Spitzer IRAC and MIPS image of NGC 2264
NASA/JPL-Caltech/P.S. Teixeira (Center for Astrophysics)
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The Evolution of Young Cluster Populations
Most low-mass stars form within large clusters or associations that disperse over time
as a result of collisional interactions, dynamic relaxation, or evaporation. The disruption
of young clusters occurs principally through collisions with giant molecular clouds, which are at least three
orders of magnitude more massive than the incident clusters. Only the most
massive clusters will survive beyond the age of the Pleiades (~100 Myr). The Sun's own early membership within
a cluster or association of stars is circumstantially supported by isotopic abundances found
in chondritic meteorites, most plausibly injected by a supernova, Wolf-Rayet star, or a nearby AGB star.
The principal objective of this research program is to document the evolution of low-mass stars as
they transition from an embedded state to the termination of the T Tauri phase. With large numbers of
dynamically pristine stars evolving under similar conditions and with identical metallicities, young
clusters in varying stages of development provide ideal locations for early stellar evolution research.
With an age of less than 1 Myr, the youngest cluster included in the survey is still embedded
within its parent molecular cloud. The stellar population of the cluster is dominated by classical
T Tauri stars (CTTS) and numerous embedded
infrared sources that lack optical counterparts. The oldest clusters in the survey have approximate ages of
10-15 Myr and are devoid of natal gas and dust. The earliest members of these clusters have already evolved
away from the zero-age main sequence (ZAMS) and strong Balmer-line emission among the low-mass population
has effectively subsided. Between these extrema of ages, young clusters and their stellar populations
experience dramatic changes as remnant molecular gas disperses, star formation halts, and the
optically-thick, inner disk regions of young, low mass stars dissipate.
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NASA/JPL-Caltech/T. Pyle (SSC)
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Circumstellar Disk Evolution and the Formation of Planetary Systems
My research at Caltech is focused upon establishing the timescale for the dissipation of
circumstellar gas and dust around young Sun-like stars, which is of critical importance for planet formation theory.
Two models of gas giant formation have gained preeminence over the last decade, each
of which is characterized by a unique timescale. Core accretion theory requires
1 to 10 Myr for large planetesimals to coalesce before accreting a massive gaseous envelope.
The disk instability mechanism suggests that protoplanetary cores
gravitationally collapse out of the disk in less than 0.1 Myr. The large number (~200) of planetary systems
discovered to date by high-precision, radial velocity surveys implies that while not ubiquitous, giant planet
formation is certainly not rare. Although strongly metallicity dependent, ~12% of all FGK-type
stars possess one or more gas giant planets within 20 AU (Marcy et al. 2005). The formation of
Jovian-like planets requires disk masses of at least 0.01 solar masses, the minimum mass solar nebula,
but even trace amounts of gas (less than one Jupiter mass) can alter system dynamics by inducing drag upon
dust and circularizing the orbits of large planetesimals. For this program, accretion diagnostics and CO fundamental band
emission are used to trace inner disk gas while Spitzer IRAC and MIPS photometry are used to
examine dust disk structure and evolution.
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Fe XII solar image (SOHO EIT)
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X-ray Emission from Class I Sources, Classical and Weak-line T Tauri Stars
Early X-ray studies by Einstein and later ROSAT recognized that pre-main sequence stars possess X-ray luminosities
that exceed those of their main sequence counterparts by one to three orders of magnitude.
The source of X-ray flux from
both classical and weak-line TTSs is believed to be soft coronal emission from enhanced solar-like
magnetic activity as well as a hard component originating from flares and magnetic reconnection events.
Accretion-produced X-ray flux is expected to be a minor contributor to the total high energy spectrum of
CTTSs given the low energies involved in viscous accretion processes. In main sequence stars, the origin
of surface magnetic activity is fairly well established to be differential rotation between the radiative
core and the convection zone. The dynamo-generated field manifests itself through the strong correlation found
between X-ray luminosity and rotation period or rotational velocity. Recent XMM-Newton and Chandra observations of star
forming regions, however, have produced two unexpected findings: first, the presence of near infrared (NIR) excess
or other disk indicators has no bearing upon measured X-ray fluxes, and second, X-ray luminosities are not
anti-correlated with rotation period, but rather weakly correlated.
The latter conclusion suggests that a different field
generation mechanism exists for pre-main sequence stars and dwarfs.
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