I am currently a fifth-year graduate student at the California Institute of Technology, where I am working toward a Ph.D in Astrophysics. I earned my undergraduate degrees in Physics, Astronomy, and Mathematics at the University of Kansas.

CV: pdf,ps

Research Interests

Multiple Star Formation

The frequency and properties of multiple star systems are important diagnostics for constraining star formation processes and calibrating stellar evolutionary models. Binary formation is a critical component of star formation since 25-75% of stars form in binary systems; any successful theory of star formation should be able to recreate the observed binary properties (frequencies, separation distribution, and mass ratio distribution) as a function of system mass and formation environment.

This has prompted numerous attempts to characterize the properties of nearby binary systems in the field. Multiplicity surveys of solar-type stars have found that they have relatively high binary frequencies (~60%) and a wide range of binary separations (up to 10,000 AU). By contrast, surveys of lower-mass M dwarfs have found marginally lower binary frequencies (~40%) and surveys near and below the substellar boundary have found substantially lower binary frequencies (10-20%) and separations (less than 20 AU) and a strong tendency toward equal-mass systems. These results suggest that the binary formation process depends on mass. However, field studies are fundamentally limited because of the uncertain age, dynamical history, and formation environment for any individual binary system.

The solution to this problem is to study binary systems in discrete stellar populations (open clusters and young stellar associations), where these properties are homogeneous. My primary thesis project is a survey of multiplicity in several nearby stellar populations using adaptive optics (to identify binaries with small separations) and a combination of seeing-limited imaging and intermediate-resolution spectroscopy (to identify binaries with large separations). My goal is to quantify and explain the role of mass and formation environment in determining the properties of binary systems.

Measuring the Fundamental Properties of Low-Mass Stars

M dwarfs are ubiquitous in the solar neighborhood and constitute the majority of stellar content in our galaxy, but their fundamental properties are not as well understood as those of their more massive brethren. These properties (e.g. mass and radius) are typically calibrated by observations of binary systems, especially eclipsing binaries, since they can be calculated directly from observed quantities. However, M dwarfs are intrinsically faint, so a very limited sample of binaries are accessible and suitable for detailed study; there are currently only ~10 known M dwarf eclipsing binaries.

I am currently conducting a program to identify new low-mass eclipsing binary systems in the catalogues of deep, wide-field variable-star surveys (e.g. the MOTESS-GNAT surveys; see below) and to determine their fundamental properties with additional photometric and spectroscopic monitoring. The end goal of this project is to calibrate low-mass stellar evolutionary models, which are still uncertain at the 10-20% level, and to explore the unconstrained roles of metallicity and stellar activity in determining stellar structure.

The Kinematics of Young Stars

Star clusters are thought to be the primary sites of star formation, and cluster evolution plays a key role in establishing the large-scale environment within which small-scale processes like star formation, binary fragmentation, and planet formation can occur. Observations show that young clusters tend to be highly structured, with numerous subgroups embedded in an overall radial density distribution. This suggests that star formation is a very complex and dynamical process, with significant spatial and kinematic differentiation on several time- and length-scales. Simulations that begin with fractal spatial distributions have some success in recreating cluster substructure, but they also suggest that the cluster and subgroup velocity distributions play a key role in determining the evolution of these substructures. High or coherent stellar velocities can delay relaxation and maintain substructure for many crossing times, and the intra-subgroup velocity distribution will strongly affect the rate of dynamical interactions.

Direct measurement of internal cluster kinematics would address the role of kinematics in cluster evolution, as well as topics regarding small-scale star formation (like the decay of unstable multiple-star systems or the rate of disruptive dynamical encounters). However, existing studies are very limited in scope because velocity measurements have been performed in one dimension (radial). Detailed analysis of individual stellar dynamics has been impossible since radial velocities are orthogonal to measurements of two-dimensional spatial distributions. Tangential velocities (as measured via proper motions) are required in order to link positions and kinematics and extrapolate individual stellar motions. The corresponding individual angular velocities (1-2 mas/yr) have been considered too small to be reliably measured, but high-resolution imaging (with adaptive optics or HST) can now deliver astrometry with the necessary precision. The next 5-10 years will see this field rapidly develop as AO systems continue to proliferate, laser guide star AO systems are adopted at more observatories, and wide-field applications like multi-conjugate adaptive optics are developed.

High Spatial Resolution Observations with Adaptive Optics

Mining Large Astronomical Datasets

Publications

The Spatial Distributions of Young Stars
Kraus, A.L. & Hillenbrand, L.A. 2008, ApJL, 686, L111

Unusually Wide Binaries: Are They Wide Or Unusual?
Kraus, A.L. & Hillenbrand, L.A. 2008, submitted to ApJ

The Disk Around CoKu Tau/4: Circumbinary, not Transitional
Ireland, M.J. & Kraus, A.L. 2008, ApJL, 678, L59

Mapping the Shores of the Brown Dwarf Desert I.: Upper Scorpius
Kraus, A.L., Ireland, M.J., Martinache, F., & Lloyd, J.P. 2008, ApJ, 679, 762

The Dynamical Mass of GJ 802B: A Brown Dwarf in a Triple System
Ireland, M.J., Kraus, A.L.. Martinache, F., Lloyd, J.P., & Tuthill, P.G. 2008, ApJ, in press

The Stellar Populations of Praesepe and Coma Berenices
Kraus, A.L. & Hillenbrand, L.A. 2007, AJ, 134, 2340

USco1606-1935: An Unusually Wide Low-Mass Triple System?
Kraus, A.L. & Hillenbrand, L.A. 2007, ApJ, 664, 1167

The Role of Mass and Environment in Multiple Star Formation: A 2MASS Survey of Wide Multiplicity in Three Nearby Young Associations
Kraus, A.L. & Hillenbrand, L.A. 2007, ApJ, 662, 413
Table 2 (Full sample) is available in PDF, TeX, or plain text.

The First MOTESS-GNAT Variable Star Survey
Kraus, A.L., Craine, E.R., Giampapa, M.S., Scharlach, W.W.G., & Tucker, R.A. 2007, AJ, 134, 1488
The catalog can be accessed at the GNAT website

Multiplicity and Optical Excess Across the Substellar Boundary in Taurus
Kraus, A.L., White, R.J., & Hillenbrand, L.A. 2006, ApJ, 649, 306

Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius
Kraus, A.L., White, R.J., & Hillenbrand, L.A. 2005, ApJ, 633, 452

Technical Pages

P200 East-Arm Echelle