On the origin of the distribution of binary-star periods

Pre-main sequence and main-sequence binary systems are observed to have periods, P, ranging from one day to 10^(10) days and eccentricities, e, ranging from 0 to 1. We pose the problem if stellar-dynamical interactions in very young and compact star clusters may broaden an initially narrow period distribution to the observed width. N-body computations of extremely compact clusters containing 100 and 1000 stars initially in equilibrium and in cold collapse are preformed. In all cases the assumed initial period distribution is uniform in the narrow range 4.5 < log10(P) < 5.5 (P in days) which straddles the maximum in the observed period distribution of late-type Galactic-field dwarf systems. None of the models lead to the necessary broadening of the period distribution, despite our adopted extreme conditions that favour binary--binary interactions. Stellar-dynamical interactions in embedded clusters thus cannot, under any circumstances, widen the period distribution sufficiently. The wide range of orbital periods of very young and old binary systems is therefore a result of cloud fragmentation and immediate subsequent magneto-hydrodynamical processes operating within the multiple proto-stellar system.Comment: 11 pages, 4 figures, ApJ, in pres

The Theoretical Mass--Magnitude Relation of Low-Mass Stars and its Metallicity Dependence

We investigate the dependence of theoretically generated mass - (absolute magnitude) relations on stellar models. Using up to date physics we compute models in the mass range 0.1 < m < 1M_sun. We compare the solar-metallicity models with our older models, with recent models computed by others, and also with an empirical mass - (absolute magnitude) relation that best fits the observed data. At a given mass below 0.6M_sun the effective temperatures differ substantially from model to model. However taken individually each set of models is in good agreement with observations in the mass - luminosity plane. A minimum in the derivative dm/dM_V at M_V = 11.5, which is due to H_2 formation and establishment of a fully convective stellar interior, is present in all photometric bands, for all models. This minimum leads to a maximum in the stellar luminosity function for Galactic disk stars at M_V = 11.5, M_bol = 9.8. Stellar models should locate this maximum in the stellar luminosity function at the same magnitude as observations. Models which incorporate the most realistic theoretical atmospheres and the most recent equation of state and opacities can satisfy this constraint. These models are also in best agreement with the most recent luminosity - (effective temperature) and mass-luminosity data. Each set of our models of a given metallicity (with 0.2 > [Fe/H] > -2.3) shows a maximum in -dm/dM_bol, which moves to brighter bolometric magnitudes with decreasing metallicity. The change in location of the maximum, as a function of [Fe/H], follows the location of structure in luminosity functions for stellar populations with different metal abundances. This structure seen in all observed stellar populations can be accounted for by the mass--luminosity relation.Comment: MNRAS (in press), 15 pages, 1 appendix, plain TeX, 9 postscript figure

Escaping stars from young low-N clusters

With the use of N-body calculations the amount and properties of escaping stars from low-N (N = 100 and 1000) young embedded star clusters prior to gas expulsion are studied over the first 5 Myr of their existence. Besides the number of stars also different initial radii and binary populations are examined as well as virialised and collapsing clusters. It is found that these clusters can loose substantial amounts (up to 20%) of stars within 5 Myr with considerable velocities up to more than 100 km/s. Even with their mean velocities between 2 and 8 km/s these stars will still be travelling between 2 and 30 pc during the 5 Myr. Therefore can large amounts of distributed stars in star-forming regions not necessarily be counted as evidence for the isolated formation of stars.Comment: 10 pages, 10 figures, accepted for publication by MNRA

A discontinuity in the low-mass initial mass function

The origin of brown dwarfs (BDs) is still an unsolved mystery. While the standard model describes the formation of BDs and stars in a similar way recent data on the multiplicity properties of stars and BDs show them to have different binary distribution functions. Here we show that proper treatment of these uncovers a discontinuity of the multiplicity-corrected mass distribution in the very-low-mass star (VLMS) and BD mass regime. A continuous IMF can be discarded with extremely high confidence. This suggests that VLMSs and BDs on the one hand, and stars on the other, are two correlated but disjoint populations with different dynamical histories. The analysis presented here suggests that about one BD forms per five stars and that the BD-star binary fraction is about 2%-3% among stellar systems.Comment: 14 pages, 11 figures, uses emulateapj.cls. Minor corrections and 1 reference added after being accepted by the Ap

Core dissolution and the dynamics of massive stars in young stellar clusters

We investigate the dynamical effects of rapid gas expulsion from the core of a young stellar cluster. The aims of this study are to determine 1) whether a mass-segregated core survives the gas expulsion and 2) the probable location of any massive stars that have escaped from the core. Feedback from massive stars is expected to remove the gas from the core of the cluster first, as that is where most massive stars are located. We find that gas expulsion has little effect on the core for a core star formation efficiency, of greater than 50%. For lower values of star formation efficiency down to 20%, a reduced core survives containing the majority of the massive stars while some of them are dispersed into the rest of the cluster. In fact we find that ejected stars migrate from radial to tangential orbits due to stellar encounters once they leave the core. Thus, the location of massive stars outside of the core does not exclude their forming in the dense cluster core. Few massive stars are expected to remain in the core for a star formation efficiency lower than 20%.Comment: 8 pages, 7 figures, accepted for publication in MNRA

The Formation of Star Clusters II: 3D Simulations of Magnetohydrodynamic Turbulence in Molecular Clouds

(Abridged) We present a series of decaying turbulence simulations that represent a cluster-forming clump within a molecular cloud, investigating the role of magnetic fields on the formation of potential star-forming cores. We present an exhaustive analysis of numerical data from these simulations that includes a compilation of all of the distributions of physical properties that characterize bound cores - including their masses, radii, mean densities, angular momenta, spins, magnetizations, and mass-to-flux ratios. We also present line maps of our models that can be compared with observations. Our simulations range between 5-30 Jeans masses of gas, and are representative of molecular cloud clumps with masses between 100-1000 solar masses. The cores have mass-to-flux ratios that are generally less than that of the original cloud, and so a cloud that is initially highly supercritical can produce cores that are slightly supercritical, similar to that seen by Zeeman measurements of molecular cloud cores. Clouds that are initially only slightly supercritical will instead collapse along the field lines into sheets, and the cores that form as these sheets fragment have a different mass spectrum than what is observed. The spin rates of these cores suggests that subsequent fragmentation into multiple systems is likely. The sizes of the bound cores that are produced are typically 0.02-0.2 pc and have densities in the range 10^4-10^5 cm^{-3} in agreement with observational surveys. Finally, our numerical data allow us to test theoretical models of the mass spectrum of cores, such as the turbulent fragmentation picture of Padoan-Nordlund. We find that while this model gets the shape of the core mass spectrum reasonably well, it fails to predict the peak mass in the core mass spectrum.Comment: Accepted by MNRAS. 28 pages, 16 figures. Substantial revision since last versio

On the Evolutionary History of Stars and their Fossil Mass and Light

The total extragalactic background radiation can be an important test of the global star formation history (SFH). Using direct observational estimates of the SFH, along with standard assumptions about the initial mass function (IMF), we calculate the total extragalactic background radiation and the observed stellar density today. We show that plausible SFHs allow a significant range in each quantity, but that their ratio is very tightly constrained. Current estimates of the stellar mass and extragalactic background are difficult to reconcile, as long as the IMF is fixed to the Salpeter slope above 1 Msun. The joint confidence interval of these two quantities only agrees with that determined from the allowed range of SFH fits at the 3-sigma level, and for our best-fit values the discrepancy is about a factor of two. Alternative energy sources that contribute to the background, such as active galactic nuclei (AGN), Population III stars, or decaying particles, appear unlikely to resolve the discrepancy. However, changes to the IMF allow plausible solutions to the background problem. The simplest is an average IMF with an increased contribution from stars around 1.5--4 Msun. A ``paunchy'' IMF of this sort could emerge as a global average if low mass star formation is suppressed in galaxies experiencing rapid starbursts. Such an IMF is consistent with observations of star-forming regions, and would help to reconcile the fossil record of star formation with the directly observed SFH.Comment: 21 pages, 7 figures, 3 tables; submitted to Monthly Notice