J/AJ/157/21 Ground-based spectroscopy of the exoplanet XO-2b (Pearson+, 2019)
Ground-based spectroscopy of the exoplanet XO-2b using a systematic wavelength
calibration.
Pearson K.A., Griffith C.A., Zellem R.T., Koskinen T.T., Roudier G.M.
<Astron. J., 157, 21-21 (2019)>
=2019AJ....157...21P 2019AJ....157...21P (SIMBAD/NED BibCode)
ADC_Keywords: Exoplanets ; Spectra, optical ; Stars, double and multiple ;
Stars, diameters
Keywords: planets and satellites: atmospheres - methods: analytical -
methods: data analysis - methods: observational -
planets and satellites: individual XO-2b - techniques: spectroscopic
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
0.4-0.3+2 ppm ([Na/H]=-0.64-0.6+0.78), 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).
Description:
Spectroscopic measurements of XO-2b were recorded with GMOS-N on 2016
January 5 at the Gemini 8.1 m Observatory on Maunakea in Hawaii
(Allington-Smith et al. 2002PASP..114..892A 2002PASP..114..892A). The spectrograph is equipped
with an e2v deep-depletion (DD) array of three CCDs that make a 6144x4608
pixel sensor with a plate scale of 0.0727"/pixel. The three CCDs are
stitched together to create a large detector with an ∼20 pixel gap between
each CCD. To reduce the read-out time to 12 s, we implemented 2x2 binning
and read out a subarray centered only on XO-2N and XO-2S. Observations
were conducted with the B600 grating (R∼1600) centered on a wavelength
of 490 nm. Clocks were synchronized with a GPS every few seconds to ensure
accurate timing. Seeing during the observations ranged from 0.64" to 1.07".
The cadence of our observations was 102.5 s. Observations began 1 hr prior
to the transit ingress and ended 1 hr after egress in order to characterize
the out-of-transit baseline.
We obtained one transit observation of XO-2b with the 4 m Mayall Telescope
on Kitt Peak in Arizona equipped with the multiobject visible-band
spectrograph KOSMOS on 2015 February 7. The spectrograph is equipped with a
2 kx4 k e2v DD CCD with a binned plate scale of 0.292"/pixel (Martini et al.
2014SPIE.9147E..0ZM). To reduce the read-out time to 19 s, the CCD was
binned 2x2. We used the blue VPH grism, which has a wavelength range of
370-620 nm and R=2100 with peak transmission of ∼0.4 near 500 nm. Clocks
were synchronized with a GPS every few seconds to ensure accurate timing.
Throughout the night, the maximum shift in the centroid along the spatial
axis on the detector of our target was less than 1 pixel due to adequate
autoguiding. Seeing in the observations ranged from 0.81" to 1.34"
throughout the observations.
Objects:
------------------------------------------------------------
RA (ICRS) DE Designation(s) (Period)
------------------------------------------------------------
07 48 06.47 +50 13 32.9 XO-2b = XO-2b (P=2.61586178)
------------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 69 60 GMOS final model parameters
table3.dat 69 59 KOSMOS final model parameters (I)
table4.dat 69 27 KOSMOS final model parameters (II)
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See also:
J/AJ/137/4911 : Six transits of the exoplanet XO-2b (Fernandez+, 2009)
J/ApJ/768/L12 : Abundances in host stars XO-2S and XO-2N (Teske+, 2013)
J/ApJ/770/36 : APOSTLE transits of XO-2 system (Kundurthy+, 2013)
J/A+A/575/A111 : GAPS V: Global analysis of the XO-2 system (Damasso+, 2015)
J/A+A/583/A135 : XO-2N and XO-2S spectra (Biazzo+, 2015)
J/ApJ/801/L10 : XO-2N and XO-2S equivalent widths (Teske+, 2015)
J/ApJ/808/13 : XO-2S and XO-2N chemical composition (Ramirez+, 2015)
Byte-by-byte Description of file: table2.dat table3.dat table4.dat
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Bytes Format Units Label Explanations
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1- 4 I4 0.1nm Wave1 [3724/6550] First value of wavelength range (Å)
6- 9 I4 0.1nm Wave2 [3956/6566] Second value of wavelength range
(Å)
11- 18 F8.6 --- Rp/R* [0.103252/0.106607] Planet-to-star radius ratio
20- 27 F8.6 --- e_Rp/R* [0.000488/0.003212] Uncertainty in Rp/R*
29- 32 F4.2 --- u1 [0.62/0.92] Linear limb-darkening coefficient
34- 37 F4.2 --- e_u1 [0.02/0.12] Uncertainty in u1
39- 43 F5.3 --- a0 [0.873/1.016] Airmass correction factor (see
Equation (3))
45- 49 F5.3 --- e_a0 [0/0.005] Uncertainty in a0
51- 57 F7.4 --- a1 [-0.0111/0.0117] Airmass correction factor (see
Equation (3))
59- 64 F6.4 --- e_a1 [0.0008/0.005] Uncertainty in a1
66- 69 I4 ppm sigma [564/3317] Residual standard deviation
σres
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History:
From electronic version of the journal
(End) Tiphaine Pouvreau [CDS] 06-May-2019