owls Release 0.1.dev23+g040b913.d20210830

Aug 30, 2021

Contents

1 Background 3 1.1 Status Update for 2021...... 4

2 Survey 7 2.1 Calibrating the 푆-index for APO...... 7

3 S-Indices 9

4 Indices and tables 11

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Exoplaneteers and stellar atmosphere enthusiasts alike can agree on at least one thing: knowing the phase and strength of a star’s activity cycle is valuable information which can only be gained through long-term, continuous monitoring from mid-sized observatories like APO. In this program, we have created a legacy survey of exoplanet-host star Ca II H & K emission line observations extending into the distant future with APO/ARCES, which we call the Olin Wilson Legacy Survey (OWLS). The time-series database of Ca II H & K measurements will be a touchstone reference for exoplaneteers to determine the extent of contamination by stellar activity in their exoplanet observations, as well as a modern update to the Baliunas et al. (1995) sample which reveals how the magnetic activity of exoplanet host stars varies with spectral type, planet orbital period, etc.

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2 Contents CHAPTER 1

Background

Fig. 1: Some example APO/ARCES spectra from our APO catalog of Ca II H & K emission line stars. The emission at the center of the calcium H & K lines is proportional to the amount of chromospheric activity, which is correlated with the phase of the stellar activity cycle.

Two of the enduring questions of stellar astrophysics are: is the Sun a typical G star, and what is the range of magnetic activity that Sun-like stars exhibit? Magnetic activity on the surfaces of stars like the Sun manifests as dark spots, where strong magnetic fields locally inhibit convection, cooling roughly planet-sized regions in the solar photosphere (Solanki 2003). While they are dark in broad optical bandpasses, they emit excess flux compared to the mean solar photosphere in certain emission lines, like the Fraunhofer lines such as CaII H & K lines (Hall 2008). The presence of these magnetic active regions can have a wide variety of implications. The Sun is a dominant source of forcing on the Earth’s climate, and small changes in the solar SED can have measurable affects on the Earth’s climate (Solanki 2013). Thinking farther from home, observations of exoplanets orbiting solar-type stars are strongly affected by the presence of starspots: correct exoplanet radii and atmospheric compositions can be only deduced if the effect of starspots is mitigated (Morris2017a, Morris2018b). Over the span of three decades, a treasure trove of spectroscopic observations of nearby solar-type stars was collected by Olin C. Wilson (Wilson 1957, 1963, 1964, 1968, 1970, 1976, 1978). Several important discoveries were made as a result of this work. Perhaps most famously, Skumanich (1972) published a relationship between stellar age/rotation

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and chromospheric activity emission indices of stars in the CaII H & K emission lines of main-sequence solar-type stars based on Wilson’s observations. This is one of the seminal works which empirically established that stellar activity declines with age, spawning much interest in: (1) rotation as the foremost driver of stellar magnetic dynamos; and (2) Ca emission as a diagnostic indicator of the phase and strength stellar activity cycles. Further observations combined with archival data mining by Baliunas (1995) revealed the diversity of stellar activity cycle morphologies which vary as a function of stellar age and mass. Stars like the Sun have periodic ∼ 11 year activity cycles, whereas young solar analogs can have chaotic, non-periodic “cycles”. Much work has been done to calibrate equivalent H & K measurements of the Sun in order to place the Sun in the stellar context (Egeland 2017). In the coming decade, optical, ground-based observations of solar-type stars which establish the phase and strength of stellar activity cycles will be critical for understanding how stellar activity influences observations of exoplanet host stars at optical wavelengths. NASA’s TESS mission is discovering a plethora of planets orbiting solar-type stars at a variety of ages, like V1298 Tau (K0V, 23 Myr) and DS Tuc (G6V, 45 Myr) (David 2019, Newton 2019). These younger stars are the keys to determining how planetary radii evolve with age as atmospheres cool and shrink (Fulton 2017); but these younger stars are also far more active than the Sun, and we can expect their starspots to contaminate spectroscopic and photometric follow-up of these compelling targets. Fortunately, JWST observations will not be strongly contaminated by stellar activity (Zellem 2017); but any complementary TESS, HST, CHEOPS and eventually PLATO observations can reach sensitivity regimes where the effects of stellar activity on the radii and transmission spectroscopy measurements of these stars can become significant. The stellar activity cycles of the planet-hosting stars may vary from periods of ∼ 1 − 10 years, and we can only know the phase and strength of the stellar activity cycles for planet-hosting stars if we measure them a few times a year with high resolution spectroscopy. APO is uniquely suited to take these activity observations – the ARC 3.5 m Telescope can efficiently collect high S/N spectra of nearby, bright solar analogs for fast activity index measurements over the next decade. A time-series database of CaII H & K measurements will be a touchstone refernece for exoplaneteers to determine the extent of contamination by stellar activity in their exoplanet observations, as well as a modern update to the Baliunas (1995) sample which reveals how the magnetic activity of exoplanet host stars varies with spectral type, planet orbital period, etc. In Morris (2019b), we observed solar-type stars with the ARCES echelle spectrograph to measure CaII H & K activity, see Figure 1. We calibrated the ARCES instrument against the Mount Wilson Observatory’s catalog of observations by Duncan (1991), enabling us to measure the 푆 indices of any star observable with APO/ARCES. In this program, we will go beyond the set of stars original observed by Morris (2019b) to create a legacy survey of exoplanet-host star H & K measurements going into the distant future with APO/ARCES, which we call the Olin Wilson Legacy Survey (OWLS). We have already developed a Python package for reducing the spectroscopy from the ARCES instrument called aesop which is modular and easily adopted by new students (Morris 2018), and we have already calibrated the ARCES instrument to produce 푆-index measurements (Morris 2017b, Morris 2019b). All that remains to do is to collect spectra of as many targets as possible over as long a time baseline as possible. All 푆- indices measured by OWLS will be immediately released online for use by astronomers characterizing the exoplanets of our host stars, all over the world. We plan for a data release announcement to occur once per year, led by a graduate student at the University of Washington.

1.1 Status Update for 2021

With the first year of OWLS observations logged in 2020, we are pleased to report that we have successfully measured the 푆-indices of 50 FGK stars with ARCES. We have reduced the observations and confirmed that we are measuring the flux in the CaII H & K cores with sufficient S/N to measure precise 푆-indices useful for chromospheric activity monitoring. A sampling of observations of 푆-indices with (very small uncertainties) are shown in Figure 2.

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Fig. 2: Visibility of 110 targets in our catalog – each row corresponds to the visibility of a single target, black pixels are when the target is not visible, white pixels are when the target is visible. You can see that there are targets observable for every month of the year, with the over-density of Kepler targets yielding a block of summer targets.

Fig. 3: Calibrated 푆-indices for 50 OWLS targets obtained in the year 2020.

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Table 1: A fraction of the 110 targets in the OWLS catalog, which we plan to observe throughout the upcoming years, with the exposure times required to reach a S/N=25 at the CaII H & K lines. Name G RA Dec Exposure [min] HD 3167 8.762 00h34m57.52s +04d22m53.3s 5 WASP-93 10.978 00h37m50.11s +51d17m19.5s 35 HAT-P-16 10.757 00h38m17.56s +42d27m47.1s 29 K2-209 10.745 00h58m45.76s +01d23m01.7s 28 K2-222 9.361 01h05m50.95s +11d45m12.3s 8 WASP-118 10.92 01h18m12.13s +02d42m10.2s 33 WASP-76 9.395 01h46m31.86s +02d42m01.9s 8 WASP-77 A 10.097 02h28m37.23s -07d03m38.5s 16 HD 17156 8.033 02h49m44.49s +71d45m11.6s 2 K2-175 10.625 03h30m00.86s +17d35m03.1s 25

Fig. 4: OWLS planet-hosting target spectroscopic properties. All stars have radii and effective temperatures consistent with single, main-sequence host stars with near-solar metallicity.

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Survey

The following text is an excerpt from Morris et al. (2017). The definition of the 푆-index has the unfortunate property that its value varies from one instrument to the next for the same intrinsic flux. Therefore 푆-index measurements must be calibrated against the stars observed in the Mount Wilson Observatory (MWO) sample before spectra from different observatories can be compared. In

2.1 Calibrating the 푆-index for APO

We reduce the raw ARCES spectra with IRAF methods to subtract biases, remove cosmic rays, normalize by the flat field, and do the wavelength calibration with exposures of a thorium-argon lamp1. We fit the spectrum of an early-type star with a high-order polynomial to measure the blaze function, and we divide the spectra of HAT-P-11 and the MWO stars by the polynomial fit to normalize each spectral order. Next the normalized spectra must be shifted in wavelength into the rest-frame by removing their radial velocities. We remove the radial velocity by maximizing the cross-correlation of the ARCES spectra with PHOENIX model atmosphere spectra (Husser 2013). To calibrate the ARCES spectra, we follow the calibration procedure developed in Isaacson (2010) for HIRES. We collect 51 spectra of 30 stars in the Duncan (1991) MWO sample with the ARC 3.5 m Telescope at APO and the ARCES spectrograph, including 22 K stars, 7 G stars and one M star. We measure the 푆-index for these stars by taking the sum of the flux in the cores of the 퐻 and 퐾 features at 3968.47 Å and 3933.66 Å, weighted with a triangular weighting function with FWHM=1.09Å. We normalize the weighted emission by the flux in pseudocontinuum regions 푅 and 푉 , which are 20 Å-wide bands centered on 3900 and 4000 Å, respectively. Then 푆 on the MWO-calibrated scale is 푎 퐻 + 푏 퐾 푆 = 퐴푃 푂 푐 푅 + 푑 푉

푆푀푊 푂 = 퐶1푆퐴푃 푂 + 퐶2,

1 An ARCES data reduction manual by J. Thorburn is available at http://astronomy.nmsu.edu:8000/apo-wiki/attachment/wiki/ARCES/ Thorburn_ARCES_manual.pdf

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where 푎, 푏, 푐, 푑, 퐶1 and 퐶2 are parameters that can be tuned to make ARCES 푆-indices match the scale of 푆푀푊 푂 (Duncan 1991). Following the example of Isaacson (2010), we chose values of 푎, 푏, 푐, 푑 so that 푆 has roughly equal flux contributions from the 퐻 and 퐾 emission lines, and roughly equal flux from the 푅 and 푉 psuedocontinuum regions in the APO spectra. Thus we set 푎 = 푐 = 1, and we choose 푏 = 2 and 푑 = 1, so that 퐻 ∼ 푏 퐾 and 퐾 ∼ 푑 푉 .

Since 푆 varies over time for each star in the sample, the linear correlation between the 푆퐴푃 푂 and 푆푀푊 푂 will have some intrinsic spread. To incorporate this into our model, we adopt the ⟨푆⟩ and the standard deviation of 푆 from Duncan (1991) as the measurement and uncertainty of the MWO values. We solve for the constants 퐶1 and 퐶2 and their uncertainties with Markov Chain Monte Carlo (MCMC) (Goodman 2010, Foreman-Mackey 2013). We find +0.99 +0.011 퐶1 = 21.26−0.83 and 퐶2 = 0.009−0.009. The software tools used to calculate calibrated 푆-indices with spectra from ARCES are publicly available2.

2 ARCES Calcium II H & K Analysis Toolkit: https://github.com/bmorris3/arces_hk

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S-Indices

The 푆-index measurements for planet hosting stars are listed below, with the date and time of observation, and the uncertainty in the S-index. If you use these S-index measurements in your work, please reach out to Brett to arrange citations of this database. [ graph ]

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Indices and tables

• genindex • modindex • search

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