Discovering Planets Around Higher Mass Stars
Introduction
Astronomers have discovered four hundred and twenty nine planets to date. Researchers discovered the first planet orbiting a normal star only fifteen years ago, but today statistical studies and new observations have allowed for a wealth of information to come forward. For example, correlations have appeared between a stellar properties and planet formation. But even with this wealth of information, these correlations are well studied over a limited range of stellar masses.
Astronomers have discovered most of the planets through the radial velocity (RV) method. According the Newton’s Third Law, each force is paired with an equal and opposite force on the other object. Therefore, as the star pulls the planet in an orbit, the planet actually moves on the star as well. Movements towards and away from us then cause Doppler shifting to the frequency of light emitted. The radial velocity can then be measured by spectroscopy to infer the presence of the planet.
The High Mass Problem
One problem to this highly successful method comes when considering higher mass stars. High rotational velocities and temperatures distort absorption features and make accurate radial velocity measurements difficult (Johnson 2007). Theoretical models predict higher percentages of planets around higher mass stars and available evidence supports this, but few data points exist for high mass stars. Therefore, to gain a more in depth model of planet formation and accurate statistics, larger mass stars must be studied.
Solution
To solve this problem, we can study stars that have just left the main sequence of stellar evolution and have stopped burning hydrogen. At this point, they cool, expand, and slow in rotation. The cooling and slowing in rotation make measurements much easier, but in return, evolved stars exhibit much more stellar jitter (pulsation and atmospheric turbulence), expand into the orbits of the most easily detected planets, and importantly have much less accurate mass estimates. Errors on mass can reach up to 1 M☉, solar mass, in the 1.5 M☉to 3 M☉ range (Lovis and Mayor) and make any modeling on the system very difficult.
The error comes from the technique that astronomers use to determine mass. The Hertzsprung-Russell diagram plots a star’s color (measured in the difference between 449nm light and 550 nm light named B-V) versus luminosity (Carroll and Ostlie p.75). Stars with a specific age and mass have the same location of the diagram. The picture in the right corner shows the possible positions for stars of the same age and a range of masses (data from CMD). The main sequence, hydrogen burning phase, is where all three lines overlap near the bottom of the plot. My mentor, Dr. Johnson, has used stars of masses up to 1.5 M☉at the point when they first leave the main sequence. As you can see in the diagram, the lines are well spread out and easy to read in this area. For stars in the 3 M☉to 1.5 M☉ range, they pass through that region in between 600 and 2,700 million years, too short a period to have a large enough sample of study (Carroll and Ostlie p.449). After that time period, stars enter an area called the “red clump”, so dubbed because stars of many different masses pass through the area multiple times and all possess the same red color. Small error in luminosity and B-V lead to huge error in mass.
Solving this mass difficulty, Lovis and Mayor used open clusters to help determine the mass of stars, and managed to find two new planets. All the stars in open clusters have similar ages, so each star in the cluster has an accurate age measurement. This eliminates the confusion between a lighter, older star and a younger, more massive star in the red clump.
My Work
This summer, I intend to expand on this technique. Lovis and Mayor used the Coralie and HARPS instruments in the southern hemisphere to view 115 stars in 13 open clusters. At Caltech we have access to the Keck telescope that could expand the observations into the northern hemisphere. In order to start this project, we need a list of objects to observe and all the necessary information compiled on them. It will be my job to formulate the list of objects and organize the information. To compile the list of objects, I will search through Alan’s Astrophysical Properties and the WEBDA database to determine clusters with the following properties:
· possess stars brighter than 13th magnitude
· exist in locations visible from Keck in summer and early fall
· possess stars with masses greater than 1.5 M☉that have evolved off the main sequence
· have accurate age measurements.
I will also need to figure which individual stars fall into the mass range and then assign each accurate mass estimates by fitting the H-R diagram for the cluster.
In addition to planning the survey, I will need to figure out the error and the feasibility of the study. A pilot project will be made using five to ten stars to be observed this summer. I will accompany the team to figure out how RV measurements are made and use that data to estimate the error from pulsation, jitter, and unclear spectral lines. Throughout the summer we will take the data, and at the end of the summer I will analyze it for feasibility of a large scale study.
Starting this summer and continuing over the next several years, Dr. Johnson will view these objects periodically using Keck’s High Resolution Echelle Spectrometer. At the end of the time period, the data will be analyzed to look for the presence of a planet. Current information and trends say that 9% of stars in the 1.5 M☉to 2.5 M☉ range will possess a planet (Johnson 2007). If I find over 100 prospective stars and this percentage holds true, the set will have a high probability of finding several planetary systems, and hopefully such discoveries will inform our understanding of planet formation.
Bibliography
Carroll, Bradley W. and Ostlie, Dale A. An Introduction to Modern Astrophysics. Second Edition. Pearson. 2007
C. Lovis and M. Mayor. “Planets around evolved intermediate-mass stars”. Astronomy and Astrophysics June 2007
CMD Version 2.2. http://stev.oapd.inaf.it/cgi-bin/cmd_2.2
Johnson, J. “International Year of Astronomy Incited Review on Exoplanets”. Draft version March 17, 2009
Johnson et all. “A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass”. The Astrophysical Journal. November 2007
