Initial Mass Function Of Stars Research Articles

Metal Absorption Lines as Probes of the Intergalactic Medium Prior to the Reionization Epoch

Winds from star-forming galaxies provide the most promising explanation for the enrichment of the intergalactic medium with heavy elements. Theoretical and observational arguments indicate that the pollution may have occurred at z 6; however, direct observational tests of such a scenario are needed. We model starburst winds in the high-redshift universe and find that the fraction of space filled by enriched material varies strongly with the assumed star formation efficiency f* and the fraction of supernova energy powering each wind fesc. We show that metals carried by these winds can be seen in absorption against bright background sources, such as quasars or gamma-ray bursts, in narrow lines with characteristic equivalent widths ~0.5 A W 5 A. We argue that a substantial fraction of the metals in high-redshift winds are likely to reside in low-ionization states (C II, O I, Si II, and Fe II), but higher ionization states (C IV and Si IV) could also provide useful probes of the winds. The number of such lines can constrain both f* and fesc. Statistics of metal absorption lines can also be used to identify whether H2 is an efficient coolant in the early universe and to study the initial mass function of stars at high redshifts.

The initial mass function of stars: evidence for uniformity in variable systems.

The distribution of stellar masses that form in one star formation event in a given volume of space is called the initial mass function (IMF). The IMF has been estimated from low-mass brown dwarfs to very massive stars. Combining IMF estimates for different populations in which the stars can be observed individually unveils an extraordinary uniformity of the IMF. This general insight appears to hold for populations including present-day star formation in small molecular clouds, rich and dense massive star-clusters forming in giant clouds, through to ancient and metal-poor exotic stellar populations that may be dominated by dark matter. This apparent universality of the IMF is a challenge for star formation theory, because elementary considerations suggest that the IMF ought to systematically vary with star-forming conditions.

Great balls of fire. Star formation: a Web supplement.

Eons before Jerry Lee Lewis recorded his ground-shaking 1957 tune “Great Balls of Fire,” human beings were pondering the stars. Lewis's single (released, aptly enough, by Sun Records) revolutionized popular music and set three generations jumping and gyrating. Astrophysicists and astronomers have had their share of hits, too. Yet they are still learning the dance by which the real balls of fire in the night sky came into being. Did luminous matter in the universe start with a single star or with a cluster of stars? Did later stars come into being through variants on a common theme (such as the collapse of molecular clouds) or in a seething mosh pit of disparate forces and mechanisms? No one knows. Still, progress is being made. In the past few years, advances in computing power and numerical techniques, as well as millions of stellar observations have improved our understanding of the mechanisms needed to initiate star formation, the physical and chemical environment in which stars form, and the evolution and clustering of stars in space and time over the history of the universe. This special section synthesizes and reviews what scientists have recently learned about star formation and what this knowledge can teach us about our solar system, other planetary systems, galaxies, clusters of galaxies, and large-scale luminous structure in the universe. In perhaps the simplest scenario, a star of a few solar masses forms in isolation from other stars. Ward-Thompson (p. [76][1]) reviews the first two stages of such stellar development, from the formation of a gravitationally bound core in a cloud dominated by molecular hydrogen to the collapse of this core under self-gravity. The trickiest part is getting the core to collapse to form a protostar despite interference from turbulence and magnetic fields. Recent simulations by Abel et al. (p. 93 and the accompanying Perspective by Rees, p. [51][2]) and others suggest that a much more massive single star may have been the first luminous object in the universe to form by the collapse of a molecular cloud. Pudritz (p. 68) reviews the formation of clusters of stars. Many experts think most stars form in clusters rather than singly, so that effects such as stellar winds and shock waves from nearby supernovae need to be considered in star-formation scenarios. Pudritz argues that the first stars formed in such turbulent cluster environments. The argument for simplicity gained force recently when astronomers synthesized data on the initial mass function; the spatial distribution of stars of a specific mass range in a given volume of space. Kroupa (p. [82][3]) reviews the initial mass function for stars in a wide range of masses. He shows that all of them, regardless of their age or environment, have similar initial mass functions. This unexpected uniformity suggests that all stars form by some common mechanism. To round out the section, News writer Robert Irion describes how two very different residents of the cosmic menagerie may shed light on broader processes of star formation. Brown dwarfs and other very-low-mass dim objects of uncertain origin are showing unexpected similarities to their more conspicuous stellar cousins. At the other end of the size and age scales lurk the ancient Population III stars; huge masses of pristine hydrogen and helium whose sheer simplicity may help astrophysicists unscramble their secrets. There's more. An online supplement to this issue ([www.sciencemag.org/feature/data/starformation/index.shtml][4]) includes links to previous Science reviews, articles, and special issues related to star formation, as well as to a wide variety of other Web resources. Star-formation researchers may still be waiting for the great hit single that will set their field ablaze, but Science readers don't have to wait. Ladies and gentlemen, let's rock and roll. [1]: /lookup/doi/10.1126/science.1067354 [2]: /lookup/doi/10.1126/science.1065713 [3]: /lookup/doi/10.1126/science.1067524 [4]: http://www.sciencemag.org/feature/data/starformation/index.shtml

A Large Spectroscopic Survey for Young Low-Mass Members of the Upper Scorpius OB Association

We present the results of a large intermediate-resolution spectroscopic survey for pre–main-sequence (PMS) stars in the Upper Scorpius OB association, the nearest region of recent massive star formation. Utilizing the 2dF multiobject spectrograph at the Anglo-Australian Telescope, we obtained spectra of 576 stars with magnitudes 12.5 ≤ R ≤ 16.5 in a 6 deg2 area in Upper Scorpius. Among these objects we were able to identify 98 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. We place the new PMS stars into the H-R diagram and find that their ages agree well with the mean age of 5 Myr determined in our earlier investigation based on a smaller sample of stars. The number of low-mass (0.1 M⊙ M* 0.5 M⊙) members is at least as high as expected from the known number of high-mass (M* 3 M⊙) association members and from recent determinations of the field star initial mass function. Thus, there is no deficit of low-mass stars in Upper Scorpius.

The Pre–Main-Sequence Stars and Initial Mass Function of NGC 2264

UBVRI and Hα CCD photometry has been performed in the southern region of NGC 2264 around the Cone Nebula. A nearly complete list of pre–main-sequence (PMS) members in the cluster has been made from the compilation of Hα emission stars identified in this study plus those selected in previous investigations, together with stars with X-ray emission. Using the H-R diagram, we tested a set of four PMS evolution models based on the PMS age and age spread estimate for the cluster and the mass-age relationship among individual stars. The resultant initial mass function (IMF) is in good agreement with the field star IMF of the solar neighborhood, with an IMF slope Γ = -1.7 in the mass range 0.3 ≤ log m ≤ 0.8.

Star Formation History of the Disk of the Large Magellanic Cloud

New optical observations of field stars in the Large Magellanic Cloud from the ground and with the Hubble Space Telescope provide the basis for investigations of the history of field star formation in our nearest galactic neighbor. Results from WFPC2 photometry of field stars indicate that the stellar initial mass function for stars around 1 M⊙ is similar to that in the solar neighborhood. Long-term variations occurred in the average SFR with enhanced activity in the past 2–4 Gyr, a fallow period about 2–10 Gyr in the past, and an initial phase of astration ~12 Gyr ago. The recent increase in field star formation is less than that for the birth of compact star clusters; the cluster formation rate does not give a complete evolutionary picture.

Recent Star Formation in Shapley Constellation III in the Large Magellanic Cloud

We present UBV photometry of four fields within Shapley Constellation III and one field on the edge of the shell. Our fields cover roughly 20% of the region, mostly in the southern half. Determinations are made of ages of the fields, the star formation densities, and the initial mass function (IMF) slopes. The field-age determinations inside the constellation show ages between 12 and 16 Myr uncorrelated with distance from the center, while the age of the field on the edge of the constellation shows an age of around 6–7 Myr. The southern part of the constellation shows star formation densities and IMF slopes typical of OB associations and giant H II regions, while the northern part shows significantly fewer intermediate-mass stars and a steeper IMF slope. We compare these properties of Constellation III with those of 30 Doradus, another LMC star-forming region of comparable size to Constellation III. Although the regions formed from roughly the same amount of gas, we estimate that 30 Doradus formed a few times more stars than Constellation III.

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

We present wide-field far-ultraviolet (FUV; 1300–1800 A) images of the Large and Small Magellanic Clouds (LMC, SMC). These data were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 (1990 December 1–10) and Astro-2 (1995 March 2–18) missions; the images provide an extensive FUV mosaic of the SMC and contain numerous regions in the LMC, covering a wide range of stellar densities and current star formation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OB associations are completely or partially included in the observed fields. FUV data can identify the hottest OB stars more easily than can optical photometry, and these stars dominate the ionizing flux, which is correlated to the observed Hα flux of the associated H II regions. Of the H II regions in the catalog of Davies, Elliott, & Meaburn (DEM), the UIT fields completely or partially include 102 DEM regions in the LMC and 74 DEM regions in the SMC. We present a catalog of FUV magnitudes derived from point-spread function photometry for 37,333 stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMC were presented recently by Cornett et al.), with a completeness limit of mUV ≈ 15 mag and a detection limit of mUV ≈ 17.5. The average uncertainty in the photometry is ~0.1 mag. The full catalog with astrometric positions, photometry, and other information is also available from publicly accessible astronomical data archives. We divide the catalog into field stars and stars that are in DEM regions. We analyze each of these two sets of stars independently, comparing the composite UV luminosity function of our data with UV magnitudes derived from stellar evolution and atmosphere models in order to derive the underlying stellar formation parameters. We find a most probable initial mass function (IMF) slope for the LMC field stars of Γ = -1.80 ± 0.09. The statistical significance of this single slope for the LMC field stars is extremely high, though we also find some evidence for a field star IMF slope of Γ ~ -1.4, roughly equal to the Salpeter slope. However, in the case of the stars in the DEM regions (the stars in all the regions were analyzed together as a single group), we find three IMF slopes of roughly equal likelihood: Γ = -1.0, -1.6, and -2.0. No typical age for the field stars is found in our data for time periods up to a continuous star formation age of 500 Myr, which is the maximum age consistent with the completeness limit magnitude of the catalog's luminosity function. The best age for the collection of cluster stars was found to be t0 = 3.4 ± 1.9 Myr; this is consistent with the age expected for a collection of OB stars from many different clusters.

Star Formation: Can There BE a Break in the IMF Near 0.1Mʘ?

The initial mass function of stars (IMF) at small masses depends on several factors. First, it depends on the mass function of cloud cores in which stars form. Second, there must be a lower limit to the core mass for contraction; very small mass cores may not contract even if they exist. This must affect greatly the IMF near its lower end. Third, not all core matter may become stars; we must determine the stellar mass M*, or the star formation efficiency M*/Mcc, as a function of the mass of the cloud core, Mcc. In this paper we discuss the second and third points.

The Initial Stellar Mass Function from Random Sampling in a Turbulent Fractal Cloud

The initial mass function (IMF) for stars is proposed to result from two distinct physical processes that determine its shape separately in two intervals of mass: random sampling of mass in a fractal cloud gives the power-law portion at intermediate to high mass, and insufficient self-gravity at the local temperature and pressure gives the cutoff at low mass. The entire function is modeled numerically, with the assumption that a star's mass is proportional to the mass of the piece of cloud in which it forms. The results typically give an IMF with the Salpeter value for the slope and a flattening at a low mass. There is little sensitivity to parameters at masses greater than the cutoff, although slightly shallower IMFs might be expected in regions with high levels of ionization and turbulence. The low-mass cutoff is essentially the thermal Jeans mass in the star-forming cloud; models with a high value of this mass produce a truncated IMF similar to that proposed for starburst galaxies. The mass of the largest star increases with total stellar mass because of the stochastic nature of the model. The star IMF is steeper than the cloud mass spectrum because of competition for mass and the density dependence of star formation.

Constraints from26Al Measurements on the Galaxy's Recent Global Star Formation Rate and Core‐Collapse Supernovae Rate

Gamma rays from the decay of 26Al offer a stringent constraint on the Galaxy's global star formation rate over the past million years, supplementing other methods for quantifying the recent Galactic star formation rate, such as equivalent widths of Hα emission. Advantages and disadvantages of using 26Al gamma-ray measurements as a tracer of the massive star formation rate are analyzed. Estimates of the Galactic 26Al mass derived from COMPTEL measurements are coupled with a simple, analytical model of the 26Al injection rate from massive stars and restrict the Galaxy's recent star formation rate to 5 ± 4 M☉ yr-1. In addition, we show that the derived 26Al mass implies a present-day Type II + Ib supernovae rate of 3.4 ± 2.8 per century, which seems consistent with other independent estimates of the Galactic core-collapse supernova rate. If some independent measure of the massive star initial mass function or star formation rate or Type II + Ib supernovae rate were to become available (perhaps through estimates of the Galactic 60Fe mass), then a convenient way to restrain, or possibly determine, the other parameters is presented.

A Theory of the Initial Mass Function for Star Formation in Molecular Clouds

We present models for the initial mass function (IMF) for stars forming within molecular clouds. These models use the idea that stars determine their own masses through the action of powerful stellar outflows. This concept allows us to calculate a semi-empirical mass formula (SEMF), which provides the transformation between initial conditions in molecular clouds and the final masses of forming stars. For a particular SEMF, a given distribution of initial conditions predicts a corresponding IMF. We consider several different descriptions for the distribution of initial conditions in star forming molecular clouds. We first consider the limiting case in which only one physical variable -- the effective sound speed -- determines the initial conditions. In this limit, we use observed scaling laws to determine the distribution of sound speed and the SEMF to convert this distribution into an IMF. We next consider the opposite limit in which many different independent physical variables play a role in determining stellar masses. In this limit, the central limit theorem shows that the IMF approaches a log-normal form. Realistic star forming regions contain an intermediate number of relevant variables; we thus consider intermediate cases between the two limits. Our results show that this picture of star formation and the IMF naturally produces stellar mass distributions that are roughly consistent with observations. This paper thus provides a calculational framework to construct theoretical models of the IMF.

THE STELLAR POPULATIONS OF DEEPLY EMBEDDED YOUNG CLUSTERS: NEAR-INFRARED SPECTROSCOPY AND EMERGENT MASS DISTRIBUTIONS

ABSTRACT The goal of this thesis is to test the following hypothesis: the initial distribution of stellar masses from a single "episode" of star formation is independent of the local physical conditions of the region. In other words, is the initial mass function (IMF) strictly universal over spatial scales d < 1 \\ pc and over time intervals Delta-tau << 3 x 106 yrs? We discuss the utility of embedded clusters in addressing this question. Using a combination of spectroscopic and photometric techniques, we seek to characterize emergent mass distributions of embedded clusters in order to compare them both with each other and with the field star IMF. Medium resolution (R=1000) near-infrared spectra obtainable with the current generation of NIR grating spectrographs can provide estimates of the photospheric temperatures of optically-invisible stars. Deriving these spectral types requires a three--step process; i) setting up a classification scheme based on near-infrared spectra of spectral standards; ii) understanding the effects of accretion on this classification scheme by studying optically-visible young stellar objects; and iii) applying this classification technique to the deeply embedded clusters. Combining near-infrared photometry with spectral types, accurate stellar luminosities can be derived for heavily reddened young stars thus enabling their placement in the H-R diagram. From their position in the H-R diagram, masses and ages of stars can be estimated from comparison with theoretical pre-main sequence evolutionary models. Because it is not practical to obtain complete spectroscopic samples of embedded cluster members, a technique is developed based solely on near-IR photometry for estimating stellar luminosities from flux--limited surveys. We then describe how spectroscopic surveys of deeply embedded clusters are necessary in order to adopt appropriate mass-luminosity relationships. Stellar luminosity functions constructed from complete extinction-limited samples can then be used to characterize emergent mass distributions of deeply embedded young clusters. Because of systematic uncertainties in these models at the low-mass end, we adopt the ratio of intermediate (10 solar mass > M* > 1.0 solar mass) to low-mass (1.0 solar mass > M * > 0.1 solar mass) stars in order to compare these mass distributions to the Miller-Scalo IMF. As an example of this analysis we present a study of the embedded cluster associated with the NGC2024 nebula. Although this cluster contains an enhanced number of intermediate mass stars, we cannot distinguish the distribution of stellar masses from the field star IMF. A detailed comparison between the stellar luminosity functions of the embedded clusters associated with the NGC2024 cluster and the embedded population found in the Ophiuchus cloud cores suggests that it is unlikely they were drawn from the same parent population. After finding the evolutionary states and accretion properties of both clusters to be similar, we interpret the difference in stellar luminosity functions as a difference in their emergent mass distributions. Synthesizing results for NGC 2024 and Ophiuchus with those from other studies of embedded clusters, we arrive at the following conclusions: i) the emergent mass distributions of most of the embedded young clusters considered are consistent with having been drawn from the Miller--Scalo IMF; and ii) there is a hint that regions of high central stellar density contain a greater proportion of intermediate mass stars.

The Intermediate-Mass Population in the Core of the R136 Star Cluster

R136, the compact, luminous star cluster at the center of the 30 Doradus nebula, represents an extreme mode of star formation. Yet it is unique among these kinds of objects in that it can be resolved into individual stars and the star formation process probed through the stellar products. In a previous paper we reported on a study of the intermediate stellar mass population in R136 and derived a stellar initial mass function (IMF) for intermediate-mass stars in the region 0.5 to 4.7 pc from the center of the cluster. We have now obtained Hubble Space Telescope images designed to probe the stellar population in the core ( < 0.5 pc) of the cluster. We have measured the IMF for stars 5-15 M☉ at radii 0.11-0.34 pc, and for 4-15 M☉ at 0.34-0.46 pc. Beyond 0.46 pc we measure stars in mass bins from 2.8-15 M☉, as before. The slope of the stellar IMF beyond 0.5 pc is found to be -1.0 ± 0.1, which is within a few sigma of the value measured previously for the same annulus. The slopes of the IMFs become shallower at smaller radii, but the uncertainties do not rule out an IMF that is constant with cluster radius. The lower stellar mass limit is ≤5 M☉ in the cluster core. Thus, R136 is found to be similar to less concentrated OB associations even into 0.1 pc from the center of the cluster.

Physical Conditions of Molecular Gas in the Galaxy

Physical conditions of molecular gas are key parameters to the formation rate and initial mass function of stars formed in molecular clouds. The ongoing Tokyo-NRO survey has been observing the Galactic CO (J=2–1) emission with a beamsize matched to the Columbia CO (J=1–0) survey. Intensities of the two lines should reflect physical conditions of the CO-emitting gas. An out-of-plane survey of the inner Galaxy which covers from 20° to 60° in galactic longitude and from −1° to +1° in galactic latitude with grid spacings of 0.25° has already been made (Sakamoto et al. 1994). Its coverage is large enough to draw conclusions on global properties of molecular gas in the inner Galaxy.

The stellar mass function of Praesepe

We have obtained V, I, and K band photometry for 290 square arcminutes in the Praesepe open cluster. The photometry reliably reaches 0.08 solar mass. We find six probable cluster members in the range 0.40 solar mass greater than M greater than 0.08 solar mass. The mass function we derive for Praesepe is similar to the field star Initial Mass Function (IMF). We conclude that dynamical processes that can lead to evaporation of low-mass members from stellar clusters have not occurred to a substantial extent in Praesepe.

Initial mass functions from ultraviolet stellar photometry: A comparison of Lucke and Hodge OB associations near 30 Doradus with the nearby field

UV stellar photometry is presented for 1563 stars within a 40 minutes circular field in the Large Magellanic Cloud (LMC), excluding the 10 min x 10 min field centered on R136 investigated earlier by Hill et al. (1993). Magnitudes are computed from images obtained by the Ultraviolet Imaging Telescope (UIT) in bands centered at 1615 A and 2558 A. Stellar masses and extinctions are estimated for the stars in associations using the evolutionary models of Schaerer et al. (1993), assuming the age is 4 Myr and that the local LMC extinction follows the Fitzpatrick (1985) 30 Dor extinction curve. The estimated slope of the initial mass function (IMF) for massive stars (greater than 15 solar mass) within the Lucke and Hodge (LH) associations is Gamma = -1.08 +/- 0.2. Initial masses and extinctions for stars not within LH associations are estimated assuming that the stellar age is either 4 Myr or half the stellar lifetime, whichever is larger. The estimated slope of the IMF for massive stars not within LH associations is Gamma = -1.74 +/- 0.3 (assuming continuous star formation), compared with Gamma = -1.35, and Gamma = -1.7 +/- 0.5, obtained for the Galaxy by Salpeter (1955) and Scalo (1986), respectively, and Gamma = -1.6 obtained for massive stars in the Galaxy by Garmany, Conti, & Chiosi (1982). The shallower slope of the association IMF suggests that not only is the star formation rate higher in associations, but that the local conditions favor the formation of higher mass stars there. We make no corrections for binaries or incompleteness.

Starburst modeling of M82 - Test case for a biased initial mass function

We compute starburst models for M82, making use of recent theoretical tracks of stellar evolution. Detailed comparisons of our models and those of others demonstrate this technique to be quite reliable, with relatively little change in output parameters as a function of the selection of theoretical tracks or of estimates of the observational characteristics of the stars along these tracks. The models are matched to the observational constraints for M 82 summarized by McLeod et al. The rate of star formation and time of observation were thoroughly optimized to produce the most favorable fit to the observations, but we still found that the recently proposed forms for the solar-neighborhood initial mass function (IMF) cannot produce starbursts adequate to fit the observations of this galaxy. We then explored adjustments to the shape of the IMF to improve the fit to M 82. We find (1) the shape of the IMF for high-mass stars need not be different from that observed locally; and (2) the most likely modification to the IMF in M 82 is that stars with masses below a few M_sun_ form much less commonly than in the solar neighborhood.

Star formation in Taurus-Auriga - The high-mass stars

The question of whether the star formation in the Tau-Aur clouds produces low-mass stars exclusively is considered. The paper identifies 29 B stars as possible kinematic members of the T association, and it is argued that the data are consistent with the existence of a single initial mass function (IMF) generating the low and high-mass stars. It is concluded that the Tau-Aur T association is related to the Cas-Tau OB association, and there is no evidence that the association IMF differs from the field star IMF.

On star formation in cooling flows of clusters of galaxies

A correlation between the masses of cooling flows in clusters of galaxies and the absolute magnitudes of the central parts of the brightest galaxies of the clusters is detected. This correlation is consistent with star formation in the cooling flows according to an initial mass function of stars truncated at stellar masses corresponding to absolute magnitudes in the most probable cases between +7 and +2.