Ground-based Spectroscopy of the Exoplanet XO-2b Using a Systematic Wavelength Calibration

Kyle A. Pearson, Caitlin Griffith, Robert T. Zellem, Tommi T. Koskinen, Gael M. Roudier

Research output: Contribution to journalArticle

Abstract

Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our solar system. These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. However, most ground-based transit spectra do not systematically resolve the wings of the features and continuum, as needed to constrain the alkali abundances. Here we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth's atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across five wavelength bins at a resolution of ∼1.6 nm and limit the abundance of Na. We obtain consistent results from each telescope with an Na amplitude of 521 ± 161 and 403 ± 186 ppm for GMOS and KOSMOS, respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between ∼1 and 100 mbar that establishes a lower limit on Na at ppm ([Na/H] = ), consistent with solar. However, we cannot rule out the presence of clouds at ∼10 mbar that allow for higher Na abundances, which would be consistent with the stellar metallicity measured for the host star ([Na/H] = 0.485 ± 0.043).

Original languageEnglish (US)
Article number21
JournalAstronomical Journal
Volume157
Issue number1
DOIs
StatePublished - Jan 1 2019

Fingerprint

extrasolar planets
transit
reference stars
spectroscopy
calibration
wavelength
stars
wings
ionospheres
ionosphere
wavelengths
atmospheres
atmospheric effects
alkali metal
atmosphere
refraction
systematic errors
solar system
alkali metals
cross correlation

Keywords

  • methods: analytical
  • methods: data analysis
  • methods: observational
  • planets and satellites: atmospheres
  • planets and satellites: individual (XO-2b)
  • techniques: spectroscopic

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Ground-based Spectroscopy of the Exoplanet XO-2b Using a Systematic Wavelength Calibration. / Pearson, Kyle A.; Griffith, Caitlin; Zellem, Robert T.; Koskinen, Tommi T.; Roudier, Gael M.

In: Astronomical Journal, Vol. 157, No. 1, 21, 01.01.2019.

Research output: Contribution to journalArticle

Pearson, Kyle A. ; Griffith, Caitlin ; Zellem, Robert T. ; Koskinen, Tommi T. ; Roudier, Gael M. / Ground-based Spectroscopy of the Exoplanet XO-2b Using a Systematic Wavelength Calibration. In: Astronomical Journal. 2019 ; Vol. 157, No. 1.
@article{27f4a4696cc84d7faef3a3c98c7b888d,
title = "Ground-based Spectroscopy of the Exoplanet XO-2b Using a Systematic Wavelength Calibration",
abstract = "Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our solar system. These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. However, most ground-based transit spectra do not systematically resolve the wings of the features and continuum, as needed to constrain the alkali abundances. Here we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth's atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across five wavelength bins at a resolution of ∼1.6 nm and limit the abundance of Na. We obtain consistent results from each telescope with an Na amplitude of 521 ± 161 and 403 ± 186 ppm for GMOS and KOSMOS, respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between ∼1 and 100 mbar that establishes a lower limit on Na at ppm ([Na/H] = ), consistent with solar. However, we cannot rule out the presence of clouds at ∼10 mbar that allow for higher Na abundances, which would be consistent with the stellar metallicity measured for the host star ([Na/H] = 0.485 ± 0.043).",
keywords = "methods: analytical, methods: data analysis, methods: observational, planets and satellites: atmospheres, planets and satellites: individual (XO-2b), techniques: spectroscopic",
author = "Pearson, {Kyle A.} and Caitlin Griffith and Zellem, {Robert T.} and Koskinen, {Tommi T.} and Roudier, {Gael M.}",
year = "2019",
month = "1",
day = "1",
doi = "10.3847/1538-3881/aaf1ae",
language = "English (US)",
volume = "157",
journal = "Astronomical Journal",
issn = "0004-6256",
publisher = "IOP Publishing Ltd.",
number = "1",

}

TY - JOUR

T1 - Ground-based Spectroscopy of the Exoplanet XO-2b Using a Systematic Wavelength Calibration

AU - Pearson, Kyle A.

AU - Griffith, Caitlin

AU - Zellem, Robert T.

AU - Koskinen, Tommi T.

AU - Roudier, Gael M.

PY - 2019/1/1

Y1 - 2019/1/1

N2 - Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our solar system. These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. However, most ground-based transit spectra do not systematically resolve the wings of the features and continuum, as needed to constrain the alkali abundances. Here we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth's atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across five wavelength bins at a resolution of ∼1.6 nm and limit the abundance of Na. We obtain consistent results from each telescope with an Na amplitude of 521 ± 161 and 403 ± 186 ppm for GMOS and KOSMOS, respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between ∼1 and 100 mbar that establishes a lower limit on Na at ppm ([Na/H] = ), consistent with solar. However, we cannot rule out the presence of clouds at ∼10 mbar that allow for higher Na abundances, which would be consistent with the stellar metallicity measured for the host star ([Na/H] = 0.485 ± 0.043).

AB - Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our solar system. These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. However, most ground-based transit spectra do not systematically resolve the wings of the features and continuum, as needed to constrain the alkali abundances. Here we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth's atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across five wavelength bins at a resolution of ∼1.6 nm and limit the abundance of Na. We obtain consistent results from each telescope with an Na amplitude of 521 ± 161 and 403 ± 186 ppm for GMOS and KOSMOS, respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between ∼1 and 100 mbar that establishes a lower limit on Na at ppm ([Na/H] = ), consistent with solar. However, we cannot rule out the presence of clouds at ∼10 mbar that allow for higher Na abundances, which would be consistent with the stellar metallicity measured for the host star ([Na/H] = 0.485 ± 0.043).

KW - methods: analytical

KW - methods: data analysis

KW - methods: observational

KW - planets and satellites: atmospheres

KW - planets and satellites: individual (XO-2b)

KW - techniques: spectroscopic

UR - http://www.scopus.com/inward/record.url?scp=85060183000&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85060183000&partnerID=8YFLogxK

U2 - 10.3847/1538-3881/aaf1ae

DO - 10.3847/1538-3881/aaf1ae

M3 - Article

VL - 157

JO - Astronomical Journal

JF - Astronomical Journal

SN - 0004-6256

IS - 1

M1 - 21

ER -