University of Minnesota
School of Physics & Astronomy

Spotlight

Investigating the Brightest Stars

Dinesh Shenoy
Dinesh Shenoy at the MMT Observatory's telescope mirror
                                                       

Dinesh Shenoy is a graduate student in astrophysics working with Professor Terry Jones. He studies eruptions and outflows from very massive stars using telescopes with some of the largest primary mirrors in the world. Shenoy uses the 6.5-meter diameter MMT telescope on Mt. Hopkins, AZ (pictured) and the 8.4-meter diameter Large Binocular Telescope on Mt. Graham, AZ. These telescopes use the novel technique of “adaptive optics” to correct the optical distortions that starlight experiences when it passes through Earth’s turbulent atmosphere.

With adaptive optics, ground-based telescopes achieve spatial resolution rivaling that of the Hubble Space Telescope. In contrast to spaced-based missions, ground-based detectors can be regularly upgraded as new components become available.

Jones’ group has designed and developed cryogenically-cooled infrared detectors for each of these telescopes, with assembly and testing done here in Tate’s Infrared Lab. Shenoy uses these detectors’ infrared imaging and polarimetry capabilities to study mass loss from massive stars. As these stars eject gas, sub-micron sized "dust" particles condense in their winds. This dusty material is detectable at infrared wavelengths because it scatters the star’s infrared light. But detecting this faint infrared signature is complicated by the material’s proximity to the very bright star. Shenoy compares it to trying to see a firefly that is right next to a flood light. Polarimetry exploits the fact that in scattering off the dust, the star’s light becomes polarized. Although much fainter than the unpolarized star, since it is polarized the scattered light can be recovered through careful differencing techniques. With MMT-Pol, the group’s imaging polarimeter custom-built for the MMT Observatory, Shenoy has successfully mass loss material that previous observers have been incapable of detecting.

Shenoy hopes to help provide some answers about the way that massive stars behave. “We know how massive stars spend the majority of their Main Sequence life. And we know that massive stars end their life off the Main Sequence in supernova explosions. But prior to their death many experience violent, episodic mass loss. This affects their final mass when they die and may influence the morphology of the supernova explosion.” The episodic mass-loss phase is a short lived phase, mandating detailed study of the few nearby examples that astronomers have. Two objects he is studying now are VY Canis Majoris, a Red Hypergiant star and IRC + 10420, a Yellow Hypergiant star. Although unimpressive to the naked eye, these objects are some of the brightest infrared stellar sources in the sky. Shenoy says that the mass loss from these stars appears analogous to our Sun’s coronal mass ejections, but on a much larger scale. Eruptions lasting a few tens of years have been observed which expel as much as 10-4 times the mass of our Sun. One unexpected result in Shenoy’s research on IRC +10420 is that the ejected mass is much hotter than thermal equilibrium calculations suggest it should be. This finding presents a challenge to theoretical models for the dust created by massive star outflows.

In the next few years, Jones’ group will extend polarimetry studies to protoplanetary discs around nearby stars. A key outstanding problem of modern astrophysics is the understanding of the conditions in protoplanetary disks during the epoch of planetesimal formation and the aggregation of planets. “One possibility we are exploring is adding and element called a chronograph, which helps further separate scattered light from the star itself and enables study of the dust in the disk.” Infrared polarimetry will help distinguish between asteroid-like dust and comet-like dust, which will help with understanding how planets form.