Massive star formation: Nurture, not nature

We investigate the physical processes which lead to the formation of massive stars. Using a numerical simulation of the formation of a stellar cluster from a turbulent molecular cloud, we evaluate the relevant contributions of fragmentation and competitive accretion in determining the masses of the more massive stars. We find no correlation between the final mass of a massive star, and the mass of the clump from which it forms. Instead, we find that the bulk of the mass of massive stars comes from subsequent competitive accretion in a clustered environment. In fact, the majority of this mass infalls onto a pre-existing stellar cluster. Furthermore, the mass of the most massive star in a system increases as the system grows in numbers of stars and in total mass. This arises as the infalling gas is accompanied by newly formed stars, resulting in a larger cluster around a more massive star. High-mass stars gain mass as they gain companions, implying a direct causal relationship between the cluster formation process, and the formation of higher-mass stars therein.Comment: 8 pages, accepted for publication in MNRAS. Version including hi-res colour postscript figure available at http://star-www.st-and.ac.uk/~sgv/ps/massnurt.ps.g

Age spreads and the temperature dependence of age estimates in Upper Sco

Past estimates for the age of the Upper Sco Association are typically 11-13 Myr for intermediate-mass stars and 4-5 Myr for low-mass stars. In this study, we simulate populations of young stars to investigate whether this apparent dependence of estimated age on spectral type may be explained by the star formation history of the association. Solar and intermediate mass stars begin their pre-main sequence evolution on the Hayashi track, with fully convective interiors and cool photospheres. Intermediate mass stars quickly heat up and transition onto the radiative Henyey track. As a consequence, for clusters in which star formation occurs on a similar timescale as the transition from a convective to a radiative interior, discrepancies in ages will arise when ages are calculated as a function of temperature instead of mass. Simple simulations of a cluster with constant star formation over several Myr may explain about half of the difference in inferred ages versus photospheric temperature; speculative constructions that consist of a constant star formation followed by a large supernova-driven burst could fully explain the differences, including those between F and G stars where evolutionary tracks may be more accurate. The age spreads of low-mass stars predicted from these prescriptions for star formation are consistent with the observed luminosity spread of Upper Sco. The conclusion that a lengthy star formation history will yield a temperature dependence in ages is expected from the basic physics of pre-main sequence evolution and is qualitatively robust to the large uncertainties in pre-main sequence evolutionary models.Comment: 13 pages, accepted by Ap

Young open clusters in the Milky Way and Small Magellanic Cloud

NGC6611, Trumpler 14, Trumpler 15, Trumpler 16, Collinder 232 are very young open clusters located in star-formation regions of the Eagle Nebula or the Carina in the MW, and NGC346 in the SMC. With different instrumentations and techniques, it was possible to detect and classify new Herbig Ae/Be stars, classical Be stars and to provide new tests / comparisons about the Be stars appearance models. Special stars (He-strong) of these star-formation regions are also presented.Comment: Proceedings IAUS266 at the IAU-GA 200

Revisiting the pre-main-sequence evolution of stars II. Consequences of planet formation on stellar surface composition

We want to investigate how planet formation is imprinted on stellar surface composition using up-to-date stellar evolution models. We simulate the evolution of pre-main-sequence stars as a function of the efficiency of heat injection during accretion, the deuterium mass fraction, and the stellar mass. For simplicity, we assume that planet formation leads to the late accretion of zero-metallicity gas, diluting the surface stellar composition as a function of the mass of the stellar outer convective zone. We adopt $150\,{\mathrm{M}_\oplus}(M_\star/\mathrm{M}_\odot)(Z/\mathrm{Z}_\odot)$ as an uncertain but plausible estimate of the mass of heavy elements that is not accreted by stars with giant planets, including our Sun. By combining our stellar evolution models to these estimates, we evaluate the consequences of planet formation on stellar surface composition. We show that after the first $\sim0.1$ Myr, the evolution of the convective zone follows classical evolutionary tracks within a factor of two in age. We find that planet formation should lead to a scatter in stellar surface composition that is larger for high-mass stars than for low-mass stars. We predict a spread in [Fe/H] of approximately $0.02$ dex for stars with $T_\mathrm{eff}\sim 5500\,$K, marginally compatible with differences in metallicities observed in some binary stars with planets. Stars with $T_\mathrm{eff}\geq 7000\,$K may show much larger [Fe/H] deficits, by 0.6 dex or more, compatible with the existence of refractory-poor $\lambda$ Boo stars. We also find that planet formation may explain the lack of refractory elements seen in the Sun as compared to solar twins, but only if the ice-to-rock ratio in the solar-system planets is less than $\approx0.4$ and planet formation began less than $\approx1.3$ Myr after the beginning of the formation of the Sun. (abbreviated)Comment: Accepted for publicatoin in A&A. 18 pages, 14 figure

The turbulent formation of stars

How stars are born from clouds of gas is a rich physics problem whose solution will inform our understanding of not just stars but also planets, galaxies, and the universe itself. Star formation is stupendously inefficient. Take the Milky Way. Our galaxy contains about a billion solar masses of fresh gas available to form stars-and yet it produces only one solar mass of new stars a year. Accounting for that inefficiency is one of the biggest challenges of modern astrophysics. Why should we care about star formation? Because the process powers the evolution of galaxies and sets the initial conditions for planet formation and thus, ultimately, for life.Comment: published in Physics Today, cover story, see http://www.mso.anu.edu.au/~chfeder/pubs/physics_today/physics_today.htm

RCW36: characterizing the outcome of massive star formation

Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds and outflows. The formation process of massive stars is poorly constrained because of their scarcity, the short formation timescale and obscuration. By obtaining a census of the newly formed stellar population, the star formation history of the young cluster and the role of the massive stars within it can be unraveled. We aim to reconstruct the formation history of the young stellar population of the massive star-forming region RCW 36. We study several dozens of individual objects, both photometrically and spectroscopically, look for signs of multiple generations of young stars and investigate the role of the massive stars in this process. We obtain a census of the physical parameters and evolutionary status of the young stellar population. Using a combination of near-infrared photometry and spectroscopy we estimate ages and masses of individual objects. We identify the population of embedded young stellar objects (YSO) by their infrared colors and emission line spectra. RCW 36 harbors a stellar population of massive and intermediate-mass stars located around the center of the cluster. Class 0/I and II sources are found throughout the cluster. The central population has a median age of 1.1 +/- 0.6 Myr. Of the stars which could be classified, the most massive ones are situated in the center of the cluster. The central cluster is surrounded by filamentary cloud structures; within these, some embedded and accreting YSOs are found. Our age determination is consistent with the filamentary structures having been shaped by the ionizing radiation and stellar winds of the central massive stars. The formation of a new generation of stars is ongoing, as demonstrated by the presence of embedded protostellar clumps, and two exposed jets.Comment: 18 pages, 10 figures, accepted for publication in Astronomy & Astrophysic