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By J. Gardner

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3 What is the Initial Mass Function at sub-stellar Masses? Does cloud fragmentation explain low-mass star formation, and is there a lower limit to the mass? How does the sub-stellar initial mass function depend on metallicity or environment? The initial mass function (IMF) is a key product of star formation (Salpeter 1955; Miller & Scalo 1979). 3 M⊙ . 3 M⊙ , studies show that the IMF flattens somewhat, but continues to increase. 1 M⊙ (100 MJUP ), the mass function starts to decline, and microlensing observations (Alcock et al.

2004; Sato et al. 2005; for a review of surveys see Gillon et al. 2005). These transiting planets are the only solar-system-aged extrasolar planets that can be physically characterized until direct imaging is possible. HST and Spitzer have detected the atmospheres of two of these planets, including sodium, an extended atmosphere of hydrogen, and thermal emission (Charbonneau et al. 2002, 2005; Vidal-Madjar et al. 2003; Deming et al. 2005). These atmosphere detections are made possible by a differential measurement of the planet and starlight combined (during primary transit or outside of secondary eclipse) compared with only the starlight (outside of primary transit or during secondary eclipse).

2003). supplemented with 10 µm photometry to separate the circumstellar emission due to disks from the intrinsic source luminosity. 5 arcmin at 1 kpc. At larger distances, high spatial resolution will eliminate crowding and resolve the lowest-mass sources from adjacent brighter, more massive neighbors. Multiobject spectroscopy at 2 to 5 µm will distinguish between true cluster members and field interlopers. These observations will also break the age-reddening-mass degeneracy found in young clusters, by determining spectral types, making it possible to accurately deredden the sources and locate them in theoretical pre-main sequence Hertzsprung-Russell diagrams.

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The James Webb Space Telescope [lg article] by J. Gardner


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