47 research outputs found
Significant primordial star formation at redshifts z ~ 3-4
Four recent observational results have challenged our understanding of
high--redshift galaxies, as they require the presence of far more ultraviolet
photons than should be emitted by normal stellar populations. First, there is
significant ultraviolet emission from Lyman Break Galaxies (LBGs) at
wavelenghts shorter than 912\AA. Second, there is strong Lyman alpha emission
from extended ``blobs'' with little or no associated apparent ionizing
continuum. Third, there is a population of galaxies with unusually strong
Lyman-alpha emission lines. And fourth, there is a strong HeII (1640 \AA)
emission line in a composite of LBGs. The proposed explanations for the first
three observations are internally inconsistent, and the fourth puzzle has
remained hitherto unexplained. Here we show that all four problems are resolved
simultaneously if 10-30 percent of the stars in many galaxies at z ~ 3-4 are
mainly primordial - unenriched by elements heavier than helium ('metals'). Most
models of hierarchical galaxy formation assume efficient intra--galactic metal
mixing, and therefore do not predict metal-free star formation at redshifts
significantly below z ~5. Our results imply that micro-mixing of metals within
galaxies is inefficient on a ~ Gyr time-scale, a conclusion that can be
verified with higher resolution simulations, and future observations of the
HeII emission line.Comment: Nature in press, March 23rd issue. Under Nature embargo. Reference
and acknowledgement adde
Current status of NLTE analysis of stellar atmospheres
Various available codes for NLTE modeling and analysis of hot star spectra
are reviewed. Generalizations of standard equations of kinetic equilibrium and
their consequences are discussed.Comment: in Determination of Atmospheric Parameters of B-, A-, F- and G-Type
Stars, E. Niemczura et al. eds., Springer, in pres
An eclipsing binary distance to the Large Magellanic Cloud accurate to 2 per cent
In the era of precision cosmology it is essential to determine the Hubble
Constant with an accuracy of 3% or better. Currently, its uncertainty is
dominated by the uncertainty in the distance to the Large Magellanic Cloud
(LMC) which as the second nearest galaxy serves as the best anchor point of the
cosmic distance scale. Observations of eclipsing binaries offer a unique
opportunity to precisely and accurately measure stellar parameters and
distances. The eclipsing binary method was previously applied to the LMC but
the accuracy of the distance results was hampered by the need to model the
bright, early-type systems used in these studies. Here, we present distance
determinations to eight long-period, late- type eclipsing systems in the LMC
composed of cool giant stars. For such systems we can accurately measure both
the linear and angular sizes of their components and avoid the most important
problems related to the hot early-type systems. Our LMC distance derived from
these systems is demonstrably accurate to 2.2 % (49.97 +/- 0.19 (statistical)
+/- 1.11 (systematic) kpc) providing a firm base for a 3 % determination of the
Hubble Constant, with prospects for improvement to 2 % in the future.Comment: 34 pages, 5 figures, 13 tables, published in the Nature, a part of
our data comes from new unpublished OGLE-IV photometric dat
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
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
Spectral analysis of four 'hypervariable' AGN: a microneedle in the haystack?
We analyse four extreme active galactic nuclei (AGN) transients to explore the possibility that they are caused by rare, high-amplitude microlensing events. These previously unknown type-I AGN are located in the redshift range 0.6-1.1 and show changes of > 1.5 mag in the g band on a time-scale of similar to years. Multi-epoch optical spectroscopy, from the William Herschel Telescope, shows clear differential variability in the broad line fluxes with respect to the continuum changes and also evolution in the line profiles. In two cases, a simple point-source, point-lens microlensing model provides an excellent match to the long-term variability seen in these objects. For both models, the parameter constraints are consistent with the microlensing being due to an intervening stellar mass object but as yet there is no confirmation of the presence of an intervening galaxy. The models predict a peak amplification of 10.3/13.5 and an Einstein time-scale of 7.5/10.8 yr, respectively. In one case, the data also allow constraints on the size of the C III] emitting region, with some simplifying assumptions, to be similar to 1.0-6.5 light-days and a lower limit on the size of the MgII emitting region to be > 9 light-days (halflight radii). This C III] radius is perhaps surprisingly small. In the remaining two objects, there is spectroscopic evidence for an intervening absorber but the extra structure seen in the light curves requires a more complex lensing scenario to adequately explain
Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star
The fundamental properties of neutron stars provide a direct test of the
equation of state of cold nuclear matter, a relationship between pressure and
density that is determined by the physics of the strong interactions between
the particles that constitute the star. The most straightforward method of
determining these properties is by measuring the gravitational redshift of
spectral lines produced in the neutron star photosphere. The equation of state
implies a mass-radius relation, while a measurement of the gravitational
redshift at the surface of a neutron star provides a direct constraint on the
mass-to-radius ratio. Here we report the discovery of signficant absorption
lines in the spectra of 28 bursts of the low-mass X-ray binary EXO 0748-676. We
identify the most signficant features with the Fe XXVI and XXV n=2-3 and O VIII
n=1-2 transitions, all with a redshift of z=0.35, identical within small
uncertainties for the respective transitions. For an astrophysically plausible
range of masses (M ~ 1.3-2.0 M_solar), this value is completely consistent with
models of neutron stars composed of normal nuclear matter, while it excludes
some models in which the neutron stars are made of more exotic matter.Comment: Published in Nature (Nov 7, 2002
A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
Most ultraluminous X-ray sources have a typical set of properties not seen in Galactic stellar-mass black holes. They have luminosities of more than 3 × 10 39 ergs per second, unusually soft X-ray components (with a typical temperature of less than about 0.3 kiloelectronvolts) and a characteristic downturn in their spectra above about 5 kiloelectronvolts. Such puzzling properties have been interpreted either as evidence of intermediate-mass black holes or as emission from stellar-mass black holes accreting above their Eddington limit, analogous to some Galactic black holes at peak luminosity. Recently, a very soft X-ray spectrum was observed in a rare and transient stellar-mass black hole. Here we report that the X-ray source P13 in the galaxy NGC 7793 is in a binary system with a period of about 64 days and exhibits all three canonical properties of ultraluminous sources. By modelling the strong optical and ultraviolet modulations arising from X-ray heating of the B9Ia donor star, we constrain the black hole mass to be less than 15 solar masses. Our results demonstrate that in P13, soft thermal emission and spectral curvature are indeed signatures of supercritical accretion. By analogy, ultraluminous X-ray sources with similar X-ray spectra and luminosities of up to a few times 10 40 ergs per second can be explained by supercritical accretion onto massive stellar-mass black holes
Formation of Supermassive Black Holes
Evidence shows that massive black holes reside in most local galaxies.
Studies have also established a number of relations between the MBH mass and
properties of the host galaxy such as bulge mass and velocity dispersion. These
results suggest that central MBHs, while much less massive than the host (~
0.1%), are linked to the evolution of galactic structure. In hierarchical
cosmologies, a single big galaxy today can be traced back to the stage when it
was split up in hundreds of smaller components. Did MBH seeds form with the
same efficiency in small proto-galaxies, or did their formation had to await
the buildup of substantial galaxies with deeper potential wells? I briefly
review here some of the physical processes that are conducive to the evolution
of the massive black hole population. I will discuss black hole formation
processes for `seed' black holes that are likely to place at early cosmic
epochs, and possible observational tests of these scenarios.Comment: To appear in The Astronomy and Astrophysics Review. The final
publication is available at http://www.springerlink.co
Measuring Dark Energy Properties with Photometrically Classified Pan-STARRS Supernovae. II. Cosmological Parameters
We use 1169 Pan-STARRS supernovae (SNe) and 195 low-z (z < 0.1) SNe Ia to measure cosmological parameters. Though most Pan-STARRS SNe lack spectroscopic classifications, in a previous paper we demonstrated that photometrically classified SNe can be used to infer unbiased cosmological parameters by using a Bayesian methodology that marginalizes over core-collapse (CC) SN contamination. Our sample contains nearly twice as many SNe as the largest previous SN Ia compilation. Combining SNe with cosmic microwave background (CMB) constraints from Planck, we measure the dark energy equation-of-state parameter w to be -0.989 +/- 0.057 (stat+sys). If w evolves with redshift as w(a) = w(0)(1 - a), we find w(0) = -0.912 +/- 0.149 and w(a) = -0.513 +/- 0.826. These results are consistent with cosmological parameters from the Joint Light-curve Analysis and the Pantheon sample. We try four different photometric classification priors for Pan-STARRS SNe and two alternate ways of modeling CC SN contamination, finding that no variant gives a w differing by more than 2% from the baseline measurement. The systematic uncertainty on w due to marginalizing over CC SN contamination, sigma(cc)(w) = 0.012, is the third smallest source of systematic uncertainty in this work. We find limited (1.6 sigma) evidence for evolution of the SN color-luminosity relation with redshift, a possible systematic that could constitute a significant uncertainty in future high-z analyses. Our data provide one of the best current constraints on w, demonstrating that samples with similar to 5% CC SN contamination can give competitive cosmological constraints when the contaminating distribution is marginalized over in a Bayesian framework