Recently it has been found that models of massive stars reach the Eddington limit in their interior, which leads to dilute extended envelopes. We perform a comparative study of the envelope ...properties of massive stars at different metallicities, with the aim to establish the impact of the stellar metallicity on the effect of envelope inflation. We analyse published grids of core-hydrogen burning massive star models computed with metallicities appropriate for massive stars in the Milky Way, the LMC and the SMC, the very metal poor dwarf galaxy I Zwicky 18, and for metal-free chemical composition. Stellar models of all the investigated metallicities reach and exceed the Eddington limit in their interior, aided by the opacity peaks of iron, helium and hydrogen, and consequently develop inflated envelopes. Envelope inflation leads to a redward bending of the zero-age main sequence and a broadening of the main sequence band in the upper part of the Hertzsprung-Russell diagram. We derive the limiting L/M-values as function of the stellar surface temperature above which inflation occurs, and find them to be larger for lower metallicity. While Galactic models show inflation above ~29 Msun, the corresponding mass limit for Population III stars is ~150 Msun. While the masses of the inflated envelopes are generally small, we find that they can reach 1-100 Msun in models with effective temperatures below ~8000 K, with higher masses reached by models of lower metallicity. Envelope inflation is expected to occur in sufficiently massive stars at all metallicities, and is expected to lead to rapidly growing pulsations, high macroturbulent velocities, and might well be related to the unexplained variability observed in Luminous Blue Variables like S Doradus and Eta Carina.
Dusty star-forming galaxies at high redshift (1 < z < 3) represent the most intense star-forming regions in the Universe. Key aspects to these processes are the gas heating and cooling mechanisms. ...Although it is well known that these galaxies are gas-rich, little is known about the gas excitation conditions. Here we examine these processes in a sample of 24 strongly lensed star-forming galaxies identified by the \textit{Planck} satellite (LPs) at z ~ 1.1 - 3.5. We analyze 162 CO rotational transitions (ranging from Jupper = 1 - 12) and 37 atomic carbon fine-structure lines (CI) in order to characterize the physical conditions of the gas in sample of LPs. We simultaneously fit the CO and CI lines, and the dust continuum emission, using two different non-LTE, radiative transfer models. The first model represents a two component gas density, while the second assumes a turbulence driven log-normal gas density distribution. These LPs are among the most gas-rich, infrared (IR) luminous galaxies ever observed (\(\mu_{\rm L}\)L\(_{\rm IR(8-1000\mu m) } \sim 10^{13-14.6} \)\Lsun; \(< \mu_{\rm L}\)M\(_{\rm ISM}> = 2.7 \pm 1.2 \times 10^{12}\) \Msun, with \(\mu_{\rm L} \sim 10-30\) the average lens magnification factor). Our results suggest that the turbulent ISM present in the LPs can be well-characterized by a high turbulent velocity dispersion (\(<\Delta V_{\rm turb}> \sim 100 \) \kms) and gas kinetic temperature to dust temperature ratios \(<T_{\rm kin}\)/\(T_{\rm d}> \sim 2.5\), sustained on scales larger than a few kpc. We speculate that the average surface density of the molecular gas mass and IR luminosity \(\Sigma_{\rm M_{\rm ISM}}\) \(\sim 10^{3 - 4}\) \Msun pc\(^{-2}\) and \(\Sigma_{\rm L_{\rm IR}}\) \(\sim 10^{11 - 12}\) \Lsun kpc\(^{-2}\), arise from both stellar mechanical feedback and a steady momentum injection from the accretion of intergalactic gas.
We report the detection of the far-infrared (FIR) fine-structure line of
singly ionised nitrogen, \Nplusa, within the peak epoch of galaxy assembly,
from a strongly lensed galaxy, hereafter ``The Red ...Radio Ring''; the RRR, at z
= 2.55. We combine new observations of the ground-state and mid-J transitions
of CO (J$_{\rm up} =$ 1,5,8), and the FIR spectral energy distribution (SED),
to explore the multi-phase interstellar medium (ISM) properties of the RRR. All
line profiles suggest that the HII regions, traced by \Nplusa, and the (diffuse
and dense) molecular gas, traced by the CO, are co-spatial when averaged over
kpc-sized regions. Using its mid-IR-to-millimetre (mm) SED, we derive a
non-negligible dust attenuation of the \Nplusa line emission. Assuming a
uniform dust screen approximation results a mean molecular gas column density
$> 10^{24}$\, cm$^{-2}$, with a molecular gas-to-dust mass ratio of 100. It is
clear that dust attenuation corrections should be accounted for when studying
FIR fine-structure lines in such systems. The attenuation corrected ratio of
$L_{\rm NII205} / L_{\rm IR(8-1000\mu m)} = 2.7 \times 10^{-4}$ is consistent
with the dispersion of local and $z >$ 4 SFGs. We find that the lower-limit,
\Nplusa -based star-formation rate (SFR) is less than the IR-derived SFR by a
factor of four. Finally, the dust SED, CO line SED and $L_{\rm NII205}$
line-to-IR luminosity ratio of the RRR is consistent with a starburst-powered
ISM.
We aim to provide new empirical clues about macroturbulent spectral line broadening in O- and B-type stars to evaluate its physical origin. We use high-resolution spectra of ~430 stars with spectral ...types in the range O4-B9 (all luminosity classes). We characterize the line-broadening of adequate diagnostic metal lines using a combined FT and GOF technique. We perform a quantitative spectroscopic analysis of the whole sample using automatic tools coupled with a huge grid of FASTWIND models. We also incorporate quantitative information about line asymmetries to our observational description of the characteristics of the line-profiles, and present a comparison of the shape and type of line-profile variability found in a small sample of O stars and B supergiants with still undefined pulsational properties and B main sequence stars with variable line-profiles. We present a homogeneous and statistically significant overview of the (single snapshot) line-broadening properties of stars in the whole O and B star domain. We find empirical evidence of the existence of various types of non-rotational broadening agents acting in the realm of massive stars. Even though all of them could be quoted and quantified as a macroturbulent broadening from a practical point of view, their physical origin can be different. Contrarily to the early- to late-B dwarfs/giants, which present a mixture of cases in terms of line-profile shape and variability, the whole O-type and B supergiant domain (or, roughly speaking, stars with M_ZAMS > 15 M_sol) is fully dominated by stars with a remarkable non-rotational broadening component and very similar profiles (including type of variability). We provide some examples illustrating how this observational dataset can be used to evaluate scenarios aimed at explaining the existence of sources of non-rotational broadening in massive stars.
The recent gravitational wave measurements have demonstrated the existence of stellar mass black hole binaries. It is essential for our understanding of massive star evolution to identify the ...contribution of binary evolution to the formation of double black holes. A promising way to progress is investigating the progenitors of double black hole systems and comparing predictions with local massive star samples such as the population in 30 Doradus in the Large Magellanic Cloud (LMC). Methods. To this purpose, we analyse a large grid of detailed binary evolution models at LMC metallicity with initial primary masses between 10 and 40 Msun, and identify which model systems potentially evolve into a binary consisting of a black hole and a massive main sequence star. We then derive the observable properties of such systems, as well as peculiarities of the OB star component. We find that about 3% of the LMC late O and early B stars in binaries are expected to possess a black hole companion, when assuming stars with a final helium core mass above 6.6 M to form black holes. While the vast majority of them may be X-ray quiet, our models suggest that these may be identified in spectroscopic binaries, either by large amplitude radial velocity variations ( > 50 km s ) and simultaneous nitrogen surface enrichment, or through a moderate radial velocity ( > 10 km/s ) and simultaneously rapid rotation of the OB star. The predicted mass ratios are such that main sequence companions could be excluded in most cases. A comparison to the observed OB+WR binaries in the LMC, Be/X-ray binaries, and known massive BH binaries supports our conclusion. We expect spectroscopic observations to be able to test key assumptions in our models, with important implications for massive star evolution in general, and for the formation of double-black hole mergers in particular.
We exploit the recent discovery of pulsations in mixed-atmosphere (He/H), extremely low-mass white dwarf precursors (ELM proto-WDs) to test the proposition that rotational mixing is a fundamental ...process in the formation and evolution of low-mass helium core white dwarfs. Rotational mixing has been shown to be a mechanism able to compete efficiently against gravitational settling, thus accounting naturally for the presence of He, as well as traces of metals such as Mg and Ca, typically found in the atmospheres of ELM proto-WDs. Here we investigate whether rotational mixing can maintain a sufficient amount of He in the deeper driving region of the star, such that it can fuel, through HeII-HeIII ionization, the observed pulsations in this type of stars. Using state-of-the-art evolutionary models computed with MESA, we show that rotational mixing can indeed explain qualitatively the very existence and general properties of the known pulsating, mixed-atmosphere ELM proto-WDs. Moreover, such objects are very likely to pulsate again during their final WD cooling phase.
We report the detection of the far-infrared (FIR) fine-structure line of singly ionised nitrogen, \Nplusa, within the peak epoch of galaxy assembly, from a strongly lensed galaxy, hereafter ``The Red ...Radio Ring''; the RRR, at z = 2.55. We combine new observations of the ground-state and mid-J transitions of CO (J\(_{\rm up} =\) 1,5,8), and the FIR spectral energy distribution (SED), to explore the multi-phase interstellar medium (ISM) properties of the RRR. All line profiles suggest that the HII regions, traced by \Nplusa, and the (diffuse and dense) molecular gas, traced by the CO, are co-spatial when averaged over kpc-sized regions. Using its mid-IR-to-millimetre (mm) SED, we derive a non-negligible dust attenuation of the \Nplusa line emission. Assuming a uniform dust screen approximation results a mean molecular gas column density \(> 10^{24}\)\, cm\(^{-2}\), with a molecular gas-to-dust mass ratio of 100. It is clear that dust attenuation corrections should be accounted for when studying FIR fine-structure lines in such systems. The attenuation corrected ratio of \(L_{\rm NII205} / L_{\rm IR(8-1000\mu m)} = 2.7 \times 10^{-4}\) is consistent with the dispersion of local and \(z >\) 4 SFGs. We find that the lower-limit, \Nplusa -based star-formation rate (SFR) is less than the IR-derived SFR by a factor of four. Finally, the dust SED, CO line SED and \(L_{\rm NII205}\) line-to-IR luminosity ratio of the RRR is consistent with a starburst-powered ISM.
