For his second-year project, graduate student Jackson Norris studied spectra taken with the High Resolution Spectrograph at Hobby-Eberly Telescope of the hot subdwarf star HD 149382.
Hot subdwarfs are a mysterious, rare class of very hot star (spectral type O or B). A hot subdwarf is a helium burning star, meaning that it has already exhausted its supply of hydrogen and has left the Main Sequence. On a cosmic timescale, it will soon run out of helium and become a white dwarf.
Normally, stars in this state of their lives are giant stars, but hot subdwarfs seem to have had their outer envelope stripped away, exposing only their hot, underlying material. This makes them hotter, smaller, and much less massive than the usual helium-burning giant star. They are called “subdwarfs” because they are less luminous than hydrogen burning stars of the same temperature (because those stars have significantly more mass, they are brighter and larger. A “dwarf star” is just a hydrogen burning star.).
The mechanism for this stripping is unknown, since at least some hot subdwarfs appear to be single stars (normally, when a star is missing its outer envelope it is because a close binary companion has gravitationally stolen it away).
There is some theoretical speculation that close-in giant planets could be the responsible parties for this stripping. Planets, being much less massive than stars, could have escaped easy notice, but some studies suggested that they could still have enough mass (and orbital energy) to have stripped away the outer layers of a giant star, creating a hot subdwarf.
In 2009, Geier et al. published a paper entitled: “Discovery of a Close Substellar Companion to the Hot Subdwarf Star HD 149382–The Decisive Influence of Substellar Objects on Late Stellar Evolution” They claimed to have detected a massive planet, 8 to 32 times the mass of Jupiter, orbiting the brightest hot subdwarf in the sky, HD 149382 (following convention, they named the planet HD 149382b). This discovery, they claimed, proved the theory that close-in planets are the mechanism for the creation of hot subdwarf stars.
We were skeptical that the observations Geier et al. had made really supported their conclusion that the star was orbited by a planet. So a team of us, including Prof. Suvrath Mahadevan and Prof. Richard Wade, applied for time on the Hobby Eberly Telescope to make our own observations of the star. The spectra were reduced by PSU graduate student Sara Gettel and analyzed by Jackson Norris. Mr. Norris determined that the star is not, in fact, orbited by such a planet, and that we can in fact rule out almost any short-period planet orbiting the star down to a mass of less than that of Jupiter.
This means that the origin of HD 149382 is still quite a mystery.
The preprint of Mr. Norris’s paper is here:
It has been accepted for publication in the Astrophysical Journal and should appear in the Nov. 20 issue.
Big news here at Penn State: we have been selected by the NSF to build a new instrument capable of finding rocky planets orbiting stars at just the right distance to harbor liquid surface water! Press release is here.