Three New Small Stars

The Big Picture

Although astronomers have excellent models for stars like the Sun, there is still plenty of room for improvement when modeling low-mass stars and brown dwarfs, objects that aren’t quite massive enough to fuse hydrogen into helium. Unlike Sun-like stars, low-mass stars and brown dwarfs are cool enough that molecular chemistry and cloud formation need to be considered when determining their temperatures and radii. Astronomers are currently working hard to develop sophisticated models that incorporate low-temperature chemistry, but testing the models is challenging because there aren’t many low-mass objects with well-established radii, masses, temperatures, and metallicities.

One way to test the models is to observe low-mass stars and brown dwarfs in binaries with Sun-like stars. Models for Sun-like stars are sufficiently advanced that astronomers can reliably determine their metallicities and masses. Since stars in binaries formed at the same time from the same material, they should have the same metallicity. Accordingly, astronomers can infer the metallicity of the low-mass star in a binary with a more massive star from the metallicity of the higher-mass companion. Astronomers can also determine the luminosity of the companion by estimating the distance to the primary star and using the distance modulus.

Paper Summary

Figure 1 from Crepp et al. 2012

Radial velocities for the three target stars: HD53665 (top), HD68017 (middle), and HD71881 (bottom). Figure 1 from Crepp et al. 2012.

The TRENDS (TaRgetting bENchmark-objects with Doppler Spectrocopy) survey aims to find low-mass companions orbiting well-characterized stars so that we can improve our understanding of low-mass objects. In this paper, the authors present three new low-mass stars in binary systems with Sun-like stars. They have been monitoring the Sun-like stars for over a decade and noticed that all three of them displayed long-term radial velocity trends (see the figure to the right), hence the somewhat contrived name of the survey. The authors determined the parameters of the target stars from high-resolution spectroscopy and then confirmed the presence of their lower-mass companions by acquiring high-resolution images of the stars.

Crepp et al. first observed the companions in February 2011 and then returned to the system in January 2012 to ensure that the companions were actually bound to the target stars and were not merely background sources. Since all three companions exhibited the same proper motion (movement relative to background stars) as the corresponding target star, the authors argue that the companions are indeed in binaries with the target stars.

They also present infrared photometry for the low-mass companions and show that the lower limits for the companion masses derived from the radial velocity curves are consistent with the masses determined by fitting evolutionary and empirical models to the data. This is good news for astronomers modeling low-mass stars, but the lower limits on the masses are currently much lower than the modeled masses. Future observations and a longer time baseline will allow the authors to determine the three-dimensional orbits of the binaries (instead of just the projection) and refine the mass estimates of the companions so that they can more rigorously test models of low-mass stars.

More Details about the Data

After establishing the presence of long-term radial velocity trends for all three target stars, the authors took high-resolution images of the stars using the Keck telescope on Mauna Kea in Hawaii. The authors used the adaptive optics system to reduce the distortions caused by turbulence in the atmosphere. The images (below) have plate scales of only 9.963 milliarcseconds per pixel and reveal the presence of a companion next to all three stars. The companions to HD 53665 (left) and HD 71881 (right) are 1.4 arcseconds (103 AU) and 0.8 arcseconds (35 AU) from the target star, respectively, so they are easy to spot in the figure. The companion to HD 68017 (center) is only 0.6 arcseconds (13 AU) from the primary, so it is almost buried in the speckles surrounding the target star.

Figure 2 from Crepp et al. 2012

Discovery images for the three low-mass companions. Figure 2 from Crepp et al. 2012.

The authors estimate the masses of the three companions as 0.65 solar masses (HD53665B), 0.16 solar masses (HD68017B), and 0.31 solar masses (HD71881B). In addition to hinting at the possibility for spectroscopic follow-up of the companions, the authors mention that continued observations of the binaries will allow them to estimate the dynamical masses of the companions instead of only lower limits. The combination of spectra and dynamical masses for these low-mass stars would be very useful for modelers and we should look out for another exciting paper about these binaries in the coming years.

About Courtney Dressing

I am a fourth-year graduate student in the Astronomy Department at Harvard University. My research interests include exoplanets, habitability, and astrobiology. I received a master's degree in astronomy and astrophysics from Harvard University and a bachelor's degree in astrophysical sciences from Princeton University. At Princeton, I worked with Jill Knapp to study the magnetic activity of M dwarfs with white dwarf companions and with Dave Spiegel to model the habitability of terrestrial exoplanets. For my senior thesis, I worked with Ed Turner, Michael McElwain, and the SEEDS (Strategic Explorations of Exoplanets and Disks with Subaru) collaboration to directly image young Jovian exoplanets using the Subaru telescope. At Harvard, I am working with Dave Charbonneau to study the properties, frequency, and detectability of small planets orbiting small stars.

Leave a Reply