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Office:
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Observational Astrophysics - HIRES Instrument Master
Principal Areas of Research:
Star formation, the evolution of circumstellar disks,
and the formation of planetary systems.
Disk Regulation of Angular Momentum
Projected rotational velocities for 20 early-type (B8-A9) and 74 late-type (F2-M8)
members of the 5 Myr old Upper Scorpius OB Association were derived from high dispersion
optical spectra obtained with the High Resolution Echelle Spectrometer (HIRES) on
Keck I and the Magellan Inamori Kyocera Echelle (MIKE) on the Magellan Clay telescope.
The spectroscopic sample was composed of stars and brown dwarfs with infrared signatures
of circumstellar disks, both primordial and debris, and non-excess sources of comparable
spectral type. The projected rotational velocities were merged with accretion diagnostics
and Spitzer Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer
(MIPS) 24 micron photometry. We find with a probability of >0.99 that M-type stars and
brown dwarfs having infrared excess suggestive of circumstellar disks rotate more slowly
than their non-excess counterparts.
Exoplanets
In collaboration with Russel White, Justin Cantrell, and Geoff Marcy,
a high precision, radial velocity survey of near solar analogs in M44 (Praesepe) has begun to
search for and ultimately characterize exoplanets.
This program is using HIRES in conjunction with the iodine cell to achieve
radial velocity precisions of a few meters per second. At 600 Myr, however,
most solar analogs will exhibit significant activity, limiting precisions to perhaps
16 m/s. The detection of hot Jupiters, i.e. giant planets orbiting in close proximity
to their host star (~0.1 AU) will be the initial objective of this survey.
Gas and Dust Disk Evolution in the Upper Scorpius OB Association
I am currently leading two projects
examining dust and gas disk structure and evolution around early
(B+A) and late-type (K+M) stars in the ~5 Myr old Upper
Scorpius OB Association. These
projects merge Spitzer InfraRed Spectrograph (IRS)
low and high resolution spectra with Keck NIRSPEC
and HIRES observations and JCMT CO J=3-2 submillimeter
spectroscopy to probe gas and dust emission
across all orbital radii (0.1-100 AU). The principal objective of the programs
is to constrain the timescale for the formation of planetary systems
by determining whether sufficient gas and dust remain for the build-up
of planetisimals and planetary cores in these evolved disk systems.
We find clear evidence for changes in the spectral characteristics of
dust emission between the early and late-type infrared excess stars. The
early-type stars exhibit featureless continuum excesses that become apparent
redward of ~8 microns. In contrast the 10 and 20 micron silicate features
are present in most low-mass stars of Upper Scorpius. Fitting the spectral
energy distributions (SED) for a sample of the low-mass excess stars, we
find that the SEDs are consistent with models having inner disk radii ranging from 0.2-1.2 AU.
Compared to Class II sources in the presumably younger (1-3 Myr old)
Taurus-Auriga star forming region, the disk population of Upper Scorpius
exhibits reduced levels of near and mid-infrared excess emission and an
order of magnitude lower mass accretion rates. These results suggest that
disk structure has changed significantly over the 2-4 Myr in age separating
these two stellar populations. The ubiquity of depleted inner disks in the Upper
Scorpius excess sample implies that such disks are a common evolutionary
pathway that persists for some time.
Coronagraphic
Adaptive Optics (AO) Imaging of Circumstellar Disks in Beta Pic and Upper Scorpius
I have recently
begun a project examining circumstellar disks in scattered light
using coronagraphic imaging in the near infrared with NIRC2 in natural guide star (NGS)
mode. The objectives of the program are to 1) place constraints
on disk structure (outer disk radius, inclination angle, and
surface brightness profiles) and 2) to identify substellar
and planetary mass companions to these disk-bearing systems.
The Evolution
of Young Clusters and Star Forming Regions
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.
X-ray Emission
from Protostars and 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.
Click here
to access the ADS link displaying a list of articles
describing some of my work.
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Keck Observatory on Mauna Kea
NASA/JPL-Caltech/T. Pyle (SSC)
Artist's conception of a debris disk around a young solar-type star. Spitzer
is an ideal platform for observations of such systems where dust emission dominates
the mid and far-infrared flux.
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Upper Scorpius:
Circumstellar disks are the progenitors of planetary systems
and dissipate over timescales on order of 10 Myr. Typically such disks are
inferred from infrared emission in excess of stellar photospheres. Shown here are the
[8.0] to [4.5] and 16.0 micron to [4.5] flux ratios for over 200 members
of Upper Scorpius observed with IRAC and IRS from Carpenter et al. (2006)
The 35 disk-bearing members of Upper Scorpius are readily apparent. The SEDs
of the excess stars are then fit with the accretion disk models of Robitaille et al. (2006)
to constrain fundamental disk properties such as disk mass, inner disk radius, and inclination
angle. |
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Spitzer Space Telescope
Young Clusters:
IC 348 (above) in the Perseus molecular cloud
and IC 5146 (below) are two young clusters that were examined in the photometric and spectroscopic
surveys of Herbig (1998) and Herbig and Dahm (2002), respectively.
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(Spitzer: L. Cieza, N. Evans, L. Rebull, and J. Jorgensen)
(J-C Cuillandre and CFHT) |
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