Topic
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.
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University of Toronto1, University of Victoria2, Pierre-and-Marie-Curie University3, Centre national de la recherche scientifique4, Instituto Superior Técnico5, University of the Mediterranean6, University of Oxford7, University of Savoy8, California Institute of Technology9, Lawrence Berkeley National Laboratory10
TL;DR: In this paper, the authors show that Type Ia supernovae are formed within both very young and old stellar populations, with observed rates that depend on the stellar mass and mean star formation rates (SFRs) of their host galaxies.
Abstract: We show that Type Ia supernovae (SNe Ia) are formed within both very young and old stellar populations, with observed rates that depend on the stellar mass and mean star formation rates (SFRs) of their host galaxies. Models in which the SN Ia rate depends solely on host galaxy stellar mass are ruled out with >99% confidence. Our analysis is based on 100 spectroscopically confirmed SNe Ia, plus 24 photometrically classified events, all from the Supernova Legacy Survey (SNLS) and distributed over 0.2 < z < 0.75. We estimate stellar masses and SFRs for the SN Ia host galaxies by fitting their broadband spectral energy distributions with the galaxy spectral synthesis code PEGASE.2. We show that the SN Ia rate per unit mass is proportional to the specific SFR of the parent galaxies—more vigorously star-forming galaxies host more SNe Ia per unit stellar mass, broadly equivalent to the trend of increasing SN Ia rate in later type galaxies seen in the local universe. Following earlier suggestions for a simple "two-component" model approximating the SN Ia rate, we find bivariate linear dependencies of the SN Ia rate on both the stellar masses and the mean SFRs of the host systems. We find that the SN Ia rate can be well represented as the sum of 5.3 ± 1.1 × 10 to the -14 SNe yr to the -1 M(.)to the -1 and 3.9 ± 0.7 × 10 to the -4 SNe yr to the -1 (M(.) yr to the -1)to the -1 of star formation. We also demonstrate a dependence of distant SN Ia light-curve shapes on star formation in the host galaxy, similar to trends observed locally. Passive galaxies, with no star formation, preferentially host faster declining/dimmer SNe Ia, while brighter events are found in systems with ongoing star formation.
526 citations
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TL;DR: In this article, a detailed study of the star formation law in a sample of 32 nearby spiral galaxies with well-measured rotation curves, H I and H_2 (as traced by CO) surface density profiles, and new Hα CCD photometry was performed.
Abstract: We report the first results of a detailed study of the star formation law in a sample of 32 nearby spiral galaxies with well-measured rotation curves, H I and H_2 (as traced by CO) surface density profiles, and new Hα CCD photometry. In this paper we present an atlas of Hα images and radial surface brightness profiles and describe a surface density threshold in the star formation law. Prominent breaks in the Hα surface brightness profiles are identified in nearly all of the actively star-forming disks, confirming previous claims of star formation thresholds based on lower quality data. We measure the ratio of the gas density to the critical density for local gravitational stability at the threshold radii. The outer threshold radii observed in Sab-Sdm galaxies are in general agreement with those expected from the Toomre Q stability criterion, confirming earlier work, but with a significant variation that appears to be weakly correlated with galaxy type. Such a trend could plausibly reflect variations in the relative contribution of the stellar disk to the instability of the gas disk across this range of galaxy types. Among disks with subcritical gas surface densities, and outside the threshold radius in star-forming disks, the number of isolated H II regions increases as the gas surface density approaches the critical density. At the thresholds, the gas surface densities span a wide range, and the atomic/molecular gas fraction is highest in the disks having the lowest total gas surface density. The simple Toomre condition fails to account for the active star formation in the inner disks of low-mass spirals such as NGC 2403 and M33. An alternative stability criterion based on the shear in the disk provides a better description of these disks but is a less accurate indicator of the outer edges of star-forming disks than the Toomre criterion. These results strongly support the view that the formation of gravitationally bound interstellar clouds regulates the onset of widespread star formation, at least in the outer regions of galactic disks.
526 citations
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TL;DR: This work reports the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value, which suggests that population III stars could still exist and that the first generation of stars also contained long-livedLow-mass objects.
Abstract: The chemical composition of the most metal-deficient stars largely reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts1,2; if totally metal-free (‘population III’) stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun3,4, leading to the suggestion5,6 that low-mass stars could form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist—that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.
525 citations
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TL;DR: In this article, the authors investigated the relation between star formation rate and gas surface densities in Galactic star-forming regions using a sample of young stellar objects (YSOs) and massive dense clumps.
Abstract: We investigate the relation between star formation rate (SFR) and gas surface densities in Galactic star-forming regions using a sample of young stellar objects (YSOs) and massive dense clumps. Our YSO sample consists of objects located in 20 large molecular clouds from the Spitzer cores to disks (c2d) and Gould's Belt (GB) surveys. These data allow us to probe the regime of low-mass star formation, essentially invisible to tracers of high-mass star formation used to establish extragalactic SFR-gas relations. We estimate the gas surface density (Σgas) from extinction (AV ) maps and YSO SFR surface densities (ΣSFR) from the number of YSOs, assuming a mean mass and lifetime. We also divide the clouds into evenly spaced contour levels of AV , counting only Class I and Flat spectral energy distribution YSOs, which have not yet migrated from their birthplace. For a sample of massive star-forming clumps, we derive SFRs from the total infrared luminosity and use HCN gas maps to estimate gas surface densities. We find that c2d and GB clouds lie above the extragalactic SFR-gas relations (e.g., Kennicutt-Schmidt law) by factors of up to 17. Cloud regions with high Σgas lie above extragalactic relations up to a factor of 54 and overlap with high-mass star-forming regions. We use 12CO and 13CO gas maps of the Perseus and Ophiuchus clouds from the COMPLETE survey to estimate gas surface densities and compare to measurements from AV maps. We find that 13CO, with the standard conversions to total gas, underestimates the AV -based mass by factors of ~4-5. 12CO may underestimate the total gas mass at Σgas 200 M ☉ pc–2 by 30%; however, this small difference in mass estimates does not explain the large discrepancy between Galactic and extragalactic relations. We find evidence for a threshold of star formation (Σth) at 129 ± 14 M ☉ pc–2. At Σgas>Σth, the Galactic SFR-gas relation is linear. A possible reason for the difference between Galactic and extragalactic relations is that much of Σgas is below Σth in extragalactic studies, which detect all the CO-emitting gas. If the Kennicutt-Schmidt relation (ΣSFR Σ1.4 gas) and a linear relation between dense gas and star formation are assumed, the fraction of dense star-forming gas (f dense) increases as ~Σ0.4 gas. When Σgas reaches ~300 Σth, the fraction of dense gas is ~1, creating a maximal starburst.
524 citations
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TL;DR: In this article, an analytical description of the physical processes associated with the origin of cored dark matter density profiles is proposed and successfully tested against new cosmological simulations. But the model is restricted to the case where only a few per cent of the baryons form stars.
Abstract: We propose and successfully test against new cosmological simulations a novel analytical description of the physical processes associated with the origin of cored dark matter density profiles. In the simulations, the potential in the central kiloparsec changes on sub-dynamical timescales over the redshift interval 4 > z > 2 as repeated, energetic feedback generates large underdense bubbles of expanding gas from centrally-concentrated bursts of star formation. The model demonstrates how fluctuations in the central potential irreversibly transfer energy into collisionless particles, thus generating a dark matter core. A supply of gas undergoing collapse and rapid expansion is therefore the essential ingredient. The framework, based on a novel impulsive approximation, breaks with the reliance on adiabatic approximations which are inappropriate in the rapidly-changing limit. It shows that both outflows and galactic fountains can give rise to cusp-flattening, even when only a few per cent of the baryons form stars. Dwarf galaxies maintain their core to the present time. The model suggests that constant density dark matter cores will be generated in systems of a wide mass range if central starbursts or AGN phases are sufficiently frequent and energetic.
524 citations