Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source libraries for a wide ...range of applications in computational stellar astrophysics. A newly designed 1-D stellar evolution module, MESA star, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very-low mass to massive stars, including advanced evolutionary phases. MESA star solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. Independently usable modules provide equation of state, opacity, nuclear reaction rates, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own public interface. Examples include comparisons to other codes and show evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets; the complete evolution of a 1 Msun star from the pre-main sequence to a cooling white dwarf; the Solar sound speed profile; the evolution of intermediate mass stars through the thermal pulses on the He-shell burning AGB phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; evolutionary tracks of massive stars from the pre-main sequence to the onset of core collapse; stars undergoing Roche lobe overflow; and accretion onto a neutron star. Instructions for downloading and installing MESA can be found on the project web site (http://mesa.sourceforge.net/).
The runaway collision scenario is one of the most promising mechanisms to explain the formation of intermediate-mass black holes (IMBHs) in young dense star clusters. On the other hand, the massive ...stars that participate in the runaway collisions lose mass by stellar winds. In this paper, we discuss new N-body simulations of massive (6.5 × 104 M⊙) star clusters, in which we added upgraded recipes for stellar winds and supernova explosion at different metallicity. We follow the evolution of the principal collision product (PCP), through dynamics and stellar evolution, till it forms a stellar remnant. At solar metallicity, the mass of the final merger product spans from few solar masses up to ∼30 M⊙. At low metallicity (0.01–0.1 Z⊙) the maximum remnant mass is ∼250 M⊙, in the range of IMBHs. A large fraction (∼0.6) of the PCPs are not ejected from the parent star cluster and acquire stellar or black hole (BH) companions. Most of the long-lived binaries hosting a PCP are BH–BH binaries. We discuss the importance of this result for gravitational wave detection.
Star formation is slow, in the sense that the gas consumption time is much longer than the dynamical time. It is also inefficient; essentially all star formation in local galaxies takes place in ...giant molecular clouds (GMCs), but the fraction of a GMC converted to stars is very small, ~5%. In the most luminous starbursts, the GMC lifetime is shorter than the main sequence lifetime of even the most massive stars, so that supernovae can play no role in GMC disruption. We investigate the disruption of GMCs across a wide range of galaxies, from normal spirals to the densest starbursts; we take into account the effects of HII gas pressure, shocked stellar winds, protostellar jets, and radiation pressure produced by the absorption and scattering of starlight on dust grains. In the Milky Way, we find that a combination of three mechanisms, jets, HII gas pressure, and radiation pressure, disrupts the clouds. In more rapidly star forming galaxies such as ``clump'' galaxies at high-redshift, ultra-luminous infrared galaxies (ULIRGs) and submillimeter galaxies, radiation pressure dominates natal cloud distribution. We predict the presence of 10-20 clusters with masses ~10^7 Msun in local ULIRGs such as Arp 220 and a similar number of clusters with M_* ~ 10^8 Msun in high redshift clump galaxies; submillimeter galaxies will have even more massive clusters. We find that the mass fraction of a GMC that ends up in stars is an increasing function of the gas surface density of a galaxy, reaching ~35% in the most luminous starbursts. Furthermore, the disruption of bubbles by radiation pressure stirs the interstellar medium to velocities of ~10 km/s in normal galaxies and to ~100 km/s in ULIRGs like Arp 220, consistent with observations. Thus, radiation pressure may play a dominant role in the ISM of star-forming galaxies.
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 ( Delta *Sgas) from extinction (AV ) maps and YSO SFR surface densities ( Delta *SSFR) 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 delta 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 Delta *Sgas 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 Delta *Sgas 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 ( Delta *Sth) at 129 ? 14 M pc--2. At Delta *Sgas> Delta *Sth, the Galactic SFR-gas relation is linear. A possible reason for the difference between Galactic and extragalactic relations is that much of Delta *Sgas is below Delta *Sth in extragalactic studies, which detect all the CO-emitting gas. If the Kennicutt-Schmidt relation ( Delta *SSFR Delta *S1.4 gas) and a linear relation between dense gas and star formation are assumed, the fraction of dense star-forming gas (f dense) increases as ~ Delta *S0.4 gas. When Delta *Sgas reaches ~300 Delta *Sth, the fraction of dense gas is ~1, creating a maximal starburst.
We present a hydrodynamical simulation of an Antennae-like galaxy merger at parsec resolution, including a multicomponent model for stellar feedback and reaching numerical convergence in the global ...star formation rate for the first time. We analyse the properties of the dense stellar objects formed during the different stages of the interaction. Each galactic encounter triggers a starburst activity, but the varying physical conditions change the triggering mechanism of each starburst. During the first two pericentre passages, the starburst is spatially extended and forms many star clusters. However, the starburst associated with the third, final passage is more centrally concentrated: stars form almost exclusively in the galactic nucleus and no new star cluster is formed. The maximum mass of stars clusters in this merger is more than 30 times higher than those in a simulation of an isolated Milky Way-like galaxy. Antennae-like mergers are therefore a formation channel of young massive clusters possibly leading to globular clusters. Monitoring the evolution of a few clusters reveals the diversity of formation scenarios including the gathering and merger of gas clumps, the monolithic formation and the hierarchical formation in sub-structures inside a single cloud. Two stellar objects formed in the simulation yield the same properties as ultracompact dwarf galaxies. They share the same formation scenario than the most massive clusters, but have a larger radius either since birth, or get it after a violent interaction with the galactic centre. The diversity of environments across space and time in a galaxy merger can account for the diversity of the stellar objects formed, both in terms of mass and size.
Context. The VLT-FLAMES Tarantula Survey has an extensive view of the copious number of massive stars in the 30 Doradus (30 Dor) star forming region of the Large Magellanic Cloud. These stars play a ...crucial role in our understanding of the stellar feedback in more distant, unresolved star forming regions. Aims. The first comprehensive census of hot luminous stars in 30 Dor is compiled within a 10 arcmin (150 pc) radius of its central cluster, R136. We investigate the stellar content and spectroscopic completeness of the early type stars. Estimates were made for both the integrated ionising luminosity and stellar wind luminosity. These values were used to re-assess the star formation rate (SFR) of the region and determine the ionising photon escape fraction. Methods. Stars were selected photometrically and combined with the latest spectral classifications. Spectral types were estimated for stars lacking spectroscopy and corrections were made for binary systems, where possible. Stellar calibrations were applied to obtain their physical parameters and wind properties. Their integrated properties were then compared to global observations from ultraviolet (UV) to far-infrared (FIR) imaging as well as the population synthesis code, Starburst99. Results. Our census identified 1145 candidate hot luminous stars within 150 pc of R136 of which >700 were considered to be genuine early type stars and contribute to feedback. We assess the survey to be spectroscopically complete to 85% in the outer regions (>5 pc) but only 35% complete in the region of the R136 cluster, giving a total of 500 hot luminous stars in the census which had spectroscopy. Only 31 were found to be Wolf-Rayet (W-R) or Of/WN stars, but their contribution to the integrated ionising luminosity and wind luminosity was ~40% and ~50%, respectively. Similarly, stars with Minit > 100 M⊙ (mostly H-rich WN stars) also showed high contributions to the global feedback, ~25% in both cases. Such massive stars are not accounted for by the current Starburst99 code, which was found to underestimate the integrated ionising luminosity of R136 by a factor ~2 and the wind luminosity by a factor ~9. The census inferred a SFR for 30 Dor of 0.073 ± 0.04 M⊙ yr-1. This was generally higher than that obtained from some popular SFR calibrations but still showed good consistency with the far-UV luminosity tracer as well as the combined Hα and mid-infrared tracer, but only after correcting for Hα extinction. The global ionising output was also found to exceed that measured from the associated gas and dust, suggesting that ~6+55-6 % of the ionising photons escape the region. Conclusions. When studying the most luminous star forming regions, it is essential to include their most massive stars if one is to determine a reliable energy budget. Photon leakage becomes more likely after including their large contributions to the ionising output. If 30 Dor is typical of other massive star forming regions, estimates of the SFR will be underpredicted if this escape fraction is not accounted for.
We have conducted a high-resolution imaging study of the Taurus-Auriga star-forming region in order to characterize the primordial outcome of multiple star formation and the extent of the brown dwarf ...desert. Our survey identified 16 new binary companions to primary stars with masses of 0.25-2.5 Msun, raising the total number of binary pairs (including components of high-order multiples) with separations of 3--5000 AU to 90. We find that ~2/3--3/4 of all Taurus members are multiple systems of two or more stars, while the other ~1/4--1/3 appear to have formed as single stars; the distribution of high-order multiplicity suggests that fragmentation into a wide binary has no impact on the subsequent probability that either component will fragment again. The separation distribution for solar-type stars (0.7--2.5 Msun) is nearly log-flat over separations of 3--5000 AU, but lower-mass stars (0.25--0.7 Msun) show a paucity of binary companions with separations of >200 AU. Across this full mass range, companion masses are well described with a linear-flat function; all system mass ratios (q=M_B/M_A) are equally probable, apparently including substellar companions. Our results are broadly consistent with the two expected modes of binary formation (freefall fragmentation on large scales and disk fragmentation on small scales), but the distributions provide some clues as to the epochs at which the companions are likely to form.
We study feedback during massive star formation using semi-analytic methods, considering the effects of disk winds, radiation pressure, photoevaporation, and stellar winds, while following ...protostellar evolution in collapsing massive gas cores. We find that disk winds are the dominant feedback mechanism setting star formation efficiencies (SFEs) from initial cores of ∼0.3-0.5. However, radiation pressure is also significant to widen the outflow cavity causing reductions of SFE compared to the disk-wind only case, especially for > 100 M star formation at clump mass surface densities cl 0.3 g cm − 2 . Photoevaporation is of relatively minor importance due to dust attenuation of ionizing photons. Stellar winds have even smaller effects during the accretion stage. For core masses M c 10 - 1000 M and cl 0.1 - 3 g cm − 2 , we find the overall SFE to be ¯ * f = 0.31 ( R c 0.1 pc ) − 0.39 , potentially a useful sub-grid star formation model in simulations that can resolve pre-stellar core radii, R c = 0.057 ( M c 60 M ) 1 2 ( cl g cm − 2 ) − 1 2 pc . The decline of SFE with Mc is gradual with no evidence for a maximum stellar-mass set by feedback processes up to stellar masses of m * ∼ 300 M . We thus conclude that the observed truncation of the high-mass end of the IMF is shaped mostly by the pre-stellar core mass function or internal stellar processes. To form massive stars with the observed maximum masses of ∼150- 300 M , initial core masses need to be 500 - 1000 M . We also apply our feedback model to zero-metallicity primordial star formation, showing that, in the absence of dust, photoevaporation staunches accretion at ∼ 50 M . Our model implies radiative feedback is most significant at metallicities ∼ 10 − 2 Z , since both radiation pressure and photoevaporation are effective in this regime.
Formation of Double Neutron Star Systems Tauris, T. M; Kramer, M; Freire, P. C. C ...
The Astrophysical journal,
01/2017, Letnik:
846, Številka:
2
Journal Article, Paper, Publication
Recenzirano
Odprti dostop
Double neutron star (DNS) systems represent extreme physical objects and the endpoint of an exotic journey of stellar evolution and binary interactions. Large numbers of DNS systems and their mergers ...are anticipated to be discovered using the Square Kilometre Array searching for radio pulsars, and the high-frequency gravitational wave detectors (LIGO/VIRGO), respectively. Here we discuss all key properties of DNS systems, as well as selection effects, and combine the latest observational data with new theoretical progress on various physical processes with the aim of advancing our knowledge on their formation. We examine key interactions of their progenitor systems and evaluate their accretion history during the high-mass X-ray binary stage, the common envelope phase, and the subsequent Case BB mass transfer, and argue that the first-formed NSs have accreted at most . We investigate DNS masses, spins, and velocities, and in particular correlations between spin period, orbital period, and eccentricity. Numerous Monte Carlo simulations of the second supernova (SN) events are performed to extrapolate pre-SN stellar properties and probe the explosions. All known close-orbit DNS systems are consistent with ultra-stripped exploding stars. Although their resulting NS kicks are often small, we demonstrate a large spread in kick magnitudes that may, in general, depend on the past interaction history of the exploding star and thus correlate with the NS mass. We analyze and discuss NS kick directions based on our SN simulations. Finally, we discuss the terminal evolution of close-orbit DNS systems until they merge and possibly produce a short γ-ray burst.
Based on color-magnitude diagrams (CMDs) from the Hubble Space Telescope Hubble Tarantula Treasury Project (HTTP) survey, we present the star formation history (SFH) of Hodge~301, the oldest star ...cluster in the Tarantula Nebula. The HTTP photometry extends faint enough to reach, for the first time, the cluster pre-main sequence (PMS) turn-on, where the PMS joins the main sequence. Using the location of this feature, along with synthetic CMDs generated with the latest PARSEC models, we find that Hodge~301 is older than previously thought, with an age between 26.5 and 31.5 Myr. From this age, we also estimate that between 38 and 61 supernovae Type-II exploded in the region. The same age is derived from the main sequence turn-off, whereas the age derived from the post-main sequence stars is younger and between 20 and 25 Myr. Other relevant parameters are a total stellar mass of \(\approx 8800\,\pm 800\)M\(_{\odot}\) and average reddening E(B\(-\)V) \(\approx 0.22-0.24\) mag, with a differential reddening \(\delta\)E(B\(-\)V)\(\approx 0.04\) mag.
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