847 research outputs found
On the simultaneous evolution of massive protostars and their host cores
Studies of the evolution of massive protostars and the evolution of their
host molecular cloud cores are commonly treated as separate problems. However,
interdependencies between the two can be significant. Here, we study the
simultaneous evolution of massive protostars and their host molecular cores
using a multi-dimensional radiation hydrodynamics code that incorporates the
effects of the thermal pressure and radiative acceleration feedback of the
centrally forming protostar. The evolution of the massive protostar is computed
simultaneously using the stellar evolution code STELLAR, modified to include
the effects of variable accretion. The interdependencies are studied in three
different collapse scenarios. For comparison, stellar evolutionary tracks at
constant accretion rates and the evolution of the host cores using pre-computed
stellar evolutionary tracks are computed. The resulting interdependencies of
the protostellar evolution and the evolution of the environment are extremely
diverse and depend on the order of events, in particular the time of
circumstellar accretion disk formation with respect to the onset of the
bloating phase of the star. Feedback mechanisms affect the instantaneous
accretion rate and the protostar's radius, temperature and luminosity on
timescales equal or smaller than 5 kyr, corresponding to the accretion
timescale and Kelvin-Helmholtz contraction timescale, respectively.
Nevertheless, it is possible to approximate the overall protostellar evolution
in many cases by pre-computed stellar evolutionary tracks assuming appropriate
constant average accretion rates.Comment: accepted for publication at Ap
Precise Astronomical Flux Calibration and its Impact on Studying the Nature of Dark Energy
Measurements of the luminosity of type Ia supernovae vs. redshift provided
the original evidence for the accelerating expansion of the Universe and the
existence of dark energy. Despite substantial improvements in survey
methodology, systematic uncertainty in flux calibration dominates the error
budget for this technique, exceeding both statistics and other systematic
uncertainties. Consequently, any further collection of type Ia supernova data
will fail to refine the constraints on the nature of dark energy unless we also
improve the state of the art in astronomical flux calibration to the order of
1%. We describe how these systematic errors arise from calibration of
instrumental sensitivity, atmospheric transmission, and Galactic extinction,
and discuss ongoing efforts to meet the 1% precision challenge using white
dwarf stars as celestial standards, exquisitely calibrated detectors as
fundamental metrologic standards, and real-time atmospheric monitoring.Comment: 25 pages, 7 figures. Accepted Modern Physics Letters
Photoevaporation of protostellar disks III. The appearance of photoevaporating disks around young intermediate mass stars
We present theoretical continuum emission spectra (SED's), isophotal maps and
line profiles for several models of photoevaporating disks at different
orientations with respect to the observer. The hydrodynamic evolution of these
models has been the topic of the two previous papers of this series. We discuss
in detail the numerical scheme used for these diagnostic radiation transfer
calculations. Our results are qualitatively compared to observed ultracompact
Hii-regions (UCHii's). Our conclusion is that the high fraction of
``unresolved'' UCHii's from the catalogues of Wood & Churchwell (1989) and
Kurtz et al. (1994) cannot be explained by disks around massive stars. In
particular, the observed infrared spectra of these objects indicate dust
temperatures which are about one order of magnitude lower than expected. We
suggest that disks around close companions to OB stars may be necessary to
resolve this inconsistency. Alternatively, strong stellar winds and radiative
acceleration could remove disk material from the immediate vicinity of luminous
O stars, whereas for the lower luminosity sources considered here this will not
occur. We also find that line profiles tracing the evaporated material
originating from the disk are not influenced significantly by the existence of
stellar winds over a wide range of wind velocities (400 - 1000 km/s). We
compare our results to the bright IRAS source MWC349A. Many of its properties,
especially its spatial appearance in high-resolution radio maps, can be well
explained by a disk surrounding a UV luminous star with a high velocity stellar
wind.Comment: 14 pages, 13 figures, corrected Fig.8, last ro
Formation of Massive Stars via Accretion
The collapse of massive molecular clumps can produce high mass stars, but the
evolution is not simply a scaled-up version of low mass star formation.
Outflows and radiative effects strongly hinder the formation of massive stars
via accretion. A necessary condition for accretion growth of a hydrostatic
object up to high masses M > 20 M_sun (rather than coalescence of optically
thick objects) is the formation of and accretion through a circumstellar disk.
Once the central object has accreted approximately 10 M_sun it has already
evolved to core hydrogen-burning; the resultant main sequence star continues to
accrete material as it begins to photoevaporate its circumstellar disk (and any
nearby disks) on a timescale of 100,000 years, similar to the accretion
timescale. Until the disk(s) is (are) completely photoevaporated, this
configuration is observable as an ultra-compact HII region (UCHII). The final
mass of the central star (and any nearby neighboring systems) is determined by
the interplay between radiation acceleration, UV photoevaporation, stellar
winds and outflows, and the accretion through the disk.
Several aspects of this evolutionary sequence have been simulated
numerically, resulting in a "proof of concept". This scenario places strong
constraints on the accretion rate necessary to produce high mass stars and
offers an opportunity to test the accretion hypothesis.Comment: 12 pages, 11 figures, IAU Symposium 221, invited major revie
Protostellar Feedback Halts the Growth of the First Stars in the Universe
The first stars fundamentally transformed the early universe by emitting the
first light and by producing the first heavy elements. These effects were
predetermined by the mass distribution of the first stars, which is thought to
have been fixed by a complex interplay of gas accretion and protostellar
radiation. We performed radiation-hydrodynamics simulations that followed the
growth of a primordial protostar through to the early stages as a star with
thermo-nuclear burning. The circumstellar accretion disk was evaporated by
ultraviolet radiation from the star when its mass was 43 times that of the Sun.
Such massive primordial stars, in contrast to the often postulated extremely
massive stars, may help explain the fact that there are no signatures of the
pair-instability supernovae in abundance patterns of metal-poor stars in our
galaxy.Comment: to appear in Science (published in online Science Express), combined
with SOM, additional images and movies are available at
http://www-tap.scphys.kyoto-u.ac.jp/~hosokawa/firststarstop_e.htm
Does the Lunar Surface Still Offer Value As a Site for Astronomical Observatories?
Current thinking about the Moon as a destination has revitalized interest in
lunar astronomical observatories. Once seen by a large scientific community as
a highly enabling site, the dramatic improvement in capabilities for free-space
observatories prompts reevaluation of this interest. Whereas the lunar surface
offers huge performance advantages for astronomy over terrestrial sites,
free-space locales such as Earth orbit or Lagrange points offer performance
that is superior to what could be achieved on the Moon. While astronomy from
the Moon may be cost effective once infrastructure is there, it is in many
respects no longer clearly enabling compared to free-space.Comment: 14 pages, 1 figure; in press Space Polic
Protostellar Feedback and Final Mass of the Second-Generation Primordial Stars
The first stars in the universe ionized the ambient primordial gas through
various feedback processes. "Second-generation" primordial stars potentially
form from this disturbed gas after its recombination. In this Letter, we study
the late formation stage of such second-generation stars, where a large amount
of gas accretes onto the protostar and the final stellar mass is determined
when the accretion terminates. We directly compute the complex interplay
between the accretion flow and stellar ultraviolet (UV) radiation, performing
radiation-hydrodynamic simulations coupled with stellar evolution calculations.
Because of more efficient H2 and HD cooling in the pre-stellar stage, the
accretion rates onto the star are ten times lower than in the case of the
formation of the first stars. The lower accretion rates and envelope density
result in the occurrence of an expanding bipolar HII region at a lower
protostellar mass M_* \simeq 10Msun, which blows out the circumstellar
material, thereby quenching the mass supply from the envelope to the accretion
disk. At the same time the disk loses mass due to photoevaporation by the
growing star. In our fiducial case the stellar UV feedback terminates mass
accretion onto the star at M_* \simeq 17Msun. Although the derived masses of
the second-generation primordial stars are systematically lower than those of
the first generation, the difference is within a factor of only a few. Our
results suggest a new scenario, whereby the majority of the primordial stars
are born as massive stars with tens of solar masses, regardless of their
generations.Comment: 5 pages, 4 figures, to be published in ApJ
Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: I. Basic Magnetic and Non-Magnetic Models and Parameter Studies
We combine dynamical and non-equilibrium chemical modeling of evolving
prestellar molecular cloud cores, and explore the evolution of molecular
abundances in the contracting core. We model both magnetic cores, with varying
degrees of initial magnetic support, and non-magnetic cores, with varying
collapse delay times. We explore, through a parameter study, the competing
effects of various model parameters in the evolving molecular abundances,
including the elemental C/O ratio, the temperature, and the cosmic-ray
ionization rate. We find that different models show their largest quantitative
differences at the center of the core, whereas the outer layers, which evolve
slower, have abundances which are severely degenerate among different dynamical
models. There is a large range of possible abundance values for different
models at a fixed evolutionary stage (central density), which demonstrates the
large potential of chemical differentiation in prestellar cores. However,
degeneracies among different models, compounded with uncertainties induced by
other model parameters, make it difficult to discriminate among dynamical
models. To address these difficulties, we identify abundance ratios between
particular molecules, the measurement of which would have maximal potential for
discrimination among the different models examined here. In particular, we find
that the ratios between NH3 and CO; NH2 and CO; NH3 and HCO+ are sensitive to
the evolutionary timescale, and that the ratio between HCN and OH is sensitive
to the C/O ratio. Finally, we demonstrate that measurements of the central
deviation (central depletion or enhancement) of abundances of certain molecules
are good indicators of the dynamics of the core.Comment: 20 pages, 15 figures, accepted for publication in Ap
Effect of OH depletion on measurements of the mass-to-flux ratio in molecular cloud cores
The ratio of mass and magnetic flux determines the relative importance of
magnetic and gravitational forces in the evolution of molecular clouds and
their cores. Its measurement is thus central in discriminating between
different theories of core formation and evolution. Here we discuss the effect
of chemical depletion on measurements of the mass-to-flux ratio using the same
molecule (OH) both for Zeeman measurements of the magnetic field and the
determination of the mass of the region. The uncertainties entering through the
OH abundance in determining separately the magnetic field and the mass of a
region have been recognized in the literature. It has been proposed however
that, when comparing two regions of the same cloud, the abundance will in both
cases be the same. We show that this assumption is invalid. We demonstrate that
when comparing regions with different densities, the effect of OH depletion in
measuring changes of the mass-to-flux ratio between different parts of the same
cloud can even reverse the direction of the underlying trends (for example, the
mass-to-flux ratio may appear to decrease as we move to higher density
regions). The systematic errors enter primarily through the inadequate
estimation of the mass of the region.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Formation of Primordial Supermassive Stars by Rapid Mass Accretion
Supermassive stars (SMSs) forming via very rapid mass accretion (Mdot >~ 0.1
Msun/yr) could be precursors of supermassive black holes observed beyond
redshift of about 6. Extending our previous work, we here study the evolution
of primordial stars growing under such rapid mass accretion until the stellar
mass reaches 10^{4 - 5} Msun. Our stellar evolution calculations show that a
star becomes supermassive while passing through the "supergiant protostar"
stage, whereby the star has a very bloated envelope and a contracting inner
core. The stellar radius increases monotonically with the stellar mass, until
=~ 100 AU for M_* >~ 10^4 Msun, after which the star begins to slowly contract.
Because of the large radius the effective temperature is always less than 10^4
K during rapid accretion. The accreting material is thus almost completely
transparent to the stellar radiation. Only for M_* >~ 10^5 Msun can stellar UV
feedback operate and disturb the mass accretion flow. We also examine the
pulsation stability of accreting SMSs, showing that the pulsation-driven mass
loss does not prevent stellar mass growth. Observational signatures of bloated
SMSs should be detectable with future observational facilities such as the
James Webb Space Telescope. Our results predict that an inner core of the
accreting SMS should suffer from the general relativistic instability soon
after the stellar mass exceeds 10^5 Msun. An extremely massive black hole
should form after the collapse of the inner core.Comment: 14 pages, 13 figures, accepted for publication in Ap
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