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

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

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 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