J/AJ/155/120 RV variability of the K-giant γ Draconis (Hatzes+, 2018)
The radial velocity variability of the K-giant γ Draconis: stellar
variability masquerading as a planet.
Hatzes A.P., Endl M., Cochran W.D., MacQueen P.J., Han I., Lee B.-C.,
Kim K.-M., Mkrtichian D., Dollinger M., Hartmann M., Karjalainen M.,
Dreizler S.
<Astron. J., 155, 120-120 (2018)>
=2018AJ....155..120H 2018AJ....155..120H (SIMBAD/NED BibCode)
ADC_Keywords: Stars, giant ; Stars, K-type ; Radial velocities ; Exoplanets
Keywords: planetary systems - stars: individual: γ Dra -
stars: oscillations - stars: variables: general -
techniques: radial velocities
Abstract:
We present precise stellar radial velocity (RV) measurements of
γ Dra taken from 2003 to 2017. The data from 2003 to 2011 show
coherent, long-lived variations with a period of 702 days. These
variations are consistent with the presence of a planetary companion
having m sin i=10.7 MJup whose orbital properties are typical for
giant planets found around evolved stars. An analysis of the Hipparcos
photometry, Ca II S-index measurements, and measurements of the
spectral line shapes during this time show no variations with the RV
of the planet, which seems to "confirm" the presence of the planet.
However, RV measurements taken from 2011-2017 seem to refute this.
From 2011-2013, the RV variations virtually disappear, only to return
in 2014 but with a noticeable phase shift. The total RV variations are
consistent either with amplitude variations on timescales of ∼10.6 year,
or the beating effect between two periods of 666 and 801 days. It
seems unlikely that both these signals stem from a two-planet system.
A simple dynamical analysis indicates that there is only a 1%-2%
chance that the two-planet system is stable. Rather, we suggest that
this multi-periodic behavior may represent a new form of stellar
variability, possibly related to oscillatory convective modes. If such
intrinsic stellar variability is common around K giant stars and is
attributed to planetary companions, then the planet occurrence rate
among these stars may be significantly lower than thought.
Description:
A total of four RV data sets were used in the analysis. The earliest
observations were made with the coude spectrograph of the 2.1 m Otto
Struve Telescope at McDonald Observatory. A 1200 grooves/mm grating was
used in second order in combination with a Tektronix 512x512 CCD. Blocking
filters were used to isolate the desired order. This instrumental setup
resulted in a spectral dispersion of 0.046 Å/pixel at the central
wavelength of 5520 Å. An 85 µm slit provided a spectral resolution
of 0.11 Å (resolving power R=λ/Δλ=50000).
The "McD-2.7" data set used the Tull Spectrograph (Tull et al.
1995PASP..107..251T 1995PASP..107..251T) at the Harlan J. Smith 2.7 m telescope. This
instrument provided a nominal wavelength coverage of 3600-10000 Å at a
resolving power of R=60000.
RV measurements were also made with the Bohyunsan Observatory Echelle
Spectrograph or BOES (Kim et al. 2006A&A...454..839K 2006A&A...454..839K) of the 1.8 m
telescope at the BOAO in South Korea. An 80 µm fiber resulted in a
resolving power of R=90000 with a wavelength coverage of 3600-10500 Å.
The TLS observations of γ Dra were made as part of the Tautenburg
Observatory Planet Search (TOPS) program using the high-resolution coude
echelle spectrometer of the Alfred Jensch 2 m telescope and an iodine
absorption cell placed in the optical path. The spectrometer is grism
cross-dispersed and it has a resolving power R=67000 and wavelength
coverage 4630-7370 Å.
Objects:
-----------------------------------------------------
RA (ICRS) DE Designation(s)
-----------------------------------------------------
17 56 36.37 +51 29 20.0 γ Dra = * gam Dra
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 39 318 The RV data
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See also:
J/A+A/574/A116 : G and K giant stars stellar parameters (Reffert+, 2015)
J/ApJ/821/89 : 12yrs of radial velocity obs. of exoplanet systems
(Bryan+, 2016)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 12 F12.4 d JD Julian Date
14- 22 F9.4 m/s RV [-285.53/249.19] Radial velocity
24- 29 F6.3 m/s e_RV [1.61/28.1] Uncertainty in RV
31- 39 A9 --- Set Data set (McD-2.1, McD-2.7 m, BOAO or TLS)
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History:
From electronic version of the journal
(End) Tiphaine Pouvreau [CDS] 06-Nov-2018