Scott E. Dahm

PhD, University of Hawaii, 2005
W. M. Keck Observatory
65-1120 Mamalahoa Highway - Kamuela, HI 96743
office: 808.881.3847 - fax: 808.881.3897 - email:

Headquarters Bldg: 107
Observing Schedule

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.
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.



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.


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.

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.
(Spitzer: L. Cieza, N. Evans, L. Rebull, and J. Jorgensen) (J-C Cuillandre and CFHT)
Curriculum Vitae  
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  Institute for Astronomy
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