Detecting Transits of Planetary Companions to Giant Stars

Abstract

Of the approximately 350 extrasolar planets currently known, of order 10% orbit evolved stars with radii R >~ 2.5 R_sun. These planets are of particular interest because they tend to orbit more massive hosts, and have been subjected to variable stellar insolation over their recent histories as their primaries evolved off the main sequence. Unfortunately, we have limited information about the physical properties of these planets, as they were all detected by the radial velocity method and none have been observed to transit. Here we evaluate the prospects for detecting transits of planetary companions to giant stars. We show that several of the known systems have a priori transit probabilities of >~ 10%, and about one transiting system is expected for the sample of host stars with R >= 2.5 R_sun. Although the transits are expected to have very small amplitudes (~few x 10^-4) and long durations (>~ 50 hrs), we argue that the difficulty with detecting these signals in broadband light is one of systematic errors and practicality rather than photon noise, even for modest aperture ~1m telescopes. We propose a novel method that may overcome these difficulties, which uses narrow-band measurements to isolate the thin ring of chromospheric emission expected at the limb of giant stars. The transit signals in these narrow bands are expected to be larger in magnitude and briefer in duration than in broad-band emission, and thus alleviating many of the difficulties with transit detection in broad-band emission. Finally, we point out that it may be possible to discover planetary companions to giant stars using Kepler, provided that a sufficient number of such targets are monitored.

34 pages, 10 figures, modified to include comments made by the referee; Accepted for publication in ApJ

References in corpus (24)

• Direct Imaging of Multiple Planets Orbiting the Star HR 8799
• Optical Images of an Exosolar Planet 25 Light Years from Earth
• The effect of red noise on planetary transit detection
• Improved parameters for extrasolar transiting planets
• Planet formation around stars of various masses: The snow line and the frequency of giant planets
• TrES-1: The Transiting Planet of a Bright K0V Star
• Sodium Absorption From the Exoplanetary Atmosphere of HD189733b Detected in the Optical Transmission Spectrum
• A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass
• Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants
• TW Hydrae: evidence of stellar spots instead of a Hot Jupiter
• Atmospheric Circulation of Hot Jupiters: Three-dimensional circulation models of HD 209458b and HD 189733b with Simplified Forcing
• Long-term tidal evolution of short-period planets with companions
• Tidal Heating of Extra-Solar Planets
• Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars
• Five Intermediate-Period Planets from the N2K Sample
• The Transit Light Curve Project. III. Tres Transits of TrES-1
• Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Del, xi Aql, and HD 81688
• A Smaller Radius for the Transiting Exoplanet WASP-10b
• CHARA Array Measurements of the Angular Diameters of Exoplanet Host Stars
• The Transit Light Curve Project. XI. Submillimagnitude Photometry of Two Transits of the Bloated Planet WASP-4b
• HD 17156b: A Transiting Planet with a 21.2 Day Period and an Eccentric Orbit
• A Planet in a 0.6-AU Orbit Around the K0 Giant HD 102272
• Predicting the Yields of Photometric Surveys for Transiting Extrasolar Planets
• Characterizing Long-Period Transiting Planets Observed by Kepler

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• The Orbital Evolution of Gas Giant Planets around Giant Stars
• Transit Timing Observations from Kepler: VII. Confirmation of 27 planets in 13 multiplanet systems via Transit Timing Variations and orbital stability
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• Re-inflated Warm Jupiters Around Red Giants
• KELT-3b: A Hot Jupiter Transiting a V=9.8 Late-F Star
• KELT-8b: A highly inflated transiting hot Jupiter and a new technique for extracting high-precision radial velocities from noisy spectra
• Exo-Planetary Transits of Limb Brightened Lines; Tentative Si IV Absorption by HD209458b
• A-type Stars, the Destroyers of Worlds: The lives and deaths of Jupiters in evolving stellar binaries
• A Posteriori Transit Probabilities
• KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V=8 Subgiant HD 93396
• KELT-10b: The First Transiting Exoplanet from the KELT-South Survey -- A Hot Sub-Jupiter Transiting a V = 10.7 Early G-Star
• KELT-6b: A P~7.9 d Hot Saturn Transiting a Metal-Poor Star with a Long-Period Companion
• KELT-4Ab: An inflated Hot Jupiter transiting the bright (V~10) component of a hierarchical triple
• TESS Delivers Five New Hot Giant Planets Orbiting Bright Stars from the Full Frame Images
• KELT-14b and KELT-15b: An Independent Discovery of WASP-122b and a New Hot Jupiter
• Two warm, low-density sub-Jovian planets orbiting bright stars in K2 campaigns 13 and 14
• KELT-12b: A $P\sim 5$ Day, Highly Inflated Hot Jupiter Transiting a Mildly Evolved Hot Star
• Cherenkov Telescopes as Optical Telescopes for Bright Sources: Today's Specialised Thirty Metre Telescopes?
• On the Transit Potential of the Planet Orbiting iota Draconis
• Non-radial oscillations mimicking a brown dwarf orbiting the cluster giant NGC 4349 No. 127
• Residual eccentricity of an Earth-like planet orbiting a red giant Sun
• Precise radial velocities of giant stars -- XVII. Distinguishing planets from intrinsically induced radial velocity signals in evolved stars
• Asteroseismology of iota Draconis and Discovery of an Additional Long-Period Companion