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.

X-ray and molecular emission from the nearest region of recent star formation

Abstract: The isolated, young, sunlike star TW Hya and four other young stars in its vicinity are strong x-ray sources. Their similar x-ray and optical properties indicate that the stars make up a physical association that is on the order of 20 million years old and that lies between about 40 and 60 parsecs (between about 130 and 200 light years) from Earth. TW Hya itself displays circumstellar CO, HCN, CN, and HCO+ emission. These molecules probably orbit the star in a solar-system-sized disk viewed more or less face-on, whereas the star is likely viewed pole-on. Being at least three times closer to Earth than any well-studied region of star formation, the TW Hya Association serves as a test-bed for the study of x-ray emission from young stars and the formation of planetary systems around sunlike stars.

The origin of the galaxy mass-metallicity relation and implications for galactic outflows

Abstract: Using cosmological hydrodynamic simulations that dynamically incorporate enriched galactic outflows together with analytical modelling, we study the origin of the stellar mass-gas-phase metallicity relation (MZR). We find that metallicities are driven by an equilibrium between the rate of enrichment owing to star formation and the rate of dilution owing to infall of unenriched gas. This equilibrium is in turn governed by the outflow strength. As such, the MZR provides valuable insights and strong constraints on galactic outflow properties across cosmic time. We compare three outflow models: no outflows, a 'constant-wind model that emulates the popular Dekel & Silk scenario, and a 'momentum-driven wind' model that best reproduces z ≥ 2 intergalactic medium metallicity data. Only the momentum-driven wind scaling simulation is able to reproduce the observed z ∼ 2 MZR's slope, amplitude, and scatter. In order to understand why, we construct a one-zone chemical evolution model guided by simulations. This model shows that the MZR in our outflow simulations can be understood in terms of three parameters: (i) the equilibrium metallicity Z g,eq = YS FR / ACC (where y = net yield), reflecting the enrichment balance between star formation rate S FR and gas accretion rate ACC ; (ii) the dilution time t d = M g /M SFR , representing the time-scale for a galaxy to return to Z g,eq after a metallicity-perturbing interaction; and (iii) the blowout mass M blowout , which is the galaxy stellar mass above which winds can escape its halo. Without outflows, galaxy metallicities exceed observations by approximately two to three times, although the slope of the MZR is roughly correct owing to greater star formation efficiencies in larger galaxies. When outflows with mass-loading factor η w are present, galaxies below M blowout obey Z g,eq ≈ y/(1 + η w ), while above M blowout , Z g,eq → y. Our constant-wind model has M blowout ∼ 10 10 M ⊙ , which yields a sharp upturn in the MZR above this scale and a flat MZR with large scatter below it, in strong disagreement with observations. Our momentum-driven wind model naturally reproduces the observed Z g ∞ M 0.3 * because Z g,eq ∞ η -1 w ∞ M 1/3 * when η w » 1 (i.e. at low masses). The flattening of the MZR at M* ≥ 10 10.5 M ⊙ observed by Tremonti et al. is reflective of the mass-scale where η w ∼ 1 rather than a characteristic outflow speed; in fact, the outflow speed plays little role in the MZR except through M blowout . The tight observed MZR scatter is ensured when t d ≤ dynamical time, which is only satisfied at all masses in our momentum-driven wind model. We also discuss secondary effects on the MZR, such as baryonic stripping from neighbouring galaxies' outflows.

Reddening and star formation in starburst galaxies

Abstract: The reddening properties and the star formation history of a sample of 19 starburst galaxies are investigated using multiwavelength spectroscopy and infrared broad band photometry. The difference in reddening between the ionized gas and the stars is explained as difference in the covering factors of the dust in front of the gas and of the stars. A ``template starburst spectrum'', derived by combining the reddening-corrected UV, optical, and infrared data of all the galaxies in the sample, is used to investigate the star formation history. Spectral synthesis models indicate that the observed UV emission can be attributed to a stellar population which is undergoing active star formation at a constant rate since ~ 20 Myr, in agreement with the supernova rates derived from the [FeII] emission line in the infrared. At least two, and probably more, intermediate age populations (age < 2 Gyr) contribute to the optical and infrared emission, while populations older than 2 Gyr do not contribute significantly to the template. Episodic star formation over the last Gyr is suggested, with star formation rates as large as or larger than the present rates. The synthetic stellar populations are generated according to an Initial Mass Function (IMF) with Salpeter slope (alpha=2.35) in the mass range 0.1--100 solar masses, and reproduce a number of observational constraints, such as the spectral energy distribution of the template spectrum, the equivalent width of the atomic hydrogen emission lines, and the mass-to-light ratios; the data, then, do not provide indication for a high-mass-star truncated or a low-mass-star deficient IMF in starburst galaxies.

The rotation rates of massive stars: the role of binary interaction through tides, mass transfer, and mergers

Abstract: Rotation is thought to be a major factor in the evolution of massive stars—especially at low metallicity—with consequences for their chemical yields, ionizing flux, and final fate. Deriving the birth spin distribution is of high priority given its importance as a constraint on theories of massive star formation and as input for models of stellar populations in the local universe and at high redshift. Recently, it has become clear that the majority of massive stars interact with a binary companion before they die. We investigate how this affects the distribution of rotation rates, through stellar winds, expansion, tides, mass transfer, and mergers. For this purpose, we simulate a massive binary-star population typical for our Galaxy assuming continuous star formation. We find that, because of binary interaction, 20+5 –10% of all massive main-sequence stars have projected rotational velocities in excess of 200 km s–1. We evaluate the effect of uncertain input distributions and physical processes and conclude that the main uncertainties are the mass transfer efficiency and the possible effect of magnetic braking, especially if magnetic fields are generated or amplified during mass accretion and stellar mergers. The fraction of rapid rotators we derive is similar to that observed. If indeed mass transfer and mergers are the main cause for rapid rotation in massive stars, little room remains for rapidly rotating stars that are born single. This implies that spin-down during star formation is even more efficient than previously thought. In addition, this raises questions about the interpretation of the surface abundances of rapidly rotating stars as evidence for rotational mixing. Furthermore, our results allow for the possibility that all early-type Be stars result from binary interactions and suggest that evidence for rotation in explosions, such as long gamma-ray bursts, points to a binary origin.

Probing the evolution of molecular cloud structure: From quiescence to birth

Abstract: Context. Probability distribution of densities is a fundamental measure of molecular cloud structure, containing information on how the material arranges itself in molecular clouds. Aims. We derive the probability density functions (PDFs) of column density for a complete sample of prominent molecular cloud complexes closer than d < 200 pc. For comparison, additional complexes at d ≈ 250−700 pc are included in the study. Methods. We derive near-infrared dust extinction maps for 23 molecular cloud complexes, using the nicest colour excess mapping technique and data from the 2MASS archive. The extinction maps are then used to examine the column density PDFs in the clouds. Results. The column density PDFs of most molecular clouds are well-fitted by log-normal functions at low column densities (0. 5m ag< AV < 3− 5m ag, or−0.5 < lnAV /AV < 1). But at higher column densities prominent power-law-like wings are common. In particular, we identify a trend among the PDFs: active star-forming clouds always have prominent non-log-normal wings. In contrast, clouds without active star formation resemble log-normals over the whole observed column density range or show only low excess of higher column densities. This trend is also reflected in the cumulative forms of the PDFs, showing that the fraction of high column density material is significantly larger in star-forming clouds. These observations agree with an evolutionary trend where turbulent motions are the main cloud-shaping mechanism for quiescent clouds, but the density enhancements induced by them quickly become dominated by gravity (and other mechanisms), which is in turn strongly reflected by the shape of the column density PDFs. The dominant role of the turbulence is restricted to the very early stages of molecular cloud evolution, comparable to the onset of active star formation in the clouds.