Science

Keck-Observatory.HatchThe Cool Star Lab focuses on observational astrophysical research of the lowest temperature and lowest mass stars in the Galaxy.  This category spans dwarf stars that look more like giant planets, to humungous giant stars at the end of their lives. Our research techniques include optical and infrared spectroscopy, high resolution and high contrast imaging, astrometry, radio interferometry, X-ray detection, ground- and space-based observatories, large survey mining, radial velocity analysis, time series analysis, spectral model fitting, and population synthesis modeling.

Here are a few of the key science areas we explore:

 

 

The Galaxy’s lowest mass stars and brown dwarfs

The vast majority (>70%) of stars in our Galaxy are cool, low luminosity sources less than 10% the mass of the Sun. These include objects that are so low mass that they are incapable of fusing hydrogen in their cores: brown dwarfs. The Cool Star Lab investigates the physical and statistical properties of low mass stars and brown dwarfs, primarily through investigations of their cool atmospheres using optical and infrared spectroscopy.

star_ejectMagnetic Activity: Low mass stars are considerably more active than the Sun, yet at the M/L transition optical and X-ray emission disappears.  Radio emission, on the other hand, persists despite the well-known correlation with X-ray luminosity for all other types of star.  We investigate this mysterious emission using the ATCA, VLA and VLBA facilities to understand what conditions lead to activity and what the magnetic field properties of these objects are.  We also investigate a rare class of “hyperactive” dwarfs that have strong and persistent magnetic emission, and remarkably appear to be relatively old.

sch_rzHalo brown dwarfs: Could brown dwarfs form in the metal-poor conditions of our early Galaxy? The answer appears to be yes, based on the discovery of metal-poor L subdwarfs by Lab PI Burgasser in 2003. Our research has revealed metallicity effects on cloud formation and temperature scales, and we have developed a parallel classification scheme for substellar subdwarfs. Work is ongoing to search for even cooler T subdwarfs and to measure the halo brown dwarf mass function.

screen-shot-2010-12-12-at-7-46-57-amYoung stars in the Solar Neighborhood: Cool Star Lab members use astrometry and radial velocities to identify and/or assign young stars in nearby moving groups. We also perform detailed studies of nearby examples, such as TWA 30AB, a nearby pair of 8-12 Myr low-mass stars in which both components are heavily obscured by circumstellar disks and exhibit jet emission.

 

 

The coolest of the cool: L, T and Y dwarfs

The spectral letter sequence of stars – O B A F G K M – has been used by astronomers since the 1940s to classify the heavens. Now we have discovered three new spectral types – the L, T and Y dwarfs – that are as cold as 300 K, and whose atmospheres harbor complex molecules and condensate clouds. In many ways, they behave more like lost giant planets. The Cool Star Lab studies varies observational aspects of these new and exciting objects to understand their origins, dissect their atmospheres, and probe the limits of star formation in the Galaxy.

This artist's conception illustrates the brown dwarf named 2MASSJ22282889-431026.Clouds in L dwarfs: The most fascinating property the L dwarfs is the presence of mineral and metal condensates in their atmospheres – clouds of dirt.  Their existence raises many questions about the climate of cool atmospheres: How do these clouds form? What is their vertical structure? How does surface gravity, metallicity or rotation influence that structure or cloud composition? What is their surface structure?  Does this structure change with time? The Cool Star Lab investigates these questions through spectroscopic analysis and modeling, time variability studies and near-infrared polarization measurements.

T-dwarf-nasa-hurtT dwarfs: These objects were the focus of PI Adam Burgasser’s PhD thesis, and the Cool Star Lab continues to study the atmospheres and kinematic properties of these methane-bearing dwarfs. Spectral model fitting techniques developed in the Lab have enabled physical characterization of individual T dwarfs to test evolutionary models, and the detection of a newly identified low temperature sulfide cloud layer in their atmospheres. T dwarf radial and rotational velocity measurements, made possible with the Magellan/FIRE instrument, are proving useful for population age determinations and studies of angular momentum evolution.

Screen shot 2013-08-22 at 1.54.27 AMThe L/T transition: The transition between the L and T classes is remarkable, with dramatic changes in spectra driven by the emergence of methane gas and the loss of mineral clouds. This transition appears to happen “suddenly”, based on the “J-band brightening” observed among early T dwarfs and a bump in multiplicity. Cool Star Lab members have studied this transition to investigate cloud disruption and atmospheric dynamics, gravity and metallicity effects on gas chemistry (relevant for exoplanet atmospheres), and brown dwarf multiplicity.

nasa-imageY dwarfs: Cool Star Lab members have contributed to the discovery of the most recent and coldest spectral class identified, the Y dwarfs. Using Magellan/FIRE, we have provided ground-based spectral characterization of candidates uncovered with WISE, and contributed to the discovery of WD 0806-661, the coldest directly imaged companion to a nearby star with an effective temperature of 300-345 K.

 

 

Stellar and substellar multiplicity

Like stars, brown dwarfs sometimes come in sets, and their pairing statistics – multiplicity rate, mass ratio distribution, separation distribution, and other orbital characteristics – are critical empirical constraints for brown dwarf formation models.

Screen shot 2013-08-22 at 2.02.36 AMResolved imaging with Adaptive Optics: The Keck Laser Guide Star Adaptive Optics system allows us to discover and characterize cold companions to brown dwarfs at separations <0.05″.  We also exploit the stability of space-based platforms (e.g., HST) and custom analysis techniques to study the resolved brown dwarf binary population.

Screen shot 2013-08-22 at 2.06.39 AMSpectral binaries: The Cool Star Lab developed the technique of finding binaries straddling the L/T transition as spectral (blended light) pairs.  This separation-independent technique has allowed the detection of some of the most closely-separated binaries, as well as systems with extreme brightness ratios.  We continue to search for these systems using our own SpeX Prism Spectral Library.

Screen shot 2013-08-22 at 2.11.45 AMSpectroscopy of resolved binary benchmarks: Using advanced AO technology – and occasionally just really good seeing – members of the Cool Star Lab have investigated resolved spectra of brown dwarf binaries as experimental benchmarks – pairs of sources at the same distance, with the same age and made of the same stuff.  These studies have allowed tight constraints on physical properties, examination of magnetic activity trends independent of age effects, and stringent tests of brown dwarf evolutionary models.

 

 

The Birth, Life, Death and Rebirth of Planetary Systems

Planets are formed, they exist, and they die, often at the whims of their host star.  And sometimes, like Phoenix rising from the ashes, planets can be reborn.  Dr. Carl Melis in the Cool Star Lab investigates the full cycle of the planetary life (and afterlife) cycle, using radio telescopes to study dusty disks from which planets are formed; mid infrared telescopes to study the evolution of these disks and the dust created from catastrophic collisions between terrestrial planets; high resolution spectroscopy of stellar remnants to measure the compositions of planets torn apart by tidal forces; and searches for disks around giant stars that signal the development of a second generation planetary system.