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.

Circumstellar Material Associated with Solar-Type Pre-Main-Sequence Stars: A Possible Constraint on the Timescale for Planet Building

Abstract: The observed frequency distribution of near-IR excesses for a sample of 83 stars in the Taurus-Auriga star-forming complex is used to provide an estimate of the number of solar-type premain-sequence (PMS) stars surrounded by circumstellar disks. The results suggest that if all such PMS stars are initially surrounded by disks, then disk lifetimes must range from much less than 3 x 10 to the 6th yr to about 10 to the 7th yr, providing a constraint on the time available for planet building. PMS stars with small near-IR excesses, but significant mid-IR and far-IR excesses, provide some evidence for changes in disk structure with time.

Different star formation laws for disks versus starbursts at low and high redshifts

Abstract: We present evidence that bona fide disks and starburst systems occupy distinct regions in the gas mass versus star formation rate (SFR) plane, both for the integrated quantities and for the respective surface densities This result is based on carbon monoxide (CO) observations of galaxy populations at low and high redshifts, and on the current consensus for the CO luminosity to gas mass conversion factors The data suggest the existence of two different SF regimes: a long-lasting mode for disks and a more rapid mode for starbursts, the latter probably occurring during major mergers or in dense nuclear SF regions Both modes are observable over a large range of SFRs The detection of CO emission from distant near-IR selected galaxies reveals such bimodal behavior for the first time, as they allow us to probe gas in disk galaxies with much higher SFRs than are seen locally The different regimes can potentially be interpreted as the effect of a top-heavy initial mass function in starbursts However, we favor a different physical origin related to the fraction of molecular gas in dense clouds The IR luminosity to gas mass ratio (ie, the SF efficiency) appears to be inversely proportional to the dynamical (rotation) timescale Only when accounting for the dynamical timescale, a universal SF law is obtained, suggesting a direct link between global galaxy properties and the local SFR

Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources

Abstract: The history of the transition from a neutral intergalactic medium (IGM) to one that is almost fully ionized can reveal the character of cosmological ionizing sources. We study the evolution of the volume filling factors of H II and He III regions in a clumpy IGM and discuss the implications for rival reionization scenarios of the rapid decline observed at z3 in the space density of optical and radio-loud quasars and of the large population of star-forming galaxies recently discovered at the same epoch. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of photons emitted above 1 ryd in one recombination time equals the mean number of hydrogen atoms. If stellar sources are responsible for keeping the IGM ionized at z=5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z≈3 and the mean metallicity per baryon in the universe 0.002 solar. An early generation of stars in dark matter halos with circular velocities, vcirc≈50 km s-1, possibly one of the main sources of UV photons at high z, could be detectable with the Next Generation Space Telescope. Models in which the quasar emissivity declines rapidly at z3 predict a late He II reionization epoch, a feature that could explain the recent detection of patchy He II Lyα at z=2.9 by Reimers et al. and the abrupt change observed by Songaila at about the same epoch of the Si IV /C IV ratio, but appear unable to provide the required number of hydrogen-ionizing photons at z≈5.

Galaxies in a simulated ΛCDM Universe – I. Cold mode and hot cores

Abstract: We study the formation of galaxies in a large volume (50 h −1 Mpc, 2 × 288 3 particles) cosmological simulation, evolved using the entropy and energy-conserving smoothed particle hydrodynamics (SPH) code GADGET-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary ‘cold mode’ accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code. Atmospheres of hot, virialized gas develop in haloes above 2–3 × 10 11 M � , a transition mass that is nearly constant from z = 3 to 0. Cold accretion persists in haloes above the transition mass, especially at z ≥ 2. It dominates the growth of galaxies in low-mass haloes at all times, and it is the main driver of the cosmic star formation history. Our results suggest that the cooling of shock-heated virialized gas, which has been the focus of many analytic models of galaxy growth spanning more than three decades, might be a relatively minor element of galaxy formation. At high redshifts, satellite galaxies have gas accretion rates similar to central galaxies of the same baryonic mass, but at z < 1t he accretion rates of low-mass satellites are well below those of comparable central galaxies. Relative to our earlier simulations, the GADGET-2 simulations predict much lower rates of ‘hot mode’ accretion from the virialized gas component. Hot accretion rates compete with cold accretion rates near the transition mass, but only at z ≤ 1. Hot accretion is inefficient in haloes

Observational studies of pre-main-sequence evolution.

Abstract: Optical scanner data from 4270 to 6710 A with a resolution of 7 A have been obtained for some 600 H..cap alpha.. emission stars listed in the compilation of T Tauri stars by Herbig and Rao and in the original H..cap alpha.. surveys. These are used to obtain spectral types, reddening corrections, and emission-line strengths and are combined with infrared broad-band measurements out to 3.5 ..mu..m to provide bolometric luminosities and effective temperatures. H-R diagrams are presented for the Taurus-Auriga complex, Orion, NGC 2264, NGC 7000/IC 5070, and the rho Ophiuchi association. The following conclusions are reached: Most H..cap alpha.. emission stars lie on convective tracks in the H-R diagram, and their distribution does not correspond to any published dynamical evolutionary tracks. Most H..cap alpha.. emission stars are of spectra type late K and luminosity class between III and V. Radii and masses derived from the H-R diagram lie between 1 and 5 R/sub sun/ and between 0.2 and 3 M/sub sun/, respectively. The value of d (log N)/d (log M) ranges from -1.35 to -2.9 and for the three best-observed groups is consistent with the -1.35 of the Salpeter initial mass function. Star formation is not coeval on amore » time scale of less than a10/sup 4/to approx.6 x 10/sup 6/ years. The overall efficiency of star formation is approx.10%.6.. The bulk of the extinction occurs within the dark clouds rather than in circumstellar envelopes. High-mass stars may therefore form preferentially in the dense cores of clouds, and low-mass stars may form throughout the cloud. It is very likely that all close pairs of T Tauri stars are binaries.« less