TOI-257b (HD 19916b): A Warm sub-Saturn Orbiting an Evolved F-type Star
Brett C. Addison, Duncan J. Wright, Belinda A. Nicholson, Bryson Cale, Teo Mocnik, Daniel Huber, Peter Plavchan, Robert A. Wittenmyer, Andrew Vanderburg, William J. Chaplin, Ashley Chontos, Jake T. Clark, Jason D. Eastman, Carl Ziegler, Rafael Brahm, Bradley D. Carter, Mathieu Clerte, Néstor Espinoza, Jonathan Horner, John Bentley, Andrés Jordán, Stephen R. Kane, John F. Kielkopf, Emilie Laychock, Matthew W. Mengel, Jack Okumura, Keivan G. Stassun, Timothy R. Bedding, Brendan P. Bowler, Andrius Burnelis, Sergi Blanco-Cuaresma, Michaela Collins, Ian Crossfield, Allen B. Davis, Dag Evensberget, Alexis Heitzmann, Steve B. Howell, Nicholas Law, Andrew W. Mann, Stephen C. Marsden, Rachel A. Matson, James O'Connor, Avi Shporer, Catherine Stevens, C.G. Tinney, Christopher Tylor, Songhu Wang, Hui Zhang, Thomas Henning, Diana Kossakowski, George Ricker, Paula Sarkis, Martin Schlecker, Pascal Torres, Roland Vanderspek, David W. Latham, Sara Seager, Joshua N. Winn, Jon M. Jenkins, Ismael Mireles, Pam Rowden, Joshua Pepper, Tansu Daylan, Joshua E. Schlieder, Karen A. Collins, Kevin I. Collins, Thiam-Guan Tan, Warrick H. Ball, Sarbani Basu, Derek L. Buzasi, Tiago L. Campante, Enrico Corsaro, Lucía González-Cuesta, Guy R. Davies, Leandro de Almeida, Jose-Dias do Nascimento Jr., Rafael A. Garcí a, Zhao Guo, Rasmus Handberg, Saskia Hekker, Daniel R. Hey, Thomas Kallinger, Steven D. Kawaler, Cenk Kayhan, James S. Kuszlewicz, Mikkel N. Lund, Alexander Lyttle, Savita Mathur, Andrea Miglio, Benoit Mosser, Martin B. Nielsen, Aldo M. Serenelli, Victor Silva Aguirre, Nathalie Themeßl
MMNRAS , 1–21 (2020) Preprint 22 January 2020 Compiled using MNRAS L A TEX style file v3.0
TOI-257b (HD 19916b): A Warm sub-Saturn on a ModeratelyEccentric Orbit Around an Evolved F-type Star
Brett C. Addison, (cid:63) Duncan J. Wright, Belinda A. Nicholson, , Bryson Cale, Teo Mocnik, Daniel Huber, Peter Plavchan, Robert A. Wittenmyer, Andrew Vanderburg, , William J. Chaplin, , Ashley Chontos, , Jake T. Clark, Jason D. Eastman, Carl Ziegler, Rafael Brahm, , , Bradley D. Carter, Mathieu Clerte, Néstor Espinoza, Jonathan Horner, John Bentley, Stephen R. Kane, John F. Kielkopf, Emilie Laychock, Matthew W. Mengel, Jack Okumura, Keivan G. Stassun, , Tim-othy R. Bedding, , Brendan P. Bowler, Andrius Burnelis, Michaela Collins, Ian Crossfield, , Allen B. Davis, Dag Evensberget, Alexis Heitzmann, Steve B. Howell, Nicholas Law, Andrew W. Mann, Stephen Marsden, JamesO’Connor, Avi Shporer, Catherine Stevens, C.G. Tinney, Christopher Tylor, Songhu Wang, , Hui Zhang, ThomasHenning, Diana Kossakowski, George Ricker, Paula Sarkis, Roland Vanderspek, David W. Latham, Sara Seager, , Joshua N. Winn, Jon M. Jenkins, Ismael Mireles, Pam Rowden, Joshua Pepper, Tansu Daylan, , Joshua E.Schlieder, Karen A. Collins, Kevin I. Collins, Thiam-Guan Tan, Warrick H. Ball, , Sarbani Basu, Derek L. Buzasi, Tiago L. Campante, , Enrico Corsaro, Lucía González-Cuesta, , Guy R. Davies, , Rafael A. García, , Zhao Guo, Rasmus Handberg, Saskia Hekker, , Daniel R. Hey, , Thomas Kallinger, Steven D. Kawaler, Cenk Kayhan, JamesS. Kuszlewicz, , Mikkel N. Lund, Alexander Lyttle, , Savita Mathur, , Andrea Miglio, , Benoit Mosser, Martin B.Nielsen, , , Aldo M. Serenelli, , Victor Silva Aguirre, & Nathalie Themeßl , The authors’ affiliations are shown in Appendix B.
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We report the discovery of a warm sub-Saturn, TOI-257b (HD 19916b), based on data fromNASA’s Transiting Exoplanet Survey Satellite (
TESS ). The transit signal was detected by
TESS and confirmed to be of planetary origin based on radial-velocity observations withthe Minerva-Australis telescope array. An analysis of the
TESS photometry, the Minerva-Australis, FEROS, and HARPS radial velocities, and the asteroseismic data of the stellaroscillations reveals that TOI-257b has a mass of M P = . + . − . M J (42 . + . − . M ⊕ ), a radiusof R P = . + . − . R J (7 . + . − . R ⊕ ), and an orbit with eccentricity 0 . + . − . and period18 . ± . = .
570 mag) somewhat evolved lateF-type star with M ∗ = . ± .
046 M sun , R ∗ = . ± .
033 R sun , T eff = ±
90 K, and v sin i = . ± . − . Additionally, we statistically validate a second non-transiting sub-Saturn mass planet on a ∼
71 day orbit using the radial velocity data. This system joins the ranksof a small number of exoplanet host stars that have been characterized with asteroseismology.Warm sub-Saturns are rare in the known sample of exoplanets, and thus the discovery ofTOI-257b is important in the context of future work studying the formation and migrationhistory of similar planetary systems.
Key words: planetary systems – techniques: radial velocities – techniques: photometric –techniques: spectroscopic – asteroseismology – stars: individual (TIC 200723869/TOI-257)
When Mayor & Queloz (1995) announced the discovery of the firsthot Jupiter, 51 Pegasi b, astronomers were baffled by the existence ofa Jovian planet orbiting its host star with such a short orbital period(about 4.2 days). That discovery revolutionized our understandingof the planet formation process, revealing the situation to be morecomplex than had been expected based on studies of the Solar system(e.g., Lissauer 1993). Radial velocity and transit surveys over the (cid:63)
E-mail: [email protected] past two decades have uncovered numerous warm and hot giantexoplanets with orbital periods shorter than 100 days (see, e.g.,Butler et al. 1997; Bayliss et al. 2013; Brahm et al. 2016; Van Eylenet al. 2018; Dawson et al. 2019; Kipping et al. 2019), and occurrencestudies based on those discoveries suggest that such planets can befound orbiting ∼
1% of all Sun-like stars (e.g., Howard et al. 2010,2012; Wright et al. 2012; Zhou et al. 2019) (in comparison to anoccurrence rate of at least 7% for more distant planets; see, e.g.,Foreman-Mackey et al. 2016; Wittenmyer et al. 2020).In addition to the Solar System lacking a hot Jupiter, it alsolacks other broad classes of planets such as super-Earths and mini- © 2020 The Authors a r X i v : . [ a s t r o - ph . E P ] J a n B. C. Addison et al.
Neptunes ( ∼ . − R ⊕ ) as well as planets larger than Neptune andsmaller than Saturn, known as sub-Saturns (which we have definedas planets with a radius between ∼ − R ⊕ ). Despite the lack ofsub-Saturns in the Solar System, they appear to be nearly twiceas common as Jovians ( ∼ − R ⊕ ) for orbital periods between5 −
100 days, which are surprisingly rare around solar type stars(2.9% for sub-Saturns versus 1.6% for Jovians, Petigura et al. 2013).Sub-Saturns are a key class of planets to study for understand-ing the formation, migration, and compositions of giant planets ingeneral. Their large size requires a significant H/He envelope thatcomprises a majority of their planetary volume, yet their massesare sufficiently small that their cores are not degenerate (unlike forplanets near the mass of Jupiter). This means that modeling theinteriors of sub-Saturns can be simplified as a planet consisting ofa high-density core surrounded by thick H/He envelope and wheremeasurements of mass and radius provide a unique solution for theplanet’s core and envelope mass fraction (e.g. Weiss & Marcy 2014;Petigura et al. 2016; Pepper et al. 2017; Petigura et al. 2017).It is commonly thought that close-in giant planets, such ashot/warm Jupiters and sub-Saturns, do not form in situ , but insteadoriginate beyond the protostellar ice line (typically located at severalastronomical units from the host star) where there is sufficient solidmaterial available to build up ∼ − M ⊕ cores (Pollack et al. 1996;Weidenschilling 2005; Rafikov 2006). In the case of Jovian planets,once their cores reach this critical mass regime, they begin to rapidlyaccrete gas from the protoplanetary disk to form their gaseous en-velopes. This process continues until the disk is dispersed (Rafikov2006; Tanigawa & Ikoma 2007), resulting in Jupiter-sized planetswith masses of ∼ − , M ⊕ . For sub-Saturns, however, therunaway accretion of gas appears to either not have occurred at allor did occur but in a gas-depleted disk (Lee et al. 2018). As a result,sub-Saturns have masses that range from ∼
10 to 100 M ⊕ . The massof a sub-Saturn is strongly correlated with the metallicity of its hoststar, but is uncorrelated with the resulting radial size (Petigura et al.2017).The sample of measurements for longer period ‘warm’ giantsand sub-Saturns thus far is small. The detection of more of thesesystems is then important to better constrain the formation andmigration mechanisms of close-in planets.One such source of warm giant planetary systems is NASA’s Transiting Exoplanet Survey Satellite ( TESS , Ricker et al. 2015),launched on 18th April, 2018. As of 6th November, 2019, the
TESS mission has delivered a total of 1361 planetary candidates – objectsthat require further observations from ground-based facilities toconfirm the existence of the candidate exoplanets . To date, suchfollow-up observations have resulted in a total of 34 confirmedplanetary discoveries (e.g. Nielsen et al. 2019; Vanderburg et al.2019; Quinn et al. 2019; Wang et al. 2019) – and it is likely thatmany more planets will be confirmed in the months to come.During its initial two-year primary mission, TESS is expectedto discover several dozen warm Jupiters, Saturns, and sub-Saturnsorbiting bright ( V <
10 mag) stars (Sullivan et al. 2015; Barclayet al. 2018; Huang et al. 2018). Those planets will be ideal tar-gets for follow-up observations to measure their masses, throughradial velocity measurements, to probe their atmospheric composi-tions, through transmission and emission spectroscopy, and to deter-mine their spin-orbit angles through measurements of the Rossiter-McLaughlin effect.In this work, we report the discovery of one such planet, TOI- Data from the NASA Exoplanet Archive, 6th November 2019
TESS , and follow-up observations using the Minerva-Australis facility at the University of Southern Queensland’s Mt.Kent Observatory (Wittenmyer et al. 2018; Addison et al. 2019).The Minerva-Australis facility is an array of five independentlyoperated 0.7 m CDK700 telescopes located at the Mount Kent Ob-servatory in Queensland, Australia (see, Addison et al. 2019, for adetailed description of the facility). Designed as a robotic observa-tory, instruments are remotely accessible and can be operated bothin manual or automatic configurations. Four of the telescopes in thearray (T1, T3, T4, T5) simultaneously feed stellar light to a singleKiwiSpec R4-100 high-resolution spectrograph via fiber optic ca-bles. The details of the spectrograph and spectroscopic observationsare provided in Section 2.3.In Section 2 we describe the
TESS photometric data, and the re-duction of the Minerva-Australis spectroscopic data and the radialvelocity pipeline, as well as radial velocities collected with otherinstruments. Section 3 presents the analysis of the data, includingthe characterization of the host star, the derived properties of theplanet, and the limits on any additional planets in the system. In Sec-tion 4 we compare TOI-257b with the demographics of the knownexoplanets, and discuss the significance of the system. We provideconcluding remarks and suggestions for future work in Section 5.
TOI-257 (HD 19916) is a bright (V = .
570 mag) late F-typestar, located at a distance of 77 . ± . . ± . Gaia
DR2, Gaia Collaboration et al. 2018a). Thestar is slightly evolved with a radius of 1 . ± . R (cid:12) , mass of1 . ± . M (cid:12) , and surface gravity of log g = . ± .
011 dex,derived from the asteroseismic analysis of the
TESS photometry inSection 3.2. The star has an effective temperature of 6075 ±
90 Kand metallicity of [ M/H ] = . ± .
10 derived from the analysisof Minerva-Australis spectra in Section 3.1 as well as a rotationalvelocity of v sin i = . ± . − in Section 3.3. TOI-257 hasrotational period of 8 . ± .
268 days based on analysis of the
TESS photometry in Section 3.3.
TESS
Photometry
The star TOI-257 (HD 19916, TIC 200723869 Stassun et al. 2019)was observed in Sectors 3 and 4 by Camera 3 of the
TESS space-craft in 2-minute cadence mode nearly continuously between 2018September 22 and 2018 November 15. The photometric data wereprocessed by the Science Processing Operations Center (SPOC)pipeline as described in Jenkins et al. (2016). Overall, three tran-sits were detected with depth of ∼ ∼ ,and the transit in sector 4 was observed during the thermal ramp.The TESS light curves were accessed from the NASA’s Mikul-ski Archive for Space Telescopes (MAST). The light curves hadbeen processed by the
TESS team using two different techniques:Pre-search Data Conditioning (PDC, the usual way of light curve See the data release notes at https://archive.stsci.edu/missions/tess/doc/tess_drn/tess_sector_03_drn04_v02.pdf
MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star Table 1.
Stellar Parameters for TOI-257.
Notes. – † Priors used in the
EXOFASTv2 global fit. (cid:63)
Upper limit on the V-band extinction from SchegelDust maps.Parameter Value SourceR.A. (hh:mm:ss) 03:10:03.982
Gaia
DR2Decl. (dd:mm:ss) -50:49:56.58
Gaia
DR2 µ α (mas yr − ) 97 . ± . Gaia
DR2 µ δ (mas yr − ) 27 . ± . Gaia
DR2Parallax (mas) 12 . ± . Gaia
DR2 A V (mag) 0 . (≤ . ) † ,(cid:63) Schegel Dust mapsBroadband Magnitudes: B T (mag) 8 . + . − . Tycho V T (mag) 7 . + . − . Tycho
T ESS (mag) 7 . ± . TESS
TIC v6 J (mag) 6 . ± .
020 2MASS H (mag) 6 . ± .
020 2MASS K s (mag) 6 . ± .
020 2MASS
W ISE . ± .
100 WISE
W ISE . ± .
033 WISE
W ISE . + . − . WISE
W ISE . + . − . WISE
Gaia (mag) 7 . + . − . Gaia
DR2
Gaia BP (mag) 7 . + . − . Gaia
DR2
Gaia RP (mag) 6 . + . − . Gaia
DR2Spectroscopic Properties derived from Minerva-Australis spectra: T eff (K) 6075 ± † iSpec; this paperlog g (dex) 3 . ± .
10 iSpec; this paper [ M/H ] (dex) 0 . ± . † iSpec; this paper R (cid:63) ( R (cid:12) ) 1 . ± .
017 isochrones; this paper M (cid:63) ( M (cid:12) ) 1 . + . − . isochrones; this paper ρ (cid:63) (g cm − ) 0 . ± .
011 isochrones; this paper L (cid:63) ( L (cid:12) ) 4 . ± .
120 isochrones; this paperAge (Gyr) 3 . ± .
46 isochrones; this paper v sin i (km s − ) 11 . ± . extraction and removal of systematics, see, Jenkins et al. 2016) andSimple Aperture Photometry (SAP, see, Twicken et al. 2010). Theseraw SAP and PDC light curves are shown in Figure 1, along withtheir detrended versions.To detrend the PDC light curves, we removed all quality-flagged data (except for stray light flag 2048), clipped 5 σ outliers,removed stellar and instrumental variability, normalized with themean of the out-of-transit flux, and merged together Sectors 3 and4. To remove the photometric variability, we used a Savitzky-Golay(SG) filter with a kernel width of 501 data points and a polyno-mial of order 2 over 3 iterations. During detrending, the planetarytransits were masked and then detrended by dividing out the inter-polated SG-filtered flux values from the out-of-transit data points.The SG detrending removed any longer-period stellar variability andsystematics, and retained any features that occurred on timescalescomparable or shorter than the duration of planetary transits (Kine-muchi et al. 2012; Jenkins et al. 2016).Two transits were recovered using the PDC technique, one inSector 3 and one in Sector 4. The transit at the beginning of Sector3 was missed by the PDC procedure since it falls on the part of thelight curve that was quality-flagged for manual exclusion during aspacecraft pointing improvement experiment. To recover this transitevent, we performed the exact same detrending procedure on theSAP version of the light curve as on the PDC light curve, the onlydifference being that the manual exclusion (flag 128) data points were not removed. The resulting detrended SAP light curve wasused for recovering the first transit observed by TESS in Sector 3but excluded from the global fit analysis as systematics were notremovable as seen in Figure 1.To include the first transit from Sector 3 in the global fit anal-ysis, we created a custom light curve following the procedures ofVanderburg et al. (2019) to obtain a cleaner light curve relatively freefrom systematics and stellar variability. We started by using a larger4.5 pixel radius aperture to extract the Sector 3 photometry, whichreduced the amplitude of the systematics observed in the early partof the light curve compared to the
TESS pipeline’s SAP light curve.We then removed systematics from a small segment of the lightcurve surrounding the first transit (2458383 . < t < .
0) bydecorrelating against the median background flux value from out-side the aperture for each 2-minute image and the standard deviationof the Q1, Q2, and Q3 quaternions within each 2-minute exposure.We excluded points during the planet transit in our decorrelationto prevent the systematics correction from biasing or distorting theshape of the transit. Next, we simultaneously fit the low-frequencyvariability (which we modeled as a basis spline) with a transit modelin a similar manner to Vanderburg et al. (2016a), except that we didnot also simultaneously fit for the systematics and we introduced adiscontinuity at BJD 24581385.95 where we switch from the cus-tom light curve to the PDC light curve. The combination of ourcustom light curve and the PDC light curve are what we use inthe final global fitting analysis with
EXOFASTv2 (Eastman et al.2013; Eastman 2017; Eastman et al. 2019). Figure 2 is the resulting30 minute binned and phase folded custom light curve along withthe PDC light curve and the individual transits color coded.
If a target star has a close companion, the additional flux from thesecond source can cause photometric contamination, resulting in anunderestimated planetary radius, or be the source of an astrophysicalfalse positive. To rule out the presence of close companions, speckleimaging observations were taken of TOI-257 with the SOAR andZorro instruments.
TOI-257 was observed with SOAR speckle imaging (Tokovinin2018) on 18 February 2019 UT, observing in a similar visiblebandpass as
TESS . The 5 σ detection sensitivity and the speckleauto-correlation function from the SOAR observation are plotted inFigure 3. Further details of the observations are available in Ziegleret al. (2020). No nearby stars were detected within 3 (cid:48)(cid:48) of TOI-257. Direct imaging observations of TOI-257 was also carried out on12 Sept. 2019 UT using the Zorro speckle instrument on Gemini-South . Zorro simultaneously provides speckle imaging in twobands, 562 nm and 832 nm, with output data products includinga reconstructed image, and robust limits on companion detections(Howell et al. 2011). Figure 4 shows our 562 nm result and recon-structed speckle image and we find that TOI-257 is indeed a singlestar with no companion brighter than about 6 magnitudes detected , 1–21 (2020) B. C. Addison et al.
Figure 1.
TESS light curves of TOI-257 from Sector 3 (left panel) and Sector 4 (right panel). The Pre-search Data Conditioning (PDC, upper panels) andSimple Aperture Photometry (SAP, lower panels) versions of the light curves before (shown in red) and after detrending (shown in black and shifted downarbitrarily to avoid overlap with the red points). The detrending function is blue and transits are grey. Top left: A single transit event was recovered by PDC inSector 3. Top right: A single transit event was recovered by PDC in Sector 4. Bottom left: Two transit events were recovered by SAP from Sector 3. Bottomright: A single transit event was recovered by SAP in Sector 4. within 1.75 (cid:48)(cid:48) . This limit corresponds to approximately an M3V starat the inner working angle of ∼ . (cid:48)(cid:48) and M5V at the outer workingangle of ∼ . (cid:48)(cid:48) . We obtained high-resolution spectroscopic observations of TOI-257 with Minerva-Australis, FEROS, and HARPS to confirm andmeasure the mass of the
TESS transiting planet candidate. Herewe describe the observations from each spectrograph and list thederived radial velocities in Table 2.
Minerva -Australis
We carried out an intensive radial velocity follow-up campaign withthe Minerva-Australis facility to confirm the planetary nature of thetransit-like signals in the
TESS photometry, measure the mass andorbital properties of the planet, search for any additional planets inthe system, and measure the stellar atmospheric properties of the host star. A total of 53 spectra (observations taken simultaneouslyfrom multiple telescopes in the array are counted as one observa-tion) of TOI-257 were obtained at 28 epochs between 2019 July 12and October 15. Telescopes T1, T3, T4, and T5 in the Minerva-Australis array simultaneously feed via 50 µ m circular fiber cablesa single KiwiSpec R4-100 high-resolution ( R = , − . Theradial velocities collected by Minerva-Australis show a ∼
10 m s − sinusoidal variation that is in phase with the photometric ephemeris MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star -15 -10 -5 0 5 10 15Hours from Mid-transit0.99850.99900.99951.0000 R e l a t i v e B r i gh t ne ss Third transitSecond transitFirst transitThird transitSecond transitFirst transit
Figure 2.
Phase folded
TESS light curve of TOI-257 binned at a cadenceof 30 minutes with the individual transits color coded showing that they areof similar depth. The first transit comes from the custom light curve wherewe removed systematics that are the result of a spacecraft pointing anomaly.The second and third transit are from the Pre-search Data Conditioning lightcurve. The red curve is the best-fit transit model. arcsec m a g n i t u d e ( I - b a n d ) [arcsec] -202 [ a r c s e c ] SOAR Speckle ACF
TOI-257
Figure 3.
The 5 σ detection sensitivity and inset speckle auto-correlationfunction from SOAR speckle observing of TOI-257 on 18 February 2019UT in I -band, which is similar to the TESS bandpass. The orientation of theinset image has North pointed up and East to the left. No stars were detectedwithin 3 (cid:48)(cid:48) of TOI-257. with an amplitude compatible with a sub-Saturn-sized planet in amoderately eccentric orbit as shown in Figures 9 and 10. Addition-ally, we measured the bisector velocity span (BVS) values usingthe cross-correlation functions (CCFs) as a check to ensure that theradial velocity variation observed is not from stellar activity or abackground eclipsing binary system. As shown in Figure 11, nocorrelations are apparent in the BVS values.
TOI-257 was observed with the FEROS instrument ( R = , E C N � Lt') TOl - o---------------------------- angular separation (arcsec) Figure 4.
Zorro speckle observation of TOI-257 taken at 562 nm. Our si-multaneous 832 nm observation provides a similar result. The red line fitand blue points represent the 5 σ fit to the sky level (black points) revealingthat no companion star is detected from the diffraction limit (17 mas) outto 1.75 (cid:48)(cid:48) within a ∆ mag of 6 to 8. (cid:48)(cid:48) across. The inset reconstructed speckleimage has north up and East to the left and is 2.5 (cid:48)(cid:48) across. formed in simultaneous calibration mode, utilizing the ThAr lampon the secondary fiber to track and remove instrumental variationsdue to changes in the temperature and pressure during the scienceexposures. The exposure times were set to 300 s, resulting in signal-to-noise ratio between 270 and 370 per resolution element. Weproduced radial velocities by cross-correlation with a G2-type bi-nary mask template using the CERES pipeline (Brahm et al. 2017),which also corrects the radial velocities for instrumental systematicsand the Earth’s motion. We monitored TOI-257 with the HARPS spectrograph ( R = , We used the Minerva-Australis spectra to determine TOI-257’s at-mospheric stellar parameters. Through the python package iSpec(Blanco-Cuaresma et al. 2014; Blanco-Cuaresma 2019), we stackedthe stellar spectra to derive the effective temperature, surface grav-ity, and overall metallicity ([M/H]) of the star. We configured theiSpec synthetic grid to incorporate a MARCS atmospheric model(Gustafsson et al. 2008) and utilized the spectrum (Gray & Corbally1994) radiative transfer code. [M/H] was derived using version 5.0of
Gaia -ESO Survey’s (GES) line-list (Heiter et al. 2015) normal-ized by solar values obtained by Asplund et al. (2009). Our syntheticspectra fit was constructed by setting initial values for T eff , log g and MNRAS , 1–21 (2020)
B. C. Addison et al.
Table 2.
Journal of radial velocity observations of TOI-257.
Notes. –M-ATel3, M-A Tel4, and M-A Tel6 are Minerva-Australis Telescope3, Tele-scope4, and Telescope6, respectively. (This table is available in its entiretyin machine-readable form.)Date RV σ Instrument(BJD) (m s − ) (m s − )2458465.539980 21.9 2.0 HARPS2458465.602650 26.7 2.0 HARPS2458465.690670 26.1 2.0 HARPS2458466.529660 24.8 2.0 HARPS2458466.591590 17.1 2.0 HARPS2458466.678080 23.5 2.0 HARPS2458466.682320 22.5 2.0 HARPS2458467.674470 12.5 5.3 FEROS2458468.663190 8.1 5.5 FEROS2458481.588670 20.9 2.0 HARPS2458481.593290 24.6 2.0 HARPS2458481.597630 24.1 2.0 HARPS2458482.673800 32.1 2.0 HARPS2458482.678140 29.7 2.0 HARPS2458493.714430 -11.4 6.2 FEROS2458497.608960 -10.3 5.7 FEROS2458500.629830 -19.3 5.7 FEROS2458505.566740 -14.6 5.9 FEROS2458677.272975 10.8 3.0 M-A Tel32458677.272975 -13.2 3.4 M-A Tel42458677.294387 24.3 3.1 M-A Tel32458677.294387 10.3 3.4 M-A Tel42458680.203692 11.6 3.6 M-A Tel32458680.203692 20.7 4.1 M-A Tel42458680.203692 -8.2 7.5 M-A Tel52458680.225093 0.1 3.9 M-A Tel32458680.225093 3.3 3.8 M-A Tel42458680.225093 5.4 8.0 M-A Tel52458681.170185 1.9 3.5 M-A Tel32458681.170185 22.1 3.6 M-A Tel42458681.170185 -8.3 4.6 M-A Tel52458681.191597 -3.8 3.3 M-A Tel32458681.191597 -11.9 3.9 M-A Tel42458681.191597 14.9 4.6 M-A Tel52458682.146655 25.4 3.9 M-A Tel32458682.146655 27.6 7.2 M-A Tel42458682.146655 12.7 5.3 M-A Tel52458682.168067 14.9 3.9 M-A Tel32458682.168067 19.6 4.6 M-A Tel42458682.168067 5.0 5.6 M-A Tel52458683.249780 6.3 3.5 M-A Tel42458683.276111 -5.9 4.6 M-A Tel3 [M/H] of 6050 K, 4.44 dex, and 0.00 dex, respectively, based on theparameters from a broadband spectral energy distribution (SED)analysis. Figure 5 depicts our observed spectra and synthetic modelproduced by iSpec. Our derived T eff , log g and [M/H] values werethen fed into the Bayesian isochrone modeler isochrones (Morton2015; Montet et al. 2015).isochrones uses nested sampling scheme called multinest(Feroz et al. 2009) to determine the stellar mass, radius, and age,which was then used to derive the stellar density and luminosity ofTOI-257. For this particular analysis, we used the stellar parameterresults from iSpec as well as the parallax value from Gaia
DR2 withG, H, J, K, V and W1 magnitudes as priors in the global fit. Thespectroscopic stellar iSpec and isochrones values can be found inTable 1 and are in good agreement with the SED analysis performedusing
EXOFASTv2 and the asteroseismology. We then incorporated
Wavelength [nm] N o r m a li s e d F l u x Fe I Si I Fe I Fe I
TOI-257Teff = 6075±90, logg =3.97±0.10, [M/H] = 0.19±0.10 obssynthresiduals + 0.4
Figure 5.
The best fit synthetic model spectrum from iSpec (the red dashedline) of TOI-257 to that of the combined stellar spectrum obtained from theMinerva-Australis spectroscopic observations (the blue solid line) for thewavelength region between 624.0 nm and 625.5 nm. The residuals of the fitare shown as the green solid line. the T eff and [M/H] values derived from the iSpec analysis of theMinerva-Australis spectra as priors in the final EXOFASTv2 globalfit of the data in Section 3.4.
To perform asteroseismic analysis on TOI-257 we produced a cus-tom light curve using the
TESS
Asteroseismic Science OperationsCenter (TASOC, Lund et al. 2017) photometry pipeline (Handberget al., in prep.), which is based on software originally developed togenerate light curves for data collected by the K2 Mission (Lundet al. 2015). The TASOC pipeline implements a series of correctionsto optimize light curves for an asteroseismic analysis (Handberg &Lund 2014), including the removal of instrumental artefacts andof the transit events using a combination of filters utilizing theestimated planetary period. The photometric performance of theTASOC light curve was comparable to the light curve produced bythe SPOC pipeline.Solar-like oscillations are broadly described by a frequency ofmaximum oscillation power ( ν max ) and a large frequency separation( ∆ ν ), which approximately scale with log g and the mean stellar den-sity, respectively (see, García & Ballot 2019). The power spectrumof the sector 3 light curve of TOI-257 displays a power excess near ∼ µ Hz (Figure 6), consistent with the spectroscopic log g andthe expected frequency range from the TESS asteroseismic targetlist (ATL, Schofield et al. 2019). An autocorrelation of the powerspectrum reveals a peak at a frequency spacing consistent with thelocation of the excess power (e.g. Stello et al. 2009). Furthermore,the amplitude of the power excess ( ∼ Kepler (Huber et al. 2011).The addition of the Sector 4 light curve reduced the significance ofthe asteroseismic detection due to the slightly elevated noise level,and was thus discarded for the remainder of our analysis.To test the significance of the detection and measure ν max and ∆ ν we used 15 independent analysis methods within working https://tasoc.dk/code/ MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star Figure 6.
Power spectrum of the Sector 3 TASOC light curve of TOI-257(grey line). The black and red lines show the power spectrum smoothed witha boxcar width of 2 µ Hz and Gaussian with a full width at half max of ∆ ν ,respectively. The inset shows the autocorrelation of the power spectrum,with a red line marking the expected value of ∆ ν based on the location ofthe power excess. group 1 of the TESS
Asteroseismic Science Consortium (e.g. Huberet al. 2009; Mathur et al. 2010; Mosser et al. 2012; Benomar et al.2012; Kallinger et al. 2012; Corsaro & De Ridder 2014; Davies& Miglio 2016; Campante 2018). All but one pipeline reported asignificant detection of solar-like oscillations. The final parametersare ν max = ± µ Hz and ∆ ν = . ± . µ Hz, with the centralvalue taken from the solution closest to the median of all solutions,and uncertainties calculated from the median formal uncertaintyreturned by individual pipelines added in quadrature to the scatterover individual methods.
We used a number of independent approaches to model the observedglobal asteroseismic parameters, including different stellar evolu-tion codes (ASTEC, GARSTEC, MESA, and YREC, Christensen-Dalsgaard 2008; Weiss et al. 2008; Paxton et al. 2011, 2013, 2015;Choi et al. 2016a; Demarque et al. 2008) and modeling methods(BeSPP, BASTA, PARAM, isoclassify, Silva Aguirre et al. 2015;Serenelli et al. 2017; Rodrigues et al. 2014, 2017; Huber et al.2017; García Saravia Ortiz de Montellano et al. 2018). Model in-puts included the spectroscopic temperature and metallicity (seeSection 3.1), ν max , ∆ ν , and the luminosity derived from the Gaiaparallax. To investigate the effects of different input parameters,modelers were asked to provide solutions with and without takinginto account the luminosity constraint.The modeling results showed a bi-modality in mass (and thusage) at ∼ . M (cid:12) and ∼ . M (cid:12) , with all pipelines favoring thehigher mass solution once the luminosity constraint was included.We adopted the solution closest to the median of all returned values,with uncertainties calculated by adding the median uncertainty for agiven stellar parameter in quadrature to the standard deviation of theparameter for all methods. This method has been commonly adoptedfor Kepler (e.g. Chaplin et al. 2014) and captures both random andsystematic errors estimated from the spread among different meth-ods. The final estimates of stellar parameters are summarized in Ta-ble 3.2.2, constraining the radius, mass, density and age of TOI-257
Table 3.
Asteroseismic Stellar Parameters for TOI-257.
Notes: † Priors usedin the
EXOFASTv2 global fit. Input ParametersFrequency of maximum oscillation power, ν max ( µ Hz), 1188 ± ∆ ν ( µ Hz), 61 . ± . T eff (K) 6075 ± . ± . L ( L (cid:12) ) 4 . ± . M (cid:63) ( M (cid:12) ) 1 . ± . † Stellar Radius, R (cid:63) ( R (cid:12) ) 1 . ± . † Stellar Density, ρ (cid:63) (cgs) 0 . ± . g (cgs) 4 . ± . t (Gyr) 3 . ± . to ∼ ∼ ∼ ∼
13 %. We emphasize that these un-certainties in stellar parameters are robust against systematic errorsfrom different stellar model grids, which are frequently neglectedwhen characterizing exoplanets. The stellar mass and radius derivedfrom this analysis is used as priors in the final
EXOFASTv2 globalfit of the data in Section 3.4.
The rotation period of TOI-257 was estimated by performing Lomb-Scargle (Scargle 1982) periodogram and auto-correlation functionanalysis (e.g., McQuillan et al. 2013) on the
TESS lightcurve, andby measuring the projected stellar rotation velocity ( v sin i ) fromMinerva-Australis spectra.We calculated the Lomb-Scargle periodograms for the raw TESS light curves from Sectors 3 and 4 individually and fromthe combined light curve of the two Sectors, after masking thetransit events. For Sector 3, the periodogram shows that the vari-ability has a period of P = . ± .
46 days and amplitude of A = ± P = . ± .
22 days and amplitude of A = ± P = . ± .
13 d, amplitudeof A = ± << .
01. Asecond very strong peak is observed at ∼ .
69 days (or 2 P /
3) in theLomb-Scargle periodograms. The FAP was computed from MonteCarlo simulations (e.g., Messina et al. 2010) and the uncertaintyin the period of variability was calculated following the procedureof Lamm et al. (2004). The variability from both sectors combinedphases-up well at a period of 4.036 days as shown in Figure 7,which indicates that the variability is likely to be astrophysical innature (from stellar rotation and star spots) and not systematics. Wetherefore have adopted the period of variability as 4 . ± .
13 d.We also performed an auto-correlation function analysis onthe light curves from the individual sectors and combined sectors,and find that the period of variability as P = . ± .
61 daysand P = . ± .
32 days for Sectors 3 and 4, respectively, and aperiod of P = . ± .
22 days for the combined light curves. Wealso find a strong secondary period in the combined light curves of ∼ . MNRAS , 1–21 (2020)
B. C. Addison et al. upper limit on the rotation period from the star’s v sin i and es-timated radius. We measured the v sin i of TOI-257 by fitting arotationally broadened Gaussian (Gray 2005) to a least-squaresdeconvolution profile (Donati & Collier Cameron 1997) obtainedfrom the sum of all the spectral orders from the combined highestS/N spectra of TOI-257. The resulting v sin i is 11 . ± . − and combined with the stellar radius from asteroseismology of R (cid:63) = . ± . R (cid:12) , sets the upper limit on the rotation periodfor the star of ∼ . .
04 day period ofvariability observed in the combined
TESS light curve to be halfthe true rotation period of 8 . ± .
26 days (which gives a v rot = ∗ π ∗ R (cid:63) / P rot = . − , consistent with the value of v sin i ).The very strong secondary peak observed at ∼ . . ± .
26 daysbeing the true rotation period since the secondary peak correspondsnicely with the P rot / .
04 days, we would haveexpected to find a strong secondary peak at ∼ .
02 days instead of ∼ . v sin i , this suggestthat the stellar obliquity is low (i.e., i (cid:63) ∼
90 deg).
To determine the system parameters for TOI-257 and its planet,we used
EXOFASTv2 (Eastman et al. 2013; Eastman 2017; Eastmanet al. 2019) to perform a joint analysis of the
TESS photometryand the radial velocity data. We placed Gaussian priors on T eff and[Fe/H] from the Minerva-Australis high-resolution spectroscopyand Gaussian priors on R ∗ and M ∗ from asteroseismology. We ap-plied an upper limit on the V-band extinction from the Schlafly &Finkbeiner (2011) dust maps at the location of TOI-257. We alsoperformed a separate SED analysis (so as not to double count infor-mation used from the asteroseismic priors) as an independent checkon the stellar parameters using catalog photometry from Tycho (Høget al. 2000), 2MASS (Cutri et al. 2003), WISE (Cutri & et al. 2013),and Gaia (Gaia Collaboration et al. 2018b) as well as MIST stel-lar evolutionary models (Dotter 2016; Choi et al. 2016b). Gaussianpriors were placed on the parallax from
Gaia
DR2, adding 82 µ as tocorrect for the systematic offset found by Stassun & Torres (2018)and adding the 33 µ as uncertainty in their offset in quadrature to the Gaia -reported uncertainty. Table 1 lists the broadband magnitudesused in the SED analysis and the stellar parameters including theones used as priors in the global analysis.The resulting best-fit models for the transit light curves areplotted in Figure 8, and for the radial velocities in Figures 9 and 10. Asummary of the best-fit model values is given in Table 4. Figure 11 isa plot of the bisector velocity span showing no correlation betweenthe bisectors and the radial velocities for the Minerva-Australisobservations, indicating that the measured radial velocity signal islikely planetary in nature and not due to stellar photospheric activity(Figueira et al. 2013).From the best-fit Kurucz stellar atmosphere model from theSED and the best-fitting MIST stellar evolutionary model, wefind that TOI-257 is a somewhat evolved late-F star with R ∗ = . + . − . R (cid:12) , M ∗ = . + . − . M (cid:12) , T eff = + − K, and log g = . + . − . (where g is in units of cm s − ). These stellarparameters are in reasonably good agreement with the parametersderived from the Minerva-Australis spectroscopy and asteroseis-mology. However, we choose not to adopt these stellar parameterssince they are not as precise as the ones derived from spectroscopyand asteroseismology and list the stellar parameters derived fromthe joint analysis of the TESS photometry and the radial velocitydata in Table 4. From the joint analysis, we find that TOI-257 hostsa sub-Saturn sized planet with a radius of R P = . + . − . R J (7 . + . − . R ⊕ ) and mass of M P = . + . − . M J (42 . + . − . M ⊕ ),on a moderately eccentric ( e = . + . − . ) ∼ . We further analyze the radial velocity data set in Table 2 with
RadVel (Fulton et al. 2018) to provide both an independent analy-sis for checking consistency in the mass and eccentricity of planetb, and to search for any additional planets. The search for additionalplanets is motivated by two reasons. First, moderately eccentricKeplerian signals can sometimes resolve into two near-circular res-onant signals with additional radial velocity data (e.g. Wittenmyeret al. 2013; Trifonov et al. 2017; Boisvert et al. 2018; Wittenmyeret al. 2019). Second, we wish to evaluate the multiplicity of systemslike TOI-257 with warm sub-Saturns.To apply
RadVel , we first median subtract and average the Min-erva-Australis radial velocities from individual telescopes acrossnights, weighted by the error bars of each measurement. The ra-dial velocity data sets are assumed to be independent in
RadVel for the statistical assessment of models. By binning the data acrossMinerva-Australis telescopes, we avoid needing to account for thesystematics common to all three Minerva-Australis radial veloci-ties. The combined analysis of the HARPS, binned Minerva-Australis, and FEROS data sets are consistent with a planet at theknown transiting period and T c from Table 4. However, the circularorbital solution is marginally favored over an eccentric model, ac-cording to the relative small-sample Akaike Information Criterion( ∆ AICc=4.67) (Akaike 1974; Burnham & Anderson 2002). We fix P and T c to the values independently derived from the ExoFASTv2 analysis of the
TESS light curve as they will not be well-constrainedfrom the RVs alone, considering the small baseline compared to theorbital period. Since we do not jointly model the light curve withthe radial velocities in
RadVel , we are not capturing the additionalinformation from the transit duration ( ∼ .
804 hr for circular orbitversus the measured 6 .
346 hr measured) and shape present in thelight curve that may contribute to the increased evidence for an ec-centric orbital solution presented earlier with
ExoFASTv2 . The bestfit semi-amplitude from
RadVel is similar to the
EXOFASTv2 re-sults when using the unbinned Minerva-Australis radial velocities(9 . ± . − ); however the eccentric model is still not exclusivelyfavored over a circular model ( ∆ AICc=3.97).The remaining scatter after removing planet b is consistentlylarger than the uncertainties of the three instruments and appearsstructured (see Figure 12). We use a custom modified version of
RadVel to generate log-likelihood periodograms (LLPs) with vari-ous orbit assumptions to search for additional planets. We start witha single planet model and generate a log-likelihood for a wide rangein fixed periods, fitting only for T c and K , as well as the relativeinstrument dependent offsets and additional radial velocity “jitter”noise terms, and then a second LLP assuming a fixed period and MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star Table 4.
Median values and 68% confidence interval for TOI-257 from the MCMC
EXOFASTv2 analysis.
Notes. – M-A T3, M-A T4, and M-A T6 areMinerva-Australis Telescope3, Telescope4, and Telescope6, respectively. ∗ The time of conjunction that is closest to the starting value supplied as a prior andis typically a good approximation for the mid transit time. (cid:5)
The optimal conjunction time is the time of conjunction that minimizes the covariance with theorbital period and therefore has the smallest uncertainty. ∧ The equilibrium temperature of the planet assumes no albedo and perfect heat redistribution. (cid:63)
Thetidal circularization timescale is calculated using Equation 3 from Adams & Laughlin (2006) and assuming a Q = . † TESS
LC1 is the
TESS light curvefrom PDC and
TESS
LC2 is the
TESS light curve produced using the Vanderburg et al. (2019) procedures.Parameter Description ValuesStellar Parameters: M ∗ . . . . . . Mass ( M (cid:12) ) . . . . . . . . . . . . . . . . . . . . . . 1 . ± . R ∗ . . . . . . Radius ( R (cid:12) ). . . . . . . . . . . . . . . . . . . . . . 1 . ± . L ∗ . . . . . . Luminosity ( L (cid:12) ). . . . . . . . . . . . . . . . . . 4 . + . − . ρ ∗ . . . . . . . Density (cgs) . . . . . . . . . . . . . . . . . . . . . 0 . + . − . log g . . . . Surface gravity (cgs) . . . . . . . . . . . . . . 4 . ± . T eff . . . . . . Effective Temperature (K). . . . . . . . . . 6096 ± [ Fe / H ] . . Metallicity (dex) . . . . . . . . . . . . . . . . . . 0 . ± . P . . . . . . . Period (days) . . . . . . . . . . . . . . . . . . . . . 18 . ± . R P . . . . . Radius ( R J ) . . . . . . . . . . . . . . . . . . . . . . 0 . + . − . M P . . . . . Mass ( M J ) . . . . . . . . . . . . . . . . . . . . . . . 0 . + . − . T C . . . . . . Time of conjunction (BJD TDB ) . . . . . 2458385 . ± . ∗ T . . . . . . . Optimal conjunction Time (BJD TDB ) 2458404 . ± . (cid:5) a . . . . . . . Semi-major axis (AU) . . . . . . . . . . . . . 0 . ± . i . . . . . . . . Inclination (Degrees) . . . . . . . . . . . . . . 88 . + . − . e . . . . . . . . Eccentricity . . . . . . . . . . . . . . . . . . . . . . 0 . + . − . ω ∗ . . . . . . Argument of Periastron (Degrees) . . 96 ± T eq . . . . . Equilibrium temperature (K) . . . . . . . 1033 + − ∧ τ circ . . . . . Tidal circularization timescale (Gyr) 1880 + − (cid:63) K . . . . . . . RV semi-amplitude (m/s) . . . . . . . . . . 8 . ± . R P / R ∗ . . Radius of planet in stellar radii . . . . . 0 . + . − . a / R ∗ . . . . Semi-major axis in stellar radii . . . . . 17 . + . − . δ . . . . . . . Transit depth (fraction) . . . . . . . . . . . . 0 . + . − . τ . . . . . . . . Ingress/egress transit duration (days) 0 . + . − . T . . . . . . Total transit duration (days) . . . . . . . . 0 . + . − . T FW H M
FWHM transit duration (days) . . . . . . 0 . ± . b . . . . . . . Transit Impact parameter . . . . . . . . . . 0 . + . − . ρ P . . . . . . Density (cgs) . . . . . . . . . . . . . . . . . . . . . 0 . + . − . logg P . . Surface gravity . . . . . . . . . . . . . . . . . . . 2 . + . − . Θ . . . . . . . Safronov Number . . . . . . . . . . . . . . . . . 0 . + . − . (cid:104) F (cid:105) . . . . . Incident Flux (10 erg s − cm − ) . . . 0 . + . − . T P . . . . . . Time of Periastron (BJD TDB ). . . . . . . 2458367 . + . − . T A . . . . . . Time of Ascending Node (BJD TDB ). 2458382 . + . − . T D . . . . . . Time of Descending Node (BJD TDB ) 2458370 . + . − . e cos ω ∗ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . − . + . − . e sin ω ∗ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . + . − . M P / M ∗ . Mass ratio . . . . . . . . . . . . . . . . . . . . . . . 0 . + . − . d / R ∗ . . . . Separation at mid transit . . . . . . . . . . . 13 . + . − . Wavelength Parameters:
TESS u . . . . . . . linear limb-darkening coeff . . . . . . . . 0 . ± . u . . . . . . . quadratic limb-darkening coeff . . . . . 0 . ± . A D . . . . . Dilution from neighboring stars . . . . ≤ . γ rel . . . . . . Relative RV Offset (m/s) . . . . . . . . . . . − . ± . . ± . . ± . . ± . − . ± . σ J . . . . . . RV Jitter (m/s) . . . . . . . . . . . . . . . . . . . . 13 . + . − . . + . − . . + . − . . + . − . . + . − . σ J . . . . . . RV Jitter Variance . . . . . . . . . . . . . . . . 175 + − + − + − + − + − Transit Parameters:
TESS
LC1 † TESS
LC2 † σ . . . . . . Added Variance . . . . . . . . . . . . . . . . . . 1 . ± . × − . + . − . × − F . . . . . . Baseline flux . . . . . . . . . . . . . . . . . . . . . 1 . ± . . ± . , 1–21 (2020) B. C. Addison et al.
Figure 7.
The left panel shows the
TESS light curve of TOI-257 from Sectors 3 and 4 with the best-fit variability. The middle panel is the Lomb-Scargleperiodogram for raw light curves from Sectors 3 and 4 combined. The right panel is the phase-folded light curve at the peak period found from the Lomb-Scargleperiodogram. − . − . − . − . . . . . . . . . . . . . . . N o r m a li ze d F l u x First transitSecond transitThird transit
Figure 8.
Phase folded
TESS light curve of TOI-257 with the individualtransits color coded similar to Figure 2. The red solid line is the best-fittingmodel. T c for planet b, but varying both semi-amplitudes to search for anadditional planet candidate TOI-257c. Anecdotally, we observe thatallowing for eccentric orbits in LLPs typically results in a noisierLLP compared to considering only circular orbits, and can partic-ularly yield false peaks where e ≈ RV /d t | located where the radial velocities are minimally sampled.These are likely non-physical orbits, so we only present circularsearches (similarly, considering only eccentricities (cid:46) . T c , yields a poste-rior probability distribution of the semi-amplitude for the secondplanet that is > σ deviant from 0, and minimally affects the sta-tistical significance of the first planet (as shown in Figure 13). Themodel comparison heavily favors the 2-planet model over the 1-planet model with ∆ AICc = .
67. This 71 day signal translatesto approximately a 0 .
2% transit depth assuming the mass-radiusrelation given by Chen & Kipping (2017) and stellar parameters R V [ m s ] FEROS HARPS MINERVA-Aus450 500 550 600 650 700 750 800 850
BJD
TDB +2.458e625025 O - C Figure 9.
Radial velocity measurements of TOI-257 as a function of time.The radial velocity measurements from each instrument have been binned byday for clarity, however, the analysis was performed using the unbinned data.Minerva-Australis radial velocities are represented by the purple filled-incircles. Radial velocities from FEROS and HARPS are the lime green andgold filled-in circles, respectively. The best-fit model is plotted as the dashedgrey line and the center-of-mass velocity has been subtracted. The bottompanel shows the residuals between the data and the best-fit model. in Table 4. Figures A1 and A2 in the Appendix shows the poste-rior distributions from
RadVel for a 1-planet and 2-planet circularmodels, respectively. We see no evidence for a transit in the
TESS light curve within the uncertainty window of the best fit T c for thepossible outer planet. While this radial velocity detection is signifi-cant, more high-precision radial velocity measurements are neededto ensure the candidate c planet signal is not an alias or possiblea result of the observing cadence, especially without an observedtransit event. MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star R V [ m s ] FEROS HARPS MINERVA-Aus0.0 0.2 0.4 0.6 0.8 1.0
Orbital phase O - C Figure 10.
Same as Figure 9 but phased to one orbital period. The units ofthe horizontal axis were chosen so that the transit mid-time corresponds toan orbital phase of 0.25. − − − −
20 0 20 40Radial Velocity (ms − ) − . − . − . − . . B i s ec t o r( k m s − ) Telescope Telescope Telescope Figure 11.
Bisector velocity span as a function of the radial velocities forthe Minerva-Australis radial velocities. There is no significant correlationbetween the bisector velocity span and the radial velocities.
Next, we consider the possibility that the excess radial velocityresiduals after modeling planet b are due to stellar activity ratherthan a second planet (or both) as presented in the previous subsec-tion.
EXOFASTv2 does not permit the inclusion of a stellar activitymodel for the radial velocities, whereas
RadVel does. With our cus-tomized version of
RadVel , we calculate LLPs using a GaussianProcess (GP) with a quasi-periodic kernel (Rajpaul et al. 2015) toapproximate any detectable stellar-activity. We re-run the MCMCanalysis for 1- and 2-planet models. We assume broad Gaussian pri-ors on the GP hyper-parameters listed in Table 5. Both ∼ The specific implementation of the quasi-periodic kernel in
RadVel can befound on https://radvel.readthedocs.io/en/latest/tutorials/GaussianProcess-tutorial.html R V [ m s ] FEROSHARPSMA2019.0 2019.2 2019.4 2019.6 2019.8
Year a) JD - 2450000 R e s i d u a l s b) Phase R V [ m s ] c) P b = 18.39 daysK b = 7.7 ± 1.8 m s e b = 0.00 Figure 12. a)
Best-fit 1-planet Keplerian orbital model for TOI-257. Themaximum likelihood model is plotted in blue. We add in quadrature the radialvelocity jitter terms listed in Table 4 with the measurement uncertainties forall radial velocities to determine individual error bars. b) Residuals to thebest fit 1-planet model. c) Radial velocities phase-folded to the period ofplanet b. Red circles are the individual velocities binned in 0.08 units oforbital phase.
GP period produce qualitatively similar LLPs and mitigate peaksless than the candidate P rot and show strong evidence for both thetransiting planet and the candidate planet near 71 days (Figure 14).However, the evidence for a GP to model the remaining scatter isminimal. Both 1- and 2-planet models favor a single per-instrumentGaussian noise model over a 4 day GP ( ∆ AICc=3.22), while the GPis only marginally favored for the 8 day period case ( ∆ AICc=0.72).However, a 2-planet model with a GP is still favored over the cor-responding 1-planet model ( ∆ AICc=6.34). Figures A3 and A4 inthe Appendix shows the posterior distributions from
RadVel with aquasi-periodic Gaussian Process for a 1-planet and 2-planet circularmodels, respectively.Despite being statistically favored ( ∼ . σ detection), we donot claim TOI-257c as a confirmed planet, and relegate it to a “sta-tistically validated” candidate status. Nava et al. (2019) has shownthat activity can introduce spurious periodogram peaks at orbitalperiods longer than the stellar rotation period over the course ofa single season, particularly for radial velocities that are unevenlysampled as is the case herein, notably for the HARPS data. How-ever, with adequately sampled data (densely sampled with nightlycadence), Vanderburg et al. (2016b) find no evidence of spuriousradial velocity periodogram peaks at periods longer than the stellarrotation period. As such, additional radial velocity monitoring overfuture seasons or novel stellar-activity mitigation approaches willbe necessary to confirm the candidate second planet signal at ∼ MNRAS , 1–21 (2020) B. C. Addison et al.
Table 5.
Gaussian and min/max priors for quasi-periodic hyper-parameters for TOI-257 used in
RadVel . Notes. –(a) These interpretations are further subject tothe specific combination of values for the hyper-parameters, notably for cases with significantly different length and timescale factors. See Angus et al. (2018)for further discussion. (b) Also used for the initial guess.Parameter Unit; Physical Interpretation a µ b σ Min Max Citation η m s − , RV amplitude 10 None 0 100 stddev. of RVs, over-estimate η days, star-spot decay time-scale 10 5 0 100 Estimated from Giles et al. (2017), Fig. 5 η days, quasi-period 4.036 ( ×
2) 0.134 ( ×
2) 0 100
TESS light curve; Section 3.3, this paper η none, period length scale 0.3525 0.044 0 100 Dai et al. (2017); Haywood et al. (2018) R V [ m s ] FEROSHARPSMA2019.00 2019.25 2019.50 2019.75
Year a) JD - 2450000 R e s i d u a l s b) Phase R V [ m s ] c) P b = 18.39 daysK b = 9.0 ± 1.5 m s e b = 0.00 Phase R V [ m s ] d) P c = 71.04 ± 0.88 daysK c = 8.7 ± 1.7 m s e c = 0.00 Figure 13. a)
Best-fit 2-planet Keplerian orbital model for TOI-257. Themaximum likelihood model(s) is plotted in blue. We add in quadrature theradial velocity jitter terms listed in Table 4 with the measurement uncertain-ties for all radial velocities to determine individual error bars. b) Residuals tothe best fit 2-planet model. c) Same, but radial velocities phase-folded to theperiod of planet b. d) Same, but radial velocities phase-folded to the periodof a possible planet c. Red circles (if present) are the individual velocitiesbinned in 0.08 units of orbital phase.
Here we have presented the discovery of TOI-257b, the first Min-erva-Australis led confirmation of a
TESS transiting planet candi-date. TOI-257b is a warm sub-Saturn planet with a radius ∼ R P = . + . − . R ⊕ ) and a mass ∼ M P = . + . − . M ⊕ ) on a moderately eccen-tric ( e = . + . − . ) orbit of P = . ± . . + . − . g cm − , Figure 14.
Log-likelihood periodograms generated using
RadVel . Shadedfrom right to left are the period of planet b (insignificant width), the esti-mated stellar rotation period P rot from Table 4 (width is ± σ ), and aliases P rot / , P rot /
3. The orange line includes a Gaussian Process (GP) to modelstellar-activity with a quasi-periodic kernel with priors listed in Table 5,while the black line models remaining jitter as per-instrument Gaussiannoise.
Top : A 1-planet circular model tested at a wide range in fixed pe-riods, fitting for K , T c and the relative instrument dependent offsets andnoise terms (or single GP). Bottom : Same, but for a 2-planet model as-suming a fixed period and T c for planet b from Table 4, but varying bothsemi-amplitudes to search for additional planets. consistent with the density of Saturn (0.687 g cm − ) and less densethan Jupiter (1.326 g cm − ). Therefore, based on the mass, radius,and bulk density of this planet, it lies within the regime of plan-ets classified as ‘Neptunian worlds’ by Chen & Kipping (2017).Further analysis of the radial velocity data also reveals strong ev-idence for a second sub-Saturn mass planet in the system with anorbit of ∼
71 days. We consider this second planet as a ‘statisticallyvalidated’ candidate.To understand the planet formation process, we must determinethe compositions of warm sub-Saturns such as TOI-257b, a classof planet which is completely absent from the Solar System. Suchobjects provide important data for models studying planetary inte-riors because their masses are sufficiently small that their cores arenot degenerate. That is, their mass and radius are dependent on eachother such that the core and envelope mass fraction has a unique so-lution (e.g. Weiss & Marcy 2014; Petigura et al. 2016; Pepper et al.2017; Petigura et al. 2017). For planets near the mass of Jupiter,cores are degenerate, and planetary radii are essentially indepen-dent of mass. Warm sub-Saturns represent an observational sweet
MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star spot where mass and radius are comparatively easy to measure, andwhere those measurements deliver a unique solution for the planet’score/envelope mass ratio. This is particularly true for sub-Saturnswith incident flux less than the ∼ . × erg s − cm − limitwhere stellar irradiation can inflate planetary radii (Demory & Sea-ger 2011). The incident flux for TOI-257b is ∼ . × erg s − cm − and is very near this limit. Thus the effects of stellar irra-diation on the radius of TOI-257b is likely negligible, allowing itsinternal structure to be modeled and highlights the significant valueof discovering other similar planets with low incident flux.Figure 15 shows the radius-density diagram for Neptunianworlds (similarly defined after Chen & Kipping 2017 as those withradii from ∼ − R ⊕ ). We show those planets for which the den-sity has been measured to a precision of better than 50%. TOI-257bhas a mean density that is comparable to other exoplanets aroundthe same size. Figure 15 also shows the apparent trend of decreas-ing bulk density as a function of planet radius, indicative of theincreasingly large volatile gas envelope up to around the radius ofSaturn.In Figure 16 we plot the orbital period versus eccentricityfor well-characterized transiting exoplanets with a measured massfrom radial velocity measurements. The size of the symbols scaleswith log of the planet mass. TOI-257b is on an eccentric orbitof e = . + . − . and lies near the upper range of eccentricityvalues for other ‘warm’ Neptune and Jovian planets that have orbitalperiods of P ≥
10 days in this sample. Figure 16 also shows thatplanets on short period orbits of P ≤
10 days tend to have nearlycircular orbits, likely due to affects of tidal interactions with thehost star circularizing the orbits that were once more eccentric(Fabrycky & Tremaine 2007). For planets orbiting beyond P ∼
10 days, tidal effects with the host star are expected to be too weakto fully circularize the orbits and a more broad distribution of orbitaleccentricity is observed.Measurements of the spin-orbit alignment for transiting warmNeptunian and Jovian worlds via the Rossiter-McLaughlin effectcan provide powerful insights into the origins and migration his-tories of these planets (e.g., Queloz et al. 2000; Chatterjee et al.2008; Winn et al. 2010; Naoz et al. 2011; Addison et al. 2018;Wang et al. 2018). Both classes of planets are strongly believedto have been formed beyond their hosts’ protostellar ice line andthen experienced inward migration through one of two types of mi-gration channels, quiescent migration through the disk (Lin et al.1996) or violently dynamical high-eccentricity migration (Fabrycky& Tremaine 2007; Ford & Rasio 2008; Naoz et al. 2011). The lattermigration mechanism is thought to be responsible for producingmany of the known hot Jupiters due to the large observed range intheir spin-orbit angles (e.g., see, Albrecht et al. 2012; Addison et al.2013, 2018). However, it is unknown if this is the case for the warmsub-Saturn and Neptunian worlds like TOI-257b with orbits greaterthan 10 days. The limited sample of spin-orbit angles measured forthese planet populations (only seven so far according to the TEP-Cat catalog , see Southworth 2011) makes it difficult to draw anyfirm conclusions and more measurements are urgently needed. Thisplanet presents a suitable candidate for studying the spin-orbit viathe Rossiter-McLaughlin effect. We predict that the radial velocitysemi-amplitude of the Rossiter-McLaughlin effect for TOI-257 tobe ∼ − based on the stellar and planetary parameters weobtained for this system. The predicted signal, while small, shouldbe detectable on very high-precision ( ∼ − ) radial velocity R ⊕ )10 − P l a n e t D e n s i t y ( g c m − ) TOI-257b
JSUN
Figure 15.
Planet radii versus density for Neptunian planets with R P = − R ⊕ and that have a density measured to better than 50%. TOI-257bstudied in this paper is labeled and plotted in red. The Solar System planetsSaturn, Uranus, and Neptune are plotted as the gold colored letter S, lightblue colored letter U, and dark blue colored letter N, respectively. Planetstaken from the NASA Exoplanet Archive ( https://exoplanetarchive.ipac.caltech.edu/ ). Orbital Period (d)0 . . . . . E cce n tr i c i t y TOI-257b
Figure 16.
Orbital period versus eccentricity for well-characterized transit-ing exoplanets and confirmed by radial velocities. TOI-257b is labeled andplotted in red. The size of the plotted circles scales with log of the planetmass. Circles in black are those with a measured uncertainty for the eccen-tricity while those in orange have no reported uncertainty on the eccentricityvalue. facilities in the south such as on HARPS (Rupprecht et al. 2004),PFS (Crane et al. 2006), and ESPRESSO (Pepe et al. 2010). Wepredict, given the stellar rotational velocity (as determined from therotational period and stellar radius) is consistent with the measured v sin i from spectroscopy (i.e., suggesting that the stellar obliquityis near 90 deg), that the projected spin-orbit angle λ when measuredshould be close to the true spin-orbit angle ψ . MNRAS , 1–21 (2020) B. C. Addison et al.
We report the discovery of TOI-257b, a R P = . + . − . R J ( R P ∼ . R ⊕ ) and M P = . + . − . M J ( M P ∼ . M ⊕ ) tran-siting planet found by TESS and confirmed using radial velocitydata from Minerva-Australis, FEROS, and HARPS as well as di-rect imaging from SOAR and Zorro. We also statistically validate anadditional non-transiting long-period ( ∼
71 day) sub-Saturn massplanet candidate orbiting TOI-257 from analysis of the radial veloc-ity data. TOI-257b belongs to a population of exoplanets betweenthe sizes of Neptune and Saturn that appears to be rare. Furthermore,TOI-257b transits a very bright star ( V = .
570 mag) on a relativelylong-period orbit of 18.423 days making it a great candidate forfuture follow-up observations to measure its spin-orbit alignment.Warm sub-Saturns such as TOI-257b are important population ofplanets to study for understanding the formation, internal structuresand compositions, and evolution and migration of giant planets.Future observational work of this planetary system will help to elu-cidate our understanding of these rare sub-Saturn planets that areabsent in the Solar System.
ACKNOWLEDGEMENTS
Minerva-Australis is supported by Australian Research CouncilLIEF Grant LE160100001, Discovery Grant DP180100972, MountCuba Astronomical Foundation, and institutional partners Univer-sity of Southern Queensland, UNSW Australia, MIT, Nanjing Uni-versity, George Mason University, University of Louisville, Univer-sity of California Riverside, University of Florida, and The Univer-sity of Texas at Austin.We respectfully acknowledge the traditional custodians of alllands throughout Australia, and recognise their continued culturaland spiritual connection to the land, waterways, cosmos, and com-munity. We pay our deepest respects to all Elders, ancestors anddescendants of the Giabal, Jarowair, and Kambuwal nations, uponwhose lands the Minerva-Australis facility at Mt Kent is situated.B.P.B. acknowledges support from the National Science Foun-dation grant AST-1909209.This research has made use of the NASA Exoplanet Archive,which is operated by the California Institute of Technology, undercontract with the National Aeronautics and Space Administrationunder the Exoplanet Exploration Program. Funding for the
TESS mission is provided by NASA’s Science Mission directorate. Weacknowledge the use of public
TESS
Alert data from pipelinesat the
TESS
Science Office and at the
TESS
Science ProcessingOperations Center. The results reported herein benefited from col-laborations and/or information exchange within NASA’s Nexus forExoplanet System Science (NExSS) research coordination networksponsored by NASA’s Science Mission Directorate. Based on ob-servations obtained at the Gemini Observatory, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under a cooperative agreement with the NSF on behalf of the Gem-ini partnership: the National Science Foundation (United States),National Research Council (Canada), CONICYT (Chile), Minis-terio de Ciencia, Tecnología e Innovación Productiva (Argentina),Ministério da Ciência, Tecnologia e Inovação (Brazil), and KoreaAstronomy and Space Science Institute (Republic of Korea). Someof the Observations in the paper made use of the High-ResolutionImaging instrument Zorro at Gemini-South. Zorro was funded bythe NASA Exoplanet Exploration Program and built at the NASA Ames Research Center by Steve B. Howell, Nic Scott, Elliott P.Horch, and Emmett Quigley.D.H. acknowledges support by the National Aeronautics andSpace Administration through the
TESS
Guest Investigator Pro-gram (80NSSC18K1585) and by the National Science Foundation(AST-1717000). A.C. acknowledges support by the National Sci-ence Foundation under the Graduate Research Fellowship Program.W.J.C., W.H.B., M.B.N. and A.M. acknowledge support from theScience and Technology Facilities Council and UK Space Agency.Funding for the Stellar Astrophysics Centre is provided by The Dan-ish National Research Foundation (Grant DNRF106). R.B. acknowl-edges support from FONDECYT Post-doctoral Fellowship Project3180246, and from the Millennium Institute of Astrophysics (MAS).H.Z. Hui Zhang is supported by the Natural Science Foundation ofChina ( NSFC grants 11673011, 11933001). A.J. acknowledgessupport from FONDECYT project 1171208 and by the Ministryfor the Economy, Development, and Tourism’s Programa Inicia-tiva Científica Milenio through grant IC 120009, awarded to theMillennium Institute of Astrophysics (MAS). A.M.S. is pastiallysupported by grants ESP2017-82674-R (Spanish Government) and2017-SGR-1131 (Generalitat de Catalunya). A.M. acknowledgessupport from the ERC Consolidator Grant funding scheme (projectASTEROCHRONOMETRY, G.A. n. 772293). R.A.G. acknowledgethe support of the PLATO grant from the CNES. S.M. acknowl-edges support from the European Research Council through theSPIRE grant 647383. T.L.C. acknowledges support from the Eu-ropean Union’s Horizon 2020 research and innovation programmeunder the Marie Skłodowska-Curie grant agreement No. 792848(PULSATION). This work was supported by FCT/MCTES throughnational funds (UID/FIS/04434/2019). E.C. is funded by the Euro-pean UnionâĂŹs Horizon 2020 research and innovation programunder the Marie Sklodowska-Curie grant agreement No. 664931.V.S.A. acknowledges support from the Independent Research FundDenmark (Research grant 7027-00096B). S.B. acknowledges NASAgrant NNX16AI09G and NSF grant AST-1514676. I.J.M.C. ac-knowledges support from the NSF through grant AST-1824644,and from NASA through Caltech/JPL grant RSA-1610091. T.D. ac-knowledges support from MITâĂŹs Kavli Institute as a Kavli post-doctoral fellow. D.B. acknowledges support from NASA through theTESS GI program (80NSSC19K0385). C.K. acknowledges supportby Erciyes University Scientific Research Projects CoordinationUnit under grant number
Software:
Astropy (Astropy Collaboration et al. 2018), Mat-plotlib (Hunter 2007), DIAMONDS (Corsaro & De Ridder 2014),isoclassify (Huber et al. 2017),
EXOFASTv2 (Eastman et al. 2013;Eastman 2017; Eastman et al. 2019)
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OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star K b = 7.73 +1.811.75 M A MA = 0.42 +1.941.93
681 01 21 4 M A MA = 9.22 +1.151.03 H A R P S HARPS = 31.88 +1.641.63 . . . . . H A R P S HARPS = 5.21 +0.960.80 F E R O S FEROS = 4.83 +3.083.19 K b F E R O S MA MA HARPS . . . . . HARPS
FEROS
FEROSFEROS = 6.69 +1.561.49
Figure A1.
Posterior distributions from
RadVel for all parameters for a1-planet circular model.
APPENDIX A:
RADVEL
POSTERIOR DISTRIBUTIONPLOTS
MNRAS , 1–21 (2020) B. C. Addison et al. b = 46.75 +322752374.70245965870.42 . . . . . K b K b = 9.02 +1.501.48 P c P c = 71.04 +0.940.81 T c o n j c +2.4587e6 T conj c = 2458707.29 +2.211.90 K c K c = 8.69 +1.661.70 M A MA = 0.87 +1.831.84 M A MA = 7.84 +1.221.12 H A R P S HARPS = 25.52 +1.461.43 H A R P S HARPS = 5.60 +0.900.77 F E R O S FEROS = 5.96 +3.133.15 . . . . b F E R O S . . . . . K b
68 70 72 74 76 P c T conj c +2.4587e6 K c MA MA
21 24 27 30
HARPS
HARPS
18 12 6 0 6
FEROS
FEROSFEROS = 5.61 +1.801.76
Figure A2.
Same as Fig. A1, but for a 2-planet circular model. MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star K b = 9.30 +2.252.25 = 9.12 +1.371.14 = 4.52 +1.991.65 . . . . = 4.02 +0.140.14 . . . . = 0.38 +0.040.04 M A MA = 0.31 +2.472.35 H A R P S HARPS = 32.41 +3.313.33 K b F E R O S .
75 4 .
00 4 .
25 4 . .
24 0 .
32 0 .
40 0 . MA
16 24 32 40
HARPS
20 10 0 10
FEROSFEROS = 4.41 +4.214.23
Figure A3.
Posterior distributions for all parameters for a 1-planet circularmodel in
RadVel with a quasi-periodic Gaussian Process to model stellar-activity. Priors for hyper-parameters are provided in Table 5.MNRAS , 1–21 (2020) B. C. Addison et al. K b = 8.99 +2.172.17 P c P c = 72.68 +2.291.98 T c o n j c T conj c = 2458707.03 +4.284.71 K c K c = 7.69 +2.412.52 . . . . = 7.81 +1.281.08 = 3.47 +1.831.58 . . . . . = 4.03 +0.140.14 . . . . = 0.37 +0.040.04 M A MA = 0.78 +1.991.96 H A R P S HARPS = 27.12 +3.763.67 K b F E R O S
56 64 72 80 88 P c T conj c K c . . . . .
50 3 .
75 4 .
00 4 .
25 4 . .
24 0 .
32 0 .
40 0 . MA
16 24 32 40
HARPS
16 8 0 8
FEROSFEROS = 4.45 +3.993.89
Figure A4.
Same as Fig. A3, but for a 2-planet circular model. MNRAS , 1–21 (2020)
OI-257b (HD 19916b): A Warm sub-Saturn on a Moderately Eccentric Orbit Around an Evolved F-type Star APPENDIX B: AUTHOR AFFILIATIONS University of Southern Queensland, Centre for Astrophysics, WestStreet, Toowoomba, QLD 4350 Australia Sub-department of Astrophysics, Department of Physics, Univer-sity of Oxford, Denys Wilkinson Building, Keble Road, Oxford,OX1 3RH, UK Department of Physics & Astronomy, George Mason University,4400 University Drive MS 3F3, Fairfax, VA 22030, USA Department of Earth and Planetary Sciences, University of Cali-fornia, Riverside, CA 92521, USA Institute for Astronomy, University of Hawai‘i, 2680 WoodlawnDrive, Honolulu, HI 96822, USA Department of Astronomy, The University of Texas at Austin,Austin, TX 78712, USA NASA Sagan Fellow School of Physics and Astronomy, University of Birmingham,Birmingham B15 2TT, UK Stellar Astrophysics Centre (SAC), Department of Physics and As-tronomy, Aarhus University, Ny Munkegade 120, DK-8000 AarhusC, Denmark NSF Graduate Research Fellow Center for Astrophysics, Harvard & Smithsonian, 60 Garden St.,Cambridge, MA 02138, USA Dunlap Institute for Astronomy and Astrophysics, University ofToronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada Center of Astro-Engineering UC, Pontificia Universidad Católicade Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago,Chile Instituto de Astrofísica, Pontificia Universidad Católica de Chile,Av. Vicuña Mackenna 4860, Macul, Santiago, Chile Millennium Institute for Astrophysics, Chile Space Telescope Science Institute, 3700 San Martin Drive, Bal-timore, MD 21218, USA Department of Physics and Astronomy, University of Louisville,Louisville, KY 40292, USA Department of Physical Sciences, Kutztown University, Kutz-town, PA 19530, USA Vanderbilt University, Department of Physics & Astronomy, 6301Stevenson Center Ln., Nashville, TN 37235, USA Fisk University, Department of Physics, 1000 18th Ave. N.,Nashville, TN 37208, USA Sydney Institute for Astronomy (SIfA), School of Physics, Uni-versity of Sydney, 2006, Australia Department of Physics, Westminster College, 319 South MarketStreet, New Wilmington, PA 16172, USA Dept. of Physics and Astronomy, University of Kansas, 1251Wescoe Hall Dr.,Lawrence, KS 66045, USA Department of Physics, Massachusetts Institute of Technology,Cambridge, MA, USA Department of Astronomy, Yale University, New Haven, CT06511, USA Space Science & Astrobiology Division, NASA Ames ResearchCenter, Moffett Field, CA 94035, USA Department of Physics and Astronomy, The University of NorthCarolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA Department of Physics and Kavli Institute for Astrophysics andSpace Research, Massachusetts Institute of Technology, Cambridge,MA 02139, USA Kavli Fellow Exoplanetary Science at UNSW, School of Physics, UNSW Syd-ney, NSW 2052, Australia
51 Pegasi b
Fellow School of Astronomy and Space Science, Key Laboratory of Mod-ern Astronomy and Astrophysics in Ministry of Education, NanjingUniversity, Nanjing 210046, Jiangsu, China Max-Planck-Institut für Astronomie, Königstuhl 17, Heidelberg69117, Germany Harvard-Smithsonian Center for Astrophysics, 60 Garden St,Cambridge, MA 02138, USA Earth and Planetary Sciences, Massachusetts Institute of Tech-nology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Department of Astrophysical Sciences, Princeton University, 4Ivy Lane, Princeton, NJ 08544, USA NASA Ames Research Center, Moffett Field, CA 94035, USA School of Physical Sciences, The Open University, Milton KeynesMK7 6AA, UK Department of Physics, Lehigh University, 16 Memorial DriveEast, Bethlehem, PA 18015, USA Exoplanets and Stellar Astrophysics Laboratory, Mail Code 667,NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Perth Exoplanet Survey Telescope, Perth, Western Australia Center for Space Science, NYUAD Institute, New York Univer-sity Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates Dept. of Chemistry & Physics, Florida Gulf Coast University,10501 FGCU Blvd. S., Fort Myers, FL 33965 USA Instituto de Astrofísica e Ciências do Espaço, Universidade doPorto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal Departamento de Física e Astronomia, Faculdade de Ciências daUniversidade do Porto, Rua do Campo Alegre, s/n, PT4169-007Porto, Portugal INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78,95123, Catania, Italy Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna,Tenerife, Spain Universidad de La Laguna (ULL), Departamento de Astrofísica,E-38206 La Laguna, Tenerife, Spain IRFU, CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette,France AIM, CEA, CNRS, Université Paris-Saclay, Université ParisDiderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette, France Center for Exoplanets and Habitable Worlds, Department of As-tronomy & Astrophysics, 525 Davey Laboratory, The PennsylvaniaState University, University Park, PA 16802, USA Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany Institute of Astrophysics, University of Vienna, 1180 Vienna,Austria Department of Physics and Astronomy, Iowa State University,Ames, IA 50011 USA Department of Astronomy & Space Sciences, Erciyes University,Kayseri, Turkey LESIA, Observatoire de Paris, Université PSL, CNRS, SorbonneUniversité, Université de Paris, 92195 Meudon, France Institute of Space Sciences (ICE, CSIC) Campus UAB, Carrer deCan Magrans, s/n, E-08193, Barcelona, Spain Institut dâĂŹEstudis Espacials de Catalunya (IEEC), C/GranCapita, 2-4, E-08034, Barcelona, Spain
This paper has been typeset from a TEX/L A TEX file prepared by the author.MNRAS000