A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host $\nu$ Octantis A

A decade or so after the first exoplanets were described (Wolszczan & Frail 1992; Mayor & Queloz 1995), radial velocity (RV) evidence for the first retrograde planet appeared from a very unexpected source, the compact single-lined spectroscopic binary $\nu$ Octantis (Ramm 2004). However, these RVs shared the same initial fate as those from $\gamma~{}{\rm Cep\,A}$ which almost hosted the first acknowledged RV-discovered planet (Campbell, Walker & Young 1988; Walker et al. 1992). $\gamma~{}{\rm Cep\,Ab}$ was eventually confirmed 15 yr later (Hatzes et al. 2003), and was then the shortest period binary with a planet ($P_{\rm\scriptscriptstyle bin}\sim 57$ yr) – about 20$\times$ longer than that for $\nu$ Oct. Both initial series of RVs led their discoverers to describe both planet and stellar rotation-related scenarios, the latter then being suspected as more likely causes. These two binaries have many other parallel details in their exoplanet histories and host star characteristics.¹¹1For example, and very significantly, both hosts were originally classified as giants, rare for early exoplanet claims – and both K0 III – which made a stellar origin for the RV signals more tenable. They also share a trivial near-polar declination detail: $\lvert\delta\rvert=77\degr$.

Host stars may provide evidence that a candidate exoplanet orbit is retrograde relative to the star’s rotation (which we label type-1), and if in a binary system, relative to the two stellar orbits (our type-2). The first acknowledged retrograde planet was HAT-P-7b, promptly identified using the McLaughlin-Rossiter effect (our type-1; Winn et al. 2009; Narita et al. 2009). This had been preceded in the same year by the first paper describing the conjectured planet $\nu~{}{\rm Oct\,Ab}$, (Ramm et al. 2009; henceforth R09), which has proven to be much more challenging for establishing its reality beyond reasonable doubt. This is almost entirely due to the unprecedented geometry of the conjectured system, whose stars are separated by only 2–3 au with the circumstellar i.e. S-type planet about midway between (see e.g. Gong & Ji 2018; Bonavita & Desidera 2020; Quarles et al. 2020). Some details for $\nu$ Oct (HD 205478, HIP 107089) and its conjectured planet (based on the persistent RV cycle with a period $P_{{}_{\rm RV}}\sim 415$ d) are listed in Table 1 and Table 2. The parallax was recently updated from Gaia observations, and critically for the planet claim, increased by about 10 per cent (Gaia Collaboration 2018; Kervella et al. 2019 - whose work specifically includes nearby stars with stellar and substellar companions).²²2Thus \nuoais less luminous and should be classified closer still to a subgiant, $\sim$ K1 IV, as now is also $\gamma~{}{\rm Cep\,A}$ (Hatzes et al. 2003; Fuhrmann 2004). § 3.5 will discuss this important revision.

The system’s geometry makes a prograde planet impossible (R09; Eberle & Cuntz 2010). An S2 (i.e. S-type, our type-2 retrograde) orbit has a significantly wider stability zone than its prograde equivalent (see e.g. Jefferys 1974; Wiegert & Holman 1997; Morais & Giuppone 2012).³³3With zero evidence for retrograde status we label the orbit type-0 i.e. prograde, and assumed if not stated. If the host provides evidence of both type-1 and 2 we would label it type-3 (1+2). The scheme is applicable to single star hosts i.e. type 0 or 1, and both S- (circumstellar) and P-type (circumbinary) planets (Dvorak 1986), i.e. type 0, 1, 2 or 3. This opportunity for $\nu~{}{\rm Oct\,Ab}$’s reality was first investigated by Eberle & Cuntz (2010), and subsequently by Quarles, Cuntz & Musielak (2012), Goździewski et al. (2013) and Ramm et al. (2016; henceforth R16), all of which found retrograde solutions with merit but without being conclusive. Meanwhile, however, multiple other tests demonstrate that all other explanations (e.g. measurement artefacts, stellar variability, $\nu$ Oct being a triple-star system) are substantially less credible than the planet (R09; Ramm 2015 - henceforth R15; and R16). If the planet is an illusion, \nuoaso far provides no evidence of its causative role other than the persistent precise cycle of so far 1437 RVs over 12.5 yr, and thus would have the alternative distinction of seriously confronting our understanding of stellar variability.⁴⁴4The RVs were obtained using two CCDs and different calibration techniques ($\rm I_{2}$-cell and thorium-argon spectra) and therefore also different reduction methods. No other star’s study using our instruments and methods provide any similar planet-like RVs. The primary star’s argument of periastron, $\omega_{1}=75\degr$, has not changed significantly in 95 yr so any claim that a hierarchical triple-star system is creating the planet-like RVs is also unfounded (see R09 and R16 for details).

Goździewski et al. identify a small number of nearby mean motion resonances for coplanar S2 orbits in $\nu$ Oct including one at the period ratio 5:2, this having been qualitatively suspected in R09. The unprecedented strong interactions suggest resonance is likely to be a contributing factor for stability, as in other multi-body systems (e.g. Campanella 2011; Robertson et al. 2012; Horner et al. 2019; Stock et al. 2020). $\nu~{}{\rm Oct\,Ab}$ would never be confined to a typical narrow orbital path but would move endlessly throughout a rather wide zone as Eberle & Cuntz (2010) first illustrated, leading to significant challenges for competing models trying to characterize different orbital geometries (Panichi et al. 2017).

Many S-type planets have now been reported, though demands for any following an S2 orbit remains rare since the binary must be quite compact to lead to this conclusion. Scenarios that may create such orbits include star-hopping (Kratter & Perets 2012), perhaps facilitated by a scattering event, or significant system or orbital changes involving stellar winds, accretion/debris discs or evolution to a white dwarf (WD) companion (see e.g. Tutukov & Ferorova 2012). The WD scenario may have support for $\nu$ Oct from spectrophotometric observations nearly 50 yr ago (Arkharov, Hagen-Thorn & Ruban 2005). They reported unusually large variability ($S_{\nu}=0.14$ mag) in the near ultraviolet (345 nm) in the 1970s. Of the 172 ‘normal’ and 176 ‘suspect variable’ stars they catalogued, only two others had larger $S_{\nu}$, both in the latter group. This may be a clue to the nature of $\nu~{}{\rm Oct\,B}$ as it seems unlikely \nuoais the source.

The first close-planet candidate for a WD host was recently reported (WD1856+534; Vanderburg et al. 2020), discovered using NASA’s Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015), another space mission that should help unravel the $\nu$ Oct mystery. WD1856 b’s orbit ($P_{\rm\scriptscriptstyle pl}\sim 1.4$ d), is presumed to be due to reconfiguration and migration processes (see e.g. Lin, Bodenheimer & Richardson 1996; Rasio & Ford 1996), which must have relevance for any orbitally retrograde planet. Together with mass loss/transfer processes inherent in WD evolution, these scenarios may further complicate attempts to understand $\nu~{}{\rm Oct\,Ab}$ but also provide some advantage since orbit reconfiguration (of the planet or the binary) would not require interaction from external or other internal masses, as anticipated with an unevolved secondary star.

Two other recent claims are also relevant here. Firstly, another S2-type planet, HD 59686 Ab, has been conjectured in this somewhat wider but rather eccentric binary (Ortiz et al. 2016; Trifonov et al. 2018; K2 III, $a_{\rm\scriptscriptstyle bin}\sim 13.6$ au, $e_{\rm\scriptscriptstyle bin}\sim 0.7$). Interestingly, HD 59686 B is also suspected of being a WD, leading Ortiz et al. to investigate one possibility that the planet formed from a second generation protoplanetary disc. It is tempting to speculate that $\nu$ Oct and HD 59686 may be two early examples of relatively compact binaries with WD secondaries that created S2-type planets. Secondly, the first planet in a binary more compact than $\nu$ Oct was recently reported (HD 42936, $\rm K0V+L$ dwarf; $a_{\rm\scriptscriptstyle bin}\sim 1.2$ au; Barnes et al. 2020). HD 42936-Ab is close to its host ($\sim 0.066$ au) so both it and WD1856 b are assumed to have reconfigured orbits, but with no evidence demanding either be retrograde.

Understandable suspicion also persists that \nuoa’s RV signal may be due to undetected surface activity. Hatzes et al. (2018) repeated this concern after $\gamma~{}{\rm Dra}$’s anticipated single-planet claim disappeared after more RV data was acquired. Oscillatory convective modes (Saio et al. 2015) were instead promoted as the variable RVs more likely origin, just as Reichert et al. (2019) speculate for Aldebaran – instead of its conjectured planet (Hatzes et al. 2015). But unlike \nuoawhich is a relatively unevolved low-luminosity star (and so far has a persistent periodic RV signal), $\gamma~{}{\rm Dra}$ ($\sim 500$$~{}\mathcal{L}_{\odot}$, 50$~{}\mathcal{R}_{\odot}$), and Aldebaran ($\sim 400$$~{}\mathcal{L}_{\odot}$, 40$~{}\mathcal{R}_{\odot}$) typify the theoretical high-luminosity expectations of a star having these convective modes. Both are classified K5 III and are reminders of the potential complications such evolved stars, both real and mistaken (e.g. \nuoaand $\gamma~{}{\rm Cep\,A}$) can present. Other radial and non-radial oscillations in stars similar to \nuoainstead have a complex power spectrum that complicate asteroseismology studies (see e.g. Dupret et al. 2009). This is not at all consistent with the planet-like RV signal \nuoahas provided to date. Hence, if \nuoais deceiving us, a more reasonable concern is that stars more similar to it are more likely to be creating well-crafted illusions, such as $\gamma~{}{\rm Cep\,A}$. This possibility would create significant complications for other exoplanet claims now and in the future, and perhaps not restricted to similar subgiants.

This background has motivated the photospheric and chromospheric studies reported here. Photospheric line variations are critical for RVs but also for the extremely temperature-sensitive method of line-depth ratios (LDRs; e.g. Hatzes, Cochran & Bakker 1998; Gray & Brown 2001; Kovtyukh et al. 2003; R15; R16). R15 and R16 demonstrate temperature constancy for \nuoato as little as $\pm 4$ K over 12 yr, with high consistency with the very low photometric variability recorded by Hipparcos (see Table 1). This latest LDR series is also relevant as, more or less concurrently, observations have also been obtained by NASA’s TESS mission which significantly support \nuoa’s photometric stability, as well as RVs from ESO’s HARPS spectrograph (Trifonov et al., in preparation).

Chromosphere activity of \nuoahas so far been limited to three eye estimates of the $\rm Ca\,{\rm\scriptstyle II}$ K line from each of two spectra (Warner 1969), a single spectrum of $\rm Ca\,{\rm\scriptstyle II}$ H & K (R09) and H-line indices from 17 spectra (R16). All three papers concluded \nuoahad minimal $\rm Ca\,{\rm\scriptstyle II}$ activity. Additionally, the Mg $h+k$ chromospheric emission-line study by Pérez Martínez, Schröder & Cuntz (2011) found \nuoa’s activity to be very near their sample’s empirically-derived basal flux of 177 cool giants. Here we report on two much larger series: 198 line indices from the $\rm Ca\,{\rm\scriptstyle II}$ H line and 1160 $\rm{H}\,\alpha$ indices.

Our work allows \nuoato be characterized in further detail, providing new evidence for the planet since significant activity is again refuted. Even if the planet is eventually proven to be an illusion, which seems to be increasingly unlikely, our work adds to the fundamental knowledge of the star at the centre of this persistently challenging system.

The data analysed here were acquired in two series, one about a decade ago and the other over the past two years. Both series were obtained at the University of Canterbury Mount John Observatory using the 1-m McLellan telescope and HERCULES, a fibre-fed, thermally insulated, vacuum-housed spectrograph (Hearnshaw et al. 2002). The CCD was a $\rm 4k\!\times\!4k\$detector cooled to $\sim-95\degr$ C which recorded the \nuoaspectra with a resolving power $R\sim 70\,000$. Further instrument details and our reduction methods for obtaining flat-fielded normalized spectra can be found in Ramm (2004), where the first tentative mention of the conjectured planet is made, and R16. Our reduction software (HRSP; see Skuljan 2004) automatically provides both vacuum and air wavelengths. To make other vacuum-to-air conversions we used the formulae of Morton (2000).

The older series comprises nearly 1200 spectra obtained from 2009.96–2013.96, i.e. four years, and allowed our chromosphere studies of the $\rm Ca\,{\rm\scriptstyle II}$ H line and $\rm{H}\,\alpha$. The spectra were almost all obtained using an iodine cell that reconfirmed the precise-RV planet-like signal (see R16). For our $\rm{H}\,\alpha$ study, 1160 spectra were included in the final analysis, made possible by the higher $S/N$ at these longer wavelengths. The initial sample size for our $\rm Ca\,{\rm\scriptstyle II}$ analysis was limited to 700 spectra as the $\rm Ca\,{\rm\scriptstyle II}$ lines were not included in the first year or so of observations. More details of these data sets will follow in the relevant sections.

A smaller set of 135 spectra were acquired more recently, from 2018.34 until 2020.42 i.e. over about 25 months. As the $\rm I_{2}$ cell was not used for these observations, we could not obtain further high-precision RVs but instead could use these spectra to obtain a new series primarily of line-depth ratios. The last spectrum obtained (2020.42; JD245 9002) has the highest $S/N$ of our entire series and was acquired to also extend that range of our $\rm Ca\,{\rm\scriptstyle II}$ spectra. The time span of all spectra considered here is from the first LDR spectrum, 2001.85, until this last observation i.e. $\sim 19$ yr.

We first present the results from our line-depth-ratio analysis of the new spectra (2018–2020) and compare them to the previously published LDR results (2001–20013 ; R15 and R16) as well as again, the Hipparcos photometry (1990–1993). The LDRs demonstrate the continued thermal stability of \nuoa’s photosphere and so are a useful prelude to our chromosphere studies.

Many spectral lines have been identified as being useful for monitoring chromosphere activity in late-type stars (see e.g. Lisogorsky, Jones & Feng 2019). One of the motivations for assessing such behaviour is the quest for increasingly precise RVs for exoplanet searches. Whilst many photosphere lines can be used as indicators for chromosphere activity, the classic ones with the strongest history are the resonance doublet lines of $\rm Ca\,{\rm\scriptstyle II}$ H and K at 3968.47 and 3933.66 Å close to the boundary of the ultraviolet and visible (see e.g. Wilson 1963; Linsky & Avrett 1970; Wilson 1978; Duncan et al. 1991; Baliunas et al. 1995). However, these two lines have the weakness, which will significantly influence our study, that they are recorded in spectral regions that have inherently low $S/N$.

At the other end of the visible spectrum is $\rm{H}\,\alpha$ ($6562.808$ Å). This has the distinct advantages of much higher $S/N$ than provided at $\rm Ca\,{\rm\scriptstyle II}$ H and K ($\sim 4-5\times$ greater), and a better defined continuum without the complications of many neighbouring metal lines as $\rm Ca\,{\rm\scriptstyle II}$ H and K also have. However, as we will show, $\rm{H}\,\alpha$ instead has the complication of telluric lines, and for a quiet star such as \nuoa, these lines dominate the line-index variability. The $S/N$ differences resulted in our having a final set of about 200 $\rm Ca\,{\rm\scriptstyle II}$ H-line indices but nearly six times as many $\rm{H}\,\alpha$ indices.

We report five new independent series of precise data that all indicate \nuoais a very quiet star: two photometric series from TESS (2019, 2020), which more or less coincide with our LDR study of its photosphere (2018–2020), and our two large surveys of its chromosphere whose origin is the $\rm I_{2}$-cell RV time series (2009–2013). The TESS photometry bounds the 1437 RVs (2001–2013) with Hipparcos observations three decades before (1989–1993). A sixth line of new evidence is the Gaia parallax that indicates \nuoais less luminous, and so more probably approximately K1 IV. This also strongly favours the quiet-star classification, as does our review of solar-like oscillation scenarios. These results are consistent with all previous surveys, for instance, more recently, two bisector studies (R09 and R16), and two other LDR studies (R15 and R16).

There is no significant variability of any of these many photometric or spectroscopic time series, and certainly none correlated with the planet-like RVs. Other facts are against $\nu$ Oct being a heirarchical triple-star system or the cause being related to \nuoa’s rotation. Thus this overwhelming evidence demonstrates all non-planetary solutions continue to be implausible, which instead we interpret is only compatible with a retrograde planet whose properties are approximated in Table 2.

With thousands of planets now described, $\nu~{}{\rm Oct\,Ab}$ remains unprecedented in terms of the system’s geometry. It still provides many opportunities for exploring unexpected dynamical models of planet formation and orbit stability, as well as exceptional observational opportunities for such a bright, short-period system’s orbital behaviour. Recently found evidence that the secondary star may be a white dwarf was reported in the Introduction which would have a significant bearing on such studies. As more evidence accumulates only in favour of the planet, the flip-side of irrefutable evidence that the planet is instead an illusion is how unexpected any non-planetary alternative would then be.

We began by confirming how thermally-stable \nuoa’s photosphere continues to be. This allowed us to confidently claim the photosphere, which has produced the enigmatic RVs for over a decade, is highly unlikely to contribute any variability to our chromosphere indices, nor be the source of the planet-like signal. Knowing the thermal stability of the photosphere is a useful advantage for any chromosphere study, a detail not commonly reported.

It is no doubt partly due to \nuoa’s quiescence that has allowed our study of its $\rm Ca\,{\rm\scriptstyle II}$ spectra with such low $S/N$ and successfully pursue our atypical methods. It is also this stability that has helped us critically examine the telluric line contribution to our $\rm{H}\,\alpha$ spectra and quite clearly reveal its quasi-periodic behaviour and origin. All our results strongly imply they record mostly random processes and not something systematically cyclic.

Because similar studies use data that may be at least moderately correlated it seems that covariance deserves closer scrutiny in such cases. Even though the covariance terms have opposite signs, the correlation coefficients may still cause the covariance terms to increase the errors significantly – as our $\rm Ca\,{\rm\scriptstyle II}$ results demonstrate. Our methods for analysing low-$S/N$ spectra may allow other so-far neglected archival material to be recovered usefully as we quite surprisingly achieved here. Finally, we introduced a simply described classification system for retrograde orbits that appears effective for our purposes, and may be of sufficient merit for use by others.

We appreciate the anonymous referee’s several suggestions that led to helpful improvements. DJR thanks M. F. Reid, then Physics & Astronomy HoD, for renewing his Research Fellow status (2018–2020) allowing continuing access to academic and IT resources including the assistance of O. K. L. Petterson. DJR is also grateful to the UC Mt. John Observatory director K. R. Pollard and the present HoS R. Marquez for the generous allocation of observing time and research support for this project that provided us the opportunity to obtain more data for $\nu$ Oct. We thank the observing technician F. Gunn for obtaining the spectra 2018–2020 and K. R. Pollard and R. Marquez of the School of Physical and Chemical Sciences, University of Canterbury for sourcing observing and technical support for this project at the UC Mt John Observatory, including funding from The Brian Mason Trust, UC Foundation, The School of Physical and Chemical Sciences and The Otago Museum. DJR thanks M. Kürster (MPIA) and K. J. Moore (Christchurch) for helpful contributions. This research made use of Lightkurve, a Python package for Kepler and TESS data analysis (Lightkurve Collaboration, 2018).

The data underlying this article will be shared on reasonable request to the corresponding author.