Embedded Clusters and the IMF

Despite valiant efforts over nearly five decades, attempts to determine the IMF over a complete mass range for galactic field stars and in open clusters have proved difficult. Infrared imaging observations of extremely young embedded clusters coupled with Monte Carlo modeling of their luminosity functions are improving this situation and providing important new contributions to our fundamental knowledge of the IMF and its universality in both space and time.Comment: 6 pages, 2 figures to appear in "The IMF@50", Kluwer Academic Press, eds. C. Corbelli, F. Palla, & Hans Zinnecke

Cluster Density and the IMF

Observed variations in the IMF are reviewed with an emphasis on environmental density. The remote field IMF studied in the LMC by several authors is clearly steeper than most cluster IMFs, which have slopes close to the Salpeter value. Local field regions of star formation, like Taurus, may have relatively steep IMFs too. Very dense and massive clusters, like super star clusters, could have flatter IMFs, or inner-truncated IMFs. We propose that these variations are the result of three distinct processes during star formation that affect the mass function in different ways depending on mass range. At solar to intermediate stellar masses, gas processes involving thermal pressure and supersonic turbulence determine the basic scale for stellar mass, starting with the observed pre-stellar condensations, and they define the mass function from several tenths to several solar masses. Brown dwarfs require extraordinarily high pressures for fragmentation from the gas, and presumably form inside the pre-stellar condensations during mutual collisions, secondary fragmentations, or in disks. High mass stars form in excess of the numbers expected from pure turbulent fragmentation as pre-stellar condensations coalesce and accrete with an enhanced gravitational cross section. Variations in the interaction rate, interaction strength, and accretion rate among the primary fragments formed by turbulence lead to variations in the relative proportions of brown dwarfs, solar to intermediate mass stars, and high mass stars.Comment: 14 pages, 3 figures, to be published in ``IMF@50: A Fest-Colloquium in honor of Edwin E. Salpeter,'' held at Abbazia di Spineto, Siena, Italy, May 16-20, 2004. Kluwer Academic Publishers; edited by E. Corbelli, F. Palla, and H. Zinnecke

UBVRI Light curves of 44 Type Ia supernovae

We present UBVRI photometry of 44 Type la supernovae (SNe la) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe la to date, nearly doubling the number of well-observed, nearby SNe la with published multicolor CCD light curves. The large sample of [U-band photometry is a unique addition, with important connections to SNe la observed at high redshift. The decline rate of SN la U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ∼40% intrinsic scatter compared to the B band

What works in preventing unintentional injuries in children and young adolescents? An updated systematic review

Over the past year, we have conducted a multi-faceted program to investigate the origin and early evolution of brown dwarfs. Using high-resolution Keck optical spectra of ~30 objects near and below the sub-stellar boundary in several star-forming regions, we present compelling evidence for a T Tauri-like accretion phase in young brown dwarfs. Our systematic study of infrared L'-band (3.8-micron) disk excess in ~50 spectroscopically confirmed young very low mass objects reveal that a significant fraction of brown dwarfs harbor disks at a very young age. Their inner disk lifetimes do not appear to be vastly different from those of disks around T Tauri stars. Taken together, our findings are consistent with a common origin for most low-mass stars, brown dwarfs and isolated planetary mass objects.Comment: to appear in proceedings of the conference on "Open Issues in Local Star Formation and Early Stellar Evolution" held in Ouro Preto, Brazil, 2003 April 5-1

GOULD’S BELT TO STARBURST GALAXIES: THE IMF OF EXTREME STAR FORMATION

Recent results indicate the stellar initial mass function is not a strong function of star–forming environment or “initial conditions ” (e.g. Meyer et al. 2000). Some studies suggest that a universal IMF may extend to sub–stellar masses (see however Briceno et al. 2002). Yet most of this work is confined to star–forming environments within 1 kpc of the Sun. In order to probe the universality of the IMF over a wider range of parameter space (metalicity, ambient pressure, magnetic field strength) new techniques are required. We begin by summarizing our approach to deriving the sub–stellar IMF down to the opacity–limit for fragmentation using NGC 1333 as an example. Next, we describe results from simulations using the observed point–spread function of the new 6.5m MMT adaptive optics system and examine the confusion–limited sensitivity to low mass stars in rich star–forming clusters out to 0.5 Mpc. We also present preliminary results from observations with this system of the W51 star–forming complex. Finally, we outline a new technique to estimate the ratio of high to low mass stars in unresolved stellar populations, such as the massive star clusters observed in interacting galaxies (e.g. Mengel et al. 2002). While evidence for variations in the IMF remains inconclusive, new studies are required to rule them out and determine whether or not the IMF is universal over the range of parameter space relevant to star–forming galaxies over cosmic time. 1