A spectro-polarimetric study of the planet-hosting G dwarf, HD 147513
Gaitee A. J. Hussain, Julian David Alvarado-Gómez, Jason Grunhut, Jean-Francois Donati, Evelyne Alecian, Mary Oksala, Julien Morin, Rim Fares, Moira Jardine, Jeremy J. Drake, Ofer Cohen, Sean Matt, Pascal Petit, Seth Redfield, Frederick M. Walter
AAstronomy & Astrophysics manuscript no. RevHD147513_spectroscopicanalysis-arxiv c (cid:13)
ESO 2018October 16, 2018
A spectro-polarimetric study of the planet-hosting G dwarf,HD 147513
G. A. J. Hussain , , J. D. Alvarado-Gómez , , J. Grunhut , J.-F. Donati , , E. Alecian , , , M. Oksala , J. Morin , R.Fares , M. Jardine , J.J. Drake , O. Cohen , S. Matt , P. Petit , , S. Redfield and F. M. Walter (A ffi liations can be found after the references) Received —–; accepted —–
ABSTRACT
The results from a spectro-polarimetric study of the planet-hosting Sun-like star, HD 147513 (G5V), are presented here. Robustdetections of Zeeman signatures at all observed epochs indicate a surface magnetic field, with longitudinal magnetic field strengthsvarying between 1.0-3.2 G. Radial velocity variations from night to night modulate on a similar timescale to the longitudinal magneticfield measurements. These variations are therefore likely due to the rotational modulation of stellar active regions rather than the muchlonger timescale of the planetary orbit ( P orb =
528 d). Both the longitudinal magnetic field measurements and radial velocity variationsare consistent with a rotation period of 10 ± (cid:48) logR (cid:48) HK = − . i ∼ ◦ . We present preliminary magnetic fieldmaps of the star based on the above period and find a simple poloidal large-scale field. Chemical analyses of the star have revealedthat it is likely to have undergone a barium-enrichment phase in its evolution because of a higher mass companion. Despite this, ourstudy reveals that the star has a fairly typical activity level for its rotation period and spectral type. Future studies will enable us toexplore the long-term evolution of the field, as well as to measure the stellar rotation period, with greater accuracy. Key words. stars: activity – stars: magnetic field – stars: solar-type – stars: individual: HD 147513 – techniques: polarimetric —techniques: radial velocities
1. Introduction
As e ff orts to find planets around other stars intensify, there is aneed to better characterise stellar magnetic activity on a rangeof timescales in moderate-activity stars. The HARPS instrumenton the ESO 3.6-m telescope at the La Silla Observatory is ahigh-precision velocimeter, enabling stability to the 1 m s − level(Pepe et al. 2005). The instrument also o ff ers a polarimetricmode, which enables the detection of stellar surface magneticfields (Piskunov et al. 2011). HARPS is thus ideally suited toboth studying stellar magnetic activity and analyse the contribu-tion of magnetic activity to radial velocity “jitter” in intensityline profiles (also see Dumusque et al. 2011).Spectro-polarimetric observations of cool stars spanning al-most three decades have yielded not only direct measurements ofmagnetic fields at the surfaces of these stars (Donati et al. 1997),but also detailed maps of large scale magnetic field topologiesthrough the technique of Zeeman Doppler imaging (Semel 1989,Brown et al. 1991, Donati & Landstreet 2009). Over 40 mainsequence G-K stars have been imaged to date, and these studiesshow general clear trends (see Vidotto et al. 2014). The slow-est rotators ( P rot >
15 d) possess the simplest and weakest largescale fields. Fields strengthen and increase in complexity in morerapidly rotating stars. Many of the maps for the most rapidly ro-tating stars also feature strong surface toroidal fields; a featurethat has no clear counterpart on the Sun and very likely indicatesa change in the underlying stellar dynamo. The BCool collabo- ration † has collected and analysed circularly polarised spectra ofover 170 solar-type stars. Marsden et al. (2014) report magneticfield detections on 67 of these stars and present these detectionsin the context of their chromospheric activity, rotation and age.We used HARPS in polarimetric mode to obtain high qualityS / N spectro-polarimetric time series of two low-moderate activ-ity planet-hosting, solar-type stars. These data enable us to char-acterise the magnetic activity properties in detail and use thesemaps to model the coronae and conditions in these young plan-etary systems. In the first study Alvarado-Gomez et al. (2015)present magnetic field maps of the planet-hosting G dwarf, HD1237. Our time series revealed a rotation period of seven daysand the resulting magnetic field maps showed a strong toroidalcomponent.We present the results for the second target in the study,HD 147513. In Section 2 we give a more detailed descriptionof the stellar system. Observations and the analysis of the chro-mospheric activity are in Sects. 3 & 4, respectively. The pho-tospheric line profiles are used to measure the longitudinal mag-netic field and radial velocity and to study the night-to-night vari-ability in both these quantities. Our conclusions are summarisedin Sect. 6, and maps based on our best estimates of the stellarrotation period are presented in the Appendix. † Bcool is part of the MagIcS international project - seehttp: // / users / donati / magics / v1 / for more informa-tion. Article number, page 1 of 7 a r X i v : . [ a s t r o - ph . S R ] O c t & A proofs: manuscript no. RevHD147513_spectroscopicanalysis-arxiv
2. HD 147513
HD 147513 (GJ 620.1 A, HR 6094) is a bright ( mV = .
4) Gdwarf at a distance of 12.9 pc (Valenti & Fischer 2005). Its mainproperties are listed in Table 1; these include both published val-ues and those determined in the analysis presented here.HD 147513 is shown to be moderately magnetically ac-tive, with an average X-ray luminosity of about 10 erg s − inthe 0.1-2.4 keV ROSAT PSPC band (Schmitt & Liefke 2004).For comparison, the average solar X-ray luminosity is approxi-mately 10 . erg s − , varying by an order of magnitude over thecourse of the 11-year solar activity cycle (Judge et al. 2003)Chromospheric activity is indicated by significant emission inits Ca II H&K profiles with a range of R (cid:48) HK values reportedin the literature ( − . < log R (cid:48) HK < − .
38) (Sa ff e et al. 2005).The rotation period of the star has been estimated based on theabove log R (cid:48) HK values, and published estimates range between4.7 Mayor et al. (2004) and 8 . ± . M sin i = . M J , P orb = . K = . ± . − with a dispersion of 5.7 m s − . Mayoret al. (2004) report that the orbital properties of this planetarysystem are characteristic of intermediate-to-long orbital periodradial velocity planets, with a semi-major axis, a = .
32 au andeccentricity, e = . ◦ M (cid:12) . This more massive star may have drivensu ffi cient mass transfer onto HD 147513 to explain its observedabundance of s-process elements. Indeed Porto de Mello & daSilva (1997) suggest further that these stars may have been partof a multiple star system, bound with the binary, HR 2047; whichis 24 pc away, a confirmed member of the UMa moving group,and which also shows evidence of barium enrichment (albeit toa lesser extent).
3. Observations
In this paper we present high S / N high resolution circularlypolarised spectra obtained using the polarimetric mode of theHARPS echelle spectrograph at the ESO 3.6-m telescope at theLa Silla Observatory (Piskunov et al. 2011, Mayor et al. 2003).The spectra encompass a wavelength range from 378 nm to691 nm, with a 8 nm gap centred at 530 nm. The data were ac-quired in 2012 July under changeable weather conditions and afull observation log is shown in Table 2. All the exposures hadthe same exposure time of 3600 s for the full circularly polarisedspectrum (Stokes V) sequence. This is obtained by combiningfour individual sub-exposures using the ratio method (see Donatiet al. 1997; Bagnulo et al. 2009) and enables a null-polarisationspectrum to be constructed in order to check for possible spu-
Table 1.
HD 147513 basic properties.
Parameter Value ReferenceSp. Type G5V Soderblom & Mayor (1993) B − V ∼ .
45 Rocha-Pinto & Maciel (1998) T e ff [K] 5930 ±
44 Valenti & Fischer (2005)log( g ) 4 . ± .
06 Valenti & Fischer (2005) M ∗ [M (cid:12) ] 1.07 ± .
01 Takeda et al. (2007) R ∗ [R (cid:12) ] 0.98 . − . Takeda et al. (2007) v sin i [km s − ] 1 . ± . i [ ◦ ] 18 − This work v R [km s − ] 13 . ± .
09 This work P rot [days] 10 . ± . R (cid:48) HK ) − . ± .
05 This worklog L X Table 2.
Journal of observations. The columns contain the date, the cor-responding Barycentric Julian Date (BJD), the start time of the observa-tions in UT, the exposure times, and the Stokes I peak Signal-to-Noiseratio (S / N).Date BJD (TT) UT Stokes I(2012) (2400000 + ) Peak S / NJul 15 56123.53315 00:40:36 660Jul 15 56124.52049 24:22:28 810Jul 17 56125.54088 00:51:56 570Jul 18 56126.53188 00:39:03 720Jul 18 56127.50182 23:55:52 734Jul 20 56128.53529 00:44:09 923Jul 22 56130.59443 02:09:32 550Jul 23 56131.71964 05:09:56 340 rious polarisation contributions to the Stokes V profiles (Donatiet al. 1997).Data were reduced using the ESPRIT package which hasbeen adapted for the HARPS instrument (Donati et al. 1997,Hébrard et al. in prep. ). This package produces an optimal ex-traction of the bias-subtracted spectra after flat-fielding correc-tions. The slit shape is averaged over each order and used tocompute the curvilinear coordinate system along which the spec-tra are extracted. The calibration frames required by the packageare the bias frames, flat field frames and a good quality ThAr arcspectrum that were acquired each night. Spectra extracted usingthe REDUCE package (Piskunov & Valenti 2002; Makaganiuket al. 2011) were almost identical compared to those reducedwith the ESPRIT, with the latter showing slightly higher S / Nlevels. As barycentric corrections are also applied to the spectrareduced by ESPRIT, this is the dataset used in the analysis pre-sented here. The extracted data have spectroscopic resolutionsvarying from 95 000 to 113 000, depending on the wavelength,with a median value of 106 000. Given the noise level of thesedata 1 m s − accuracy should be achievable in the radial velocitymeasurements.
4. Chromospheric activity
In order to characterise the chromospheric activity level of thestar at the observed epoch, we present an analysis of the Ca II H(396.8492 nm) & K (393.3682 nm) lines, converting the fluxes
Article number, page 2 of 7ussain et al.: Spectro-polarimetric study of HD 147513
Fig. 1.
Chromospheric activity in HD 147513: Comparison between CaII K observed with HARPS in 2012 July 18 (red dotted line) and archiveFEROS spectra from 2006 July 15 (black solid line). The HARPS spec-tra have been rebinned to enable a better comparison with the FEROSspectra, which have a lower spectroscopic resolution. to the classic Mount Wilson S-index, S MW . This index is definedas follows: S MW = H + KR + V . (1)Here H and K represent the fluxes measured in each of the Ca IIline cores using 0 .
105 nm wide spectral windows. R and V arethe fluxes measured in the continuum over 2 nm windows cen-tred at 390 . . α , for a se-ries of standard stars observed with HARPS that also have pub-lished S MW values. The spectra from a number of G and K-typestars were renormalised to the continuum level in the same wayand aligned using a high S / N HARPS solar spectrum as a tem-plate. The largest contribution to the measurement errors is likelydue to slight di ff erences in the continuum normalisation for eachindividual exposure. We find that the renormalisation introducesa typical error of 5%. The errors on the values of the S-index arehowever dominated by the conversion factor, α , and we refer toAlvarado-Gomez et al. (2015) for further details. Their conver-sion factor, α = . ± .
65, is used to compute the HARPSS-index using the measured HARPS fluxes, H , K , R , and V : S MW = α (cid:18) H + KR + V (cid:19) H (2)Sample spectra from our dataset are shown in Fig. 1 (redline), clearly illustrating emission in the cores of both Ca II H&Kprofiles due to significant chromospheric heating and indicatinga moderate magnetic activity level. An average S-index of 0.23 ± .
01 is computed for our dataset. No significant variability isfound in the Ca II H&K fluxes over eight days, indicating a con-stant contribution from the chromospheric active regions even asthe star rotates (with estimated rotation periods in the literatureranging from 4.7–8.5 d).The S-index is converted to the chromospheric activity in-dices, R HK and R (cid:48) HK applying the colour and photospheric cor-rections for main sequence cool stars and a B − V of 0.62 (Mid-delkoop 1982; Noyes et al. 1984). This conversion results in anaverage log R (cid:48) HK of -4.64 ± .
06 (from log R HK = − . ± . R (cid:48) HK values in theliterature, ranging from − . − .
38 over a period spanningalmost 20 years (from 1983 to 2004; Soderblom & Clements1987, Sa ff e et al. 2005). In order to investigate whether long-term changes (e.g., due to magnetic activity cycles) might beat the root of these di ff erent measurements we searched publicarchives for spectra of HD 147513 that cover the relevant wave-length range. Fig. 1 shows a comparison between spectra ac-quired in 2006 (archive FEROS spectra) with our 2012 HARPSspectra. It is clear that there is little variability over these twoepochs and the corresponding R (cid:48) HK indices are therefore iden-tical within the measurement errors. While this cannot excludeintrinsic variability over a wider range of timescales it is likelythat the chromospheric activity level is more stable than sug-gested from the range of published measurements. We concludethat these variations are likely dominated by di ff erences in con-versions to the Mt Wilson index from spectra acquired from arange of instruments.It is possible to estimate the rotation period of the star withinabout 20% accuracy using its log R (cid:48) HK index and the conversionfactors presented by Noyes et al. (1984). The Rossby number ofthe star is computed using its R (cid:48) HK index, while the convectiveturnover timescale, τ c can be estimated from the star’s B − V .We find a period of 12.4 d, for our value (log R (cid:48) HK = − . R (cid:48) HK of − . R (cid:48) HK = − .
38 (Mayor et al. 2004). Combining these esti-mates for the stellar rotation period with its projected rotationalvelocity, v e sin i , and radius (Table 1), it is possible to computethe inclination angle of the star. For HD 147513 a relatively lowinclination angle is expected; using the range of 4.7–12.4 d pe-riods and radius (Table 1) the star’s inclination angle must bebetween 10–25 ◦ .
5. Photospheric line profiles & stellar magnetic field
As the large scale magnetic field in cool stars such as HD 147513is expected to be relatively weak ( (cid:28) / N by afactor of ∼
30, compared to the original spectrum in this way.The mask used in the LSD analysis is constructed from anatomic line list extracted for a star with the same basic pa-rameters ( T e ff , log g ) as HD 147513 from the VALD database † (Kupka et al. 2000). This downloaded line list is first “cleaned”of all strong lines, including any diagnostics that are likely tohave significant contributions from the chromosphere (e.g., CaII H&K, H α ). This list is then further tailored to the star by ad-justing the individual depths to fit those of the spectral lines ofHD 147513. As discussed by Alvarado-Gomez et al. (2015) this“clean-tweaking” method (see Neiner et al. 2012) is most com-monly employed when applying LSD to hot (OB) stars. As coolstars have thousands of photospheric line profiles, this techniquedoes not have as significant an impact on the LSD profiles butdoes increase them in S / N by between 5-10% compared to theoriginal “clean” line mask (Alvarado-Gomez et al. 2015). LSD † http://vald.astro.uu.se/ – Vienna Atomic Line Database(VALD3) Article number, page 3 of 7 & A proofs: manuscript no. RevHD147513_spectroscopicanalysis-arxiv is then applied to our spectro-polarimetric dataset using thesetailored clean-tweaked masks, cutting o ff at a depth of 0.1. Thisresults in almost 4500 lines being used in the deconvolution. Thevelocity step used is 0.8 km / s, which corresponds to the averagepixel size of the CCD. Fig. 2 shows the time series of the derived LSD profiles of HD147513 over 8 days.The Stokes I (unpolarised) profile is shownin the left column, while the Stokes V (circularly polarised) pro-files for each epoch are compared to the mean profile on theright. The noise level in the Stokes I LSD profiles remains fairlyconstant ( ∼ − ) over the whole dataset. Definite positivemagnetic field detections are found in each of the circularly po-larised (Stokes V) profiles. From Fig. 2 it is clear that the shapeof the Stokes V profiles is largely unchanged over the courseof the observations, showing a classic antisymmetric shape withrespect to the centre. There does however, appear to be a modula-tion in the amplitude of the Stokes V profiles which is indicativeof a small level of inhomogeneity and non-axisymmetry in thelarge scale field of the star. Fig. 2.
LSD Profiles of HD 147513.
Left:
Stokes I (intensity) and right:
Stokes V (circularly polarised) profiles. The mean Stokes V profilecomputed over this dataset has been overplotted (red line) to investi-gate variability from night to night. The dashed vertical lines denote thevelocity limits over which the B (cid:96) measurements were calculated. The derived LSD profiles can be used to compute the surfaceaveraged longitudinal magnetic field ( B (cid:96) ). This quantity is mea-sured with respect to the intensity line profile, using the centralwavelength λ (0.519 µ m) and the mean Landé factor, ¯ g (1.197)of the LSD profiles according to the following formula (Donatiet al. 1997; Wade et al. 2000). B (cid:96) = − (cid:82) v V( v ) dv λ ¯ g (cid:82) [1 − I( v )] dv , (3)The measurements of B (cid:96) for HD 147513 are calculated be-tween 5.0 and 21.2 km s − from the line centre and show vari-ability from night to night. The uncertainties on these values are determined via standard error propagation from the spectra. Therange of values shown in Fig. 3 is higher than the range typicallyseen on the Sun, where | B (cid:96) | is predominantly under 1 G; solar | B (cid:96) | values can get as high as 3-4G but only very rarely (Kotov et al.1998). As noted in Sect. 1, the chromospheric and coronal mag-netic activity levels of the Sun and HD 147513 are very di ff erent.It is therefore highly likely that these surface magnetic field mea-surements have a di ff erent origin; HD 147513 should have muchlarger, stronger active regions at the stellar surface compared tothe Sun. The B (cid:96) modulations observed in Fig. 3 are significant(over 3 σ ) and their timescale is consistent with that expected byrotational modulation of active regions. Fig. 3 strongly discountsthe possibility of a period shorter than 8 d and hence excludesthe previously published value of 4.7 d. Naturally this argumentassumes that the variability is not driven by the emergence ofnew flux. Studies of active cool stars tracing starspot lifetimestypically show that the large scale field should remain stable overa period of several weeks and so this appears to be a reasonableassertion (Barnes et al. 1998, Hussain 2002, Strassmeier 2009). Fig. 3. B (cid:96) measurements of HD 147513 show significant variability overthe 8-day span of the dataset. The x-axis shows the time in BarycentricJulian Date (see Table 2) and the red and black dashed lines denote themean B (cid:96) (2.0 G) and 0 G levels respectively. We measured the radial velocity (RV) at each observation epochby least-squares fitting Gaussians to the Stokes I LSD profiles.The resulting measurements are shown in Fig. 4, the errors aresmaller than the symbol sizes ( ± − ). We find significantvariations about a mean RV, ¯ v = . ± .
009 km s − over8-days. As the span of our observations is so much smaller thanthe orbital period of the planet it is clear that these variations arestellar in origin.This is clearly demonstrated in the comparisonof the measured RV variability with the expected RV contribu-tion caused by HD147513b in Fig. 4. This was computed usingthe ephemeris reported by Mayor et al. (2004). We note that thetimescale of these variations is consistent with that shown by the B (cid:96) measurements (Fig. 3). Unfortunately, as less than one full ro-tation period is sampled in these observations, it is not possibleto establish whether this is definitively due to rotational mod-ulation of active regions; although this appears to be the mostlikely explanation. In particular, as these spectra were obtainedby integrating over 1 hour, shorter timescale phenomena (e.g., Article number, page 4 of 7ussain et al.: Spectro-polarimetric study of HD 147513
Fig. 4.
RV variability in m s − over the 8-day span of the dataset aboutthe mean (dotted line – 13.232 km s − ). The red line shows the predictedRV contribution of the planet over the same timeframe using the re-ported ephemeris (Mayor et al. 2004). As in Fig. 3, the x-axis shows thetime in Barycentric Julian Date. pulsations, granulation) are unlikely to contribute significantlyto these RV measurements (Dumusque et al. 2011).
6. Discussion and conclusions
We have presented an analysis of spectro-polarimetric data ofthe planet-hosting barium-rich G dwarf, HD 147513. We mea-sured an S-index of 0.23 using the Ca II H&K lines and usethis to compute a mean chromospheric activity index, log R (cid:48) HK , of − .
64. We compared our HARPS spectra with archive FEROSCa II H&K spectra acquired six years previously and find anidentical level of chromospheric emission in the cores of the pro-files, which indicates that the level of the activity remains moreconstant than the range of published log R (cid:48) HK indices would sug-gest (Sa ff e et al. 2005).We obtain robust magnetic field detections at all observedepochs and note that this star belongs to the group of low-moderate activity stars as classified in the “Bcool” project. Mars-den et al. (2014) find that stars with a similar S-index typicallyhave a 60% chance of a definite magnetic field detection. Inthe same paper they conclude there is a smaller, 40% chance,of detecting a magnetic field in stars with similar v e sin i val-ues ( < − ). The activity measurements for HD 147513(log R (cid:48) HK of − .
64 and longitudinal magnetic field values, 0 . < B (cid:96) < . R (cid:48) HK index. Our mea-surements indicate a period of 12.4 days. Least-squares fittingof sine-curves to the B l and RV measurements (Figs. 3 & 4)reveal rotation periods of 10.4 d and 9.3 d respectively, thoughlonger periods cannot be excluded. Fitting both together we finda rotation period of 10 d, which is the period adopted for thereconstruction in the Appendix. We can therefore definitivelyexclude the 4.7 d reported by (Mayor et al. 2004). A 10-d pe-riod is consistent with an age of ∼ ff erent activ-ity saturation-threshold braking laws (Krishnamurthi et al. 1997, Reiners & Mohanty 2012). However, as large spreads are foundin rotation periods of stars with similar masses at these ages, itis simply noted that the gyrochronological age derived for HD147513 is consistent with the age of the UMa group.Significantly longer periods ( >
20 d) are ruled out due to therelatively high values of the chromospheric and coronal activityindices. The low v e sin i of 1.5 km s − is therefore likely due toa low inclination angle. With a period of 10 d we compute aninclination angle of 18 ◦ , which is in good agreement with 15 + ◦− ,as determined by Watson et al. (2010). Based on the maximumand minimum likely values of the radius, rotation period esti-mates and vsini measurements we find that the inclination canvary between 10 to 30 ◦ .Even though a good quality time series of HD 147513 wasacquired, as the period covered appears to be less than the rota-tion period of the star, we cannot definitively measure the rota-tion period of the star using ZDI as done in previous studies (re-cent examples; Je ff ers et al. 2014, Alvarado-Gomez et al. 2015).A map of the large-scale surface magnetic field is produced us-ing our best estimate of the period and the technique of ZDIin the Appendix (Fig. A.1). We investigate how the large scalestructure is a ff ected by the rotation period, varying the periodbetween 8–12 days, and find that some aspects of the large scalefield change (e.g., the magnetic field strength and energy). Re-gardless of the rotation period used no significant toroidal com-ponent is required and the Stokes V signatures can be adequatelyfit assuming a purely poloidal field. This is di ff erent to the ZDIanalysis of the G8V star, HD 1237 ( P rot = (cid:15) Eri and ξ Boo, the toroidal field component mayhave a stronger contribution at other epochs (Je ff ers et al. 2014,Morgenthaler et al. 2012).We computed the expected RV signature of the planet andfound this to be almost constant over the 8 d timescale probedby our observations. As noted, the measured RV variations showa similar modulation to that traced by B (cid:96) and are consistent withthe same period. It is therefore likely that the RV variability isstellar in origin and due to magnetic activity, e.g., due to a darkspot aligned with the dipolar field. Both surface spots and plageand broadening e ff ects due to the small scale local field arefound to a ff ect the shape of the line profiles albeit in di ff erentways (e.g., Dumusque et al. 2014, Hebrard et al. 2014).We calculate a RMS of 9 m s − in these RV measurements.This is 50% larger than the reported σ ( O − C ) of ± . − (Mayor et al. 2004). Those measurements were based on 30observations acquired over 1690 d whereas ours have been col-lected on a timescale closer to the star’s rotation period. The K -velocity amplitude due to the planetary orbit is 29 . ± . − .This is a higher level of activity jitter than previously reportedand further observations would be necessary to confirm whetherthis level of jitter is typical for the star but would likely not sig-nificantly a ff ect the planet detection.We note that the RV RMS we measure is of the same orderas that reported in the moderately active M2.5 dwarf, GJ 674,by Bonfils et al. (2007). Whereas the spot causing the RV vari-ability in GJ 674 shows a clear correlation with chromosphericand photospheric spectral indices, no such correlation is foundfor HD 147513. In HD 147513, the chromospheric activity in-dex remains constant over the eight-day span of the observations.This di ff erent relationship between the RV signature of the active Article number, page 5 of 7 & A proofs: manuscript no. RevHD147513_spectroscopicanalysis-arxiv region and the chromospheric activity index may be due to thedi ff erent spectral types of the stars, the lower chromospheric ac-tivity index of HD 147513 or di ff erences in the geometric prop-erties of the spot signatures in these two stars. Further studiescombining spectro-polarimetry with velocimetry are necessaryto better understand the dependence of RV jitter on these param-eters. Acknowledgements.
Based on observations made with ESO Telescopes at theLa Silla Paranal Observatory under the programme ID 089.D-0138 and usingspectra downloaded from the ESO Science Archive Facility under the requestnumber GHUSSAIN-162114. We also thank the IDEX initiative at UniversitéFédérale Toulouse Midi-Pyrénées (UFT-MiP) for funding the “STEPS” collab-oration through the Chaire d’Attractivité programme, which enables GAJH tocarry out regular research visits to Toulouse.
Appendix A: Magnetic field maps of HD 147513
We present here the maps of the large scale surface magneticfield of HD 147513 assuming the 10-d rotation period that pro-vided the best fit to the longitudinal field and RV variability re-ported earlier in the paper. These have been reconstructed us-ing the ZDI code presented in Hussain et al. (2002); this de-scribes the field in terms of spherical harmonics and allows forboth poloidal and toroidal field components. The local line pro-file has been modelled using a Milne-Eddington profile whosewidth and amplitude were adjusted to fit that of HD 147513 andthe equivalent width of 67 mÅ, following the approach of Do-nati et al. (2008). More specifically a Voigt profile was used inorder to better fit the wings of the Stokes I LSD profiles in thislow v e sin i star. The width of the local profile was adjusted tofind the best fit to the integrated Stokes I profile in agreementwith the published v e sin i value. The model fits shown here allassume a linear limb darkening law, with a limb darkening coef-ficient of 0.65 (Sing 2010).The resulting maps fit the observed data to a reduced χ of1 and are shown in Fig. A.1. By restricting the solution to adipolar solution ( l max =
1) convergence cannot be found beyond χ r of 1.4. We adopt a maximum spherical harmonic degree of l max = ◦ . Magnetic field regions downto − ◦ latitude should contribute to the observed line profiles instars with inclination angles of 18 ◦ .The region below the equa-tor is mostly not visible due to the low inclination angle. Never-theless field is reconstructed in the unobserved hemisphere dueto the low order spherical harmonics used. Most of the energy(70%) is concentrated in the aligned dipolar, quadrupolar andoctupolar components (25%, 24% and 21% respectively), withthe rest predominantly divided between the l = , m = l = , m = ξ Boo (Alvarado-Gomez et al. 2015) and (Morgenthaler et al. 2012) as the signa-ture would still be unambiguously detectable. ξ Boo is itself arelatively low inclination star ( i = ◦ ), and shows a dominant toroidal field component at its highest activity states. Future ob-servations of HD 147513 are necessary to reveal whether or notthere is a similar change in the relative strength of the toroidalfield component with its activity level.We analysed how the uncertainty in the rotation period maya ff ect the large scale field by reconstructing maps assuming peri-ods of 8 and 12 days. The general structure remains very similarto that shown in Fig. A.1 with the main features being either con-centrated or smeared out with the shorter and longer period. Themain di ff erence is in the strength of the magnetic flux, whichis 20-25% stronger and weaker in the maps derived for the 8-d and 12-d periods respectively. This is expected as the phasecoverage is more sparse in the 12-d map. Further observationsspanning 16–20 days would be necessary to ascertain the periodwith greater accuracy. References
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Fig. A.1.
Magnetic field maps and fits to the Stokes V data. The firstthree panels show the radial, azimuthal and meridional field compo-nents respectively. Red and blue represents ±
25 Gauss. The bottompanel shows the fits to the Stokes V profiles ( χ r = European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748Garching bei München, Germany e-mail: [email protected] Institut de Recherche en Astrophysique et Planétologie, Université deToulouse, UPS-OMP, F-31400 Toulouse, France Universitäts-Sternwarte München, Ludwig-Maximilians-Universität,Scheinerstr. 1, 81679 München, Germany CNRS, Institut de Recherche en Astrophysique et Planétologie, 14Avenue Edouard Belin, F-31400 Toulouse, France Univ. Grenoble Alpes, IPAG, F-38000, Grenoble, France CNRS, IPAG, F-38000, Grenoble, France LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Univ. ParisDiderot, 5 place Jules Janssen, 92190 Meudon, France LUPM-UMR 5299, CNRS & Université Montpellier, Place EugéneBataillon, 34095 Montpellier Cedex 05, France INAF-Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123Catania Italy SUPA, School of Physics and Astronomy, University of St Andrews,St Andrews KY16 9SS, UK Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,Cambridge, MA 02138, USA Department of Physics and Astronomy, University of Exeter, StockerRoad, Exeter EX4 4QL, UK Astronomy Department, Van Vleck Observatory, Wesleyan Univer-sity, 96 Foss Hill Drive, Middletown, CT 06459, USA14