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.

Irac observations of taurus pre-main-sequence stars

Abstract: We presented infrared photometry obtained with the IRAC camera on the Spitzer Space Telescope of a sample of 82 pre-main-sequence stars and brown dwarfs in the Taurus starforming region. We find a clear separation in some IRAC color-color diagrams between objects with and without disks. A few "transition" objects are noted, which correspond to systems in which the inner disk has been evacuated of small dust. Separating pure disk systems from objects with remnant protostellar envelopes is more difficult at IRAC wavelengths, especially for objects with infall at low rates and large angular momenta. Our results generally confirm the IRAC color classification scheme used in previous papers by Allen et al. and Megeath et al. to distinguish between protostars, T Tauri stars with disks, and young stars without (inner) disks. The observed IRAC colors are in good agreement with recent improved disk models, and in general accord with models for protostellar envelopes derived from analyzing a larger wavelength region. We also comment on a few Taurus objects of special interest. Our results should be useful for interpreting IRAC results in other, less well studied star-forming regions.

The galaxy stellar mass-star formation rate relation: evidence for an evolving stellar initial mass function?

Abstract: The evolution of the galaxy stellar mass-star formation rate relationship (M*-SFR) provides key constraints on the stellar mass assembly histories of galaxies. For star-forming galaxies, M*-SFR is observed to be fairly tight with a slope close to unity from z ∼ 0 → 2, and it evolves downwards roughly independently of M*. Simulations of galaxy formation reproduce these trends, broadly independent of modelling details, owing to the generic dominance of smooth and steady cold accretion in these systems. In contrast, the observed amplitude of the M*-SFR relation evolves markedly differently than in models, indicating either that stellar mass assembly is poorly understood or that observations have been misinterpreted. Stated in terms of a star formation activity parameter α sf ≡ (M* /SFR)/(t Hubble -1 Gyr), models predict a constant α sf ∼ 1 out to redshifts z ∼ 4+, while the observed M*-SFR relation indicates that α sf increases by approximately three times from z ∼ 2 until today. The low α sf (i.e. rapid star formation) at high z not only conflicts with models, but also difficult to reconcile with other observations of high-z galaxies, such as the small scatter in M*-SFR, the slow evolution of star-forming galaxies at z ∼ 2-4 and the modest passive fractions in mass-selected samples. Systematic biases could significantly affect measurements of M* and SFR, but detailed considerations suggest that none are obvious candidates to reconcile the discrepancy. A speculative solution is considered in which the stellar initial mass function (IMF) evolves towards more high-mass star formation at earlier epochs. Following Larson, a model is investigated in which the characteristic mass M where the IMF turns over increases with redshift. Population synthesis models are used to show that the observed and predicted M*-SFR evolution may be brought into broad agreement if M = 0.5 (1 + z) 2 M ⊙ out to z ∼ 2. Such IMF evolution matches recent observations of cosmic stellar mass growth, and the resulting z = 0 cumulative IMF is similar to the 'paunchy' IMF favoured by Fardal et al. to reconcile the observed cosmic star formation history with present-day fossil light measures.

DEMOGRAPHICS AND PHYSICAL PROPERTIES OF GAS OUTFLOWS/INFLOWS AT 0.4 < z < 1.4

Abstract: We present Keck/LRIS spectra of over 200 galaxies with well-determined redshifts between 0.4 and 1.4. We combine new measurements of near-ultraviolet, low-ionization absorption lines with previously measured masses, luminosities, colors, and star formation rates to describe the demographics and properties of galactic flows. Among star-forming galaxies with blue colors, we find a net blueshift of the Fe II absorption greater than 200 km s{sup -1} (100 km s{sup -1}) toward 2.5% (20%) of the galaxies. The fraction of blueshifted spectra does not vary significantly with stellar mass, color, or luminosity but does decline at specific star formation rates less than roughly 0.8 Gyr{sup -1}. The insensitivity of the blueshifted fraction to galaxy properties requires collimated outflows at these redshifts, while the decline in outflow fraction with increasing blueshift might reflect the angular dependence of the outflow velocity. The low detection rate of infalling gas, 3%-6% of the spectra, suggests an origin in (enriched) streams favorably aligned with our sightline. We find that four of these nine infalling streams have projected velocities commensurate with the kinematics of an extended disk or satellite galaxy. The strength of the Mg II absorption increases with stellar mass, B-band luminosity, and U - B color,more » trends arising from a combination of more interstellar absorption at the systemic velocity and less emission filling in more massive galaxies. Our results provide a new quantitative understanding of gas flows between galaxies and the circumgalactic medium over a critical period in galaxy evolution.« less

Formation of Massive Black Holes in Dense Star Clusters. I. Mass Segregation and Core Collapse

Abstract: We study the early dynamical evolution of young dense star clusters by using Monte Carlo simulations for systems with up to N = 107 stars. Rapid mass segregation of massive main-sequence stars and the development of the Spitzer instability can drive these systems to core collapse in a small fraction of the initial half-mass relaxation time. If the core-collapse time is less than the lifetime of the massive stars, all stars in the collapsing core may then undergo a runaway collision process leading to the formation of a massive black hole. Here we study in detail the first step in this process, up to the occurrence of core collapse. We have performed about 100 simulations for clusters with a wide variety of initial conditions, varying systematically the cluster density profile, stellar initial mass function (IMF), and number of stars. We also considered the effects of initial mass segregation and stellar evolution mass loss. Our results show that, for clusters with a moderate initial central concentration and any realistic IMF, the ratio of core-collapse time to initial half-mass relaxation time is typically ~0.1, in agreement with the value previously found by direct N-body simulations for much smaller systems. Models with even higher central concentration initially, or with initial mass segregation (from star formation) have even shorter core collapse times. Remarkably, we find that, for all realistic initial conditions, the mass of the collapsing core is always close to ~10-3 of the total cluster mass, very similar to the observed correlation between central black hole mass and total cluster mass in a variety of environments. We discuss the implications of our results for the formation of intermediate-mass black holes in globular clusters and super star clusters, ultraluminous X-ray sources, and seed black holes in proto-galactic nuclei.

Timescales for Planetary Accretion and the Structure of the Protoplanetary Disk

Abstract: No self-consistent scenario for all stages of planetary accretion which satisfies observational constraints currently exists. An attempt is accordingly made here to resolve the timescale problems and to outline a planet formation scenario consistent with current theories of star formation as well as related models of the protoplanetary disk. For accretion to have proceeded in the manner presently hypothesized, the surface mass density of planetessimals would have had to to be relatively uniform in the Venus-Jupiter region of the protoplanetary disk, consistent with viscous accretion disk models of the solar nebula. The outer regions of the nebula would still have contained enough solid matter to account for the growth of Uranus and Neptune in 5 to 500 million years.