H{sub 2} Fluorescence in M Dwarf Systems: A Stellar Origin

Journal Article · Thu Aug 10 04:00:00 EDT 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA7B79· OSTI ID:22875950

Kruczek, Nicholas; France, Kevin; Loyd, R. O. Parke; Youngblood, Allison; Fleming, Brian; Hoadley, Keri ^[1]; Evonosky, William ^[2]; Roberge, Aki ^[3]; Wittenmyer, Robert A. ^[4]

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1. Laboratory for Atmospheric and Space Physics, University of Colorado, 600 UCB, Boulder, CO 80309 (United States)
2. Department of Physics, University of South Florida, Tampa, FL 33620 (United States)
3. Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
4. University of Southern Queensland, Computational Engineering and Science Research Centre, Toowoomba, QLD 4350 (Australia)

Observations of molecular hydrogen (H{sub 2}) fluorescence are a potentially useful tool for measuring the H{sub 2} abundance in exoplanet atmospheres. This emission was previously observed in M dwarfs with planetary systems. However, low signal-to-noise prevented a conclusive determination of its origin. Possible sources include exoplanetary atmospheres, circumstellar gas disks, and the stellar surface. We use observations from the “Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanet Host Stars” Treasury Survey to study H{sub 2} fluorescence in M dwarfs. We detect fluorescence in Hubble Space Telescope spectra of 8/9 planet-hosting and 5/6 non-planet-hosting M dwarfs. The detection statistics, velocity centroids, and line widths of the emission suggest a stellar origin. We calculate H{sub 2}-to-stellar-ion flux ratios to compare flux levels between stars. For stars with planets, we find an average ratio of 1.7±0.9, using the fluxes of the brightest H{sub 2} feature and two stellar C iv lines. This is compared to 0.9±0.4 for stars without planets, showing that the planet-hosting M dwarfs do not have significant excess H{sub 2} emission. This claim is supported by the direct FUV imaging of GJ 832, where no fluorescence is observed at the expected star–planet separation. Additionally, the 3σ upper limit of 4.9 × 10{sup −17} erg cm{sup −2} s{sup −1} from these observations is two orders of magnitude below the spectroscopically observed H{sub 2} flux. We constrain the location of the fluorescing H{sub 2} using 1D radiative transfer models, and find that it could reside in starspots or a ∼2500–3000 K region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H{sub 2} in exoplanets orbiting M dwarfs.

• OSTI ID: 22875950
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 845; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CHROMOSPHERE
COMPARATIVE EVALUATIONS
DETECTION
ELEMENT ABUNDANCE
FLUORESCENCE
HYDROGEN
LINE WIDTHS
MASS
PLANETS
RADIANT HEAT TRANSFER
SPACE
SPECTRA
STARS
STARSPOTS
STATISTICS
TELESCOPES
ULTRAVIOLET RADIATION
VELOCITY