J/A+A/702/A139 HD208487 TERRA, TERRA2, PFS and AAT RV curves (Rubenstein+, 2025)
Destruction of "peas in a pod?"
A candidate multi-planet system around the nearby, bright star, HD 208487.
Rubenstein R.I., Jenkins J.S., Pena R.P.A., Charalambous C., Tuomi M.,
Alves D.R., Vines J., Diaz M.R., Saha S., Butler R.P., Crane J.D.,
Shectman S., Teske J.K., Osip D., Essack Z., Montet B.T., Feinstein A.D.,
Petrovich C.
<Astron. Astrophys. 702, A139 (2025)>
=2025A&A...702A.139R 2025A&A...702A.139R (SIMBAD/NED BibCode)
ADC_Keywords: Stars, nearby ; Stars, double and multiple ; Exoplanets ;
Radial velocities ; Optical
Keywords: planets and satellites: detection -
planets and satellites: dynamical evolution and stability -
planets and satellites: formation
Abstract:
We reinvestigate the HD 208487 star system to test the reality of the
proposed HD 208487c world. We also search for additional companions
using applied Bayesian statistics and 15+ years of new radial velocity
(RV) data from the HARPS and the PFS instruments that were taken
post-discovery of HD208487b, as part of our continued study of bright
Sun-like stars within 50pc of the Sun.
RV data were analyzed with generalized Lomb-Scargle periodograms,
followed by Bayesian analysis techniques using the EMPEROR code. We
also scrutinised various stellar activity indices to search for any
corresponding peaks in the power spectra, correlations with the RV
measurements, or significant signals from a Bayesian analysis
methodology. Finally, photometric data was also checked to test for
any transits or possible activity manifestations that could lead to
possible false RV signals or excess noise.
Our analysis points towards a candidate second planet in the system,
positioned near the period of a previously proposed and subsequently
challenged signal. This signal, HD208487c, would relate to a cool
Saturn world with an orbital period of 923.06+2.02-2.76d and a
minimum mass of Mj*sini=0.32±0.01Mj . Our analysis also gives rise
to a newly discovered candidate planet, HD208487d, which if confirmed
would be the result of a cool super-Neptune/sub-Saturn with a period
of 1380.13+19.20-8.25d and a minimum mass of Mj*sini=0.15±0.01Mj
. Neither stellar activity indices nor photometric data show signals
statistically matching these periods. We find that stellar activity is
indeed affecting the RVs, yet our joint RV+activity indicator modeling
argues they are Doppler in nature. We show that the RV models are
stable over long timescales, and these signals are independent of
wavelength-dependent noise. The relative contributions of the data to
the model were also examined.
We uncovered a candidate three-planet system that would consist of an
inner gas giant, a central Saturn and an outer
super-Neptune/sub-Saturn. Extensive analysis of both photometric and
spectroscopic data as activity proxies strongly supports the planetary
system hypothesis, yet more long-term RV data would help add more
statistical weight to the reality of candidate planets c and
d.Assuming our model best represents reality, a dynamical analysis
suggests that gravitational scattering of an initially ordered,
equally-spaced system in a long resonant chain of six Neptunes can
explain the current architecture of HD208487 - a moderately-eccentric,
inner massive planet with an outer nearly resonant (Pd/Pc=1.495) gas
giant and super-Neptune. More RVs may also shed light on the reality
of a fourth Doppler signal uncovered in the data that sits close to
the 2:1 period-ratio with signal of HD208487c.
Description:
We used a total of 219 high-precision Doppler spectroscopic RV
measurements of HD 208487 spanning from JD 2451034.18, August 8, 1998,
to JD 2459508.64, October 21, 2021 for a total baseline of 8474 days
(23.2 years).
The data comes from the University College London Echelle
Spectrograph (UCLES) instrument installed on the 3.9m Anglo-Australian
Telescope at the Siding Spring Observatory (SSO), the High Accuracy
Radial Velocity Planet Searcher (HARPS) installed on the 3.6m ESO
Telescope (before and after the fibre upgrade in 2015) at La Silla
Observatory, and the Carnegie Planet Finder Spectograph (PFS) mounted
on the 6.5m Magellan II Telescope at Las Campanas Observatory.
Radial velocity time series for HD 208487 from four instruments:
TERRA, TERRA2, PFS, and AAT. Columns include Julian Date, radial velocity,
uncertainties, and activity indices where available.
Objects:
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RA (2000) DE Designation(s)
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21 57 19.84 -37 45 49.0 HD208487 = TIC 197686484
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
tablea1.dat 30 48 Complete AAT data
tablea2.dat 116 20 Complete TERRA data
tablea3.dat 43 45 Complete PFS data
tablea4.dat 119 106 Complete TERRA2 data
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 13 F13.5 d JD Julian Date
15- 20 F6.2 m/s RV Mean-subtracted radial velocity
22- 26 F5.2 m/s e_RV Mean-subtracted radial velocity error
28- 30 A3 --- Inst [AAT] Instrument
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Byte-by-byte Description of file: tablea2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 20 F20.12 d JD Julian Date
22- 40 F19.15 m/s RV Mean-subtracted radial velocity
42- 59 F18.16 m/s e_RV Mean-subtracted radial velocity error
61- 79 F19.17 --- Sindex Chromospheric activity index
81- 91 F11.9 --- FWHM Full width at half maximum
93-110 F18.15 --- BIS Bisector span
112-116 A5 --- Inst [TERRA] Instrument
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Byte-by-byte Description of file: tablea3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 F13.5 d JD Julian Date
15- 23 F9.5 m/s RV Mean-subtracted radial velocity
25- 31 F7.5 m/s e_RV Mean-subtracted radial velocity error
33- 39 F7.5 --- Sindex Chromospheric activity index
41- 43 A3 --- Inst [PFS] Instrument
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Byte-by-byte Description of file: tablea4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 20 F20.12 d JD Julian Date
22- 42 F21.17 m/s RV Mean-subtracted radial velocity
44- 61 F18.16 m/s e_RV Mean-subtracted radial velocity error
63- 81 F19.17 --- Sindex Chromospheric activity index
83- 93 F11.9 --- FWHM Full width at half maximum
95-112 F18.15 --- BIS Bisector span
114-119 A6 --- Inst [TERRA2] Instrument
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Acknowledgements:
Rafael Rubenstein, rafael.rubenstein(at)fulbrightmail.org
(End) Patricia Vannier [CDS] 25-Aug-2025