Kelle Cruz, NSF Post-Doctoral Fellow

Introduction

The observational study of low-mass stars and brown dwarfs at AMNH is multi-faceted and includes proper-motion studies, spectroscopic analysis, search for companions, search for subdwarfs. As part of these projects, observations are obtained with ground-based optical, near-infrared, and radio telescopes; space-based observatories including HST and Spitzer; as well as archived plate material.

Low-mass stars are just that, stars that are low in mass and also low in temperature. Brown dwarfs, on the other hand, are objects that form in the same way stars do, but which are not massive enough to sustain fusion in their core. While stars reach an equilibrium temperature and shine at a constant brightness for billions of years, brown dwarfs gradually cool and dim with time. Low-mass stars and brown dwarfs are the most numerous star-like objects in galaxies. However, even though they are ubiquitous, they do not significantly contribute to the total mass.

The Solar Neighborhood is the laboratory we use to study low-mass stars and brown dwarfs. Because these objects are intrinsically low in temperature and very faint, we are only able to study them in detail when they are physically near to us. The objects in the Solar Neighborhood are the ones we know best and are the basis for the stellar luminosity function, the mass-luminosity relation, the stellar contribution to the local mass density, the velocity distribution, and the stellar multiplicity statistics. An accurate census of the Solar Neighborhood in all mass regimes is vital to many areas of astronomical study.

The distribution of stars and brown dwarfs in a typical slice of the Milky Way. Even though there are many more brown dwarfs, they do not constitute a significant fraction of the mass. This artwork was designed by Robert Hurt.

While brown dwarfs have been thought to exist for over fifty years, only recently have they actually been discovered. The past ten years has seen an revolution in our knowledge of the low-mass end of the HR diagram. The OBAFKM stellar classification system has been extended to include both type 'L' and type 'T' dwarfs. All T dwarfs are brown dwarfs, while M and L dwarfs are a mix of both low-mass stars and brown dwarfs.

Optical spectra of low-mass stars and brown dwarfs. Both molecular bands and atomic lines are present. The morphological change from type M through L is dominated by the pressure broadening of the K I line.

Projects

Completing the Census

We are searching for nearby late-type objects (M and L dwarfs) using various methods including follow-up of proper-motion surveys and culling the Two Micron All-Sky Survey (2MASS) database. This project is near completion and has more than doubled the number of L dwarfs known with 20 pc. Candidate nearby objects were targeted with NOAO facilities 2.1-m and 4-m telescopes on Kitt Peak and their sister 1.5-m and 4-m telescopes on Cerro Tololo. We are currently using IRTF on Mauna Kea and the Gemini telescopes to finish the faintest part of this sample to identify the coolest L dwarfs. This sample provides the basis for most of the project described below.

Mid-IR Properties of L Dwarfs with Spitzer

Using the Spitzer space telescope, we are obtaining spectra and photometry of the nearest L dwarfs. These data will constrain theoretical models of L dwarfs and provide insight on the role clouds and dust play in their atmospheres. In addition, we will be mapping out the mid-infrared properties of these objects.

Uncovering Companions

Using both HST, ground based adaptive-optics systems, and archival plate material, we are searching for companions to low-mass stars and brown dwarfs. Binary systems are essential to the study of how these objects form and evolve. Multiple systems provide an opportunity to make age and mass estimates, both crucial to the understanding of brown dwarfs.