, HST/FOC and HST/STIS
datasets. Narrow-band imaging confirms that A and B knots represent the primary
sites of Wolf-Rayet (WR) stars, whilst HST imaging reveals that both regions
host multiple clusters. Clusters A1 + A2 within region A host WR stars. altough
is not clear which cluster within region B hosts WR stars. Nebular properties
infer an LMC-like metallicity. LMC template WN5-6 & WC4 spectra are matched to
the observed optical WR bumps of A1 and B, permitting the contribution of WC
stars to the blue bump to be quantified. We obtain N(WN5-6:WC4)=105:20, 55:0,
40:20 for clusters A1, A2 and B1+B2, a factor of 3 lower than optical studies,
as a result of a lower E(B-V). Using Starburst99 models to estimate O star
populations for individual clusters, we find N(WR)/N(O)=0.2 for A1 and 0.1 for
A2 and B1+B2. From Halpha imaging, the O star content of the Giant HII regions
A and B is found to be a factor of 5-10 times higher than that derived
spectroscopically for the UV/optically bright clusters, suggesting that NGC3125
hosts optically obscured young massive clusters, further supported by VLT/ISAAC
K band imaging. Archival HST/STIS UV spectroscopy confirms the low E(B-V)
towards A1, for which we have determined an SMC extinction law, in preference
to an LMC or starburst law. We obtain N(WN5-6)=110 from the slit-loss corrected
HeII 1640 line flux, in excellent agreement with optical result. However, this
is a factor of 35 times lower than that inferred from the same dataset by
Chandar et al. due to their use of a starburst extinction law. Highly
discrepant stellar populations may result in spatially resolved star forming
regions from UV and optical studies through use of different extinction laws.Comment: 12 pages, 8 figures, accepted for MNRAS (fixes previous error in
Table 2
The Very Massive Star Content of the Nuclear Star Clusters in NGC 5253
The blue compact dwarf galaxy NGC 5253 hosts a very young starburst
containing twin nuclear star clusters, separated by a projected distance of 5
pc. One cluster (#5) coincides with the peak of the H-alpha emission and the
other (#11) with a massive ultracompact H II region. A recent analysis of these
clusters shows that they have a photometric age of 1+/-1 Myr, in apparent
contradiction with the age of 3-5 Myr inferred from the presence of Wolf-Rayet
features in the cluster #5 spectrum. We examine Hubble Space Telescope
ultraviolet and Very Large Telescope optical spectroscopy of #5 and show that
the stellar features arise from very massive stars (VMS), with masses greater
than 100 Msun, at an age of 1-2 Myr. We further show that the very high
ionizing flux from the nuclear clusters can only be explained if VMS are
present. We investigate the origin of the observed nitrogen enrichment in the
circum-cluster ionized gas and find that the excess N can be produced by
massive rotating stars within the first 1 Myr. We find similarities between the
NGC 5253 cluster spectrum and those of metal poor, high redshift galaxies. We
discuss the presence of VMS in young, star-forming galaxies at high redshift;
these should be detected in rest frame UV spectra to be obtained with the James
Webb Space Telescope. We emphasize that population synthesis models with upper
mass cut-offs greater than 100 Msun are crucial for future studies of young
massive star clusters at all redshifts.Comment: 11 pages, 7 figures, accepted for publication in Astrophysical
Journa
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
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
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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
A survey of the Wolf-Rayet population of the barred, spiral galaxy NGC 1313
We present a VLT/FORS1 survey of Wolf-Rayet (WR) stars in the spiral galaxy
NGC 1313. In total, 94 WR candidate sources have been identified from
narrow-band imaging. Of these, 82 have been spectroscopically observed, for
which WR emission features are confirmed in 70 cases, one of which also
exhibits strong nebular HeII 4686 emission. We also detect strong nebular HeII
4686 emission within two other regions of NGC 1313, one of which is a possible
supernova remnant. Nebular properties confirm that NGC 1313 has a metal-content
log(O/H)+12=8.23+/-0.06, in good agreement with previous studies. From
continuum subtracted Halpha images we infer a global star formation rate of 0.6
Msun/yr. Using template LMC WR stars, spectroscopy reveals that NGC 1313 hosts
a minimum of 84 WR stars. Our census comprises 51 WN stars, including a rare
WN/C transition star plus 32 WC stars. In addition, we identify one WO star
which represents the first such case identified beyond the Local Group. The
bright giant HII region PES 1, comparable in Halpha luminosity to NGC 595 in M
33, is found to host a minimum of 17 WR stars. The remaining photometric
candidates generally display photometric properties consistent with WN stars,
such that we expect a global WR population of ~115 stars with N(WR)/N(O)~0.01
and N(WC)/N(WN)~0.4.Comment: Accepted for publication in MNRAS. Finding charts omitted, full
version available by anonymous ftp (ftp:
hydra.shef.ac.uk/pub/lh/ngc1313-fullversion.pd
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
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)
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