In this paper, we investigate scaling relations between star formation rates and molecular gas masses for both local Galactic clouds and a sample of external galaxies. We specifically consider ...relations between the star formation rates and measurements of dense, as well as total, molecular gas masses. We argue that there is a fundamental empirical scaling relation that directly connects the local star formation process with that operating globally within galaxies. Specifically, the total star formation rate in a molecular cloud or galaxy is linearly proportional to the mass of dense gas within the cloud or galaxy. This simple relation, first documented in previous studies, holds over a span of mass covering nearly nine orders of magnitude and indicates that the rate of star formation is directly controlled by the amount of dense molecular gas that can be assembled within a star formation complex. We further show that the star formation rates and total molecular masses, characterizing both local clouds and galaxies, are correlated over similarly large scales of mass and can be described by a family of linear star formation scaling laws, parameterized by functionof sub(DG), the fraction of dense gas contained within the clouds or galaxies. That is, the underlying star formation scaling law is always linear for clouds and galaxies with the same dense gas fraction. These considerations provide a single unified framework for understanding the relation between the standard (nonlinear) extragalactic Schmidt-Kennicutt scaling law, that is typically derived from CO observations of the gas, and the linear star formation scaling law derived from HCN observations of the dense gas.
Context.
The star cluster R136 inside the Large Magellanic Cloud hosts a rich population of massive stars, including the most massive stars known. The strong stellar winds of these very luminous ...stars impact their evolution and the surrounding environment. We currently lack detailed knowledge of the wind structure that is needed to quantify this impact.
Aims.
Our goal is to observationally constrain the stellar and wind properties of the massive stars in R136, in particular the wind-structure parameters related to wind clumping.
Methods.
We simultaneously analyse optical and ultraviolet spectroscopy of 53 O-type and three WNh-stars using the F
astwind
model atmosphere code and a genetic algorithm. The models account for optically thick clumps and effects related to porosity and velocity-porosity, as well as a non-void interclump medium.
Results.
We obtain stellar parameters, surface abundances, mass-loss rates, terminal velocities, and clumping characteristics and compare them to theoretical predictions and evolutionary models. The clumping properties include the density of the interclump medium and the velocity-porosity of the wind. For the first time, these characteristics are systematically measured for a wide range of effective temperatures and luminosities.
Conclusions.
We confirm a cluster age of 1.0–2.5 Myr and derived an initial stellar mass of ≥250
M
⊙
for the most massive star in our sample, R136a1. The winds of our sample stars are highly clumped, with an average clumping factor of
f
cl
= 29 ± 15. We find tentative trends in the wind-structure parameters as a function of the mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped. We compare several theoretical predictions to the observed mass-loss rates and terminal velocities and find that none satisfactorily reproduce both quantities. The prescription of Krtička & Kubát (2018) matches the observed mass-loss rates best.
ABSTRACT Star formation on galactic scales is known to be a slow process, but whether it is slow on smaller scales is uncertain. We cross-correlate 5469 giant molecular clouds (GMCs) from a new ...all-sky catalog with 256 star-forming complexes (SFCs) to build a sample of 191 SFC-GMC complexes-collections of multiple clouds each matched to 191 SFCs. The total mass in stars harbored by these clouds is inferred from WMAP free-free fluxes. We measure the GMC mass, the virial parameter, the star formation efficiency ϵ and the star formation rate per freefall time ϵff. Both ϵ and ϵff range over 3-4 orders of magnitude. We find that 68.3% of the clouds fall within and about the median. Compared to these observed scatters, a simple model with a time-independent ϵff that depends on the host GMC properties predicts . Allowing for a time-variable ϵff, we can recover the large dispersion in the rate of star formation. This strongly suggests that star formation in the Milky Way is a dynamic process on GMC scales. We also show that the surface star formation rate profile of the Milky Way correlates well with the molecular gas surface density profile.
ABSTRACT The initial mass function (IMF), binary fraction, and distributions of binary parameters (mass ratios, separations, and eccentricities) are indispensable inputs for simulations of stellar ...populations. It is often claimed that these are poorly constrained, significantly affecting evolutionary predictions. Recently, dedicated observing campaigns have provided new constraints on the initial conditions for massive stars. Findings include a larger close binary fraction and a stronger preference for very tight systems. We investigate the impact on the predicted merger rates of neutron stars and black holes. Despite the changes with previous assumptions, we only find an increase of less than a factor of 2 (insignificant compared with evolutionary uncertainties of typically a factor of 10-100). We further show that the uncertainties in the new initial binary properties do not significantly affect (within a factor of 2) our predictions of double compact object merger rates. An exception is the uncertainty in IMF (variations by a factor of 6 up and down). No significant changes in the distributions of final component masses, mass ratios, chirp masses, and delay times are found. We conclude that the predictions are, for practical purposes, robust against uncertainties in the initial conditions concerning binary parameters, with the exception of the IMF. This eliminates an important layer of the many uncertain assumptions affecting the predictions of merger detection rates with the gravitational wave detectors aLIGO/aVirgo.
Context.
Understanding the conditions in which stars and stellar clusters form is of great importance. In particular, the role that stellar feedback may have is still hampered by large uncertainties.
...Aims.
We aim to investigate the role played by ionising radiation and protostellar outflows during the formation and evolution of a stellar cluster. To self-consistently take into account gas accretion, we start with clumps of tens of parsecs in size.
Methods.
Using an adaptive mesh refinement code, we ran magneto-hydrodynamical numerical simulations aimed at describing the collapse of massive clumps with either no stellar feedback or taking into account ionising radiation and/or protostellar jets.
Results.
Stellar feedback substantially modifies the protostellar cluster properties in several ways. We confirm that protostellar outflows reduce the star formation rate by a factor of a few, although the outflows do not stop accretion and, likely enough, do not modify the final cluster mass. On the other hand, once sufficiently massive stars have formed, ionising radiation efficiently expels the remaining gas and reduces the final cluster mass by a factor of several. We found that while HII radiation and jets barely change the distribution of high density gas, the latter increases the dense gas velocity dispersion again by a factor of several in a few places. As we are starting from a relatively large scale, we found that the clusters whose mass and size are, respectively, of the order of a few 1000
M
⊙
and a fraction of parsec, present a significant level of rotation. Moreover, we found that the sink particles that mimic the stars themselves tend to have rotation axes aligned with the cluster’s large-scale rotation. Finally, computing the classical
Q
parameter used to quantify stellar cluster structure, we infer that when jets are included in the calculation, the
Q
values are typical of observations, while when protostellar jets are not included, the
Q
values tend to be significantly lower. This is due to the presence of sub-clustering that is considerably reduced by the jets.
Conclusions.
Both large-scale gas accretion and stellar feedback, namely HII regions and protostellar jets, appear to significantly influence the formation and evolution of stellar clusters.
ABSTRACT We present a model of star formation in self-gravitating turbulent gas in which the turbulent velocity, vT, is a dynamical variable and is adiabatically heated by the collapse. The theory ...predicts the run of density, infall, and turbulent velocity and the rate of star formation in compact massive clouds. The adiabatic heating ensures that the turbulent pressure is dynamically important at all radii. The system evolves toward a coherent spatial structure with a fixed run of density, ; mass flows through this structure onto the central star or star cluster. We define the sphere of influence of the accreted matter by , where m* is the stellar plus disk mass in the nascent star cluster and Mg(r) is the gas mass inside radius r. Both vT and the infall velocity, decrease with decreasing r for ; , the size-line-width relation, with , explaining the observation that Larson's Law is altered in massive star-forming regions. The infall velocity is generally smaller than the turbulent velocity at . For , the infall and turbulent velocities are again similar, and both increase with decreasing r as , with a magnitude about half of the free-fall velocity. The accreted (stellar) mass grows superlinearly with time, , with φ a dimensionless number somewhat less than unity, the clump mass, and the free-fall time of the clump. We suggest that small values of p can be used as a tracer of convergent collapsing flows.
We present observations of CO(3-2) and 13CO(3-2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The ...0 3 resolution images reveal an unusual molecular cloud, "Cloud D1," that is coincident with the radio-infrared supernebula. The ∼6 pc diameter cloud has a linewidth, Δ v = 21.7 , that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 × 105 . The cluster appears to have a top-heavy initial mass function, with M* 1-2 . Cloud D1 is optically thin in CO(3-2), probably because the gas is hot. Molecular gas mass is very uncertain but constitutes <35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ∼1500-2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster.
We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that ...star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25-0.6 power, and that the maximum size over which star formation is physically correlated ranges from ∼200 pc to ∼1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are close to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy's shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.
We analyze K2 light curves for 794 low-mass ( 1 M * 0.1 M ) members of the 650 Myr old open cluster Praesepe and measure rotation periods (Prot) for 677 of these stars. We find that half of the ...rapidly rotating 0.3 M stars are confirmed or candidate binary systems. The remaining 0.3 M fast rotators have not been searched for companions, and are therefore not confirmed single stars. We previously found that nearly all rapidly rotating 0.3 M stars in the Hyades are binaries, but we require deeper binary searches in Praesepe to confirm whether binaries in these two co-eval clusters have different Prot distributions. We also compare the observed Prot distribution in Praesepe to that predicted by models of angular-momentum evolution. We do not observe the clear bimodal Prot distribution predicted by Brown for >0.5 M stars at the age of Praesepe, but 0.25−0.5 M stars do show stronger bimodality. In addition, we find that >60% of early M dwarfs in Praesepe rotate more slowly than predicted at 650 Myr by Matt et al., which suggests an increase in braking efficiency for these stars relative to solar-type stars and fully convective stars. The incompleteness of surveys for binaries in open clusters likely impacts our comparison with these models since the models only attempt to describe the evolution of isolated single stars.
Combining the precise parallaxes and optical photometry delivered by Gaia’s second data release with the photometric catalogues of Pan-STARRS1, 2MASS, and AllWISE, we derived Bayesian stellar ...parameters, distances, and extinctions for 265 million of the 285 million objects brighter than G = 18. Because of the wide wavelength range used, our results substantially improve the accuracy and precision of previous extinction and effective temperature estimates. After cleaning our results for both unreliable input and output data, we retain 137 million stars, for which we achieve a median precision of 5% in distance, 0.20 mag in V-band extinction, and 245 K in effective temperature for G ≤ 14, degrading towards fainter magnitudes (12%, 0.20 mag, and 245 K at G = 16; 16%, 0.23 mag, and 260 K at G = 17, respectively). We find a very good agreement with the asteroseismic surface gravities and distances of 7000 stars in the Kepler, K2-C3, and K2-C6 fields, with stellar parameters from the APOGEE survey, and with distances to star clusters. Our results are available through the ADQL query interface of the Gaia mirror at the Leibniz-Institut für Astrophysik Potsdam (gaia.aip.de) and as binary tables at data.aip.de. As a first application, we provide distance- and extinction-corrected colour-magnitude diagrams, extinction maps as a function of distance, and extensive density maps. These demonstrate the potential of our value-added dataset for mapping the three-dimensional structure of our Galaxy. In particular, we see a clear manifestation of the Galactic bar in the stellar density distributions, an observation that can almost be considered direct imaging of the Galactic bar.
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