Stars

Maintained by WW

Low-Mass Stars

Dan Huber's research group at IfA Manoa focuses on the structure and evolution of solar-type and low-mass stars using observational techniques such as asteroseismology, optical long-baseline interferometry, spectroscopy and broadband photometry, as well as the discovery and characterization of exoplanets. He uses data from space-based and ground-based telescopes such as Kepler, K2, TESS, Keck, Subaru, the CHARA Array, as well as various ground-based photometric and spectroscopic surveys.

The image shows Kepler light curves of four stars in different evolutionary states. By measuring the frequencies and amplitudes of the oscillations in these light curves, we can constrain the sizes, masses and interior structure of these stars, and characterize exoplanets that orbit them.

Planetary Nebulae

Roberto Mendez studies planetary nebulae in our galaxy (the Milky Way) and in other galaxies. Studies in our galaxy are focused on the central stars. How many of these are currently binary systems? Can we gain direct information about the stellar masses? What are their chemical abundances, and how do they compare to the abundances measured in the ionized gas around them? The answers to all these questions require many high-resolution stellar spectrograms, which in turn require a lot of observing time with large telescopes like those on Maunakea.

The planetary nebula NGC6543.
Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

Spinning Stars

Jennifer van Saders focuses of the interiors and evolution of stars, with a particular emphasis on the inference of stellar ages and stellar rotation. All stars are born rotating; over time, Sun-like stars lose angular momentum via magnetized stellar winds and spin down, providing a rotational clock. These rotation-based ages fits into a tapestry of other age inference techniques (isochrone ages, kinematic ages, chemical ages, and age inference from asteroseismology) that allow us to put a timeline to astrophysical systems. The physics underpinning these dating techniques has deep ties to the evolution of stellar structure and magnetism with time, and provides a window into uncertain physical processes in stellar interiors.

Models of Milky Way stellar populations are coupled with with prescriptions for stellar spin-down (shaded colors) to reproduce the observed distribution of ~34,000 stars with measured rotation periods from Kepler (contours). The tuning required to make the model distribution match the observed distribution provides insight into the magnetic behavior of stars are they age.

Light Elements in Stars

Ann Merchant Boesgaard uses high-resolution spectrographs on several telescopes, but primarily on the Keck I 10-meter telescope, to investigate the evolution and structure of stars. Of particular interest to her are the abundances of the rare light elements – lithium, beryllium, and boron – in stellar atmospheres. Those three elements are readily destroyed in stellar interiors by nuclear fusion reactions, yet some amounts of those elements are found on the surfaces of stars where their abundances can be measured. This provides very valuable information on processes occurring in the unobservable interior regions of stars, including internal rotation and mixing.

The images on the right shows lithium and beryllium surface abundances in stars of different surface temperatures in the comparatively young Hyades cluster. Stars with surface temperatures of around 6700 K show the greatest depletion of these elements, especially lithium, as a result of their being transported down into the inner regions of the star, where they are liable to be destroyed by nuclear fusion reactions at about 2½ million K. The differing depletions of lithium, beryllium and boron provide information about conditions at different depths inside the stars.

This work also has cosmological implications since the abundance of Li in the oldest stars helps to provide information on the conditions in the 15 minutes after the Big Bang, more than 14 billion years ago.

IRTF Extended Spectral Library

John Rayner is working with Alexa Villaume (UC Santa Cruz) and Charlie Conroy (CfA, Harvard) on extending the IRTF Cool Stars spectral library to include a wider range of stellar metallicities. The new IRTF Extended Spectral Library contains 207 stars with metallicities in the range -1.7 <[Fe/H] < 0.3 observed at a resolving power of 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, and covering the wavelength range 0.7-2.5 microns. The stars are chosen from the from the medium-resolution Isaac Newton Telescope library of empirical spectra (MILES) that covers optical wavelengths. The combined spectra cover the full optical to near-infrared wavelength range (0.4-2.5 microns),

The purpose of the IRTF Extended Spectral Library is to study the unresolved stellar populations in galaxies by synthesizing the spectra of stars to match observed (unresolved) spectra of galaxies at different evolutionary epochs (red shifts), and in particular the optically obscured regions of galaxies.

Examples of infrared spectra of stars with
different surface temperatures