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Register a new userStar formation with disc accretion and rotation I. Stars between 2 and 22 Msol at solar metallicity
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Added by
Lionel Haemmerl\\'e
Publication date
2013
fields
Physics
and research's language is
English
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The way angular momentum is built up in stars during their formation process may have an impact on their further evolution. In the frame of the cold disc accretion scenario, we study for the first time how angular momentum builds up inside the star during its formation and what are the consequences for its evolution on the main sequence (MS). Computation begins from a hydrostatic core on the Hayashi line of 0.7 Msol at solar metallicity (Z=0.014) rotating as a solid body. Accretion rates depending on the luminosity of the accreting object are considered varying between 1.5e-5 and 1.7e-3 Msol/yr. The accreted matter is assumed to have an angular velocity equal to that of the outer layer of the accreting star. Models are computed for a mass-range on the zero-age main sequence (ZAMS) between 2 and 22 Msol. We study how the internal and surface velocities vary as a function of time during the accretion phase and the evolution towards the ZAMS. Stellar models, whose evolution has been followed along the pre-MS phase, are found to exhibit a shallow gradient of angular velocity on the ZAMS. Interestingly, for masses on the ZAMS larger than 8 Msol, there exists a maximum surface velocity that can be reached through the present scenario of formation. Typically, for 14 Msol models, only stars with surface velocities on the ZAMS lower than about 45% of the critical velocity can be formed. To reach higher velocities would require to start from cores rotating above the critical limit. We find that this upper velocity limit is smaller for higher masses. In contrast, below 8 Msol, there is no restriction and the whole domain of velocities, up to the critical one, can be reached.
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Observations have revealed massive (logM*/Msun>11) galaxies that were already dead when the universe was only ~2 Gyr. Given the short time before these galaxies were quenched, their past histories and quenching mechanism(s) are of particular interest. In this paper, we study star formation histories (SFHs) of 24 massive galaxies at 1.6<z<2.5. A deep slitless spectroscopy + imaging data set collected from multiple Hubble Space Telescope surveys allows robust determination of their spectral energy distributions and SFHs with no functional assumption on their forms. We find that most of our massive galaxies had formed > 50% of their extant masses by ~1.5 Gyr before the time of observed redshifts, with a trend where more massive galaxies form earlier. Their stellar-phase metallicities are already compatible with those of local early-type galaxies, with a median value of logZ*/Zsun=0.25 and scatter of ~0.15dex. In combination with the reconstructed SFHs, we reveal their rapid metallicity evolution from z~5.5 to ~2.2 at a rate of ~0.2dex/Gyr in log Z*/Zsun. Interestingly, the inferred stellar-phase metallicities are, when compared at half-mass time, ~0.25dex higher than observed gas-phase metallicities of star forming galaxies. While systematic uncertainties remain, this may imply that these quenched galaxies have continued low-level star formation, rather than abruptly terminating their star formation activity, and kept enhancing their metallicity until recently.
The formation of planets within a disc must operate within the time frame of disc dispersal, it is thus crucial to establish what is the dominant process that disperses the gaseous component of discs around young stars. Planet formation itself as well as photoevaporation by energetic radiation from the central young stellar object have been proposed as plausible dispersal mechanisms. [abridged]. In this paper we use the different metallicity dependance of X-ray photoevaporation and planet formation to discriminate between these two processes. We study the effects of metallicity, Z, on the dispersal timescale, t_phot, in the context of a photoevaporation model, by means of detailed thermal calculations of a disc in hydrostatic equilibrium irradiated by EUV and X-ray radiation from the central source. Our models show t_phot propto Z^0.52 for a pure photoevaporation model. By means of analytical estimates we derive instead a much stronger negative power dependance on metallicity of the disc lifetime for a dispersal model based on planet formation. A census of disc fractions in lower metallicity regions should therefore be able to distinguish between the two models. A recent study by Yasui et al. in low metallicity clusters of the extreme outer Galaxy ([O/H] ~- 0.7dex and dust to gas ratio of ~0.001) provides preliminary observational evidence for shorter disc lifetimes at lower metallicities, in agreement with the predictions of a pure photoevaporation model. [abridged] We finally develop an analytical framework to study the effects of metallicity dependent photoevaporation on the formation of gas giants in the core accretion scenario. We show that accounting for this effect strengthens the conclusion that planet formation is favoured at higher metallicity. [abridged]
Aims: We study the influence of rotation and disc lifetime on lithium depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of rotational mixing and of the hydrostatic effects of rotation on lithium abundances are investigated by computing non-rotating and rotating PMS models that include a comprehensive treatment of shellular rotation. The influence of the disc lifetime is then studied by comparing the lithium content of PMS rotating models experiencing different durations of the disc-locking phase between 3 and 9 Myr. Results: The surface lithium abundance at the end of the PMS is decreased when rotational effects are included. During the beginning of the lithium depletion phase, only hydrostatic effects of rotation are at work. This results in a decrease in the lithium depletion rate for rotating models compared to non-rotating ones. When the convective envelope recedes from the stellar centre, rotational mixing begins to play an important role due to differential rotation near the bottom of the convective envelope. This mixing results in a decrease in the surface lithium abundance with a limited contribution from hydrostatic effects of rotation, which favours lithium depletion during the second part of the PMS evolution. The impact of rotation on PMS lithium depletion is also found to be sensitive to the duration of the disc-locking phase. When the disc lifetime increases, the PMS lithium abundance of a solar-type star decreases owing to the higher efficiency of rotational mixing in the radiative zone. A relationship between the surface rotation and lithium abundance at the end of the PMS is then obtained: slow rotators on the zero-age main sequence are predicted to be more lithium-depleted than fast rotators due to the increase in the disc lifetime.
We study the evidence for a connection between active galactic nuclei (AGN) fueling and star formation by investigating the relationship between the X-ray luminosities of AGN and the star formation rates (SFRs) of their host galaxies. We identify a sample of 309 AGN with $10^{41}<L_mathrm{X}<10^{44} $ erg s$^{-1}$ at $0.2 < z < 1.2$ in the PRIMUS redshift survey. We find AGN in galaxies with a wide range of SFR at a given $L_X$. We do not find a significant correlation between SFR and the observed instantaneous $L_X$ for star forming AGN host galaxies. However, there is a weak but significant correlation between the mean $L_mathrm{X}$ and SFR of detected AGN in star forming galaxies, which likely reflects that $L_mathrm{X}$ varies on shorter timescales than SFR. We find no correlation between stellar mass and $L_mathrm{X}$ within the AGN population. Within both populations of star forming and quiescent galaxies, we find a similar power-law distribution in the probability of hosting an AGN as a function of specific accretion rate. Furthermore, at a given stellar mass, we find a star forming galaxy $sim2-3$ more likely than a quiescent galaxy to host an AGN of a given specific accretion rate. The probability of a galaxy hosting an AGN is constant across the main sequence of star formation. These results indicate that there is an underlying connection between star formation and the presence of AGN, but AGN are often hosted by quiescent galaxies.
We present a simplified chemical and thermal model designed to allow computationally efficient study of the thermal evolution of metal-poor gas within large numerical simulations. Our main simplification is the neglect of the molecular chemistry of the heavy elements. The only molecular chemistry retained within the model is the formation and destruction of molecular hydrogen. Despite this major simplification, the model allows for accurate treatment of the thermal evolution of the gas within a large volume of parameter space. It is valid for temperatures 50 < T < 10000 K and metallicities 0 < Z < 0.1 Z_solar. In gas with a metallicity Z = 0.1 Z_solar, and in the absence of an incident ultraviolet radiation field, it is valid for hydrogen number densities n_H < 500 / t_char cm^-3, where t_char is the size in Myr of the characteristic physical timescale of interest in the problem. If Z << 0.1 Z_solar, or if a strong ultraviolet radiation field is present, then the model remains accurate up to significantly higher densities. We also discuss some possible applications of this model.
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