Star formation

About: Star formation is a research topic. Over the lifetime, 37405 publications have been published within this topic receiving 1808161 citations. The topic is also known as: astrogenesis.

The Rate of Neutron Star Binary Mergers in the Universe: Minimal Predictions for Gravity Wave Detectors

Abstract: Of the many sources which gravitational wave observatories might see, merging neutron star binaries are the most predictable. Their waveforms at the observable frequencies are easy to calculate. And three systems which will merge in less than a Hubble time have already been observed as binary pulsars: two in the disk of the Galaxy, and one in a globular cluster. From the lifetimes and positions of these, a lower limit to the merger rate in the Galaxy and globular cluster system are inferred with confidence. Taking the merger rate in other galaxies to scale with the star formation rate, the merger rate expected in the local universe is computed. An ultraconservative lower limit to the rate gives three per year within 1 Gpc. The best estimate, still conservative in that it considers only systems like those already observed, gives three per year within 200 Mpc. An upper limit of three mergers per year within 23/h Mpc is set by the rate of Type Ib supernovae. The rates of black hole binary mergers and black hole-neutron star binary mergers are model-dependent, but could be comparable to the given rate of neutron-star binary mergers.

Massive star formation near the Galactic center and the fate of the stellar remnants

Abstract: Several points are made regarding massive stars, star formation, and stellar remnants in the Galactic center region, particularly the inner 1-10 parsecs. First, in light of the processes which act to inhibit or suppress star formation there, it is argued that those stars which do form tend to be formed by an externally caused compression of their parent clouds rather than by spontaneous cloud collapse. As a result of this, and because of the particular characteristics of the interstellar medium near the Galactic center, it is likely that the initial mass function (IMF) favors more massive stars than that in the Galactic disk, or at least that the lower mass cutoff of the Galactic center IMF is relatively large

The enrichment of the intergalactic medium with adiabatic feedback - I. Metal cooling and metal diffusion

Abstract: A study of metal enrichment of the intergalactic medium (IGM) using a series of smooth particle hydrodynamics (SPH) simulations is presented, employing models for metal cooling and the turbulent diffusion of metals and thermal energy. An adiabatic feedback mechanism was adopted where gas cooling was prevented on the timescale of supernova bubble expansion to generate galactic winds without explicit wind particles. The simulations produced a cosmic star formation history (SFH) that is broadly consistent with observations until z � 0.5, and a steady evolution of the universal neutral hydrogen fraction (HI) that compares reasonably well with observations. The evolution of the mass and metallicities in stars and various gas phases was investigated. At z=0, about 40% of the baryons are in the warm-hot intergalactic medium (WHIM), but most metals (80%-90%) are locked in stars. At higher redshifts the proportion of metals in the IGM is higher due to more efficient loss from galaxies. The r also indicate that IGM metals primarily reside in the WHIM throughout cosmic history, which differs from simulations with hydrodynamically decoupled explicit winds. The metallicity of the WHIM lies between 0.01 and 0.1 solar with a slight decrease at lower redshifts. The metallicity evolution of the gas inside galaxies are broadly consistent with observations, but the diffuse IGM is under enriched at z � 2.5. Galactic winds most efficiently enrich the IGM for halos in the intermediate mass range 10 10 M � - 10 11 M � . At the low mass end gas is prevented from accreting onto halos and has very low metallicities. At the high mass end, the fraction of halo baryons escaped as winds declines along with the decline of stellar mass fraction of the galaxies. This is likely because of the decrease in star formation activity and decrease in wind escape efficiency. Metals enhance cooling which allows WHIM gas to cool onto galaxies and increases star formation. Metal diffusion allows winds to mix prior to escape, decreasing the IGM metal content in favour of gas within galactic halos and star forming gas. Diffusion significantly increases the amount of gas with low metallicities and changes the density-metallicity relation.

Evolution of Massive Protostars with High Accretion Rates

Abstract: Formation of massive stars by accretion requires a high accretion rate of to overcome the radiation pressure barrier of the forming stars. Here, we study evolution of protostars accreting at such high rates by solving the structure of the central star and the inner accreting envelope simultaneously. The protostellar evolution is followed starting from small initial cores until their arrival at the stage of the Zero-Age Main-Sequence (ZAMS) stars. An emphasis is put on evolutionary features different from those with a low accretion rate of , which is presumed in the standard scenario for low-mass star formation. With the high accretion rate of , the protostellar radius becomes very large and exceeds 100 R ☉. Unlike the cases of low accretion rates, deuterium burning hardly affects the evolution, and the protostar remains radiative even after its ignition. It is not until the stellar mass reaches 40 M ☉ that hydrogen burning begins and the protostar reaches the ZAMS phase, and this ZAMS arrival mass increases with the accretion rate. These features are similar to those of the first star formation in the universe, where high accretion rates are also expected, rather than to the present-day low-mass star formation. At a very high accretion rate of >3 × 10–3 M ☉ yr-1, the total luminosity of the protostar becomes so high that the resultant radiation pressure inhibits the growth of the protostars under steady accretion before reaching the ZAMS stage. Therefore, the evolution under the critical accretion rate 3 × 10–3 M ☉ yr-1 gives the upper mass limit of possible pre-main sequence stars at 60 M ☉. The upper mass limit of MS stars is also set by the radiation pressure onto the dusty envelope under the same accretion rate at 250 M ☉. We also propose that the central source enshrouded in the Orion KL/BN nebula has effective temperature and luminosity consistent with our model and is a possible candidate for such protostars growing under the high accretion rate.

Connecting Dense Gas Tracers of Star Formation in our Galaxy to High-z Star Formation

Abstract: Observations have revealed prodigious amounts of star formation in starburst galaxies as traced by dust and molecular emission, even at large redshifts. Recent work shows that for both nearby spiral galaxies and distant starbursts, the global star formation rate, as indicated by the infrared luminosity, has a tight and almost linear correlation with the amount of dense gas as traced by the luminosity of HCN. Our surveys of Galactic dense cores in HCN 1-0 emission show that this correlation continues to a much smaller scale, with nearly the same ratio of infrared luminosity to HCN luminosity found over 7-8 orders of magnitude in, with a lower cutoff L(IR) around 10(4.5) L(circle dot) of infrared luminosity. The linear correlation suggests that we may understand distant star, formation in terms of the known properties of local star-forming regions. Both the correlation and the luminosity cutoff can be explained if the basic unit of star formation in galaxies is a dense core, similar to those studied in our Galaxy.