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Overview

I am an observational astrophysicist studying the formation and evolution of low-mass stars. Once overlooked as merely smaller cousins of solar-type stars, we now know these fainter, redder stars possess interiors and atmospheres quite distinct from those of our Sun, and represent extremely promising targets for the detection of habitable Earth-like planets. My primary research efforts are guided by a desire to understand how young low-mass stars accrete mass, shed angular momentum, and eventually disperse their circumstellar disks; I am also interested in understanding the evolution of stellar magnetic fields over time.

To shed light on these topics, I make observations at wavelengths ranging from X-rays to the sub-millimeter, and use the results to test theoretical models of the formation and evolution of stars. I make extensive use of survey scale datasets, both photometric and spectroscopic, and am involved in preparations for next-generation surveys such as the Large Synoptic Survey Telescope.

The Angular Momentum Evolution of Young Low-Mass stars

Conservation of angular momentum plays a fundamental role in the star formation process, most notably by driving the formation of a circumstellar disk (Tereby et al. 1984). Coupling of the stellar magnetic field to this disk allows contracting young stars to transfer angular momentum to their disks via magnetic field lines (Shu et al. 1994), only conserving angular momentum and spinning up once their disks have dissipated. Observations indicate that stellar rotation does indeed evolve during the star formation process: my analysis of high resolution near-infrared spectra proved that optically invisible protostars lose half their angular momentum content during their transition to the optically visible T Tauri phase (see Fig. 1 below, and Covey et al. 2005); recent observations confirm the connection between circumstellar disks and T Tauri star rotation (Rebull et al. 2006, Cieza et al. 2007).

Theoretical models of mass accretion and angular momentum transfer predict observable correlations between stellar properties and signatures of the accretion process (e.g., Macc ~ [R*4 facc] / [M* Prot]; Johns-Krull 2002, Hartmann 2002). To quantify the impact of active mass accretion on stellar rotation, I am analyzing spectra of hundreds of young Tauri stars. Modeling each star's optical/infrared photometry provides an estimate of its mass (M*) and the properties of its circumstellar disk (ie, the radius of its inner edge, Rdisk); the spectra identify each star's projected rotation velocity (v sin i), and mass accretion rate (Macc, measured from Hα emission). By demonstrating if stars spin up only after the disk is fully dissipated, or immediately following the cessation of mass accretion, this sample will identify if proto-stellar rotation is more sensitive to ongoing mass accretion or the presence of circumstellar disks.

Figure 1 - Observed projected rotation velocity as a function of mid-infrared SED slope (alpha). Larger values of alpha indicate redder, more deeply embedded stars. Symbols show objects in the Taurus (squares), rho Ophiuchi (triangles) and Serpens (diamonds) star forming regions; filled symbols indicate a rotation velocity upper limit. The dashed vertical line shows the canonical dividing line between embedded objects, on the right, and optically revealed sources, on the left. While a variety of rotation rates are seen at every evolutionary stage, the mean and maximum observed rotation velocities decline as sources become more optically visible, indicating angular momentum must be extracted from these sources as they emerge from their protostellar envelopes. (Data from White et al. 2004, Covey et al. 2005 and references therein)

The Origin of Stellar Masses

The mass function (MF) is a fundamental property of stellar systems, describing the number of stars as a function of stellar mass. This statistical measure of the star formation process succinctly characterizes a large population of stars, informing our understanding of the structure and dynamical evolution of stellar clusters, the Milky Way and other galaxies. Understanding the physical processes that govern the shape of the stellar MF is one of my core research goals.

Using accurate, multi-color catalogs of stars detected in wide field imaging from the Two Micron All Sky Survey (2MASS) and Sloan Digital Sky Survey (SDSS), I was able to determine that the MF of the Galactic disk peaks at 0.17 solar masses. My analysis of ~30,000 stars across 30 square degrees of sky exceeded the scale of previous efforts by nearly an order of magnitude, and included a careful spectroscopic analysis of ~13,000 of these stars to quantify potential errors due to incompleteness, contamination, and bias. This measurement provides a strong constraint for theoretical models using the physics of molecular cloud fragmentation to explain the shape of the MF (Covey et al. 2007, Covey et al. 2008a). Having verified the feasibility of this type of analysis, I am collaborating with John Bochanski (MIT) and Suzanne Hawley (Univ. of Washington) on a similar analysis of the entire SDSS database; with 15 million low-mass stars in our sample, we are able to measure not only the shape of the MF, but also if it varies as a function of position in the Galaxy.

In the next decade, I will analyze multi-epoch data from the Large Synoptic Survey Telescope (LSST) to create a detailed history of star formation in the Galactic disk. This groundbreaking analysis is made possible by the new technique of `gyrochronology' \citep{Barnes2007}, which uses stellar clusters to calibrate the tight relationship between stellar age and rotation period due to angular momentum loss from stellar winds. Starting in 2014, LSST will image the entire visible sky every four nights, obtaining thousands of observations of hundreds of millions of stars over its decade long mission. Using gyrochronology to derive accurate (~10\%) ages from rotation periods LSST measures for these stars, I will construct an incredibly detailed picture of the formation history of the Galactic disk; this measurement will not only clarify how the Milky Way formed and evolved, but also inform cosmological models of structure formation in a universe dominated by un-seen dark matter and dark energy. To ensure this success of this and other LSST projects, I am a founding member of the LSST stellar populations science collaboration; we are using detailed simulations of various observing strategies to optimize the LSST cadence for stellar/Galactic science.

Magnetic Activity in Low-Mass Stars

Despite their diminutive stature, low-mass stars generate strong magnetic fields capable of heating their outer atmospheres to temperatures of millions of degrees, producing strong X-ray and Balmer line emission which are commonly used as tracers of a star's overall `magnetic activity'. The physical mechanism powering magnetic activity in low-mass stars remains poorly understood, particularly because many low-mass stars lack the internal radiative/convective zone boundary that plays a central role in the solar dynamo. My collaboration with Andrew West (MIT) has produced Halpha line strength measurements for more than 45,000 low-mass stars with SDSS spectra, revealing that the characteristic level of stellar magnetic activity decreases with height in the Galactic disk. Since stars are born close to the mid-plane of the Galaxy, and then slowly have their vertical velocity dispersion increased by dynamical interactions, the median stellar age increases with Galactic height; simple physical models of this process have allowed us to infer the magnetic activity lifetime as a function of stellar mass, placing new constraints on dynamo models for low-mass stars (West et al. 2004, West et al. 2006, West et al. 2008).

Following up on this work, I have constructed a catalog of serendipitously detected X-ray emitters by cross-correlating detections from the Chandra Multi-wavelength Project (ChaMP) with SDSS. Using this sample, we were able to confirm that the weakest X-ray emitters possess colors indicative of old stars, confirming that the age activity relation holds for X-ray emitting field stars as well (Covey et al. 2008). Building on this work I am now working to incorporate magnetic activity into my models of the stellar population in the Galactic disk. By requiring that the model simultaneously reproduce X-ray and optical star counts detected in my matched catalog and by deep, extragalactic Chandra/HST surveys that probe multiple lines of sight through the Galaxy, I will provide a detailed portrait of the evolution of stellar X-ray emission over time.

The Formation of Gas Giant Planets Around Young Stars

Since the discovery of the first extra-solar planets orbiting main sequence stars (Mayor et al. 1995, Marcy & Butler 1996, Butler & Marcy 1996), more than 314 planets in 253 systems have been detected (exoplanet.eu). These systems have taught us about the frequency and physical properties of mature gas giants, and induced sweeping changes to theories of planet formation that adopted our solar system as a template. These new theories, however, still rely upon inferences drawn from mature planetary systems; measuring the physical and orbital properties of gas giants in young (t < 10 Myrs) planetary systems could force yet another wholesale revision of planet formation theories.

In recent years, several young stars have been identified whose circumstellar disks appear to possess unusually large inner holes or gaps. These `transition disks' are identified either from spatially resolved imaging, or inferred from the shape of the star's spectrum across many wavelengths. Some believe photoionization of the disk by the central star may be causing these holes, but clearing of the inner disk due to dynamical interactions with newly formed planets is the mechanism most commonly invoked to explain the presence of these holes and gaps. Attempts to detect planets around these young stars, however, face several hurdles. Most perniciously, large sunspots common to young stars can distort the velocity structure of optical spectral lines, mimicking the radial velocity signals which are most often used to detect planets (Huerta et al. 2008).

In the near infrared, however, the contrast between Rayleigh-Jeans emission from the starspot and the surrounding photosphere is too low to distort the shapes of spectral lines; near infrared Doppler monitoring of young stars revealed they are free of radial velocity anomalies at the ~20 m/sec level, sufficient to detect gas giant planets. To test if planet formation is indeed responsible for clearing gaps and holes in circumstellar disks, I have initiated a targeted search for planets around transition disk stars. My collaborators (Andres Jordan, Pontificia Universidad Catolica De Chile, and Andreas Seifahrt, University of Gottingen) and I have proposed to use the most stable and precise near infrared spectrograph currently available, CRIRES on the Very Large Telescope in Chile, to carry out a Doppler survey of IRS 48. This T Tauri star and transition disk source has a moderately inclined circumstellar disk with a significant gap (rinner ~ 60 AU, sin idisk ~ 0.5; Geers et al. 2007). This new component of my research program builds on my previous experience with NIR spectroscopy (Covey et al. 2008, in prep., Covey et al. 2005, Covey et al. 2006) and will provide a direct, empirical indication of the mechanism responsible for clearing transition disks, demonstrating if the gap in the disk around IRS 48 and similar stars is being cleared by a short period hot Jupiter, a large stellar companion, or by non-dynamical means (ie, photo-evaporation).