67 research outputs found
Infrared imaging and spectroscopy of the Luminous Blue Variables Wra 751 and AG Car
We present ground-based infrared imaging and ISO spectroscopy of the luminous blue variables Wra 751 and AG Car. The images show in both cases a detached shell with a roughly circular distribution of emission. The infrared images of AG Car coincide very well with the optical images. The optical (H[FORMULA]) image of Wra 751 is different from the infrared image; the H[FORMULA] nebula is suggested to be a scattering nebula containing cold dust particles.
Fitting both the images and the spectra consistently with a 1-D radiative transfer model, we derive properties of their dust shells. Wra 751 is surrounded by a dust shell with inner and outer radii of 0.17 and 0.34 pc respectively and a dust mass of 0.017 [FORMULA]. The dust shell of AG Car has inner and outer radii of 0.37 and 0.81 pc respectively and a total dust mass of 0.25 [FORMULA]. Dust mass-loss rates during the formation of the shells are 2.7[FORMULA] and 3.4[FORMULA] [FORMULA] yr-1, respectively. The total dust mass and hence the derived dust mass-loss rates are uncertain by at least a factor of two. For AG Car, the derived dust mass and mass-loss rate are higher than previous estimates. This is mainly caused by the fact that a contribution of very large grains ([FORMULA] 10 µm) is needed to explain the flux levels at longer wavelengths.
Dust models for both objects fail to explain the flux shortward of 15 to 20 µm: a population of small warm grains, not in thermal equilibrium with the central star is necessary to explain this excess. Similarities between dust shells around Wolf-Rayet stars and Wra 751 and AG Car (mass, grain size population, morphology) suggest a similar formation history and imply an evolutionary connection. A similar connection with red supergiants is suggested on the basis of the dust composition and derived time-averaged mass-loss rates
Stellar winds from Massive Stars
We review the various techniques through which wind properties of massive
stars - O stars, AB supergiants, Luminous Blue Variables (LBVs), Wolf-Rayet
(WR) stars and cool supergiants - are derived. The wind momentum-luminosity
relation (e.g. Kudritzki et al. 1999) provides a method of predicting mass-loss
rates of O stars and blue supergiants which is superior to previous
parameterizations. Assuming the theoretical sqrt(Z) metallicity dependence,
Magellanic Cloud O star mass-loss rates are typically matched to within a
factor of two for various calibrations. Stellar winds from LBVs are typically
denser and slower than equivalent B supergiants, with exceptional mass-loss
rates during giant eruptions Mdot=10^-3 .. 10^-1 Mo/yr (Drissen et al. 2001).
Recent mass-loss rates for Galactic WR stars indicate a downward revision of
2-4 relative to previous calibrations due to clumping (e.g. Schmutz 1997),
although evidence for a metallicity dependence remains inconclusive (Crowther
2000). Mass-loss properties of luminous (> 10^5 Lo) yellow and red supergiants
from alternative techniques remain highly contradictory. Recent Galactic and
LMC results for RSG reveal a large scatter such that typical mass-loss rates
lie in the range 10^-6 .. 10^-4 Mo/yr, with a few cases exhibiting 10^-3 Mo/yr.Comment: 16 pages, 2 figures, Review paper to appear in Proc `The influence of
binaries on stellar population studies', Brussels, Aug 2000 (D. Vanbeveren
ed.), Kluwe
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Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
Star clusters in the solar neighborhood: a solution to Oort's problem
In 1958 Jan Oort remarked that the lack of old clusters in the solar neighborhood (SN) implies that clusters are destroyed on a timescale of less than a Gyr. This is much shorter than the predicted dissolution time of clusters due to stellar evolution and two-body relaxation in the tidal field of the Galaxy. So, other (external) effects must play a dominant role in the destruction of star clusters in the solar neighborhood. We recalculated the survival time of initially bound star clusters in the solar neighborhood taking into account: (1) stellar evolution, (2) tidal stripping, (3) perturbations by spiral arms and (4) encounters with giant molecular clouds (GMCs). We find that encounters with GMCs are the most damaging to clusters. The resulting predicted dissolution time of these combined effects, t_dis=1.7 (Mi/10^4 M_sun)^0.67 Gyr for clusters in the mass range of 10^2 < M < 10^5 M_sun, is very similar to the disruption time of t_dis=1.3+/-0.5 (M/10^4 M_sun)^0.62 Gyr that was derived empirically from a mass limited sample of clusters in the solar neighborhood within 600 pc. The predicted shape of the age distribution of clusters agrees very well with the observed one. The comparison between observations and theory implies a surface star formation rate (SFR) near the sun of 3.5x10^-10 M_sun yr^-1 pc^-2 for stars in bound clusters with an initial mass in the range of 10^2 to 3x10^4 M_sun. This can be compared to a total SFR of 7-10x10^-10 M_sun yr^-1 pc^-2 derived from embedded clusters or 3-7x10^-9 M_sun yr^-1 pc^-2 derived from field stars. This implies an infant mortality rate of clusters in the solar neighborhood between 50% and 95%, in agreement with the results of a study of embedded clusters
Luminous pre-main sequence stars in the LMC?
We report the serendipitous discovery of seven luminous irregular variables in the LMC by the EROS project. The stars have V similar or equal to 15(m) to 17(m), (B(E) - R(E)) similar or equal to 0(m), and vary by about 0.4(m) on timescales of 10 to 40 days. The variations in B(E) and R(E) are about equal but the stars are slightly bluer when they an fainter. Medium resolution optical spectra show strong H alpha emission, weak H beta emission and absorption lines of the higher Balmer lines. Absorption lines of He I, C III, O II and Si IV are present in the spectra of some of the stars. The spectral types derived from the presence or absence of the lines are between late-O and late-B. Effective temperatures and luminosities were derived from the energy distributions and independently from the EROS photometry combined with the approximate spectral types. The luminosity, derived from V, E(B - V), and the distance of the LMC, are in the range of 10(3) to 10(4) L.. The strong Hey emission, the spectral types, the luminosities and the irregular variations are similar (but not the same) to those of the Galactic pre-main sequence HAeBe stars (i.e. the Herbig Ae/Be stars). However the stars are about a factor 10 more luminous than the luminosity upper limit for Galactic HAeBe stars. This might be due to a shorter accretion timescale (tau(accr) = M(*) /(M) over dot), or to the smaller dust-to-gas ratio in the LMC
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