2,860 research outputs found
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
Realistic Ionizing Fluxes for Young Stellar Populations from 0.05 to twice solar metallicity
We present a new grid of ionizing fluxes for O and Wolf-Rayet stars for use
with evolutionary synthesis codes and single star H II region analyses. A total
of 230 expanding, non-LTE, line-blanketed model atmospheres have been
calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 solar) using the
WM-basic code of Pauldrach et al. (2001) and the CMFGEN code of Hillier &
Miller (1998). The stellar wind parameters are scaled with metallicity for both
O and W-R stars. We incorporate the new models into Starburst99 (Leitherer et
al. 1999) and compare the ionizing outputs with Schaerer & Vacca (1998) and
Leitherer et al. (1999). The changes in the output ionizing fluxes are
dramatic, particularly below 228 A. We also find lower fluxes in the He I
continuum for Z > 0.4 solar and ages < 7 Myr because of the increased line
blanketing. We test the accuracy of the new models by constructing
photoionization models. We show that for the dwarf O star grid, He I 5876/H
beta decreases between Z = 1 and twice solar in a similar manner to
observations (e.g. Bresolin et al. 1999) due to the increased effect of line
blanketing. We therefore suggest that a lowering of the upper mass limit at
high abundances is not required to explain the observations. For the case of an
instantaneous burst, we plot the softness parameter "eta prime" against the
abundance indicator R_23. The new models are coincident with the data of
Bresolin et al. (1999), particularly during the W-R phase, unlike previous
models which over-predict the hardness of the ionizing radiation.Comment: 21 pages, 15 postscript colour figures, includes mn2e.cls. To be
published in MNRAS. Revised version containing modifications to Tables 1-
Teachers as leaders in a knowledge society: encouraging signs of a new professionalism
[Abstract]: Challenges confronting schools worldwide are greater than ever,and, likewise, many teachers possess capabilities, talents, and formal credentials more sophisticated than ever. However, the responsibility and authority accorded
to teachers have not grown significantly, nor has the image of teaching as a profession advanced significantly. The question becomes, what are the implications for the image and status of the teaching profession as the concept of knowledge society takes a firm hold in the industrialized world? This article addresses the philosophical underpinnings of teacher leadership manifested in case studies where schools sought to achieve the generation of new knowledge as part of a process of whole-school revitalization. Specifically, this article reports on Australian research that has illuminated the work of teacher leaders engaged in the IDEAS project, a joint school revitalization initiative of the University
of Southern Queensland and the Queensland Department of Education and the Arts
The P Cygni supergiant [OMN2000] LS1 â implications for the star formation history of W51
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO) DOI: 10.1051/0004-6361/200911980Aims. We investigate the nature of the massive star [OMN2000] LS1 and use these results to constrain the history of star formation within the host complex W51. Methods. We utilised a combination of near-IR spectroscopy and non-LTE model atmosphere analysis to derive the physical properties of [OMN2000] LS1 , and a combination of theoretical evolutionary calculations and Monte Carlo simulations to apply limits on the star formation history of W51. Results. We find the spectrum of [OMN2000] LS1 to be consistent with that of a P Cygni supergiant. With a temperature in the range of 13.2â13.7 kK and log( ) , it is significantly cooler, less luminous, and less massive than proposed by previous authors. The presence of such a star within W51 shows that star formation has been underway for at least 3 Myr, while the formation of massive O stars is still on going. The lack of a population of evolved red supergiants within the complex shows that the rate of formation of young massive clusters at ages 9 Myr was lower than currently observed. We find no evidence of internally triggered, sequential star formation within W51, and favour the suggestion that star formation has proceeded at multiple indepedent sites within the GMC. Along with other examples, such as the G305 and Carina star-forming regions, we suggest that W51 is a Galactic analogue of the ubiquitous star cluster complexes seen in external galaxies such as M51 and NGC2403.Peer reviewe
Metallicity in the Galactic Center: The Arches cluster
We present a quantitative spectral analysis of five very massive stars in the
Arches cluster, located near the Galactic center, to determine stellar
parameters, stellar wind properties and, most importantly, metallicity content.
The analysis uses a new technique, presented here for the first time, and uses
line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared
spectra of late-type nitrogen-rich Wolf-Rayet stars and OfI+ stars in the
cluster. It relies on the fact that massive stars reach a maximum nitrogen
abundance that is related to initial metallicity when they are in the WNL
phase. We determine the present-day nitrogen abundance of the WNL stars in the
Arches cluster to be 1.6% (mass fraction) and constrain the stellar metallicity
in the cluster to be solar. This result is invariant to assumptions about the
mass-luminosity relationship, the mass-loss rates, and rotation speeds. In
addition, from this analysis, we find the age of the Arches cluster to be
2-2.5Myr, assuming coeval formation
Crowded field 3D spectroscopy of LBV candidates in M33
We present integral field spectroscopy of the LBV candidate stars B416 and
v532 in the local group galaxy M33. B416 is surrounded by an elongated
ring-like nebula, which has a projected radius of 20x30 pc. From the datacube
we create ionization and radial velocity maps of the nebula. The excitation of
the gas decreases towards the outer part of the ring, while the inner part of
the nebula is filled with a more excited gas. In the EW direction the ring is
seen to expand with a maximum projected velocity amplitude of about 40 km/s.
The eastern part approaches the observer. We estimate the nebula dynamical
lifetime 8*10E5 years. It could be a residual MS bubble, which indicates a
main-sequence or pre-LBV status of the star. We classify B416 as an "iron star"
or B[e]-supergiant. In v532 an elongated nebula has been marginally detected.
The total projected size of the nebula along the main axis is 30 pc, and the
total radial velocity gradient is 44+/-11 km/s. v532 exhibits both strong
photometric and spectral variability. At the time of our observations it was in
an intermediate brightness state with a rich nitrogen spectrum. We classify
v532 as an LBV, showing LBV Ofpe/WN transitions. We stress the importance
of integral field spectroscopy as the optimal technique for studying nebulae
and the evolution of LBV-like stars in nearby galaxies.Comment: 12 pages, 10 Postscript figures, A&A accepte
The Arches cluster revisited: II. A massive eclipsing spectroscopic binary in the Arches cluster
We have carried out a spectroscopic variability survey of some of the most massive stars in the Arches cluster, using K-band observations obtained with SINFONI on the VLT. One target, F2, exhibits substantial changes in radial velocity; in combination with new KMOS and archival SINFONI spectra, its primary component is found to undergo radial velocity variation with a period of 10.483+/-0.002 d and an amplitude of ~350 km/s-1. A secondary radial velocity curve is also marginally detectable. We reanalyse archival NAOS-CONICA photometric survey data in combination with our radial velocity results to confirm this object as an eclipsing SB2 system, and the first binary identified in the Arches. We model it as consisting of an 82+/-12 Mâ WN8-9h primary and a 60+/-8 Mâ O5-6 Ia+ secondary, and as having a slightly eccentric orbit, implying an evolutionary stage prior to strong binary interaction. As one of four X-ray bright Arches sources previously proposed as colliding-wind massive binaries, it may be only the first of several binaries to be discovered in this cluster, presenting potential challenges to recent models for the Arches' age and composition. It also appears to be one of the most massive binaries detected to date; the primary's calculated initial mass of >~120 Mâ would arguably make this the most massive binary known in the Galaxy
A Spectroscopic Study of a Large Sample of Wolf-Rayet Galaxies
We analyze long-slit spectral observations of 39 Wolf-Rayet (WR) galaxies
with heavy element mass fraction ranging over 2 orders of magnitude, from
Zsun/50 to 2Zsun. Nearly all galaxies in our sample show broad WR emission in
the blue region of the spectrum (the blue bump) consisting of an unresolved
blend of N III 4640, C III 4650, C IV 4658 and He II 4686 emission lines. Broad
C IV 5808 emission (the red bump) is detected in 30 galaxies. Additionally,
weaker WR emission lines are identified, most often the N III 4512 and Si III
4565 lines, which have very rarely or never been seen and discussed before in
WR galaxies. These emission features are characteristic of WN7-WN8 and WN9-WN11
stars respectively.
We derive the numbers of early WC (WCE) and late WN (WNL) stars from the
luminosities of the red and blue bumps, and the number of O stars from the
luminosity of the Hbeta emission line. Additionally, we propose a new technique
for deriving the numbers of WNL stars from the N III 4512 and Si III 4565
emission lines. This technique is potentially more precise than the blue bump
method because it does not suffer from contamination of WCE and early WN (WNE)
stars and nebular gaseous emission.
The N(WR)/N(O+WR) ratio decreases with decreasing metallicity, in agreement
with predictions of evolutionary synthesis models. The N(WC)/N(WN) ratios and
the equivalent widths of the blue bump EW(4650) and of the red bump EW(5808)
derived from observations are also in satisfactory agreement with theoretical
predictions.Comment: 49 pages, 9 figures, to appear in Astrophys.
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